Treatment Methods for Scratches on Plastic Mold Surfaces

Scratches appearing on mold cavities, slides, ejector blocks and other molding surfaces will be replicated on plastic products during injection molding, resulting in defective appearance parts. Deep scratches further cause stress concentration and scraping during demolding. Medical and transparent plastic products have extremely strict requirements for mold surface finish, and even minor scratches will lead to mass scrapping of finished goods. Scratches are mainly caused by friction from molten plastic, collision during mold disassembly and assembly, improper polishing operations, and erosion by impurities in raw materials. The standardized treatment follows three steps: grading scratch depth, repairing and polishing, and implementing long-term preventive measures, balancing repair precision and mold service life.

I. On-site Grading and Identification of Scratch Depth

Fine micro-scratches: Visible to the naked eye without obvious concave-convex texture when touched by fingertips. Such scratches only damage the surface polishing layer without penetrating the metal substrate, mostly generated by tiny plastic debris or slight friction during ejection, featuring low repair difficulty without heavy grinding equipment.

Moderate scratches: Distinct grooves can be felt when sliding fingertips over the surface, with a depth ranging from 0.005mm to 0.02mm. These scratches cut through the polishing layer and reach the metal base material. Carbon deposits and dirt easily accumulate inside the grooves during long-term production, leaving permanent linear marks on transparent plastic products. Partial grinding and sequential polishing are mandatory for elimination.

Deep hard scratches: Clear grooves with burrs on edges, exceeding 0.02mm in depth. They are generally formed by hard tool collision, scouring of metal residues, or scraping by rigid components during mold handling. Simple polishing cannot erase such deep damage; laser cold welding repair followed by grinding and full re-polishing is required.

II. Standardized Repair Procedures for Scratches of Different Depths

Rapid repair of fine micro-scratches

Thoroughly wipe off oil stains and carbon deposits on the mold surface with dust-free cloth soaked in anhydrous cleaning solvent. Select diamond polishing paste of matching fineness paired with low-speed wool polishing heads, polishing from coarse to fine grits sequentially: 800 mesh, 2000 mesh, then 5000 mesh. Control polishing pressure strictly to avoid expanding the polished area beyond the scratch range. After scratches are completely eliminated, wipe residual polishing paste with mold-specific cleaner to prevent contamination of subsequent molded parts. For mirror-finish molds, finish with 10000-mesh diamond paste to restore the original Ra surface roughness standard.

Complete repair of moderate grooved scratches

Use 400# oil stones to grind unidirectionally along the scratch direction to flatten raised edges of grooves, limiting the grinding area to 2–3mm wider than the scratch itself to avoid damaging intact polished surfaces nearby. Replace with 800#, 1500# and 3000# fine oil stones for cross grinding to erase linear grinding traces, and fully clear metal dust after switching each grit grade. Complete the polishing sequence with diamond paste of increasing fineness until uniform reflection without texture is achieved on the molding surface. After repair, recheck fitting clearance of slides and parting surfaces to prevent flash caused by excessive grinding.

Welding repair for deep substrate scratches

Direct grinding on deep scratches will alter local mold dimensions. Adopt mold-specific laser cold welding with welding rods matching the mold steel grade. Apply low-current layered welding to minimize heat input and prevent mold deformation or substrate temper softening. After welding, grind down welding protrusions with oil stones to level the welded area with the original mold surface, then carry out full sequential polishing following the process for moderate scratches. For high-precision appearance molds, conduct hardness testing on welded zones to avoid rapid re-scratching during mass production due to insufficient local hardness.

III. Key Repair Points for Scratches on Special Positions: Slides, Ejector Pins and Runner Channels

Scratches on sliding fitting surfaces: Besides polishing repair, slightly increase matching clearance after treatment, and coat high-temperature dry lubricant before sliding tests to reduce friction resistance and prevent secondary scraping. Avoid over-grinding which enlarges clearance and triggers flash.

Scratches on gate and inner runner walls: High-speed molten plastic scouring deepens grooves easily. Polish runners to mirror finish and add smooth arc transitions after scratch repair to reduce erosion. For molds processing PC and transparent resins, mirror polishing of runners lowers adhesion of precipitates and carbon residues.

Scratches on splicing gaps of ejector blocks and inserts: Align splicing seams after repair, slightly chamfer sharp edges to prevent plastic debris from jamming into gaps and scratching molding surfaces during production.

IV. Long-term Preventive Control Measures Against Mold Scratches

Source control during production: Install magnetic iron removers and filter screens on raw material feeding pipelines to block metal impurities; equip crushers with sealed dust removal systems to fully filter recycled crushed materials. Ban rigid metal tools for cavity cleaning; only copper sheets, bamboo scrapers and soft dust-free brushes are permitted for residue removal.

Protection during disassembly, assembly and storage: Wrap molds with soft protective pads during hoisting and handling to avoid cavity collision by traveling cranes or spanners. Apply uniform anti-rust oil before offline storage, and cover molding surfaces with soft protective films. Handle molds gently during workshop transfer to eliminate impact damage.

Standard daily maintenance: Blow debris out of cavities with air guns after each production shift; conduct full polishing maintenance regularly. For molds with continuous mass production, complete comprehensive polishing every 100,000 shots to eliminate subtle wear traces in advance. Optimize injection molding speed with low-speed filling at gate areas to reduce high-speed melt scouring damage to mold surfaces.

Conclusion

The core principle of scratch treatment on plastic molds is classified layered repair: direct polishing for fine scratches, sequential oil stone grinding plus polishing for moderate scratches, and cold welding repair followed by full grinding for deep grooves. Restrict grinding scope strictly during repair to preserve original mold dimensions and matching precision. Repair operations only eliminate existing scratches; complete prevention requires coordinated control of raw material impurity filtration, standardized mold cleaning tools, protective handling during mold transfer, and optimized molding processes. Comprehensive implementation of above measures maintains high surface finish of molds to guarantee qualified appearance of plastic products, extends intervals of mold polishing maintenance, and cuts downtime losses caused by mold repair.