Note: Descriptions are shown in the official language in which they were submitted.
7~)
IMPROVED METHOD OF FORMING BLANKS FOR lh~
SOLID-PHASE FORMING OF THERMOPLASTIC ARTICLE$
U.S. Patent No. 4,323,531 discloses the
formation of thermoplastic articles from a polymeric
resinous powder which is compr~ssed into a briguette,
sintered and then forged and, if desired, formed into
S an article. The process is essentially scrap-free, and
the need to go through a melt-forming stage is avoided.
While the aforesaid process can readily form satis-
factory products from most thermoplastic resinous
powders, difficulties in doing so a~ acceptable
production speeds with some resinous powders and for
large parts can be experienced. The present invention
improves the process so that forging of ~lanks (from
bri~uettes~ into preforms which have essentially no
voids, cracks or defects and have the prerequisite
toughness can be accomplished with satisfactory
rapidity even with large parts. Accordingly, the
present invention resides in an improved essentially
scrapless process in which the steps of the operation
of forming are simplified. In the process of the
27, 818-F -1-
/
invention, thermoplastic resinous powders are formed at
production speeds directly into products of high quality
even where th~ resinous powders are basically difficult
to form.
More par-ticularly, the present invention
comprises a process fox forming thermoplastic resinous
powders directly into products at production speeds
without reguiring processing through a complete melt
stage. One particular advantage of the process of the
invention is that large parts can be readily manufactured.
The steps of the process include the formation of a
briquette from a sufficient quantity of thermoplastic
polymPric resinous powder for making t~e desired article
by compressing the powder into a briquette, sintering
the briquette for a-time and temperature to accomplish
more than about 20 percent melting but less than about
90 percent melting of the crystallinity of the original
resinous powder, recompressing the briquette in a warm
and, preferably, vented repress tool to form a blank
and then forging the blank into a preform which is then
thermoformable by standard thermoforming techniyues
into a finished article, unless the resulting preform
is already in the configuration of the final article
desired. The percent melting in the sintering step
should be from 20 to 90 percent while the preferred
range of melting in sintering step can be from 40 to 80
percent. The powder which is formed into briquettes
can be at room temperature or can be preheated where it
is determined that such will accelerate the total
processing operation. In one alternate method of the
invention, sintering can take place after recompressing,
with preheating of the briquette prior to the recompressin~
- step.
27,818-~ -2-
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-3- 4693-3514
The invention particularly resides in a process for
forming an arti.cle from a thermopl.astic polymeric resinous powder,
wherein a quantity of said powder sufficient for making said
article is compressed into a briquette having green strength,
comprising the steps of
(a) preheating said briquette such that the temperature
in the center of the briquette is less than the melt temperature of
the powder forming the briquette,
(b) repressing the briquette while at about the same
temperature to form a blank;
(c) applying additional heat to the blank such that the
center thereof is at a temperature range higher than that of step
(a) bu-t still less than the melt temperature,
(d) maintaining the higher temperature for a period
sufficient to soften and sinter the blank to accomplish more than
about 20% melting but less -than about 90% melting of the
crystallinity oE the resinous powder; and
(e) forging the blank into said article while the blank
is at said higher temperature to effect substantial plug flow
deformation of the blank and obtain substantial fusion of the powder
forming said blank.
The invention additionally resides in a process for Eorm-
ing an article Erom a thermoplastic polymeric resinous powder,
wherein a quantity of said powder sufficient fo:r making said
article is compressed into a briquette having green strength, com-
prising the steps of
(a) heating the briquette to a temperature in the range
from above room temperature to less than the melt temperature of
. . ~
-4- 4693-3514
the resinous powder and maintaining said temperature for a period
sufficient to soften and sin-ter the briquet-te to accomplish more
than about 20% melting but less than about 90% melting of the
crystallinity of the resinous powder;
(b) repressing the briquette at the same temperature
range to form a blank; and
(c~ forging the blank into said article while the blank
is at a temperature within the temperature range to effect sub-
stantial fusion of the resinous powder forming said blank.
Figure 1 is a diagrammatic representation of the steps
of the prior invention over which this present invention is an
improvement. The steps are designated A through E, wherein a
powder of a resinous material is formed into a preform and then
into a container.
Figure 2 is a similar diagrammatic representa-tion of the
steps of the present invention designated as AA through GG, wherein
a powder of a resinous material is formed into a preform and sub-
sequently into a container as in Step E of Figure 1, if desired~
Figure 3 is a similar diagrammatic representation of the
steps of a modified process of -this invention herein designated as
steps AAA through FFF. Here again, subsequent thermoforming into
containers can also be achieved as in Step E of Figure 1, if desired.
