SPI to VDI 3400 to Mold-Tech Conversion Chart

There is no official conversion between SPI, VDI 3400, and Mold-Tech. The three mold texture systems can only be related through surface roughness (Ra). Converting the SPI roughness values published by DME with the VDI formula shows where they meet: SPI A and B finishes sit below the VDI reference scale, C-1 and D-1 land near VDI 8 to 10, C-2 and D-2 near VDI 16 to 18, C-3 near VDI 20 to 21, and D-3, the coarsest SPI grade, near VDI 34 to 35. Large parts of the VDI scale, from about VDI 24 to 33 and above VDI 36, have no SPI counterpart at all. Mold-Tech has no numeric equivalent. The chart below gives every SPI grade with its Ra and approximate VDI position.

SPI to VDI 3400 conversion chart

The table uses the approximate Ra published for each SPI grade in DME's mold finish guide and converts it into a position on the VDI 3400 scale. Positions below VDI 12 are calculated points on the underlying scale; they are not reference grades and are not specified on drawings.

SPI grades with DME reference Ra and approximate VDI 3400 position

SPI grade	Finishing method	Ra (µin, DME)	Ra (µm, approx.)	Approx. VDI 3400 position
A-1	#3 diamond buff	0 to 1	Up to 0.025	Below VDI scale (polish)
A-2	#6 diamond buff	1 to 2	0.025 to 0.05	Below VDI scale (polish)
A-3	#15 diamond buff	2 to 3	0.05 to 0.076	Below VDI 0
B-1	600 grit paper	2 to 3	0.05 to 0.076	Below VDI 0
B-2	400 grit paper	4 to 5	0.10 to 0.13	About VDI 0 to 2
B-3	320 grit paper	9 to 10	0.23 to 0.25	About VDI 7 to 8
C-1	600 stone	10 to 12	0.25 to 0.30	About VDI 8 to 10
C-2	400 stone	25 to 28	0.64 to 0.71	About VDI 16 to 17
C-3	320 stone	38 to 42	0.97 to 1.07	About VDI 20 to 21
D-1	Dry blast, #11 glass bead	10 to 12	0.25 to 0.30	About VDI 8 to 10
D-2	Dry blast, #240 aluminum oxide	26 to 32	0.66 to 0.81	About VDI 16 to 18
D-3	Dry blast, #24 aluminum oxide	190 to 230	4.8 to 5.8	About VDI 34 to 35

Approximate, not official. Published SPI tables disagree on roughness, especially for the C and D grades, and a chart built on different SPI values will give different VDI matches. Two finishes with the same Ra can also look different: a stoned C surface has a direction, a blasted D surface does not, and neither looks exactly like an EDM-textured VDI surface. Confirm with a physical sample and your toolmaker.

VDI 3400 to SPI: the reverse lookup

Read from the VDI side, the two systems meet only at a few points, and most VDI reference grades have no SPI grade at the same roughness.

VDI 3400 reference grades and the nearest SPI grade by Ra

VDI 3400 grade	Ra (µm)	Nearest SPI grade
VDI 12	0.40	No direct match, between C-1 / D-1 and C-2 / D-2
VDI 15	0.56	Just below C-2 / D-2
VDI 18	0.80	Upper end of D-2
VDI 21	1.12	Close to C-3
VDI 24 to 33	1.60 to 4.50	No SPI grade at this roughness
VDI 36	6.30	Close to D-3
VDI 39 to 45	8.90 to 18.00	Coarser than any SPI grade

This is the practical reason textured parts are specified in VDI 3400 or as a Mold-Tech pattern rather than in SPI: SPI has no grade for most of the matte and textured range that VDI 3400 covers in steps. For the full VDI chart with Rz, microinches, and the conversion formula, see VDI 3400 surface finish.

Worked examples

SPI D-2 on a drawing, toolmaker working in VDI. DME gives D-2 about 26 to 32 microinches, or 0.66 to 0.81 µm. The formula places that at about VDI 16 to 18, so the nearest reference grade is VDI 18 (Ra 0.80 µm). A clear callout for that toolmaker is "VDI 18 (Ra 0.80 µm), to match approved SPI D-2 sample", which keeps both the number and the visual reference.

