US 3694129 A
An apparatus for casting sheets of plastic is shown. It includes a casting zone defined by two glass surfaces and a marginal sealing gasket, and an adjacent temperature control cell. The cell is an enclosed chamber comprised of a rigid frame having two extended, semi-rigid sides which may be of sheet metal or glass and which are flexible and responsive to pressures acting upon them. In one embodiment, the cell frame has a rigid brace attached within it, positioned adjacent to the inner surface of its sheet metal or glass side. The brace prevents inward deflection of that side so that the casting zone adjacent to that side is not distorted by the hydrostatic pressure of the liquid charged to it. In this embodiment, there is no need for charge measuring apparatus, the side of the casting zone is free to deflect outwardly and follow the cast sheet as it shrinks during polymerization and higher temperature and higher pressure media can be used in the temperature control cell. In the preferred embodiment the side of the temperature control cell is glass bonded to the sides of the frame at a series of novel bonding zones. The amount of liquid plastic charged to the casting zone is controlled by first adjusting the head of temperature control liquid in the adjacent control cell to a height which just balances the head of liquid plastic in the full casting zone.
Description (OCR text may contain errors)
United States Patent Daddona 1541 APPARATUS FOR CASTING PLASTIC SHEET  Inventor: Paul R. Daddona, Burning Tree Road, Greenwich, Conn. 06830  Filed: Aug. 24, 1970  App1.No.: 66,357
52 us. C1. ..'....425/405, 264/102, 249/82 51 Int. Cl. ..B29d 7/08 581 Field of Search....l8/26 R; 249/81, 82; 264/102,
 References Cited UNITED STATES PATENTS- 3,551,541 12/1970 Rossetti 18/26 R X 3,165,786 1/1965 Yogi et a1. .;264/219 X 3,072,971 l/l963 Kish ..264/2l9'X 3,425,885 2/1969 Webb ..264/l02 X Primary Examiner-Robert L. Spicer, Jr. Attorney-Joseph Levinson, Esq.
[ 5 7 ABSTRACT An apparatus for casting sheets of plastic is shown. It
[4 1 Sept. 26, 1972 includes a casting zone defined by two glass surfaces and a marginal sealing gasket, and an adjacent temperature control cell. The cell is an enclosed chamber comprised of a rigid frame having two extended, semirigid sides which may be of sheet metal or. glass and which are flexible and responsive to pressures acting upon them. In one embodiment, the cell frame has a rigid brace attached within it, positioned adjacent to the inner surface of its sheet metal or glass side. The
brace prevents inward deflection of that side so that the casting zone adjacent to that side is not distorted by the hydrostatic pressure of the liquid charged to it. In this embodiment, there is no need for charge measuring apparatus, the side of the casting zone is free to deflect outwardly and follow the cast sheet as it shrinks during polymerization and higher temperature and higher pressure media can be used in the temperature control cell. In the preferred embodiment the side of the temperature control cell is glass bonded to the sides of the frame at a series of novel bonding zones. The amount of liquid plastic charged to the casting zone is controlled by first adjusting the head of temperature control liquid in the adjacent control cell to a height which just balances the head of liquid plastic in the fullcasting zone.
17 Claims, 9 Drawing Figures PATENTED I972 3.894.129
sum 1 0F 5 PATENTEUSEPZS I972 SHEET 2 BF 5 I\\ EN TOR PAUL R. DADDONA A TTORNERS PATENTEU E I9 3.694.129
SHEET u 0? 5 IXTEN'TOH. PAUL R. DADDONA A T TOR XE Y5 PATENTEDSEPZS I972 SHEET 5 BF 5 www INVENTOR. PAUL R. DADDONA 1 WWW A TTORNE Y5 1 APPARATUS FOR CASTING PLASTIC SHEET This invention relates to an apparatus and a method for casting articles from liquid casting fluids. More specifically, this invention relates to an apparatus and to a method for casting and curing plastic and still more specifically to an apparatus and method for the simultaneous casting of a plurality of flat plastic sheets of uniform thickness from a monomer or pre-polymer fluid.