Figures 4 to 6 illustrate various mechanical properties of
the products made by this invention as inflwenced by the sintering
step.
The following terms used in this application have the
following meaning:
..b.
~5~ ~2~
"Sinterlng" is the process by which an
assembly of particles, compact~d under pressure,
physically and/or chemically bond themselves across
contacting particle interfaces or boundaries into a
5 coherent body under the influence of elevated tempera-
ture for a pariod of time, without complete melting
generally occurring.
"Forging" is the process whereby resin
particles are fused into a preform or article which has
10 generally the same density and generally the same or
improved mechanical properties that it would have if
made by conventional melt forming processes.
"Plug flow" is the condition in which a blank
deforms in an essentially multi-axial stretching mode
15 such that the velocity gradient through the thickness
of the material is relatively constant. This is in
contrast to the usual parabolic flow pattern observed
in conventional molding of poly~ers in a viscous state
where the velocity varies from zero at the mold surface
20 to a maximum near the mold center. Plug flow is the
condition where the relative constant velocity through
the thickness of the material is achieved by reducing
the frictional drag at the mold surface. This can be
conveniently accomplished by placing a lubricant
25 between the blank and the contacting metal surfaces
since few presently known resins are sufficiently self~
lubricating for this purpose.
"Green strength" means having a compactness
and adhesion of resinous powder in the briquette
~ 30 sufficient to enable the powder to be moved as a unit
J without support.
27,818-F _5_
6 ~ 7~
.
"Crystallinity" means the extent to which the
material in a given sample is arranged in generally
reyular, periodic arrays, commonly known as crystals.
Determinations of crystallinity are usually made by
measurement of sample density, heat absorbed on melting,
or intensity of discrete x-ray diffraction patterns.
"Degree of melting" means the percentage of
the original crystallinity in a crystalline thermoplastic
material which is melted during a heat treating step.
For example, an unheated specimen will have a degree of
melting equal to zero percent while a completely melted
specimen will have a degree of melting equal to 100
percent.
"Repressing" or "recompressing" means a
recompaction of the resinous material powder without
significant deformation of the original briguette which
was initially compacted into a briguette. The material
of the briquette, while warm, is further densified
during such recompaction to form a blank.
"Melting point" or "peak melt temperature"
(Tp) means that temperature indica-ted by the maximum in
the melting endothermic peak as seen in the customary
differential scanning calorimeter (DSC) measurement.
"Alpha transition temperature" (T~j for
amorphous polymers is considered to be the glass
txansition. In the case of crystalline polymers, it is
taken as an energy loss peak associated with the
crystalline region often observed at a temperature of
from 50C to 100C below the melting point of the
polymer.
27,818-F -6-
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"Preheating" means heating a bri~uette at a
temperature below Tp, preferably such that little or no
melting of the crystal structure of the resinous material
powdex occurs.
Figure 1 is a schematic depictation of a
solid phase process for forming preforms directly from
a resinous material powder 10 as described in detail in
U.S~ Patent No. 4,323,531. For purposes of this
description, such process is identified as Method I.
Briefly, a quantity of a resinous material powder 10
sufficient to make a finished product is measured at
Step A and is thereafter compressed in Step B into a
briquette 12. The briquette is heated in Step C to a
temperature in the range from about the alpha transition
temperature to less than the melt temperature thereof
and is thereafter maintained at that temperature for a
period of time sufficient to soften the bri~uette and
sinter the same short of substantial fusion thereof to
form a blank. The blank 12 is then forged in Step D
between plattens 16 into a preform 18 (or a final product
where the preform can take the shape of the finishecl
article desired). While the blanX is forged, it is at
a temperature within the above stated temperature range
to affect sl~stantial plug flow deformation of the
blank and obtain substantial fusion of the powder
comprising the blank to form the preform or final
product. If formed as a prefonm, the preform 18 can be
transferred to a thermoforming die 22 and can then be
formed into a container 20 or other product by con-
ventional thermoforming techni~ues as illustrated atstep E.
While Method I works well for some resinous
material powders, it has been found to have some
27,818-F -7-
-8~
limitations in forming products from certain resinous
powders. Some powders are difficult to compact into a
briguette. Some develop voids during forging. Long
heating times are required because of resin expansion
before densification of the resin particles. The
process of Method I can sometimes be slower than desired
for production applications. The processes of the
present invention, hereafter described, have been able
to accomplish the formability of such difficult-to-handle
thermoplastic materials and especially large parts
while at the same time speeding up the ~orming process.
