Apparatus for and method of pressing plastic sheets

A peripheral edge-containing member is provided to reduce the length of the projectile path of fragmented particles expelled outwardly from the peripheral edge of a frangible pressing plate which is broken during the press-stretching or press-polishing of acrylic sheets. The acrylic sheets are fabricated for use in light weight transparencies.

FIELD OF THE INVENTION 
This invention relates to pressing plastic sheet between pressing plates, 
and more particularly, to press-stretching and/or press-polishing the 
surface of a plastic sheet between glass pressing plates. 
DISCUSSION OF THE TECHNICAL PROBLEM 
It is known in the fabrication of light-weight transparencies, e.g., 
aircraft transparencies, to utilize at least one sheet of a plastic sheet, 
e.g., acrylic, either monolithically or as part of a laminate structure. 
In general, a plastic sheet may be formed from an as-cast acrylic blank by 
press-stretching the acrylic blank between a pair of complementarily 
shaped pressing plates to a required useable thickness. The optical 
properties of the plastic sheet may be improved by pressing it at an 
elevated temperature and pressure between a pair of complementarily shaped 
pressing plates having opposed optically smooth surfaces to minimize 
surface deformations. For a detailed description of these techniques, 
reference may be had to U.S. Pat. Nos. 3,632,841 to Fortin and 3,681,483 
to Moore. Pressing plates employed in the above-mentioned techniques are 
commonly formed of glass sheets because glass may conveniently be formed 
to have the desired surface smoothness. 
Due to the elevated pressures reached during press stretching and/or press 
polishing and in the interest of durability, the glass pressing plates are 
generally tempered for increased strength. However, tempered glass, if 
broken, tends to explode into a large number of relatively small 
particles, e.g., one-half inch diameter (1.27 cm.). During press 
stretching and/or press polishing, the peripheral edges of the tempered 
glass pressing plates are generally exposed. When the tempered glass 
pressing plates are broken under pressure, e.g., during press-stretching 
or press-polishing, a number of the relatively small glass particles are 
forcefully expelled outwardly from the exposed peripheral edges, and 
depending upon the pressure being utilized, have been known to travel up 
to thirty feet (10 meters). This type of occurrence, of course, poses 
personnel safety problems as well as substantial clean-up burdens. 
In addition, the exposed peripheral edges of tempered glass pressing plates 
may be damaged during the handling to which they are subjected. Minor edge 
damage may lead to fracture during subsequent pressing operations. 
In view of the above, it would be advantageous to provide an apparatus for 
and a method of press-stretching and press-polishing a plastic sheet which 
diminishes or eliminates the limitations presently associated with 
utilizing tempered glass pressing plates. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus for and a method of applying 
pressure to opposed major surfaces of a plastic sheet with a pair of 
complementarily shaped pressing plates, at least one of which is formed of 
a fracturable material, e.g., tempered glass. An adhering material, e.g., 
a silicone adhesive, is applied to exposed portions of the peripheral edge 
of the fracturable pressing plate to form an edge-containing layer 
therearound to reduce the length of the projectile path of particles which 
are outwardly expelled from the peripheral edge when breakage occurs. In 
this convenient and cost-effective manner, operational safety is improved 
and the burden of cleaning up a broken pressing plate is substantially 
diminished. In addition, the peripheral edges of the pressing plate is 
protected from much damage which might occur during handling by the 
interposition of the edge-containing layer.

DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, there is shown a press 10 having an upper 
pressing platen 12 and a lower pressing platen 14 mounted in facing 
relation and conveniently moveable relative to one another, e.g., the 
upper platen 12 moveable toward and away from the stationary lower platen 
14 to press an acrylic member therebetween. A carrier member 16 is 
positioned upon the upwardly facing surface of the lower pressing platen 
14, and a subassembly 20 formed of a lower pressing plate 22, an acrylic 
blank 24, and an upper pressing plate 26 is positioned thereon. The upper 
platen 12 and lower platen 14 are then generally biased one toward the 
other, either to impress an optically smooth surface onto the acrylic 
blank 24 during a press-polishing mode of operation, or to generate a 
thickness reduction in the acrylic blank 24 in a press-stretching mode of 
operation. 
The invention will be more fully described as it relates to the 
press-stretching mode of operation which is illustrated in FIG. 1, 
although it is to be understood that the invention is not limited thereto. 