Plastic sheet is conventionally cast between two flat, smooth mold surfaces separated by a spacer or by a gasket and clamped together to provide a liquid-tight assembly. A prepared monomer or prepolymer such as a catalyzed methyl methacrylate syrup is poured into the casting zone and the casting assembly is placed into an air oven or water bath to effect polymerization and cure. Heat generated during the exothermic polymerization reaction is removed by air or water convection and depending on the thickness of the sheet and the plastic, up to 20 hours may be-required to harden the sheets and polymerize the residual monomer. 4
The mechanical steps required to assemble the casting zone, feed monomer to it, convey it to a suitable heat transfer bath, control the temperature within the zone, dismantle the zone and remove finished product are a substantial source of manufacturing difficulty. The preferred mold surface is glass because it has a smooth, easy to clean surface and is available in many different sizes. Plate, sheet or tempered glass are commonly used. The problems encountered in conveying brittle glass sheets from one plant area to another, however, without damage are considerable and although ingenious mechanical conveyers for glass molds have been devised, they are expensive, space consuming and not altogether satisfactory. Another weakness of conventional casting processes is in the very substantial amounts of hand labor required to assemble the glass sheets and gaskets into casting zones. Still further, the thickness tolerance of large plastic sheet produced by conventional methods is poor due to non-uniform stresses produced by the clips used to seal the zones.
Conventional processes have been further complicated by the temperature control equipment required to effect polymerization of the monomer charge. The filled casting zones must be subjected to heat in an oven or bath for sufficient time to effect polymerization. in the casting of methyl methacrylate sheets, for example, the glass casting zones are mainvtained at a temperature between 50 and 75C. for a period of from 6 to 20 hours if the cure is effected by heat in an oven, and like or smaller amount of time if the cure is effected in a water bath. The actual time required depends on the thickness of the sheet and the nature of the catalytic additives added to the monomer charge. After polymerization, the temperature is run up" in order to complete polymerization of residual monomer and this may require 3 to hours more at a temperature of 100C.
After polymerization, the casting zones must be disassembled and the plastic sheets extracted. More time and laboris required and the plates usually must be washed before reuse in order to remove polymerized material or other contaminents which can ruin the surface quality of the cast sheet.
A further inadequacy of conventional processes for casting plastic sheets is in the method of feeding monomer or partial polymer to the casting zones. Typically, each charge must be weighed and then transferred to the casting zone. Separate weighing of each charge is time and labor consuming and inefficient in that inevitable losses occur through leakage, adhesion, evaporation and emission of objectional fumes into the casting area. Furthermore, when air is absorbed into the charge, the final product is invariably voided.
An apparatus which has greatly simplified these problems includes an elevated support track with a plurality of plastic casting zones suspended vertically from it. The casting zones consist of two flat glass surfaces in confronting relationship separated by a resilient marginal sealing gasket which serves as an edge closure. The gasket may be independently supported on a frame suspended from the support track or may be affixed to the outer rim of one of the casting zone surfaces. The
glass casting zone surfaces are suspended from the elevated support track and are movable along it. These surfaces may be individual sheets of glass or they may be the surface of one or more temperature control cells, the cells comprising an enclosed chamber with inlet and outlet ports for the passage of a heat transfer fluid. The temperature control cell has at least one side wall suitable as a casting zone surface and in the preferred embodiment, both side walls are suitable as casting zone surfaces. A clamp in contact with the exterior of the plurality of casting zones formed from temperature control cell( s) and/or individual sheets of glass tightly seals the casting zone surface against their intermediate marginal sealing gaskets in order to provide liquid-tight casting zones.
This apparatus reduces the problems and expense, among others, of glass breakage, voided sheet, excess labor requirement and raw material spillage. The apparatus includes equipment to measure, weight or otherwise meter casting fluid into the casting zones including tanks, piping, weighing stations and associated equipment, or, preferably, a plurality of identical volumetric charge vessels joined at their base by common conduits. These vessels permit a single weighed charge of fluid to be evenly distributed among themselves.
A volumetric measurement is required for each casting zone because the casting zones are expanded and are non-linearly distorted by the hydrostatic pressure of the casting fluid in them. The casting zone surfaces bow outwardly into the adjacent, empty temperature control cells. Nor is it possible to employ rigid casting zone surfaces as it is necessary that the surfaces of the casting zones be flexible so that they can follow the shrinking sheet as the plastic polymerizes.
