How materials influence part design in thermoforming

Introduction

How do you select the right material for a thermoformed part? Factors to consider include a resin’s chemical structure and features. But key design criteria include functional requirements, processability, and (ultimately) cost. 

This article was originally part of Profile’s Design Guide. Plastics industry legend Glenn Beall wrote our first edition in 1985. Hidden away in the company archives for years, the concepts presented here can offer valuable insight for creating successful parts today.

How does heat impact part design?

Any thermoplastic can be formed into parts. However, an ideal material for thermoforming is one with enough hot melt strength to support itself during transfer from the oven to the forming station. 

The high heat characteristics of the material make the difference. The plastic must become soft enough to be stretched into a larger shape without becoming so weak that it separates during the process.

Look at a material’s structure: amorphous vs crystalline

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

Crystalline materials with clearly defined melting points, such as polyethylene, nylon, polypropylene, acetals and polyesters are more difficult to thermoform. However, they can be thermoformed successfully. Some of these materials find wide usage in thin-walled packaging applications.

The polymers which have found widest usage in pressure forming include: 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.

Considering special materials?

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.

Special sheet stock can be made from materials which exhibit or incorporate: 

• EMI-RFI shielding
• fire retardants
• ultraviolet inhibitors
• anti-bacteria stats
• lubricants
• transparency
• special colors

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 pressure forming process can crush the foam during forming.

Select the right material

Selecting the ideal plastic material for pressure forming a part is a complex subject. Simply put, the designer must decide 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 processable without undue difficulty. 

Simultaneously, the final cost (the material cost and forming costs) must fall within a range that is acceptable in the marketplace.

Remember that the original equipment manufacturer is responsible for the suitability of the chosen plastic material for the application. With that in mind, 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 comprehensive material identification would be:
Borg-Warner ABS Cycolac GSE B lack #4500
Starting Gauge Thickness .187″ ± .010

Conclusion: Lean into experienced knowledge

Selecting the optimum plastic material for a specialized process such as pressure forming is not easy. However, the advice of an experienced pressure former can be very helpful when you need to choose the right material. Find a discussion of the factors that go into this decision on Profile’s material expertise page.