Rimnetics: Polyurethane Reaction Injection Molding & Rapid Prototypes

RIM Materials

Design Release

RIM Systems

Design Guide

Chemical Resistance

CAD Technology

Part Design Guide

Introduction
The Design Guide provides basic design criteria for producing a quality Reaction Injection Molded (RIM) polyurethane structural foam part at the lowest molding cost.

Material Densities
RIM polyurethane structural foam densities can be altered from (.6) to (.9) specific gravity. Typical parts are molded to (.85) specific gravity which provides best physical properties. Insert pull out strength decreases directly with reduction in density.

Adhesive Bonding
For bonding RIM polyurethane parts, epoxy or cyanoacrylate adhesives work well. The bonding area of a lap joint should be roughly three times the wall thickness.

Tolerances:
Part: First 12" +/- .020 thereafter .002 in./in.
Inserts: First 12" +/- .010 thereafter .001 in./in.
Flatness: First 12" +/- .015 thereafter . 0015 in./in.

Video Switcher control
panel manufactured by Rimnetics.

The RIM Process
RIM polyurethane is a material with an integral, low density, cellular core, and solid, high density skin. RIM structural foam has a high strength to weight ratio and excellent chemical resistance.

Versatility of RIM
RIM is low pressure (about 60 psi), and temperature (about 150^o F), permitting a lower tooling cost and shorter lead time than most other molding processes. A variety of shapes can be achieved without “molded in” stresses. Rapid thickness variations are accommodated without “sink marks.” Substrates of wood, steel and other materials can be encapsulated for fillers or for increased strength. Rimnetics, Inc.'s approximate part size capacity is from one (1) square foot surface area to a four (4) foot x six (6) foot area with depths to twenty (20) inches.

Material Properties
Available RIM material systems are varied and range from flexible, to rigid, to structural composite. It is important to match physical properties with the needs of the specific application. In general, RIM best serves the larger, thick-walled (1/4”+), structural parts. Please refer to the Typical Properties Data Sheet Supplement covering the rigid system used by Rimnetics, Inc. for enclosures.

Wall Thickness Design
RIM wall thickness can comfortably vary from 1/4” to 1/2” without unduly altering molding cost. Wall thickness above 1/2”, and up to 1”, are accommodated by longer molding times. When thicker wall sections are desired, “encapsulation” should be considered to control molding time and material usage. Wall thickness can be reduced to a minimum of 1/8” in local areas.

Reinforcement Ribs can and should be used as part stiffeners.

Image of approved rib configuration. Tall and Narrow vs. Short and Wide: Tall and narrow rib designs are stronger and have less impact on molding time than short and wide designs

Figure A: Rib Configuration...Tall and Narrow vs. Short and Wide

Rib design, however, should be controlled to avoid increasing molding time. Tall and narrow rib designs are stronger and have less impact on molding time than short and wide designs. (see Figure A). Rib design should be in the direction of material flow. (see Figure B).

Image of approved location of ribs to guide direction of material flow in reaction injection molding

Figure B: Location of Ribs...in Direction of Material Flow

Draft Angles
Draft of 2^o is preferred for best molding practice. Draft angles of less than 1^o per side should be avoided. This is especially important on interior walls. If the part design demands 0^o draft on one side only, this may be accomplished by providing extra draft on the opposite wall without significant impact on molding time. Please consult Rimnetics, Inc. for design guidance.

Radii
In the RIM process, the material can be thought of as water flowing through the mold. Smoother contours reduce material flow turbulence, resulting in stronger parts and better mold fill. For best molding practice, interior radii should not be less than 1/8”, although smaller radii can be accommodated. Exterior radii, other than a parting line, should not be less than 1/16”.

Bosses and Inserts
For best molding practice, bosses should have 2^o+ draft and located along part perimeter or within ribs. If design criteria requires free standing bosses exceeding 1/4” in height, down stream gussets must be incorporated. (see Figure C). Molded in pilot holes accurately locate threaded inserts or self tapping screws. Proper boss diameter and pilot hold size are important for achieving highest pull out strength. Boss diameter, molded in pilot hole size and threaded insert pull out strength values are provided in Rimnetics, Inc.'s Part Design Guide Supplement.

Image of approved bosses location. For best molding practice, bosses should have 2o+ draft and located along part perimeter or within ribs. If design criteria requires free standing bosses exceeding 1/4
Figure C: Boss Location...along part perimeter, within rib, free standing.

A self tapping screw's (often used in one time applications) pilot hole diameter and depth varies with the chosen fastener. Please consult with Rimnetics.