Repair Methods for Coating Peeling on Injection Mold Surfaces

In the injection molding industry, the surface coating of a mold is critical to ensuring product appearance, smooth demolding, and long service life. Common coatings include hard chrome plating, PVD, DLC, TiN, and nitrided layers. Under long-term production, frequent mold movement, high-temperature melt scouring, and chemical corrosion, these coatings often suffer from scratches, peeling, blistering, and substrate exposure. Once the coating fails, it directly causes defects such as flow marks, sticking, uneven gloss, and dimensional instability. It also accelerates wear and rust on the mold base, increasing production costs significantly. A complete and standardized repair process is therefore essential for maintaining stable production, reducing downtime, and extending mold life.

Failure Judgment

Mold coating failure generally falls into two categories. Localized small-area peeling, spot exposure, or linear damage is usually caused by mechanical scratches, stress concentration at sharp corners, long-term erosion by release agents, or accumulated carbon deposits. Large-area peeling, edge lifting, and overall flaking are mostly due to insufficient pre-treatment, such as residual oil, oxide scale, or porosity on the steel surface. They can also result from excessive mold temperature, corrosive materials like PVC, or severe abrasion from glass-filled compounds, all of which destroy coating adhesion. Accurate diagnosis helps prevent repeated failure after repair.

Repair Preparation

Pre-treatment is the most important step that determines coating durability. First, completely remove the damaged coating. For small areas, use fine sandpaper, oilstone, or diamond paste. For large areas, use chemical stripping, electrolytic stripping, or light sandblasting until clean, uniform steel is exposed. Next, clean the surface thoroughly with a professional mold cleaner and ultrasonic equipment to remove oil, dust, and moisture. Finally, blend the transition area between the damaged and intact coating into a gentle slope to eliminate steps and stress points, preventing the new coating from cracking or lifting at the edge.

Graded Repair

For minor damage such as shallow scratches and small spots without lifting, direct polishing plus local chrome plating is fast and cost-effective, ideal for standard and mirror molds. For moderate peeling with visible wear, PVD coatings such as CrN or TiN are recommended for high adhesion, wear resistance, and temperature stability, especially for molds running glass-filled materials. For severe large-area peeling, full removal and re-coating are necessary; partial patching is not reliable. Mirror molds require precision polishing and chrome plating, while molds requiring excellent release performance benefit most from DLC coating.

Post-Treatment Reinforcement

After repair, low-temperature stress relief is highly recommended to reduce internal stress and prevent cracking. During production, avoid excessively high mold temperature and injection pressure to reduce impact on the cavity. Minimize the use of corrosive release agents and rust preventives, as they can penetrate coating edges and weaken adhesion. Any small scratches or wear found during production should be polished immediately to prevent further expansion.

Daily Maintenance

Regular cleaning of the cavity, runners, and vents prevents carbon buildup and chemical corrosion. Protect the mold surface during handling, installation, and storage to avoid scratches. Periodic light polishing maintains smoothness and low friction. For molds processing corrosive or abrasive materials, shorten the inspection interval to detect issues early.

Summary

Repairing mold coating peeling is a systematic process that includes failure analysis, proper pre-treatment, graded repair, post-treatment, and daily maintenance. A scientific procedure restores mold performance quickly, improves product quality, reduces maintenance costs, and extends service life. Establishing a standardized maintenance system greatly improves production stability and supports high-efficiency, high-quality injection molding.