Resin Transfer Molding (RTM)

Summary

Resin Transfer Molding (RTM) is a closed-mold composite manufacturing process that injects liquid resin into a mold cavity preloaded with dry fiber reinforcements. It offers excellent control over part geometry, material properties, and surface finish, making it an increasingly popular method for producing high-performance carbon fiber bicycle frames and components.

Key Facts

  • Introduced: 1960s (aerospace and marine industries)
  • Category: Technology / Manufacturing
  • Also known as: RTM, Resin Injection Molding
  • Used by / Found on: High-end carbon bicycle frames, forks, rims, and small parts
  • Core advantage: Precision fiber placement and void-free resin infusion
  • Alternative to: Bladder molding, filament winding, vacuum bagging
  • Common materials: Pre-cut carbon or glass fiber reinforcements, epoxy or polyester resin systems

Overview

In modern carbon frame construction, not all manufacturing methods are created equal. Bladder molding and vacuum bagging remain widely used for their efficiency and decent repeatability. But when top-tier performance, precision, and consistency are the priority, Resin Transfer Molding (RTM) becomes a compelling choice.

RTM is a closed-mold composite process that begins with dry fiber material — usually carbon or glass fiber — laid into a rigid mold. Once the fibers are in place, resin is injected under pressure into the sealed mold cavity, where it saturates the fiber layup and cures into a finished part.

This method flips the script compared to traditional wet lay-up techniques, where resin is applied by hand or via vacuum infusion. With RTM, fiber orientation and placement can be optimized in a dry state before any resin is introduced. This allows for meticulous control of structural properties and yields composite parts with minimal void content and high fiber-to-resin ratios — essential traits in performance-driven applications like bike frames, forks, and wheel rims.

In the cycling world, RTM is still relatively niche due to its tooling costs and slower production cycle. However, several high-end brands have adopted it to produce frames and components with exceptionally tight tolerances, clean internal surfaces, and consistent structural performance.

How It Works

At its core, RTM is a two-step process: dry reinforcement loading followed by resin injection. Here’s how it works in detail:

1. Mold Design

RTM requires a two-part rigid mold — often CNC-machined aluminum or steel — that forms the external shape of the finished component. The mold must be airtight and able to withstand injection pressure and elevated cure temperatures.

Channels, vents, and gates are built into the mold to control resin flow, prevent air entrapment, and ensure complete fiber wet-out.

2. Preforming

Before resin enters the picture, dry fiber reinforcements are carefully arranged in the mold. These may take the form of:

  • Pre-cut carbon fiber sheets
  • Woven or stitched fabrics
  • 3D preforms using thermoformed fabrics
  • Inserts or foam cores for local stiffness

This stage is key to performance. Engineers can precisely control fiber orientation, layering, and thickness without dealing with sticky resin, which simplifies handling and improves quality control.

3. Mold Closure

Once the fiber stack is loaded, the mold is closed and sealed. Clamping pressure is applied to maintain shape and integrity during injection.

4. Resin Injection

Liquid thermoset resin — typically epoxy, polyester, or vinyl ester — is injected into the mold under pressure. The resin flows through the preform, displacing air and saturating the fibers.

Resin can be pushed in using:

  • Vacuum-assisted RTM (VARTM): Resin is pulled into the mold via vacuum pressure
  • High-pressure RTM (HP-RTM): Pumps inject resin quickly for faster cycles and denser parts

Flow control is critical. Poor resin flow can cause dry spots or air pockets, compromising strength.

5. Cure and Demolding

Once the mold is full, the resin is cured using heat (room temperature or elevated depending on the resin system). After curing, the mold is opened and the finished part is removed — often with little or no post-processing.

Applications in Cycling & Benefits

While RTM originated in the aerospace and marine sectors, its precision and repeatability have made it increasingly attractive in cycling — especially at the high-performance and luxury end of the market.

Key Applications

  • Carbon frames with complex geometry and thin-wall tubing
  • Carbon forks and stays
  • One-piece cockpits and rims
  • Dropouts, swingarms, and linkages for full-suspension bikes

Benefits for Bike Manufacturing

1. Superior Surface Finish
Because both sides of the part cure against polished mold surfaces, RTM yields exceptionally clean external and internal surfaces — reducing the need for sanding or painting.

2. High Fiber Content
Dry fiber loading followed by pressurized resin injection allows for high fiber-to-resin ratios, which translates to lighter and stiffer components.

3. Lower Void Content
The closed mold and controlled resin flow minimize voids, which can be a hidden weakness in other molding techniques.

4. Improved Repeatability
Every part is molded from identical fiber layouts and mold conditions, enhancing quality control — a major selling point for premium brands.

5. Structural Precision
RTM excels at producing thin-walled, load-specific structures with tight tolerances, ideal for performance bike frames with demanding stiffness profiles.

Limitations

RTM is not without drawbacks:

  • High Tooling Costs: Molds are expensive to produce and maintain
  • Longer Cycle Times: Slower than bladder molding in volume production
  • Less Flexibility: Design changes require new molds

As such, RTM is more common in limited production runs, custom geometry builds, or ultra-premium frames.

Notable Implementations

  • Look 795 Blade RS: The French brand uses RTM for its road frames to achieve race-grade stiffness and clean integration.
  • TIME Alpe d’Huez Disc: Manufactured using “RTM Carbon Matrix” tech, yielding exceptionally refined frame internals.
  • BMC Impec Lab Frames: Early experiments with RTM to explore next-gen carbon layups.
  • Scott Addict RC (prototype editions): Rumored use of RTM in specific fork and rear triangle elements.
  • Specialized S-Works test rigs: Internal projects exploring RTM for rim and dropout structures.

Related Terms

References

  • Look Cycle: RTM Manufacturing Overview
  • TIME Sport Engineering White Papers
  • Composites World: RTM in Bicycle and Aerospace Applications
  • BikeRadar: “How Premium Frames Are Made” Feature
  • Hexcel Technical Documentation on Epoxy Resin Systems
  • Specialized R&D Interviews (internal fabrication techniques)
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