Volumes of production
Suited to high-volume production, however, because of the potential to use inexpensive, low-strength molds, low-volume production is also a realistic possibility.
Unit price vs. capital investment
This is a low-pressure process with low tooling costs compared to those of standard injection molding (see p.196). However, setup costs are high, so large numbers of units need to be produced in order to be economical.
Speed
This is not a rapid process, unlike standard injection molding. Cycle times are considerably longer and, depending on the size and complexity of the part, can take several minutes, as opposed to seconds, per part.
Surface
The foams produced using this process are “self-skinning” and sometimes form a hard skin similar in quality to that formed in standard injection molding, while retaining the foam core.
Types/complexity of shape
Large and complex solid shapes are possible, with the potential to create varying wall thicknesses in the same component. Typical wall thicknesses of RIM parts are a chunky 3/8 inch.
Scale
Suited to large-scale components up to 6½ feet long.
Tolerances
High tolerances.
Relevant materials
RIM is often used to form dense polyurethane foams. Other common materials include phenolics, nylon 6, polyester, and epoxies.
Typical products
Large foam moldings, rigid and flexible alike, are manufactured using RIM for use in products such as car bumpers and trim, industrial pallets, casings for large-scale electronics, and refrigerator door panels.
Similar methods
Injection molding (p.196) and transfer molding (p.176). Also, gas-assisted injection molding (p.201), which allows for complex, large lightweight parts, although it is not suited to foams.
Sustainability issues
Material use is considerably lower than with conventional injection molding as the expansive nature of the foam produces a predominantly hollow structure that better resists shrinkage while maintaining excellent strength. In addition, the heating temperatures are considerably lower, which results in reduced energy consumption and emissions. One disadvantage is that the slower cycle times mean energy use is less efficient than it is with conventional injection molding. Materials, however, are not recyclable.
Further information
www.pmahome.org
www.rimmolding.com
www.plasticparts.org
Gas-Assisted Injection Molding
Product |
Air-Chair |
Designer |
Jasper Morrison |
Materials |
polypropylene, with glass-fiber reinforcement |
Manufacturer |
Magis |
Country |
Italy |
Date |
1999 |
This stackable chair, while sturdy enough to withstand considerable bulk, is lightweight, hollow, and economical, all of which are the advantages of using gas-assisted injection molding.
In standard injection molding, thermoplastics are heated and injected into a mold (see p.196). Channels in the mold act to cool the plastic part before it is released from the mold. During cooling, the part shrinks and moves away from the walls of the mold and, to compensate for this, more material is injected into the mold.
An alternative to this widely used method is to inject gas, usually nitrogen, into the mold cavity while the plastic is still in its molten state. This internal force counteracts the shrinkage by inflating the component, forcing it to remain in contact with the surface of the mold until it solidifies, resulting in parts with hollow sections or cavities.
There are two types of gas-assisted injection molding: internal and external molding. The former is the most widely used, with the external method being used when greater definition in the part, or a larger surface area, is required. This is achieved by injecting a very thin layer of gas between one surface of the plastic and its adjoining mold cavity.
Exploiting the reduction in weight provided by this process, the Italian manufacturer Magis has produced a range of furniture that redefines the rules for designing large-scale plastic products. The ubiquitous low-grade garden chairs, produced by standard injection molding, that you find in your local hardware store are made with a thin cross-section and have a strong, stable structure. By contrast, the Magis range, designed by Jasper Morrison, appears to be solid, but the inside is hollow.
– Allows for components to be made with variable wall thicknesses.
– Reduced cycle times.
– Reduced weight.
– Less sink marking than in conventional injection molding (see p.196).
– Consumes 15 percent less energy than standard injection molding.
– Because of the extra parameters involved—handling of gas, regulation of pressure, and cooling—potential problems need to be addressed in advance, requires experience, and often, a fairly complicated setup.