SPI finish

In practice

Textured D grades hide sink and scratches better but need more draft. A-grade finishes also show every flow line.

Where it comes up

• Plastic injection moldingMolten thermoplastic injected into a steel or aluminum mold. Lowest piece price for plastic parts at 5,000+ a year.

Related terms

Draft angle

Draft angle is the taper applied to part walls parallel to the direction of mold opening so the part releases without dragging. Injection molded parts typically need 1–2° of draft, plus about 1–1.5° more per 0.001 in (0.025 mm) of texture depth. Die castings typically need 0.5–2°.

Surface roughness (Ra)

Surface roughness (Ra) is the arithmetic average of a surface profile's deviations from its mean line, reported in microinches or micrometers. Typical machined surfaces run 63–125 µin (1.6–3.2 µm) Ra, ground surfaces 16–32 µin (0.4–0.8 µm), and seal surfaces often 16 µin (0.4 µm) or finer.