Another remaining limitation of the system is in the speed with which polymerization and curing processes can be carried out. It is known that the rate of polymerization reactions increases exponentially with temperature. However, increasing the temperature heat, to be used directly in the cell nor can fluids be pumped through the cells at high flows and attendant high pressure differentials. Thirdly, the smooth walls of provide a method for using such apparatus without charge measuring equipment.
It is still a further object of this invention to provide an apparatus for casting plastic fluids into sheet which has a fixed volume casting zone during charging and therefore requires no fluid charge metering equipment but which nontheless permits the flexible movement of the casting zone surfaces during shrinkage of the cast sheet.
It is still a further object of this invention to provide a temperature control cell for use in a plastic casting apparatus, whose side walls are fixed during charging but are deflectible during polymerization so that they can follow the shrinking sheet.
It is a further and related object of this invention to provide a plastic casting apparatus and more specifically a temperature control cell for such an apparatus which permits high temperature and high pressure fluids to be used within the temperature control cells thereby reducing the time required to effect polymerization and cure.
'It is yet a further object of this invention to provide a temperature control cell of sturdy and dependable construction which can be used-in a plastic casting apparatus. I 1 I It has been found that these and other objects of this invention are achieved in an apparatus including a casting zone and an adjacent temperature control zone wherein the temperature control zone comprises an enclosed cell with a semi-rigid side deflectible in response to pressures acting upon it and having rigid bracing means affixed within it and positioned adjacent to the inner surface of the semi-rigid side in order to stopthe inward deflection of that semi-rigid side.
In one embodiment, the semi-rigid side of the temperature control cell is a casting zone element, i.e., a sheet of glass. In another embodiment, the temperature control cell includes an extended semi-rigid side of sheet metal with a casting sheet of glass outwardly adjacent to it. Bracing means are affixed to the frame within the cell and are positioned adjacent to the inner surface of the semi-rigid side. If two sides of the cell are extended, semi-rigid, sheet metal or glass pieces, the cell can be used in an apparatus for casting a plurality of plastic sheets simultaneously, i.e., one cell can provide temperature control for two casting zones located on either side of it.
It has also been found that a most satisfactory temperature control cell can be formed, without rigid bracing means, from a frame describing a closed rectangular area and two sheets of glass. The glass is bonded to the frame along two flat sides thereof. In order to form a pressure tight cell and yet permit the outward deflection-of the glass sheets during the curing process, the bond should be along at least two continuous bonding zones about the periphery of the frame. These bonding zones are recesses in the surface of the frame and have enlarged areas below the surface. Desirably, the recess has a cross-section which is a truncated triangle and the bonding agent is a two-part silicone adhesive.
With the latter temperature control cell, it has been found, that the liquid plastic can be charged to the casting zone without volumetric charge measuring equipment although such equipment can be used where particularly close tolerances are required or where otherwise such measurement is deemed desirable. The temperature control medium is introduced into the adjacent temperature control cell to a pre-determined height, a height at which the head of said liquid exactly balances the height of plastic charge liquid in the casting zone when the latter is entirely filled. Thenthe castingzone is filled. Since the hydrostatic heads in both the casting zone and in the temperature control cell are balanced, there is no expansion of the casting zone. An exact volumetric charge is obtained.
IN THE DRAWINGS FIG. 1 is a side elevation view of the plastic sheet casting and curing apparatus;
FIG. 2 is a front elevation view of the apparatus of FIG. 1;
FIG. 3 is a side elevation view, partially broken away, of an improved temperature control cell, according to this invention, through which curing fluid is circulated and of a single plate of glass which defines a casting zone with the improved temperature control cell;
FIG. 4 is a sectional view taken along lines 4 4 of FIG. 3;
FIG. 5 is a fragmentary exploded view of a temperature control cell and a pair of gasket frames which hold the marginal sealing gaskets;
FIG. 6 is a side elevation detail of a preferred cell;
FIG. 7 is a perspective view with portions broken away, of an improved temperature control cell;
FIG. 8 is a partial, side sectional view of the lower right hand corner of the temperature control cell of FIG. 7; and
FIG. 9 is a top sectional view of the lower corner of the temperature control cell taken along lines 9 9 of FIG. 8.
