The RIM process utilizes very low viscosity liquids ranging from 500 to 1500 mPa s, low processing temperatures of 30 to 40 °C, low mold temperatures of 30 to 40 °C and low internal molding pressures between 3 and 10 bar. The low viscosity, low temperatures and low pressures provide some very distinct benefits or advantages for the RIM process compared with other plastic processing methods.
1. Very Large Parts
The flowability of the liquid polyurethane components enables them to fill molds for very large parts. The rigid structural foam roof for a Deere & Company combine measures 1550 mm by 1700 mm and weighs approximately 250 N. The size of the part that can be molded depends on the speed of the reactivity profile of the polyurethane formulation and the throughput of the metering unit - the pounds per minute that can be dispensed.
2. Encapsulation of Inserts
Inserts of many types can be placed into a mold prior to injection of the RIM material. And the RIM material can encapsulate many inserts during molding. In one example, an aluminum frame is placed into the mold and an elastomeric RIM system is injected and encapsulates the frame to form the decking for a snowshoe. Steel, aluminum shapes and frames, window glass, glass preforms, electronic sensors, PC boards and wiring harnesses are some examples of materials that have been encapsulated using the RIM process.
3. Thick and Thin Walls
Producing variable wall sections within the same molded part are a definite problem with many plastic processing methods and materials, such as thermoplastic injection molding, blow molding, sheet molding compound (SMC) and other polymers. But the RIM process offers you the flexibility to design parts with significant wall thickness variations. Wall thickness ranges between 6 mm and 30 mm are possible cross-sections in the same molded part.
4. Class-A Surfaces
The surface finish of parts molded with the RIM process allows manufacturers to produce Class A painted parts. For example, automotive manufacturers are able to produces fenders, spoilers and fascia parts that can match the high-gloss painted metal parts they are mounted next to in the final assembly.
5. In-mold Decorating
With the RIM process, it's possible to apply gel-coats and two-component polyurethane in-mold paints into the mold prior to injection. The injected polyurethane material bonds to the gel-coat or paint during molding, allowing a decorated part to be produced in the mold. This can greatly reduce secondary finishing costs. An extension of this technique that is under development allows the molder to use pre-decorated films, vinyl or fabric preforms as the decorating material during molding.
6. Low Cost Tooling Options
The low injection pressures of the RIM process allow mold builders to use a variety of less expensive mold materials other than steel. Alternate materials range from machined or cast aluminum, to cast Kirksite, nickel shell and even some plastic composite materials.
These six process advantages give engineers and designers tremendous freedom to develop creative styling, including designs not possible with other manufacturing methods. A survey conducted by Bayer Polyurethanes found that the most successful applications of the polyurethane RIM process incorporate one or more of these six advantages.
In general, the RIM process uses less energy to make the same product than injection-molded thermoplastics and often requires less equipment and floor space. RIM processing also is more automated than other thermoset molding alternatives.
In addition to high strength and low weight, polyurethane RIM parts exhibit heat resistance, thermal insulation, dimensional stability and a high level of dynamic properties. They also offer resistance to organic and inorganic acids, as well as many other potentially damaging materials and chemicals - including a large number of solvents. Polyurethane RIM parts also exhibit resistance to weathering and aging, though extended exposure to the sun's ultraviolet rays typically results in a color shift on the surface of the part.