METHOD AND APPARATUS FOR FORMING EXPANDED PANELS

METHODE ET DISPOSITIF DE FACONNAGE DE PANNEAUX EN MATIERE EXPANSEE

English Abstract

ABSTRACT
In order to make large panels of expanded
thermoformable material in a process in which a blank of the
material is placed between two heated mold plates, the blank
is adhered to the plates by hot tack adhesion; the plates are
separated and the expanded material is cooled, while maintaining
commercial tolerances; the mold plates are supported on
elongated mounting studs.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
l. In a device for expanding the cross sec-
tion of a blank of thermoformable material which includes
a machine having first and second opposed press platens;
means for controlling the spacing of the press platens,
first and second molding plates; and means attaching the
molding plates to the press platens, the improvement compris-
ing the means attaching the press platens comprising:
a) a plurality of elongated studs
distributed over the area of the
molding plates the length of said
studs being such as to satisfy the
equation: D = C <IMG> where
D equals one-half the maximum devia-
tion permissible in panel thickness,
C the length of the studs and X the
amount of thermal expansion in one
half of the mold based on the mold
material used and the temperature
differential to be employed;
b) means for attaching one end of
each of said studs to a press platen;
and
c) means for attaching the other end of each
of said studs to a mold plate.
2. Apparatus according to claim l wherein
said thermoformable material has a length of approximate-
ly 10 feet and wherein said studs are approximately 16
inches long.

3. Apparatus according to claim 1 wherein
said press platens are equipped with openings through
which said studs may pass and wherein said means for at-
taching to said press platens comprise lock nuts on each
side Or said openings.
4. Apparatus according to claim 1 wherein
said means for attaching said studs to said molding
comprise threaded blocks rigidly attached to said molding
plates and a lock nut for locking said studs in place
after being screwed into said threaded blocks.
5. Apparatus according to claim 1 wherein
said studs are arranged in rows.
6. Apparatus according to claim 5 wherein
the two studs on each end of each row are spaced closer
to each other than the remaining studs.
7. Apparatus according to claim 6 wherein
said thermoformable material is approximately 10 feet
long and 4 feet wide and wherein each row of studs com-
prises seven studs.
8. Apparatus according to claim 7 wherein
said end studs of each row are spaced approximately 8
inches apart and the remaining studs approximately 13
inches apart.
9. Apparatus according to claim 8 wherein said
rows are spaced approximately 17 inches apart.

Description

11185~2
This invention relates to processes in which a
plate or press must be held in a fixed position during a
wide range of thermal cycling such as in the production
of expanded thermoformable materials and products in
general and more particularly to an improved method and
apparatus for maintaining a press or plate so fixed.
With respect to producing thermoformable products, it
permits producing larger panels of such materials than
was heretofore possible.
In general terms, an important type of process
and apparatus to which the present invention relates is
that disclosed in a series of U.S. Patents 3,919,445;
3,919,380; 3,919,381 and 3,919,378, all issued Novem-
ber 11, 1975 to Walter H. Smarook. Basically, as dis-
closed in U.S. Patent 3,919,445 the process is one in
which a blank of thermoplastic polymeric material is
placed between two molds of a heated press. The tem-
perature of the mold plates is about five or ten degrees
Celsius above the temperature at which the thermoplastic
material exhibits hot tack adhesion. The mold plates
are separated apart and the adhesive forces of the poly-
meric material to the mold plates are greater than the
cohesive flow properties of the polymeric material itself
during the plate separation or expansion step. Thus, it
is possible to mechanically move the mold plates a cer-
tain distance apart with the polymeric material bonded
to the surfaces thereof without causing a rupture o~ the
adhesion between the surfaces and the fused material.
The plates are separated causing the panel to take or. 2
new internal geometry and the plates then cooled to

