Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED SHOT PEENED PRESS PLATE
AND METHOD OF PRODUCING SAME
This is a divisional of Canadian Patent Application
Serial Number 2,453,325.
FIELD OF THE INVENTtON
This invention relates to a textured press plate for manufacturing
decorative laminates and a. method for producing same. Specifically, a press
plate
may be simultaneously blasted by a mixture of at least two sizes of shot at
high
velocity to produce a textured press plate with a desirable combination of
texture
and gloss.
BACKGROUND OF INVENTION
Press plates are used in a well known process to prepare decorative
laminates. These laminates are used for surfaces such as walls, table tops,
furniture,
doors, kitchen cabinets, countertops, flooring tiles, and the like. The
decorative
_etting resin
laminates are formed by compressing and bonding together thermps
impregnated pigmented or printed decorative cellulosic surface papers to a
variety
of core materials, such as particle board or phenolic resin impregnated kraft
paper
filler sheets and the like between press plates, with the press plate
imparting a
surface finish to at least one side of the pressed laminate. The materiai
layers are
first placed adjacent to a stainless steel press plate. The laminate
comporients and
press plate assembly are then pressed at high pressures and elevated
temperatures
to fuse the laminate materials together. After final cure, a consolidated
iaminate
sheet or board is formed. Once cooled, the iaminates are separated from the
press
plates, which are returned to the press for use in the manufacture of
subsequent,
laminates.
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The resulting laminate surface finish is an exact mirror image replication
of the press plate surface finish against which it is pressed. Press plates
are thus
an important feature in the production of consistently acceptable commercial
laminate surfaces, as the laminate surfaces faithfully adopt the surface
finish of the
press plate.
There are two general categories of press plates, polished and textured.
The scope of the present invention is directed. to textured press plates used
to
manufacture matte or suede laminate finishes. A variety of methods result in a
desired textured press plate finish. A common method is chemical etching.
While
this is an effective method for producing the desired textures and overall
finish, it
is both costly and difficult to implement. Another method of texturizing
includes
shot peening, such as that disclosed in U.S. Patent No. 4,076,566.
Shot peening techniques are primarily used for their mechanical benefits,
as the cumulative effect of individual particle impacts on the workpiece
surface cold
works the article. As the shot particles are propelled or blasted at high
velocities
onto the metal surface, the resulting plastic flow of the metal somewhat
relieves
residual tensile stresses near the surface of the article by inducing
compressive
~= stresses: The mechanical benefits are improved fatigue.strength:.and_*~fs~
por!'o~icm :
resistance, or if desired, reshaping of flat pieces. Two major parameters must
be
controlled to achieve the desired results -- surface coverage and peening
intensity.
Surface coverage is a measure of the percentage of the article's surface
area impacted by the shot particles. The cumulative impacts of the shot
particles
should nearly or fully saturate the surface area to obtain the mechanical
enhancements normally sought through peening. Peening intensity is a measure
of
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the amount of force of each individual impact and the cumulative work done on
the
surface of the article. Peening intensity is thus a function of the impact
velocity and
particle size and density (i.,,,, mass). The optimum peening intensity and
resultant
compressive stress layer depth is also related to the metal alloy and hardness
of the
material being peened.
However, in contrast to the typical applications of shot peening, the use
of shot peening in the manufacture of textured press plates does not primarily
seek
enhancement of mechanical properties nearly as much as providing the desired
press
plate surface finish aesthetics, including its texture, gloss, and overall
appearance
while retaining its requisite flatness. Strict adherence to surface coverage
and
peening intensity criteria, as in _mechanicai applications of shot peening,
are not
critical except from the standpoint of consistently obtaining the desired
textured
plate and resultant laminate appearance. Maintaining plate flatness during
peening
and obtaining uniform gloss and roughness over the surface of the plate are
Important considerations.
Peened textured press plates typically begin as highly polished plates
having a surface finish known in the press plate and laminate industry as a#7
mirror
õfi,[rishr ;T-be.&e plates-are predominantly fabricated, from AI,$1.41 p
stair.iless steel.This;=
aiioy offers relative hardness, damage resistance, and corrosion resistance,
as well
as dimensional stability after repeated thermal cycling. Shot peening an
initial #7
mirror plate finish has been found to inevitably lower the gloss of the plate.
This
results in a duller surface finish due to the myriad indentations which cause
increased light scattering as perceived either visually or instrumentally (i
gi, through
use of an electronic gloss meter)... In general, the greater the shot coverage
'up to
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saturation, the lower the resultant gloss, as the total area of shiny
"islands" (non-
impacted areas) on the surface decreases. Importantly, saturation of the
surface can
be achieved relatively easily when smaller or finer shot grades are used,
eliminating
an overall speckled appearance and obtaining a uniformly dull "matte" (lightly
textured) finish.
For aesthetic reasons, laminates with "rougher" textures to simulate a
"suede" finish are sometimes desired. "Suede" textures have been found
attainable
through the use of larger shot grades and/or increased impact velocity,
both=of which
as discussed above contribute to greater peening intensity. However, problems
occur when larger shot grades are used.
In "rougher" textures obtained through the use of larger shot grades
(creating greater impact crater depths through greater impact forces), courser
peak
structures appear. Further, since press plates are large, thin, and flat
articles, they
become more and more dimensionally unstable with greater peening intensity.
For
example, peening one side of the plate with a large shot grade at high
velocities
causes plates to warp with a convex curvature on the peened face. Although
this
can be somewhat solved by "balanced" shot peening machines which equalize face
~tb 'face irOtiCeddor9piresaive- stresses,by simultaneously btasting
both.sides: of. .ft == . : ; .,. , . : :. =
plate, larger shot grades still tend to heighten dimensional instability.
Additionally, with larger shot grades, it has been found progressively
more difficult to achieve a saturated surface free of what is often referred
to as
objectionable "sparkle." Peening machines essentially process a given mass of
shot
per unit time. Therefore, the number of pellets striking.a plate surface per
unit time
surface coverage) Is an inverse cubic function of increased shot diameters,
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necessitating either significant slowing down of the peening speed, multiple
passes,
propelling a greater mass of beads per unit time, or a combination of the
above.'The
former two approaches have serious drawbacks from a practical commercial
viewpoint, and the latter alternative is governed by machine design
limitations. To
achieve an acceptably uniform plate appearance approaching saturation with
larger
shot grades, all three alternatives in effect result in a greater peening
intensity and
associated stress relief problems if not perfectly balanced.
Moreover, producing fully saturated, roughly peened surface textures is
not usually difficult to achieve in a laboratory or pilot plant environment.
However,
serious surface coverage and peening intensity restrictions contributing to
overall
roughness limitations are often encountered with large production size plates.
These
press plates must remain nearly perfectly flat after peening if they are to be
used in
a conventional high pressure decorative laminate manufacturing process, which
use
a plurality of plates during each pressing operation.
Thus, the process described above using a single size range or grade
steel shot to texture a plate has serious limitations in Its ability to
achieve the higher
roughnesses desired, which requires the use of larger shot grade, while still
achieving
adeq.uate+caver,a.gearicha uniform fftnish;(without excessive sparkle) vYithi~
r~asg,nabfeu~..,
processing times.
U.S. Patent 4,076,566 largely resolved the above diametrically opposed
roughness and coverage constraints imposed when processing full size
production
plates. It disclosed first using a relatively large shot grade to obtain the
desired
roughness and texture structure, followed by a second smaller grit or shot
grade to
achieve a fully saturated sparkle-free surface appearance. Although the
desired
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macro-texture or roughness is first developed using large shot grade peening,
full
coverage does not result. The plate is then blasted with smaller shot grade
(with
much more efficient covering ability) to obtain saturation and elimination of
residual
high gloss islands without altering the initially formed macro-texture of the
plate.
Importantly, the need for subsequent electropolishing and preferred chroming
is
eliminated with this process.
Moreover, the use of steel shot in the peening process of the prior art
causes several problems. Steel shot with the preferred hardness is brittle and
prone
to shatter. Softer steel alloys tend to flatten and cannot be readily
separated from
the shot peening machines. Moreover, each hardness of steel shot has a
different
effect on the roughness of the finished press plate. Steel is also quite
dense,
requiring powerful blasting machines which result in greater cost and machine
wear.
Additionally, steel shot leaves an iron/iron oxide residue within the
impact craters generated on the surface of the press plates. These deposits,
particularly when exposed to moisture, quickly oxidize and result in an
objectionable
overall rusty or "blackened" appearance to the plate surface. The oxide
residue is
difficult to remove by normal washing techniques and often requires special
treatment-:=pFior: to artyr;,fi.nat,.p~cessing::.(g;,g~;:-chrome plating) or
laminate::pressiag...;_ ...: ,. ,.,.:,~ ..;
Another conventional shot peening material is glass beads, particularly
used to avoid the contamination associated with steel shot when processing
stainless
steel and non-ferrous metals. However, glass beads have three distinct
disadvantages compared to other shot materials, particularly as applied, to
texturing
or overblasting stainless steel press plates. First, being =less dense than
either steel
or ceramic shot, it is difficult with glass shot to obtain blasting
intensities required
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to achieve relatively deep texturing, even at high impact velocities. Second,
glass
beads are very brittle and are prone to breakage and shattering into a fine
glass
"dust", particularly at high impact velocities. This dust is very aggressive
towards
the plate surface, behaving in a manner similar to sharp angled "grit" rather
than the
preferred spherical shot, resulting in micro-scarring of the plate surface and
undesirable light scattering and "grayness" of the surface of laminate pressed
from
it. In fact, glass beads shatter so quickly that. during the processing of a
single
plate, a severe gloss gradient can develop along its length due to the
progressive rise
in abrasive blasting with an accompanying decrease in the preferred shot
peening.
