How to Improve the Polishing Quality of Injection Molds
2026-03-26 11:49:28
Injection Molds
The polishing quality of injection molds directly determines the surface finish, demolding performance and service life of molded parts, and is one of the most critical processes in mold manufacturing. Common defects such as orange peel, wavy lines, uneven gloss and residual tool marks are mostly caused by poor pre-processing, skipped procedures, improper tools or inconsistent operating force. To achieve stable and high-quality polishing, a standardized, step-by-step approach is essential. Based on practical production experience, the following methods can effectively improve polishing quality.
1. Establish a Solid Pre-Machining FoundationPolishing quality depends heavily on previous processing. During CNC semi-finish milling, a uniform polishing allowance of 0.05–0.15 mm must be reserved. An uneven allowance will lead to difficult polishing or dimensional deviation. For deep cavities, sharp corners and complex profiles, smooth tool paths are required to avoid deep and irregular tool marks.
After EDM, a hard modified layer and micro-pits remain on the steel surface, which must be removed with fine oilstone before polishing. Different mold steels such as S136 and NAK80 have different hardness and toughness, so pre-processing parameters must be adjusted accordingly to avoid hard spots and deep scratches, which are hard to eliminate in later polishing.
2. Follow a Graded Polishing Process StrictlySkipping grit sizes is the main cause of hidden lines and poor gloss. Polishing must be carried out in a progressive order: oilstone rough polishing, sandpaper fine polishing, and diamond paste mirror polishing.
Rough polishing with oilstone removes tool marks and EDM traces with consistent direction and even force. Sandpaper polishing follows the sequence 400#→600#→800#→1000#→1500#, with the direction changed 90 degrees at each stage to ensure full removal of previous scratches. For final polishing, diamond pastes such as W3.5 or W1.5 are used with wool wheels, bamboo or wooden tools at low speeds to prevent overheating and burning. A clean, dust-free environment is necessary to avoid particle scratches.
3. Optimize Operating Skills and Tool ControlManual polishing requires stable and uniform force to prevent wavy surfaces and uneven areas. For narrow grooves, fillets and ejector pin areas, small and flexible tools should be used to avoid over-polishing or damaging corner profiles.
In mechanical polishing, the rotational speed of wool wheels should be controlled at 1000–1500 rpm. Excessively high speed causes surface deformation and discoloration. Abrasive debris and residual paste must be cleaned frequently to avoid secondary scratches. Worn oilstone, sandpaper and wool wheels should be replaced in time to maintain stable polishing quality.
4. Adapt to Different Steels and Strengthen Final InspectionPolishing methods vary for different materials. NAK80 tends to produce wire-drawing marks, so flexible tools and lubricating oil are recommended. S136 has high hardness and requires lighter force and finer steps to avoid pitting and chipping.
After polishing, the cavity must be cleaned with mold cleaner and coated with anti-rust oil immediately to prevent oxidation and rust. Surface roughness is measured to ensure quality: Ra ≤ 0.8 μm for conventional high-gloss molds and Ra ≤ 0.2 μm for mirror molds. Visual inspection under light is also performed to check for uniform gloss and no dark lines or uneven areas. Establishing a standardized process sheet helps reduce operator differences and maintain consistent quality.
In short, high-quality mold polishing relies on stable pre-processing, standardized graded procedures, careful operation and targeted material treatment. By eliminating rushed work and improper operations, molds can achieve excellent surface finish, improve product appearance and stability, reduce defects in mass production, and extend service life.
