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Surface Finish Standards Guide

Surface finish is one of the most commonly specified quality requirements for injection molded parts. The right surface finish affects appearance, feel, functionality, and cost. This guide explains the major surface finish standards and how to specify them correctly.

SPI Surface Finish Standards

The Society of the Plastics Industry (SPI) classification is the most widely used standard in North America and Asia.

SPI A — Diamond Buffed (High Gloss)

SPI A finishes are produced by diamond polishing the mold surface. They produce the highest gloss (mirror-like) finish on molded parts. Three grades: A-1 (highest, #3 diamond), A-2 (#6 diamond), A-3 (#15 diamond). Typical Ra: 0.012-0.025μm. Applications: optical lenses, transparent parts, cosmetic consumer products.

SPI B — Paper Buffed (Medium Gloss)

SPI B finishes are produced by buffing with abrasive paper. Three grades: B-1 (600 grit), B-2 (400 grit), B-3 (320 grit). Typical Ra: 0.025-0.05μm. Applications: general cosmetic parts, automotive interior trim, appliance housings.

SPI C — Stone Polished (Semi-Gloss)

SPI C finishes are produced by stone polishing. Three grades: C-1 (600 stone), C-2 (400 stone), C-3 (320 stone). Typical Ra: 0.05-0.10μm. Applications: structural parts with moderate cosmetic requirements, under-hood components.

SPI D — Dry Blast (Matte)

SPI D finishes are produced by sandblasting the mold surface with fine glass beads or aluminum oxide. Three grades: D-1 (fine blast), D-2 (medium), D-3 (coarse). Typical Ra: 0.15-0.40μm. Applications: textured surfaces, grip surfaces, parts where gloss is undesirable.

VDI 3400 Standard

The VDI 3400 standard is commonly used in Europe and specifies surface texture using a numerical scale from 0 (polished) to 45 (very rough). This standard corresponds to specific Ra values:

VDI Grade	Ra (μm)	Equivalent SPI	Typical Application
VDI 0-15	0.01-0.10	SPI A / B	Polished, high gloss
VDI 18-24	0.20-0.40	SPI C	Fine texture, semi-gloss
VDI 27-33	0.56-1.00	SPI D	Medium texture, matte
VDI 36-42	1.60-4.00	—	Coarse texture, grip surface

Ra vs. RMS vs. Rz

Ra (Arithmetic Average): Most common roughness parameter. Average deviation from the mean line.
Rq/RMS (Root Mean Square): Similar to Ra but more sensitive to peaks and valleys. Approximately 1.1x Ra.
Rz (Average Maximum Height): Average of the highest peaks minus lowest valleys over five sampling lengths. Approximately 4-6x Ra.

Cost Considerations

Higher quality surface finishes cost significantly more because they require longer polishing time and higher skill levels. A cavity polished to SPI A-1 can cost 3-5x more than one with SPI C-3 finish. Specify the lowest acceptable finish grade to control costs.