What Finishing Processes Are Used to Remove Tool Marks on Injection Molds

CNC cutting tool marks on mold cavity and core will transfer onto plastic products, creating appearance defects and increasing polishing workload. Severe marks may lead to stress concentration and demolding scratches. According to mold material, surface shape and product appearance requirements, select matched finishing processes to remove cutting traces step by step, control surface roughness and balance machining efficiency and dimensional precision.

1. High-speed Milling Finishing Process

High-speed milling serves as the pre-process to reduce tool marks after rough CNC machining. Use small ball end mills and reduce cutting step distance to lower residual cutting height. Optimize spindle speed and feed rate to avoid vibration and joint marks. For pre-hardened steel S136 and NAK80, set step distance at 0.03~0.08mm; for hardened steel H13 and SKD11, narrow it to 0.02~0.05mm. Minimize tool overhang to reduce shaking. High-speed milling removes coarse marks but leaves micro scratches, which need further polishing.

2. EDM + Polishing Process

For deep cavities, narrow slots and tiny corners unreachable by cutters, adopt EDM. EDM surface forms re-melt layer and fine discharge texture that cannot be directly used for molding. Use low-loss copper or graphite electrodes for multi-stage discharge, apply low energy parameters in fine machining to reduce discharge pits. Remove brittle re-melt layer after EDM, then grind with oil stones and sandpaper from coarse to fine grit. This method fits mold corners and thin rib structures.

3. Ultrasonic Grinding Finishing Process

Ultrasonic grinding is a non-cutting finishing method. High-frequency abrasive vibration performs micro-cutting to eliminate micro tool marks after CNC milling with minimal dimensional deformation. Match the ultrasonic tool head with mold curved surfaces and use diamond abrasive paste for uniform movement. It delivers consistent surface texture compared to manual work and avoids local surface sinking. It is suitable for precision free-form molds, though efficiency is low for large flat areas.

4. Laser Polishing Process

Laser polishing melts steel surface locally to smooth micro tool marks with high automation and stable consistency. It contains rough laser polishing for obvious scratches and fine polishing for low roughness. It works well on stainless mold steel like S136 and STAVAX, handling curved surfaces and texture borders and cutting manual polishing time. Control laser energy strictly, because excessive energy changes metal grain and forms oxide layer, which affects later texture etching.

5. Traditional Manual Lapping and Polishing Process

Manual polishing is the widely adopted final finishing method. Operators use oil stones, graded sandpaper and diamond paste for progressive grinding. Alternate grinding directions to erase unidirectional cutting marks and keep steady force to prevent over-polishing and dimension collapse. Mirror polishing with ultra-fine paste is used for high-gloss molds. It adapts to complex curved surfaces, yet the processing cycle is long and surface quality relies on operator skill.

6. Process Selection Judgment Points

For simple planes and large curved surfaces, choose high-speed milling plus manual polishing. Deep narrow cavities use EDM combined with grinding. High-volume precision curved molds use high-speed milling with ultrasonic grinding. High-gloss stainless steel molds can use high-speed milling and laser polishing. Check steel hardness before processing and control cutting parameters to reduce tool marks at source. Detect roughness and dimensions after finishing to guarantee quality.