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solidify the expanded panel. During the process it is
mandatory that the plate surfaces be parallel to each
other within a few thousands of an inch, first during
the melting stage, or else the plastic adhesion will not
become accomplished, and finally, during the cooling
state, or else the final pattern will not meet commercial
standards of thickness uniformity. Typically, tempera-
ture cycling can range between 60 and 700F; a more com-
mon range being from 100 to 400F. Thus, a fundamental
problem in this process is that the metal mold plates,
when subjected to temperature changes, also undergo
dimensional changes and, when fixed to a supporting
frame which is, of course, necessary they tend to bend
so as to lose their parallelism. In addition, buckling
of the supporting frame can also occur. Most develop-
mental work in these processes described in aforemen-
tioned patents was done on six inch squares. Maintain-
ing parallelism with such a small size is not particu-
larly difficult. However, attempts to make larger size
panels, up until the time of the present invention, have
not been particularly successful. Even panels in a size
as small as 14 x 24 inches have proven problematic.
There is, however, a need for larger panels, for example,
panels of sizes up to and larger than 4 x 10 feet. For
example, such panels can be used as floating roof covers
in chemical and petroleum tanks to reduce pollution and
evaporation loss. There are, of course, many other ap-
plications for such panels, some of which require high
tolerances, particularly where a plurality of panels are
bonded or fitted together, e.g. office partitions and
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desk tops.
Thus, it can be seen that there is a need for
an improved method and apparatus for carrying out an ex-
pansion process with thermoformable material so as to
produce larger size products, such as panels, which are
commercially acceptable.
In general terms, there is a need for a method
and apparatus for holding large presses or plates fixed
during ther~al cycling.
The present invention provides a solution to
this problem. Deviating from the previous manner of
mounting in which I beams were mounted directly to the
parallel press platens of the press machine-used to move
the molds apart and together, an arrangement which per-
mits relative movement between the machine and the mold
is used in the method and apparatus of the present inven-
tion. In accordance with the present invention, each of
the molds is connected to the machine frame utilizing a
plurality of elongated mounting studs. It has been dis-
covered that when making thermoformable panels of a size
of 4 x 10 feet, a length of these mounting studs greater
than twelve inches is necessary in order to obtain molded
panels which meet tolerance requirements and to achieve
repeatability.
The construction of the present invention
avoids problems found in other mounting arrangements.
For example, a mounting arrangement in which the mount-
ing of the molds was made directly to I beams which were
in turn directly mounted to the machine frame with the
mold mounting bolts disposed so as to permit free sliding
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111857Z
movement was found to be completely unsatisfactory re-
sulting in a bowing which caused a three-sixteenths
deviation between the ends and the middle. This is
thought to be the result of the bolts not sliding freely
and heat from the mold warping the I beams and machine
frame. Even attempts at improving the sliding of the
mounting bolts through the use of Teflon washers and
insulating the mold from the I beams with suitable in-
sulators does not give effective results. Although such
steps will result in improvement, it is still difficult
to obtain panel repeatability which meets commercial
standards. Furthermore, even the use of mounting bolts
of the type used in the present invention which are
twelve inches long or shorter will not permit repeatably
satisfactory results when making panels which have a
size of 4 x 10 feet. Only when the mounting bolt length
is increased above twelve inches, for example, sixteen
inches, can tolerances be maintained in this size of
panel. With sixteen inch mounting studs, thickness de-
viation has been reduced to 10 mils with the apparatusand process of the present invention.
With the mold mounting of the present inven-
tion, utilizing sixteen inch studs, thermal effects on
the frame are reduced to insignificance. The predominant
distortion effect is the horizontal movement of the
mounting studs. Typically, with a horizontal mold plate
and a 300 temperature difference there will be a change
in length of approximately 0.23 inches on each side.
The effect of this horizontal change on the vertical
position has been calculated to be approximately 1.6
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*Trademark

3572
mils for each mold plate or a total of 3.2 mils. This
is consistent with measurements made on actual molded
panels and is well within the tolerances required for
commercial grade panels.
In accordance with the foregoing objects, there
is provided:
In a process for expanding the cross sec-
tion of a blank of thermoformable material while in a
thermoformable state by steps comprising positioning the
material between two mold plates while the material in a
thermoformable state and heated to a temperature at which
it will adhesively bond to the mold plates, adhesively
bonding the material by hot tack adhesion to each of the
two mold plates, pulling the mold plates apart while the
material is so bonded and in a thermoformable state and
cooling the expanded material to a temperature below the
heat distortion point of the material, the improvement
comprising supporting each of the two mold plates being
supported on a plurality of elongated studs, the studs
having a length such as to satisfy the following equa-
tion: D = C _ ~ c2 _ x2 where D equals one-half the
maximum deviation permissible in panel thickness, C the
length of the bolts and X the amount of thermal expansion
in one half of the mold based on the mold material used
and the temperature differential to be employed.
There is also provided:
In a device for expanding the cross sec-
tion of a blank of thermofor~able material which includes
a machine having first and second opposed press platens;
means for controlling the spacing Or the press platens,
first and second molding plates; and means attaching the