A final problem is the extreme hygroscopicity of glass beads, causing
cohesion between the beads and sporadic or total interruption of the bead flow
to
the blast guns. This further promotes an unacceptably non-uniform blast
pattern and
gloss differential over the surface of a plate. Conversely, the flow fluidity
of other
shot materials, especially ceramic shot, are unaffected by ambient moisture
conditions.
Even after.the finished textured press plates are obtained in any of the
methods discussed above, refinishing -remains a large concern. After a number
of
pressings; . the :~piater:..-*ften. :d.evicelo.ps.y=IcFoscratches induced by
laminate- surface
materials, especially by the presence of hard grit particles which are now
more
frequently being added to the laminate surface to improve wear and mar-
resistance.
Also, macroscopic imperfections may occur during routine handling in the form
of
burnishing, rub marks and small scratches. These defects on the plate surface
in
turn tend to degrade the surface finish of the laminates, which faithfully
reproduce
the press plate surface finish. When the microscratches and visible defects
become
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too pronounced, the plates must be replaced or refinished. Thus, the texturing
process must also be capable of consistently providing a method to refinish
the press
plates.
As an aspect of the present invention, it has been recognized that the
combined texture and gloss achieved by mixing different grades of shot
provides
equivalent or improved results compared to using a discrete shot grade and
peening
a plate several times, each peening operation. using a different sized shot
grade.
Thus, mixing different sized shot grades is a superior method than shot
peening a
plate multiple times with different sized shot grades each time. The former
method
1.0 avoids the need for separate overbiast steps. . Furthermore, mixing
various shot
grades avoids having to empty the blasting machine of large shot grades and
refilling
the machine with small shot grades or the converse. This conversion process is
time
consuming, particularly if a large quantity of plates are to be processed.
Mixing shot
grades also avoids the alternative of dedicating equipment to each grade of
shot
used In the process to maintain productivity.
It has thus been discovered that the two-step process of large shot
grade peening followed by small shot grade overblasting as disclosed in U.S.
Patent
No._ 4,076,566 can be simplified to a,hav'~r,1gsjg.~ificantly fewer blasting
i.............. 1... .. ._. _.,r... ... .. . ..... - .. ,. ....... .-. . .e ..
. . . .,...... . . .
operations white achieving essentially the same results, wherein a mixture of
a
predominantly large shot grade (preferably 75-90%) and a small shot grade
(preferably 10-25 %) can be used simultaneously to effectively texture the
press plate
and provide the f911 coverage and sparkle-free surface desired. In combination
with
the improvements sought, the further use of ceramic shot rather than
conventional
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steel shot further avoids the need for chemical decontamination after peening
and
the disadvantages of glass shot.
Thus, the present invention contemplates using a mixture of at least a
first shot grade and a second shot grade in the plate finishing process. Large
shot
grade peening and small shot grade overblasting are simplified to a one step
process
while achieving the same results as multiple passes with different shot
grades.
Furthermore, mixtures of various grades of "larger" shot may be used in that
portion
of the total mix to form a more visually complex and pleasing textured
structure and
pattern, wherein a wider variety of intermixed large shot grade impact crater
diameters form a continuous three dimensional roughness matrix.
The further advantages of the use of mixed shot grade peening and
ceramic shot will become apparent to those versed in the art, such that the
above
disclosures set forth should not be construed as limiting to the scope of the
instant
invention as claimed.
SUMMARY OF INVENTION
This invention involves a method for obtaining a peened textured finish
-a' press plate with desirable glosi;' features''Abd~'oiVbratl appearartae~-
and with at
least 95% coverage. The present invention contemplates shot peening with a
mixture of at least two size ranges or grades of shot to obtain this surface
finish,
including particularly ceramic shot. The untreated press plate is impacted
with a
mixture of shot grades, wherein the mixture with at least the two different
grades
of shot are fed into a propelling device such as a blast gun or wheel located
a fixed
distance from the plate surface. .. The gun or wheel thus projects the shot
grade
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mixture against the surface of the press plate. After the
desired texture and gloss is achieved, the finished press
plate is removed from the blasting or shot peening equipment.
Moreover, the present invention contemplates and discloses a
method for improving the gloss and general appearance of worn
used plates.
According to a broad aspect of the invention, there
is a press plate for producing decorative laminate having a
textured surface of at least 95% peening coverage formed by
shot peening the surface simultaneously with a mixture of
shot particles, the mixture of shot particles including a
first quantity of shot which is at least 60 percent of the
total shot mixture and which comprises shot which is
essentially 250 m or larger in diameter for imparting a
texture having an Ra of between 1.26 m and 1.80 m and a Rz
of between 8.88 m and 13.86 m to the surface and a second
quantity of shot which is up to 40 percent of the total shot
comprising shot which is essentially between 5 and 250 m in
diameter for adjusting a gloss level of the surface of the
plate to between ISO 89 - 145.
According to another broad aspect of the invention
there is a press plate for producing decorative laminate
having a textured surface having at least 95% peened coverage
formed by shot peening the surface simultaneously with a
mixture of ceramic shot particles, the mixture of ceramic
shot particles including a first quantity of ceramic shot
comprising shot grades for imparting a texture having an Ra
of between 1.26 m-1.80 m and a Rz of between 8.88 m-13.86
m to the surface and a second quantity of ceramic shot
comprising shot grades for adjusting a gloss level of the
surface of the plate to between ISO 89 - 145.
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According to a further broad aspect of the
invention there is a method for texturing the surface of a
press plate used in the manufacture of decorative laminate
with at least 95 percent peening coverage, characterized in
that the surface of the press plate is shot peened at least
one time with a mixture of shot particles which include a
first quantity of shot which is at least 60 percent of the
total mixture, the diametrical sizes of the particles
thereof being essentially 250 m or larger in diameter, and
a second quantity of shot which is up to 40 percent of the
total mixture, the diametrical sizes of the particles
thereof being essentially between 5 and 250 m in diameter.
According to yet a further broad aspect of the
invention there is provided a method for finishing press
plates to obtain a textured surface by shot peening, the
method comprising the steps of: supporting the surface to
be finished; loading shot having a mixture of shot grades
into a peening device located a fixed distance from the
surface, where the mixture of shot particles includes a
first quantity of shot comprising shot grades for imparting
. ,,.. . : . _ _,
a texture to the surface of the plate and a second quantity
of shot having shot grades for adjusting a gloss level to
the surface of the plate; propelling the shot mixture from
the shot peening device onto the surface; and removing the
press plate for subsequent use in decorative laminate
manufacture.
According to yet a further broad aspect of the
invention there is provided a method for texturing a surface
of a press plate used in the manufacture of decorative
laminate with at least 95% surface peening coverage by shot
peening, the method comprising the steps of: supporting the
surface in a chamber; loading shot having a mixture of shot
grades into a shot peening device located a fixed distance
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from the surface, where the mixture of shot grades includes
a first quantity of shot grade having a first range of
diametrical sizes for imparting a texture to the surface and
a second quantity of shot grade having a second range of
diametrical sizes for adjusting a gloss level to the surface
of the plate; propelling the shot mixture from the shot
peening device onto the plate surface; and removing the
press plate for subsequent use in decorative laminate
manufacture.
According to yet a further broad aspect of the
invention there is provided a method for finishing press
plates to obtain a textured surface by ceramic shot peening,
the method comprising the steps of: supporting the surface
to be finished; loading ceramic shot having a mixture of
ceramic shot grades into a peening device located a fixed
distance from the surface, where the mixture of ceramic shot
particles includes a first quantity of ceramic shot
comprising shot grades for imparting a texture to the
surface of the plate and a second quantity of ceramic shot
having shot grades for adjusting a gloss level to the
surface of the plate; propelling the ceramic shot mixture
from the shot peening device onto the surface; and removing
the press plate for subsequent use in decorative laminate
manufacture.
According to yet a further broad aspect of the
invention there is provided a method for texturing a surface
of a press plate used in the manufacture of decorative
laminate with at least 95% surface peening coverage by
ceramic shot peening, the method comprising the steps of:
supporting the surface in a chamber; loading ceramic shot
having a mixture of ceramic shot grades into a shot peening
device located a fixed distance from the surface, where the
mixture of ceramic shot grades includes a first quantity of
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ceramic shot grade having a first range of diametrical sizes
for imparting a texture to the surface and a second quantity
of ceramic shot grade having a second range of diametrical
sizes for adjusting a gloss level to the surface of the
plate; propelling the ceramic shot mixture from the shot
peening device onto the plate surface; and removing the
press plate for subsequent use in decorative laminate
manufacture.
According to yet a further aspect of the present
invention there is provided a method for re-finishing a
surface of a press plate used in the manufacture of
decorative laminate with at least 95% surface peening
coverage by ceramic shot peening, the method comprising the
steps of: supporting the surface in a blast chamber;
loading ceramic shot having a mixture of ceramic shot sizes
into blast guns located a fixed distance from the surface,
where the mixture of ceramic shot particles includes a
quantity of ceramic shot having a range of diametrical sizes
essentially between 5 and 250 pm in diameter for adjusting a
gloss level to the surface of the plate; propelling the
ceramic shot mixture from the blast guns onto the plate
surface; and removing the press plate for subsequent use in
decorative laminate manufacture.
Ceramic shot has only recently become commercially
available. As late as 1982, no reference was made of the
use of ceramic shot in that year's ASI Metals Handbook
(Volume 5, Ninth Edition) section on Shot Peening. It has
been found that ceramic shot offers significant advantages
over conventional shot media for peening and press plate
texturing. Smaller ceramic shot grades also offer
advantages over small steel shot or glass bead sizes for
"non-texturing" repair of grit induced micro-scratches or
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other superficial damage (i.e. refinishing) incurred by
textured plate surfaces during use, where the macro-texture
is not appreciably altered during the overblasting.
