Thermoforming

Thermoforming is the most commonly used method for volume production today [10]. In this method a special type of reinforcement that is already impregnated with a preforming powder is heated before it is clamped in a cold preforming tool [11], The most common type of reinforcement is continuous strand mat which is manufactured by e.g. Vetrotex and Owens Coming. 

Woven fabrics are also available, however, with preforming powder (e.g., from Brochier SA). The formability of woven fabrics is limited and only moderately double-curved shapes have been formed commercially. Highly formable woven fabrics suited for thermoforming, however, have recently been introduced on the market (Brochier SA). This may increase the use of woven fabrics in industrial-scale preforming. 

Thermoforming is faster and higher fiber contents can be achieved than they can for the spray-up technique. Moreover, if continuous strand mat is used in thermoforming, it can be combined with patches of directional reinforcement. The major drawback with thermoforming is the relatively high level of waste material. 

Thermoforming is a generic term encompassing many techniques for producing useful plastic articles from flat sheets. This process 

In this process, PLA sheet is heated to soften the polymer, forced either pneumatic and/or mechanically against the mold, allowed to cool, removed from the mold, and then trimmed. The radiant heater of the thermoforming line for PLA must be adjusted to very low temperatures. Preheating is not absolutely necessary however, it has the general advantage that the sheet is homogeneously preheated. 

Thermoforming is a fabrication technique that first requires the polyethylene sample to be converted into a flat film or sheet of variable thickness. Once this intermediate film/sheet is formed, the thermoforming step takes place by heating the film/sheet to a softening point within a mold that replicates the shape of the final product. A vacuum within the molding step is usually utilized to evacuate trapped air from the mold and to force the softened polyethylene sheet into the contours of the mold. 

Thermoforming is commonly used to fabricate disposable cups, containers, lids, trays and other items used in the food, medical and retail packaging applications. Thick gauge polyethylene sheets may be used to thermoform very large parts such as refrigerator panels or liners. 

Heating of PLA sheet for thermoforming is generally achieved by infrared (IR) radiation from heater elements, although convection and conductive heating may also be used. Because each polymer has an optimum IR absorbance 

FIGURE 14.18 Thermoforming of heated PLA sheet over a female mold using plug-assist drawing and a male mold using reverse drawing processes. 

A simple approximation of the thermoforming process is based on a mass balance principle. To illustrate this concept, let us consider the thermoforming process of a conical object, as schematically depicted in Fig. 6.25. 

For the solution, we assume the notation presented in in Fig. 6.25. As shown in the figure, at an arbitrary point in time the bubble will contact the mold at a height z and will have a radius R, which is determined by the mold geometry 

If we perform a mass balance as the bubble advances a distance As, we get 

This equation can be extended to simulate the thermoforming process of a truncated cone, which is a more realistic geometry encountered in the thermoforming industry. 

In many applications of conventional thermoforming, low-cost tooling is used compared to that of other processes, particularly in cases of limited production and/or the forming of very large parts (Fig. 5-2). Thermoforming of thin parts has an advantage over most other processes, where very thin walls cannot be produced. An example of its use is in skin and blister packaging. 

With most forming (not including bending) there can be up to 50 percent scrap trim or web. This material could be wasted, but it is usually recycled and blended with virgin resin. Individual sheet stock formed into round shapes could have 50 percent or more scrap. With square forms, there could be up to 25 percent scrap. 

Compressed air thermoforming is a technique borrowed from sheet steel processing its main use in plastics processing is in plastic cup production. Machines are also coming into use for the production of engineering components, such as computer covers. A flexible-pressure pad is used to press the hot sheet into the mold cavity, with the aid of an air cushion maintained under pressure. Compared with vacuum forming, this 

Mold pushes sheet into box and then vacuum ts applied 

The main reason for the use of plasticizer in thermoforming material is to decrease the temperature required for thermoforming. This is done by the selection of the type 

Many problems may be encountered in thermoforming, some of these are related to plasticizers or they can be eliminated by changes in plasticizer type and amount Problems related to plasticizers include die lines or dull spots (plate-out buildup), dark specs and streaks (polymer degradation), excessive sag, distortion after part removal, warpage, poor wall thickness distribution, and part sticking to mold. 

Fibers and other non-spherical fillers change their orientation during thermoforming. Small strains are sufficient to orient fibers. Experiments have demonstrated that particles of talc orient themselves parallel to the surface of thermoformed parts.The crystallites are oriented in a direction perpendicular to the either the talc or the mold surface. This is because the mechanism of crystallite growth begins on the surface of talc and grows outwards. 

