Molds for Thermoforming

As is the case in molds for thermoforming, the design of injection molds must provide for venting to remove trapped air, which otherwise will mar the finish of the molded object, and may, in extreme cases, prevent accurate mold filling. The simplest way to accomplish this is to build vent holes into the mold parting line, where the core and the cavity come together. For large molded objects, this may be insufficient, and additional vents may be needed. 

Molding employs a mobile prepolymer that may be thermoset or using a thermoplastic polymer. The polymer can be injection-molded (often for solid objects), blow-molded (for hollow objects such as bottles), rotation-molded, compression-molded, transfer-molded, or thermoformed. Casting is closely related to molding, except that the pressure is typically not used. 

Figure 7.72 Comparison of production costs per product for a polymer tray formed using (1) thermofoiming in a wooden mold, (2) thermoforming in an aluminum mold, (3) blow molding with a single mold, (4) compression molding, and (5) injection molding. Reprinted, by permission, from P. C. Powell and A. J. I. Housz, Engineering with Polymers, p. 58. Copyright 1998 by Stanley Thomed Publishers.

Finally, another equibiaxial deformation test is carried out by blowing a bubble and measuring the pressure required to blow the bubble and the size of the bubble during the test, as schematically depicted in Fig. 2.52. This test has been successfully used to measure extensional properties of polymer membranes for blow molding and thermoforming applications. Here, a sheet is clamped between two plates with circular holes and a pressure differential is introduced to deform it. The pressure applied and deformation of the sheet are monitored over time and related to extensional properties of the material. 

Consequently, their use is best confined to short run or prototype use. In normal production, the improved heat transfer capability of a metal mold will more than repay the greater cost. Aluminum is most commonly used for thermoforming molds other options include cast or sprayed low melting point alloys, porous sintered metals, and copper alloys (Chapter 17). 

Blow molded or thermoformed containers for chemicals, food, personal care 0.2-0.6 <0.959 2030 90,000... 

Cycolac . [GE Plastics GE Plastics Ltd.] ABS resins fs inj. molding, extrusitm, thermoforming for business machine, computer equip., elec, apfdics., house-wares, luggage, automotive, api iances, decnative c ce and home furnishings, hardware, toys. 

Sheets of PVC/ABS blends are often used for thermoforming large parts such as automotive or aircraft interiors. These blends have also been formed by molding and extrusion processes as low cost alternatives to flame-retarded ABS. The low thermal stability of PVC requires strict control of the processing conditions. 

Body panels with somewhat less demanding aesthetic requirements include tmck fairings. Extremely large parts such as these are often made by thermoforming as opposed to injection molding. For this reason, good melt strength is essential. Currentiy PPE/PA materials are used in these applications. 

V., Dow Chem. Co., Mitsui Toatsu Chem., Ltd., Neste Oy) Poly-lactic acid (PLA) for thermoforming, blow molding, extrusion of fibers and films. The available products include compostable bags, food containers, cafeteria plates, cutlery, medical and pharmaceutical articles,... 

Molds for the thermoforming process may be made of wood, metal, or epoxy and are relatively cheap. They are provided with vents to allow trapped air to escape and release possible pressure buildup. Temperature control, as we shall see in the following discussion, determines part quality. It is, therefore, crucial that mold temperature is controlled properly. The mold is consequently provided with channels for the passage of the cooling liquid. 

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