Ceramic shot is about twice as hard as standard
cast steel shots, the latter with typical hardnesses between
Rockwell C 40-50, which in turn are only slightly harder
than typical 410 stainless steel press plates (38-45 HRC).
Special hard steel or iron shots are available (57-65 HRC),
but these shots are quite brittle and are easily broken upon
impact. However, even though extremely hard, ceramic shot
is not prone to breakage as is hard steel shot, nor does it
deform upon impact as does softer, more durable steel shot.
Deformation, such as slight flattening into a non-spherical
condition, is undesirable, yet shot so effected is not
easily separated from the system. Further, ceramic shot
offers a smoother initial surface finish than steel shot.
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As such, ceramic shot provides hardness and lack of deformation upon
impact, little breakage with ease of separation if breakage does occur, and an
inherently very smooth surface, which all contribute to consistently well
formed
hemispherical impact craters, which in turn minimize light scattering and
resultant
haziness or grayness (loss of color fidelity) of laminates pressed from
ceramic shot
blasted press plates.
Further, in contrast to steel shot, ceramic shot is chemically inert and
does not leave any embedded residue on the plate surface, obviating the. need
for
any chemical "decontamination" process for the removal of residual oxide
materials
after blasting.
Also, while ceramic shot has about twice the hardness as steel shot,
it is only half as dense. Thus, for equivalent shot size ranges or grades,
ceramic
shot requires less shot "tonnage" and transport capacity and allows for the
use of
smaller, simpler and less costly (and more easily maintained) blasting
equipment to
achieve about the same degree of coverage (or saturation) and peening
intensity
(impact force), since the lighter ceramic shot can be propelled to higher
impingement
velocities. Particularly in the case of large steel shot grades, very
powerful, motor
driven, high speed centrifugal wheel "slinger" machines are required to obtain
.the=
desired impact velocity for the shot. Similar or greater velocities can be
achieved
with ceramic shot using simpler and more versatile compressed air blast guns
in
which the shot is injected into a high velocity air stream. Due to ceramic
shot's
lower density (and greater hardness) compared to steel, it was found to be
more
effectively transported and projected by simpler, less expensive air-blast
type peening
machines (rather than centrifugal wheel type equipment), resulting in faster
coverage
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rates and comparable finish textures. The ceramic shot can be fed to the blast
guns
either by a vacuum suction venturi effect, or injected directly under pressure
into the
compressed air stream, which then propels the shot towards the target with
very
high impact velocities, especially with smaller shot sizes.
The higher striking velocity and lower density of ceramic shot compared
to steel shot thus tend to off-set each other and, for equivalent shot size
ranges,
result in about the same amount of work applied to the plate surface. However,
with press plates, enhancement of mechanical properties is not an important
consideration compared to the aesthetics (i.e., the texture, gloss,
"structure," and
overall appearance of the surface). Strict adherence to surface coverage and
peening
intensity criteria are not critical except for consistency in obtaining the
desired
textured plate and resultant laminate "look". Maintaining plate flatness
during
peening, and obtaining uniform gloss and roughness, however, remain important
considerations.
Moreover, it has also been discovered that, unlike peening with other
media such as steel shot or glass beads, gentle overblasting with small
ceramic shot
grades at low velocities can actually adjust w r s the gloss of a peened plate
(the
degree of gloss rise "being highly dependent on the intrinsic texture
geome:try;
roughness and initial gloss of the plate). Heretofore, peening has been solely
a
dulling or matting process, with the magnitude of the effect dependent on the
severity of the processing conditions employed. However, it was found that
when
small ceramic spheres strike the plate surface with just enough force to cause
plastic
flow of the metal, minute microscratch fissures are "sealed" and the
microfinish
imperfections are obliterated, effectively reducing light scattering and
increasing the
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perceived gloss level. This, ability to upwardly adjust the plate gioss using
miid
impact velocity (low propellent air pressure) with small ceramic shot grades,
and
preferably B60 shot, was wholly unexpected and has the distinct advantage of
allowing gloss recovery of used press plates that have become microscratched
by
inclusion of hard grit materials in the laminate surface to enhance their
physical
surface properties, and in particular, wear properties related to abrasion,
mar,
scratch, scrape and scuff resistance during exposure to fabrication and end-
use
installation conditions. This process reforms the microtexture and restores-
the plate
finish to its original quality.
Ceramic shot gloss adjustment also provides a method, within limits, to
readjust plate gloss to the desired level (either downwards or upwards) after
additional surface treatment (ea., chrome plating), where gloss shifts away
from the
prescribed level are often encountered.
Likewise, ceramic shot is a superior alternative to glass beads. It is
denser than glass and thus may be used to achieve higher blasting intensity
for
deeper texturing. Also, ceramic shot does not shatter and as such does not
produce
the fine dust produced by glass beads. Finally, ceramic shot does not absorb
moisture and thus'does-not clurnp andlinterfWahot flow. -'
A further benefit is that since ceramic shot minimizes microscopic plate
surface disruptions (and resultant laminate haziness), the peened press plates
produced with ceramic shot do not need to be additionally chrome plated to
ensure
releasability of the finished laminate after pressing.
Accordingly an object of the present invention Is to obtain the desired
uniform texture and gloss finish on a press plate.
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Another object of the present invention is to minimize the number of
passes needed to texture the press plate, using a mixture of shot grades
simultaneously.
It is another object of the invention to use a shot material to avoid the
problem of impact shattering and resulting micro-scarring of the press plate
surface.
It is also an object of the invention to use a shot media capable of
increasing the gloss of a press plate after blasting.
It is a further object of the invention to obtain a shot = material
particularly suited for refinishing textured press plates.
It is still another object of the invention to use a shot material that is
easily handled by conventional blasting machinery.
It is yet another object of the invention to use a shot material to avoid
the problem of iron residue contamination.
Still another object of the present invention is to use a shot material
which is at least as hard as steel shot but less dense.
The above and other related objects and features of the present
invention will be apparent from a reading of the following detailed
description of the
referred embodiment -and the appended. = -
p clairras - . . ..
DESCRIPTION OF THE DRAWINGS
The present invention may be better understood with reference to the
drawings in which:
Figure 1 is a vacuum-type air-blast shot peening machine for processing
press plates utilizing the method of the present invention;
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Figure 2 is the plate infeed lift table, vertical support, and trolley
mechanism for holding and transporting the press plate for the machine in
Figure 1;
Figure 3 is the blast cabinet of the shot peening machine of Figure 1;
Figure 4 is an overhead view of the configuration of the blast guns in
the blast cabinet of Figure 3;
Figure 5 is the reclaimer, shot feed supply, classifier and dust collector
components of the shot handling system for the machine in Figure 1;
Figure 6 is a cross sectional view of the shot feed hopper of -the shot
handling system of Figure 5;
Figure 7 is a cross sectional view of one of the vacuum-type
compressed air blast guns shown mounted in Figure 4; and
Figure 8 Is a side view of a daylight opening of a hydraulic laminating
press containing a pack of laminates interieaved with press plates obtained in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings, wherein reference characters designate like
or corresponding parts throughout the ~~ews;; Figure "~ illustrates >~ shatr-
peentng.
machine 2 suitable for utilizing the method of the present invention and for
producing
a press plate according to the present invention. The shot peening machine 2
has
a blast cabinet 4, which is provided with a pair of access doors 6, 8 each
having an
observation window 10, 12. Mounted above and around the blast cabinet 4 is a
gun
tower 14 comprised of a metal frame structure. ' The gun tower~~14 has a
central
column 16 as a mount and guide for a gun carriage 18. The gun carriage 18
CA 02617480 2008-02-07
'76668-7E
extends into the blast cabinet 4 and reciprocates up and down the central
column
16 as will be explained below. The blast cabinet 4 is used for the actual shot
peening operation to contain and collect the expended shot.
As can be further seen in Figure 2, the blast cabinet 4 is attached on
either side to a support frame 20 which facilitates movement of the press
plate 42
through the blast cabinet 4. A plate rail 22 supports a plate trolley 48 which
transports the press plate 42 through the blast cabinet 4. Multiple slotted
rollers
24 are interspaced along the plate rail 22 to guide the cross-sectional T-
shaped plate
trolley 48 and vertically held press plate 42 through the blast cabinet 4.
Further,
- plate guide 26 is lowered over the top edge of the plate 42 to maintain the
plate in
a vertical position through the blast cabinet 4.
As the press plate 42 is transported through the blast cabinet 4, It is
blasted by shot from guns mounted on the lower end of the gun carriage 18.
This
operation will be described in detail below. The shot is supplied to the guns
via feed
hoses 28, as shown in Figure 1. In the preferred machine shown, six separate
guns
each require a separate feed hose 28. The feed hoses 28 are preferably made
from
natural gum rubber to minimize wear. The spent shot is removed -from the blast
, .. .:.aabinet 4,by=ah-airend, shot collection outlet-pip -man#foJdt 30
located:in xhe;.bostam.,.,_, ....; .
of the blast cabinet 4. The collection outlet pipe manifold 30 is attached to
an air
and shot collection hose 32 through automatic shot refill hopper 80.
Spent shot is continuously recycled by the shot peening machine for
reuse by the guns. The spent shot in the air collection hose 32 Is fed into a
cyclone
reclaimer 34 for separation of dust and fine shot fragments. The dust and fine
shot
fragments are then sent to a dust collector 36 for removal from the system.
