Thermoforming mold materials

Thermoforming is one of the major processes used to produce prototype parts that may be made other ways, such as injection molding. Many materials are used to produce molds that are serviceable for a few to a few himdred parts. Traditional mold materials include wood, plywood, hard plaster such as Hydrocal (US Gypsum), and medium density fiberboard. Sprayed and cast white metal are used on occasion. 

Aluminum is the most commonly used material for thermoforming molds. The advantages of aluminum as a mold material are its good thermal conductivity and workability. Specific alloys help meet the stiffness requirements. Modern computer-aided milling techniques allow small series of this material. Large-volume molds are made from aluminum blocks cast under vacuum. 

For all their apparent simplicity, molds for rotational molding are subjected to more rigorous demands than those used in other processes. Injection, blow, and thermoforming molds are used to shape molten or softened plastic. Rotational molding tools are used to heat the raw material from ambient conditions, shape it, and then cool it back close to ambient conditions again. This thermal cycle presents a dichotomy for the mold maker in that the mold must be thin enough to... 

The thermoforming mold is usually single-surfaced. Aluminum is the preferred material for longer production runs for both thin- and thick-gauge thermoformers. The mold serves several purposes and has several requirements. 

There are many occasions when only a few to a few dozen products are needed. Traditional vacuum forming uses low pressures and relatively low sheet temperatures. As a result, many common materials often serve as thermoforming molds. There are two general types of prototype molds. 

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). 

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). 

This work was motivated by cracking of a thermoformed part while cooling on the mold, the complexity of the problem could be immediately appreciated since the effect was sensitive to very delicate changes in material composition. Due to coupling between the heat transfer and stress evolution, both problems were solved simultaneously ... 

At room temperature, atactic polystyrene is well below its glass transition temperature of approximately 100 °C. In this state, it is an amorphous glassy material that is brittle, stiff, and transparent. Due to its relatively low glass transition temperature, low heat capacity, and lack of crystallites we can readily raise its temperature until it softens. In its molten state, it is quite thermally stable so we can mold it into useful items by most of the standard conversion processes. It is particularly well suited to thermoforming due to its high melt viscosity. As it has no significant polarity, it is a good electrical insulator. 

Although some molding techniques are adaptable to the production of thermoset materials, thermoforming is carried out using thermoplastic materials. 

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