In sequential order, a blank 24 of plastic sheet, e.g., acrylic, is 
preheated in an oven to its softening temperature range, e.g., about 
300.degree. F. (150.degree. C.), for between about a 3/4 hour to about a 3 
hour period. The carrier member 16 and pressing plates 22 and 26 are also 
preheated to a comparable temperature, and a lubricant film, e.g., 
polytetrafluoroethylene in a colloidal dispersion sold under the trademark 
Mold-Wiz F-57 by Axel Plastics Research Lab, Inc., is applied to both 
major surfaces of the pressing plates 22 and 26. Upon reaching the desired 
temperature conditions, the carrier 16 is positioned between the platens 
12 and 14, and the preheated and lubricated sandwich 20 is positioned atop 
the carrier 16. The upper platen 12 is biased downwardly from its rest 
position (shown in solid in FIG. 1) toward its pressing position (shown in 
phantom) to exert between about 300 pounds per square inch (p.s.i.) 
(2.1.times.10.sup.6 pascal) and about 500 p.s.i. (3.5.times.10.sup.6 
pascal) pressure upon the acrylic blank 24. The acrylic blank 24 gradually 
undergoes a thickness reduction and a corresponding length and width 
increase, and the pressure is maintained for a time and at a magnitude to 
produce desired physical characteristics, e.g., about 15-20 minutes of 400 
p.s.i. (2.8.times.10.sup.6 pascal) pressure. The reduced-thickness acrylic 
blank 24 is maintained under pressure and controllably cooled to a 
temperature below its softening temperature to preserve the desired final 
physical characteristics and avoid the affects of "plastic memory", e.g., 
for about 15 minutes to a temperature of about 150.degree. F. (65.degree. 
C.). Additional details of the press-stretching process may be found in 
U.S. Pat. No. 3,632,841 to Fortin, which is hereby incorporated by 
reference. After the cooling process is completed, the pressing assembly, 
i.e., carrier 16 and pressing plates 22 and 26, are cleaned of residual 
lubricant and relubricated in preparation for a subsequent pressing cycle. 
As before mentioned, the pressing plates 22 and 26 are commonly formed of 
glass which is tempered for increased strength and durability. Prior to 
the present invention, such tempered glass pressing plates have been 
utilized with their peripheral edges exposed during all phases of the 
press-stretching operation, and it has been known to happen that the 
pressing plates break, most commonly while pressure is being applied 
thereto. In such an event, the tempered glass pressing plate fragments 
into a large number of relatively small particles, e.g., 1/2 inch (1.25 
cm.) in diameter, a substantial number of which are forcefully expelled 
outwardly from the peripheral edge of the pressing plate. 
With reference to FIGS. 1-4, the present invention provides facilities for 
obviating the above-discussed difficulty, in the form of a peripheral 
edge-containing member 30, which is secured about the periphery of each 
tempered pressing plate to retain particles of fragmented glass within a 
localized area during inadvertent breakage of the pressing plate. 
With particular reference to FIGS. 2 and 3, the edge-containing member 30 
may take the form of a continuous bead 32 of an elastomeric material 
secured to the peripheral edge of the pressing plate 22 or 26. Preferably, 
the bead 32 is applied to the peripheral edge to completely cover same, 
while avoiding engagement with the marginal edge portions of the pressing 
plate 22 or 26, to avoid introducing any foreign body onto the major 
surfaces of the pressing plates 22 or 26 which might generate a nonuniform 
pressure condition. As shown in FIG. 3, the bead 32 may be conveniently 
formed in a generally semi-circular shape when viewed in cross-section by 
the use of an appropriate rounded template tool. Preferably the bead 32 is 
formed of a material which is able to withstand repeated temperature 
cycling in the applicable temperature range, which adheres strongly to 
glass, which cures quickly to a non-tacky finish, and which is 
economically attractive. Although not limiting to the invention, useful 
elastomeric materials may include silicone adhesives and sealants, 
polysulfide adhesives and sealants, structural elastic hot melts, or 
polyurethane adhesives. 
With reference to FIG. 4, another embodiment of the present invention is 
shown, including an edge-containing member 30 taking the form of a layer 
34, e.g., between about 0.005 inch (0.013 cm.) to 0.016 inch (0.04 cm.) 
thick, of an appropriate paint. As it is the object of the present 
invention to diminish or eliminate the expulsion of fragmented particles 
from the peripheral edge of the pressing plates 22 or 26, it has been 
determined that the peripheral edge of the pressing plates 22 or 26 may be 
conveniently painted to reduce the length of the projectile path of 
fragmented particles. Again it is preferred that superior temperature 
cycling properties and glass adhesion properties be provided in the paint 
selected for use in this embodiment of the invention. 
Although not limiting to the invention, paints which may be found useful in 
the practice of the present invention may include latex paints, 
epoxy-based paints, and preferably elastomeric polyurethane based coatings 
such as those taught in U.S. Pat. Nos. 3,912,790 and 4,147,679, which 
teachings are incorporated herein by reference. 