The
1-6
CA 02617480 2008-02-07
' 76668-7E
remaining shot is then sent through a classifier 86 as will be described in
more detail
below, with the good shot separated and returned to a feed hopper 38, to which
the
feed hoses 28 are attached and which re-supplies the guns with a continuous
flow
of shot.
Figure 2 shows the support frame 20, plate rail 22, plate guide rollers
24, and top plate guide 26 of the plate handling system. The support frame 20
has
an infeed table 40 which is a fabricated steel frame attached through hinges
41 to
rotate from a horizontal to a vertical position. An unfinished press plate 42
is loaded
onto the infeed table 40 with the infeed table 40 in a horizontal position
resting on
table support frame 45. The infeed table 40 is fitted with bristled pads 44 on
which
the unfinished plate 42 lies to avoid scratches. The frame of the infeed table
40
may be provided with position markers to aid in properly locating various
sizes of
unfinished plates.
Once the plate 42 is positioned on the infeed table 40, the infeed table
40 and plate 42 are rotated into the vertical position by an air cylinder (not
shown).
The lower edge of the plate 42 is inserted in a narrow lengthwise groove (not
shown) in-the plate trolley 48 mounted on the plate rail 22 when the infeed
table 40
:-~ .: t MaVu4w, siatrW.tioiW>j~"ition.. ,The .plate 42 is then locked. into:
the uertice.k-ppsitipn.
by means of lowering the plate guide 26 onto the top edge of the plate 42. The
plate guide 26 is a V-shaped top edge guide extending the entire length of the
shot
peening machine on both sides of the blast cabinet 4. The plate guide 26 is
moved
into position by two sets of motorized screw drives (not shown).
A variable speed drive motor (not shown) lacated under the plate rail 22
and mounted to the support frame 20 slowly drives the plate trolley 48 with
the
17
CA 02617480 2008-02-07
.76668-7E
plate 42 through the blast cabinet 4. A second fixed speed fast forward and
reverse
drive motor (not shown) is used only to quickly position the press plate 42
prior to
blasting and retracting it after processing, while the variable speed slow
drive system
is used during the actual blasting operation. Of course, other methods and
devices
may be utilized to position and hold the plate 42, and drive it through the
blast
cabinet 4.
Figure 3 is a detailed view of blast cabinet 4 with the plate 42 partially
inside for the blasting operation. Figure 4 is an overhead view of the plate
42 in the
blast cabinet 4. The plate 42 has a first surface 56 and a second surface 58
which
are blasted and textured according to the present invention. The lower end of
the
gun carriage 18 extends into the interior of the blast cabinet 4 from the gun
tower
14. The lower end of the gun carriage has a first gun battery 60 having a gun
mount 62 and three compressed air blast guns 64, 66, 68. Located on the other
side of the plate 42 is a second gun battery 70 having a gun mount 72 and
three
compressed air blast guns 74, 76, 78. The gun batteries 60, 70 are attached to
the
gun carriage 18 so that they are diametrically opposed to each other, with the
guns
64, 66, 68 located at a fixed distance from the first surface 56 and the guns-
74,
.,...g6"~6+16 ,.,~
.' , p,.~.. .,srEe gurts=fi4, 66, 68 at the same fixed
distance<frram..thwsec,oW~;.:~~.ri..õ .., .: ,
surface 58. The plate trolley 48 and the plate 42 are driven through the blast
cabinet 4 at a slow and continuous rate of speed (i g, 3 to 4 in/min). Once
the
plate 42 enters the blast cabinet 4, the gun carriage 18 and blast guns 64,
66, 68,
74, 76, 78 are activated, rapidly (jg, 90 to 110 ft/min) reciprocating up and
down
and evenly blasting the plate surfaces 56 and 58'with shot.
18
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The gun carriage 18 reciprocates up and down along the central column
16 by means of a dual chain and sprocket drive system, with a gun carriage 18
counter weight attached to both chains. The two chains are independently
driven
by separate motor and air clutch assemblies (not shown). The gun carriage 18
is
guided by a series of sealed bearing rollers spaced around the central column
16.
The sealed rollers prevent bearing contamination from the shot. Two air
clutches
move the gun carriage 18 in opposite directions.. When a first air clutch is
engaged,
the gun carriage 18 moves upward until it reaches the top of the central
column 16
and the gun batteries 60, 70 reach the top of the blast cabinet 4, triggering
a first
proximity switch (not shown). The first proximity switch disengages the first
air
clutch and engages a second air clutch which moves the gun carriage 18
downward
until it reaches the bottom of the central column 16 and the gun batteries 60,
70
reach the bottom of the blast cabinet 4 and triggers a second proximity
switch.
The second proximity switch disengages the second air clutch and re-engages
the
first air clutch which moves the gun carriage 18 upward again. The rapid
reciprocation cycle is repeated continuously as the plate 42 slowly traverses
through
the blast cabinet 4. In this manner, all areas of the surfaces of the plate 42
are
eqiiaf~~(~Fasted; a~ev~i~~:flv~rage isx-neoessary far a uniform
finish._of:#tte
and to maintain its flatness. Of course, other means and methods may be
utilized
by those skilled in the art to position and move the guns in relation to the
plate 42.
After the surfaces 56, 58 of the plate 42 are blasted, the spent shot
falls and is collected at the bottom of the blast cabinet 4. The spent shot is
removed through the outlet pipe manifold 30 and drawn through the air and shot
collection hose 32 by suction created by a blower 37 in the dust collector 36.
The
19
CA 02617480 2008-02-07
76668-7E
recycling and shot feed system may be seen better in Figure 5. The air and
shot
collection manifold outlet 30 from the blast cabinet base is connected to an
automatic shot refill hopper 80, which is in turn connected to the air and
shot
collection hose 32 return line, which transports new shot as required from the
automatic refill hopper 80, along with the spent shot, to the cyclone
reclaimer 34.
The cyclone reclaimer 34 separates out dust and fine shot fragments produced
by
shot breakage. This waste is removed and sent.to the dust collector 36 via a
dust
exhaust hose 84.
The remaining shot is preferably filtered by a classifier 86 which uses
a series of vibrating screens to separate out oversized shot and broken shot
pieces
Into waste bins. The remaining good shot is then sent by a pipe 88 to the shot
feed
hopper 38. Various numbers of screens and sieve size combinations can be
utilized
depending on the shot grade mix used. Alternately, the classifier 86 section
of the
shot recirculation system may be bypassed if so desired, and the spent shot in
the
system periodically disposed of or screened and good shot recovered external
to the
system by other means. The above devices for recycling and sorting shot may be
replaced by other methods and devices which perform the same function of
ensuring
cont~.nu$ ;u~e ,a~fy gg~od~~aal+#y::~it~ ..~ < _ ..wv t , w., .
The shot feed hopper 38 has a level probe 90 which senses the level
of shot in the system. If the shot level falls below a preset point, a control
signal
is sent to the automatic refill hopper 80, actuating a valve to add more shot
to the
collection hose 32. At the bottom of the shot feed hopper 38 directly below
three
isolation cylinders 92 are three pairs (six total) of shot. feed orifice tees
94. Each
orifice tee 94 feeds shot to a separate blast gun, with each of the three
cylinders 92
CA 02617480 2008-02-07
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mounted on the shot hopper 38 capable of isolating a pair of two blast guns.
The
six shot feed hoses 28 are attached to the six feed tees 94, and shot is
transmitted
to the guns 64; 66, 68, 74, 76, 78 as described above. The blast gun shot flow
rates and feed hopper shot level may be monitored by lowering the isolation
cylinders
92.
Figure 6 is a cross sectional view of the shot feed hopper 38 and one
of the isolation cylinders 92 and orifice tees 94 used to feed shot to the
guns. The
shot 100 falls from the reclaimer 34, through the classifier 86 to the base of
the
shot hopper 38. The shot then is drawn into one of the six feed tees 94
through
an orifice 102 by suction generated at the guns 64, 66, 68, 74, 76, 78. An
orifice
plate 104, which has a hole drilled in it, is installed on the opposite end of
the feed
tee 94 from the outlet 105 to which the feed hose 28 is attached.
The orifice plate 104 is used to restrict and control the amount of air
wash and optimize the rate of shot feed flow to the guns. For example, for
optimal
balance of the gun types described below, a 5/32 inch diameter orifice 104 on
each
tee 94 results in a shot flow rate of about 3 kg/min per gun. Alternately, a
common
air wash manifold (not shown) can be connected to all of the feed tee 94 air
inlets
instead of individ.ual: flr.ifice-piat$s 10"o4itisure identical air wash and -
shot -flow.to .=:.y
each gun. With such a manifold, air wash and shot flow rates can be adjusted,
gg by a needle valve, to compensate for various gun compressed air operating
pressures.
The shot flow to each pair of opposed blast guns may be isolated while
the shot peening operation is ongoing by means of lowering the cylinder 92. A
removable cap 108 allows an operator to observe the shot flow to the
particular feed
21
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76668-7E
tee 94. If a blockage occurs, the operator may clear it through the cylinder
92. A
removable plate 110 is also provided on the side of the feed hopper 38 for
access
to the chamber for maintenance or other adjustments.
Each feed tee 94 outlet 105 is attached to a feed hose 28, which is in
turn attached to one of the compressed air guns. A cross section of the
preferred
compressed air gun 64 is shown in Figure 7. The compressed air gun 64 has a
gun
body 112 and a nozzle 114 for emitting shot. . The shot is sucked from the
feed
hose 28 through a hose adapter 118 into a chamber 116. Compressed air from a
compressed air regulator is fed through an air hose fitting 120 into the gun
64 and
through the air injector 124 port. The flow of compressed air through the
injector
124 creates a vacuum in the chamber 116 by a venturi effect. This vacuum sucks
and meters shot from the shot hopper 38 through the feed hose 28 into the
chamber
116. The compressed air from the injector 124 then propels the shot in the
chamber
116 out the nozzle 114 at high velocity to Impinge upon and blast the plate
surface
56.
