Cost Considerations

The cost of a plastic part is obviously of primary concern to the design engineer. The final cost of a plastic part is made up of (1) the cost of the material, (2) the cost of the tool amortized over a given quantity of parts*, and (3) the cost of processing the material into a three-dimensional part which includes the processor's overhead, rejects, scrap, and profit. Each of these three factors makes an important contribution to the total cost of a plastic part.

The three processes that are most often compared with Thermo Pressure Forming are injection molding, structural foam, and reaction injection molding. The following briefly reviews the cost factors of these three processes and Thermo Pressure Forming.

Injection molding and structural foam start with the plastic in pellet form which is ready for molding. Structural foam requires the addition of a chemical or gaseous blowing agent which raises the raw material cost a small amount. However, these two processes generally have the lowest material cost. Reaction injection molding starts out with two liquid components which react together during the molding process to produce a thermosetting material. The material cost is only slightly greater than for the same material if it were injection molded. However, the machinery required to automate this process is relatively costly.

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

Injection molding machines utilize high pressures to force a thick, viscous plastic into a cavity. Modern injection molding machines are capable of developing pressures up to as much as 20,000 psi.

The clamping portion of these machines which holds the two halves of the mold closed must be strong enough to withstand these high pressures. The machines which are capable of developing these high injection and clamping pressures are relatively costly and have a limit to their maximum size due at least in part to the capital-intensive nature of the equipment.

The molds which define the shapes of injection molded parts must also be strong enough to withstand these same high pressures without deformation. As a result, injection molds are among the most costly of plastic processing tools. This is especially true as the size of the part becomes large.

The low pressure structural foam process normally utilizes machined aluminum and in special cases, prehardened steel molds. The cavity packing pressures are in the range of 500 psi or lower. The low pressures do not need the stronger and therefore more costly molds or molding machines that are required for injection molding.

Reaction injection molding produces cavity packing pressures in the range of 50 to 100 psi. The most common type of tooling is cast aluminum. However, machined aluminum and prehardened steel tools are used for long runs. Sprayed metal, kirksite and fiberglass molds are sometimes used for short runs and prototype work. Reaction injection molds are less costly than those required for either injection or structural foam molding. However, the mixing, metering, pumping and injection equipment are sophisticated and relatively costly.

Thermo Pressure Forming is normally done at pressures below 100 psi and these machines are relatively light-duty. Production Thermo Pressure Forming dies are generally cast and/or machined aluminum. Prototype and short run tools are occasionally made of filled fiberglass and sprayed metal. The relatively short deliveries and low initial financial investment required for Thermo Pressure Forming tools and machines provide a definite advantage over other more capital-intensive processes. These advantages become even more significant as the size of the parts becomes progressively larger.

The designer may think that the cost of the machinery is of little concern to him. However, the higher machine costs reflect higher machine hour rates and that has a direct effect on part cost.

The relatively low initial investment in tooling and the short delivery times that are possible with Thermo Pressure Forming allow the process to be used for products which, if successful, will eventually be produced in larger quantities by more capital-intensive processes. The ability to make this conversion with no outward change in appearance is a major advantage of Thermo Pressure Forming.

Table 2 compares the cost of tooling and part cost for the part shown in Figure 22 . Comparative costs have been developed for this relatively small part and a part twice as large. In all cases, the plastic material is medium impact ABS. The one exception is reaction injection molding which utilizes polyurethane. The starting gauge thickness for the Thermo PressureFormed part is .125 inches. The injection molded part has average wall thickness of .125 inches. The structural foam and reaction injection molded parts have an average thickness of .250 inches with an average formed density reduction of 25%.

The tools are fabricated aluminum for Thermo Pressure Forming and cast kirksite for the reaction injection molding. The structural foam mold is machined aluminum. The injection mold is prehardened (P-20) steel.

The injection molded and reaction injection molded parts are as molded. The structural foam parts are as molded and painted. The Thermo Pressure Formed parts are trimmed and machined but unpainted.

Ongoing developments in the relatively new reaction injection molding process and the recent advancements made in the counter pressure structural foam molding process (Smooth SkinŽ) are beginning to produce parts that do not require the traditional finishing and painting procedures normally associated with these processes.

Table 2 lists the tool delivery, cost and amortization factor for the total production quantities of from 500 to 20,000 parts for the two sizes of parts shown in Figure 22.

Referring to the chart, it is obvious that the combined tooling and part cost, or the total cost of Thermo Pressure Forming compares favorably with reaction injection and structural foam molding of parts in all quantities for both sizes.

Injection molding, with the highest tooling costs, become cost effective in quantities of 10,000 for the small part, and 20,000 for the larger part.