One embodiment of the process of the present
invention is Method Il, illustrated in Figure 2. In
this method an amount of resinous powder 24, sufficient
to form a desired articIe, is provided at Step AA. The
powder is at room temperature and is compacted at Step BB
into a briquette 26 having green strength. Compaction
pressures and compaction dwell times are basically the
same as those used in the prior art; i.e., Method I of
7 20 Figure 1. The bri~uette 26 is then placed into an
enyironment for heating, such as a circulating air oven
or an in~ra-red oven or a radio frequency heater, where
it is preheated at Step CC. When the temperature at
the center of the briquette 26 reaches a temperature
which is preferably from 15 to 35 degrees below Tp, the
preheated briguette is removed from the oven and
repressed in Step DD at a temperature above room
temperature in a repress tool to form a blank.
Obviously, the repress tool can also be at about the
same temperature as the briquette itself. Venting or
evacuation can be used during either the compaction or
repressing steps.
.
27,818-F -8-
-9~
Immediately after repressing, the hot blank
26 obtained at Step DD, can he cooled at Step EE for
further processing at a later time or can be taken
directly to another oven, for example, like that used
in Step CC. Here, however, the blank 26 is kept in the
oven until it has reached a degree of melting of the
resinous material in the blank of from 20 to 90 percent.
; Noxmally, this will put the center of the blank at a
temperature no greater than 1 or 2 degrees below Tp,
and no significant melt flow is occurring. The sintered
blank can then be forged (solid phase formed) at Step GG
into a preform or article 28. A traditional thermoforming
step can be added if preform 28 is not in the shape of
the finished article at Step GG. For example, for
typical high molecular weight high density polyethylene,
th~ repress tool can be heated to a temperature of from
100 to 135C. Repress pressures are customarily between
210 to 700 kg/cm2 with a dwell period of about 5 seconds.
~epress tool temperatures are not critical provided the
blank is not chilled before the- repressing occurs.
High repress tool temperatures, while possible, may be
undesirable since the blank could stick to the repress
tool and woulc~ thus be hard to remove. ~owever, lubri-
cakion of the tool surfaces with a lubricant such as a
silicone coating can significantly eliminate sticking.
A major difference between Method II of this
invention and Method I of Figure l, is that the bric~ette
before sintering is preheated and repressed in a wa~m
tool. The reason that repressing is not done at a melt
phase temperature is so that no significant plasticizing
occurs at khe repressing step in this solid-phase forming
process. Thus, the basic shape of the bxic~uette is not
changed during the repress operation. The repressing
27,818-F -9-
~10-
step serves to densify the briquette (to form the blank)
in a way that helps minimize voids, cracXs or defects
which might otherwise occur. This results in a blank
requiring a shorter sinter time.
Larger briquettes made by the above method
still tended to crack occasionally and the extra preheat
time and additional heating equipment still is somewhat
of a disadvantage in the manufacturing operation of
large blanks. A modified process of this invention whlch
is more rapid and can produce satisfactory large as well
as small blanks, is illustrated in Method III of Figure 3.
Here, a resinous material powder 30 sufficient to make
an article is provided in Step AAA and compressed in
Step BBB into a briquette 32 as in Method II described
before. ~ow~ver, the sintering Step CCC occurs before
the repressing Step DDD instead of afterwards, thereby
eliminating at least one step in the process. The
repressing and sintering steps can be done under
- substantially the same conditions as in Method II, but
only in a reverse order. An optional cooling ~tep EEE
can be included should it be desired to forge the
repressed blank 32 into a preform in Step FFF at a
later date. Otherwise, the blank 32 from Step DDD is
directly transferred to the forging apparatus of
Step FFF for forging an article or preform. Subsequent
thermoforming of the preform into a container can be
achieved as illustrated in Step E of Method I, if
desired.
27,818-F -10-
Example
Using Method III (Figure 3) of -this invention,
50 and 100 gram portions of a high denisty polyethylene
~HDPE? in powder form were compression formed into
briguettes having a diameter of 6.35 cm. ~he briquettes
were heated and sintered ~Step CCC~ in a circulating
air oven at atPJmperature of from 133 to 137C. The
samples (shown in Table I below) were then repressed
(at Step DDD) at a temperature of 130C in a repress
tool with a vacuum applied before and during the
repress step. The cycle was conducted for 15 seconds
in a vacuum and for 15 seconds of repressing at a
pressure of 700 kg/cm2. The 50 gram briquettes were
heated and sintered for 80 minutes and the 100 gram
briquettes for 120 minutes. After repressing, hot
~lanks formed from the briquettes were hand carried and
placed in a 19.7 cm diameter forging tool and forged
; into a preform. The conditions and results are
summarized in the following Table 1.
