Liquid Resin Casting for Prototype to Production
By Walter O'Hearn
Manager, Marketing and Sales
Resin Systems Corporation
We live in a world with many different industries, each with its own requirements for product performance, durability, appearance, compliance with standards, reasonable cost, ease of manufacture, short production deadlines, and so on.
Though unique in some ways, these industries share some common challenges in developing prototypes and short production runs because of the costs of machining, tooling, injection molding, and other forms of fabrication.
There is at least one solution--liquid resin casting. It is an ideal process for prototypes, as well as small-to-medium volume production runs.
Casting with Liquid Resin
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 some secondary operations.
Resin material is selected, or custom formulated, according to predefined performance needs. Such needs 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 volume.
For example, in some instances the mold may be made of RTV silicone that has been formed, with great fidelity to detail, about a master pattern. In other cases, the mold may be a comparatively sophisticated machined aluminum tool, with extraordinarily tight tolerances.
Among its most important advantages, liquid resin casting offers:
- virtually unlimited customizability in terms of configuration, material, appearance, texture, precision, and physical, electrical and chemical performance properties
- superior part-to-part and lot-to-lot repeatability
- easy adaptability to changing needs
- fast and economical implementation
The process is an excellent method for making prototype parts (often in days or weeks rather than months), and subsequently for producing short-to-medium production runs--from the tens to the low thousands of parts annually.
Molds can be produced and revised quickly and economically. Similarly, resin materials can be chosen, formulated, test-cast, evaluated, reformulated, and re-evaluated in short times.
Ultimately, as quantities rise, it may become more economical to use high volume plastic production processes instead of liquid resin casting. For example, injection molding may make sense when long production runs of many thousands of parts can balance out the characteristically high tooling costs of that process.
Resin Systems Corporation in New Hampshire has adapted liquid resin casting to meet the unique requirements of many customers in different industries. Three examples are described below.
Epoxy Housing for Laser Printing
Markem Corporation in New Hampshire makes the industry's most extensive line of in-plant marking and decorating equipment. Best known for the systems and supplies it provides to the electronics industry, Markem also offers a broad range of systems for other industries, including packaging, textiles, apparel, food processing, and pharmaceuticals.
One of the newest systems from Markem is their programmable, digitally controlled Q2000 Optimark laser marking system. This device performs high-speed, high-quality film printing on integrated circuit packages. It uses a combination of a focused laser beam, and a film drive mechanism, to deposit opaque dry ink precisely and crisply onto the surface of integrated circuit packages.
A critical part of the laser marking system is the cartridge housing for the film drive mechanism. "In designing the system," explained David Georgis, Senior Production Engineer at Markem, "we needed a dimensionally stable housing to ensure proper tracking of the film mechanism and accurate alignment of the laser optics. The housing also had to be sufficiently lightweight and rugged to withstand repeated handling by system operators, who regularly unlock and remove it to replace the film that conveys the ink supply. Cosmetics were important too because the housing mounts externally and is always visible."
Because of comparatively low-volume production requirements--not high enough to justify the cost of plastic injection molding--Markem considered several alternatives for manufacturing the housing: aluminum machined from block, cast Nylatron, and cast liquid resin.
"Working with aluminum had significant limitations," notes Georgis. "An aluminum housing would weigh up to 60 percent more--and would also cost more to produce than a comparable housing made of plastic.
"Cast Nylatron was not competitive due to tooling costs and unit cost, and was not pursued beyond initial inquiries."
Liquid resin casting offered a way to overcome these difficulties and produce the desired housing quickly, efficiently, and economically.
Markem had previously worked with Resin Systems on cast liquid resin parts for other marking systems. Although those parts were smaller and less complex than the Q2000 film drive housing, Markem approached Resin Systems to determine if the same process was applicable to larger parts.
Working together, engineers from both companies adjusted the housing design to make it more readily manufacturable by liquid resin casting. According to Stuart McCord, Resin Systems' Engineering Manager, "The design team considered various materials including a urethane elastomer, a filled urethane, and epoxy. We settled upon a toughened epoxy because of its superior handling characteristics, suitability for casting into complex shapes, durability, minimal shrink factor, and its ability to withstand moderate heat."
Resin Systems then produced the machined-aluminum tooling for molding the housing within six weeks. It took only another two days to cast the first housing, perform required secondary CNC machining, and make delivery to Markem.
"The quality and precision of the cast resin housing was so good, with tolerances as tight as 0.001 inch, that production could begin immediately," Georgis recounts. "In fact, within a week after receiving the first housing from Resin Systems, we installed it on a Q2000 system and shipped the system directly to a customer."
BYK-Gardner USA in Maryland designs and manufactures state of the art quality control instrumentation: spectrophotometers, colorimeters, and gloss meters. These are used in many different industries for measuring appearance characteristics--color, gloss, and texture--of many products. Typical users of these instruments include manufacturers of paints, inks, plastics, textiles, food and pharmaceuticals.
One of BYK-Gardner's newest systems, the color-view spectrophotometer, has a large, fully visible, protective enclosure measuring 12 by 15 by 6 inches, with a uniform shell thickness of 1/4 inch and tolerances as close as 0.005 inch. Because the instrument has a self-supporting chassis, the enclosure needs only moderate structural strength. Its primary functions are to keep the internal components of instrument clean and dust-free, and to enhance the appearance of the system.
