Surface finish on plastic composites can vary a great deal, depending on the physical and chemical properties of the polymer blend, as well as the parameters of the injection molding process.
The first objective is working with the client to determine how important the surface finish is for the appearance and/or performance of the final product. For example, does the product need to be eye-catching or simply functional? Depending on the answer, the material selected and the desired finish will determine the settings for the injection molding process, as well as any secondary finishing operations that might be required.
There are also functional tradeoffs to be considered. For example, the coefficient of friction and resistance to wear can be affected by the surface finish; if these characteristics are important to product performance, the design team needs to select a material and process that will create compatible finish. Experienced injection molders have a vast body of scientific data on material chemistry and behavior and processing conditions and can cross-reference this information to accurately determine surface finish according to process parameters.
It is critical to determine surface finish during the design stage because it will impact the types of material, tooling, and processing decisions that can be considered. The texture on the mold steel will limit the part surface finish. Sometimes a rougher texture can be used to hide other surface imperfections, such as sink. Surface finish can also affect the draft required on the part—without appropriate draft, the surface finish could be destroyed during ejection.
Two key surface characteristics—gloss and roughness—can be impacted by the material selected, additives, and different injection molding parameters such as fill rate, pressure, and temperature. Material type is especially important—for example, higher melt temperatures for products made from reinforced crystalline resins increase gloss and reduce roughness, creating a smoother surface. If reinforced amorphous resins are being injection-molded, however, a higher mold temperature will increase gloss and reduce roughness.
Another important consideration for surface finish is additive compounds that are mixed into the resin. This is where knowledge of material science really comes into play—for example, fiberglass content results in a lower gloss. Other additives like carbon black or mica can reduce surface roughness—these are additives to avoid (or find substitutes for) if a certain surface roughness must be maintained for product function or appearance. Adding particulate fillers may increase surface roughness. Engineers can mix and match the right combination of resins and additives to produce specific surface qualities.
Then there’s speed—faster injection speeds, combined with higher melt or mold temperatures, will further enhance the gloss and smoothness of the surface. In fact, a faster injection speed always improves gloss and smoothness, regardless of the other production parameters that have been determined. For any reinforced composite, the fast injection speed combined with high melt and mold temperatures will always provide the glossiest and smoothest part (although a slow injection speed, combined with cooler mold and melt temperatures, may meet other product specifications, they will typically result in a poor surface texture). Quick filling of the mold cavity can help minimize fiber orientation, making the weld line less visible and boosting the aesthetic quality of the product.
These are just some of the methods for controlling surface finish during the injection-molding process. Be sure to consult with your injection molder in the earliest design stages to fully determine surface finish needs—getting this worked out up front will result in the most efficient production process and likely reduce the need for any secondary surface preparations.