Polybutylene terephthalate (PBT) material has a chemical structure similar to PET material used for beverage bottles and is suitable for continuous use at temperatures up to 190 °C.
Compared to polyamide resin (nylon resin), it offers superior strength and rigidity despite having equivalent heat resistance. While PBT exhibits no dimensional change due to water absorption, it also features significant warping and deformation in molded parts.
Therefore, in automotive applications, it is primarily used for connectors in electrical wiring the size of a fingertip. Glass fiber-reinforced PBT material offers excellent cost performance, and the material is well known for its effectiveness in large products. However, it is difficult to handle, making it hard to utilize.
We achieved the application of such PBT material to large exterior parts measuring 700 mm in length, leveraging its high heat resistance, through the following methods.
1. Resin injection design that minimizes warping and deformation
Due to the short flow length and low flow resistance of small parts, a pinpoint gate can be used to inject resin through a small opening. With this method, the contact area between the product and the runner is small, making it easy to separate the product from the runner.
However, using the pinpoint gate method in large parts results in high flow resistance, which leads to insufficient filling. This will also cause significant deformation due to strain. Therefore, we adopted the direct gate method, which allows the large injection port to be positioned anywhere.
This injection method is also used in the molding of helmets made from polycarbonate resin, which is prone to warping, and can reduce flow resistance at the injection port to a fraction of the original value. Direct gate molding tends to cause roughness on the surface near the injection port area, but this issue was resolved by modifying the mold structure and exterior design.
2. Product design that leverages the warping
The end of the resin flow in large products is prone to significant warping. Typically, we try to suppress the warping with reinforced structure such as ribs, but this time we decided to use the warping as part of the shape. Our resin flow simulation achieves exceptional accuracy because the predicted amount of warping in the analysis is highly precise.
The ultimate reward of product design is understanding the strengths and weaknesses of promising yet challenging materials and fully leveraging their potential.