China Plastic Molds – Surface Roughness Control Specification for Electroplated Plastic Molds

After CNC machining, EDM processing and polishing, plastic mold cavities, inserts and mold components often undergo hard chrome electroplating treatment. The electroplated coating will change the original surface roughness of the mold substrate, which directly affects plastic part appearance, demolding performance and wear resistance. Improper control of coating thickness, pre-treatment process and base surface roughness easily leads to surface defects such as pitting, orange peel effect and inconsistent gloss. Systematic full-process control standards are required to stabilize the surface quality of electroplated molds.

1. Graded Control Standard of Surface Roughness

Surface roughness testing is carried out on molding surfaces of cavities after electroplating, using a surface roughness tester, with measuring direction perpendicular to processing texture. Classification standards are formulated according to the appearance grade of molded plastic products.

For Grade A mirror surface plastic parts, Ra ≤ 0.025μm after electroplating; the base material roughness after polishing should be controlled within Ra ≤ 0.020μm, reserving allowance for roughness rise caused by electroplating deposition.

For Grade B high-gloss appearance plastic parts, Ra ≤ 0.050μm after electroplating, and the original base roughness is limited to Ra ≤ 0.040μm.

For Grade C matte and texture surfaces, the post-electroplating roughness is maintained between 0.10μm and 0.40μm. Texture surfaces shall avoid excessive polishing to prevent texture from being filled and smoothed by plating layers.

Functional areas including parting surfaces, exhaust slots and sliding fitting surfaces require Ra ≤ 0.8μm after electroplating, balancing sealing performance and wear resistance. All inspections shall be conducted at normal temperature after electroplating cleaning and drying. At least three key positions on each cavity are sampled and recorded.

2. Base Material Pre-treatment and Roughness Control

Electroplating cannot make up for severe surface defects of mold substrates. Tool marks from machining, EDM spark lines and local corrosion pits must be completely removed before plating. Rough EDM processing shall reserve sufficient polishing allowance; it is forbidden to carry out electroplating directly on surfaces with residual spark textures. Polishing shall follow progressive grit transition without skipping abrasive grades, to avoid hidden scratches.

After polishing, mold surfaces need thorough cleaning to remove polishing paste, oil and dust. Residual contaminants will cause peeling, pinholes and abnormal fluctuation of roughness. Substrates with rust, cracks and pits must be re-polished and re-inspected after defect repair before being transferred to electroplating procedures.

3. Electroplating Process Parameters Affecting Surface Roughness

Hard chrome coating thickness for plastic mold cavities is generally controlled between 0.03mm and 0.08mm. Excessively thick coatings tend to form chrome nodules and edge protrusions, increasing surface roughness; too thin coatings lead to insufficient service life. Stable current density is essential. Excessive current produces coarse crystal structures and surface particles, while low current results in insufficient surface gloss.

Activation time before electroplating needs strict management. Over activation will corrode mold substrates and form micro pits to raise Ra value. Electroplating solution requires regular filtration to avoid suspended impurities attaching to mold surfaces. Reasonable mold hanging mode prevents air bubbles from accumulating in blind areas, which would cause uneven surface texture.

4. Post-electroplating Trimming and Re-inspection Requirements

Comprehensive roughness testing shall be performed immediately after electroplating. Tiny chrome particles and slight protrusions can be gently trimmed and polished with corresponding whetstones. Trimming areas cannot form obvious boundary traces, and roughness needs re-verification after finishing. Large-area heavy polishing is prohibited, as it may wear through plating layers and lose anti-corrosion and wear-resistant functions.

Textured molds can only have surface deposits lightly cleaned after electroplating. Large-scale polishing will destroy original textures and change surface tactility of plastic products. For molds assembled with multiple inserts, the roughness difference between components should be controlled to avoid visible gloss dividing lines on molded parts.

5. Defect Judgment and Improvement Methods

If measured roughness exceeds the specified upper limit, the workpiece is judged non-conforming. Widespread high roughness should be traced to base polishing quality, cleanliness of electroplating liquid and current parameters. Local out-of-tolerance mainly results from residual scratches, air bubbles and surface particle adhesion. Once batch surface quality abnormalities occur, suspend electroplating operation instead of relying on massive post-polishing for remedy. Track pre-polishing procedures, degreasing processes and tank maintenance records to find root causes. Mold trial production feedback about plastic part gloss shall be continuously collected to optimize base polishing standards and electroplating parameters.

6. Daily Maintenance of Electroplated Mold Surfaces

During mass production, regularly clean carbon deposits and plastic precipitates inside cavities. Soft grinding tools are adopted for cleaning to prevent scratches on electroplated layers and continuous deterioration of surface roughness. Roughness sampling records shall be kept in mold maintenance files. When surface roughness keeps rising accompanied by corrosion spots and scratches, evaluate whether to remove chrome coating, re-polish and re-plate.