RTM-Lite Fiberglass Molding: Cost-Effective Closed Mold Solutions

When your project calls for a clean, consistent fiberglass part—but without the tooling expense of traditional RTM—RTM-Lite offers an ideal solution. Short for “Light Resin Transfer Molding,” RTM-Lite is a closed molding process that delivers smooth, finished surfaces on both sides of the part, accurate thickness control, and repeatable performance. At Arrowhead, we offer RTM-Lite fiberglass molding in-house, giving you a high-quality, cost-effective option for low- to mid-volume composite production.

What is RTM-Lite?

RTM-Lite is a vacuum-assisted, closed-mold fiberglass molding method. It uses a rigid “A-side” base mold and a lightweight, semi-rigid “B-side” counter mold. The fiber reinforcement—typically fiberglass—is placed into the A-side mold. Once the B-side is placed on top, vacuum pressure is applied to clamp the two mold halves together, sealing the flanges and creating a closed cavity. Resin is then injected into the cavity and allowed to cure.

The process is automated in many cases, with resin delivery controlled by computer-monitored pumps that regulate flow rate and temperature for consistent, repeatable results. Once cured, the part is removed from the mold and trimmed, typically using precision tools like Arrowhead’s 5-axis CNC trimming system.

When to Use RTM-Lite

RTM-Lite excels in situations where part quality matters—but full RTM tooling costs can’t be justified. It’s a great fit when:

• You need a part with smooth surfaces on both sides
• Mid-level production volumes don’t warrant metal tooling
• Consistent part thickness is required
• Traditional hand lay-up or spray-up methods won’t meet finish or dimensional needs
• You need a mold design that allows some production flexibility without starting from scratch

If your project has outgrown the limitations of open molding, but full RTM seems like overkill, RTM-Lite is often the perfect middle ground.

Benefits of RTM-Lite Over Other Molding Methods

Closed Mold, Smooth Surfaces
Unlike hand lay-up or spray-up processes, RTM-Lite delivers a finished, cosmetic surface on both the front and back of the part—ideal for panels, covers, and enclosures where appearance and fit both matter.

Lower Tooling Costs
Traditional RTM requires heavy, rigid tooling on both sides. RTM-Lite reduces that cost significantly by using a semi-rigid counter mold—perfect for low- to mid-volume production without compromising quality.

Controlled Thickness
With built-in hard stops in the flange, RTM-Lite molds help maintain a more controlled thickness across every part. This is a major advantage over open molding, where material thickness can vary due to operator technique.

Cleaner Production
Because the process is enclosed and vacuum-assisted, it produces less airborne resin and fiber than open mold techniques. That helps maintain a cleaner shop environment and reduces waste.

Repeatable Process
Computer-assisted resin injection ensures consistent temperature, flow, and cure times from cycle to cycle, improving part quality and production reliability.

Applications for RTM-Lite Fiberglass Molding

RTM-Lite is well suited to a wide range of industries and part types, including:

• Transportation and bus panels
• Agricultural equipment covers
• Marine access doors and hatches
• Industrial machine guards and enclosures
• Architectural panels or cladding
• Vehicle or equipment body components
• Utility boxes, control panel covers, and fairings

If your part requires a refined finish, accurate dimensions, and reliable mechanical strength—but doesn’t require full-scale production—RTM-Lite provides a perfect balance of cost and performance.

Best Materials for RTM-Lite

RTM-Lite typically uses fiberglass reinforcements such as chopped strand mat, woven roving, or stitched fabrics, depending on strength and weight requirements. Resins commonly used include:

• Polyester – General-purpose and cost-effective
• Vinyl Ester – Offers improved corrosion resistance
• Epoxy – For high-performance structural parts

Gel coat can be applied to one or both mold surfaces for improved cosmetics and UV resistance, depending on your application’s needs.