--~` 111857Z
mold plates to the press plates, the improvement compris-
ing the means attaching the press platens comprising:
a) a plurality of elongated studs
distributed over the area of the
molding plates the length of said
studs being such as to satisfy the
equation: D = C _~c2 _ x2 where
D equals one-half the maximum devia-
tion permissible in panel thickness,
C the length of the bolts and X the
amount of thermal expansion in one
half of the mold based on the mold
material used and the temperature
differential to be employed;
b) ~eans for attaching one end of
each of said studs to a press platen;
and
c) means for attaching the other end
of said studs to a mold plate.
There is further provided:
In a process in which a flat planar member
such as a press or plate must be held in a fixed position
during thermal cycling through large ranges, a method of
avoiding bending of the member due to the thermal cycling
comprising supporting the member on a plurality of elon-
gated studs, the studs having a length such as to satisfy
the following equation: D = C -~C 2 _ X2where D equal
the maximum deviation permissible in bending of the member,
C the length of the bolts and X the amount of thermal ex-
pansion in the member based on the material used therein
and the temperature differential to be employed.
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8S7Z
There is also provided:
In a device which includes a machine having
first and second flat planar members; means for controlling
the spacing of the planar members, and means attaching the
planar members to a support structure, the improvement com-
prising the means attaching the planar members comprising:
a) a plurality of elongated studs
distributed over the area of the planar
members the length of said studs being
such as to satisfy the equation:
D = C _~c2 _ x2 where D equals the
maximum deviation per~issible in bend-
ing of the member C the length of the
bolts and X the amount cf thermal ex-
pansion in the planar members based on
the material of which they are made and
the temperature differential to be employed;
b) means for attaching one end of each
of said studs to the support structure;
and
c) means for attaching the other end of
said studs to a planar member.
Fig. 1 is a plan view of a portion of the ma-
chine platen illustrating the location of the mounting
studs.
Fig. 2 is a side view of the arrangement of
Fi~. 1 also showing the mold plate attached to the ma-
chine platen.
Fig. 3 is a detail of Fig. 2 illustrating in
more detail the mounting arrangement.
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~ 111857Z
FiEs. 1 and 2 illustrate the general arrange-
ment of the apparatus of the present invention. Fig. 1
is a view of the attachment of the mounting studs 11 of
the present invention to the machine frame platen 13,
more specifically the upper platen 13 of the machine
frame. The machine frame platen illustrated comprises
two 8 inch steel channels 12 with transverse ~ inch
steel I beams 14 disposed therebetween. As illustrated,
threé rows of mounting bolts are used. With the excep-
tion of those on the ends, the mounting bolts have equal
spacing in the horizontal direction equal to b". At the
ends there is only a spacing of b' between mounting
studs. The three rows of mounting studs 11 are separated
fro~ each other by a distance designated a. Typically,
for a 4 x 10 panel, i.e. where the di~ension A is 4 feet
and the dimension B 10 feet, the distance a will be 17
inches with the middle row of mounting studs 11 on the
center line, the distance b' will be 8 inches and the
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` -~ 1118572
distance b" 13 inches. Fig. 2 illustrates the machine
platen 13 once again along with the mounting studs 11
and, at the end of the mounting studs 11, the mold plate
15. Shown also is a portion of the bottom mold plate
15a which is attached with similar bolts 11a to the op-
posing machine frame platen (not shown). Finally, Fig.
3 illustrates in more detail the attachment of the mold
15 to the machine frame 13. As illustrated, each mount-
ing stud 11, which may be a 5/8-11 threaded rod is at-
10 tached to a threaded block 17 which is welded or other- ~-
wise fastened to the mold plate with a lock nut 19 used
to secure the threaded rod 11 in place. The threaded
rod is passed through a suitable hole in the machine
platen 13 with an opening provided thereabove so that
lock nuts 21 can be fastened on opposing sides of the
machine frame. The distance between the bottom of the
machine frame 13 and the top of the mold plate 15, in-
dicated as the distance C on the figure, when molding
plates which are 4 by 10 feet should be at least 16
inches. In general terms, the amount of thermal expan-
sion X of a plate in either direction as measured from
the center line would be equal to B/2 times the coeffi-
cient of thermal expansion times a t, where a t is the
temperature change. For a 4 x 10 foot panel the equation
25 becomes X = 60 in. x 13 x 10 6 in/in/F x 300F = 0.23
inches. The change D in vertical dimensions because of
such a horizontal expansion will be equal to C _ ~c2 _ x2.
In the present example, C is equal to 16 inches and
X equals .23 inches. Solving for the above equation
results in the vertical distance D being equal to 1.6
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` ~ 1118S72
mils. In general termæ, when panels of a different size
are being made, the length of the bolts should be se-
lected to satisfy the equation D = C - ~C~ _ x2 where
D equals one-half the maximum deviation permissible in
panel thickness, (since there are two molds, each will
make a contribution to the thickness deviation), C the
length of the bolts and X the amount of thermal expan-
sion in one half of the mold based on the mold material
used and the temperature differential to be employed.
AlthouEh the present invention has been dis-
closed in connection with a process for thermoformable
materials, it will be recognized by those skilled in the
art that it is ~enerally applicable to any process where
a plate or press must be held fixed such as in reaction
injection molding (RIM).
. ~