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Armstrong Rapid Manufacturing

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Applications For Reaction Molded Polyurethane Parts

Plastics Reaction Injection Molding

Reaction injection molded polyurethane parts are finding use in the medical diagnostic field for cabinetry, containers, keyboards and structural components.

Typical is the blood analysis apparatus market where short runs, low tooling costs and short tooling lead times are a necessity. Other advantages of this process are the ability to make product and tool changes and modifications with ease. The product has uniform shrink and dimensional stability. Molds can be built from machined aluminum, cast aluminum, kirksite, cast filled epoxy and cast polyurethane. Armstrong Rapid Manufacturing employs all of these mediums in its in-house tooling and pattern shop. Most of the parts are painted with the textured catalyzed polyurethane paint over a primer coat leading in popularity.

EMI shielding with a nickel acrylic paint on the inside of the part is frequently requested. The size of the parts can range from 2" x 2" x 6" to 50" diameter x 14" deep. Cabinets 56" long x 14" wide x 6" deep with molded-in louvers are possible. Part geometry and sizes made by this process are untenable or not economically feasible by most other processes. Wall thicknesses from 3/16" to 1/4" are recommended. Most common densities are from 38 to 40 pounds per cubic foot. This gives a uniform foam sandwiched within a sound skin on the outside surfaces. Large wall sections can intersect normal wall sections without causing sink marks or part stresses. Screw threads are attained by inserting brass screw inserts into molded holes after the part is molded. This allows for easy apparatus assembly with machine screws and allows for future apparatus maintenance.

Blood analysis apparatus frequently needs insulating properties to maintain blood or reagents at near 34°F temperatures. Here the RIM molded urethanes have wall thicknesses from 3/4" to 2" with product density ranging from 8 to 12 pounds per cubic foot.

Here again, uniform wall thickness is not necessary. All of these parts are weighed after molding to insure the required density, which is the key insulating factor ('K' factor). Here again paintable skins are possible. Also, metal inserts and structures can be encapsulated into the mold part.

Most customers demand Underwriters Lab's compliance. Another frequent requirement is UL94-VO or UL94-5V flame retardency. RIM molded elastomers and cushionable skinned parts are also sometimes requested.

The Process

Reaction Injection Molding (RIM) produces parts with intricate detail that are dimensionally stable, chemically resistant, physically tough, and wear resistant. This process is an excellent choice for larger plastic parts produced in short run or low volume production quantities.

The process takes its name from a chemical reaction that occurs within the tool. The plastics used are thermosets, either polyurethanes or foamed polyurethanes. The two components that produce the polyurethane are mixed just prior to injection into the tool. With the low viscosity and low injection pressures, large, complex parts can be produced economically in low quantities.

Considerable design freedom is possible, including thick and thin wall sections that are not good for injection molding, due to the uniform shrink characteristics. Foamed polyurethanes are natural thermal and acoustic insulators. Excellent flowability allows for the encapsulation of a variety of inserts.

Reaction injection molding is used in many industries for many types of parts. While bumpers for vehicles are produced in this process, most applications are for large, complex parts produced in quantities less than 5,000 units. Examples include panels for electrical equipment, enclosures for medical devices, and housings for computer and telecommunications equipment.

This technical information has been contributed by
Armstrong Rapid Manufacturing

Click on Company Name for a Detailed Profile

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