Mold Material Selection And Fabrication for RTM (Resin Transfer Molding) Process

Resin Transfer Molding (RTM) is a closed-mold process in which resin is injected into a sealed mold to impregnate reinforcement materials and form the final composite part through curing. RTM is ideal for producing high-quality, complex-shaped components in medium-volume production runs. It offers numerous advantages, including compatibility with a wide variety of fiber reinforcements and resin systems, excellent surface finish, high fiber content, low emissions during molding, minimal environmental impact, strong adaptability to automation, relatively low investment cost, and high production efficiency. As a result, RTM is widely used in industries such as automotive, aerospace, defense, mechanical equipment, and electronics.

The mold is one of the most critical factors determining the quality of RTM products. RTM molds typically consist of a male and female mold pair, making surface finish and dimensional accuracy of both mold halves essential to the final product quality.

Material Selection

The quality of an RTM mold begins with material selection, tailored to meet the specific demands of the RTM process.

1. Gelcoat Layer
   RTM generates significant exothermic heat during curing — for instance, a 4 mm thick part can reach over 120°C. Therefore, the gelcoat resin must offer heat resistance, thermal shock stability, and gloss retention. In this process, a vinyl ester-based tooling gelcoat is selected, offering a heat distortion temperature between 160°C and 172°C and excellent mechanical performance.

2. Surface Layer
   This layer must provide heat and crack resistance. A combination of 30 g/㎡ surface mat and 300 g/㎡ E-glass chopped strand mat is used as reinforcement, paired with a bisphenol-A epoxy vinyl ester resin. This resin features excellent high-temperature performance and low shrinkage.

3. Reinforcement Layer
   Focused on mechanical strength and low shrinkage, this layer utilizes 0.4 mm E-glass woven fabric and 300 g/㎡ chopped strand mat as reinforcements, with a zero-shrinkage resin as the matrix.

4. Structural Layer
   To enhance the overall rigidity of the mold and facilitate mold opening/closing operations, a steel frame reinforcement structure is employed.

Master Mold Fabrication

Traditionally, FRP master molds have been made using materials like plaster, wood, cement, or wax through manual processes. However, these materials and methods often fall short in meeting the high precision and surface quality requirements of RTM molds. Their limitations include difficulty achieving A-class surfaces, inconsistent dimensional accuracy, complex workflows, long production cycles, and a high risk of defects — making them unsuitable for RTM applications.

To meet the stringent demands of RTM tooling, machinable block resin, commonly known as tooling board, is now widely used. This material is typically made from a blend of epoxy resin, ABS, glass microspheres, aluminum oxide, and carboxymethyl fiber. After thorough mixing and heating into a paste-like consistency (with additional agents like dibutyl phthalate), the mixture is vacuum-degassed and cast into molds. Once cured through heating, it forms a stable solid with excellent machinability and thermal properties.

When using tooling board to fabricate the master model, the process begins with creating a 3D digital model using CAD software such as Pro/E, UG, or CATIA. To ensure precision, shrinkage allowances are considered based on the selected resin system. The master mold is then CNC machined to reflect the exact shape, size, and thickness of the final product, eliminating the need for cavity modeling and significantly improving dimensional accuracy.