The above described shot circulation system involves two opposed
vacuum systems. The primary vacuum is generated by the venturi effect from the
hi -h volunYe of -com ressect iif-vvt~rich's
9 p rr ~4fihie' g~Urts 64; 66, 68, 74r 76, 78, For-. _. ..~_
example, the guns in the preferred. embodiment each have a rated air flow of
36
c.f.m. A common air pressure regulator and manifold supplies all of the guns.
The
velocity of the shot can thus be controlled by the regulator, where lowering
the air
pressure reduces the suction and shot fed to the guns, and also results in a-
lower
propulsion of the shot and impact velocity on the plate.
22
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~
76668-7E
To adjust for small air supply pressure fiuctuations from the air
compressor and downstream accumulator, a pilot feedback regulator is fitted to
the
main compressed air regulator. The feedback regulator senses fluctuations in
pressure on the downstream side of the system and adjusts the main regulator
accordingly. In this manner, a constant dampened air pressure supply to the
guns
is achieved.
Of course, other types of blasting equipment for shot peening may be
used for the method of the present invention. The vacuum-type compressed air
blast
gun system as described above may be replaced if the use of denser or larger
shot
grades or higher velocities are desired. For example, a "pressure pot" type
air blast
system, where the compressed air is directly connected to the shot feed
hopper,
which then feeds the shot via hoses to unconstricted blast guns may be used
for
greater force. Blasting may also be accomplished by a centrifugal wheel or
slinger
machine having a vaned or bladed wheel attached to a motor to propel the shot.
Also, different numbers of blast guns in different arrangements may be
utilized.
Importantly, it has been found that circulating ceramic shot generates
several hundred thousand volts of static electricity which must be dissipated
or the
~.;>~,., ... .:., - ,... sfiot,.~will: . tend to . ,,agglomerate_.
aruk..4.behava.wl1k-e,"vwt:.sand., , blo,cking.:-=the shot
recirculation system and flow to the guns. An extensive grounding system is
therefore required for all isolated components of the above machine. For
example,
even the plate rail 22 is grounded using a spring loaded carbon motor brush
for
positive contact while It is=moving, since a static charge may accumulate on
the rail
and lower portion of the plate to which excessive shot.will then adhere.
23
CA 02617480 2008-02-07
76668-7E
In most high pressure decorative laminate commercial applications, the
press plates 42 are interleaved between successive layers of thermosetting
resin
treated laminate materials 200, separator sheets 201, and press plates 42 in
separate back to back laminate assemblies or doublets, as shown in Figure 8,
permitting the production of a plurality of laminate sheets in one pressing
operation.
As noted above, the textured press plates are used in direct contact with
composites
of resin treated paper sheets placed therebetween, with the decorative surface
sheets facing the plates and backed by filler sheets facing each other. -
Multiple
layers of laminate materials 200 and press plates 42 form "packs" 202,
supported
and transported on carrier trays 207, which are then loaded into a press 204
with
multiple openings 206 for curing and pressure treatment between
heating/cooling
platens 205. Note that as the clearance for material movement Into the press
204
is limited by the press opening 206, It is apparent from Figure 8 that warpage
or
curvature of press plates 42 can interfere with the ability to move the pack
202 into
the press 204.
Another form of decorative laminate is low pressure faced board, in
which thermosetting resin treated decorative papers are fused and bonded
directly
1.1nrrto OFre =-or-'bat-h sides of substrates-suct:asparticlabaar~-;,or Fwii-
umi~density
fiberboard. Most commonly, a single board is laminated between two press
plates
42. The pressing operation occurs within a press 204 with a single opening 206
and
two isothermal heating platens 205 using relatively low pressure and a very
short
press cycle time compared to the high pressure decorative laminate process.
The
details of the composition, construction and processing methods for each type
of
24
CA 02617480 2008-02-07
"76668-7E
product, Jg high pressure decorative laminate and low pressure faced board,
will be
well known to those skilled in the art.
General Exarnples
As a baseline example, using a vacuum-type compressed air shot
peening machine of the same design as described in detail above, a large, 4x10
foot
production size press plate 42, polished to a#7 finish, was shot peened with a
50:50 mixture of "large" diameter Grade B20 (essentially 600-850 Nm (0.024-
0.033
inch)) and "medium" diameter Grade B30 (essentially 425-600 pm (0.017-0.024
inch)) S.E.P.R. Zirblast ceramic shot. (it should be apparent to those
skilled in the
art that specifying a range of diameters for any particular grade of shot
will, of
course, possibly include a very small number of individual shot particles
having
diameters outside of the recited range of diameters, where the recited range
is
typically of a normal diameter distribution. The recitation of diameter sizes
as used
herein and for establishing a particular shot grade, however, has been found
by the
industry to represent a meaningful method of categorizing ranges of shot sizes
for
a number of purposes.
~~ -: ~~~~>= ~ Afrer=~o~veratl
ly-.:_-:,
passes at maximum bbast 9un. press-ura=arrd. a reiative
slow 3-4 inch/min plate traversing speed, about only 75% of the surface area
of the
press plate had been covered by peening, with little evidence of further
measurable
improvement possible. As such, the surface finish was considered unacceptable
because of the distinct sparkle appearance.
CA 02617480 2008-02-07
76668-7E
The plate was subsequently overblasted with "small" diameter Grade
B60 (essentially 125-250 pm (0.005-0.010 inch)) ceramic shot, which eliminated
the shiny islands and resulted in a desirable uniform gloss and visual surface
aspect.
In contrast, using the same equipment, an identical press plate 42
prepared in accordance with the present invention was simultaneously peened
with
a tertiary mixture of shot in the ratio 37.5 percent B20, 37.5 percent B30,
and 25
percent B60 S.E.P.R. Zirblast ceramic shot (i.~.,.75% medium to large size
shot and
25% small shot). As noted above, the large to medium shot grades provide a
textured finish while a small shot grade is utilized to achieve essentially
complete
overall coverage (saturation) and the desired plate gloss level. After only a
few
passes, the plate was satisfactorily textured by the larger shot grade mix and
at the
same time exhibited a desirable uniform gloss appearance.
In fact, while both plates looked quite similar in structure and overall
appearance and exhibited no objectionable residue from the peening operation,
the
plate using the present Invention was obtained with fewer passes. Both plates
were
subsequently used (without any post-treatment such as electropolishing or
chrome
plating) to press full size sheets of solid color laminates. Both plates
exhibited
- ~= exbe#tert~~~fe~~e -#rvi~"~he larl~inates u on se ar-axiori--witk~ -no
e~ridence:~fx: =
P P Pirkangi. . , z a . . . .
(traces of laminate surface resin and paper fibers remaining adhered to the
plate) or
physical sticking of the plate and laminate together. The laminate finish was
independently judged to be surprisingly similar in appearance to the
conventional
suede finish produced with LC-55 release/texturing paper supplied by Ivex Inc.
(formerly L&CP Inc.), which is widely used in the U.S. Jaminating industry.
26
CA 02617480 2008-02-07
76668-7E
Simultaneous mixed texturing and overblasting offers advantages over
the separate discrete processes of texturing and overblasting by reducing the
number
of passes required to satisfactorily process a plate (here, by at least one
pass).
Further, It eliminates the need for time consuming change-overs of the peening
machine from one grade of shot to another and back again, or the expensive
option
of dedicating equipment to each shot grade used in the process.
Snecific Examoles
A series of plate texturing shot peening triais were conducted to
demonstrate the utility and versatility of simultaneous mixed shot grade
texturing and
overblasting, rather than the use of discrete texturing and overblasting steps
as
taught in the prior art. The various shot grade mixtures tested are shown In
Table
1. All of the trials used a binary mixture of relatively large and medium size
shot
grades, to which was added a proportion of a small shot grade when
simultaneous
overblasting was practiced.
Examples #1 and #7 used ceramic shot and a second, discreet overblast
step to achieve acceptable peening coverage in a'manner similar to that taught
in the
.:. .,,- .
praRt: !t,=.neat,example..noted above. ; , r n _. . , ...
Examples #11 and #13 are similar, except cast steel shot was used to
simulate the texturing process taught by the prior art. Again, however, a
mixture
of large and medium shot grades rather than a single large shot grade was
used.
These examples illustrate the limited coverage obtained when attempting to
generate
a relatively rough texture without employing some form of overblasting
(whether
simultaneous or separate) to achieve adequate coverage and obliterate the
residual
27
CA 02617480 2008-02-07
76668-7E
myriad unpeened shiny "islands" which result in a generally commercially
unacceptable sparkle or mottled appearance. The use of binary mixtures of
relatively
large and medium shot grades were chosen for the plate texturing trials
described
herein, rather than individual shot grades as practiced by the prior art, to
obtain a
more diverse or "blended" peened appearance considered more aesthetically
attractive.
The ceramic shot used in these examples were various grades of
Zirblast fused ceramic beads, produced by Societe Europeenne des 'Produits
Refractaires in France and sold by their subsidiary S.E.P.R. Ceramic Beads and
Powders in the United States. The shot is essentially spherical, has high
impact
resistance and a low breakage rate, and is almost entirely free of metallic or
iron
oxide contamination. S.E.P.R.'s Zirshot , a still more highly refined (and
expensive)
range of ceramic beads, is also useful in the practice of this invention. Both
types
of shot have a typical composition of about 67% Zirconia (ZrOZ) and a 33%
glassy
phase (comprised of 30% SiOz and 3% impurities). It should be understood that
other ceramic shot compositions and grades from other suppliers might be
available
and are considered within the scope of the present invention. The cast steel
shot
used In these exeitiptes, uwith'~a'-typicai'' Mardriess of Rockwell C 40-50,
is a wiefel y . r,. .
used peening medium and is readily available from many suppliers.