In FIGS. 1, 2 and 3, referencenumber 10 designates a plastic sheet casting station 10. The component parts of casting station 10 are supported by a frame, identified by reference numeral 12. The frame ineludes, at its two ends, upright members 14, 16 and 18 and spanning members 20, 22 and 24. These are in turn joined by several cross members, among them members 26 and 28. A first pair of elongated channels 30a and 30b and a second pair of elongated channels 32a and 32b are welded to the underside of the rear and front upper cross members. The four channels which comprise an elevated support track accommodate a plurality of casters 34. Frame 12 is tilted slightly off vertical for reasons which are discussed below.
A plurality of rectangular mold members, commonly designated by reference numeral 36 are individually, vertically suspended from channels 30a, 30b and 32a,
32b via casters 34. Pairs of casters 34, support each mold member 36 at spaced points along the elevated track, adjacent mold to and from one another.
Mold members 36 include rectangular temperature control cells designated by reference numeral 40, alternating with rectangular sheets of tempered glass 42 commonly known as lights. The latter are held inaluminum frames 43. Each light and cell is attached at spaced points along their upper edges to support rods 45 and fixtures 47 carried by each of casters 34. These are shown schematically in FIG. 4.
Cell 40 comprises a rigid rectangular frame 44 of aluminum and an extended semi-rigid side 46. Side 46, as shown in FIG. 3 and 4, is sheet metal preferably between 0.060 and 0.375 inches and is deflectible, ex-
members being readily movable casting zones 72.
cept as noted below, inwardly or outwardly (to-the right or left as shown in FIG. 4) in response to pressures acting upon it. Side 46 is attached and sealed to frame 44 by screws 49 so that a pressure tight cell is formed. Inlet and outlet ports 48 and 50 are provided in cell 40 e to permit passage of a temperature control fluid such as water, steam or heat transfer oil.
Cell 40 has a rigid brace 52 attached to frame 44 consisting of horizontal and vertical cross members 54 and 56 which may be of suitable steel bar stock or rods. Cross members 54 and 56 are firmly intersected or otherwise connected at cross points 58 and are firmly attached to the rigid frame 44 at points 60. Brace 52 is positioned adjacent to and in contact with the inner surface of semi-rigid side 46 in the straight planar position shown.
Adjacent tosemi-rigid side 46 of cell 40 is a glass sheet designated by reference numeral 62. This glass sheet may be supported, as shown in FIG. 6, on a shelf 68 attached to the lower'surface of frame 44 or in any other suitable manner. Gaskets 70, strips of a suitable material such as vinyl elastomer, are interposed as continuous marginal seals between each'glass plate 62 and 42. Thus, each gasket 70 with a surface of a glass sheet 42 and a surface of one of the glass sheets 62 forms a plastic sheet casting zone 72. With the assemblage of the cells 40 and the glass sheets 42 in side-by-side relationship, the plurality of casting zones 72 are eachadjacent on one side to a temperature. control zone.
Gaskets 70 also serve as spacers and by their size,
establish the thickness of the sheet cast. Although the drawings display a sheet metal semi-rigid. side 46, the apparatus can be successfully operated .without it. Where a sheet metal side is not used, glass sheet 62 is lights can be used to suit particular demands and manufacturing procedures.
Each gasket 70 is provided with a groove 74 throughout its length. An elongated, flexible flange 76 extends from a gasket frame 80 which attaches to aluminum frame 64 or alternatively is independently suspended from channels 30a, 30b and 32a, 32b. The free edge of each flange 78 is beaded for insertion in gasket groove 74 in order to support gasket 70. One comer of each gasket frame 78 is cut back, as indicated at 80, to facilitate the pouring of plastic charge into the The mold members 36 are clamped together in sandwich relationship by spider clamps 82a and 82b located at either end of the assemblage. Each spider clamp has a central hub 83 joined to a rectangular clamping frame 84 by a plurality of spokes 85. The rectangular configuration of each clamping frame conforms to the configuration of the marginal sealing gaskets 70 so that clamping force is exerted directly over the gaskets and evenly along all four sides of the casting zones 62. In practice, it is desirable to affix a resilient strip on the inner face of clamping members 84 in alignment with gaskets 70 to limit the clamping forces to the width of such gaskets. In this manner, clamping forces which would tend to bow the glass plates are avoided, casting zone leakage is avoided, and cast plastic sheet of improved thickness tolerance is obtained.
