High-gloss plastic molds can produce spray-free bright plastic parts directly after molding, and the quality of mirror polishing determines the glossiness of finished products, visibility of orange peel textures and weld lines, as well as mold demolding smoothness. The complete polishing process is divided into four stages: rough polishing, intermediate polishing, fine polishing and final mirror polishing, always following the principle of progressing from coarse to fine abrasives and completely removing grinding marks left by the previous procedure. Precisely finished mold base and thorough cleaning before polishing serve as essential premises to prevent pits and scratches during polishing operation.
1. Pre-Polishing Preparation
After the mold core completes CNC and EDM machining, inspect the cavity dimensions, flatness and perpendicularity with a coordinate measuring instrument to confirm all dimensions stay within tolerance ranges without deformation, collapse or local depressions. If EDM leaves white layers, pits and subtle machining lines, trim raised areas with diamond files to eliminate deep textures that cannot be removed in subsequent polishing stages. Fix the mold core on a constant-temperature polishing workbench with the cavity facing upward, wipe the entire cavity surface with dust-free cloth dipped in anhydrous ethanol to remove cutting fluid, powder, oil stains and residual grease from human hands, then place the mold core in a dust-free static drying area for air drying. The workshop must maintain a clean and dust-free environment, as airborne dust falling on polished surfaces will create dense tiny scratches during polishing. Prepare small polishing grinding heads separately for corners, deep ribs and narrow insert gaps that are difficult to grind, and reserve a machining allowance of approximately 0.002mm for sharp edges to prevent edge collapse during polishing which would cause rounded deformation of plastic parts.
2. Rough Polishing for Removing Machining Textures
The purpose of rough polishing is to eliminate CNC tool marks, EDM discharge textures and white machining layers, and unify the surface roughness of the cavity rapidly. Use 800-mesh to 1500-mesh diamond sandpaper with a pneumatic reciprocating polishing machine under low hydraulic pressure. Grind along a single direction at a constant speed, then rotate 90 degrees for cross grinding after finishing one complete area, and replace sandpaper of higher mesh only after all textures from the previous process are completely erased. Lay sandpaper flat for polishing large flat areas, and wrap soft polishing heads with sandpaper to grind corners and rib positions along the shape of the mold. Continuously spray special polishing coolant during polishing to avoid annealing of mold steel caused by frictional heating, which would lead to decreased local hardness and easy pit formation in later service. After rough polishing, the roughness of the cavity reaches Ra 0.8μm, with no obvious machining lines visible to the naked eye and only uniform fine grinding textures remaining.
3. Intermediate Polishing for Surface Texture Refinement
Intermediate polishing follows rough polishing to refine grinding traces and lay a foundation for mirror effect. Gradually polish with 2000-mesh, 3000-mesh and 5000-mesh diamond sandpaper while reducing polishing force continuously, still adopting the cross grinding path. Before switching to higher-mesh sandpaper, clean the cavity surface with alcohol to wash away coarse particles shed from the previous sandpaper, preventing hard particles from being trapped between sandpaper and the mold to form deep scratches. For curved high-gloss areas, switch to wool polishing wheels matched with diamond polishing paste whose particle size corresponds to the current sandpaper mesh, polish gently along the arc direction of curved surfaces, and avoid heavy pressing which may cause collapse of molded surfaces. After intermediate polishing, the mold surface presents a matte bright texture with roughness below Ra 0.2μm, and no orange peel patterns from diffuse reflection under strong light irradiation.
4. Fine Polishing to Approach Mirror Finish
Sandpaper is no longer used in fine polishing, which entirely relies on ultra-fine polishing paste paired with soft polishing media for construction. Select 8000-mesh and 15000-mesh diamond polishing paste matched with wool felt polishing heads and suede polishing cotton respectively, switch the polishing equipment to a small hand-held vibration polisher and reduce vibration intensity to prevent wave textures formed by vibration. Polish each area slowly in small circular motions with consistent polishing duration to guarantee uniform material removal. Manually perform fine polishing around mold right angles and parting lines with cotton swabs wrapped in polishing cotton dipped in polishing paste to avoid abrasion of edge boundaries by machine polishing. Clean the cavity regularly during polishing to prevent caked polishing paste from scratching polished surfaces. After fine polishing, clear reflections can be formed on the mold cavity under light without scratches, pits or hairline marks.
5. Final Mirror Polishing and Anti-Rust Protection
Conduct final mirror polishing manually with 30000-mesh nano-diamond polishing paste and high-purity degreased suede in gentle circular motions, focusing on rib positions, corners and weld line areas until the mold surface achieves mirror standard with undistorted reflections under light and no hazy fog. The surface roughness can reach within Ra 0.01μm to meet the molding requirements of high-gloss spray-free plastic parts. After all polishing work, wipe the cavity repeatedly with dust-free paper and anhydrous ethanol to completely remove residual polishing paste, blow dry moisture, then evenly coat a layer of special mirror anti-rust oil to avoid rust spots formed by mold rust during storage and assembly. Do not touch mirror cavity surfaces by hand during mold closing and assembly, as fingerprint grease will cause printed spots on plastic parts during injection molding. Perform mold trial after finishing the whole set of polished mold cores, inject transparent high-gloss plastic raw materials, inspect plastic parts for scratches, orange peel and pits, and conduct local rework fine polishing for subtle defective points.
6. Prohibited Operations During Polishing
Skipping mesh grades during polishing is forbidden throughout the process; for example, directly using tens of thousands of mesh polishing paste after rough polishing leaves deep machining textures intact, inevitably resulting in orange peel patterns on molded plastic parts. Heavy pressing polishing is prohibited, as high-gloss mold steel has high hardness and heavy pressure easily causes plastic deformation of molded surfaces leading to dimensional out-of-tolerance of plastic parts. The polishing environment must be dust-proof, since diamond abrasive debris generated by grinding and workshop dust falling on polished surfaces will form irreversible scratches. The complete polishing process is applicable to common mirror mold steel such as S136 and STAVAX, while polishing pressure should be further reduced in fine polishing and mirror stages for mold steel with low hardness to prevent wrinkled textures on the steel surface.
Summary
Mirror polishing of high-gloss molds follows a progressive flow: rough polishing removes textures, intermediate polishing refines textures, fine polishing forms a bright base, and manual final mirror polishing completes the finish. Removing EDM white layers and machining textures in advance, gradually replacing polishing consumables and maintaining a dust-free cleaning environment are core points to avoid polishing defects. Standardized mirror polishing procedures can not only make mold cavities reach mirror high-gloss grade to produce high-brightness traceless plastic products, but also prevent molded surface deformation and edge collapse caused by over-polishing, extend the service life of high-gloss molds, and reduce mass production failures such as mold sticking and pits on plastic parts during injection molding.
