Reaction Injection Molding for Prototypes and Short Runs
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RIM Offers One-step Alternative to High-pressure Injection Molding

A patent-pending method permits the molding of pivot joints as a single manufactured part, produced in a single-step molding process. It is said to result in relatively low manufacturing costs and no slack or looseness in the joints.

By Mark Shortt
Editorial Director, Design-2-Part Magazine

Reaction injection molding (RIM) is a plastic molding process that uses aluminum and hard epoxy tooling for prototypes and short runs. A significant difference, however, exists between RIM and standard injection molding for short run work. The distinction is that RIM uses low heat and low pressure, which allows its tooling to be used for many production cycles, often 30,000 to 50,000. If used with high-pressure injection molding, the same tooling would usually last only a few thousand production cycles due to the high pressure and high heat generated by the process.

And while standard injection molding is also applied for short runs and prototypes using soft aluminum tooling, it is only used for small parts with thin wall thicknesses. When parts need to be larger than 6 x 6 x 2 inches, the aluminum tooling needs to be made stronger and heavier to accommodate thicker part walls and other larger features.

The RIM process is ideal for a product manufacturer that needs only a small quantity of parts for a given product on a yearly basis. When the product begins to sell widely and larger quantities are needed, it is more economical to build hard steel tooling and run the parts on standard injection molding equipment. In the interim, RIM is ideal for a long-range, short-run production capacity.

Although RIM is very popular in Europe, only a handful of contract manufacturers in the U.S. perform this unique molding specialty. One such company is Premold Corporation in Oconomowoc, Wisconsin.

"We specialize in custom, short run, and prototype reaction injection molding for clients who only want about 500 or 600 parts per year, or a few thousand at most, but who want precision and complex design features," states Ken Schweitz, the president of Premold Corporation. "We make a lot of structural parts, both large and small, where the look of the part is very important, like machine bases and frames, enclosures, covers, and control panels. With our RIM process, a part can have very deep cavities, with internal ribs, bosses, and thin walls, or any other complex geometry."

Schweitz says that very few manufacturing engineers and industrial designers are familiar with the RIM process, so education about its versatility and cost effectiveness often takes place initially. "We can help industrial designers design very appealing products with interesting and unique features because RIM is a very versatile process and the tooling is inexpensive," Schweitz continues. "In fact, we've helped several designers get styling and design awards for their products with the RIM process."

Much of Premold's work is for medical and lab equipment, electronics instruments, recreation and leisure products, and custom plumbing hardware. The company opened its doors in 1980, and now operates out of a new 20,000-sq-ft facility with its own tooling department and 20 full-time staff members. Unlike many RIM processors, Premold works with a range of part sizes, and routinely does shot sizes from several ounces to 25 pounds. The firm also recently took delivery of a new resin dispensing unit, which gives it the ability to inject color pigments into materials.

Designed specifically for reaction injection molding, RIM equipment operates differently from standard injection molding equipment. Some of Premold's machines are automatic, CNC equipment, while others are operated manually. With this arrangement of equipment and human power, Premold is able to hold tolerances of +/- 0.005 inch on most of the work it performs. On large parts that are several feet long, the company reportedly can hold +/- 0.002 inch per inch of length. By achieving close tolerances, the molder can produce most parts in a net-shape configuration that is ready for assembly, unless the parts need secondary operations.

Finishing work includes the trimming of flow gates and vents from the part after molding. In addition, Premold performs light sanding, painting, EMI coating, and silk screening for labels or logos. It can also procure extra parts, perform assembly, and deliver a completely finished assembly to an OEM.

RIM Pivot Joint Eliminates Assembly

One especially interesting RIM demonstration project-for which Premold has a patent pending with the U.S. Patent Office-has been in the development process for the past year. The company is using the patent-pending method to mold a complete, fully articulated pivot joint in one molding cycle. According to Schweitz, a complete pivot joint has never been molded in plastic by any process without several molding steps.

With the low temperatures and pressures of RIM, all of the pivot joint pieces can be produced in one cycle of the molding machine. To do this, a plastic bushing is inserted into the mold cavity and then polyurethane is molded around it. In addition, the pivot joint can be tooled up for about $3,000 or $4,000, whereas hard steel tooling would range from about $6,000 to $12,000.

Schweitz says that the pivot joint concept could be applied to many different applications, with an unlimited number of configurations. The patent could be used for any folding or pivoting type of application. The same concept could be used for molding doors within their frames, for joints on workholding trays for dental uses, and even for action figure toys that have moving joints for arms and legs.

"Using our joint eliminates the need for any additional assembly on any application," Schweitz reveals. "It could be used for parts that are now metal, that need bushings, pins, retainer clips, and sleeves made of metal that will corrode. This joint needs no lubrication or adjustments since parts mesh tightly; it's totally sealed to keep out contaminants, and it's lighter than a comparable metal component," he adds. "These benefits also reduce labor costs significantly. We can produce a complete component with all of these parts in plastic, in a variety of shapes and configurations-many more than with a metal joint."

