Plastic Fabrication — Reaction Injection Molding (RIM)

May 5, 2024

Plastic Fabrication comes in various forms depending on the quantity needed. Reaction Injection Molding is a cross between between injection molding and compression molding. Its commonly used to manufacture medium to large quantities of plastic parts. Here is an article written by our engineering team that will provide you insightful information about RIM Molding (Reaction Injection Molding)

Do check out our previous articles on Additive manufacturing listed below for a better understanding of how the Additive manufacturing works if you’re looking for smaller quantity manufacturing instead.

Surface Finishing for Metal and 3D Printed Parts

3D Printing Technologies

Design for 3D Printing

Factorem’s guide to Additive Manufacturing plastics

Low quantity to medium quantity manufacturing

For smaller quantities of plastic manufacturing, parts may be CNC-ed, laser cut or 3D printed. But for larger quantities, one might consider injection molding, compression molding, transfer molding, extrusion, blow molding, rotational molding, calendaring laminating, reinforced molding and foam molding.

Unlike smaller quantity manufacturing, Reaction injection molding is for creating parts that are easily made using a mold. While much of the manufacturing cost for smaller parts are accounted by manual labor, machine tool cost and material costs, when moving to higher quantity production, majority of the initial costs comes from creating the mold for the part application. These molds are usually made with high precision and high wear life with its material varying depending on the injected mold mixture.

As these molds require high precision and fabricating mold cores using materials such as DAC10 to obtain the molds in the first place, the initial cost of these molds could cost anywhere from SGD$1000 to SGD$50000.

What is Reaction Injection Molding?

Injection molding often produces components that require little to no additional work after the part is manufactured. RIM is a subset of injection molding that uses chemically bonded plastics before they are injected into the mold. One of the differences between injection molding and reaction injection molding is plastic polymerization altering the properties of the base material.

Reaction injection molding is a plastic forming process which thermoplastic or thermoset plastic liquids are mixed to obtain a consistent mixture. This mixture is then injected into a mold to form the shape from the mold. This process is typically used for medical, industrial, and automotive industries to manufacture large parts in large quantities. The parts that are obtained from RIM are often strong and light weight parts which are easily paintable.

The RIM process is a repeated cycle which starts by dispensing high pressure polymer liquids from the storage tank into a multi-stream mix-head and back to the storage tank. After mixing, the liquid is dispensed into the tool cavity.

Overall process for Reaction Injection Molding
Overall process for Reaction Injection Molding

Simplified photo of Reaction Injection Molding
Simplified photo of Reaction Injection Molding

Physical properties

Some Physical properties that can be achieved using RIM includes the following:

  • High tensile strength
  • Heat resistant
  • Resistance to corrosion

By considering if your design would benefit from the above properties, one can better decide between regular Injection Molding, RIM or other plastic manufacturing processes.


RIM is similar to injection molding due to the similarities in process, except there are differences in the type of plastic usable for the process as well as the aforementioned additional properties achieved by using this manufacturing technique. There are some advantages for choosing RIM compared to Injection molding such as

  • RIM allows for lighter weight and more intricate designs compared to normal injection molding.
  • Larger parts
  • Encapsulated inserts
  • Thin and thick walls

Industrial Application

RIM is commonly used in the industry for the following reasons

  • Large components with variable wall thickness and smooth surface finishes
  • High turnover rate (Low-Mid volume projects)
  • Fast manufacturing process
  • Low-cost tooling manufacturing

While RIM shares lots of similarities with injection molding and compression molding, it can be thought of as injection molding which allows it to create large components with variable wall thickness and smooth surface finishes. The polymerization increases the strength of the resultant plastic which is similar to how compression molding increases the strength of the part, without the downside of removing the flesh from the mold. This process is used in various industrial applications such as in the automotive industry, aviation industry and even household appliances.


It may be difficult to identify which parts are made using RIM against . Some examples of what are commonly made with RIM include the following:

  • Car Tires
  • Steering wheels
  • Armrests on seats


Reaction Injection molding should be used for parts which require higher strength while requiring a fast turnover rates. The following is a simple overview of what was covered in this whitepaper.

Thank you for reading!