The blasting or peening machine used for these trials was a Matrasur
Model PHF03-60 pressure-pot type compressed air blasting machine, more
powerful
than disclosed above, but on a much smaller scale suitable only for laboratory
testing
(Lg,, 10x10 cm. samples were used in the trials). All operating conditions
were held
constant during this trial, with the exception of air pressure as noted in
Table I. The
28
CA 02617480 2008-02-07
76668-7E
combination of conditions set, including a gun air pressure of 1.5 kg/cmZ,
simulates
reasonably well the peening intensity and coverage rate obtained with the full-
scale,
vacuum-type air-blast peening equipment disclosed above and used to process
full
size press plates according to the preferred practice of this invention.
Since the trial apparatus is not suited to dual side blasting, being
equipped with only a single blast gun, asymmetric blasting of one side of each
plate
sample occurred. As expected, unbalanced compressive stresses resulted in the
plates having a curvature which otherwise would be a major problem with large,
thin
production size press plates. However, for small plate samples, the equipmerit
is
convenient to use while obtaining generally reproducible results. The plate
samples
in the examples shown in Table I all exhibited convex warpage of the peened
face
with about a 1.0-2.5 mm. corner deflection.
The press plate samples peened and textured as recited in the Table
series were all A.I.S.I. 410 alloy stainless steel, with a hardness of 38-45
on the
Rockwell C scale. To prepare. the samples, a full-size production plate was
first
ground (commonly referred to as abrasive belt polishing) on a flexible flat-
bed
grinding machine using a series of belts of decreasing grit size (from course
180 grit
down,to-Une<320..gM-~,=anrith-cutting~nila-s hibrIcation, to remove deep
scratches and
other imperfections. The plate was then processed on a flat-bed cutting buff,
using
a hard buffing mop with a relatively aggressive cutting rouge (i.g_, a buffing
compound consisting primarily of alumina abrasive grit blended with animal
based
greases and fats) to remove residual grinding marks. Finally, the plate was
color
buffed with a soft "floppy" mop and fine rouge to a lustrous, scratch-free
condition
referred to in the art as a #8 mirror finish. The 10x10 cm. samples used for
the trials
29
CA 02617480 2008-02-07
7 6668-7E
were each cut from this plate. Thus, each sample was essentially identical to
the
others, the only variable being the shot grade mixtures used in the trials,
and in a
few special cases, the gun pressure selected.
It should be apparent to those versed in the art that less exacting
finishes than the "commercial" quality #8 mirror finish described herein are
also
suitable for shot peen texturing by the method of this invention, since the
peening
process itself obliterates and masks certain minor defects (which would
otherwise
be objectionable in a polished plate were it used as such to produce glossy
finish
product), and uniformly dulls the plate surface anyway. Thus, plates with
lesser
quality finishes requiring less stringent color buffing, and resultant gloss
and
reflectivity standards (gg, finishes commonly designated #6 and #7 in the
art), and
therefore less preparation effort, can also be used with the method of this
fnvention.
High gloss #8 finish plate samples were used for the trials summarized in
Table I to
facilitate judging the resultant peening coverage obtained.
All of the trials conducted using the process of this invention (except
Examples #1, #7, #11 and #13 as controls) used a binary mixture of relatively
large
and medium "texturing shot" grades to obtain a perceptible texture roughness;
with
a single -sfiot -gfade,,aCrnuci=i s-maiW -rovepbl~*asting, "sh t",ta achieve
the -requisite-...- ...
peening coverage and finish uniformity, in a combined single blasting
operation.
These examples are not to be construed as restrictive in terms of the shot
grade
mixtures within the scope of this invention. For example, use of a tertiary
mixture
of texturing shot grades In conjunction with an overblasting shot grade, such
as 25%
B16 (essentially 850-1180 /.rm (0.034-0.047 inch)), 25% B20, 25% B30, and 25%
B60 is also possible. Likewise, a single texturing shot grade, in combinatiori
with
CA 02617480 2008-02-07
'76668-7E
an overblasting shot grade (g,g:, 75% B20 and 25% B125 (essentially 5-125 pm
(0.0002-0.005 inch))) can also be used, as well as a binary mixture of
overblasting
shot grades.
Many combinations of various shot size grades offer the benefits of the
present invention. Overblasting shot grades are by definition significantly
smaller
than texturing shot grades, and herein are arbitrarily defined as essentially
having a
diameter of 250 pm or less. Conversely, relatively large texturing shot grades
are
herein arbitrarily defined as essentially having a diameter of 425 pm or
greater,
although It should not be construed that shot with diameters in the range 250-
425
Nm would not be useful. Texturing shot should comprise the majority of the
shot
grade mixture, preferably at least 60%.
The texturing shot grades used in the examples of Table I were limited
to size distributions essentially with a diameter of 1000 pm (1.0 mm) or less.
This
should not be construed as restrictive, but only as practically Illustrative.
First, these
shot grades as used in the examples are the most popular shot grades used
commercially. Second, the density and size of shot that can be practically
used
(i.,, transported and propelled) is limited by- the type of blasting equipment
.M3li:...'w;.'.,.n...-.;.. ..',.........ti.. --::._'sv.LA.i..:b'....ibl.f..-
;..:~.V. ...... ... . ia. -. . ... .. .... ........, -
.employed for. these trials.
Vacuurn-type air-blast machines, as described In detail herein, are the
least powerful and are the most limited in shot mass transport or "throw
rate," such
that ceramic shot sizes essentially over about 800 Nm or any size steel shot
present
difficulties.
Pressure-type air-blast machines similar to the Matrasur equipment used
for these trials, are significantly more powerful with greater shot mass
transport
31
CA 02617480 2008-02-07
76668-7E
capabilities. Ceramic shot sizes essentially up to about 1000 pm can be used
with
good results, as can smaller grades of denser steel shot. The mass transport
limitations of air-blast peening machines in general (i.e., their inability to
convey and
propel a sufficient quantity of large shot grades, particularly denser steel
shot, at a
sufficient rate to achieve the desired peening coverage) dictate the types of
shot that
can be used, and thereby controls the range of textures that can be produced
by
peening and overblasting with these types of machines. At any given mass
transport
rate (g, the mass per unit time of shot expended), coverage rate is inversely
proportional to the density (linear function) and diameter (cubic function) of
the shot
used. The larger and denser the shot Is, the less individual particle impacts
(coverage) per unit time results.
The third variety of blasting machine in common use are the centrifugal
wheel type, which mechanically "sling" the shot at the target. These machines
are
capable of propelling a very large volume of shot. Therefore, while air-blast
machines can generally accelerate shot to greater velocities than can
"slinger" wheel
machines (although with shot size and density limitations), the mechanical
"slinging"
operations have much greater mass transport capability and can easily handle
large
steel shot grades , (with the -size af> the wheel and motor being . 'practical
considerations). Steel shot is most commonly used with this type of machine,
with
shot diameters often exceeding 1000 Nm (1.0 mm), and up to 2000 pm (2.0 mm)
diameter or greater. Thus, the type and sizes of shot used for texturing shot
mixtures is limited only by the capabilities of the blasting equipment
available to the
practitioner.
32
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'76668-7E
However, ceramic shot, and specifically ceramic shot grade mixtures
essentially having diameters less than nominally 1000 Nm (1.0 mm), are
preferred
because of the finer textures achieved, non-contamination of the peened press
plates, and ease of handling of the shot, which allows simpler, less
expensive, lower
maintenance blasting equipment to be used.
The overall finish of a peened press plate and the laminate subsequently
produced from it, to be aesthetically and commercially acceptable, is
established by
its visual appearance (i.g_, its surface texture or structure, gloss, and
overall
uniformity). The texture can be described quantitatively by its roughness and
peak
count, while gloss and uniformity are partially a function of the peening
coverage
achieved. The smaller the shot grade and the lower its density, the greater
the
equipment mass transport capability, and the longer the exposure time (i.e.,
the
slower the plate traverses past the blast gun or multiple guns), the greater
will be
the coverage during a single peening pass.
The percent of coverage obtained, as shown in Table I for each trial,
may be visually determined against pre-established reference standards. For
very
exacting peening applications, several quantitative techniques have been
developed,
,. -., ,.-.-: -., =. suchas~he-Straub projection, Peenscanultraviofet
Fight:sensitive dye.rlFalentfne-grain
growth metallographic examination, and Almen strip arc methods. All are
relatively
difficult and tedious to perform. Therefore, visual inspection is most often
employed .
in the art, and was used during the trials described herein. The visual,
qualitative
method to determine peening coverage of press plates is quite adequate, since
mechanical property enhancement of press plates, such as improved fatigue
strength
or stress corrosion resistance, is of little interest with press plates.
