Polishing of injection molded surfaces
The quality of the injection molded surface finish directly impacts the product’s appearance and performance. This is especially true for high-precision products like transparent and decorative parts, where the polishing process is a crucial step in determining product quality. By gradually refining the abrasive grit, the polishing process removes machining marks from the mold cavity surface, resulting in a smooth, even surface. This not only enhances the product’s gloss but also reduces melt flow resistance, making demolding easier. Common polishing methods include mechanical polishing, chemical polishing, electrolytic polishing, and ultrasonic polishing. Mechanical polishing is widely used due to its ease of use and low cost. The polishing process should follow a “coarse to fine, step-by-step” approach, gradually improving the surface finish from rough grinding to fine polishing, ultimately achieving a mirror-like finish (e.g., Ra 0.02μm or less).
Preparatory work before mechanical polishing is crucial to the subsequent polishing results. First, inspect the surface finish of the mold cavity to ensure there are no obvious scratches, pores, or cracks. Otherwise, repairs such as repair welding or grinding are necessary. The choice of polishing tools depends on the polishing stage: For rough grinding, grinding wheels or emery cloth wheels, typically with a grit of 80#-240#, can be used to remove marks left by milling or EDM. For semi-finishing, aluminum oxide grinding wheels or sandpaper with a grit of 400#-800# are used to further refine the surface roughness. For fine polishing, silicon carbide grinding wheels or diamond paste with a grit of 1000#-2000# are used to gradually improve the surface finish. Before polishing, the mold cavity must be cleaned to remove impurities such as oil, iron filings, and other impurities to prevent scratching during the polishing process. For complex cavities, specialized polishing tools (such as contoured grinding wheels or wool wheels) are required to ensure even polishing of every corner.
Mechanical polishing procedures directly impact polishing efficiency and surface quality. During the rough grinding stage, a low speed (1000-1500 rpm) should be used to avoid surface oxidation caused by excessive grinding temperatures. At the same time, uniform pressure should be applied, and each grinding pass should be short to prevent local overheating. During semi-finishing grinding, any rough grinding marks on the mold cavity surface must be completely removed. A cross-grinding method (grinding in one direction first, then in a perpendicular direction) can be employed to ensure a smooth surface. During the fine polishing stage, a soft polishing wheel (such as a wool wheel) and abrasive paste should be used. The speed can be increased to 2000-3000 rpm. The abrasive paste should be evenly applied to the polishing wheel to avoid excessive buildup. During the polishing process, the surface should be cleaned frequently with alcohol or acetone, and the polishing results should be checked to prevent secondary scratches caused by residual abrasive. For mirror polishing, diamond abrasive paste (0.5-1 μm grit) can be used in the final stage, combined with ultrasonic vibration, to achieve a surface roughness of less than Ra 0.01 μm.
Chemical polishing and electrolytic polishing are suitable for mold cavities with complex shapes or those difficult to mechanically polish. Chemical polishing involves immersing the mold in a polishing solution composed of acids, oxidants, and other agents. A chemical reaction dissolves surface protrusions, smoothing the surface. A commonly used chemical polishing solution formulation includes: phosphoric acid (60%-70%), sulfuric acid (20%-30%), and nitric acid (5%-10%). The temperature is controlled at 80-100°C, and the treatment time is 10-30 minutes. Chemical polishing does not require complex equipment and can evenly treat complex surfaces. However, the polishing effect is significantly affected by the solution concentration and temperature, requiring regular adjustment of the solution composition. Electrolytic polishing uses the mold as the anode, passing electricity through a specific electrolyte. This electrochemical process dissolves surface roughness, resulting in high polishing efficiency and a surface finish of up to Ra 0.02μm. It is suitable for corrosion-resistant mold steels such as stainless steel. The electrolyte for electrolytic polishing is typically a phosphoric acid-chromic anhydride system, with a voltage of 10-30V, a current density of 10-50A/dm², and a treatment time of 5-15 minutes.
Post-polishing quality inspection and maintenance are the final steps in ensuring product quality. Surface finish can be tested using a roughness meter. For mirror-polished parts, the surface finish must be below Ra 0.02μm, with no visible scratches, pitting, or other defects. Appearance inspections should be conducted under natural light or white light, observed from a distance of 500mm. The surface should be uniformly bright, with no uneven reflections. Molds for transparent products also require light transmittance testing to ensure that the polished cavity surface does not affect the product’s optical performance. Polished molds require rust-proofing treatment, either by applying a special anti-rust oil or chrome plating to prevent rust during storage or use. During production, the mold cavity must be regularly cleaned and maintained to prevent residual plastic melt or wear that could affect the surface finish. If necessary, secondary polishing and repair should be performed to ensure product consistency and stability.
