Reaction Injection Molding: Processes, Materials, and Applications

What is Reaction Injection Molding (RIM)?

Reaction Injection Molding Process

RIM molding is a low-pressure plastic molding process. It uses two storage tanks for the storage of low-viscosity polymers. Then, a pressurized pump is used to transfer a mixture of the two polymer liquids into a mold with a predetermined cavity shape. The mixing of the two polymers inside the injection mold causes a chemical reaction that generates heat and forms thicker components.

Reaction Injection Molds & Components

The mixing of polymer liquids is critical to the quality of the final part. As a result, the mixing chamber operates at high pressure. At the same time, some additives, such as fiber reinforcements, are added according to the structural requirements. In addition, the low injection pressure ( 0.5~10 MPa pressure helps to foam during curing.

How is RIM molding different from standard injection molding?

Reaction injection molded parts

First, the RIM molding process involves two or more reactive liquid materials, while standard injection molding involves injecting the material in a molten form without any mixing or similar mechanisms. Second, polymerization can produce closed-cell foam-structured items with unique properties. Thirdly, the injection pressure of injection molding can be as high as 100MPa. The pressure of reaction injection molding does not exceed 10 Mpa.

RIM molding process

Steps in the RIM process

1.Reactant preparation

First, the liquid polymer (usually polyols and isocyanates) is stored in two separate tanks to avoid premature reactions. The main concern of this step is to maintain the temperature and pressure of the liquid inside the tank. Temperature fluctuations can affect viscosity and thus the molding process.

2.Homogenize and mi

Material mixing in the RIM process

The next step is to prepare a homogeneous mixture so that the parts are consistent for each production cycle. It involves pumping the liquid from two tanks into a high-pressure mixing head chamber. In addition, a metering system connected to this chamber exists to maintain a definite ratio of the two liquids.

3.Injection and polymerization

Next, the mixed reactants are injected into the mold cavity. The mixture then fills the entire mold and captures all the intricate details of the mold design. At the same time, the reactants undergo a chemical reaction inside the mold, which is called the polymerization process. Polymerization causes the material to swell and solidify.

4.vulcanization

Mold curing is critical to achieving full polymerization and final performance. The curing time depends on the injection molding material used and the desired properties of the final product. The state-of-the-art RIM system uses sensors and feedback loops to monitor the curing process in real-time, so operators can make adjustments as needed to maintain optimal conditions.

5.Release

Once cured, the mold is opened and gently removed using a thimble or automated system. Subsequently, the ejected parts are subjected to post-processing such as trimming, grinding or polishing to achieve the required tolerances and finished product quality.

Reaction injection molding materials

RIM primarily utilizes polyols and isocyanate mixtures, known as polyurethanes. Its excellent flexibility, impact resistance, and durability have made it popular. In addition, it can be formulated to be rigid or flexible.Other materials and their respective properties include;

• Epoxy resins: high mechanical strength, excellent adhesion, good chemical resistance.
• Silicone materials: biocompatible, heat resistant and flexible.
• Polyurea: Fast curing time, strong abrasion and chemical resistance.
• Nylon: High tensile strength, excellent abrasion resistance, good chemical stability.
• Composites: Reinforced materials, such as glass or carbon fiber, to increase strength and stiffness.