Start with the number. Draft angle = base 1–1.5° + (actual texture depth ÷ 0.025 mm) × 1°. A 0.02 mm texture gets 2°. A 0.05 mm texture gets 3°. That is the industry's working safety line, and the rest of this article is about the places where the line is not enough.
Take the grade most of our enclosures land on: VDI 3400 #30, Ra 4 μm. The real peak-to-valley depth sits between 0.016 and 0.024 mm, so the formula returns a little over 2°. On drawings we write 2.5°. A coarse #36 runs 0.04–0.06 mm deep and needs 3.5–4.5° on the side wall. Go under that and the first T0 shot tends to come out with scuffed, whitened walls — and the corrective round costs far more than the single degree you saved.
The texture number is a Ra value, not a depth
VDI 3400 is the German standard for spark-eroded and etched textures. Its grade numbers — 24, 27, 30, 33, 36 — map to Ra (arithmetic mean roughness): 1.6 μm, 2.5 μm, 4 μm, 6.3 μm, 10 μm. Those five figures are public. But Ra is an average, not the depth of the grooves. Draft calculation needs peak-to-valley depth, and the working conversion is 4–6 times Ra.
Get that factor wrong and the arithmetic is off by five. A client once read Ra 4 μm as a depth of 0.004 mm. The formula returned 1°, the number went onto the drawing, the texture shop did not flag it, and it went into the tool. At T0 all four side walls came out scuffed. Adding angle and re-texturing the cavity took three weeks. Texture shops quote Ra because that is the convention of their trade; the moment you feed that number into a draft calculation, you have to convert it.
The VDI 3400 table we keep on the wall
| VDI 3400 grade | Ra (published standard) | Practical depth | Minimum draft |
|---|---|---|---|
| #24 | 1.6 μm | 0.006–0.010 mm | 1.5–2° |
| #27 | 2.5 μm | 0.010–0.015 mm | 2–2.5° |
| #30 | 4 μm | 0.016–0.024 mm | 2.5–3° |
| #33 | 6.3 μm | 0.026–0.038 mm | 3–3.5° |
| #36 | 10 μm | 0.040–0.060 mm | 3.5–4.5° |
The Ra column comes from VDI 3400 itself. The depth column is the 4–6× conversion — it drifts with the process (chemical etch, laser, EDM), so confirm the real depth with the texture shop before the cavity is cut. The draft column already carries about half a degree of margin on top of the formula, for the reasons in the next section.
What the formula does not cover
The formula only handles geometric interference in the direction perpendicular to the grain. Three other things cause the same scuffing, and none of them is in the equation.
Shrinkage grip. As plastic cools it shrinks toward the cavity centre, which clamps the part onto the core. That adds normal pressure between side wall and cavity insert, and friction rises with it. Crystalline resins (PP, PA, POM) shrink more than amorphous ones (ABS, PC): at the same 2°, a PP part feels noticeably heavier to eject. On crystalline enclosures we add 0.5–1° on top of the formula value.
Texture depth is not uniform. In chemical etching, deep pockets and corners exchange etchant poorly, so the texture there comes out shallower or patchy. Laser texturing drifts in energy density where curvature changes sharply. The same side wall therefore has different depths at different points — calculate from the deepest point, not the average.
Ejection balance. Even with enough angle, too few ejector pins or badly placed ones let the part tilt as it leaves the core, and a local side wall becomes a negative angle scraping hard against the texture. We learned this on a walkie-talkie housing: the wall had 3°, and one side still came out with band-shaped drag marks. Two extra flat ejector pins and a rebalanced ejection fixed it. The angle was never the problem.
So when a textured part sticks, the angle is only one of four suspects: angle, shrinkage, depth variation, ejection.
When there is no room for angle: three fallbacks
Appearance parts regularly hit a dead end: wall height is fixed, the envelope is fixed, and every extra degree of draft eats wall thickness or overall size. At that point we work down a priority list.
