Chinese Plastic Mold Complete High‑Gloss Mirror Polishing Procedure

In China’s plastic mold industry, high‑gloss mirror finishing is essential for manufacturing high‑grade home appliance shells, automotive exterior parts and transparent plastic products. The mirror quality directly determines the appearance grade of injection parts. Common defects such as flow lines, pitting, scratches and orange peel cannot be eliminated by injection adjustment alone. High‑gloss mirror processing is a systematic and step‑by‑step manufacturing process. Every working procedure from rough machining to final fine polishing must be strictly controlled to ensure uniform, transparent and flawless mirror surfaces.

1. Rough Machining and Polishing Allowance Control

Rough machining adopts high‑speed CNC milling with carbide tools to remove bulk material efficiently. Cutting parameters are optimized to reduce tool vibration and uneven tool marks. For Chinese standard high‑gloss plastic molds, the unilateral polishing allowance is controlled between 0.15 mm and 0.25 mm. Excessive allowance will increase polishing time and easily cause mold surface deformation; insufficient allowance fails to eliminate deep tool traces, leaving hidden defects for final finishing.

After roughing, the cavity surface must be inspected completely. Deep tool grooves and collapsed areas need welding and re‑milling instead of relying on polishing removal. All sharp edges and burrs are removed to prevent secondary scratching in subsequent grinding processes.

2. Semi‑finishing to Improve Mold Substrate Quality

Semi‑finishing is used to erase uneven rough machining tool marks and create a uniform base surface for polishing. High‑precision CNC milling reduces step spacing and machining ripple height, ensuring no visible coarse tool lines remain on the cavity surface. Uniform substrate texture guarantees consistent material removal speed in later polishing and avoids local surface depression.

After semi‑finishing, stress relief tempering is mandatory to eliminate internal cutting stress and prevent mold deformation during polishing and mass production. High‑gloss mold steels such as NAK80 and S136 must reach standard hardness before grinding. Insufficient hardness will directly cause orange peel and foggy mirror effects during fine polishing.

3. Progressive Sandpaper Grinding to Remove Machining Texture

Sandpaper grinding follows a strict coarse‑to‑fine sequence: 800#, 1200#, 1500# and 2000#. Each previous grinding trace must be completely eliminated before using finer sandpaper. Skipping grades will leave invisible deep textures that cause later mirror distortion and pitting.

Grinding movements are kept stable and uniform, fitting the cavity radian without fixed‑point over‑grinding. Special care is taken for narrow grooves, sharp corners and fine curved surfaces to prevent shape deformation. Kerosene is used as lubrication and cleaning medium to prevent sand particle embedding. The mold cavity is fully cleaned after each grinding stage to avoid residual grit causing polishing defects.

4. Oil Stone Lapping for Micro Surface Refinement

After sandpaper grinding, progressive oil stone lapping from 800# to 3000# is performed to refine micro surface texture. Cross lapping at 90‑degree directions effectively reveals residual fine lines from previous processes and ensures balanced material removal.

Lapping pressure is kept even to avoid local over‑processing. Stress‑concentrated areas such as parting lines and runner inlets require repeated careful lapping. Before changing oil stone grades, the mold surface is thoroughly cleaned to eliminate abrasive particles and maintain consistent surface fineness.

5. Diamond Paste Fine Polishing for Ultimate Mirror Effect

Final high‑gloss mirror effect is achieved through multi‑stage diamond paste polishing. Gradations from 9 μm, 6 μm, 3 μm, 1 μm to 0.5 μm are adopted for high‑standard molds. Soft wool or copper polishing tools are used to avoid hard contact scratches.

Polishing follows the principle of “small amount and multiple times”. Long‑time fixed‑position polishing is strictly prohibited to prevent orange peel layers and foggy surfaces. The polishing environment must be dust‑free, because tiny floating dust will form pitting after polishing. After each grade upgrade, the mold is fully cleaned to remove coarse particles. Final inspection uses oblique light scanning to check for scratches, waves and micro pits.

Summary

Chinese plastic mold high‑gloss mirror processing depends on standardized full‑process control rather than single final polishing. Reasonable roughing allowance, qualified semi‑finished substrate, stable heat treatment and gradual refining grinding are the core guarantees of high‑transparency mirror surfaces. Skipping procedures and over‑polishing are the main causes of orange peel, sink marks and material texture defects. Strict implementation of complete processing standards ensures stable and high‑quality mirror molds, meeting the mass production requirements of high‑end plastic appearance parts in China’s mold industry.