Draft Angle Interactions with Textured Mold Surfaces (Adding 1° per 0.02mm Depth)

Some toolrooms add about 1° of draft for every 0.02 mm of texture depth. Mold-Tech cards often publish a similar pairing of depth versus recommended draft (their numbers are in inches and degrees; the 0.02 mm rule is the millimetre-shaped cousin). It is a useful napkin. It is not a material law, and it was not written on a 20 Shore A LSR core with a robot waiting.

Texture is a field of micro-undercuts. Draft is how those undercuts unhook as the part moves along the draw. Silicone complicates the thermoplastic story because the part can elongate. That lets you cheat the rule. It also lets you tear a grain off a thin wall and call it “release.”

Untextured draft for LSR steel is a different baseline (often 0.5-2° on free walls). This article is the interaction: what happens when you add VDI 3400 or an MT 11xxx grain on top of that baseline.

Where the 1° / 0.02 mm rule comes from

Decorative etch houses learned the hard way that a 0.08 mm leather on 0.5° of plastic draft smears. Depth times a factor gives a starting draft that lets the peaks clear. 0.02 mm of depth → +1° is easy to remember. A 0.06 mm grain would then ask for about +3° on top of the untextured draft.

VDI 3400 grades do not all publish a single depth the way an MT code does. Fine VDI is shallow satin; coarse VDI is heavy crater. Translating a VDI number into millimetres of depth via a blog table is how you add 5° you did not need or 0° you did. Use a plaque and, if the house gives a depth, use that depth in the napkin. Do not invent a 45-row chart.

The rule assumes:

• A rigid part (thermoplastic) that cannot stretch off a barb.
• A strip along the draw, not a peel.
• Grain on a wall that is tall enough for the undercut to matter.
• A texture that actually has that depth after polish, wear, and spotting.

Silicone breaks all four assumptions on some jobs.

When the thumb rule is too conservative (you can use less)

• Soft LSR, high elongation, short walls. A 20 Shore A keypad skin on a 3 mm textured wall may strip at the untextured 1° plus a little. Extra 4° of draft changes the look of a logo more than it saves the strip.
• RTV jackets. The mold peels. Draft on the *cavity wall* is often 0°. Texture still lives on the master; a deep grain on a 0° master can tear the *jacket*, which is the opposite part. The 1°/0.02 mm rule is then about protecting the silicone *mold*, not the product.
• Very fine grain (MT 11000-class depths around 0.01 mm, fine VDI). The extra degree is in the noise next to polish tack and vacuum lock. Breaking the polish may matter more than the extra degree.

If you still add 5° “because texture” on a shallow, soft, optical-adjacent part, the defect is a wedge the designer did not want and a grain that stripped anyway because the core was a mirror between the peaks.

When the thumb rule fails the other way (you need more)

• Hard LSR or HCR (60-80 Shore A) with deep leather. Elongation is lower. The part behaves closer to plastic. Catalog draft or more.
• Tall cores with grain all the way down. Height times texture drag. 1°/0.02 mm at 8 mm height is not the same as at 40 mm. The napkin has no height term; reality does. Add draft or lose grain at the mouth.
• Robotic ejection. No peel, no pause, no thumbnail. Design for the strip vector. Extra draft is cheaper than air damage on a textured lip.
• Self-bonding grades that tack to steel. Texture plus chemistry. The rule does not know about adhesion promoters.
• Female pits (raised bumps on the part) that gas-trap and then drag. Fill and vacuum first; draft second.
• Undercut-like leather with overhanging etch. Depth on the card is not the undercut angle of the crater. Some grains need more than the depth rule, or they need to sit on a face that is not a deep draw.

If you ignore height and durometer, the defect is a smeared VDI that looks “polished” on the last 10 mm of a core.

How to apply it without cargo-culting

1. Set base draft for the process (LSR steel vs HCR vs RTV master).
2. Get texture depth from the house or plaque, not from a VDI-to-MT conversion spreadsheet.
3. Add +1° per 0.02 mm as a *starting* extra on walls that are both tall and textured.
4. Adjust: minus for short, soft, hand-stripped LSR; plus for hard, tall, robotic, self-bonding, or heavily undercut grains.
5. Keep grain off shut-offs. Extra draft on a land does not stop LSR flashing through 0.005 mm of etch.
6. Buy off a textured sample in the production durometer, stripped the production way.

ISO 3302-1 will not save a smeared grain. Put texture and draft on the same drawing view so they cannot be specified by two people who never met.

RFQ notes for draft-plus-texture

Faces, texture system and code (VDI 3400 *or* MT code), depth if known, base draft, extra draft you already added, durometer, strip method (hand, air, robot), process. If you write “texture TBD, draft 0.5°,” you invited a T1 smear. If you write “add 1° per 0.02 mm” with no height and no grade, you invited a fight with CAD.

FAQ

Is +1° per 0.02 mm of texture depth a standard I can put in a title block?

It is a shop rule of thumb, cousins with etch-house draft cards, not an ISO requirement. Use it to start a number, then adjust for durometer, wall height, and how the part is stripped. Putting it as a law on every face will over-draft some and under-draft others.

Does VDI 3400 follow the same depth-to-draft math as Mold-Tech?

Only if you have a depth for that VDI finish. VDI is an EDM roughness character, not an MT catalog depth. Fine VDI may need almost no extra draft; coarse VDI can act like a deep grain. Use a plaque and a measured depth, not a conversion table.

Can soft LSR skip extra draft on a deep leather grain?

Sometimes on short walls with a hand peel. On tall cores, robots, and thin walls, the leather still barbs and tears. Softness is elongation, not a solvent for undercuts; trial the production strip rather than assuming 20 Shore A repeals the card.

Why did we add the catalog draft and still smear the texture?

Height, tack, polish in the valleys, or a grain with more undercut than its published depth. Also: spotting and wear that flattened peaks into hooks. Extra draft cannot fix a shut-off that was etched or a core that vacuum-locks between peaks.

Should RTV masters get the same extra draft as LSR steel?

The risk is different. RTV peels, and the jacket can tear on a 0° deep grain on the master, while LSR steel needs the part to leave a rigid hole. Apply extra draft where the *rigid* half will scrape. For RTV, protect the jacket with master draft or a shallower grain, not by copying a LIM core recipe blindly.