① Add local stock, 0.3–0.5 mm. Reserve material outside the textured face and trade thickness for angle; after ejection it sits where assembly hides it. The cost is uneven wall thickness and a sink risk. Do not do this on transparent or high-gloss parts.
② Move the parting line so the pull direction follows the grain. A lot of "scuffing" is really pull direction fighting grain direction. Texture is etched evenly over the cavity surface, with grooves running roughly perpendicular to the surface; if the parting line makes one face a wipe direction, no amount of angle will save it. Moving the parting line is more effective than adding angle — the price is a new tool structure.
③ Drop the grade, or remove texture locally. Going from #33 to #27 cuts the depth by more than half, and the draft requirement falls from the mid-3° range to the low 2s. At normal viewing distance the appearance difference is not obvious, while the process window widens a lot. We did exactly this on a small-appliance panel, and the client accepted it after seeing samples.
Steel hardness before texturing, and the order of operations
One item gets skipped more than any other: the texture is only as good as the steel under it. Pre-hardened P20 (around HRC30) etches inconsistently, with depth varying across the surface. For production tools we ask for heat treatment to HRC48 or above before texturing. Low hardness also wears: a few hundred shots and the grain polishes bright, so appearance consistency will not survive the production run. Confirm this with the tool shop at design stage — discovering at T0 that the texture looks washed out is a far longer recovery than changing an angle.
Short-life, low-volume parts can sometimes get away with pre-hardened steel plus laser texturing, but agree that with both shops before anyone cuts steel.
The order of operations. Fix the grade → ask the texture shop for the actual depth, not the Ra → run the formula for the base angle → add margin for shrinkage and parting direction → check ejection balance last. Reverse the order and the drawing bounces back from the texture shop.
FAQ
How hard does tool steel need to be before texturing?
HRC48 or above for chemical texturing. Pre-hardened steel such as P20, which sits around HRC30, etches unevenly and is not something we would use directly on a production tool. Below the hardness threshold the texture also wears away — a few hundred shots and the grain polishes bright, and appearance consistency cannot be held across a production run. Short-life, low-volume parts can sometimes use pre-hardened steel with laser texturing, but agree it with both shops first — steel selection belongs in the design stage, not in a T0 correction.
The wall already has 2° and it still scuffs. What do I check next?
First have the texture shop measure the actual depth (not the Ra) and confirm it is inside the range the formula assumed. Then check ejection balance, then whether the resin shrinks more than expected, then whether the parting direction conflicts with the grain direction. Those four checks find the cause in more than 80% of cases. Our own 2° case turned out to be too few ejector pins — the part tilted on ejection and a local wall briefly went negative and scraped the texture. Two extra flat pins solved it; the angle never changed. If the scuffing concentrates in one corner it is usually ejection or local depth; if the whole face scuffs evenly, the angle itself is short.
Do VDI, SPI and Mold-Tech convert to each other?
There is no official conversion table. VDI 3400 publishes Ra values, SPI grades by processing method (A1 to D3), and Mold-Tech is a vendor's own texture library. When crossing systems, ask the texture shop for both the grade number and the actual depth, not the grade alone. We have seen the same grade come out at noticeably different depths at different shops, so "VDI 30" on a drawing is not enough — add a permitted depth tolerance and have the shop sign off a sample. That signature removes a lot of later argument.
Is there a relationship between texture depth and wall thickness?
Keep texture depth clearly below wall thickness — under about 1/10 of it. At 1.5 mm wall thickness, be careful with #33 and coarser grades. Too deep a texture on too thin a wall produces show-through (internal structure visible as shadows on the outer face) and a local loss of strength, and it shows up worst on light-coloured resins and thin walls. On a remote-control top cover we had 1.2 mm wall with #33, and the rib positions were faintly visible on the cosmetic face; we added local thickness and dropped the grade. Check wall thickness and texture grade together at design stage, not at sampling.
Originally published at https://www.hezidesign.com/gongyi/1228.html
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