In resin transfer molding (RTM), a low-viscosity polymer resin is injected at low pressure between matched dies containing fiber reinforcement. Pressure and heat from an exothermic reaction allow the structure to cure. When compared to RTM Light, equipment and molds require a higher investment, but more sophisticated geometries can be realized.
Advantages
When compared to hand lay-up, advantages include
- Two polished surfaces rather than one
- Less material loss
- Higher levels of accuracy
- A faster cycle due to a higher production rate
- Reproducibility and sound parts
- Scalable for small or medium production
- Lower VOC emission
Disadvantages
- More mold-design knowledge
- Higher investment costs due to the use of two molds
- The degree of automation leads to rising investment
Production Equipment
- Mold
- Mold clamping unit, consisting of screw, snap fasteners, hydraulic cylinders or a hydraulic press
- Mixer, either a hand stirrer or semi-automatic closed mixer for resin formula
- Resin injection system, comprised of
- Pressurized tank
- Metering pump
- Optional vacuum pump
Specific Aspects of Processing
Upstream
- Create preforms or reinforcements
- Formulate resins
Molding
- Prepare the mold
- Cleaning and applying release agent
- Gelcoating
- Placing cores
- Place reinforcements
- Closed mold
- Inject resin, removing excess by a peripheral channel
- Cure
- Open mold and demold
Downstream
- Finish
- Conduct quality control
Industrial RTM applications utilize custom preformed reinforcements produced by hot-pressure forming continuous-filament mats, and then cooling them. Fibers and binders can also be sprayed on a suitable form. Metal or sandwiched components can be inserted in molded parts.
RTM is most cost-effective for runs of 100 - 500 pieces.