China Plastic Mold: High-Gloss Mold Processing Specification for PMMA Transparent Parts
PMMA transparent plastic parts are widely used in optical devices, electronic displays and high-end appearance products due to their excellent light transmittance, high surface gloss and stable physical properties. Unlike ordinary plastic materials, PMMA is extremely sensitive to tiny surface defects. Minor tool marks, polishing scratches, dust residues and micro pits on the mold cavity will be completely replicated on the molded parts, resulting in foggy surfaces, reduced light transmittance and defective appearance. Therefore, high-gloss molds for PMMA transparent products must follow strict and standardized processing procedures covering steel selection, precision machining, EDM control, graded mirror polishing, dust-free assembly and mold protection, so as to ensure stable optical performance and flawless surface quality of finished products.
1. Mold Steel Selection and Heat Treatment Standards
High-purity anti-corrosion mirror mold steel is the basic guarantee for PMMA high-gloss mold manufacturing. Preferred steel grades include S136 and STAVAX, which feature low impurity content, uniform internal structure and excellent corrosion resistance and polishing performance. Unqualified steel with pores, inclusions and uneven hardness is strictly prohibited, as internal defects will form tiny pits after fine polishing and affect the transparency of plastic parts. Pre-hardened steel is controlled at HRC 38–42, while quenched and tempered stainless steel reaches HRC 48–52. Complete heat treatment and deep cryogenic stabilization treatment are required to fully release internal stress, effectively avoiding cavity deformation and surface peeling during long-term repeated molding cycles. All blanks must be inspected by ultrasonic testing before processing to eliminate hidden material defects and ensure the stability of subsequent high-gloss finishing.
2. CNC and EDM Precision Machining Control
High-precision CNC high-speed milling is adopted as the main processing method for PMMA high-gloss cavities to minimize the use of EDM processing, because electric discharge will form a micro white layer and tiny cracks on the steel surface, which are difficult to eliminate completely. Ultra-fine grain carbide tools are used with high-speed, low-feed and small-cutting-depth processing strategies to reduce tool marks. The uniform single-side polishing allowance is controlled between 0.03 mm and 0.06 mm to avoid uneven allowance leading to local over-polishing and surface distortion. If EDM processing is unavoidable, high-purity copper electrodes and low-impurity special spark oil must be used with low-energy finishing parameters. After discharge, ultrasonic cleaning is carried out to remove carbon residues and oxide layers. All cavity workpieces are protected with soft films during transportation to prevent collision scratches.
3. Graded Mirror Polishing Process Specifications
Polishing is the core procedure that determines the gloss and transparency of PMMA molds, and it must follow the strict coarse-to-fine graded polishing principle without skipping any level. Firstly, 800# and 1200# oil stones are used to remove machining tool marks, followed by 1500# and 3000# sandpapers with cross polishing method to eliminate directional texture. After finishing sandpaper grinding, 9 μm and 3 μm coarse diamond paste are used for rough polishing, and 1 μm and 0.5 μm ultra-fine diamond paste for final mirror finishing. Soft wool polishing heads are used throughout the process to avoid hard contact damage. The polishing workshop maintains a dust-free environment, because floating fine dust will form pinpoint pits on the mirror surface. Uniform and gentle force is required to prevent local overheating and orange peel texture. After polishing, the cavity reaches SPI A2–A3 optical mirror standard with no visible scratches, pits or textures under strong light inspection.
4. Dust-Free Cleaning and Assembly Management
All molded components, inserts and ejector parts must undergo multi-stage ultrasonic degreasing and pure water cleaning before assembly to completely remove cutting oil, polishing powder and metal debris. Low-temperature hot air drying is adopted to prevent water stains from forming permanent imprint on the mirror surface. The assembly area is kept clean with anti-static rubber pads. Operators wear dust-free gloves, and only copper tools and plastic tools are allowed to adjust components, avoiding hard steel impact on the high-gloss surface. Before mold closing, the cavity, parting surface and ejector pin gaps are thoroughly purged to eliminate residual particles. Special silicon-free low-precipitation high-temperature lubricating grease is used for movable parts to prevent grease precipitation from causing foggy defects on PMMA products.
5. Mold Trial and Finished Mold Protection Standards
Before trial molding, strong light multi-angle inspection is conducted to confirm no hidden defects on the cavity surface. PMMA raw materials must be fully dried to eliminate moisture-induced bubbles and silver streaks. The first trial adopts low-speed and low-pressure filling to observe the molding surface state and distinguish mold defects from material defects. After passing trial production, the high-gloss cavity is coated with professional silicon-free anti-rust agent and covered with protective film. The mold is stored in a dry environment to avoid surface oxidation and fogging. Regular maintenance is implemented during long-term use, and only dust-free soft cloth and neutral cleaning agents are allowed for cavity cleaning.
Conclusion
The processing of PMMA high-gloss transparent molds is a systematic fine manufacturing project, which relies on high-purity mold steel, stress-relief heat treatment, low-texture precision machining, graded dust-free mirror polishing and standardized assembly protection. Every processing detail directly determines the light transmittance and appearance quality of transparent plastic parts. Implementing unified high-gloss mold processing specifications can effectively eliminate common defects such as scratches, orange peel, pits and foggy surfaces, reduce repeated mold repairs and debugging costs, and provide stable and high-quality mold support for mass production of high-end PMMA optical transparent products.
