Mirror Polishing Practice of Injection Molds for Painted Appearance Parts in China
Plastic parts for painting have special requirements on mold surface status, different from ultra-high-gloss non-painted parts. Non-painted mirror surface pursues perfect smoothness without any texture. For painted substrates, mirror polishing does not aim for the highest optical mirror grade. Its core goal is to form uniform and delicate molding surfaces, reduce pinholes, bubbles, flow marks and weld lines on plastic parts, and control micro roughness to guarantee primer adhesion, so as to avoid orange peel, shrinkage and paint peeling after spraying. Over-polishing will lead to overly smooth surfaces and poor coating adhesion. Insufficient polishing will leave tiny textures on plastic parts that show through paint and form uneven brightness. Mastering polishing grade, operation process and risk avoidance is critical for mold development of painted appearance parts.
Polishing Grade Selection and Roughness Target
A2 and A3 mirror grades are commonly used for painted plastic parts, corresponding to roughness Ra 0.02~0.05μm, different from A1 ultra-mirror for non-painted products. A1 mirror has extremely few micro textures and low surface tension, which easily causes paint shrinkage and poor adhesion. It is not recommended for painted substrates unless specified by customers. A3 mirror balances surface fineness and surface tension, which is the preferred choice for most ABS, PC/ABS and HIPS painted shells. For soft rubber and modified flame-retardant plastics with poor fluidity and high gas precipitation risk, polishing grade can be appropriately reduced, and exhaust optimization is prioritized instead of blindly chasing high mirror finish to avoid blocking exhaust channels. Polishing areas should be clarified in technical agreements. Parting lines, vents and insert joints are not mirror polishing areas, to prevent sealing and exhaust structures from being damaged during polishing.
Pre-process Control Before Polishing
Polishing quality relies on previous machining. If tool marks and spark marks left by CNC and EDM are too deep, they cannot be fully removed in later polishing, and local surface collapse and profile deformation will occur. The cavity reserves proper polishing allowance: 0.015~0.03mm on one side for mirror areas. Insufficient allowance cannot eliminate machining traces, while excessive allowance easily collapses curved surfaces and damages product contours. Low-loss fine EDM parameters are adopted for electric discharge, and spark mark depth after fine discharge is controlled within 0.008mm. High-speed CNC milling is preferred for large appearance surfaces to reduce EDM usage. S136, NAK80 and STAVAX are mainstream steel grades for painted appearance molds. The steel must be fully tempered for stress relief, with hardness controlled at HRC48~52. Low hardness causes scratches easily during polishing, while excessive hardness brings low polishing efficiency and edge chipping. Steel surface defects such as sand holes, pinholes and rust should be repaired before polishing.
Graded Polishing Operation Process
Polishing follows the principle from coarse to fine. Do not use fine sandpaper or diamond paste directly, otherwise traces will be locked inside the steel surface. First, use 800# and 1200# sandpaper to remove CNC tool marks and coarse spark marks, grind along curved surfaces with uniform force, and protect sharp corners, radii and engraved features to prevent edge collapse. Second, switch to 1500# and 2000# sandpaper to further refine traces and eliminate visible machining marks. Third, use 3000# and 5000# diamond paste with wool polishing heads for fine polishing to remove sandpaper traces and form delicate mirror base. Fourth, adopt 7000# and 10000# diamond paste for final finishing with soft sheepskin. Cross grinding movement is adopted to avoid directional textures, which will turn into light and dark stripes after painting. Special polishing cleaner is used all the time to remove debris, because hard particles will leave permanent scratches.
Risk Control During Polishing
Curved surfaces and transition zones are prone to polishing collapse. Do not polish one fixed spot for a long time, as local overheating will cause surface tempering softening and rapid wear in mass production. Sharp corners, thin ribs and small undercuts are protected with isolation and polished lightly. Over-polishing causes out-of-tolerance dimension and flash. Vents are strictly protected and must not be flattened or shallowed. Gas released by plastic materials will form pinholes on parts and turn into black spots after painting if vents are blocked. Inserts are polished under unified reference to realize smooth transition at joints and avoid visible lines on plastic parts. Polishing workshops should be clean and dust-free, because dust will scratch mirror surfaces.
Post-polishing Verification and Trial Molding Matching
After polishing, inspect the surface under oblique strong light from multiple angles to check scratches, orange peel and uneven polishing. Sampling roughness test is carried out to confirm Ra value meets requirements. Trial molding uses customer mass production raw materials. Check hidden pinholes and gas marks on first articles. If pinholes appear frequently, do not upgrade mirror grade blindly. Check exhaust, drying and injection parameters first. If paint shrinkage or poor adhesion occurs, it means the mirror is over-polished, and the micro texture can be adjusted by lowering polishing grade. Anti-rust treatment and dust film wrapping are required after polishing to prevent scratches during assembly and transportation.
Mirror Maintenance in Mass Production
During long-term production, mirror surfaces are contaminated by volatile substances and flame retardant precipitates, turning foggy and dull. Clean the surface gently with special mirror paste and dust-free cloth. Do not use sandpaper or hard tools. Vents are disassembled and cleaned regularly. Local re-polishing is adopted for minor scratches with allowance control to avoid surface profile change. Whole-surface refurbishment can be arranged for severely foggy mirrors, followed by painting verification. Mirror polishing for painted parts is not about pursuing maximum brightness. It balances molding quality, exhaust capacity and paint adhesion. Stable pre-machining, graded polishing and strict surface protection help produce qualified plastic substrates for painting and reduce scrap rate in spraying processes.
