Design vs. Material

All thermoplastic materials can be and have been thermoformed at least experimentally. However, an ideal plastic material for thermoforming is one which has enough hot melt strength to support itself while it is being transferred from the oven to the forming station. The high heat characteristics of the material must allow it to become soft enough to be stretched into a larger shape without becoming so weak that it separates during the forming or stretching process.

The broad softening and eventual melting temperatures of the amorphous plastics such as acrylic, ABS and polycarbonate make them easier to handle in the thermoforming process.

The clearly defined melting points of crystalline materials such as polyethylene, nylon, polypropylene, acetals and the polyesters are more difficult to thermoform. However, these materials can be thermoformed successfully, and some of these materials find wide usage in thin walled packaging applications.

The plastic materials which have found wide usage with the Thermo Pressure Forming process are acrylic, ABS, impact styrene, cellulosics, polycarbonate, polysulfone, polyvinyl chloride, PPO and PPE.

Blends or alloys of PVC and ABS or PVC and acrylic have been used where improved fire retardance is required. Laminated sheets of various combinations of materials are available for especially demanding applications. A thin layer of ultraviolet light-resistant material on the outer side of a thicker sheet of lower cost material is just one example.

Styrofoam sheets and structural foam laminates with two solid skins separated by a layer of foam have been formed. However, this is tricky as the higher pressures associated with the Thermo Pressure Forming process tend to crush the foam during the forming process. Special sheet stock can be made from materials which exhibit or incorporate EMI-RFI shielding, fire retardants, ultraviolet inhibitors, anti-bacteria stats, lubricants, transparency, and of course, colors.

The selection of the ideal plastic material for a part for Thermo Pressure Forming is a complex subject which is beyond the scope of this booklet. However, it suffices to say that the designer must make his decision based on at least three sets of criteria which must all be satisfied. First and most importantly, the chosen material must satisfy the functional requirement for which the part was created. Secondly, the material must be able to be processed without undue difficulty. Simultaneously, the combination of the cost of the material and resulting cost of forming it into a sellable product must be within a range that is acceptable in the marketplace.

Selecting the optimum plastic material for a specialized process such as Thermo Pressure Forming is not easy. However, the advice of an experienced thermo pressure former can be very helpful when the time comes to select the specific material.

It must be remembered that the original equipment manufacturer is responsible for the suitability of the chosen plastic material for the application. Considering that fact, the part drawing or supplemental specification sheet must clearly identify the plastic material that is to be used to produce the part. An example of a thorough material identification would be: