Using RIM Molding for Complex Parts


If you are a student or otherwise new to the processing and manufacturing world, you might be wondering what RIM molding is and what the differences between RIM molding and injection molding are, or even structural foam molding vs injection molding.


Considering plastic generation in the United States topped 34 million tons for items such as insert molding magnet in 2015, there is good reason to want to understand the world of encapsulation and other molding procedures.


RIM stands for Reaction Injection Molding. RIM molding can be used to create all sorts of plastic parts from insert molding magnets and other plastic goods. RIM and injection molding are very similar, so there are even times when experienced engineers and designers get them mixed up. However, the difference is in the “reaction.” RIM uses low-viscosity liquid polymers in a thermoset (not thermoplastic) process. Using chemical reactions, the polymers expand, thicken, and harden after they are injected into a mold. This allows more intricate designs and complex features.


Some Advantages of RIM Molding


Creating Large Parts: When creating large parts, RIM offers two major advantages: low viscosity of component chemicals and strength of polyurethane. This allows for both structural and aesthetic advantages in the finished product. Low viscosity allows large molds to be filled quickly and completely, making large molds as a single piece, rather than requiring multiple molds to be assembled. This also gives the ability to include fine details in the mold.


Very High Quality Finish to Surface of Plastic: Because the RIM liquid component chemicals have such a low viscosity, fine details can be accurately and consistently applied to the molding. One of the reasons manufacturers love this is because it saves time and money in finishing the product. Additionally, it gives freedom to the designers to create unique textures for their parts. RIM parts are also excellent at mixing in paint or other coatings to apply a specific color finish. In the automotive industry, for example, RIM parts are used because of their ability to match painted mental parts perfectly, while having the flexibility and impact resistance afforded to RIM molded parts.


Part Encapsulation: This is especially important for insert molding magnet and other similar items. When using an insert molding magnet, the magnet often needs to be encapsulated within a mold. RIM is used extensively for encapsulation of other materials and parts because it enhances the strength of the RIM part and protects the item being encapsulated. Additionally, because RIM polyurethane is highly adhesive, it allows complex parts to be encapsulated without deforming, separating, or damaging components. It is common to embed metal, carbon fibers, and even glass fibers using RIM molding techniques. This can add strength and infrastructural integrity to parts.


Wall Thickness Variations: RIM molding allows for easy variable thickness of walls. Thermoforming and metal don’t even compete with RIM in this category as it is very difficult to achieve variable wall thickness using these methods. Polyurethane remains in a liquid state as it fills a RIM mold, which allows a 1/8 inch wall cavity to be filled just as easily as a two-inch wall cavity. This is especially important for complex geometric shapes and structural features. Curves, raised surface details, ribs, and wall thickness all add variable challenges that RIM molding handles perfectly


Lower Costs for Tooling: RIM Molding can even save significant money. The cost of tooling is much cheaper for RIM molds than it is for thermoplastic injection or structural foam molding (because the latter require very high temperatures and pressures to create parts). RIM molding, on the other hand, requires low temperatures and an insignificant amount of pressure, which lowers the cost of mold materials, requires smaller presses, and less energy consumed. This is also great for environmental reasons as there is less energy waste in creating a product. Because the tooling costs are lower, turnaround is usually much shorter and the molds can be more compact.


While it is important to be familiar with many different types of molding, RIM molding holds many key advantages over counterparts. Being familiar with the advantages can help you design the best molds and save you money.