Reinforced reaction injection molding (RRIM) is an alternative to the standard RIM process used to create lightweight, durable parts in larger sizes than ever before. It has made great strides within the automotive and transportation industries, allowing polyurethane injection molding to be used to make body panels, fascia, bumpers, spoilers, floor panels and more.
RRIM differentiates itself from standard reaction injection molding in that fibers, usually glass or carbon, are added to the resin during the molding process. The fibers reinforce the thermoset polymer and create a stronger, more impact resistant overall product.
Recently, long fiber injection molding (LFI) has emerged as a competitor to RRIM in terms of creating strong, lightweight, durable materials. However, RRIM is still a cost-efficient process which is extremely useful in the production of large parts for automobiles and heavy machinery.
The Reinforced Reaction Injection Molding Process
Reinforced reaction injection molding is a two-step process similar to its non-reinforced variation. It utilizes a closed mold, often created from a lightweight, low-cost material such as aluminum. Low pressure can be used, but high temperatures (often between 300 and 350 degrees Fahrenheit / 149 and 177 degrees Celsius) are required due to the presence of the reinforcing fibers.
As with standard RIM, the RRIM process begins with two components, polyol and isocyanate, stored in separate containers in liquid form. Both are injected into a mold, where they are mixed using a combination of low pressure and high velocity and then allowed to cure and form a solid thermoset polymer.
However, where RRIM differs from RIM is in the addition of reinforcing fibers to the polyol before its injection into the mold. Traditionally, glass fibers are used, however, recently, some RRIM processes have begun to use carbon fiber instead. Carbon fiber is both stronger and more lightweight than glass, and many believe that it may represent RRIM. However, glass is still a cost-effective option that is extremely viable especially in larger production runs.
The fibers which are used during the RRIM process are either milled or chopped using a specialized machine before they are added to the polyol resin. The standard length of fibers utilized in RRIM-produced parts is extremely short, between 0.009 in and 0.02 in (0.2 and 0.5mm). In addition, some RRIM manufacturers choose to use glass flake rather than the standard chopped or milled fibers.
Only certain resins are compatible with the RRIM process. The most popular are epoxy, polyester, nylon, and polyurethane, with polyurethane emerging as the most widely used RRIM resin in recent years. Typically, thermoset polyurethane in an elastomeric variety is used rather than thermoplastic due to its shorter cure time and minimal need for post-mold processing.
Benefits of Reinforced Reaction Injection Molding
Parts produced using reinforced reaction injection molding feature all the benefits associated with the non-reinforced RIM process. These include light weight, flexibility, durability and toughness. In addition, RRIM parts boast superior strength and impact resistance when compared to products manufactured via the standard process.
The combination of strength and light weight makes RRIM uniquely suited to producing large parts such as body and floor panels. RRIM-produced body panels are long-lasting and highly resistant to wear, deformation, and impact.
In addition, RRIM boasts a number of benefits uniquely provided by the presence of the reinforcing fibers, which cannot be achieved via other, similar processes. Parts manufactured using RRIM experience far less shrinkage when removed from the mold, especially when compared to more traditional processes such as compression molding. The reinforcing fibers also decrease thermal expansion as well as the droop and sag that plastics frequently suffer at extremely high or extremely low temperatures.
Reinforced reaction injection molding also provides a number of aesthetic advantages similar to other varieties of RIM. In-mold painting is possible, significantly decreasing the time and labor requirements necessitated by a lengthy post-molding painting process. Both glass and carbon fiber reinforced products are capable of featuring high gloss Class A finishes directly out of the mold, further increasing the time- and labor-efficiency of the process.
Reinforced Reaction Injection Molding vs. Long Fiber Injection Molding
Recently, long fiber injection molding (LFI) has emerged as a competitor to RRIM. LFI is a one-step process, often surpassing even the highly praised quick cycle times and rapid curing of RRIM. In addition, the longer fibers (traditionally between 0.5 inch to 4 inches, or 12.5mm to 100mm) provide an ever greater strength and durability without sacrificing weight and flexibility.
When choosing a manufacturing method for your next product, it is important to carefully consider the advantages and drawbacks of both reinforced reaction injection molding and long fiber injection molding. Contact us today for more information about which of these processes might be right for you!