Draft vs Texture: 1° per 0.02 mm on RTV Masters

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 — or on a 12A RTV glove peeling a 0° leather master.

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.”

On this site the rigid half is usually the master. Extra draft is how the *jacket* survives a deep grain. 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 — on steel *or* on a master.

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. RTV peels, which breaks them on purpose.

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 — or tear the RTV skin 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. Platinum RTV against a dirty or inhibiting master is a sticky face, not a draft problem.
• Female pits that gas-trap and then drag. Fill and vacuum first; draft second. Degas the RTV pour the same way.
• 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 — or they need a two-part jacket.

If you ignore height and durometer, the defect is a smeared VDI that looks “polished” on the last 10 mm of a core — or a jacket that left its grain in the master well.

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; on RTV, plus where the jacket would tear.
5. Keep grain off shut-offs and keys. Extra draft on a land does not stop LSR flashing through 0.005 mm of etch, and it does not stop a dump leaking a textured split.
6. Buy off a textured sample in the production durometer, stripped the production way — peel for RTV, robot strip for LIM.

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.

RUUIPON Mold Structure Decision: Deep grain plus peel? If the silicone cannot peel safely → two-part / plug. Flat back, shallow grain? → open-back, maybe skip extra draft. Large wet dump? → mother mold so the grain does not stretch as the tray sags.

Common mix-up: catalog draft written for PC on a 12A master

A lot of RFQs paste the MT card onto a glove-mold drawing. In the shop that usually means over-drafted art and a jacket that still tore because the well was 0° and glossy between peaks.

What to ask when you're buying

• Faces, texture system and code, depth if known?
• Base draft, extra draft already added, wall height?
• Durometer and strip method (hand peel, air, robot)?
• Process: RTV master vs LSR/HCR steel?
• Grain kept off keys and lands?

Worked example

You're texturing a 40 mm deep mascot master, leather ~0.06 mm, 12A platinum glove, 6 cavities. Napkin extra draft ≈ +3° on those walls as a *start*; trial the peel. Keep grain off keys. If the same grain sits on a 70A LSR steel core 40 mm tall with a robot, budget catalog draft or more — different factory.

We would not recommend putting +1° per 0.02 mm in the title block as a law

We would not recommend a VDI-to-MT depth table, extra 5° on a shallow optical face, or grain on the split. We would not copy a LIM core recipe blindly onto an RTV master.

Typical shop values

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

| Item | Typical shop value | | --- | --- | | Thumb rule | About +1° extra draft per 0.02 mm texture depth | | Untextured LSR steel walls | Often 0.5–2° baseline | | MT 11000-class depth | On the order of 0.01 mm — extra degree often noise | | RTV cavity Shore | 10A–20A peel; deep 0° grain can tear the jacket | | LSR leak band | About 0.005 mm — keep grain off lands | | ISO 3302-1 | M2 common; smeared grain is visual |

Copy this into a draft-plus-texture RFQ

• 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, RTV peel)
• Process: RTV master or LSR/HCR steel
• If you write “texture TBD, draft 0.5°,” you invited a T1 smear

Send the tall textured walls and the Shore. We'll start from the napkin and adjust for peel vs robot. Custom RTV molds.

Related: VDI vs MT 11000, deep-draw demold, texture options.

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 (or jacket) 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. Same warning for 12A RTV on a deep 0° master.

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, or split the tool.