Texture Mapping on Silicone Molds: VDI/MT11000 Standards Explained for Toolmakers

VDI 3400 and MT 11000 are not two names for the same sandpaper.

VDI 3400 (Verein Deutscher Ingenieure) is a spark-erosion / EDM roughness scale used on mold steel. Grades are numbered (commonly discussed as VDI 12 through VDI 45, sometimes written N12-N45). Each step is a defined texture character and roughness from fine satin to heavy orange peel. It is a *standard*, process-tied, and relatively cheap to specify on an industrial LSR or HCR tool.

MT 11000 is not a committee standard. It is a Mold-Tech (Standex Engraving) catalog code in the MT 11xxx family: chemically etched or laser-mapped grains with a published depth and a supplier-recommended draft. MT 11000 itself is a very fine grain in that family (on the order of 0.01 mm depth in Mold-Tech’s published card, with a small recommended draft). Neighboring codes (MT 11010, 11020, and so on) are different depths and different looks. “MT 11000” on a drawing that meant “any Mold-Tech grain” is how you get the wrong plaque.

SPI polishing grades (A-1 through D-3) are a third language, aimed at polish and blast, not at decorative etch. Do not write “SPI A-2 or VDI 24 or MT 11000” as if those were equivalent. Pick the system that matches how the steel will actually be finished, then map it to the *silicone part* you will demold.

What the part sees is the inverse of the steel

Silicone copies. A pit in the steel is a bump on the part. A peak in the steel is a crater in the rubber. LSR will print a fingerprint. That is not the same as surviving strip, filling the grain at the end of flow, or holding the grain after 100,000 shots of a medical washer.

For LSR/HCR steel tools:

• VDI via EDM is consistent, non-directional, and available from any decent toolroom that owns a sinker and a comparator plaque. Fine VDI is a matte that hides mill marks. Heavy VDI is a grip and a thousand micro-undercuts.
• Mold-Tech (or equivalent etch houses: Yick Sang, Standex, local laser texture) is how you get leather, technical grains, logos-as-texture, and brand-matched interiors. The house needs the steel type, hardness, and a mask. Inserts must be designed so the etchant or laser can see the face.
• Laser texture can put VDI-like roughness or a named grain on geometry EDM cannot sit on. It is still a specified plaque, not a Photoshop filter.

For RTV molds (platinum or tin jackets from a master): the texture lives on the master. You do not EDM a 20 Shore A jacket. Finish the master (or the 3D print, or the SLA sealer) to the grain you want copied. A VDI callout on an RTV tool drawing is a category error unless you mean the *mother tool* that makes the master.

If you ignore the inverse, the defect is a grip you specified as “recessed leather” that came out as raised warts because someone textured the wrong half.

Do not publish a fake conversion table

Shops circulate charts that say VDI 21 = MT 11020 = SPI D-2. Those charts are marketing, plus Ra overlap, plus wishful thinking.

Ra overlap is real: a medium VDI and a fine MT 11xxx can share a roughness number and still look different under a customer’s thumb. EDM spark texture is cratered. Chemical etch follows the microstructure of the steel and the film. Leather grains have direction and undercut. A clear LSR keypad will show that difference immediately. A black 70 Shore A boot might hide it.

What to do instead:

• Call one system on the face: “VDI 3400 - 24” or “Mold-Tech MT 11030” (or the actual code you meant), plus a physical plaque to sign off.
• If the customer’s plastic housing uses MT 11000-family grain and the silicone overmold must *match*, send the plastic plaque and the silicone plaque through the same light. Matching is a visual buy-off, not a VDI number you reverse-engineered from a blog.
• Write depth only when the house publishes it and you need it for draft. Do not invent depths for every VDI step on the drawing.

This library will not print a 45-row conversion. If a vendor’s quote depends on one, ask for their plaque, not their spreadsheet.

Draft, fill, and silicone-specific traps

Texture is draft. A common thermoplastic rule of thumb is about +1° of draft per 0.02 mm of texture depth (Mold-Tech cards often quote draft versus depth in the same spirit). It is a rule of thumb. Soft LSR can strip more grain than PC. Hard LSR, deep leather, tall walls, and robotic ejection can need more than the card. A separate article in this cluster treats that interaction; do not assume the MT card was written for 20 Shore A.

Fill: female pits in steel (raised bumps on the part) trap gas. Heavy grain at last fill needs vacuum and vents that do not flood. Male peaks (recessed grain on the part) strip-drag and tear thin skins.

Wear: EDM VDI on hardened stainless holds up in LSR better than a deep etch on mild steel that will polish off in the shut-off zone. Texture across a flash land is how you leak. Keep grain off the seal face unless the seal *is* the grain, and then you are in a different DFM fight.

Medical and food parts: texture that traps residue can fight cleanability even when the grade is ISO 10993 or FDA 21 CFR 177.2600. A matte VDI to hide scratches is not free on a surgical grip that must wash.

RFQ notes for texture callouts

Name the faces, the system and code (VDI 3400 grade *or* MT code *or* SPI polish), the plaque for sign-off, extra draft already added, process (LSR steel, HCR steel, or RTV from a textured master), and whether the grain must match a plastic neighbor. If the code is “MT 11000” confirm they want *that* fine grain, not “the 11000 series.” If you ignore which half is textured, you will buy the inverse part.

FAQ

Is VDI 3400 the same as Mold-Tech MT 11000?

No. VDI 3400 is an EDM roughness scale; MT 11000 is one fine grain in Mold-Tech’s catalog family, with a published depth and recommended draft. They can overlap in Ra and still look different. Specify one system and buy off a plaque.

Can I convert every VDI grade to an MT number from a chart?

You can find charts; they are approximate at best. Spark craters, chemical etch, and leather undercuts do not map 1:1. For a visible silicone face, use the actual plaque. For a hidden industrial face, a VDI grade is usually enough and cheaper than a decorative etch.

Should an RTV silicone mold be specified in VDI 3400?

Only if you mean the rigid master or mother tool that will be spark-eroded. The flexible jacket copies the master. Put the grain on the master finish. A VDI callout on a poured platinum mold, with no master process, will be ignored or guessed.

Does MT 11000 need extra draft on LSR the same as on plastic?

The catalog draft is a thermoplastic-oriented starting point. Soft LSR may strip a fine grain with less draft. Deep grains, hard durometer, tall cores, and robots often need more. Treat +1° per 0.02 mm depth as a thumb rule to check, not as a law (see the draft-and-texture article in this cluster).

Will texture on the parting line help hide flash?

It will help hide a witness and will help LSR leak. Keep grain off shut-offs and sealing lands unless you have a flashless tool and a reason. Hide flash by closing the land, not by etching a camouflage into a gap.