There are many methods for processing injection molded surfaces, with mechanical processing being a common approach. These methods, including grinding, polishing, and honing, can effectively reduce surface roughness and enhance the appearance and performance of plastic parts. Grinding is suitable for surfaces requiring high flatness and dimensional accuracy. Using a high-speed grinding wheel, the surface can be cut to a roughness of Ra 0.8-1.6μm. For example, surface grinding can ensure perpendicularity between the end faces of plastic gears while reducing roughness and minimizing frictional losses during transmission. Polishing, on the other hand, involves finely preparing the surface using tools such as sandpaper and polishing paste. Depending on the abrasive grit used, varying degrees of roughness can be achieved. Rough polishing using 800-1200 grit sandpaper can reduce roughness to Ra 1.6-3.2μm. Fine polishing using diamond polishing paste can achieve a surface roughness of Ra 0.02-0.1μm, making it suitable for processing transparent plastic parts such as optical lenses. Honing is a process that uses a grinding stone to perform micro-cutting on the surface through the relative movement of the honing head and the workpiece, which can obtain a uniform cross-texture surface with a roughness generally between Ra0.2-0.8μm. It is often used for surface processing of plastic seals in hydraulic systems to improve sealing performance.
Chemical finishing methods also play an important role in injection molding surface treatment, primarily including chemical polishing, etching, and anodizing (for metal inserts). These methods can improve surface roughness without compromising the dimensional accuracy of the part. Chemical polishing involves immersing the part in a specific chemical solution. A chemical reaction dissolves microscopic surface protrusions, resulting in a smooth, flat surface. This method is suitable for complex, difficult-to-machine parts. For example, chemical polishing with a mixture of nitric and hydrofluoric acids can reduce the roughness from Ra 3.2μm to Ra 0.8μm while preserving the integrity of the pattern on decorative plastic parts. Etching, in which a chemical solution corrodes the surface, creates a texture with a defined roughness. This is often used in applications requiring improved surface adhesion, such as pre-painting plastic parts. Etching increases the surface roughness to Ra 1.6-6.3μm, enhancing the bond between the paint film and the part. It is important to carefully control the solution concentration, temperature, and treatment time during chemical finishing to avoid excessive corrosion that could lead to dimensional deviations or surface defects.
Electrochemical machining (ECM) is primarily used for the surface treatment of injection molded parts with metal coatings. Electrolysis modifies the surface, achieving extremely high surface quality and uniform roughness. Electropolishing is the most commonly used electrochemical machining method. The plastic part acts as the anode and is placed in an electrolytic cell. Direct current dissolves metal ions on the surface and deposits them at the cathode, removing microscopic surface bumps and reducing roughness. For example, electropolishing can reduce the surface roughness of chrome-plated plastic door handles from Ra 0.8μm to Ra 0.04μm, achieving a mirror-like finish. Furthermore, electrolytic etching can create uniform, microscopic pits on the metal coating by controlling the current density and electrolysis time, adjusting the surface roughness. This method is suitable for applications requiring a specific friction coefficient, such as the metal coating of plastic bearings. Electrolytic etching can achieve a roughness of Ra 0.4-0.8μm, ensuring good lubrication. The advantages of electrochemical machining are high processing efficiency and uniform surface quality. However, its disadvantages include high equipment investment, the need to handle the electrolyte, and high environmental requirements.
Laser processing is a high-precision surface treatment method that modifies injection-molded surfaces through the high-energy action of a laser beam, enabling precise roughness control and complex surface texture creation. Laser engraving can create micron-scale grooves or projections on the surface of plastic parts. Depending on the laser power and scanning speed, a roughness range of Ra 0.1-6.3μm can be achieved, making it suitable for the production of anti-slip plastic handles. Laser engraving creates a uniform texture, improving friction while ensuring surface aesthetics. Laser polishing utilizes the thermal effect of the laser to melt and resolidify the surface material, filling microscopic pits and reducing roughness. For transparent polycarbonate (PC) plastic parts, laser polishing can reduce roughness from Ra 1.6μm to Ra 0.02μm without affecting the material’s optical properties. Laser processing has the advantage of being non-contact and does not induce mechanical stress on the plastic part, making it suitable for thin-walled or easily deformed parts. However, its equipment cost is relatively high, and processing efficiency is relatively low. It is suitable for high-precision, small-batch surface treatment.
Process control during the injection molding process has a direct impact on surface roughness. Optimizing molding process parameters can reduce subsequent processing and even directly achieve satisfactory surface quality. The surface roughness of the mold cavity is crucial to determining the initial roughness of the molded part. If the mold cavity is mirror-polished (Ra 0.02μm), under appropriate process conditions, the surface of the molded part can directly achieve Ra 0.04-0.1μm, without the need for subsequent processing. Controlling melt temperature and injection speed is particularly important. Too low a melt temperature can lead to inadequate filling, cold spots on the surface, and increased roughness. Too high a temperature can cause material degradation and scorch marks, also affecting surface quality. For example, when molding ABS parts, controlling the melt temperature between 220-240°C and using a moderate injection speed (50-80 mm/s) can achieve a surface roughness below Ra 0.8μm. Holding pressure and time also affect surface quality. Insufficient holding pressure can lead to surface shrinkage and increased roughness, while excessive holding pressure can cause flash, making subsequent processing more difficult. Furthermore, mold temperature uniformity significantly impacts surface roughness. Using a hot oil circulation temperature control system keeps mold temperature fluctuations within ±1°C, ensuring even cooling of the part surface and minimizing roughness variations caused by uneven shrinkage. By optimizing the molding process, part surface quality can be significantly improved, reducing subsequent processing costs.