The hubs 83 of spider clamps 82a and 82b are suitably keyed to the ends of screw shafts 86 and 87 which are slidingly received in sleeves 88 which extend through braces 22 of frame 12. Shafts 86 and 87 are thus axially fixed in position. A crank or wheel 90 threaded on shaft 87 and bearing against the end of sleeve 88, is rotated to advance shaft 87 to the left and thereby clamp the assemblage of mold members 36 together between spider clamps 82a and 82b.
In operation, a single master charge vessel may be used to provide sufficient monomer or prepolymer to fill each of the casting zones 72. Monomer flows from directly in contact with brace 52 and the same effect is achieved.
Eleven water cells 40 are shown in FIG. 2 and between each pair of such cells are two casting zones 72 separated by glass sheets 42. Twenty plastic sheets can therefore be cast simultaneously. It has been found that this arrangement is efficient for casting plastic sheets up to thicknesses of one-quarter inch. For thicker plastic sheet, one-half inch or more, it is recommended, in order to achieve efficient curing, that the intermediate glass sheet 42 be removed, and a single sheet be cast between adjacent temperature control cells. This arrangement disposes a temperature control cell 40 adjacent to both sides of each casting zone. Other arrangements of temperature control cells and the vessel, not shown, via a flexible tube 90 into the casting zone. The flexible tube 90 extends into each Y casting zone through a gap between the ends 92 and 94 of gasket at the elevated corner 80. When zones 72 are being filled, tube extends nearly to the bottom.
' Mold members 36 are clamped together by clamps 82a and 82b during the filling operation and preferably the casting zones 72 are filled simultaneously. The tilting of the casting zones 62 facilitates the bleeding of air during and after filling.
As the casting zones are filled, the glass sheets 62 are subjected to force, due to the hydrostatic pressure of the plastic in the casting zone, which tends to deflect them into the temperature control cells 40. Were it not for rigid braces 52 which stop this deflection, the casting zones would be expanded and bowed. If the casting zones were permitted to bow due to the hydrostatic pressure, the volume of the zone would increase and charge measuring apparatus would be required to control the amount of plastic fed to the casting zone. With the rigid brace 52 maintaining glass sheet 62 and semirigid side 46 in an even, vertical, planar position, however, the volume of each casting zone is fixed during filling and there need be no measurement of the charge. When the zone is filled, the tube 90 is withdrawn. Ends 92 and 94 of gaskets 70 are brought together leaving very small gaps through which additional air may escape.
In FIGS. 7, 8 and 9, reference numeral 100 refers to an alternative and preferred temperature control cell. Control cell 100 is comprised of a rectangular, planar frame 102 which'is made of a light weight material such as aluminum. The members comprising frame 102 are hollow and are of rectangular cross-section as seen in reference numeral 116. As shown in FIG. 9, recesses I 116 have an expanded volume below the surfaces 104 and 105 of frame 102. Their cross-section is that of a truncated triangle. It has been found that a firm and reliable bond is formed between glass sheets 106 and 108 and the surfaces 104 and 105 of frame member 102 if bonding zones, having an expanded volume below the surface of the frame, are provided. The bonding agent is preferably a two-part silicone adhesive and it is believed that the firmness of the bond stems from the numerous points of sealing provided in each bonding zone. Good sealing is effected between the glass and the surface of the frame at flat portions 118, at each of the two surface corners of the recess, at each of the two corners at the base of the recess and along the bottom of the recess. Thus, there are six or more points of effective sealing associatedlwith each of the bonding zones. It has been found that at least two such bonding zones should be provided and preferably three should be used. The bond between the glass plates and the surface of the frame extends throughout their common area, however, the most effective sealing is at the bonding zones.