33
CA 02617480 2008-02-07
'76668-7E
The relationship of the percentage of coverage obtained after peening,
and the number of processing passes used, can be expressed by the equation:
C"= 1-0-C,
where C, is the percentage of coverage after one pass (expressed as a
decimal), C"
is the percentage of coverage after n passes (expressed as a decimal), and n
is the
number of passes. By visually determining the percentage of coverage after
several
(n) passes, the percentage of coverage per pass (C,) can easily be calculated
by the
rearranged equation:
C,=1-"1-C"
and the number of passes necessary to obtain any final degree of coverage (C")
desired can then be extrapolated by the equation:
n = log(1-C")/log(1-C, )
This relationship shows that 100% theoretical peening coverage is only reached
asymptotically, although in practical terms, visual inspection can discern
"100%"
coverage as the absence of any obvious unpeened areas or islands on the
surface
of the processed workpiece. Since in practice even the most precise
quantitative
analytical methods cannot accurately measure peening coverages above 98%, this
= ~:v~tee h~s beer~ ~tbitra~'ily established by the art-es>defirring ful(
cover,age~t=s$fiuWatie-rtT ;,,.
peening.
For mechanical applications, it is extremely important to obtain
saturation coverage if the property enhancements sought are to be realized.
However, when considering the cosmetic qualities of press plates, and the
decorative
laminates produced therefrom, lesser degrees of coverage are often acceptable.
It
is generally accepted in the industry that 95% coverage or better produces an
34
CA 02617480 2008-02-07
76668-7E
essentially uniform and sparkle-free press plate finish appearance, although
for some
applications, less coverage may be considered appropriate. For the peening
trials
summarized in Table I, at least 95% coverage was considered optimal to produce
a
plate and laminate finish aesthetically pleasing for most commercial
applications.
Another quantifiable plate surface feature, gloss level, is usually
measured with a 600 gloss meter, of which many makes and models are readily
available, and all of which generally work on - the same principle. A beam of
calibrated intensity light is projected onto the surface of the plate at an
angle 60
to normal of the plate surface (i.e., an incidence angle of 30 from the plane
of the
plate surface). Geometrically opposite to the light source is a photodetector
device
which measures the intensity of the light beam reflected from the plate, which
is
then transposed by the meter into a gloss value (either in analogue or digital
form).
The gloss meter is initially calibrated against a high gloss black tile
standard with a known gloss value (usually 94 1 gloss units or degrees)
supplied
with the meter. By convention in the United States, per NEMA standards which
regulate the high pressure decorative laminate industry, and specifically NEMA
Test
Method LD3.13.1 (1991) prescribing the procedure for measuring gloss, for
historical
, w. ~..<.,
-reasons~thea meter~trit ra'riirlfiaisdly} calibrated to an 82 gloss urtit-
readi%g-agaifW- - ~
the black tile primary working standard.
However, for the gloss measurements made for the plate finishes
produced during the peening trials summarized in Table I, the more universally
used
and increasingly accepted industry-wide ISO (International Standards
Organization)
method was empioyed, in which the gloss meter was calibrated to agree
precisely
with the gloss value of the black tile standard. It must be understood by
those
CA 02617480 2008-02-07
76668-7E
skilled in the art that several factors influence the relative gloss of a
plate finish, the
most important of which are the microtexture (degree of polish) and
macrotexture
(structure) of the surface, which have confounding effects on the gloss level
measured by a meter.
Except for a perfect mirror finish, some quantity of light intensity is lost
from the meter's light source through reflection to the photodetector because
of light
scattering caused by non-planer irregularities on the plate surface. The
irregularities
are present on a nanometer (nm) scale with respect to a plate surface's
"microfinish"
as well as on a micron (pm) or even millimeter (mm) scale when related to a
plate
texture structure's roughness and topography (i.e., peaks and valleys, or in
the case
of a peened plate surface, ridges and craters, respectively). All of these
disruptions
scatter some amount of light and reduce the perceived gloss level of the
finish, both
instrumentally and visually, although both "measurements" are not necessarily
always in total agreement. In general, the greater the microtexture and
macrotexture
of the surface finish, the more light will be scattered and the lower will be
its gloss
level.
Another confounding factor is the effect of surface gloss uniformity as
-re1a'eecf'to' tti~--d6gri5"a*'df' corr&age"ofthe:.rpeened plates of this
inventioft: -The-plate
finish, prior to peening, is essentially a planar surface, which due to
polishing and
buffing, has a relatively smooth microtexture and glossy appearance. The
subsequent impact of the shot particles during peening disrupts the planarity
of the
plate surface by creating myriad Impact craters, providing the macrotexture as
well
as a more subtle microtexture resulting from embossing of the shot's finish
itself into
the plate surface. In general, the greater the peening coverage and intensity,
the
36
CA 02617480 2008-02-07
'76668-7E
rougher the texture will be and the lower the gloss level will become. The
gloss
level obtained is aiso influenced by the type of shot.. used, in terms of its
smoothness, sphericity and size. With less than complete peening coverage,
residual
unpeened, flat and relatively high gloss areas or "islands" will remain
interspersed
within the overall peened texture. If these islands cover an appreciable
percentage
of the surface area of a peened plate (.P., more than 5% of the total area,
corresponding to less than 95% coverage), a generally less desirable non-
uniform
gloss, sparkle appearance may be perceived.
However, a typical gloss meter measures a relatively large area of a
plate surface, with a beam focus of about 1 cm. diameter, and as such only
senses
the aggregate or average gloss or reflectance of the dull peened areas and
shiny
unpeened islands. Therefore, while gloss can be quantified, it Is important to
understand that there are many interactive factors influencing the vaiues
measured
which are not easily separated.
Finally, the macrotexture can be quantified in terms of the elevation and
depth of the peaks and valleys, and their frequency, commonly referred to as
the
texture's roughness and peak count, respectively. A profilometer determines
these
-,.. _ _ ... .. ara- ~rbters; tn.wh~Pr'e-erS,
w ~ ,s~etrsftive,>.
~- styitrs==traces -the topography of 3he.. surfaae,-,. ..<,- .......
structure and converts the profile geometry into electronic signals from which
roughness values and peak count can be calculated by defined equation
algorithms.
The instrument used to measure the roughness and peak count values
reported in Table I was a Perthometer Model M4P. All measurements were made
using a 15 mm. trace length, with a 2.5 mm. cut-off, such that signals from
the first
1.25 mm. of the trace and the last 1.25 mm. of the trace are ignored, and only
the
37
CA 02617480 2008-02-07
76668-7E
middle 12.5 mm section is considered in the measurement. Additionally, for
peak
count determinations, a slice setting of 0.5 pm was used, which is the total
distance
about the mean roughness between the upper and lower slice thresholds, above
or
below which peaks and following valleys that extend are measured and
considered
significant profile deviations. Roughness and peak count are interrelated, and
are
affected by the shot grade and peening intensity used, as well as the coverage
achieved.
For textured press plates, the most descriptive and useful roughness
parameters-are considered to be Ra and Rz. Ra, the average arithmetic
roughriess
(in the past often denoted AA), is the arithmetic mean of all deviations of
the
roughness profile along the total 12.5 mm. measuring length. Rz, the average
roughness depth, is the mean of the individual roughness depths in five
successive
individual 2.5 mm. measuring lengths comprising the total 12.5 mm. measuring
length, where each roughness depth is determined as the vertical distance
between
the highest and lowest points of the roughness profile within each segment.
Peak
count is simply a measure of the number of peaks (followed by their successive
valleys) that together extend above and below the predetermined slice
threshold of
t0.25 .,
pm abduf "the "fdug'hriess' prd'fW'r'iOaf"'i "V9Tud~i i ~pectively. " While
these = ' =' =
parameters are useful in quantifying a plate texture's structure, and
meaningful
conclusions can often be made based on them, like gloss measurements, they do
not
in themselves completely define the overall visual aspect of a plate finish.
The peened textured plates,produced by the method of this invention,
whose properties are described in Table I, can be generally described as
"average"
in terms of their roughness compared to the prevalent commercially available
38
CA 02617480 2008-02-07
76668-7E
laminate finishes popular in the marketplace. The peened finishes of these
examples
generally have an average roughness (Ra) in the range 1.0-2.5 pm and roughness
depths (Rz) in the range of 8.0-15.0 Nm. Table II shows a comparison of this
type
of finish and the other major finishes that are commonly produced by the high
pressure decorative laminate industry today.
TABLE 11
Plate Generic Finishing Typical Roughness (ium)
Finish Name Method Ra Rz
A Glossy (Lacquer) Buffing 0.02-0.05 0.2-0.8
B Matte (Satin) Shotblasting 0.2-0.5 1..0-3.0
C Peened* Shotblasting 1.0-2.5 8.0-15.0
D Suede Chemical Etching 2.0-4.0 15.0-30.0
or Shotblasting
E Deep Texture Peening, Etching, > 5 >30
(Dimensional) Machining, or
Sculpturing
= Produced by the method of the present invention.
Type A "glossy" finish is produced by the methods previously described
for -preparation of the- #8 mirror finish press.plate used in the:.peening
trials
summarized in Table I, and is the smoothest of the finishes produced.
Type B "matte" finish has a low gloss (i.e., dull or matte appearance)
and is essentially planar with no apparent visual macrotexture. It is produced
by
blasting a Type A finish plate with very fine glass beads or grit materials
such as
sand or alumina. Glass beads in the range 70-110 Nm, are commonly used, which
readily shatter upon impact.with the plate surface such that, with
recirculation, the
39
CA 02617480 2008-02-07
'76668-7E
plate surface is essentially being microtextured with fine, sharply fragmented
glass
powder. This finish has become popular as a replacement for the older "dull
rubbed"
finish, in which the surface of a glossy finish laminate after pressing is
subsequently
abraded by the action of brushes and a slurry containing abrasive material to
reduce
its overall gloss level.
The Type D "suede" finish is most commonly produced by a chemical
etching process, although moderately powerful, centrifugal wheel type blasting
machines and relatively large shot grades can be used to produce finishes with
such
roughnesses.
Finally, a Type E finish with a deep exaggerated texture can be
produced by either prolonged, controlled chemical etching or excessive shot
peening
with powerful, centrifugal wheel machines and large steel shot grades.
