What are the SPI surface finish grades for injection moulding?

SPI grades run from A1 (mirror polish, Ra about 0.012-0.025 µm) through B (paper-polished semi-gloss), C (stone-finished matt) to D (bead-blasted textured, up to Ra ~18 µm). A1 is the most expensive; C and D grades hide defects and fingerprints well.

What is VDI 3400 in injection moulding?

VDI 3400 is the European standard for EDM-textured mould surfaces, numbered VDI 12 (finest, Ra ~0.4 µm) to VDI 45 (coarsest, Ra ~18 µm). Each step increases roughness by about 40%. It is the usual texture reference on European tooling quotes, including Nordmould's.

How much draft angle does a textured surface need?

The common rule is 1° of additional draft for every 0.025 mm of texture depth, on top of the base 1-1.5°. Light textures work with 1.5-2° total; heavy textures can need 3-5° or more per side. Underestimating texture draft is a frequent cause of drag marks.

Which surface finish should I choose for a cosmetic part?

Match the finish to function: A-grade polish for optical and clear parts, B-grade for visible glossy housings, C-grade for general matt parts, and VDI/MT textures for grip, glare reduction or hiding flow marks and fingerprints. Specify finish per face, not for the whole part.

Does surface finish affect mould cost?

Yes, significantly. Mirror polishing a large cavity to SPI A1 can add hundreds to thousands of euros in hand labour, and chemical or laser texturing adds cost per textured area. Specifying high finishes only on visible faces is one of the easiest tooling savings.

Can texture hide injection moulding defects?

Partly. A medium texture (VDI 24-30 or equivalent MT) masks minor flow lines, gloss variation, fingerprints and small sink shadows. It cannot hide warpage, short shots or deep sink marks — those need design or process fixes, not surface treatment.

Injection Moulding Surface Finishes: SPI, VDI 3400 & Mold-Tech Explained

The surface finish of an injection-moulded part is the surface finish of the steel that made it. Every moulded part replicates the cavity: polished steel gives glossy parts, textured steel gives textured parts. That is why finish is specified on the tool, per face, before machining — and why changing it later means reworking steel.

Three naming systems cover nearly everything you will see on a European tooling quote: SPI grades, VDI 3400 numbers and Mold-Tech (MT) textures. Nordmould confirms the finish map — which faces get which finish — as part of every DFM review.

What are the SPI finish grades?

The SPI (Society of the Plastics Industry) scale is the oldest and most widely quoted system, running from A1 mirror to D3 coarse blast.

SPI grade	Process	Typical Ra (µm)	Appearance	Typical use
A1	Diamond buff, grade 3	0.012-0.025	True mirror	Lenses, optical parts, clear PC/PMMA
A2	Diamond buff, grade 6	0.025-0.05	High gloss	Premium cosmetic housings
A3	Diamond buff, grade 15	0.05-0.10	Gloss	Visible consumer parts
B1	600-grit paper	0.05-0.10	Semi-gloss	General housings, low sheen
B2	400-grit paper	0.10-0.15	Semi-gloss, slight sheen	Interior trim, covers
B3	320-grit paper	0.28-0.32	Dull semi-matt	Non-cosmetic visible faces
C1	600 stone	0.35-0.40	Fine matt	Functional parts, easy release
C2	400 stone	0.45-0.55	Matt	Hidden structures, clip features
C3	320 stone	0.63-0.70	Coarse matt	Non-visible faces
D1	Dry blast, glass bead	0.80-1.00	Satin texture	Grips, glare control
D2	Dry blast, oxide	1.60-2.80	Flat texture	Rugged housings, tools
D3	Dry blast, coarse	3.20-18.0	Heavy texture	Industrial, hidden robustness

Ra values are typical published ranges, not gauge readings — the standard defines the process and appearance, and actual roughness varies with steel and operator. Use Ra as a comparison between grades, not as an acceptance number.

What is VDI 3400?

VDI 3400 is the European reference for surfaces textured by EDM (spark erosion). It is a numeric scale where each step multiplies roughness by roughly 1.4 — so the numbers are memorable and the progression is logarithmic.

VDI	Ra (µm)	VDI	Ra (µm)
VDI 12	0.40	VDI 30	3.15
VDI 15	0.56	VDI 33	4.50
VDI 18	0.80	VDI 36	6.30
VDI 21	1.12	VDI 39	9.00
VDI 24	1.60	VDI 42	12.50
VDI 27	2.24	VDI 45	18.00

VDI 18-27 reads as a fine satin texture; VDI 30-36 is a medium, clearly visible grain; VDI 39-45 is a heavy industrial texture. Because EDM texturing happens as part of cavity machining, a VDI finish usually costs little or nothing extra — unlike hand polishing. For parts produced through Nordmould's network, VDI is the default texture language on European-built tools.

What are Mold-Tech (MT) textures?

Mold-Tech textures are chemically etched patterns with their own numbering (MT-11000 series and up), applied by specialist texture houses. They range from fine leather grains to deep geometric patterns, and they are the standard way to get a designed, repeatable aesthetic rather than a generic blast.

Etching adds a per-area cost and a few days of lead time, and deep patterns demand more draft. Specify MT numbers on the cosmetic drawing, with the texture boundary marked — etch stopping mid-face leaves a visible witness edge, so boundaries should sit on natural edges or grooves.

How much draft does a texture need?

Texture locks the part against the steel on every micro-undercut of the grain. The widely used rule: add 1° of draft for every 0.025 mm (25 µm) of texture depth, on top of the base draft.

Finish	Approx. depth	Recommended draft per side
SPI A/B polish	~0	0.5-1.5°
SPI C / VDI 12-18	0.01-0.03 mm	1.5-2°
VDI 24-30 / light MT	0.03-0.08 mm	2-3°
VDI 33-39 / medium MT	0.08-0.15 mm	3-4°
VDI 42-45 / heavy MT	0.15-0.30 mm+	4-6°+

Insufficient texture draft is one of the most common causes of drag marks and scuffed cosmetic faces. If your enclosure walls are near-vertical and textured, raise it in the DFM review before the tool is cut — adding draft in CAD costs nothing.

How does finish choice affect cost and quality?

Three cost levers matter. First, polishing labour: A-grade mirror polishing is skilled hand work priced by the hour, and large or deep cavities multiply it. Second, texturing processes: EDM texture is nearly free, chemical etch is moderate, laser texture is premium. Third, steel and heat treatment: only hardened, through-quality steels take and keep a true A1 polish.

Quality effects run both ways. Glossy surfaces expose every flow line, weld line and sink shadow; a VDI 24-30 grain hides them. Matt and textured surfaces reduce visible fingerprints and glare but hold dirt more readily. Clear materials (PC, PMMA) effectively require A-grade polish — any texture on the cavity becomes optical distortion in the part.

The practical rule Nordmould applies in quoting: specify finish per face on the 2D drawing — cosmetic faces get what marketing wants, everything else gets the cheapest finish that releases cleanly.