Summary
Resin Transfer Molding (RTM) is a composite manufacturing process in which liquid resin is injected into a closed mold containing dry fiber reinforcement. It can produce complex carbon structures with controlled dimensions, clean surfaces, and consistent resin distribution. Although established in aerospace and automotive manufacturing, RTM remains relatively uncommon in bicycle production.
Key Facts
- Category: Manufacturing
- Also known as: RTM
- Reinforcement: Dry carbon, glass, aramid, or hybrid fibers
- Matrix: Low-viscosity thermoset resin, usually epoxy in performance applications
- Tooling: Closed, matched mold
- Bicycle use: Select carbon frames and components
- Current notable user: TIME Bicycles
- Alternative to: Prepreg bladder molding and resin infusion
- Related processes: HP-RTM, VARTM, liquid resin infusion
Overview
Most carbon bicycle frames are manufactured from prepreg carbon: sheets of fiber already impregnated with resin. The sheets are arranged around a bladder or mandrel, placed inside a mold, and consolidated using heat and internal pressure.
RTM separates these stages. Dry reinforcement is first arranged into the required structure. The mold is then closed, air is evacuated, and liquid resin is injected through the fibers before curing.
This approach allows manufacturers to use braided, woven, stitched, or otherwise preformed fiber structures that would be difficult to produce from conventional prepreg sheets. It can also provide accurate wall thickness and good surface quality on both sides of a component when rigid internal and external tooling is used.
RTM should not be treated as a quality grade by itself. Frame performance still depends on fiber orientation, resin chemistry, structural design, molding control, and inspection. A well-made prepreg frame can outperform a poorly executed RTM frame.
How It Works
1. Fiber Preforming
Dry fibers are arranged into a preform approximating the finished component’s shape. Depending on the design, this may include:
- Woven carbon sheets
- Braided fiber sleeves
- Unidirectional reinforcement
- Stitched or three-dimensional fabrics
- Local inserts and reinforcement patches
- Foam, wax, or removable internal cores
Fiber orientation remains critical. The resin binds the structure together, but the fibers carry most of the significant loads.
2. Mold Loading
The preform is placed between matched mold surfaces. The mold establishes the part’s dimensions and compresses the reinforcement to the intended thickness.
Hollow bicycle structures require some form of internal tooling or mandrel. TIME, for example, forms dry braided fibers around internal wax cores before placing them in external molds. The wax is removed and reused after molding.
3. Air Removal and Resin Injection
The sealed mold is normally evacuated to remove air. Low-viscosity resin is then injected through carefully positioned gates while vents allow displaced air to escape.
Injection pressure varies by process. Conventional RTM commonly uses moderate pressure, while High-Pressure RTM uses faster injection and more substantial tooling for shorter production cycles.
Resin flow must be carefully controlled. If it moves too slowly, curing may begin before the preform is fully saturated. Excessive flow can disturb the fibers, while poor gate or vent placement can produce dry areas, trapped air, or resin-rich pockets.
4. Curing and Demolding
Once the preform is saturated, heat activates the resin’s curing reaction. The mold remains closed until the component has developed enough strength to be removed.
The part may then require trimming, bonding, machining, painting, and inspection. RTM can produce near-net-shape components, but it does not necessarily eliminate finishing work.
RTM Compared With Other Processes
| Process | Fiber condition | Resin introduction | Tooling |
|---|---|---|---|
| RTM | Dry preform | Injected after mold closure | Rigid matched mold |
| Prepreg bladder molding | Fiber already contains resin | Present before molding | External mold with internal bladder or mandrel |
| Vacuum infusion | Dry reinforcement | Drawn through by vacuum | Usually one rigid surface and a flexible vacuum bag |
| HP-RTM | Dry preform | Injected rapidly at higher pressure | Heavy matched tooling |
VARTM and vacuum infusion are sometimes described loosely as RTM, but they generally use a flexible vacuum bag instead of two rigid mold surfaces. This reduces tooling cost but provides less dimensional control on the bagged side.
Manufacturing Advantages
Dimensional Control
Matched tooling can produce accurate wall thicknesses and repeatable interfaces. This is valuable around bottom brackets, head tubes, bearing seats, and bonded joints.
Dry-Fiber Architecture
Dry fibers can be braided or preformed into continuous structures before resin is introduced. This gives engineers additional options for managing fiber direction and transitions around complex shapes.
Surface Quality
Where both sides contact finished mold surfaces, RTM can create clean external and internal surfaces with less wrinkling or uncontrolled resin accumulation.
Part Integration
Bearing surfaces, inserts, reinforcements, and other features may be incorporated into the preform before injection, reducing the number of later bonding operations.
Repeatability
A controlled mold, preform, injection schedule, and cure cycle can reduce manufacturing variation. Low void levels are achievable, but they depend on proper material preparation and process control.
Limitations and Failure Modes
RTM also introduces significant manufacturing challenges:
- Matched molds and injection equipment are expensive
- Resin gates and flow paths require extensive development
- Low-viscosity resin systems must cure within a controlled processing window
- Fiber movement during injection can change the intended layup
- Poor saturation can create dry fibers or internal voids
- Resin-rich regions add weight without providing equivalent reinforcement
- Mold leakage can disrupt pressure and resin flow
- Design changes may require new tooling or preforms
RTM is therefore not automatically cheaper or faster than prepreg construction. High-pressure variants can support automated production, but handcrafted bicycle applications may remain slow and labor-intensive.
Use in Bicycle Manufacturing
TIME Bicycles is the most established current example. Its frames use braided dry-fiber sleeves placed over internal cores, followed by resin injection under heat and pressure. RTM is used as part of a complete manufacturing system rather than as a single isolated feature.
The BMC Impec was a notable historical application. Its manufacturing system produced braided carbon tubes that were injected with epoxy before being assembled into a frame.
RTM may also be used for smaller composite parts, but manufacturers do not always publish enough information to distinguish true matched-mold RTM from bladder molding, vacuum infusion, or other resin-injection processes.
What It Means for Riders
The molding method alone does not determine ride quality, weight, durability, or safety. Those results depend on the complete frame design and manufacturing execution.
When a company promotes RTM, useful questions include:
- Is RTM used for the complete frame or only selected parts?
- Are the fibers braided, woven, or assembled from conventional sheets?
- How are hollow internal surfaces formed?
- What inspection and quality-control methods are used?
- Does the process provide a measurable benefit beyond manufacturing terminology?
RTM frames can still suffer impact damage, delamination, or resin cracking and require the same careful inspection as other carbon structures. Their repairability depends more on the location and construction of the damage than on the original resin-injection method.
Related Terms
- Prepreg Carbon
- Bladder Molding
- Fiber Preform
- Braided Carbon
- Fiber Volume Ratio
- Void Content
- Composite Layup
- Vacuum Infusion