The bonding zones may be of different cross-sec-. tional configurations provided, however, that there is an expanded volume beneath the surface of the frame. Thus, the bonding zones may be of a diamond, crosssectional configuration or such other configuration as may be determined.
With bonds as described, it has been found, that the glass sheets do not pull away from the frames when they are subjected to the internal pressures caused by the temperature control fluids. The expanded volumes beneath the surface of the frames, with their multiple sealing points, act as locking devices and prevent the adhesive from pulling away from the frame.
Temperature control fluids are introduced into the temperature control cell formed by frame 102 and glass sheets 106 and 108 via an inlet port 120 and an outlet port 122. Inlet port 120 leads to the hollow top member tribution ports 125 for receiving the temperature control fluid.
Thus, a temperature control fluid such as water passes into the hollow, upper member of frame 102, passes downwardly through the distribution ports provided in the lower side of that member, passes through the outlet distribution ports in upper side 123 of the lower frame member and finally passes out of the temperature control cell via port 122 in the lower side of the lower frame member.
The surfaces of outer glass sheets 106 and 108 are casting surfaces and since there are no rigid bracing means on the interior of temperature control cell 100, sheets 106 and l08'will bow inwardly'into cell 100 under the hydrostatic head of plastic fluid in the adjacent casting zones when those casting zones are filled prior to the casting operation. Accordingly, when using temperature control cells of the configuration shown in FIGS! 7, 8 and 9, it is necessary to either use volumetric charge measuringequipment or, alternatively, to use the novel filling method of this invention.
It has-been found that the use of volumetric charge measuring equipment is optional if the casting apparatus is charged as follows. First, a temperature control liquid, such as water or heat transfer oil, is introduced via port 120 into temperature control cell 100. The outlet port 122 is closed off by suitable valve means and the level of temperature control liquid is permitted to build up in cell 100. The level in cell 100 is permitted to build up to a level predetermined as exactly sufficient to hydrostatically balance a full charge of plastic feed liquid in an adjacent casting zone. It may be necessary to experiment several times with the particular plastic feed material and with the particular temperature control liquid that are used, as the differences in specific gravity will determine the height of temperature control liquid at which the hydrostatic of temperature control cell 100. The lower side of this member is provided with a plurality of spaced distribution ports which distribute the temperature control fluid evenly downward into the temperature control cell. Likewise, the upper side 123 of the lower member of frame 102 is provided with a plurality of spaced disheads are balanced. Typically, a temperature control liquid, such as water, has a greater density than the organic, plastic liquid feed, and the height of water in the temperature control cell is less than that of liquid monomer or prepolymer in the adjacent casting zone.
After the temperature control liquid has reached the predetermined level, plastic feed liquid is introduced to the casting zone. When the casting zone is exactly full, its sides, i.e. glass sheets 106 and 108 will be straight and parallel, as the hydrostatic head pressure on both sides is balanced.
The plastic is cured under temperature conditions which are determined by the nature of the plastic and the catalyst added to it. For methyl methacrylate, for example, the temperature may range from 55 to C. I-Ieated water, steam or heat transfer oil such as Dowtherm is passed through the cells for sufficient time to take the sheets cast in zones 72 through the exotherm phase of the curing process, after which the charge, methyl methacrylate for example, is substantially polymerized. During the exotherm phase, the temperature of the cast sheets tends to rise as a result of reaction taking place. The circulating medium carries off excess heat, maintaining the cast sheets at substantially uniform curing temperature. As the sheets cure, the polymerizing mass shrinks in volume; methyl methacrylate, for example, shrinks approximately 22 percent during polymerization. As shrinking occurs,
the glass plates 62 of cell 40 or the glass plates 106 and 108 of cell 100 follow the shrinking mass and complete contact between the glass plates and the surfaces of the cast sheet is maintained. If the glass sheets were not free to deflect outwardly from the temperature control cells separation of cast sheet from glass sheet would occur and the cast sheet would be voided.
Some residual monomer remains in the cast sheets, even after curing, and to polymerize this residual monomer, the temperature must be run up to 100C. The final curing temperature is maintained for approximately one-half hour to an hour, or longer according to the thickness of the cast sheets and in any event until the sheets in the zones 72 are completely polymerized.
