Classification Standard of Ra Surface Roughness for Chinese Plastic Molds
Surface roughness is one of the most critical technical indicators for plastic injection molds, directly determining the surface gloss, appearance quality, demolding performance and batch consistency of molded plastic products. In Chinese mold manufacturing industry, Ra (arithmetic mean deviation of roughness profile) is adopted as the core quantitative evaluation index, combined with international mainstream SPI polishing standards and VDI 3400 texture standards to form a localized and unified grading system. This standardized classification eliminates inconsistent evaluation criteria between design, processing and quality inspection, unifies factory acceptance specifications, and adapts to the high-precision manufacturing requirements of domestic consumer electronics, medical devices and automotive plastic parts.
I. Optical Mirror Grade (Ra ≤ 0.10μm, SPI A Series)
The optical mirror grade is the highest surface standard for high-end transparent and high-gloss plastic molds in China, widely applied in PSU, PC, PMMA and other transparent engineering plastic products. Any tiny scratches, pits or polishing lines on the mold cavity will be completely copied on the plastic surface, resulting in foggy marks, bright lines and poor light transmission. Domestic high-standard mirror molds strictly implement SPI A1 to A3 grading with matched Ra values and polishing processes.
SPI A1 super mirror (Ra 0.012–0.025μm) adopts ultra-fine 6000# diamond paste polishing, achieving distortion-free mirror imaging effect. It is mainly used for optical lenses, light guide plates and ultra-high transparency medical windows. SPI A2 fine mirror (Ra 0.025–0.05μm) is the most commonly used mirror standard for domestic transparent molds, suitable for 3C electronic shells and automotive lamp transparent parts. SPI A3 high gloss mirror (Ra 0.05–0.10μm) delivers uniform high-gloss effect without obvious mirror reflection, meeting the appearance requirements of ordinary high-gloss plastic shells. All mirror-grade molds require dust-free polishing environment and multi-angle strong light inspection to ensure zero visible surface defects.
II. Semi-gloss Fine Polishing Grade (Ra 0.10–0.40μm, SPI B Series)
As the mainstream appearance standard for civil plastic products in Chinese factories, the semi-gloss grade balances appearance effect, demolding performance and manufacturing cost. Polished with fine sandpaper instead of high-cost diamond paste, this grade effectively weakens micro machining traces and properly covers minor molding defects such as faint weld lines and slight sink marks.
SPI B1 (Ra 0.05–0.10μm) presents soft semi-high gloss, commonly used for home appliance panels and charger shells. SPI B2 (Ra 0.10–0.20μm) features low uniform gloss for daily plastic appearance parts. SPI B3 (Ra 0.20–0.40μm) is applied to non-visible inner surfaces of products. Domestic processing specifications stipulate unified one-way polishing along the demolding direction to avoid cross texture, ensuring consistent surface uniformity of mass-produced molds.
III. Matte Stone Polishing Grade (Ra 0.40–1.60μm, SPI C & VDI12–VDI24)
Matte grade surfaces are formed by uniform oil stone grinding, featuring non-reflective and delicate texture, which are widely used in Chinese automotive interior parts, instrument shells and industrial equipment accessories. This grade perfectly hides conventional molding defects and improves product texture and anti-fingerprint performance.
In domestic unified matching standards, SPI C1 (Ra 0.35–0.40μm) corresponds to VDI12 fine matte, SPI C2 (Ra 0.80–1.00μm) corresponds to VDI18 medium matte, and SPI C3 (Ra 1.25–1.60μm) corresponds to VDI24 coarse matte. Most domestic medium and large batch matte molds prioritize EDM spark texture forming matching VDI standards to reduce manual polishing workload. Meanwhile, the demolding draft angle is appropriately increased according to roughness parameters to prevent surface galling during mass production.
IV. Sandblasting and Spark Texture Grade (Ra ＞1.60μm, SPI D & VDI27+)
This rough texture grade includes EDM spark patterns and dry sandblasting textures, which are widely used for non-appearance surfaces, anti-slip structural parts and industrial plastic shells in domestic molds. It features strong wear resistance and excellent defect hiding ability.
SPI D1 fine sandblasting (Ra 1.6–3.2μm), SPI D2 medium sandblasting (Ra 3.2–6.3μm, corresponding to VDI30) and SPI D3 coarse sandblasting (Ra 6.3–18μm, corresponding to VDI36) constitute the mainstream domestic rough texture grading system. Chinese mold processing specifications clearly point out that single Ra value cannot define surface effect. Spark texture and sandblasting texture with the same Ra value present completely different visual effects, so engineering drawings must mark both Ra value and processing standard to avoid delivery disputes.
V. Domestic Industrial Application Specification and Inspection Standards
Based on national manufacturing characteristics, Chinese mold industry has formed targeted inspection specifications. First, Ra value is used as the basic detection index, while Rz parameter is supplemented for high-precision transparent molds to avoid local sharp protrusions scratching products. Second, mass-production molds reserve 10%–15% roughness margin during acceptance to offset micro wear in long-term operation. Third, all grading inspections comply with ISO 4287 testing standards, adopting professional roughness meters combined with 300mm-distance visual inspection under multi-angle strong light to ensure comprehensive quality control.
Conclusion
The Chinese Ra classification standard for plastic mold surface roughness forms a complete grading system covering optical mirror, semi-gloss fine polishing, matte grinding and rough texture. It combines international SPI and VDI standards with domestic industrial mass-production characteristics, realizing unified definition from design parameter marking, processing technology formulation to finished product acceptance. Standardized roughness grading effectively improves mold processing consistency, reduces rework and revision rates, stabilizes plastic product appearance quality, and provides standardized technical support for the high-quality development of China’s precision mold manufacturing industry.
