Views: 0 Author: Site Editor Publish Time: 2026-10-27 Origin: Site
When we quote an injection mold, surface finish and draft angle are two essential items that customers frequently inquire about. Mold surface treatment mainly falls into two categories: SPI polish finish and etched texture (graining). Different finishes carry different requirements for draft angle.
Draft angle is the slight taper designed on the side wall of plastic parts. After plastic cools and shrinks, it will clamp tightly around the mold core. The taper allows the part to slide out smoothly during ejection, preventing drag marks, whitening, tearing and sticking to the mold.
Polishing only removes steel marks, without creating grain grooves. Contact between plastic and steel is smooth surface friction only.
SPI A (Mirror / High gloss): 0.5°~1° draft. Used for transparent parts and bright cosmetic housing.
SPI B (Semi-gloss sand polish): 1°~1.2° draft. Common for home appliance covers.
SPI C (Oil stone matte): 1°~1.5° draft, for hiding minor steel scratches.
SPI D (Sand blast matte): 1.5° draft. Slight pits on steel surface, weak interlock.
Texture is etched concave-convex grooves on mold steel. Plastic will flow into these grooves during injection and form mechanical interlock after shrinkage.
VDI12 / VDI15 Extra fine grain: ≥1.2°
VDI18 Fine grain (most popular): ≥1.5°
VDI21 / VDI24 Medium grain: ≥2.0°~2.5°
VDI27 / VDI30 Deep grain: ≥3.0°
VDI33+ Heavy deep grain: ≥3.5°~4°
MT11000 Fine sand grain: ≥1.5°
MT12000 Lychee grain: ≥2.0°
MT13000 Leather grain: ≥2.5°~3°
MT14000 Rough leather grain: ≥3°~4°
Polished steel has no grooves. There is only simple sliding friction between plastic and steel, so small draft works. Texture creates countless tiny peaks and valleys. Melted plastic fills every groove. After cooling shrinkage, the plastic locks into these indentations — this is mechanical interlock, similar to velcro hooks.
Shallow texture has shallow grooves. The plastic only lightly engages with the grain, so moderate draft is enough to release. Deeper texture means deeper grooves. The plastic sinks deeper into the grain and gets firmly hooked. If we keep the same small draft angle as polished surface, the plastic cannot separate from the bottom of grooves during ejection. The texture will drag and scratch the cosmetic surface, resulting in whitening, tearing or stuck parts.
The deeper the grain, the stronger the interlocking force, hence a larger draft angle is required.
Material reminder: Soft materials like PP, POM and TPE scratch easily. For the same texture, add an extra 0.5° draft compared with ABS / PC.
If the customer insists on deep texture together with a very small draft angle, we must inform the risk in advance. It will cause unstable mass production and high scrap rate. Two practical solutions: increase the draft angle, or select a shallower texture.
