THERMOFORMING OF THIN POLYSTYRENE FOAM SHEET

Abstract: A co-thermoforming process for forming thermoplastic foam sheet of low density and small cell size. The sheet is formed from multiple webs of thinner gauge polystyrene foam which permits improved control of cell size and resultant functionality of product thermoformed from the sheet. Adhesion of the webs is achieved in the thermoforming process. The co-thermoformed products have improved ductility and surface characteristics.

Description: THE MOFORMING OF THIN POLYSTYRENE FOAM SHEET This invention relates to a process for providing improved thermoformed parts made from thin polystyrene foam sheet prepared with a blowing agent which preferably comprises only or predominately carbon dioxide (C0 2) . The invention is particularly suited for making such polystyrene foam sheet having a thickness of less than about 1/2 inch (12.7mm) which is to be thermoformed into products like containers such as trays, cups, tubs and the like. It has become increasingly important to provide polystyrene foam sheet with blowing agents other than halogenated or aliphatic hydrocarbons for environmental reasons and to be in compliance with regulations. One solution is to produce such foam using a blowing agent which is comprised essentially of carbon dioxide as described in U.S. Patent 5,250,577 of which one of the i nventors hereof is the i nventor. It has been found when extruding polystyrene foam with C0 2 as the sole blowing agent it becomes increasingly difficult to control gauge variation at higher gauges, such as greaterthan about 0.16 inches (4.1 mm), and Iow density, that is less than about 4 pounds per cubic foot (64.1 kg/cu. meter), due to the higher levels of C0 2 concentration and the subsequent increase in the foaming rate. Such gauge variation is sometimes referred to as corrugations which not only adversely affects cell size, surface characteristics, and overall tray functionality measured in terms of brittleness and corresponding container overwrapability, but final part aesthetics as well. It is therefore desirable to manufacture high gauge, low density thermoformed polystyrene foam sheet products which have preferred cell size, adequate surface characteristics and physical performance. This invention takes advantage of the easier processing window associated with lower gauge C0 blown polystyrene foam sheet to manufacture a superior high gauge low density thermoformed polystyrene foam sheet product with better physical and aesthetic characteristics than heretofore possible. As the gauge levels increase the difficulty in extruding increases. The present invention takes advantage of the fact that it is easier to extrude a lower gauge product with minimal gauge variation and smaller cell size The improved result is achieved by extruding two or more webs of lower gauge than the finai desired gauge and joining these thinner webs into a single sheet of desired gauge prior to thermoformmg. The thermoforming provides a slight compression and ordinariiy sufficient heat to the foam webs to not only permit the web sheet formed from the material to be thermoformable in a conventional manner but also to obtain good adhesion between the adjoining component web inside surfaces. The process concludes by the thermoformed parts being trimmed from the sheet in a conventional manner. Figure 1 is a schematic representation of a thermoforming process for forming products in accordance with the present invention; and Figure 2 is an exploded broken away section of the process of Figure 1 , indicated as section 2-2 thereof, showing multiple webs being brought together for forming into sheet during the thermoforming process. Polymers useful in the present invention as well as blowing agentsin the process for forming thin polystyrene foam sheet are described in U.S. Patent 5,520,577. For purposes of illustration the foam sheet described herein will be that of a polystyrene foam made with carbon dioxide as the sole blowing agent. Although this invention may be applicable to other resins and blowing agents it has been found to be of particular benefit with such foam sheet. A preferred embodiment of the process of this invention is illustrated schematically in Figure 1. However variations in the process may be employed and still obtain the beneficial effects of this invention. Figure 1 illustrates rolls 10 and 12 containing webs 14 and 16 of thin extruded polystyrene foam sheet of about half the gauge thickness of the resulting foam sheet desired to be thermoformed. For purposes of illustration, the webs 14 and 16 are made of polystyrene resin foamed with C0 as essentially the sole blowing agent. While webs 14 and 16 