Machining
or sculpturing of plate "masters" with deep textured designs are other
techniques
commonly used. It should be recognized by those skilled in the art that press
plates
with more pronounced textures of the Type D and Type E finishes may have to be
further processed after primary texturing with electropolishing to deburr and
smooth
the surface, or additionally, chrome plating to further improve the
releasability of the
.... __ , . _
"~ "~ ' ~late ftom'tfie laminate face during sepat'eti6ri' after p-fas$ifig
'and "curing has 'taken
place. While electropolishing and/or chrome plating of Type C plates of this
invention is not necessary for acceptable release properties, they may
optionally be
so treated to further refine the finish, enhance releasibility, or improve
wear
resistance.
The press plate shot peening, simultaneous texturing and overblasting
trials summarized In Table I were conducted with various mixtures of
commercially
CA 02617480 2008-02-07
'= 76668-7E
available grades of either ceramic or cast steel shot. It was attempted to
match the
size distributions of the various ceramic and steel shot grades as closely as
possible,
and sizes for both were restricted to those grades of shot essentially 1000 pm
diameter or less, because of limitations in the mass transport capability of
the
blasting equipment used. All the examples used a binary mixture of large and
medium texturing shot grades.
Examales 1 and 1 A
Examples #1 and #1A is similar to the prior art process, except that a
mixture of large and medium shot grades, in lieu of a single large shot grade,
was
used to generate the plate macrotexture. After four passes, 90% coverage was
achieved. Applying the general coverage equation previously discussed,
extrapolation
indicates that six passes would be required to obtain the minimum acceptable
95%
coverage, and an additional pass (7 total) would be necessary to reach 98%
coverage or saturation. After the initial four passes with larger shot grades,
the plate
was overblasted with a small shot grade, resulting in full peening coverage
and a
very uniform, pleasing plate appearance.
Examoles 2. 3 and 4
; ,, .... , .. . : .., ,
Ezample"s "#2, #3 and #4 were coriducted in accordance- with =the
present invention and employed the same mixture of large and medium ceramic
shot
grades as Example #1, to which was added decreasing quantities of the same
overblasting small ceramic shot grade as Example #1 A to simultaneously
overblast
the plate samples while texturing them. As-Fan be seen, even with the least
amount
of=averblast shot grade of Example #4, 95% coverage was obtained in only four
passes. With the greatest percentage of overblasting shot grade (Example' #2),
41
CA 02617480 2008-02-07
= 76668-7E
100% visual coverage was achieved in four passes, rather than the seven passes
which would otherwise be needed without overblasting.
Examples 5 and 6
Examples #5 and #6 used the same large and medium ceramic shot
grade mixture as the preceding examples, but was combined with even a smaller
grade of overblasting ceramic shot. In both trials, essentially 100% coverage
was
obtained in only three passes, compared to the four passes needed to obtain
95%
coverage with the larger overblast shot grade used in comparative Examples #3
and
#4.
Examoles 7 and 7A
Example #7 and #7A again represents a modification of the-prior art,
using a binary mixture of texturing shot grades with an even wider range of
large
and medium shot grades than in Example #1 and #1 A, followed by the same
discrete, secondary overblasting step. Prior to overblasting, the plate
exhibited 90%
coverage after five passes. The lower coverage rate compared to Example #1
(five
passes versus four passes) is attributable to the presence of larger
particles. Since
the blasting equipment transports a given mass of shot at a constant rate
regardless
of -strcytygraft; targershot grades provide fewer individual particles
t'trpruiiel'trgWnst ''
the plate per unit time or pass. Hence, a poorer coverage rate results. The
general
- coverage equation indicates that for the large shot grade mixture used in
Example
#7, seven passes would be required to obtain the minimum acceptable 95%
coverage level, and nine passes would be needed to achieve saturation or 98%
coverage.
42
CA 02617480 2008-02-07
~ =
76668-7E
Examples 8 and 10
Examples #8 and #10 used the same mixture of large and medium
ceramic texturing shot grades as did Example #7, to which was added two
different
grades of smaller ceramic overblasting shot in accordance with the present
invention.
A 100% visual coverage level was obtained with the larger (B60) overbiast shot
grade in four passes, while 100% coverage resulted in only three passes when
the
smaller ().g, B125) overblast shot grade was used (as was the case with
Example
#5).
Exam Ip e 9
Example #9 used the largest overall ceramic texturing shot grade
distribution with B60 shot for the simultaneous overblasting. Again, 100%
coverage
was achieved in only four passes. The slightly higher roughness and lower
gloss
compared to Examples #8 and #10 is consistent with the use of the larger shot
grade mix and absence of intermediate size B30 texturing shot grade.
Overblasting, whether simultaneous or discrete, has a slight smoothing
effect on the plate texture, wherein the near saturation or saturation
coverage of the
small shot can flatten and smooth the large shot crater rims to some extent.
This
-' 'effect N'rtfilMed 'Tri a'd ecrease in 'the plate textUre's average-
rouugflness 'tRar' end
gloss level as illustrated when the plates of Examples #1 and #7 are
discretely
overblasted, where =resultant Ra values are very similar to those for all the
other
ceramic shot simultaneously overblasted plates, with the exceptions of Example
#4,
where the least amount of overblasting shot was used, and Example #9, where
the
largest size texturing shot distribution was used.
43
CA 02617480 2008-02-07
76668-7E
AII the gloss values were quite similar after overblasting, with the
exception of Example #3, which appears to be an anomalous reading.
Examples 11-16
Trials were also conducted with cast steel shot of comparable grades
to the ceramic shot used in the previous examples. Using identical blasting
machine
conditions as with the ceramic shot trials, Examples #11 versus #12 and
Examples
#13 versus #14 illustrate the relative benefits of simultaneous overblasting
compared
to the prior art, the only difference being use of a binary mixture of
texturing shot
grades rather than a single shot grade. However, it is clear that poorer
coverage is
obtained with steel shot compared to a comparable mixture of ceramic shot (g,
95% coverage is achieved in four passes in Example #3, whereas only 85%
coverage is obtained after eleven passes in Example #12). Similarly, in
Example #8,
100% coverage results after four passes, while in Example #14, after eleven
passes
only 859'o coverage is obtained. Using the steel shot mixtures of Examples #12
and
#14 under these operating conditions, the general coverage equation predicts
that
eighteen passes would be needed to achieve a minimum acceptable 95% coverage,
and twenty-three passes would be required to obtain a full 98% coverage or
.... .: , ..,..._.a ._. _ . _ : :.... _ ._
saturation.
The reason for the poorer coverage efficiency is largely related to mass
transport. While ceramic shot has a density of about 3.75-3.85 gms/cm', cast
steel
shot typically has a density of about 7.70-7.80 gms/cm3. With steel shol being
more than twice as dense as ceramic shot, less than half the number of
individual
pellets will theoretically be projected per unit time, contributing to a much
poorer
coverage rate. The poor coverage rate associated with steel shot is further
44
CA 02617480 2008-02-07
76668-7E
aggravated by its poorer "fluid" flow properties, since it is heavier and
typically less
perfectly spherical, with a rougher surface texture than ceramic shot. While
the
lower density of ceramic shot reduces its peening intensity at a given impact
velocity
vis-a-vis steel shot, air-blast machines can more easily accelerate ceramic
shot to
much higher velocities, thus negating much if not all of the disadvantageous
density
effect. The slightly lower roughness values obtained in Examples #12 and #14
with
steel texturing shot using a simultaneous overblasting steel shot, compared to
the
ceramic shot counterparts (Examples #3 and #8, respectively), suggest slightly
less
striking force or peening intensity was achieved with the steel shot. Further,
the
lower peak counts, as with Examples #1 and #7, are indicative of incomplete
coverage.
In Examples #15 and #16, the same shot grade mixtures as in Examples
#12 and #14, respectively, were used with much higher air pressure to increase
shot
flow, coverage and impact velocity. The increased peening Intensity and
coverage
achieved in Examples #15 and #16 is evidenced by the significantly higher
texture
roughnesses and peak counts compared to Examples #12 and #14. Hovvever, the
4.0 kg/cm' (57 psig) air pressure used with the last two examples is at or
near the
operating limit of the blasting equipment employed. Still higher peening
intensities
or use of even larger steel shot grades than Examples #15 and #16, with
resultant
greater roughnesses attainable, would require more powerful blasting equipment
of
the centrifugal wheel type. Vacuum-feed air blast machines of the type used
for
texturing of the large, production plates of this invention and described in
detail
herein, are less effective at increasing shot flow and coverage at high
operating air
pressures than the direct pressure feed air-blast machines of the type
employed for
CA 02617480 2008-02-07
r y 76668-7E
this trial, since with the former, shot feed rate is regulated by suction
created by the
blast gun air injector venturi effect, which is not as linear as direct
pressure
regulation. However, when using the lighter ceramic shot of the grades
employed
in Examples #2 through #6, even the least powerful vacuum machines are quite
effective in uniformly texturing stainless steel press plates.
The aforementioned description is not to be interpreted to exclude other
arrangements advantageously employing the method and product of the present
invention. For example, the use of a mixture of different grades of ceramic
shot may
be used to finish other surfaces of other materials. It may also be
utilized'on
products other than press plates for laminates. Different mixtures of
different grades
of shot may be used which utilize the principles of the present invention. It
is to be
understood that the above described method and product is merely an
illustrative
embodiment of the principles of this invention and that numerous other
arrangements
and advantages may be devised by those skilled in the art without departing
from
the spirit and scope of the invention.
46
CA 02617480 2008-02-07
'76668-7E
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