Engineering Answers / Design & DFM
Plastic design guides say one to two degrees and stop there. Silicone is elastic enough that zero draft sometimes strips cleanly — and stiff enough that the same zero will tear a tall wall. Height is the variable that matters.
For silicone, 0.5–1° per side covers walls up to about 10 mm, 1–2° covers 10–25 mm, and textured surfaces need 2–3°. Zero draft is workable only on short features — roughly under 3 mm — that can bump off, and only at hardness below about 60 Shore A. A one-degree draft on a 20 mm wall displaces the surface by about 0.35 mm, so the draft also has to be accounted for in the tolerance stack.
A polypropylene wall needs draft because the plastic is rigid: it cannot stretch over a core, so the tool has to be tapered enough that the part slides off. The rule of thumb — one to two degrees per side — is really a statement about friction and shrinkage in a stiff material.
Silicone does not behave that way. Unfilled silicone stretches 300–600% before it breaks, and it recovers almost completely. A short vertical wall can be pulled over a core and snap back with no measurable set. That is why a silicone part can carry features a plastic part cannot, and why copying a plastic design guide into a silicone drawing produces a tool that is more expensive than it needs to be.
But the same softness that allows stripping is also a limit. The strain during demolding concentrates wherever the geometry changes fastest — the base of the wall, a sharp internal corner, a thin-to-thick transition. Strip a tall wall and the material at the base is asked to carry the entire elongation of the wall above it.
| Wall feature | Draft per side | Why |
|---|---|---|
| Wall up to 10 mm, 40–60 Shore A | 0.5–1° | Enough to break the friction without a visible taper |
| Wall 10–25 mm | 1–2° | Strain at the base climbs with height; the taper shares it |
| Wall over 25 mm | 2° and a polish | Deep draws need both taper and a low-friction cavity |
| Short feature under 3 mm | 0° possible | The stretch is small enough to recover fully |
| Textured surface | 2–3° | Texture grips; a matte or grained face needs more release angle |
| Wall above 70 Shore A | 1–2° minimum | Less elongation available, so draft does the work |
Two of those rows are worth arguing about with your supplier, because they are the ones where "just add draft" hides a cost.
Zero draft on a short feature is a legitimate design choice and we mold it. A 2 mm vertical lip that has to sit flush against another part cannot carry a 1° taper without creating a gap. Strip demolding handles it. What we will not do is accept zero draft on the same lip at 6 mm height, because the strain at the base goes up with height and the failure is a torn edge that reads as a molding defect.
Texture plus draft is where drawings most often conflict with themselves. A grained surface specified at 1° draft will drag, scuff or tear on the way out, because the texture interlocks with the steel. If the surface is cosmetic and textured, 2–3° is the working number.
This is the part that gets missed. Draft is not free geometry — it consumes dimensional tolerance.
At 1° per side, the surface moves 0.0175 mm for every millimetre of height. On a 20 mm wall that is 0.35 mm. If the drawing calls that wall at ±0.1 mm and does not say which end is the datum, the part cannot be both drafted and in tolerance.
The fix is procedural and cheap:
A parting line splits the tool into two halves that open in opposite directions. That means a wall crossing the line needs a taper in one direction on the upper half and the opposite direction on the lower half. The result is a ridge at the line — small, but present on every part, and the reason a "zero draft" wall through a parting line is never really zero.
On a sealing face this matters more than the draft number itself. A 0.02 mm step across a seal is a leak path. The design answer is to place the parting line at the end of the wall rather than at its midpoint, so the seal sits on one continuous drafted surface instead of straddling a step.
A section view with the draft angles annotated takes ten minutes and prevents the two failures that cost real money: a torn base on a tall wall, and a part that cannot meet its own tolerance because the taper ate the band. When we review a drawing, draft is one of the first four things we measure — after wall thickness, parting line placement and undercut strategy.
If you are unsure whether a wall will strip, send the section and the wall height. The answer usually comes down to one number: strain at the base of the wall during demolding.
On a short feature, yes. Silicone elongates 300–600% before it breaks, so a 2–3 mm vertical wall at 50 Shore A will stretch over the core and recover without visible damage. The limit is not the material's willingness to stretch, it is whether the stretch is uniform: a tall wall, a wall with a sharp base radius, or a wall at 70 Shore A concentrates the strain at one line and tears there. We treat 0° as a deliberate exception for shallow features, not as a default.
It makes demolding easier and the part less accurate. Draft is a taper, so every degree removes material from the base of the wall and adds it at the top. On a 20 mm wall, 1° moves the surface about 0.35 mm per side. If the drawing tolerances that wall at ±0.1 mm without saying which end is the datum, the draft alone exceeds the tolerance band and the part is out of specification before it is molded.
It reverses. A wall drafted one degree above the parting line has to be drafted the opposite way below it, because the two halves of the tool open in opposite directions. That reversal produces a visible step at the parting line and, on a sealing surface, a leak path. Where a wall crosses the parting line and the surface is functional, the usual fix is to move the parting line to the end of the wall rather than to accept the step.