Ray Krieger
Rimnetics Inc.
Mountain View, Calif.
Polyurethane structural foam has long been used to form enclosures. But this versatile substance is now serving in a number of innovative applications that would be impractical or too costly if more traditional engineering materials were employed.
The technique used to form polyurethane foam is reaction-injection molding, a process in which two or more liquid-chemical components from separate tanks are metered through high pressure supply lines to a chamber. They mix in the chamber via high-velocity impingement. The resulting mixture is then injected into a mold at low pressure, about 60 psi, and at low temperature, about 150°F. This leads to a lower tooling cost and less lead time than for most other molding processes.
The process is similar in concept to an ordinary two-part epoxy mix. RIM reactants become a low-viscosity, exothermic, expanding material which readily flows into a mold. The mold-clamping machine typically is equipped with electronics that turn the mold, so that material more easily flows into all cavities. When a successful series of movements are perfected, they are stored as a program to ensure uniform molding of the part.
As the mold fills, the reactants are still releasing heat while tiny bubbles swell the plastic into the finest details of the mold. Cooling coils built into the mold transfer heat to maintain an ideal working temperature. The reactants quickly harden and parts can be removed in a few minutes.
RIM polyurethane has an integral, low density, cellular core and a solid, high-density skin. RIM structural foam has a high strength-to-weight ratio and excellent chemical resistance. A variety of shapes can be molded this way without molded-in stresses.
The process accommodates rapid thickness variations, without sink marks. Substrates of wood, steel, or other materials can be encapsulated for fillers or for increased strength. The part sizes that can be handled with RIM polyurethane depend on the clamping capacity of the molder, but typical dimensions are from 1 to 4 × 6 ft2 surface area with depths to 20 in. Typical part tolerances are 0.002 in./in., with insert and flatness tolerances at 0.001 and 0.0015 in./in. respectively.
Available RIM material systems are varied and range from flexible to rigid to structural composite. In general, RIM best serves in larger, thick-walled (0.25 in. or more) structural parts.
When designing a polyurethane foam part for lowest molding cost, a few simple guidelines are in order. RIM wall thickness can comfortably vary from 0.25 to 0.5 in. without unduly altering molding cost. Wall thickness above 0.5 up to 1.0 in. are handled through longer molding times.
When designs call for thicker wall sections, encapsulation should be considered to minimize molding time and material usage. Wall thickness can drop to a minimum of 0.125 in. in local areas.
Ribs can and should be used as part stiffeners. Rib design, though, must be handled carefully to avoid boosting mold time. Rib designs that are tall and narrow are stronger and have less impact on molding time than short and wide features. Ribs should also be designed so they are in the direction of the material flow.
A draft of 2° is preferred for best molding practice. Angles below 1° per side should be avoided. This is especially important on interior walls. It is possible to handle part designs that demand 0° draft on one side only by providing extra draft on the opposite wall. This will not significantly boost molding time.
In RIM, material can be visualized as water flowing through the mold. Smoother contours cut material flow turbulence, resulting in stronger parts and better mold fill. Best molding practice calls for interior radii not less than 0.125 in., although it is possible to handle smaller radii. Exterior radii, other than a parting line, should be no less than 0.0625 in.
Best molding practice also dictates that bosses have 2° plus draft and sit along part perimeters or within ribs. If design criteria requires free standing bosses higher than 0.25 in., the part must incorporate downstream gussets.
Molded-in pilot holes accurately locate threaded inserts or self-tapping screws. Proper boss diameter and pilot hole size are important for getting highest pull-out strength. Boss diameter, molded-in pilot hole size and threaded insert pull-out strength values are generally available from the molder. Similarly, a self-tapping screw’s pilot hole diameter and depth varies with the fastener chosen. The molder can generally provide guidance on this issue as well.
RIM polyurethane structural foam density can be altered from 0.6 to 0.9 specific gravity. A typical figure is 0.85 specific gravity which provides the best physical properties for enclosures. Insert pull-out strength decreases directly as density drops. Epoxy or cynoacrylate adhesives work well for bonding RIM polyurethane parts. The bonding area of a lap joint should be roughly three times the wall thickness.