Scratches on polished mold steel will directly affect the appearance, bright surface and matte effect of plastic products, and easily lead to mold sticking, strain and poor demolding.
Scratch Type and Cause JudgmentCommon scratches after polishing include deep mechanical scratches, fine sand lines, polishing paste residual scratches, EDM lines and material impurity scratches. Mechanical scratches are mostly caused by excessive sandpaper size jump and single grinding direction. Fine sand marks are caused by insufficient sandpaper mesh or uneven polishing force. EDM lines are caused by uncleaned discharge processing layer. Material impurity scratches are caused by insufficient steel purity. Confirm scratch depth and source before repair to avoid worse polishing due to blind polishing.
Superficial Scratch Repair (Depth Less Than 0.02 mm)Superficial scratches are mostly seen on mirror and high-gloss surfaces, visible to the naked eye but no obvious concave-convex feeling by hand. First clean oil and dust on the mold surface with clean dust-free cloth to ensure no secondary scratch by impurities. Then use high-mesh sandpaper, transition from 1500 mesh to 2000 mesh, grind lightly in the same direction with uniform force and no back-and-forth grinding to prevent new lines. Then replace fine-grained diamond paste, polish step by step from 2.5 microns to 1.5 microns, cooperate with wool wheel or copper brazing low-speed polishing, keep moving smoothly until scratches disappear and surface brightness is consistent. Finally wipe clean with clean cotton cloth, check no fine lines.
Medium Depth Scratch Repair (Depth 0.02 to 0.05 mm)Medium depth scratches have obvious concave-convex feeling by hand, and the damage layer must be removed first before fine polishing. First level gradually with coarse sandpaper, generally start from 1000 mesh to 1200 mesh, grind scratches flat in one direction to ensure the flatness of the whole area. Then gradually increase sandpaper mesh, 1500 mesh and 2000 mesh transition in turn, completely remove sand marks left by the previous level at each step. Then enter the oilstone fine repair stage, use fine oilstone to push gently along the molding surface lines to eliminate sandpaper marks. Finally use 1.0 micron or 0.5 micron diamond paste with soft polishing wheel, low-speed light pressure polishing to restore mirror or high-gloss surface. Check with strong light oblique at any time during the process to ensure no dark marks and no wavy lines.
Deep Scratch and Local Damage Repair (Depth Greater Than 0.05 mm)Deep scratches have obviously damaged the mold surface, and cannot be repaired by simple polishing, needing processing before polishing. First grind the scratch area flat with fine grinding wheel or oilstone. If necessary, use milling machine or engraving machine to slightly reduce the surface to ensure flat base without steps. After processing, tool marks and discharge layers must be removed, and then gradually grind from 800 mesh or 1000 mesh sandpaper. Carry out rough grinding, medium grinding and fine grinding step by step, eliminate traces of previous process at each step, and prohibit excessive jump numbers. Polish from coarse to fine, diamond paste particle size from large to small, polishing tools from hard to soft to ensure smooth transition between repair area and surrounding surface, no steps, no shadows and no local depressions.
Special Repair Points for Mirror Mold ScratchesMirror molds have extremely high surface requirements, and scratch repair must be more precise. Strictly clean the environment before repair to reduce dust interference. Sandpaper must be brand new and clean, do not use consumables contaminated with impurities. Keep polishing direction uniform, adjust 90 degrees for each mesh change to facilitate observing whether the previous trace is removed. Polishing tools select high-density wool wheel, bamboo brazing, copper brazing, speed control at low speed to avoid high temperature causing mold surface annealing and softening. Use high-purity diamond paste throughout the process, gradually polish to 0.5 micron or 0.25 micron, and finally polish with clean cotton cloth to ensure clear mirror, no orange peel, no scratches and no material lines.
Key Precautions to Avoid Polishing ScratchesAll sandpaper, oilstone and polishing wheels must be kept clean, and do not mix consumables of different particle sizes. Grinding direction is along the product demolding direction or cavity long direction as much as possible, do not randomly horizontal polishing. Sandpaper mesh increases step by step, do not jump too much to prevent deep trace residue. Polishing force is uniform and stable, no local heavy pressure to avoid wavy surface. EDM metamorphic layer is removed first when processing steel surface, rough processing lines should be regular to reduce later polishing difficulty. Regularly check the wear of polishing tools and replace in time to prevent scratches caused by the tools themselves.
Inspection and Protection After RepairAfter scratch repair, use strong light to irradiate obliquely to ensure no visible scratches, sand lines, dark marks and wavy lines. Mirror molds need to achieve high gloss and high clarity, without impurities and pits. Clean residual polishing paste on the surface in time after passing acceptance, apply anti-rust oil to avoid oxidation, rust and dust scratches. Confirm the surface state again before mold on-machine to ensure no appearance defects of produced plastic products and stable qualification.
