Surface Finish & Texture Standards — SPI, VDI 3400, and Mold-Tech Explained

An engineer writes “SPI B-1” on a drawing and moves on. The mold maker reads it and has three questions: SPI B-1 achieved how — by polishing, by stoning, or by EDM? On what steel hardness? On a surface that is flat, contoured, or inside a deep rib? Each answer changes the cost, the lead time, and how the surface actually looks and feels.

Surface finish specifications sit at an uncomfortable intersection in injection molding. They are aesthetic — the customer sees and touches the surface. They are functional — the surface affects release, friction, and coating adhesion. And they are expensive to get wrong — refinishing a mold surface after the tool is built costs hundreds to thousands of dollars and delays T1 by a week. Getting the spec right the first time is worth the effort.

This guide covers the four surface finish standards you will encounter on part drawings — SPI, VDI 3400, Mold-Tech, and Ra — with the practical information an engineer needs to specify them correctly and a buyer needs to understand the cost implications.

1. SPI Surface Finish Standards

The SPI (Society of the Plastics Industry) standard is the most widely used surface finish classification in North America and Asia. It divides mold finishes into four grades — A, B, C, D — based on the manufacturing method used to produce the surface on the mold steel.

SPI Grade A — Diamond Polish (High Gloss)

Produced by diamond buffing the mold steel to a mirror finish.

Where it is used: Optical lenses, transparent covers, high-gloss cosmetic surfaces, LED light guides. Anywhere the plastic part needs to transmit light or reflect with mirror clarity.

Cost impact: A-grade polishing is the most expensive surface finish. A mirror-polished A-1 surface on a flat area adds 2–4 hours of hand polishing per cavity. On a contoured surface or inside a rib, the time doubles because the polisher cannot use a flat lap and must work with small rotary tools and compound. Budget an additional $150–400 per cavity for A-1, $100–250 for A-2, $60–150 for A-3, depending on geometry complexity.

Design rule: A-grade polish amplifies every imperfection. A weld line that is invisible on a B-grade surface becomes a visible streak on an A-2 surface. A slight sink mark over a rib that measures 2 μm in depth is below the threshold of feel on any other finish — and plainly visible as distortion in the reflection on A-grade. If specifying A-grade, the part geometry must be designed to avoid the features that produce these defects, or the part must be gated so the defects fall on a non-cosmetic surface.

SPI Grade B — Paper / Stone Polish (Semi-Gloss)

Produced by sanding the mold with increasingly fine grit paper or stone.

Where it is used: Consumer electronics housings, appliance fascias, automotive interior trim. B-1 is the default cosmetic surface for most injection molded products.

Cost impact: B-grade is the baseline. It is what a mold maker produces as standard finish work on cavity surfaces. A B-1 finish adds no cost premium to the mold — it is priced into the tooling quote. B-2 and B-3 are marginally cheaper because less polishing time is required, but the savings are small ($30–60 per cavity) and rarely worth specifying a lower grade purely for cost.

SPI Grade C — Grit Stone Finish (Rough Matte)

Produced by using coarse grit stones on the mold surface.

Where it is used: Internal structural parts, surfaces that will be painted or coated, non-cosmetic surfaces where a uniform appearance is still required.

Design rule: C-grade surfaces provide excellent adhesion for paint and coatings. The surface roughness creates mechanical interlock for the coating layer, improving cross-hatch adhesion test results compared to B-grade or higher. If the part will be spray painted, specifying C-1 or C-2 on the painted surfaces is technically correct — it improves paint adhesion without adding cost.

SPI Grade D — Blast Finish (Textured)

Produced by media blasting the mold surface — glass bead, aluminum oxide, or sand.

Where it is used: Tool housings, industrial equipment covers, surfaces where grip is required, areas where minor molding defects need to be hidden.

Cost impact: D-grade finishes are generally cheaper than A or B grade because blasting is faster than polishing. However, the finish is not durable — glass bead blasted surfaces wear faster than polished surfaces over the life of the mold. A D-1 finish on a mold running glass-filled material may degrade visibly within 50,000 shots.

2. VDI 3400 Texture Standards

VDI 3400 is the German standard for mold surface textures, produced by electrical discharge machining (EDM) rather than polishing or blasting. It is the dominant texture standard in European manufacturing and widely used in Asia.

VDI textures are produced directly by the EDM process — the spark erosion creates a uniform pitted surface whose roughness is controlled by the EDM parameters (current, pulse duration, electrode material). A VDI 12 texture is produced with fine EDM settings. A VDI 45 texture is produced with aggressive settings that create deeper, larger craters.

