What Should Be Paid Attention To When Designing RTM Molds

 

RTM mold design is also a very important part. Generally, the following aspects should be paid attention to.

 

(1) Material. Depending on the batch and complexity of the product, the mold material can be steel, aluminum alloy, copper alloy and epoxy composite material. At present, epoxy composite material is the most commonly used, followed by steel and aluminum. Steel alloy has only recently started feasibility testing and has the best thermal performance.

 

Due to the slow filling speed and low filling pressure of RTM (about 140~345KPa), the mold clamping force is also low (under the same conditions, it is about 1/40 of SMC and 1/100 of thermoplastic composite injection molding). Therefore, the mold strength requirement is not high. In addition, the heating temperature of RTM mold is low (about 50~60℃), so epoxy resin (the middle and frame can be reinforced with wood and aluminum alloy) RTM mold has been widely used. For RTM parts with large batches or high surface quality requirements, steel, zinc-aluminum alloy or nickel-plated steel shell molds can be used. Using metal molds can make the turning point with high surface quality of RTM more prominent.

 

(2) Sealing. The sealing of RTM molds is not difficult. However, if it is not properly solved, serious quality problems will occur (such as bubbles on the surface or inside). Currently, RTM molds use two forms of seals (Figure 8-3). Figure 8-3 (a) is an O-ring seal, which is mainly used for sealing the mold parting surface of complex shapes. Figure 8-3 (b) is a clamping seal, which is mainly used for sealing the mold parting surface of a simple mold (such as the parting surface is on the same plane).

 

(3) Exhaust port and discharge port. The exhaust port of RTM mold is generally designed at the same position as the outlet, which is a major feature of RTM. Unlike thermoplastic composite injection molding molds, RTM molds are not a closed cavity (relative to the very small exhaust gap), and their outlets are generally designed at the end of the filling material flow. This is because the RTM filling speed is low, and its outlet can be used as both an exhaust port and a window to observe whether the filling is complete. When making high-quality RTM parts, it is even considered that the filling is completely completed only after a certain amount of resin flows out of the outlet. This is because it can improve the degree of resin impregnation into the fiber and eliminate microscopic bubbles. It not only improves the physical bonding strength between the fiber and the resin, but also improves the density of the part. However, the cost is the waste of resin (generally up to 1/3~2/3 of the quality of the molded product).

 

(4) Demolding. Another important issue in RTM mold design is part demolding. Especially when using epoxy resin molds, internal mold release agents (already added to the RTM resin) have almost no effect, and external mold release agents must be used. These external release agents are usually applied to the mold surface in the form of a coating. They mainly include wax, silicone and polyvinyl alcohol (PVA). Different RTM resin systems have different requirements for release agents. For example, polyester usually needs to be re-sprayed with release agent after every few moldings.