Rapid tooling meets prototyping needsRapid tooling meets prototyping needs

Editor's note: When an OEM needs a functional prototype made of a production material, sometimes the only choice is to develop prototype tooling and mold the parts. As rapid tooling technologies emerge to fill this need, molders and moldmakers are able to reduce the time it takes to bring a project from concept to production. A recent effort to design and prototype heat exchangers using cast epoxy tools underscores this point. Scott Hay, president of 3Dimensional Engineering (Pompano Beach, FL), and Mike Downs, injection molding supervisor at Ralph S. Alberts Co. (Montoursville, PA), presented the details of this project at the recent RP&M 2000 conference.

To successfully introduce a new product in today's marketplace, companies need more than a great idea. They must be prepared to move faster than the competition. A key element in reducing the time-to-market cycle is being able to create working prototypes for comprehensive physical testing and analysis early in the design process. For many projects, this means that prototype parts must be injection molded from the production material before a design is finalized. 

In one such project, designers at Peregrine Industries, a leading supplier of equipment used to heat swimming pools, began with the idea of creating an all-plastic heat exchanger. They theorized that replacing conventional metal heat exchangers with a high-performance plastic unit could decrease size, weight, and price while increasing corrosion resistance. It would also allow Peregrine to reduce significantly the size of the cabinet needed on pool heating equipment, decrease shipping and packaging costs, and improve part performance in the moist, hot, chemically corrosive environment that characterizes pool heating enclosures. 

Product development began in 1999. The part Peregrine designed is a patent-pending heat exchanger, made from a proprietary highly filled liquid crystal polymer, for installation in pool pump assemblies. The prototyping challenge was injection molding the multiribbed, thin-walled heat exchangers in a cast epoxy tool. 

Concept Development
The first step was to identify a corrosion-resistant, high-temperature, thermally conductive plastic. Peregrine chemists established that a liquid crystal polymer provided the required performance under high-heat, corrosive conditions. 

Next, they examined thermal conductivity issues. The standard method for increasing thermal conductivity in polymers, which are inherently poor conductors, is to add carbon fibers. The drawback of these materials is cost. Adding carbon fibers would have made the heat exchanger prohibitively expensive. To find a more cost-effective method, Peregrine began testing a number of different additives and fillers. The result was a new, patent-pending polymeric system-a liquid crystal polymer filled with a proprietary carbonaceous material. 

Part design efforts followed material development. As the name implies, heat exchangers must transfer heat. Typically, this is a function of surface area, so heat exchangers are usually built with a series of fins that increase the part's surface area. In general, the more thin fins a heat exchanger has, the greater its ability to transfer heat. 

For the new polymer heat exchanger, Peregrine designed a part containing 150 fins (or ribs), ranging in thickness from .024 inch to .062 inch, surrounded by a .062-inch-thick wall. Dimensions were given to 3Dimensional Engineering, a product design and development firm, to produce a stereolithography (SLA) master. 

With this master, and a carton of the proprietary, compounded liquid crystal polymer, 3Dimensional Engineering met with Ralph S. Alberts Co., a custom molder and moldmaker, and asked Alberts' toolmakers to build an epoxy injection mold and to form as many prototype parts as they could with the supplied material. (The two companies work together regularly in a virtual partnership arrangement.) 

As with most product development projects, Peregrine wanted the parts as quickly as possible so that it could begin the physical testing phase of the program. Together with 3Dimensional Engineering, it wanted to determine whether the polymer heat exchanger, which looked promising in laboratory tests, actually satisfied the required performance characteristics. 

Part Prototyping
Alberts Co. toolmakers quickly identified several potential obstacles to molding the parts. The first was the large number of thin ribs used for the heat transfer. Difficult to mold under any circumstances, the ribs would be even tougher to form in prototype epoxy tooling. Further complicating the project was the 68 percent filled liquid crystal polymer, which has a melt temperature of 760F and requires an injection pressure of nearly 20,000 psi. 

Building tools. Given these parameters, Alberts Co. moldmakers began evaluating rapid tooling epoxies that could withstand the high temperatures and pressures needed to shoot the liquid crystal polymer parts. The decision was made to cast the tools from an aluminum-filled epoxy alloy from Ciba Specialty Chemicals called Cast-IT 2000. The product, introduced to the U.S. market last year, had been used by Ciba customers in Japan for more than five years to build nearly 20,000 injection molds for short-run prototyping, as well as for molding thousands of thermoplastic production parts. 

Alberts Co. moldmakers compared the physical properties of the new rapid tooling epoxy with conventional casting materials (Table 1, p. 96). They found that it had the combination of high strength, high glass transition temperature, and thermal properties that would be required to produce injection molds to make the heat exchanger prototypes. The epoxy also could be polished to achieve a metal-like finish and exhibited the low shrinkage and dimensional stability needed to mold extremely accurate parts. 

Once the tooling material was selected, moldmaking began with master preparation. All surfaces on the SLA model were smoothed to a fine finish. Then, several coats of sealer and release agent were applied to prevent mold sticking in the many thin, ribbed sections of the design. 

Next, molds were cast, incorporating several aluminum inserts in high-tolerance areas. The tools were poured at room temperature on a vibrating table to eliminate voids at the rib edges. Then, traditional Ren RP 4037 R/H tooling epoxy was poured behind the Cast-IT 2000 to form a durable back-support structure. Completed molds were postcured in an oven. Finally, a flash gate was installed to optimize mold filling and produce fully formed parts. 

Molding parts. To run the parts, molds were heated to 190F in an oven and then mounted in a 150-ton press. Several parts were shot at different temperatures and pressures, with the best results achieved using a melt temperature of 760F and injection pressure of 18,500 psi (Table 2). Cycle time was 4 minutes, with 30 seconds for part injection and curing and 3 minutes and 30 seconds for mold cooling. (Compressed air was used to maintain surface temperatures of the tool at 240 to 270F.) With these molding conditions, Alberts Co. produced 300 parts with no failure on the epoxy mold surface. 

Future developments. The LCP prototype heat exchangers were used by Peregrine for physical testing and field evaluation. This work is still under way and is expected to yield the details required to produce a final part design. Although the cost to fabricate the LCP parts has not met Peregrine's goals yet, designers estimate that it will be cheaper than aluminum once material development costs are amortized. Ralph Alberts and 3Dimensional Engineering continue to work on other possible solutions. For example, the prototype parts must be adhesively assembled into a 15-part array at present. By turning to soluble cores in the epoxy tool, it may be possible to consolidate parts so that fewer need to be assembled.