Cost impact: Within the VDI 12–27 range, there is negligible cost difference — the EDM settings are adjusted in the machine program. Above VDI 30, larger electrodes and longer burn times increase EDM cost by 10–25%. Below VDI 12, the surface transitions to a polish-grade finish that cannot be achieved by EDM alone — it requires manual polishing after EDM, adding cost.

Design rule: VDI textures on a cavity surface must account for draft angle. A VDI 33 texture on a vertical wall without adequate draft will cause the part to stick during ejection. Minimum draft for VDI textures:

• VDI 12–18: 0.5° minimum
• VDI 21–27: 1.0° minimum
• VDI 30–36: 1.5° minimum
• VDI 39–45: 2.0–3.0° minimum

3. Mold-Tech Texture Standards

Mold-Tech is a brand name — not a standard — but it has become the de facto reference system for chemical-etched textures in injection molding. Mold-Tech textures are produced by chemically etching the mold steel using a photo-resist process: a pattern is printed on a film, transferred to the mold surface, and acid-etched through the resist to create the texture depth and pattern.

Unlike SPI (mechanical polish/blast) and VDI (EDM), Mold-Tech textures are capable of reproducing specific patterns — leather grain, wood grain, geometric patterns, carbon fiber weave. This makes Mold-Tech the standard for automotive interior parts, consumer product styling, and any application where the texture must mimic a natural material.

Common Mold-Tech references:

Cost impact: Chemical etching a Mold-Tech texture onto a cavity costs $300–1,200 per cavity depending on the pattern complexity, cavity surface area, and steel hardness. Harder steels (H13, S136 at HRC 48–52) etch more slowly and cost more than P20 (HRC 28–32). Deep textures (>0.100 mm) require longer etch times and higher acid concentrations, increasing cost by 30–50%.

Critical design constraint: Chemical etching increases the cavity dimensions by the texture depth. If a 100.00 mm cavity surface is etched to 0.075 mm depth, the cavity becomes 100.15 mm — the plastic part will be larger by the texture depth. The mold design must account for this by cutting the cavity undersized before etching. If the texture is specified after the mold is built, the cavity must be re-cut or the texture depth must be compensated elsewhere — both expensive corrections.

4. Ra (Roughness Average) — The Universal Metric

Ra is the arithmetic average of surface roughness deviations from the mean line, measured in micrometers (μm). It is the most common quantitative surface finish specification and appears on engineering drawings regardless of which texture standard is used.

Ra is measured with a profilometer — a stylus dragged across the surface that records the vertical deviations. The result is a single number that describes the average roughness but says nothing about the pattern, directionality, or visual appearance.

Correlating Ra to the standards:

What Ra does not tell you:

• Directionality. A surface with Ra 0.40 μm produced by linear sanding feels and reflects light differently than one produced by random-orbital EDM at the same Ra value. Two surfaces with identical Ra can look completely different.
• Peak height vs valley depth. Ra averages everything. A surface with a few deep scratches and an otherwise smooth field may have the same Ra as a uniform fine texture — but the customer will see the scratches.
• Visual appearance. Ra has zero correlation with gloss, color uniformity, or perceived quality. Use Ra as a process control metric, not as a visual specification.

5. How to Specify Surface Finish on a Drawing

A correct surface finish specification answers four questions:

1. What standard? SPI, VDI 3400, Mold-Tech, or Ra.
2. What grade or pattern? B-1, VDI 24, MT-12000, Ra 0.40 μm.
3. Where on the part? Which surfaces. “All cosmetic surfaces SPI B-1” is acceptable if accompanied by a drawing that marks which surfaces are cosmetic.
4. What is the verification method? Visual comparison to a reference sample, profilometer measurement, or both.

Acceptable drawing callouts:

• SPI B-1, all A-side surfaces
• VDI 24, cavity surfaces per texture map
• Mold-Tech YS-1285, instrument panel visible surface
• Ra ≤ 0.10 μm, lens surface only

Ambiguous callouts that will generate questions:

• “SPI finish” — which grade?
• “Textured” — what pattern? What depth?
• “Smooth” — SPI A-3 smooth or SPI B-1 smooth? The difference is visible and costs money.
• “As machined” — a milled surface at Ra 0.80 μm or an EDM surface at Ra 3.15 μm? Both are “as machined.”

Surface finish is specified in two lines on a drawing and produces consequences that run through tooling cost, cycle time, part quality, and mold maintenance for the life of the program. The standards exist so that an engineer in Stuttgart and a mold maker in Shenzhen have the same understanding of what “VDI 24” means. Use them precisely, and the surface that comes back on T1 is the surface you expected.

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