What is a draft angle in injection moulding?

A draft angle is a small taper applied to faces parallel to the mould opening direction, so the part releases without scraping. The baseline recommendation is 1° per side for smooth surfaces, increasing with feature depth and surface texture.

What is the minimum draft angle for injection moulding?

For smooth, polished surfaces on shallow parts, 0.5° per side can work with moulder approval; 1° is the safe default. Textured surfaces need 1° extra per 0.025 mm of texture depth, and deep cavities over 75 mm usually need 2° or more.

Why do textured parts need more draft?

Texture is thousands of micro-undercuts that grip the part during ejection. The standard rule adds 1° of draft for every 0.025 mm of texture depth — a medium VDI 30 grain needs roughly 3° total, and heavy textures 4-6° per side.

Can an injection-moulded part have zero draft?

Yes, selectively. Zero-draft faces are possible with highly polished cavities, generous radii, flexible materials, or by using side actions and stripper plates — each adding cost. Any zero-draft face must be agreed before tooling, because retrofitting draft changes the part geometry.

What happens if a part has insufficient draft?

The part grips the cavity as it shrinks onto cores and walls: drag marks and scuffing on cosmetic faces, ejector-pin damage or pushed-through pins, cracked thin features, extended cycle time, and accelerated mould wear. It is one of the cheapest problems to prevent and one of the most expensive to fix in steel.

Do ribs and bosses need draft angles too?

Yes. Ribs typically need at least 0.5-1° per side, and deep bosses more. Remember that rib draft tapers the rib thinner toward its tip — check that the tip still meets the minimum fillable thickness for your material. Nordmould checks rib tip thickness as part of DFM review.

Draft Angles in Injection Moulding: Rules, Minimums & Texture Adjustments

Draft angle is the taper that lets a frozen part slide out of the tool it shrank onto. Every face parallel to the mould-opening direction needs it: walls, ribs, bosses, snap hooks, deep pockets. The design baseline is 1° per side on smooth surfaces, adjusted upward for depth and texture — and agreed explicitly for any face that must stay at zero.

What are the standard draft-angle rules?

Four rules of thumb cover most parts:

Situation	Recommended draft per side
Smooth/polished faces, shallow parts (<50 mm depth)	0.5-1.0°
General default, smooth surfaces	1.0°
Deeper features — add per 25 mm of depth	+1.0°
Textured faces — add per 0.025 mm texture depth	+1.0°
Ribs and thin upstands	0.5-1.0° minimum
Deep cavities (>75 mm)	2.0°+
Soft elastomers (TPE/TPU, LSR)	can run lower — confirm per grade

The depth rule compounds: a 75 mm tall smooth boss wants about 3° total, and a medium texture on top of that pushes it to 4-5°. Draft is applied per side, so a 2° drafting wall is 4° narrower at the top across two opposite walls — check fit-critical dimensions at the right end.

Why does texture multiply the requirement?

A polished wall contacts the part on a smooth plane; a textured wall contacts it through thousands of micro-undercuts in the grain. During ejection every one of those asperities is a tiny hook resisting the draw.

The industry rule converts texture depth to draft: 1° for every 0.025 mm (25 µm) of grain depth. A fine VDI 18 grain (~~0.01-0.02 mm deep) adds under 1°; a VDI 30 grain (~~0.06-0.08 mm) adds about 3°; a heavy leather-grain MT texture can demand 5-6°. The full texture-to-draft mapping is in the surface finishes guide. Specifying texture on near-vertical walls without this adjustment is one of the most common causes of scuffed first articles.

Where does draft go wrong in real parts?

Three recurring cases. First, cosmetic enclosures with zero-draft styling faces: the ID sketch demands straight walls, and nobody negotiates with the moulding process until the T1 parts come back scuffed. Second, ribs and bosses copied from a library feature at 0° — they eject hard, whiten at the root, and eventually crack. Third, deep text or logos: fine engraved lettering needs draft on every character wall, which is why raised text (drafted by default) is preferred over recessed.

The fix order in CAD: apply draft to all pull-direction faces as the last modelling step before release, run the CAD draft analysis with the intended parting direction, and mark any face that cannot take draft on the 2D drawing as a discussion item.

What are the options when a face must stay at zero draft?

Zero draft is achievable — at a price. The options, cheapest first: highly polished cavity with generous radii and a forgiving material (works for shallow faces); stripper-plate ejection that pushes the part off evenly; side actions or lifters that move the steel sideways instead of pulling the part along it; and collapsible cores for internal features.

Each mechanism adds tooling cost, build time and maintenance — side actions typically add thousands of euros per direction. The right question in a Nordmould DFM review is therefore not "can this face be zero draft" but "what does zero draft on this face cost, and does the function need it".