TABLE 1
Appearance of Preforms
Forged by Method III
50 gm Sample 100 gm Sample
Sinter80 Minute 120 Minute
25 TemperatureSinter TimeSinter Time
133C
135C 2 II
137C 2 III
1 - Some small voids, complete lip.
I - Some small voids, incomplete lip.
2 or II - No voids, complete lip, excellent preform.
III - No voids, complete lip, overheated blank.
27,818-F
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TABLE 1 Continued
Conditions: Material - HDPE
Repress tool temperature - 130C
Forging platen temperature - 130C
Forging Force - 91 metric tons
Forging Dwell time 1 second
For -this particular material and forging
tool, a temperature of 135C appeared to be the ideal
oven temperature for sintering. Additional 150 gram
forged preforms made at a t~mperature of 135C had full
well-defined lips, glossy surfaces, no thin spots and
- no voids. While lower repress die temperatures were
not used in this run wi-tA high speed equipment and
integrated tooling, it is expected that lower repress
die temperatures would eliminate any sticking without
chilling of the surface of the blank.
.
A series of additional tests were made with
similar results. There seemed to be little difference
between the 100 gram and the 50 gram blanks. Repressing
the heated bri~uette in a warm repressing tool under
vacuum increased -the density about 18%. When cooled,
the repressed sample looked like a compression molding.
It was firm, had glossy surfaces, had a convexed top
and bottom surfaces and concave vertical surfaces.
When these blanks were forged into preforms immediately
after repressing, excellent preforms were obtained.
The forged preforms were then formed into parts wi-thout
di~ficulty. The parts were genarally uniform in
cross-sections and were free of voids. When a
repressed blank was allowed to cool to room temperature
and then reheated in an oven for a sufficient period of
time, forged and formed, the resulting part had white
27,818-F -12-
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blemishes. However, these blemishes did not have the
appearance of voids in the center. The cause of the
difference is unknown.
It has been found that the guality of the
forged preform improves as the compaction pressure uqed
to make the preforms was increased from 70 to about 350
kg/cm2. Above this pressure, little differences were
found in processing times, although compaction pressures
up to about 1750 kg/cm2 can make briquettes stronger
and more abbrasion resistant.
It is desirable to keep the heating times to
a minimum in production operations. The use of radio
frequency (RF) to heat the interior of a sample and
circulating air oven to heat the exterior surfaces has
been employed with considerable success. It is also
po~sible to decrease the heating time by using thin
blanks which are heated individually in a circulating
air oven ~nd then stacked upon one another to form a
full weight blank. The only problem is that once the
hot thin blanks come into contact with one another,
they tend to stick so that proper alignment must be
maintained.
In the present invention hot briquettes are
repressed into blanks without shaping and excellent
void-free forged preforms can be made from the blanks
which can then be formed into high ~uality pre~orms or
articles. If preforms are made they can thereafter be
*hermoformed into containers and other articles by
conventional the~moforminy techniques.
27,818-F -13-
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The high density polyethylene resin which is
used in the practice of this invention is a ine, fluffy,
low bulk density powder. It has been found to be formabl~
with some dlfficulty, but is still quite operable and
practical. Other high-density pol~ethylene powders,
including ultra-high molecular weight powders have been
evaluated and have been found to be satisfactory. When
compacting such powders into a briguette, heating and
repressing it into a blank according to the present
invention, a defect-free part can be satisfactorily
forged and formed. This resin used in the specific
examples can be compacted into a briquette using a cold
compaction die and no vacuum. ~owever, better results
are often obtained when a warmer compaction die and
some vacuum is used.
Some comparative tests were run of Methods I,
II and III using a somewhat different xesinous powder.
Forty gram portions of ultra high molecular weight
polyethylene (UE~W) powder were compacted into briquettes
which were about 6.35 cm in diameter and abqut 2.54 cm
high. The conditions were as follows:
Method I
A. The bri~uettes were sintered by placing
them in a convection oven set at a peak melting
temperature (143C) for 120 minutes.
Method II
A. The bri~uettes were preheated by plac:ing
them in a convection oven at a temperature of 110C for
60 minutes (about Tp-33C).
27,818-F -14-
-15~
B. The bri~uettes were then repressed at a
pressure of 7Q0 kg/cm2 for 15 seconds in a compression
tool at a temperature ~f 136C to form blanks.
C. The blanks were then cooled to room
temperature.
D. The blanks sintering were as in Method
IA, above.
Method III
_
A. The briquettes were sintered as in
Method IA, abov~.
B. The briguettes were then repressed at a
pressure of 700 kg/cm2, for-15 seconds in a compression
tool at a temperature of 136C to form blank.
