Cost Considerations In Designing for Thermoforming

Introduction

During product development, the cost of a part comes down to materials, tooling, and process. Added together, the lower total cost of pressure forming offers a distinct advantage compared to injection molding, structural foam, or reaction injection molding.

This article originally appeared as 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 still offer valuable insight for creating successful parts today.

What goes into cost?

The cost of a plastic part is obviously of primary concern to the design engineer. The final cost of a plastic part combines: 

1. the material cost 
2. the tooling cost, amortized over a given quantity of parts
3. the process cost, turning plastic into a final part 

Each of these factors contributes to the total cost of a plastic part. The process cost also includes the processor’s overhead, rejects, scrap, and profit.

Three plastic processes are often compared with pressure forming: injection molding, structural foam, or reaction injection molding. This article briefly reviews the cost factors of these three processes and pressure forming. 

At the core: materials

Injection molding and structural foam start with plastic pellets, ready for molding. Structural foam requires the addition of a chemical or gaseous blowing agent which raises the raw material cost somewhat. However, these two processes generally have the lowest material cost. 

Reaction injection molding starts with two liquid components which react together during the molding process to produce a thermoset material. The material cost is only slightly greater than using injection molding. However, the machinery required to automate this process is costly. 

Pressure Forming starts with a sheet of plastic material which has already been produced by extrusion, casting, or calendaring. As a result, the material costs are somewhat higher than the other three. 

Add the mold and pressure

Injection molding machines use high pressures to force thick, viscous plastic into a cavity. Modern equipment can develop pressures up to 20,000 psi, but these costly machines have a limit to their maximum size. 

Plus, the injection molds themselves must be strong enough to withstand these high pressures. They are usually made of pre-hardened steel. As a result, these molds are among the costliest plastic processing tools, especially as the part size increases. 

Structural foam uses somewhat lower pressure. To pack the cavity, pressures range up to 500 psi. This process doesn’t require the stronger and more costly molds or machines needed for injection molding. The molds are usually made of machined aluminum or pre-hardened steel.

Reaction injection molding only needs cavity packing pressures in the range of 50-100 psi. Cast aluminum is the most common type of tooling. However, machined aluminum and pre-hardened steel tools are used for long runs. For prototyping, molds are sometimes made of sprayed metal, kirksite, or fiberglass. 

While reaction injection molds are less costly than those required for injection or structural foam molding, the sophisticated mixing, metering, pumping and injection machines are costly.

Pressure forming normally uses pressures below 100 psi. These machines are relatively light duty. Production molds are generally cast or machined aluminum. For prototyping and short runs, tools are occasionally made of filled fiberglass or sprayed metal. 

How tooling impacts design

Designer may not think about the cost of the machinery. However, higher machine costs reflect higher machine hour rates, which have a direct effect on part cost. Plus, tooling costs need to be amortized. 

The relatively low initial investment in tooling and short delivery times possible with pressure forming provide a definite advantage over more capital-intensive processes. The benefits grow for larger size parts. 

Pressure forming also allows initial production of parts which, if successful, can eventually be produced in larger quantities by more expensive processes. The ability to switch processes with no outward change in appearance delivers a major cost advantage. 

The cost of finishing up

Finishing adds to part costs. Injection molded and reaction injection molded parts can be used as molded. Structural foam parts are painted. Pressure formed parts need to be trimmed but don’t require painting. 

Ongoing advancements across these processes are beginning to produce parts that require less traditional finishing and painting procedures.

Conclusion: The final comparison

Cost comparisons developed for typical plastic parts show advantages for different processes based on quantity and size when other factors like material and wall thickness are maintained. 

Injection molding, with the highest tooling costs, becomes cost effective in larger quantities. However, larger parts require much higher quantities to achieve similar effectiveness.

The total cost, combining tooling and part costs, of pressure forming compares favorably with reaction injection and structural foam molding of parts in all quantities for across sizes.