With continued reference to FIG. 4, the edge-containing member 30 may take 
the form of a layer of adhesive tape applied in a continuous manner to the 
peripheral edge of the pressing plates 22 or 26. Preferably the tape 
selected would exhibit superior temperature cycling properties, would 
adhere well to glass, and preferably would be resilient. 
Although not limiting to the invention, tapes which may be found useful in 
the practice of the present invention may include polyester tapes, a 
polyethylene tape having an acrylic adhesive such as the one available 
from the Venture Tape Corporation of Randolph, Massachusetts as 910, and 
selected metal-backed tapes. 
As will be better appreciated from a consideration of the experimental work 
set forth in the following Examples, it is expected that the elastomeric 
materials will be preferred in the practice of the present invention, 
although each of the embodiments disclosed herein may be practiced to 
advantage. 
EXAMPLE I 
A 12 inch (30.5 cm.).times.12 inch (30.5 cm.).times.0.5 inch (1.25 cm.) 
flat glass plate was thermally tempered and placed centrally within an 
open-topped compartment having bottom dimensions of about 24 inches (61 
cm.).times.36 inches (91.5 cm.). The peripheral edges of the glass plate 
were left exposed and untreated. A planar sheet was placed over the 
exposed major surface of the glass plate to simulate the sandwich 
condition which exists during a press-stretching operation, and the planar 
sheet was struck near its center with one end of a 3 lb. (1.36 kg.) metal 
bar with a force sufficient to fracture the underlying glass plate. A 
large number of small glass particles were expelled outwardly, confined 
along the plane of the major surfaces of the plate by the sandwich 
condition, and some had velocity sufficient to bombard all walls of the 
compartment in which the glass plate was contained. Example I illustrates 
the condition which exists absent the practice of the present invention, 
and will be used for comparison purposes hereinafter. 
EXAMPLE II 
A tempered flat glass plate of the type utilized in Example I had its 
peripheral edges wrapped by a single 0.002 inch (0.005 cm.) thick layer of 
polyester tape having a silicone adhesive on one side thereof available 
from the 3M Company as 8403. The taped-edge glass plate was positioned and 
sandwiched in the compartment and fractured by a blow with the end of the 
metal bar as in Example I. Upon fracture, the plate fragmented into a 
large number of relatively small particles, and the polyester tape severed 
at one point along its length, permitting a number of glass particles to 
be expelled outwardly in three directions, but restricting the expulsion 
of glass particles in the direction toward the peripheral edge opposite 
the position of tape breakage. Thus, three of the compartment walls were 
bombarded with glass particles, while one was protected by the polyester 
tape. Thus, an appropriate tape may prove useful in the practice of the 
present invention, particularly if the tape can be prevented from severing 
at a point. 
EXAMPLE III 
A tempered glass plate as in Examples I and II had a single layer of latex 
poster paint, available from Leisure Crafts, Co., of Compton, California 
under the trademark Leisure Tone, applied and dried upon its peripheral 
edge. The plate was positioned in the compartment as in the previous 
Examples and was fractured by a blow with the metal bar near its center, 
as before. Upon fracture, the plate was fragmented into a large number of 
relatively small particles which were expelled outwardly along the plane 
of the major surface of the sheet, but the projectile path of the expelled 
glass particles was substantially shorter than those of Examples I and II, 
none reaching the walls of the compartment. The expelled glass particles 
finally formed a substantially square array of glass particles in the 
bottom of the compartment measuring about 21.3 inches (54 cm.).times.21.3 
inches (54 cm.). 
EXAMPLE IV 
A tempered glass plate as in the previous Examples had two of its 
peripheral edges coated with a bead of GE RTV Clear Silicone Rubber 
Adhesive Sealant #108, which was cured thereon, and two of its peripheral 
edges coated with a layer of a sealant available from Products Research 
Manufacturing of Glendale, California as PR1221-B 1/2. As in the previous 
Examples, the plate was positioned in the compartment and fractured by a 
blow near its center. Upon fracture, the plate fragmented into a large 
number of relatively small particles, but the glass particles were 
retained substantially intact by the edge-containing function of the 
silicone adhesive and sealant layers. Only a very few glass particles were 
actually expelled from the fractured glass plate, and the intact but 
fractured plate was expanded only slightly from its original 12 
inch.times.12 inch dimension. Additionally, no significant difference 
could be noted between the final condition of the silicone adhesive-coated 
edges and the sealant-coated edges. In practice, however, the silicone 
adhesive is generally preferred due to its quick cure time and excellent 
adhesion to glass. 
Of course the invention is not intended to be limited by the specific 
embodiments disclosed in the Examples, but rather, only by the claims 
which follow.