Analysis of Mold Surface Coating Peeling Causes and Targeted Improvement Solutions
Nitriding, hard chrome plating, electroless nickel-phosphorus and other surface coatings on plastic molds easily peel off, directly resulting in product drag marks, surface pits and dimensional out-of-tolerance, drastically shortening mold service life. Precision molds for consumer electronics with mirror or textured surfaces impose extremely high requirements on coating adhesion. Coating peeling is never caused by a single factor, covering four major dimensions: substrate pre-treatment defects, improper electroplating process control, wear and corrosion from injection molding working conditions, and incorrect post-mold repair and polishing operations. This paper breaks down root causes of peeling and provides implementable improvement solutions respectively.
I. Mold Substrate Pre-Treatment Defects (Primary Root Cause of Coating Peeling)
Inadequate surface preparation of mold steel prevents stable bonding between coating and base material, leading to large-area peeling under thermal and mechanical loads during molding.
Residual oil contamination: Polishing wax, anti-rust oil and grease remain in deep cavities, ribs and blind corners after polishing. The oil film isolates coating from substrate, triggering spot peeling after electroplating.
Oxidation and EDM deteriorated layers: Rust and loose metamorphic white layers left by electrical discharge machining are not fully removed. Coatings attach to fragile oxidized layers and peel off entirely under minor force.
Unbalanced surface roughness: Over-polished mirror molds lack micro texture for mechanical interlocking, weakening coating grip force; textured molds trap contaminants within texture gaps, also reducing adhesion.
Unrelieved substrate internal stress: Internal stress accumulates from mold quenching and CNC machining. High temperature during electroplating induces slight steel deformation, pulling coatings to generate cracks and peeling.
Corresponding Improvement Measures
After polishing and EDM processing, apply ultrasonic degreasing with alkaline cleaning agents for over 30 minutes, paired with high-pressure air flushing for deep cavities and tiny ribs. Sand all EDM areas with fine white corundum blasting to eliminate white layers and oxidation, forming uniform micro rough surfaces to boost mechanical interlocking. New molds receive tempering after heat treatment to release internal stress, plus low-temperature stress relief baking before plating. Short-time acid activation removes temporary oxide films and activates steel surfaces prior to electroplating.
II. Improper Electroplating Production Process Control
Unbalanced electroplating parameters and unreasonable coating structures drastically reduce bonding strength, divided into pre-treatment, electroplating parameters and coating thickness issues.
Over-etching during pickling: Excessively long acid activation corrodes steel surfaces to form loose micropores, creating unstable foundations for coatings.
Unstable electroplating current: Fluctuating current generates coarse crystalline coatings with internal pinholes and residual stress, expanding micro cracks under repeated cold-hot cycles.
Excess single-layer coating thickness: Hard chrome plating over 0.08 mm creates massive internal stress, causing cracking and flaking.
Absence of transition layers in composite coatings: Direct hard chrome plating on steel substrates generates large thermal expansion coefficient differences between two materials, leading to delamination after cyclic molding temperature changes.
Corresponding Improvement Measures
Strictly control acid activation duration at 1–3 minutes based on steel grades to avoid over-corrosion. Adopt constant segmented current during plating: low initial current forms fine transition underlayers, followed by standard current to build coating thickness. Limit hard chrome thickness to 0.03–0.06 mm; precision molds prioritize nickel-chromium composite coatings with electroless nickel transition layers to buffer thermal expansion differences. Conduct hydrogen embrittlement relief baking at 200–220°C for 4 hours post-plating to eliminate internal coating stress and prevent crack propagation and peeling.
III. Coating Damage and Delamination Induced by Injection Molding Working Conditions
Continuous temperature fluctuation, pressure friction and chemical corrosion during mass production gradually damage coatings, categorized as service-stage peeling triggers.
Alternating thermal stress: Large differences in thermal expansion coefficients between steel and coatings create cyclic tensile stress from frequent temperature rise and fall, gradually expanding micro cracks in coatings.
Abrasion from filled plastics: Glass fiber and mineral reinforced plastics continuously scour mold cavities, wearing through coating surfaces and triggering edge warping and peeling.
Chemical corrosion from plastic decomposition: Corrosive plastics including PVC, flame retardant PA and POM release acidic decomposition gas, corroding bonding interfaces between coating and substrate to form corrosive gaps and full-area peeling.
Local friction wear: Frequent sliding of ejector pins, slides and lifters abrades coatings at corners, which become priority peeling zones.
Storage rust: Long-term shutdown without anti-rust protection allows water vapor penetration into coating micropores; internal rust expansion lifts coatings from substrates.
Corresponding Improvement Measures
Standardize injection machine temperature parameters to minimize drastic temperature fluctuations. Molds processing glass-filled plastics adopt thickened nickel transition layers to upgrade wear resistance. Molds for corrosive plastics use corrosion-resistant nickel-phosphorus coatings, with regular cleaning of mold cavity decomposition residues. Grind matching sliding components post-coating to lower friction resistance. Thoroughly clean mold cavities and spray neutral anti-rust oil before long shutdown, sealing molds to isolate moisture.
IV. Improper Post-Mold Repair and Polishing Operations
Incorrect grinding and polishing during mold maintenance for flash and drag marks directly destroy coating integrity.
Coating thinning and cracking from rough abrasives: Coarse sandpaper and oil stones used for heavy grinding thin local coatings and crack edges, accelerating peeling expansion during subsequent molding.
Instant high temperature from over-speed polishing: Excessively high polisher rotation speed generates instantaneous friction heat, debonding coating-substrate interfaces.
Patch welding without full old coating removal: Partial repair welding retains residual original coating, causing delamination between old and new plating layers and immediate peeling under heat.
Texture etching damage: Texture rework etchant penetrates coating gaps and corrodes base bonding surfaces.
Corresponding Improvement Measures
Only use 1500-grit or finer sandpaper and diamond polishing paste for light polishing during coated mold maintenance, lowering grinding intensity and polishing rotation speed. For partial repairs, fully grind away all old coatings in the target area, complete full pre-treatment before local re-plating. Seal intact coating zones before texture rework, limiting etching liquid contact only to substrate areas to avoid interface corrosion. Conduct coating adhesion inspection via tape pull test post-repair before resuming mass production.
Conclusion
Mold coating peeling results from superposition of substrate pre-treatment defects, unregulated electroplating processes, abrasion and corrosion from molding conditions, and improper maintenance polishing. Residual contaminants and steel deteriorated layers during pre-treatment are congenital root causes of peeling; unbalanced electroplating parameters and over-thick coatings embed hidden internal stress risks. High temperature, chemical corrosion and mechanical abrasion during mass production accelerate coating damage, while improper repair grinding directly breaks intact coating structures. Improvement follows the priority of front-end control: new molds strictly implement sandblasting, degreasing and stress relief procedures; electroplating adopts nickel-chromium composite transition layers plus hydrogen relief baking; select matching coating materials based on plastic raw material properties and stabilize molding temperatures during production; adopt refined light polishing processes for mold maintenance. Multi-dimensional synchronized control eliminates coating flaking and peeling from the source, extending mold service life and stabilizing plastic product quality during mass production.
