Silicone Membrane Resin Transfer Molding (SRTM) for Fiberglass

At Arrowhead, we offer several closed-mold fiberglass processes, and one of the most efficient for mid-volume production is Silicone Membrane Resin Transfer Molding (SRTM). This process uses a rigid base mold and a flexible, reusable silicone membrane to form high-quality composite parts with improved consistency, reduced labor, and excellent surface finishes.

If you’re looking for a fiberglass molding process that delivers accuracy, repeatability, and better laminate control than open molding—without the high cost of traditional RTM—SRTM is a smart solution.

What is SRTM?

SRTM is a variation of resin transfer molding that uses a flexible silicone membrane in place of a rigid “B-side” mold. The silicone membrane acts as the top half of the mold and is designed to seal tightly over the rigid base mold using vacuum pressure.

Here’s how it works:

• Dry fiberglass reinforcement is placed into the rigid mold.
• The silicone membrane is closed and vacuum is applied to fully seal the cavity.
• Resin is introduced into the mold under controlled pressure, saturating the reinforcement.
• Once cured, the part is removed and the membrane is reset for the next cycle.

Because the membrane is reusable and doesn’t require re-bagging like traditional vacuum bag molding, SRTM streamlines the production process and improves cycle times.

When to Use SRTM

SRTM is best suited for parts that need:

• A finished surface on one side and a burr free, smooth backside
• Dimensional accuracy and fairly consistent thickness
• Shorter production cycles than hand lay-up
• Tighter control over laminate quality and significantly reduced susceptibility to air pockets
• Medium production volumes where reusable tooling adds value

It’s especially useful when a vacuum bagging process becomes too labor-intensive or inconsistent, but full RTM isn’t required.

Benefits of Silicone Membrane Molding

Reusable Membrane
The flexible silicone membrane can be reused for hundreds of cycles, reducing waste and setup time compared to traditional bagging films.

Improved Laminate Control
SRTM provides a more consistent fiber-to-resin ratio than open molding and part thickness is more consistent due to controlled cavity spacing and full vacuum sealing.

Improved Laminate structure

Due to the tooling being closed and sealed and vacuum applied, the glass and resin become more fully compacted resulting in a very high glass to resin ratio.

Clean Surface Finish
Because the mold is fully enclosed, parts can achieve a smoother finish on both sides with minimal print-through or surface defects.

Faster Setup
Compared to traditional vacuum bagging, SRTM allows for faster part setup and faster cycle times, helping reduce labor and increase throughput.

Lower Tooling Cost Than RTM
SRTM avoids the need for a full rigid “B-side” mold, which keeps tooling investment lower while still benefiting from a closed-cavity process.

Typical Applications for SRTM Fiberglass Molding

SRTM is used in a variety of industries where lightweight, precision-molded composite parts are required. Common applications include:

• Panels and hatches for marine and transportation
• Enclosures, covers, and guards
• Structural skins with smooth finish requirements
• Reinforced parts with embedded inserts
• Brackets and internal vehicle components
• Agricultural and commercial equipment covers

Because the process balances repeatability, surface quality, and cost control, it fits well within a wide range of fiberglass molding programs.