Complete Composite SolutionsRequest Information

Structural Reaction Injection Molding

Structural reaction injection molding (SRIM) is one of the two most common variations on the reaction injection molding (RIM) process. While the other most common variant, reinforced reaction injection molding (RRIM) primarily focuses on increasing the strength and size of the resultant products, SRIM instead focuses on increasing the stiffness.

Like RRIM, SRIM reinforces the molded part via the use of glass or carbon fibers. However, while RRIM utilizes short milled or chopped fibers, SRIM relies on fiber mats, meshes and preforms, which are traditionally placed into the mold prior to the injection of resin.

Romeo RIM’s signature long fiber injection molding (LFI) technology is in many ways an improved, upgraded version of SRIM, as it provides similarly strong and stiff parts via a quicker and more efficient process. However, SRIM still sees wide use in the automotive, medical, aerospace and recreational equipment industries for the creation of stiff parts such as doors, panels and shelves.

The Structural Reaction Injection Molding Process

Like both standard reaction injection molding and its reinforced variant, structural reaction injection molding is a two-step process. Resin is stored in two separate containers, one for polyol and one for isocyanate. The two parts are then combined within an impinging mixer utilizing high velocity, heat and some pressure. Similar to RRIM, less pressure is needed due to the reliance on chemical reactions, but high temperatures (often between 300 and 350 degrees Fahrenheit / 149 and 177 degrees Celsius) are still required.

Once the mixing has completed, the combined resins are then injected into a closed mold. Because of the lower temperatures utilized in the reaction injection molding process, these molds can be crafted of lightweight, cost-effective but sturdy material such as aluminum.

The formerly liquid substance reacts and cures within the mold, emerging as a fully solid thermoset polymer. The process has an extremely short cycle time, with a finished part usually created every 3-5 minutes. It is also labor-efficient due to the high mechanization of each step of the process.

Unlike other variations of reaction injection molding, SRIM utilizes glass mats, preforms and fiber meshes placed into the mold prior to the injection of the resin. The injected resin saturates the fibers, creating a stiff composite.

When manufacturing products utilizing structural reaction injection molding, is important to use a low-viscosity resin. Low-viscosity materials flow better into the mold, ensuring that all of the fiber mat or mesh is properly covered and no sections remain dry. It is the interaction between the resin and fiber which produces the high strength and stiffness of the finished product, so it is important that a full saturation is achieved. For this reason, thermoset polyurethane has become the most widely used resin in the SRIM process.

Benefits of Structural Reaction Injection Molding

Products manufactured using structural reaction injection molding boast an unparalleled stiffness when compared with other molded parts. The average flexural modulus of a SRIM part is between 400 and 1500 ksi, while the tensile strength is between 7 and 18.5 ksi. In addition to this stiffness, the parts are tough, durable, strong, and highly impact resistant. This has led to SRIM being widely used within the automobile and aerospace industries as well as in the production of various types of recreational equipment and machinery.

The stiff parts produced by SRIM are also an excellent choice for anything which requires encapsulation. Excellent adhesion occurs between the fiber-reinforced polyurethane and the encapsulated materials. Even typically difficult to insert materials such as textiles can be successfully encapsulated utilizing the structural reaction injection molding process.

One common criticism of SRIM is its similarity to resin transfer molding (RTM), which also features a liquid resin saturating a skeleton of reinforcing fibers within the confines of a closed mold. However, SRIM boasts a number of advantages when compared to this more traditional process. Because it relies on chemical reactions rather than merely thermal influences to cure the final product, SRIM utilizes both lower heat and lower temperature than RTM. This, in addition to the lesser labor requirements, makes SRIM a significantly more cost-effective and time-effective process overall.

It is important to note that in-mold painting (IMP) is not possible with SRIM products, due to the necessity of laying the mat, mesh or preform into the mold at the beginning of the manufacturing process. Parts produced using SRIM typically require a post-mold paint job, and most often do not feature Class A finishes. Because of this, SRIM has seen use in the manufacture of parts such as dashboards and shelves where a high-quality finish is not always required.

Structural Reaction Injection Molding vs. Long Fiber Injection Molding


Long fiber injection molding (LFI) is a process frequently compared to structural reaction injection molding. Both types of molding involve utilizing fibers, most frequently glass, to create a reinforcing “skeleton” which stiffens and strengthens the finished product as well as increasing its impact resistance.

However, LFI has recently shown a number of benefits even when compared with the similar SRIM process. It completes the product in one step rather than the traditional two, as the long fibers are dispensed directly into the mix-head. Because Romeo RIM employs robots in its LFI process, it is also labor- and cost-effective even when contrasted with the already affordable RIM.

In addition, LFI technologies are compatible with in-mold painting (IMP) and can produce high- or low-gloss Class A finishes directly out of the mold. This further saves on time, eliminating the need for lengthy, messy (and potentially health-hazardous, depending on the paint) post-mold painting processes.

Structural reaction injection molding is a useful process which boasts a number of advantages for the creation of stiff parts which do not require a Class A finish. However, Romeo RIM’s long fiber injection molding technology is competitive with SRIM, providing similar benefits with additional perks such as lowered cost and labor needs. Contact Romeo RIM today to learn more about both SRIM and LFI and how one or both of these processes could be perfect for your next manufacturing project!

Romeorim design guide