Upon completion of each of the above Methods,
the blanks were forged into 19.1 cm diameter disc
shaped parts between flat patens at a temperature of
136C by application of a pressure of 315 kg/cm2.
Tensile test specimens were then cut from -these disc
shaped parts and tested according to standard ASTM
method D~638. The results are presented below:
Elongation Strength Modulus
Method (~ (kq/cm2) (k~/cm2)
I 95 658 14,350
II 120 917 17,850
25III 100 1008 16,100
27,818-F -15
-16~
Tests of the three methods were also run at
different heating cycles some of which produced
unsatisfactory forgings because heating times were too
short, or because heating times were too long to melt
the parts. The above test was selected to typify
comparative results where satisfactory forgings for all
three methods were obtained. What it shows is that
both Methods II and III resulted generally in forgings
with better properties than those made by Method I with
this particular material. Method II was found to yield
somewhat better properties generally than Method III,
but Method III still yielded blanks having excellent
properties and had the advantage of being less complex
than Method II.
The efect of the degree of melting during
sintering on the mechanical properties of a preform
during solid-phase forming has been found to be
significant. The process of sintering is a progressive
one in which three stages may be identified. During
the initial stage, the entire briquette is being heated
to within a few degrees of Tp or the melting range.
- - Blanks forged after sintering to this stage commonly
produce grossly defective preforms in t~at they tend to
not fill the mold, have very low values of ultimate
extension and are generally quite cloudy due to
incomplete fusion of the particles. In the second
stage, when the deyree of melting approaches about ~0
percent, the particles at the exterior surface o~ the
briguette become completely molten and begin conducting
significantly more hea-t to the interior. Forgings
produced from blanks at this second stage are normally
well~formed and have consistent physical properties.
It has been discovered that if the sintering process is
27,818-F -16-
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-17~
allowed to proceed to a third stage wherein one exceeds
about 80 percent and approaches about 90 percent or so
of the original crystallinity being melted, the physical
properties normally change rapidly with further sintering.
Ultimate elongation (Figure 6) is increased significantly
while modulus (FigurP 5) can be decreased by as much as
30 percent. The reasons for these dramatic changes in
the final stage of sintering is not clear but may be
related to a complete loss of the crys-tal morphology
originally presen-t in the resinous material powder.
The effects of degree of melting during sintering on
tensile strength, for example, are illustrated in
Figure 4. A preform, such as a preform 18, 28 or 34 of
Figures 1 to 3, respectively, would have its tensile
strength measured after forging where the degree o
melting at the conclusion of the sintering step would
be measured with the results shown in Figure 4. It was
discovered that the maximum tensile strength was
reached when the degree of melting of the crystals in
the samples was between 20 and 90 percent.
The tensile modulus of these same samples was
also measured during the sintering process. From
Figure 5 is is evident that the modulus r~emained high
until the degree of melting approached about 90
percent.
The tensile elongation of the samples was
also measured and again it was found that a satisfaçtory
level of elongation was achieved when the degree of
melting approached about 20 percent until it approached
about 90 percent, as evident Erom Figure 6.
27,818-F -17-
~.2~
-18-
Balancing the mos-t desirable tensile strength,
tensile modulus and tensile elongation mechanical
properties it is evident that the physical parameters
are subject to rather little change and are in a satis-
factory range at melting levels from 20 to 90 percent.
When the degree of melting is in excess of about 90
percent, all three physical parameters were found to
change dramatically. For example, between 90 and 100
percent, the ultimate elongation increased by a factor
of 2 while the strength decreased by a factor of nearly
1/2. These observations have led to the conclusion
that optimal sintering conditions for briquettes of a
nature similar to that of high density polyethylene in
Method I as well as in Methods II and III of this
present invention are such that the degree of melting
is between 20 and 90 percent. To insure ma~imum
advantages of the processes of this invention, it is
preferable to sinter where the degree of melting
achieved is from 40 to 80 percent. When a forced air
convection oven is used for heating, the oven
temperatures can be set to about 3 degrees Centigrade
above Tp for faster heating cycles without hindering
the process, but residence times for the preforms
should not be such that Tp is reached at the center of
th b tt
s rlque e.
With the present invention numerous materials
heretofore not readily formable can be formed in a
solid-phase system directly from many crystalline and
perhaps some amorphous resinous material powders into
thermoplastic preforms and articles. While certain
representative embodiments and details have been shown
for the purposes of illustrating the invention, it will
be apparent to those skilled in the art that various
27,818-F -18-
" -19-
changes and modifications can be made therein without
departing from the spirit and scop~3 of the invention.
27, 818-F -19-
