Design for manufacture
6 min read
Draft angles are a conversation with the mould
Why one and a half degrees decides whether a part releases cleanly, and how we choose draft before the first sketch is dimensioned.
Author
Tomás Reyes
Role
Mechanical lead
Published
Reading
6 min read
The part has to leave the tool
Every injection-moulded part spends its first second of life being pushed out of steel. If its walls are perfectly vertical, they scrape along the cavity, drag marks appear and the ejector pins leave white stress rings. A small angle, the draft, lets the part break free the moment the mould opens.
We treat draft as a design input, not a finishing touch. It is far cheaper to sketch a wall at 1.5° than to discover at the first trial that the toolmaker needs it and the enclosure no longer fits the board.
How much is enough
There is no single number, but there are good defaults. We start from the texture and the depth of the wall, then check what the material and the toolmaker prefer.
1° for smooth, polished surfaces up to 50 mm deep
1.5° to 2° for light textures
3° or more for heavy textures and deep ribs
About 1° more for every extra 25 mm of depth
Where draft goes wrong
The usual mistakes are quiet ones. Draft applied in the wrong direction locks the part in the cavity. Ribs without draft stick and tear. And a parting line moved late in the project flips every angle on one side of the part.
Our habit is simple: decide the pull direction and the parting line on day one, show them on every drawing, and review draft with a colour map before any part goes to quote.
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