Rimnetics: Polyurethane Reaction Injection Molding & Rapid Prototypes

RIM Works Well For Enclosures

Reaction injection molding (RIM) is an excellent choice for moderate-volume production of medium-to-large structural parts. Designers of equipment enclosures find RIM particularly useful for developing strong, lightweight, complex polyurethane parts that are durable, chemical- and impact-resistant, and non-conductive. When you compare sheet-metal enclosures with RIM enclosures, the latter are most cost-effective for parts approaching 2ft² and larger, for annual production volumes of 250-2,000 units, and for designs having complicated contours. Complex design details that are very costly in sheet metal are easily achieved in molding. For new products, sheet metal may be specified for an enclosure to avoid the tooling costs of plastics molding processes—even when the piece-part price is considerably higher for sheet metal. As production volume increases, however, RIM often becomes more competitive economically.

How RIM works
In RIM, two reactive liquid components—a polyol resin blend and an isocyanate—are metered, mixed, and injected into a closed mold at low pressure. The liquid reactants are fed from separate, temperature-controlled tanks, through supply lines to metering units, and then to the mixing head, as shown in Figure 1. When the shot is made, valves in the mixing head open and the reactants (at pressure of 1,500 - 3,000 psi) enter a chamber where they are mixed by high-velocity impingement.

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They then flow into the mold at approximately atmospheric pressure, where they react to form a polyurethane structural-foam part having a high-density, durable skin, and low-density cellular core. After the shot, a piston cleans the mixing chamber and seals the mold. The mixing head is attached directly to the mold so all the reacted material can be demolded with the part, allowing for the next shot with minimum preparation. The mixing head also is self-cleaning, so solvent-flushing and its associated environmental and disposal problems are avoided.

Advantages of RIM
Because RIM materials start as low-viscosity liquids, the process can fill complex molds and dependably produce parts—even of unusual thickness or varying cross-sections—without inducing the flow-generated internal stresses that result in sink marks or read-through of reinforcing ribs. Part shrinkage is minimized by low mold temperatures of 130-160 ^oF, and internal mold pressures of less than 100 psi allow use of inexpensive, light-weight molds. Mold cost savings increase with increases in part size. RIM also saves time. Lead times average approximately one-half to two-thirds those for injection molding.

RIM success story

The advantages of using RIM for enclosure designs are demonstrated by the successful evolution of the NS 6000 UNIX Computer Network Server by Auspex Systems (Santa Clara, California), shown here.

Image: Computer equipment molded by Rimnetics Inc., using reaction injection molding for Auspex Systems.

Converting to a molding for the 76 X 24 X 39 inch UNIX server's two front doors made economic sense, in light of increased production volume. Anticipated demand for the product, however, did not support use of the expensive tooling required to injection mold the enclosure. Neither was continuing to use sheet metal acceptable, due to the material's excessive weight and the cost of required assembly and service operations. Also, metal's aesthetics did not meet the company's objectives for the unit's feel and appearance.

The unit's enclosure's size, UNIX's production volume, and a reasonable delivery time of 10-12 weeks all made the RIM process particularly suitable for the NS 6000. The unit is now enclosed with two mated, reaction injected-molded doors. The doors include a molded-in, stepped design (an enlarged version of the company's logo); perforated sheet is bonded into windows after molding to produce vents.

Using RIM allowed the OEM to reduce enclosure costs by 30% compared with the sheet-metal version, and to recover its tooling investment in less than a year.

By A. Banks