Grade Standards and Application Fields of Mirror Polished Molds in China
Mirror polishing is a core surface treatment process for injection molds, which removes machining traces, spark patterns and tiny surface defects by sequential grinding and diamond powder polishing to obtain low-roughness cavity surfaces. The polishing grade directly decides the visual texture of plastic products. Multiple evaluation systems are widely used in the Chinese mold industry, and confirming the polishing standard in the early DFM review can effectively avoid disputes over appearance quality and reduce repeated polishing rework and schedule loss.
Three mainstream evaluation systems are adopted in China: SPI, VDI3400 and GB/T 14234. The SPI standard divides surfaces into 12 grades, and category A represents mirror finish. SPI A1 is the highest grade with Ra ≤0.012μm, with no scratches or pinholes on the surface. SPI A2 reaches Ra ≤0.025μm, the most widely used mass-production mirror grade, and minor defects can only be seen under strong angled light. SPI A3, with Ra ≤0.05μm, is an economical mirror finish for common high-gloss products. The VDI3400 specification is popular in automotive and medical mold projects, and VDI06, VDI07 and VDI08 belong to the mirror range. VDI07 is equivalent to SPI A2, which is preferred for most high-gloss shells. GB/T 14234 is the domestic standard, which takes Ra roughness value as the core basis. It is important to note that these standards can only be roughly compared, and a single standard should be marked on drawings to prevent inconsistent inspection results.
Processing procedures and steel requirements for different mirror grades
Ultra-mirror finish such as SPI A1 and VDI06 requires high-purity mirror steel including S136 and STAVAX with low sulfur and low impurity. Internal inclusions in steel will form pinpoints after polishing. The polishing process is completed in a dust-free constant-temperature workshop, and diamond powder is gradually refined from coarse to ultra-fine. No EDM modification or heat treatment can be carried out after polishing, otherwise orange peel defects will appear. SPI A2 mirror also uses stainless steel mirror steel, with lower control difficulty and larger fault tolerance, becoming the mainstream choice for mass-produced high-gloss appearance parts. SPI A3 simple mirror has wider material options, and qualified P20 pre-hardened steel can meet the requirements under good impurity control. Ordinary P20 and 718H steel cannot stably reach A2 mirror standard.
Application scenarios for each mirror grade
SPI A1 ultra-mirror molds are mainly used for optical parts such as lenses and light guides. Tiny surface defects will cause light scattering and affect optical performance. SPI A2 mirror is used for home appliance panels, smart device shells and automotive interior high-gloss decorative parts requiring paint-free high-gloss appearance. SPI A3 mirror is applied to secondary decorative surfaces and non-main viewing surfaces, where faint polishing lines are acceptable under oblique light. Semi-gloss and spark grain surfaces are used for hidden inner structures or parts that need subsequent painting and texture etching.
Selection principles and supporting restrictions
Mirror cavities are prone to trapped air, sticking and air marks. The depth of vent slots must be strictly controlled to avoid vent marks on the glossy surface. Filler-reinforced materials are not suitable for high-grade mirror cavities, as fillers will continuously wear the polished surface and make it foggy. It is not advisable to blindly pursue the highest polishing grade. Excessive standards will increase mold cost, extend processing cycle and raise the risk of molding defects. Reasonable matching of polishing grade, steel quality, vent design and raw material characteristics helps to balance cost, delivery cycle and product yield for domestic mold manufacturers.
