Reinforced plastics: common fillers

Glass Fiber

A common way to improve the mechanical properties of a plastic material is to add glass fibers. These fibers improve structural properties like strength and stiffness and reduce shrinkage of the part. Glass-filled resins are relatively inexpensive and can be added to most plastics, such as nylon.

In terms of drawbacks, glass fibers can make the part brittle and reduce impact strength. They also decrease the life of the mold and wear on the barrel and nozzle of the molding machine. Glass-filled resins will also raise the material’s viscosity, making the mold harder to fill. Glass fibers can be added to various colored resins, but they will affect the appearance.

Carbon Fiber

Carbon fiber fillers can improve the mechanical properties of a plastic material. Carbon-filled plastic parts have similar mechanical properties as glass-filled plastic, but result in stronger and lighter parts. Carbon fibers are conductive, so carbon-filled parts have better electromagnetic shielding properties. Even more than glass fibers, carbon fibers improve structural properties, like strength and stiffness, and reduce the shrinkage of the part.

The main drawback of carbon-filled parts is that they are expensive, and like glass fibers, carbon fibers will make the part brittle and reduce impact strength; decrease the life of the mold and cause wear on the barrel and nozzle of the molding machine. Carbon fibers will also raise the viscosity of the material, making the mold harder to fill. Keep in mind that for carbon-filled materials, the part color will be limited to black only. Some resins also need to have the mold temperature run very high, which could require expensive auxiliary equipment.

Design tip for fiber-filled parts: The length of the fibers will align with the direction the material flow, creating a grain direction. The material will be stiffer in bending along the direction of the length of the fibers. For this reason, it’s helpful to have an understanding of the tooling design and mold flow, in order to make gating or design modifications as necessary.