The method can be used to mold hinges, wheels, retractable arms, holders of all types, and virtually any other object used in a folding or pivoting application. It could also be used to mold interconnected joints or a series of joints, including joints that have different axes of rotation relative to one another.

Using Liquid instead of Heated Plastics

In standard injection molding, the machine first melts the plastic before injecting it into the mold under high pressure; the part must then cool into a solid before being ejected from the machine. Acrylonitrile-butadiene-styrene (ABS) and polypropylene are common plastic materials that are heated in high-pressure molding machines before injection. In RIM, the hot plastics are substituted with a chemical mixture of liquid isocyanate and polyol resin, which combine to form polyurethane. The two liquids are mixed just before being injected into the mold. The polyol determines the physical characteristics-including density, impact strength, flex modulus, and color-of the molded part. The isocyanate reacts with the polyol to form a thermoset polyurethane plastic. Specific gravity and mix ratios are controlled during molding.

Low-cost tooling is the primary benefit of RIM, especially for start-up companies and those that only need a small quantity of parts for a particular product line. Tooling costs are lower because machine pressure is much lower (about 25 psi to 50 psi) compared to high-pressure molding at several thousand psi, and because molds are only heated to 170°F, instead of 200°F or higher. Premold can make tooling from several different materials that offer different price ranges for clients. The company can also build bridge tooling when a product starts out with short runs and then grows into larger production runs.

"The RIM process allows us to do large and small parts with equal ease," Schweitz explains. "We can do large parts that are six feet long, four feet wide, and four feet tall, with a special holding clamp-five feet by five feet in size-that we use on the molding machine. The process lends itself to reduced setup time and costs, and consequently just-in-time delivery is made easier. We can machine tooling out of hard epoxy, which is still very accurate tooling since it comes from CAD models, or we can use different grades of aluminum for moldmaking. We can use these softer materials and still get precision molds because of the low heat and low pressure."

One disadvantage of the RIM process would be that per part costs are more expensive. However, on small runs of a few hundred parts, the lower cost of tooling far outweighs the per-part cost. If a customer requires several thousand parts per month, it is less costly per part to use expensive, hard steel tooling with high-pressure molding. Another downside to the RIM process is that it's limited to using one material (polyurethane), whereas many different plastic materials can be used with high-pressure molding. However, polyurethanes are available with different material properties, Schweitz adds.

Creative Molding

Premold takes great pride in its ability to solve problems for clients. One customer that manufactures custom plumbing products was working on a pump housing for an aircraft application. Initially, the company was casting a metal housing for the pump and then performing several machining operations on it. Bearings, shafts, and seals then had to be installed. But the main housing was too expensive to make out of metal, and too heavy for an aircraft application. The customer was also having problems with corrosion.

Premold personnel sat down with the client's design engineers, who were not very familiar with the RIM process, to help them work out design details. They also worked with the client's manufacturing engineers to come up with the best tooling and operational options. The parts that Premold helped them design ended up with complex geometry, but tooling cost was still kept down to several thousand dollars.

"We were able to use a creative mold approach to mold the housing into a net shape out of polyurethane, and thus eliminate all of the machining," Schweitz affirmed. "Some of the boring had to be +/-0.002 inch to fit in a bearing, which our mold came within. And we were able to it all in one molding step, whereas it took them several steps for the casting and several machining steps."

The weight of the new plastic part was less and the corrosion problems ceased. The part was also able to meet the FAA's stringent flammability ratings. "One of their initial worries was strength, but they found that the plastic part met their requirements," says Schweitz. "And most importantly, we were able to save them 33% of the cost of the metal version, reduce the weight in half, and cut out 30 to 60 minutes of machining time. These efficiencies allowed them to get the parts into assembly much more quickly, maybe a whole day for each production lot, which also saved them time and money."

In another instance, Premold worked with a customer that wanted to build, in one piece, a large cover for a piece of dental equipment. With vacuum or pressure forming, the part would have had to be produced in three pieces. Moreover, the designer wasn't pleased with the original design: The customer would have had to put attachments into the mold, thereby changing the design substantially.

"We were able to save them money by doing it in a one-step mold, and were able to keep their original design with no modifications at all," Schweitz commented. "They even ended up receiving design awards for the part. Likewise, we were able to hold unequal wall thicknesses within the part, and our part eliminated a lot of assembly work, since parts had to be glued on the vacuum-formed cover."

Elimination of the assembly work saved the OEM $50 to $100 per part, and it was stronger without the glued-on parts. "Premold's tooling cost was about $45,000, yet it would have cost about $225,000 to $250,000 to tool up for the other processes," Schweitz summarized.

In addition to producing low-volume, custom molded polyurethanes with reaction injection molding, Premold offers custom painting, EMI shielding, and secondary machining. Design assistance and mold design and build services are also available.

For more information, contact Premold Corp.

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