Injection-Molding Thermoplastic Parts from Composite Board

Prototyping is today the only way to effectively validate many new design concepts. Because manufacturers emphasize consolidation of components to reduce costs, there has been an overall increase in part complexity. Although computer models can indicate the theoretical success of a design, it is through the generation of solid models and prototypes that engineering tests and marketing feedback can be obtained.

One widely used prototyping method uses rapid modeling and tooling techniques to form cast injection molds for running components from production thermoplastics. The benefit of forming production-intent prototypes from the end-product material is that the parts feature the same properties, such as weight, density, feel, and flexibility, as the final part. But a drawback has been in the moldmaking products and processes used for rapid tooling.

The Automotive Interiors Division (formerly Prince Corp.) of Johnson Controls, Inc. (JCI) initiated a research program with Ciba Specialty Chemicals to develop a tooling material that could produce highly accurate core and cavity injection molds using fewer construction steps. The program led to the subject new composite board.

Among the goals and objectives of the program were:

• Machine a mold that could withstand injection molding pressures and temperatures used for engineering thermoplastics such as polypropylene, acrylonitrile-butadiene-styrene copolymer (ABS), and polycarbonate. Specifically, the moldmaking material has to exhibit good compressive and flexural strengths, in excess of about 8000 psi, and must be able to withstand the required temperatures, approximately 350-600^oF.
  
• Produce an injection mold via high-speed machining at substantial feed rates.
• Develop a tool that could hold part accuracy to within approximately 0.005 inch.
• Injection mold up to 250 thermoplastic production-representative parts.
• Attain a machined surface finish equal to or better than that produced with electrodes.
• Develop a surface that could mold thermoplastic parts without secondary surface treatments.
• Create a surface that could be polished or textured.

The new composite board developed by JCI and Ciba meets each of these criteria, with the exception that machined surfaces do require secondary treatment.

Further development involved designing molds using a computer-aided design software package that analyzed the configuration of a simple test part. The computer program was then used to aid in the creation of a reverse image of the part, incorporating mold features such as parting line, venting, sprue and runners, and ejectors.

When mold design was completed, preparations for high-speed CNC machining were made, including setup parameters as defined in cutter pathing for part geometry. During initial test runs, the preferred cutting tool for use on the composite board was a solid carbide ball-nosed end mill with one-and-one-half degree taper per side for draft. Spindle speed for machining the board was generally 6000 to 10,000 revolutions per minute (RPM) for roughing and 15,000 RPM for finishing. Cutter wear was negligible.

To enhance the performance of the resulting composite board tool, a pocketed steel support structure was attached to the core and cavity molds. This frame was used to produce additional mold strength, to help the tool withstand the force and stress of injection molding, and to generate a greater number of parts without degrading.

When the workability of the composite board for initial injection molds was verified, a series of different thermoplastic parts was formed to confirm mold performance under a variety of conditions. Three of the prototype parts Prince selected were an automobile visor bracket cap, a visor track extender, and a visor backbone.

The bracket cap was deemed a realistic candidate to produce a functional part in molds machined from the new composite board. To test the machinability and molding capabilities of the composite board, core and cavity mold inserts were machined from the new product as well as from conventional electrode carbon and aluminum moldmaking materials. The table demonstrates the significant time savings that accrued from machining the bracket cap mold from the board versus aluminum. Nonstructured testing confirmed that parts from the carbon and composite tools were virtually identical.

The third part tested during the research program was a visor backbone, an aesthetic structural member for the foam visor core and mountings for other functional items. The design of the part was ideal for demonstrating the strengths of the new tooling board because it featured deep ribs, as shown in the figure, that required significantly more moldmaking operations for aluminum tools, including more finish cutting, electrode prep, electrical discharge machining (EDM), and final polishing. With the composite board all of the required details were cut in one setup, saving almost 50 hours over aluminum molds and also increasing core and cavity accuracy. Preliminary dimensional testing indicated that the prototypes run in the composite molds were virtually identical to the parts produced on the aluminum inserts.

For more information on the new composite board, contact the author of the SME paper based on the above material, Ken Filipiak, lead tool engineer/tool room manager at JCI; (616) 394-6432; fax: (616) 394-6464, or Mahesh Kotnis of Ciba Specialty Chemicals.