have been pre-extruded and inventoried as rolls 10 and 12, it is conceivable that in a fully continuous process webs 14 and 16 could instead be directly extruded from two extruders. Whether sourced from rolls 10 and 12 or directly from extruders, the webs 14 and 16 thereafter meet but are not necessarily joined at a junction 18. When the webs H and 16 are combined in an unheated condition, an inside surface heater 20 may be employed if a high degree of adhesion between the webs is desired. However the use of an inside surface heater 20 is optional in that it has been discovered that sufficient heat and pressure may be obtained in the thermoforming process itself to provide adequate heat substantially to achieve mutual adhesion of the webs in the final product. Idle rollers 22 help bring the webs 14 and 16 generally together at juncture 18, their respective interfaces 26 and 28 being in close proximity to form the elements of sheet 24. The webs forming sheet 24 thereafter pass through a conventional thermoformer heat tunnel 30 comprised of heating elements 32 and 34 with chain rails 36 and pin chains 38 (like bicycle chains with protruding pins) gripping edges of sheet 24 to move the sheet 24 to a forming station 40. Forming station 40 is comprised of conventional thermoforming match molds 42 and 44 having a male plug 46 and a female cavity 48 of a configuration conforming to a container part desired to be formed. The compression pressure and residuai heat occasioned by the match molds closing also serve to ordinarily cause the interfaces 26 and 28 of the webs to securely adhere to one another, if not earlier adequately joined in the process. Thus the thermoforming station simultaneously provides the dual function of causing web adhesion and shaping in this process as illustrated. When molds 42 and 44 close in a conventional way and engage sheet 24, a container of the desired configuration is formed in the sheet as illustrated at section 50 of the sheet. Sheet 24 then advances to a conventional trim station 52 including trimming knives 54 on mold part 56 and receptacles 58 in matching mold 60 adapted to receive the trim knives 54 to trim the desired part 62 from the sheet to form a finished part 62. A plurality of finished parts 62 are thereafter discharged and may be stacked at stacking station 64 or removed in any conventional manner well known in the art. The process of forming a single sheet from multiple thinner webs is hereinafter referred to as "co-thermoforming" or "co-thermoformed" sheet by the process of co- thermoforming. As previously alluded to, the current invention takes advantage of the fact that a lower gauge product at a Iow density with minimal gauge variation is more readily extrudable than is the same density product at higher gauge, when extruding polystyrene 10 foam made with C0 2 asthe sole or primary blowing agent. By extruding thinner gauges one has the advantage of being able to use higher nucleation levels and obtain smaller cell sizes resulting in a smoother foam surface without excessive gauge variations due to corrugations. A part formed from highly corrugated sheet can result in unsatisfactory wastage due to a high scrap rate when wrapping food trays too brittle, for example. Large cell sizes induce brittleness 5 while small sell sizes induce ductility and tray toughness in products thermoformed from polystyrene foam sheet. EXAMPLE I A specific example of co-forming by practice of the present invention was practiced by combining two webs, such as webs 14 an 16, of about 0.1 inch (2.55mm) gauge to 0 form a polystyrene foam sheet of 0.2 inch (5.1 mm) gauge for thermoforming. Table 1 discloses the details of the conditions and results using the process generally illustrated in Figure 1 (Col. B) as compared with the prior practice (Col. A). TABLE 1 Col. A Col. B Standard Co- 100% CO thermoforming Polystyrene 100% CO 2 Foam Sheet Polystyrene Foam Sheet Gauge, in. (mm) 0.200 (5.1) 0.098 (2.5) Sheet Density, cu. ft/lb. 3.0 2.9 (kg/cu. meter) (48.1) (46.5) Tray Cell Size, mm, MD, CD, VD* 0.54, 0.54, 0.54 0.33, 0.27, 0.18 Tray Bottom Gauge, in. (mm) 0.220(5.7) 0.220 (5.6) Tray Stiffness good good Tray Surface Appearance average excellent *MD = Machine direction, CD = Cross direction, VD = Vertical direction As can been seen in Table 1 , instead of producing a polystyrene sheet product made with C0 2 blowing agent of 3 pounds per cubic foot (48.1 kg/cu. meter) density and about 0.2 inch (5.1 mm) gauge (Col. A), C0 2 blown polystyrene foam sheet webs were produced (Col. B), with a similar density, that is about 3 pounds per cubic foot (48 1 kg/cu. meter) but with about half the gauge thickness, of about 0.1 inch (2.54mm), 0.098 inch (2.49mm). Two webs of the thinner gauge sheet were unwound from rolls 10 and 12 and brought together at juncture 18 to o loosely form sheet 24 which was then fed into the chain rails 36. Once the pin chains 38 have gripped the sheet edges, the sheet 24 was then pulled forward through the thermoformer heat tunnel 40 where the outside surfaces of web 24 were heated The sheet 24 comprised of the webs 14 and 16 was then indexed to forming station 52 where the matched molds closed to a specified mold gap to form parts 62 which can be trays having defined walls. This slight 5 compression of the heated foam sheet caused the webs forming sheet 24 to not only be formed but to also provide excellent adhesion between the two i nside surfaces 26 and 28 of webs 14 and 16, respectively, forming sheet 24. The final thermoformed parts were then trimmed from the sheet 24 at trimming station 52 in a conventional manner. The results shown in Table 1 illustrate that the desired smaller cell size can be 0 achieved by this invention for improved product performance and appearance By employing the thermoforming process of the present invention small cell sizes of less than about 0 4mm (about 0 3mm) were obtained in the co-thermoformed product. This is highly preferable to cell sizes greaterthan about 0.4mm, such asthe 0.5mm, common with standard polystyrene foam products thermoformed in a conventional manner. While both products tested as illustrated in 5 Table 1 showed satisfactory or good tray stiffness it should be appreciated that the failure rate would be expected to be higher in tray wrapping as the standard sheet (Col. A) would be more brittle than the smaller cell sized co-thermoformed sheet (Col. B) produced By producing a foam sheet without excessive gauge variation and smaller cell sizes a smoother more functional " and desirable finish is obtained and a generally perceived as well as actually strengthened 0 product due to the lack of gauge variations throughout the formed product is provided Additionally, it should be evident that the webs forming sheet 24 can be extruαed at a higher rate because less residence time is required for cooling Although the co-thermoforming process of this invention has been particularly described with respect to the thermoforming of polystyrene foam sheet where C0 is the 5 primary or sole blowing agent, it is conceivable that this invention would also provide advantages with other resin materials and biowmg agents The resulting products can be containers such as for food products ana the like, but may nave applicaDiiity to proαucts other than containers.

Claims: WHAT IS CLAIMED IS: 1. A co-thermoforming process for producing a thermoformable thermoplastic foam sheet comprising the follow steps: a. producing a plurality of thin webs of polystyrene foam the total thickness of which substantially equals the total thickness of the sheet to be thermoformed into a product each of said webs presenting an interface to the other, b. bringing said thin foam webs into close proximity at their interfaces to form said sheet, c. heating said sheet to a thermoforming temperature, d. thermoforming the sheet into products of a desired configuration with said webs adhered to one another, and e. trimming and collecting said thermoformed products. 2. The process of Claim 1 , wherein the interf aces of the thin webs are preheated prior to being joined to form the sheet. 3. The webs of Claim 1 being comprised of polystyrene foam prepared with a carbon dioxide blowing agent. 4. The process of Claim 3, wherein the carbon dioxide blowing agent is the sole blowing agent used in forming the thermoplastic webs. 5. The process of Claim 1 , wherein the co-thermoformed product has a cell size of less than about 0.4mm. 6. The process of Claim 1, wherein the webs have a density of less than about 4 pounds per cubic foot (64.1 kg/cu. meter). 7. The process of Claim 1, wherein the thickness of each of the thermoformed webs is less than about 0.16 inches (4.1 mm) and the gauge thickness of the sheet formed from combining the webs is greater than about 0.16 inches (4.1 mm). 8. Products formed from the foam sheet made by the process of Claim 1. 9. A co-thermoformed product having foam walls comprised of two or more foam webs joined together, wherein said walls are of a Iow density and small cell size. 10. The co-thermoformed product of Claim 9, wherein the walls are composed of polystyrene foam having cell sizes of less than about 0.4mm. -3-

Inventors: DARTEE MARCEL, WELSH GARY C, DARTEE, MARCEL, WELSH, GARY, C.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1995
Main classification: B29C51
IPC codes: B29C51, B29K25, B29K105, B29L22