VDI 27 on a drawing, supplier working in SPI. VDI 27 is Ra 2.24 µm. No SPI grade sits at that roughness: C-3 is near 1 µm and D-3 near 5 µm. The right answer is not to force an SPI grade but to keep the VDI callout and supply a VDI 27 reference plaque.

Why there is no exact conversion

Each system was built for a different job. SPI, from the Society of the Plastics Industry (now the Plastics Industry Association), grades a mold finish by the finishing method used, from diamond buffing to blasting. VDI 3400, from the Verein Deutscher Ingenieure, grades EDM-textured roughness by a single Ra-based number. Mold-Tech, from Standex, is a catalog of named decorative patterns. Because they define surfaces by method, by roughness, and by pattern respectively, they can only be bridged through Ra, and Ra alone does not capture gloss, pattern, or texture character.

That is also why published conversion charts disagree. They take SPI roughness from different sources, some report microinches and others micrometers, and some match SPI D grades to coarse VDI grades by appearance rather than by Ra. This chart states its basis, DME's published values and the VDI formula, so the result can be checked and reproduced.

Where Mold-Tech fits

Mold-Tech textures are identified by pattern codes, not by roughness grades. Two Mold-Tech textures with similar Ra can look completely different because the pattern, depth, and gloss level define the result. For that reason there is no numeric Mold-Tech to VDI or Mold-Tech to SPI conversion; a Mold-Tech callout is matched by pattern sample, not by number. See Mold-Tech textures for how the system is organized.

How to use a conversion in practice

• Specify in the system that fits the surface. Use SPI for polished, stoned, and blasted cavities, VDI 3400 for EDM matte and textured surfaces, and Mold-Tech for decorative patterns.
• Convert only to communicate. When a supplier works in another system, use the chart to find a starting point, then agree on a physical sample.
• Write the basis on the drawing. If a conversion is used, note the Ra target as well as the grade, for example "VDI 24 (Ra 1.6 µm)".
• Check draft at the same time. A coarser equivalent grade needs more draft. See draft angle by texture.

Frequently asked questions

Is there an official conversion between SPI and VDI 3400?
No. The two standards are related only through Ra, and any conversion is approximate.

What VDI grade is SPI D-3?
On DME's published value of about 190 to 230 microinches (4.8 to 5.8 µm), SPI D-3 sits near VDI 34 to 35. Other published tables give D-3 a wider range.

What VDI grade is SPI C-1 or C-3?
On DME values, C-1 sits at about VDI 8 to 10, below the VDI reference grades, and C-3 at about VDI 20 to 21.

What SPI grade matches VDI 12?
None directly. VDI 12 (Ra 0.40 µm) falls between C-1 and D-1 on one side and C-2 and D-2 on the other.

Can SPI A grades be expressed in VDI?
Not usefully. SPI A finishes are polishes with Ra well below 0.10 µm, below the point where the VDI scale is used.

How do I convert Ra to a VDI grade?
VDI = 20 × log₁₀(10 × Ra in µm). For example, Ra 1.6 µm is VDI 24.

Is Mold-Tech the same as VDI?
No. Mold-Tech is a pattern library from Standex with no numeric cross-reference to VDI or SPI; it is matched by sample.

Why do conversion charts online disagree?
They take SPI roughness from different sources, mix microinches and micrometers, or match by appearance instead of Ra.

Related standards and design pages

For the individual standards, see VDI 3400 surface finish, SPI surface finish, SPI A glossy finish, SPI D textured finish, and Mold-Tech textures. Terms used here are defined in the glossary, and the mold texture standards hub lists every standard we cover.

Normative References & Sources

• DME, Micro Finish Mold Tool Finish Guide, SPI finish designations with approximate Ra: dme.net
• Verein Deutscher Ingenieure (VDI), VDI 3400 guideline: vdi.de
• Plastics Industry Association (formerly SPI): plasticsindustry.org
• ISO 1302, indication of surface texture in technical product documentation: iso.org
• VDI 3400 and Charmilles CH reference comparison scales (Ra per grade).

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Reviewed by Marcel Dias, Technical Editor, with over 28 years in mold texturing. Conversions between standards are approximate and provided for reference only; verify against the primary standard and your toolmaker before production. See our Disclaimer. Last updated: October 9, 2026.