Processing, thermoplastics sheet

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

As with SMC, appHcations are limited to high volume because of the capital investment in equipment and tooling. Thermoset compression molders require additional heating and material Handling equipment to adapt their process to thermoplastic sheet fabrication. AppHcations include automotive bumper beams, load floors, radiator supports, battery trays, and package shelves. Chair sheUs, military containers, material Handling pallets, trays, and concrete foaming pans are also produced. 

The rigidi d thermoplastic sheet - Tiocess combines thermoplastic sheet vacuum forming with a spray-up or cold press molding process to add a thermoset composite stmctural backing to a decorative thermoplastic skin. Large parts such as bathtubs, hot tubs, recreation vehicle components, and camper tops have been produced by this process. 

Table 2.17 analyses some examples of the relative processing cost versus the annual production in units. The various technologies listed are not suitable for all materials or parts machining is used with numerous materials rotational moulding uses liquid resins vacuum forming uses thermoplastic sheets blow moulding uses special grades of thermoplastics... 

The major bulk processed thermoplastic using calendering is PVC sheets and films including blends and copolymers. A sample recipe to produce PVC sheet might include a plasticizer such as a dialkyl phthalate, pigment, filler, lubricant, and stabilizer. 

The calendering process is commonly used for shaping high melt viscosity thermoplastic sheets and is particularly suitable for polymers susceptible to thermal degradation or containing substantial amounts of solid additives. This is because the calender can convey large rates of melt with a small mechanical energy input (compared to an extruder). 

Different processes have been developed with a few in use. Their designs usually start with an extruded thermoplastic sheet. Circular blanks cut from the sheet are compression molded into the desired preliminary shape. During the compression action, the blank can be simultaneously stretched, or being thermoplastic can be stretched after compression molding. They generate no trim scrap when formed... 

Thermoforming has close similarities with vacuum forming, except that greater use is made of air pressure and plug assisted forming of the softened sheet. The process is invariably automated and faster cycle times are achieved than in the vacuum forming process. Only thermoplastic sheet can be processed by this method. 

Thermoforming a material for a unit change in temperature. Values for plastics range from 0.01 to 0.2 mil/in. Any process of forming thermoplastic sheet which consists of heating the sheet and pulling it down onto a mould surface. 

Alcryn . [DuPont DuPont UK] Halo-genated ethylene inteipolymer alloy melt processable thermoplastic elastomer for seals, gaskets, weatherstripping, coated fabrics, sheet goods, belt-... 

Thermoforming - The process of heating a thermoplastic sheet to a point at which it softens and flows, then applying differential pressure to make the sheet conform to the shape of a mold or die. 

Thermoforming is a process for forming moderately complex shaped parts that cannot be injection molded because the part is either very large and too expensive or has very thin walls. It consists essentially of two stages elevation of the temperature of a thermoplastic sheet material until it is soft and pliable and forming the material into the desired shape using one of several techniques. 

In the thermoforming process, heat and pressure are applied to thermoplastic sheets which are placed over moulds to adopt their shapes (Figure 3.35). Pressure can be applied using air, compression or vacuum (vacuum forming). Compression is a low cost process because the moulds used are inexpensive and automation allows a production cycle of less than 2 seconds. Details are... 

Figure 5.54 Processing sequence for compression stamping glass fiber reinforced thermoplastic sheets...

In rolling (roll milling) (Figure 36-3) and calendering (Figure 36-4), elastomers or plasticized thermoplasts are processed to sheet or film, or fabrics and films are coated with elastomers, thermoplasts, or thermosets by these processes. The rollers are heated and run at different speeds. The milled... 

With regard to methods of fabrication, all processes in Table 1.1 that are applicable to unfilled, unmodified thermoplastics can also be used for discontinuous systems (with the exception of expandable bead molding). In addition to thermoforming, hot stamping of reinforced thermoplastic sheets mostly containing randomly oriented continuous or discontinuous fibers is used for the production of large semistructural parts. Fillers can also be used in the thermoset processes in Table 1.1, often in combination with the primary continuous fiber reinforcement. The content and inherent properties of the additive, as well as its physical/chemical interactions with the matrix, are important parameters controlling the processability of the composite. 

Calendering n. The process of forming thermoplastics sheeting and films by passing the material through a series of rigid, heated rolls and rolling to press out voids. Four rolls are typical. The gap between the last pair of heated rolls determines the thickness of the sheet. Subsequent chilled rolls cool the sheet. The plastic compound is usually premixed and plasticated on separate equipment, then fed continuously into the nip of the first pair of calender rolls. 

Deep drawing n. The process of forming a thermoplastics sheet in a mold in which the ratio of depth to the shortest lateral opening is 1 1 or greater. 

Dicing n. The process of cutting thermoplastic sheets (or square strands) into cubical pellets for further processing. 

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