"Originally we considered forming the enclosure by plastic injection-molding," explains Rick Trawick, BYK-Gardner's Manufacturing Engineering Manager. "But, the cost of tooling for injection molding was prohibitive. So the prototypes for the enclosure--and the initial production runs--were assemblies fabricated from machined and glued PVC plates."
According to Trawick, "These fabricated enclosure assemblies were expensive to manufacture and somewhat fragile. Also, delivery often fell several months behind schedule, and there was a significant part-to-part inconsistency problem." These problems threatened to affect BYK-Gardner's ability to keep up with its backlog of new orders for the color-view spectrophotometer.
The solution to BYK-Gardner's problem came when a sales representative for Resin Systems proposed using liquid resin casting for the enclosure instead of plastic fabrication. "Although we had never used liquid resin cast parts before, we thought the process had some compelling advantages that justified giving it a try," says Trawick.
To ease installation on the color-view spectrophotometer, the cast resin enclosures incorporated cast-in threaded inserts and need some minor secondary CNC machining.
"By using an aluminum mold, we could provide excellent dimensional accuracy ( 0.015 inch over a 14 inch distance, with some tolerances as tight as 0.005 inch over five inches) and achieve the required part-to-part consistency and repeatability," according to McCord. "During a short prototype development period of two weeks, we modified the resin formulation and molding techniques. Made of filled urethane, the enclosure is produced at a rate of 250 parts per year per mold."
"The change to liquid resin casting immediately resolved the back-order bottleneck," says Trawick. "The cast resin covers have excellent uniformity from part-to-part and lot-to-lot, and BYK-Gardner is receives steady, reliable, and predictable deliveries against our production schedule. I wish we had found out about liquid resin casting years ago. There are many things we could have done with this process. For us, it opens a lot of new avenues for production."
Elastomer Parts for Ophthalmologic Instrument
Computed Anatomy Corporation in New York is an electronic engineering firm that develops and markets specialized medical diagnostic instruments for ophthalmologic and optometric applications. In particular, the company pioneered systems to measure and evaluate the shape and surface quality of the cornea of the eye.
Video images captured by the instrument are computer-analyzed and used for such applications as diagnosing eye diseases and custom-fitting contact lenses. Computed Anatomy's newest offering, the Topographic Modeling System-2 (TMS-2), is a third-generation instrument that produces a color-coded topographic map of the cornea.
Computed Anatomy was concerned about three components critical to the operator's and patient's interfaces with the instrument. For the patient, it was a chin rest to keep the head steady during the measurement procedure. For the operator, it was the handle of the joy stick control used to position the instrument's measurement apparatus, and the knob used to adjust the patient chin rest.
"Because these three parts touch people, it is important they have the right feel," says Roy Maus, Computed Anatomy's Vice President of Administration and Manufacturing.
"From the outset, getting the desired qualities of feel and texture argued against using machined or cast metallic materials," Maus explains. "We considered using injection molded plastic parts, but there were overriding limitations to that approach too. Injection molding was very costly for the production quantities we anticipated. Also, we had a tight time line to get the parts into production. And tooling up for injection molding is time consuming.
"Even if there were no cost and time problems, attaining the proper feel in the material might have been difficult. In addition, the parting lines from injection molds would have been unsightly. We would have had to perform secondary machining to remove them.
"Liquid resin casting allowed us to move the parts from design into production in just two months. Tooling-up for resin casting was much faster than injection molding--two to four weeks compared to 8-16 weeks. And the silicone rubber molds for resin casting were about one tenth the cost of machined metal injection molds."
The liquid resin casting process yields parts that accurately replicate the finish of the original master parts. "For Computed Anatomy's parts, we formulated a flexible elastomeric urethane material, color-coordinated to the TMS-2 instrument," says Stuart McCord. "This material allows us to produce parts with precisely the right feel and texture to optimize comfort for the patient and the instrument operator."
"We were able to mold a threaded brass insert directly into the chin rest, and a Delrin shaft into the adjustment knob, saving additional assembly steps. Plus, the total absence of mold parting lines, gate marks and sink marks eliminates the need for secondary machining." McCord adds, "The finely detailed surface finish was duplicated from an aluminum master. Machined plastic and stereolithography masters don't adequately develop the surface finish for highly aesthetic parts."
"Our objective was to make the human-to-instrument interface of the TMS-2 to be perfect," concluded Maus, "and Resin Systems helped us to meet this objective."
Founded in 1954, Resin Systems pioneered the development of custom epoxy and urethane resin castings for dependable performance in countless demanding applications and environments. The company offers broad expertise in computer-aided design, pressure and vacuum casting, and CNC precision machining for close-tolerance secondary operations.
The company 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 parts.
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 Corporation, he is responsible for all the company's sales and consulting activities. He is also responsible for marketing programs, including advertising, trade shows and public relations. Before joining Resin Systems, Walter served in various management roles in other manufacturing businesses, including ten years of managing his own precision machining business.
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