In principle there are two techniques to process porous substrates with thermoforming. A novel method was shown by Giselbrecht et at [23], who processed pre-ion-beamed foils with thermoforming. Later the formed structures were etched to achieve porous microstructures. The advantage of this method is the easy process setup. 

It will be noted that injection moulding in all these respects appears to be a superior technique. Nevertheless, modem thermoforming techniques (of which simple vacuum forming is the most primitive) have become of considerable technical significance because they are at a decided advantage over injection mouldirig in three important areas  

42 Thermoforming with matching moulds (a) the heated sheet in position over the negative (female) mould (b) the male is rammed down forcing the sheet into the female mould and (c) the form is cooled and removed from the mould (after Modem Plastics Encyclopedia). 

The technical developments that have brought this about can be grouped under three headings. 

Pressure vacuum forming gives a pressure differential, at most, of 1 atmosphere. The use of positive pressures (up to 10 atmospheres) with or without vacuum on the other side, has led to the achievement of much superior forms. Because of the pressures involved, the machines become somewhat massive as the area of the forming increases. 

43 Thermoforming the four stages in the pressure-bubble vacuum-snapback technique (after Modem Plastics Encyclopedia). 

The aim of this section is to describe some of the techniques used to process polymers. We have already mentioned the stretching of fibres and films, so we shall not refer to these processes again. However, many commonly encountered polymer objects are processed by thermoforming, extrusion and injection. 

We have already observed that amorphous polymers above their glass transition temperatures have low moduli and are therefore deformable. We also noted that subsequent cooling above Tg will fix the applied deformation. This is easily demonstrated for polystyrene, either in the form of a transparent goblet or picnic knife. Dipping the object into boiling water, it softens. It can then be curved and allowed to cool in the deformed state. Once back at ambient temperature, the shape conferred upon it while hot is retained. This is ther- 

Yoghurt pots provide an illustration in everyday life of this kind of preparation. They are produced from thin polystyrene sheets, heated then stamped out and cooled. Curved caravan windows are made in this way from poly(methyl methacrylate) plates. Large objects such as chairs, beds and baths are made by thermoforming polystyrene or poly(methyl methacrylate) plates. 

Undercuts and reentrant shapes are possible in many designs. They require movable or collapsible mold members, but with small undercuts they can often be sprung from a female mold while the formed product is still warm. This type of action works best when the plastic has some flexibility, as do the TPEs, or the material is very thin. Guidelines for the maximum amounts of undercutting that can be stripped from a mold are as follows 0.04 in. (0.1 cm) for acrylics, PCs and other rigid plastics 0.060 in. (0.15 cm) for PEs, ABSs, and PAs 0.100 in. (0.25 cm) for flexible plastics such as the PVCs. 

When female tooling is split to permit the removal of products with undercuts, a parting 

The best products have smooth, natural curves and drawn sections that are spherical or nearly so in shape. Their walls will be more uniform, they will be more rigid, their surfaces will be less apt to show tool marks, and their tooling and molds will be lower in cost. Notches or square holes should be avoided when punching formed products. Round holes are preferred to oval ones for minimizing stress buildup. 

Some draft is required in side walls to facilitate the easy removal of the product from the mold. Female molds require less draft since products tend to pull away from mold walls as they shrink during cooling. With female or male tooling, for most plastics the draft on each side wall should be at least 1 degree (Fig. 3-39). 

Tolerance Thermoformed products lack the dimensional accuracy of processes such as injection and compression molded products. 

Fluoropolymers such as PVDF, FEP, PFA, and modified polytetrafluoroethylene can be thermo-formedl J by vacuum forming, pressure forming, and matched-die forming (Fig. 7.19). In all methods, a 

Blowmolding(4). Blowmolding equipment used for HDFE may be used with these resins with minor modifications to the dies and molds. Actually, PVC molds with low shrinkage allowances are more suitable than those for HDPE, because the shrinkage characteristics of these resins are similar to those for PVC. 

ACS Symposium Series American Chemical Society Washington, DC, 1974. 

Either reciprocating screw or continuous parison extrusion may be used. These resins have less diameter die swell than HDPE and have a higher melt temperature range (195-215 C). 

1 Basic principles of the compression molding process. Mold is open on the left and elosed on the right 

Low-volume production of hudding panels, automotive body panels, and hampers is thermoformed from engineering thermoplastics. ABS, polycarbonate, polycarhonate/ABS, polyamide, and polyesters are thermoformed into in-mold decorating films which are rohoticaUy placed into an injection molding mold. The substrate is back-molded onto the film. 

