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Resin Transfer Molding (RTM)

If you need to manufacture large parts with a high strength to weight ratio, resin transfer molding (RTM) might just be the process you need. Resin transfer molding is a form of composite liquid molding in which resin is mixed with a catalyst or hardener before being injected into a closed mold filled with reinforcing fibers.

RTM’s strength, durability and high glass-to-resin ratio make it perfect for creating large, strong parts. In addition, RTM is also a viable option for molding complex shapes, up to and including compound curves.

The Resin Transfer Molding Process

Resin transfer molding is a closed-mold process utilizing matched male and female molds. Molds for RTM processes can be made out of a variety of materials, including steel, aluminum, nickel or composite.

In transfer molding, the mold is filled with reinforcing fibers, often in the form of a matrix, before injecting the resin. The most common type of fiber used in this process is glass, though carbon fibers, Kevlar and even some organic fibers such as hemp may also be used.

Once the fibers have been placed inside the enclosed mold, a liquid thermoset resin such as polyurethane is then injected. The resin saturates the fibers, creating a strong, durable product with an excellent bond between the two materials.

The mold is kept closed during the process using bolts, toggle clamps or a press. Typically, the mold is heated using a liquid heating system, with the average temperature used during RTM being 104 degrees Fahrenheit (40 degrees Celsius).

When compared to injection molding, RTM uses similarly low temperatures but higher amounts of pressure. The two processes have similarly quick cycle times, as the reinforced thermoset cools directly in the mold. Like injection molding, RTM is both time- and labor-efficient when compared to other processes such as standard compression molding.

A variation on standard resin transfer molding also exists. It is known as vacuum-assisted resin transfer molding (VARTM). In the VARTM process, a partial vacuum is used to pull the resin into the mold. This achieves a complete saturation of the strengthening fibers within the mold. Because VARTM uses lower plunging forces when compared with its standard counterpart, it is extremely economic, especially in regards to the creation of larger parts or parts which utilize thicker fibers.

Benefits of Resin Transfer Molding

Resin transfer molding initially gained popularity for the strength of the parts which it is capable of producing. Transfer molded parts boast a high strength to weight ratio, a benefit which is not sacrificed even when manufacturing large items.

The matrix formed by the reinforcing fibers allows the resulting parts to withstand high amounts of stress. Resin transfer molded products typically contain a fiber load of 25-50%, making them both extremely lightweight and highly durable.

In addition, resin transfer molding offers aesthetic advantages. Transfer molds are capable of producing extremely complex geometric shapes regardless of size. The resulting products also feature 2 finished surfaces, outside and inside, with a high glass-to-resin ratio and multi-color capability. Gel coat can be added to the interior of the mold during the process to further improve the quality of the finished surfaces.

Resin transfer molding is a cost-effective process, as the materials are typically less expensive than those utilized in other types of molding. In particular, money can be saved during the creation of the molds, which can be made from lightweight materials such as aluminum due to the low temperatures used during molding.


When selectin materials and crafting molds for the resin transfer molding process, it is important to avoid common defects. In resin transfer molding, the highest risk is the accidental creation of empty spaces or holes called voids due to the resin improperly filling all parts of the mold.

To avoid defects, it is recommended to select a low-viscosity thermoset polymer resin. Low-viscosity polymers tend to have better “flowability” within the mold, and can more easily fill even the most complex of shapes.

It is also important to ensure that pressure distribution remains uniform throughout the process, as areas of too-low pressure are another common cause of voids. Fiber should also be spread and distributed uniformly throughout the mold prior to the injection of the resin, to keep the glass to resin ratio relatively even and avoid resin-rich low-fiber zones that could decrease the part’s overall strength.

Sharp corners in the design of the mold could also potentially cause defects in resin transfer molded parts. Too-sharp corners can cause the reinforcing fibers to break, resulting in both aesthetically and structurally weak areas on the surface of the finished part.

However, when the process is carried out correctly and defects are avoided, resin transfer molding results in large, strong, durable parts with an aesthetically pleasing finish and an excellent glass to resin ratio. Contact us today to learn more about resin transfer molding and its role in the creation of high-quality large products.

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