This technical information has been contributed by
Resin Systems Corporation

Liquid Resin Casting Trims Costs, Production Time

The process offers unique and tangible benefits with regard to accelerating production cycles and reducing production costs for low- to medium-volume applications.

Plastics - Liquid Resin Casting

By Walter O'Hearn
Resin Systems Corporation
Amherst, New Hampshire

One of industry's most innovative and timesaving design prototyping tools is stereolithography. In this process, data from special CAD files (in an .stl file format) are used to control a computer-driven laser beam that is directed upon a tank full of a liquid resin monomer. The laser alternately scans (and thus photo-cures) cross-sections of the part, after which the part is lowered to a prescribed depth (typically a few thousandths of an inch). Then, scanning resumes.

In this manner, stereolithography creates a series of photo-cured, cross-sectional layers superimposed upon and bonded to each other. When the process is complete, the result is a solid, three-dimensional representation of the CAD part. Such parts are usually called SLA masters, referring to the Stereo Lithographic Apparatus used in the process.

Under normal circumstances, SLA masters are used to prove the form and fit characteristics of a design concept. Subsequently, conventional manufacturing processessuch as machining, composite lay-up, or injection moldingare usually used for production runs of the actual parts.

However, Resin Systems Corp., of Amherst, New Hampshire, has recently used SLA masters provided by clients to create molds for short-production-run, liquid resin cast parts. The major benefit is that tooling time and costs are substantially reduced, and actual parts can enter production much faster than otherwise possible.

An example of Resin Systems' work in this area is the case of Crew Systems Corporation (Carbondale, Pennsylvania), a developer of life-support equipment designed primarily for military aviation applications in the U.S. and abroad. Typical products of Crew Systems Corp. include flight helmets, oxygen masks, connector blocks, NBC (nuclear-biological-chemical) respirators, and integrated helmet-mounted sight-and-display systems.

"We have recently been involved in the design of a totally new type of aviator's integrated flight helmet/NBC protective respirator," said Gary Piorkowski, Vice President and General Manager at Crew Systems. "Each helmet incorporates numerous parts, ranging in size from a 1/2 in3 up to 10 inches x 10 inches x 4 inches-all necessarily lightweight because of the head-borne application and most quite complex in terms of surface geometry.

"One crucial requirement was that we faced a very aggressive development schedule to get the helmet from the design stage through qualification and flight testing before entering production," explained Mr. Piorkowski. "Because the anticipated production runs for qualification testing were to be low-volume, we determined that the costs and lead-times for conventional injection mold tooling or composite lay-up would be prohibitive."

The company, seeking a more timely and cost-effective mode of short-run production, approached Resin Systems. "We agreed upon the feasibility of Resin Systems using SLA masters of each part as the basis for creating the required resin casting molds," Mr. Piorkowski recounted. "By using this approach, Resin Systems went from CAD file to tool to production in only 21/2-to-four weeks, depending on the specific part, compared to three-to-five monthsor morefor other manufacturing techniques, and at a fraction of the cost."

Resin Systems makes products ranging from intricate components for biomedical and scientific applications to 500-pound high-voltage system castings with specialized high-performance insulating properties. In a typical run of 25 to 2500 pieces, the company offers a wide range of capabilities, including rapid manufacture of complex shapes, specialized material formulations, custom colors, cast-in undercuts, threads, inserts, surface textures, component encapsulation, and prototypes from stereolithography master parts.

Founded in 1954, the company pioneered the development of custom epoxy and urethane resin castings for dependable performance in countless demanding applications and environments. It also offers broad expertise in computer-aided design, pressure and vacuum casting, and CNC precision machining for close-tolerance secondary operations.

Replacing Machined Components with Resin Cast Parts

Falmouth Scientific, Inc. (FSI), Cataumet, Mass., has built a reputation as an international leader in the development and manufacture of high-quality precision oceanographic instrumentation. One of FSI's newest instruments is its Shallow Water Two Dimensional Acoustic Current Meter (ACM), introduced in 1998.

Compact and highly accurate, this comparatively low-cost unit is designed for general oceanographic fieldwork, fjord and estuary research, fish farming, underwater engineering, and environmental research and monitoring, among other applications.

"In designing the 2D-ACM, we had two major objectives," said Paul Smith, FSI's operations manager. "We wanted to ensure that the product delivered high-precision measurement performance comparable to our field-proven 3D-ACM instrument, and that we could get the product to our customers at the lowest possible selling price.

"In the initial versions of the 2D-ACM," Mr. Smith explained, "we employed a variety of parts that were CNC-machined from blocks of plastic, including velocity "fingers," the base on which the velocity fingers mount, and the housing for the current meter. In addition, there was an end cap of machined aluminum that incorporated an electrical connector to interface with PCBs in the housing.

"For FSI to get the cost of the instrument down to where we wanted it," Mr. Smith said, "we needed a way to minimize manufacturing costs without sacrificing quality or precision. Having Resin Systems furnish us with cast resin parts to replace the more expensive machined parts proved to be the answer."

Citing one notable cost reduction, Mr. Smith said: "Resin Systems replaced the complex machined-aluminum end cap with a more easily produced cast resin end cap incorporating a molded-in electrical connector, and the cost immediately dropped by about a third.

"Besides providing equivalent quality at as much as 60% less than the cost of machined parts, Resin Systems also gave FSI another important advantagewe could get the parts we needed in two to three weekstypically a week faster than with the machined parts," he concluded.

About the Author

Walter O'Hearn has over two decades of experience in liquid resin casting and precision machining, serving diversified customers ranging from microwave to medical. As Manager, Marketing and Sales for Resin Systems Corp., he is responsible for all of the company's sales and consulting activities. He is also responsible for advertising, trade shows, and public relations.

A Liquid Resin Casting Primer

(The following explanation is extracted from the author's article Liquid Resin Casting for Prototype to Production.)

Liquid resin casting involves mixing and pouring or injecting a thermoset resin, such as epoxy or polyurethane, into a mold to cure. Heat, vacuum, and pressure are often used to enhance the quality of the casting. When cured, the casting is removed from the mold, cleaned of any mold release agents, and prepared either for immediate use or for secondary operations.

Resin material is selected or custom formulated according to predefined performance needs. These can include tensile and flexural strength, density, rigidity, flexibility, surface texture, color, chemical resistance, required tolerances, dielectric strength, and temperature stability.

The casting process may employ any of various types of molds. The type depends on numerous factors, such as part complexity, size, weight, critical dimensions, need for cast-in inserts, and production volumes. For example, the mold may be made of RTV silicone that has been formed, with great fidelity in detail, about a master pattern. Or it might be a sophisticated machined aluminum tool with extremely tight tolerances.

Among its most important advantages, liquid resin casting offers the following:

  • Virtually unlimited customizability
  • Superior part-to-part and lot-to-lot uniformity
  • Easy adaptability to changing needs
  • Fast and economical implementation

This technical information has been contributed by
Resin Systems Corporation

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