Blow-moulded products are associated with certain inherent limitations concerning shapes and wall thickness. While the blow-moulding process normally yields one-piece, closed, hollow products, pairs of open, hollow products have been made by splitting closed mouldings. 

SPS sheet can be thermoformed only in its amorphous state by rapid cooUng from the melt during sheet extrusion to prevent crystallization. During ther-moforming, the amorphous sheet is heated to a temperature that allows 

If the thermoformed sheet is still amorphous, its chemical resistance will be similar to that of atactic polystyrene sheet. Therefore, after thermoforming one has to apply an additional heating step to obtain crystallinity (and thus chemical resistance) or use heated molds (180 °C). The food packaging industry utilizes similar technology with crystalline poly(ethylene terephthalate) (CPET) to make oven-capable food trays, which can also be prepared from SPS. 

The whole process can be broken down into three stages (a) extrusion, (b) inflation, and (c) cooling. 

Mathematical modeling can provide valuable insights into mold design and process improvement. The objective of computer simulation of thermoforming is 

Three types of modeling techniques have been used in the past (a) simple mass balances. These models involve only geometrical considerations and are independent of material behavior [160] (b) finite element analysis with the membrane approximation. This is applicable for thin-walled parts for which bending resistance is negligible [149,161] (c) finite element analysis without the membrane approximation. 

This is applicable for thick-walled parts and multilayer thermoforming [162, 163]. 

Biaxial extensional behavior is important. Strain-hardening, that is, increasing resistance of the resin to extension as deformations increase, cannot be neglected anymore. Otherwise, inaccurate results would be calculated. POLYFLOW [16] has implemented differential viscoelastic models in 2 D and K- B KZ viscoelastic models in 3D in order to take this behavior into account. 

The term thermoforniing incoroporates a wide range of possibilities for sheet forming but basically there are two sub-divisions - vacuum forming and pressure forming. 

In some cases Negative Forming would not be suitable because, ftNT example, the shape formed in Fig. 4.51 would have a wall thickness in the comers which is considerably less than that close to the clamp. If this was not acceptable then 

A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4.S3. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum. 

Thin sheets are also stamped similar to sheet metal stampings where a punch pushes the sheet into a matching die cavity. In this case, the sheet is usually softened first by preheating. 

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Processing. SAN copolymers may be processed using the conventional fabrication methods of extmsion, blow mol ding, injection molding, thermoforming, and casting. Small amounts of additives, such as antioxidants, lubricants, and colorants, may also be used. Typical temperature profiles for injection mol ding and extmsion of predried SAN resins are as follows (101). 

Thermoformability is a property required by the many sheet materials used in the thermoforming industry. These properties are unique for the specific forming methods used, and are best determined by actual thermoforming tests on smaU-scale equipment. The softening or drape temperature of the material, residual stress in the sheet from its manufacture, and its melt strength and viscosity are important parameters relating to this use. 

A continuous extmsion process, as weU as mol ding techniques, can be used as the thermoforming method. A more rapid rate of cure is then necessary to ensure the cure of the mbber before the ceUular stmcture coUapses. The stock is ordinarily extmded at a temperature high enough to produce some curing and expansion and then oven-heated to complete the expansion and cure. 

Nylon. Nylon is the designation for a family of thermoplastic polyamide materials which in film form are moderate-oxygen barriers. The gas-barrier properties are equal to odor and flavor barrier properties important in food appHcations. Nylon films are usually tough and thermoform able, but are only fain moisture barriers (see Polyamides). 

Nylon films are used in lamination or coated form to ensure heat sealabiHty and enhance barrier properties. The largest uses are as thermoforming webs for twin-web processed meat and cheese packagiag under vacuum or in an inert atmosphere. Other uses include bags for red meat, boil-ia-bags, bag-in-box for wine, and as the outer protective layer for aluminum foil in cookie and vacuum coffee packages. 

Some heavier gauge flexible materials, usually containing nylon, are thermoformed, ie, heated and formed into three-dimensional shapes. Such stmctures are used to provide high gas-barrier, heat-sealable containment for processed meat or cheese. 

Sheet Extrusion and Thermoforming. Sheet for thermoforming and analogous operations is usually formed by extmding the melt through a slot die onto a set of poHshed chill roUs. The sheet is usually approximately 150 cm wide. After rapid cooling, the web is coiled or cut into sheets. Polystyrene, PVC, polyethylene, polypropylene, and filled polypropylene are prepared in sheet form by extmsion. 

