Processing thermoforming

The thinnest nominal walls are created by the stretching processes thermoforming and blow molding. In each case, the material is first processed by some other method before it is shaped. Thermoforming uses extruded sheet which is then heated and stretched over a tool. Blow molding uses an extruded tube or injection moldment which is then blown into a mold. Plastic thins as it is stretched, and therefore, the effect of this thinning must be carefully considered in the part design. 

Low-MW acrylics, styrene/acrylics and poly(a-methylstyrene) blend easily with PVC and improve processing, thermoformability, gloss and ductility [33]. Commercial PMMA systems have been found to be miscible with PVC [34]. Miscibility allows improved processability by breaking the structures of PVC particles. 

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). 

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 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. 

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. 

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). 

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). 

Thermoforming is perhaps the process with the lowest unit cost. Examples of thermoformed articles are refrigerator-door and food-container liners, containers for dairy products, and luggage. Some of the largest formed parts are camper/trader covers and liners for refrigerated-radroad-car doors (236). 

Figure 19. Moving bed catalytic crackers (A) Thermoform moving bed process (B) Houdry catalytic cracking process.

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. 4.53. 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. 

Designers and processors to produce products at the lowest cost have unconsciously used the basic concept of the FALLO approach. This approach makes one aware that many steps are involved to be successful, all of which must be coordinated and interrelated. It starts with the design that involves specifying the plastic, and specifying the manufacturing process. The specific process (injection, extrusion, blow molding, thermoforming, and so forth) is an important part of the overall scheme and should not be problematic. 

Plastic films represent the largest worldwide market for plastics with practically all extruded (6). They are used to meet different performance requirements particularly for its major packaging market. Worldwide just for biaxial oriented (Chapter 8) polypropylene consumption is about 5 /2 billion lb. Their use includes tape, food, tobacco, and confectionery. Thermoforming film (and extruded sheets) is a major processing technique producing all kinds of products. 

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

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