To separate the cast sheets from the glass of the cast- -ing zones, the clamped assembly is first cooled. Cold water or other cold medium is passed through the temperature control cells. This cooling process requires to 10 minutes and effectively separates the cast sheets from their casting surfaces.
With separation achieved, the assemblage of mold members 36 is unclamped by turning the crank 90 andmoving the shaft 87 to the right. The spider clamp 82b can then also be moved to the right. With clamping pressure removed, the adjacent mold members 36 are readily separated and the cast sheets are easily removed. Separation of the mold members is simplified by their suspension from casters 34 laterally movable in channels a, 30b and 32a, 32b. The cast sheets are extracted, with gaskets 70 adhered and the empty gasket frames are also removed. .By having an additional set of gasket frames which can be fitted with gaskets during the curing process, a newly gasketed frame-can be immediately substituted for the empty gasket frame, thus minimizing down time. As soon as all of thecast sheets have been extracted, and the gasket frames replaced, the next casting process can'be immediately instituted. The apparatus of this invention can be used to cast numerous differently shaped and sized plastic products from a variety of monomer or partial polymer starting materials. Cast articles can be made from methyl methacrylate, styrene, allyl diglycol carbonate, copolymers of methyl methacrylate with allyl diglycol carbonate, glycol dimethacrylate or similar cross-linking agents, styrene, styrene-acrylonitrile copolymer and other monomeric resins. The apparatus is not limited to the casting of sheets although it is best adapted for this purpose.
The advantages inherent in the apparatus are manifold. The reduction in labor required to prepare the monomer charge and cast the finished product is dramatic since there is virtually no movement of molds from one place to another or conveyance of raw material from one place to another. There are savings in the reduced breakage and scratching of glass and in the storage of the glass mold surfaces. The rate at which plastic sheet is produced is increased because of the far greater heat transfer coefficients achieved in the temperature control cells are compared with the curing ovens or water baths of conventional processes. The rapid passage of water or steam through the temperature control cells, at high temperature and pressures, if desired, substantially reduces the periods of time required to effect a cure. Alternatively, it is possible to polymerize and cure thicker sheets than has heretofore been possible.
Since it is possible to produce different sized sheets onthe same assembly, merely be changing the area enclosed by a gasket, it is possible to cast different size sheets using the same temperature control cells and lights. This flexiblity substantially reduces the finished stock inventory required and likewise reduces working capital.
The product produced by the apparatus and with the method of this invention is improved both in regard to thickness tolerance, surface quality and internal homogeneity. Casting takes place in vertically oriented casting zones and thus, there is no distortion or bowing of the glass due to sag in the frames or glass mold surfaces. Impurities or inclusions incorporated in the monomer charge fall to the bottom edge of the casting zone and do not ruin the surface or the interior of the case sheet. By trimming the gasket and a small amount of the edge of the cast sheet, all the inclusions are removed. The result is high yield on raw material and high quality product. further, the glass mold surfaces need not be cleaned after each casting since the dirt and impurities remove themselves from the casting zone.
The controlled volume of the casting zone during charging with either cell 40 or cell makes the need for charge metering apparatus and associated equipment optional. Such apparatus can be used where extremely close thickness tolerances are required or when otherwise deemed useful. The casting zone can be filled with monomer rather than a partially polymerized syrup as was heretofore necessary to avoid aeration of the charge during pouring procedures. The use of monomer is advantageous in that it does not adhere to the side walls of the containers and there is no evaporation loss as was experienced during the boiling of monomer in conventional processes. Emission of fumes is greatly reduced, the monomer flows faster into the casting zones and there is decreased opportunity for the material to set up. The feed procedure is simplified' in that partial polymerization to a predetermined viscosity isobviated and the optics of the final product sheet are morejuniform.
What is claimed is:
1. In a casting apparatus including a casting zone and an adjacent temperature control zone the improvement which comprises: an enclosed temperature control cell having a rigid rectangular frame and at least one semirigid side in sealable contact with 'said frame which is deflectible in response to pressures acting upon it, said cell having rigid bracing" means consisting of a plurality of horizontal and vertical cross members which are firmly connected at cross-over points affixed to said frame and positioned within said cell adjacent to the inner surface of said semi-rigid side, said bracing means providing a stop against the inward deflection of said side.