Thermoforming includes the extmsion of sheets, thicker than 0.25 mm, followed by forming a reheated sheet in an open-face mold by pressure, vacuum, or both. Sheet of less than 0.25 mm thick is therm oformed in-line, and filled and sealed with contents such as processed meats, cheeses, and pastas. 

Thermoform able sheet may be mono- or multilayer with the latter produced by lamination or coextmsion. Multilayers are employed to incorporate high oxygen-barrier materials between stmctural or high water-vapor barrier plastics. Both ethylene vinyl alcohol copolymers and poly(vinyhdene chloride) (less often) are used as high oxygen-barrier interior layers with polystyrene or polypropylene as the stmctural layers, and polyolefin on the exterior for sealing. 

Thermoforming. Thermoforming is the most common method of fabricating sheet into three-dimensional packaging. In conventional thermoforming, the sheet is heated to its softening point or just below the melting temperature. The softened plastic is forced by differential air pressure into an open-top mold to assume the shape of the female mold. The mold is chilled and the plastic sheet solidifies and is then removed from the mold. 

Thermoforming is used for gauges above 6 mm in some nonpackaging appHcations for packaging appHcations gauges are between 0.6 and 2.5 mm. 

In commercial practice, packaging is produced from continuous web on intermittent-motion thermoforming/die-cut machines. The web edge is clamped and conveyed into a heating box. If a plastic with a narrow softening temperature is used, heating is carefully controUed from top and bottom. 

Conventional thermoforming of polystyrene and PVC is the most widely used technique for making packages for dairy products and for disposable cups and trays. 

In the cuspation—dilation thermoforming process developed in AustraHa, sheet formation is promoted by expanding blades extending into aU areas and distributing the material uniformly throughout the mold. This process is claimed to deHver uniform distribution of high barrier components of sheet coextmsions and laminations. The process also permits almost vertical side waUs to cups (2). 

Post-forming type is an HPDL similar to the general-purpose type but is capable of being thermoformed under controlled temperature and pressure in accordance with the laminate manufacturer s recommendations. 

Thermoforming and Extrusion. Improved equipment and polymers have increased the capabiUty to extmde and thermoform polypropylene however, consumption of polypropylene in these areas has not grown dramatically. Drinking straws are commonly extmded from polypropylene, however most larger diameter tubes, such as pipes and conduits, are predominantly extmded from other thermoplastics. Extmded sheet is thermoformed into food containers and trays polypropylene is used when microwavabiUty is desired. 

J. L. Throne, Thermoforming, Hanser Pubhshers, Munich, Germany, 1987. 

Conventional thermoforming of sheet and film is appHcable to the production of skylights, radomes, signs, curved wiadshields, prototype production of body parts for automobiles, skimohiles, boats, etc. Because BPA polycarbonate is malleable, it can be cold-formed like metal, and may be cold-roUed, stamped, or forged. 

Some time earlier, Eastman-Kodak has been working on a novel polyester as an entry into the important polyester fiber market and had devised a new ahcychc diol, 1,4-cydohexanedimethanol [105-08-5] effectively made by exhaustive hydrogenation of dimethyl terephthalate. Reaction of the new diol with dimethyl terephthalate gave a crystalline polyester with a higher melting point than PET and it was introduced in the United States in 1954 as a new polyester fiber under the trade name Kodel (5). Much later the same polyester, now called PCT, and a cyclohexanedimethanol—terephthalate/isophthalate copolymer were introduced as mol ding resins and thermoforming materials (6). More recentiy stiU, copolymers of PET with CHDM units have been introduced for blow molded bottie resins (7). 

Thermoformed acryHc sheet is displacing gel coats in some bathtub appHcations spas have converted almost exclusively to formed acryHc sheet reinforced with glass-reinforced polyester laminations due to higher temperatures and higher stmctural requirements. 

Polysulfones are easily processible by other thermoplastic fabrication techniques, including extmsion, thermoforming, and blow mol ding. Extmsion... 

AH forms and compositions of reinforcements, ie, mats, woven roving, glass, carbon, and aramid, are commonly used with these processes. Special continuous glass strand mats with a thermoplastic binder aHow preforms to be made using thermoforming techniques. These processes are used for tmck and autobody components, medical equipment cabinets, transportation seating, and other parts needed in the intermediate volume range (1,000—10,000 parts/yr). 

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