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How much draft does an injection moulded part need?

In most situations an injection-moulded part needs about 1 to 2 degrees of draft per side, with more for deep walls and textured surfaces, according to Protolabs' design guidance.

Draft is the slight taper on any face that runs in the direction the mould opens. It lets the part release from the steel instead of dragging along it. A first-time designer often models perfectly vertical walls because that is how CAD starts; adding draft early saves a round of rework when the toolmaker reviews the design.

Why draft matters

Plastic shrinks onto the core of the mould as it cools, so a vertical wall grips the steel. Without draft the part has to be pushed off against friction, which can leave drag marks or scuffs on the surface, stress the part, or make it stick in the tool. Komaspec's design guide notes that polished surfaces need more draft because polish increases both the vacuum effect and how visible any scratch will be. Its practical advice: if you are debating whether to add half a degree to a rib, add it.

Published draft angles

The table below collects the general figures from three established moulding references. Where a cell says "not stated", the source did not give a figure for that case.

Wikipedia's article on the design of plastic components gives a similar picture: 1° per side is usually enough, 2 to 5° per side is preferable, 0.5° per side where 1° will not fit, and even 0.25° is better than none.

Where the sources disagree

The minimum is the main point of difference. Protolabs strongly advises at least 0.5° and says as little as 0.25° can work in some cases, while Komaspec sets 1° per side as the minimum for untextured surfaces in unfilled resin. Designing to 1° or more where the part allows it avoids the question.

The shutoff figure also differs: Protolabs gives 3° as the minimum where steel slides on steel, while Fictiv gives 5 to 7° for a shutoff on a stepped parting line. These may describe different geometries, so ask the toolmaker which applies to your part.

Texture needs extra draft

Texture is the most common reason a part ends up needing far more draft than the designer drew. The texture leaves tiny undercuts in the surface, and without enough taper the mould scrapes them off during ejection.

Published rules for texture depth:

• Fictiv: about 1.5° of draft per 0.025 mm (0.001 in) of texture depth.
• Komaspec: add an extra 1° of draft for every 0.025 mm (0.001 in) of texture depth.
• Fictiv: leathery or snakeskin-like finishes can need 10° or more.

The two per-depth rules differ by half a degree per 0.025 mm, which adds up on deep grains. Komaspec gives an example of a moderately deep grain that might need up to 5° even though only 1° was drawn. Texture suppliers publish draft recommendations for each of their patterns, so choose the texture early and check its specific requirement.

Deep features need more

Draft is an angle, so the same angle gives more clearance on a deep wall than a shallow one. Protolabs' rule of thumb of 1° per inch of cavity depth, and Komaspec's 1° per 25 mm for deep cores and pockets, both mean deeper walls should get more draft. Protolabs' draft article also includes a table relating wall depth and thickness to the draft it suggests; for example, it pairs a 1 in deep wall of 0.080 in thickness with 1° of draft.

A step-by-step draft check

1. Set the pull direction (the direction the mould opens) before adding any draft.
2. Apply at least 1° per side to every face parallel to the pull, unless the toolmaker agrees to less.
3. Add draft for depth, using roughly 1° per inch (25 mm) of cavity depth as a starting point.
4. Decide on cosmetic finishes now: high polish and texture both need more draft than a standard finish.
5. For textured faces, look up the texture's own draft requirement and use the per-depth rule as a cross-check.
6. Check ribs and bosses separately; they still need draft, typically at least 0.5° per side.
7. Mark any face that truly cannot have draft and discuss it with the toolmaker; slides or other mould actions may be needed.

Draft figures here are planning guidance from published design guides. The toolmaker's design review and the texture supplier's chart for your chosen finish take priority.

Questions buyers ask

Can my part have zero draft?

Some features can be moulded with very little draft in particular cases, and Protolabs mentions that as little as 0.25° can work in some cases. Truly vertical faces usually need special mould actions or risk drag marks and sticking. Ask the toolmaker before committing to zero draft on any face.

Does draft change the dimensions I specified?

Yes. A drafted wall is thicker at one end than the other, or an opening is wider at one end. State on the drawing where the nominal dimension applies, for example at the top or bottom of the feature, so the toolmaker and inspector read it the same way.

Why did the toolmaker ask for more draft than I drew?

Common reasons are a textured or highly polished finish, a deep wall, or a shutoff. Komaspec notes that a moderately deep grain might need up to 5° even when only 1° was drawn. Adding it at the design stage costs far less than changing steel later.

Do inside and outside walls need the same draft?

The published rules are given per side, and both faces need draft to release. The part shrinks onto the core, so the inside faces are often where sticking shows up first. Confirm the split with the toolmaker for your geometry.

Sources

• Protolabs, Improving part moldability with draft: https://www.protolabs.com/resources/design-tips/improving-part-moldability-with-draft/
• Komaspec, Plastic part design for injection molding: https://www.komaspec.com/about-us/blog/plastic-part-design-for-injection-molding/
• Fictiv, Draft and texture in plastic injection molded parts: https://www.fictiv.com/articles/draft-and-texture-in-plastic-injection-molded-parts
• Wikipedia, Design of plastic components: https://en.wikipedia.org/wiki/Design_of_plastic_components