2. An apparatus as described in claim 1 wherein the said semi-rigid side of said cell is a casting zone element of said adjacent casting zone.
3. An apparatus as described in claim 1 wherein the outer surface of said semi-rigid side of said cell is in contact with a casting zone element of said adjacent casting zone.
4. In anapparatus for casting flat sheets of plastic including a casting zone defined between glass surfaces and an adjacent temperature control zone, the im provement which comprises: an enclosed hollow, substantially unobstructed temperature control cell defined by a rigid frame and at least one extended, semi-rigid side connected thereto, said side being deflectible in response to pressure acting upon it; and rigid bracing means affixed to said rigid frame and positioned in said cell adjacent to and in contact with the inner surface of said semi-rigid side, said bracing means providing a stop against inward deflection of said'side.
5. The apparatus of claim 4 wherein said cell includes two extended semi-rigid sides connected to said rigid frame, each of said. semi-rigid sides being braced against inward deflection by rigid bracing means.
6. The apparatus of claim 4 wherein said semi-rigid side is a casting zone elementof said adjacent casting zone. 7
7. The apparatus of claim 4 wherein said semi-rigid side of said cell is in contact with a casting zone element of said adjacent casting zone.
8. The apparatus of claim 4 wherein said semi-rigid side is sheet metal.
9. The apparatus of claim 4 wherein said semi-rigid side is a sheet of glass.
10. In a casting apparatus including a casting zone and an adjacent, temperature control cell, the improved temperature control cell comprising: a frame describing a closed area, and having at least one flat surface adapted to receive and to have bonded to it a glass sheet; a plurality of bonding zones comprising continuous recesses in said surface, each of said recesses having an expanded volume beneath the said surface; means forming an enclosure with said frame including a glass sheet bonded to said frame at said bonding zones; and means associated with said frame for introducing a temperature control medium into and out of said temperaturecontrol cell.
1 l. The temperature control cell of claim 10 wherein said frame is planar and generally rectangular in area and includes a first hollow structural member having an inlet port for said temperature control medium and a plurality of spaced outlet ports for distributing said medium into the enclosed cell and a second hollow member spaced opposite to said first hollow member having a plurality of spaced inlet ports for receiving said temperature control medium and an outlet port for said medium.
12. The temperature control cell of claim 10 wherein said frame is comprised of hollow members of rectangular crosssection, said hollow members having two flat surfaces on either side thereof having said bonding zones to which two glass sheets are bonded.
13. In a plastic sheet casting apparatus including a casting zone for plastic sheet and an adjacent temperature control cell, the improved temperature control cell comprising: a planar frame describing a closed rectangular area and consisting essentially of hollow members of rectangular cross-section; a plurality of continuous, female bonding zones on opposite side surfaces of said hollow members, each of said bonding zones having an expanded volume below the said side surfaces of said hollow members; two flat sheets of glass positioned on either side of said frame and bonded to said hollow members at said bonding zones thereby forming an enclosed cell; and means for introducing and distributing a temperature control medium into and out of said enclos d cell.
12. The improved temperature control cell of claim 13 wherein two of said hollow frame members, opposite one another, include an inlet and outlet port respectively on their exterior sides and a plurality of distribution ports on their interior sides.
15. The apparatus of claim 13 wherein said bonding zones are of a truncated triangular crosssection.
16. The apparatus of claim 13 wherein the bonding agent is a two-part silicone adhesive.
17. In an apparatus for casting plastic sheet comprising a plastic casting zone bounded by two parallel glass sheets sealed by a marginal gasket, and a temperature control cell adjacent to said plastic casting zone, said cell having a wall deflectible under pressure, means for controlling the volume of the liquid plastic charge to the said casting zone, including: introducing a temperature control liquid into said temperature control cell until the height thereof reaches a height predetermined as sufficient to hydrostatically balance the head of liquid plastic in said casting zone when the latter is fully charged; and charging said casting zone with liquid plastic until the latter is totally filled.