Draft is the slight taper on every face of a moulded part that runs in the direction the tool opens. It exists because a part with vertical walls grips the core as it shrinks onto it, and pushing it off then scrapes the surface, distorts the part or leaves it stuck in the tool. Draft is the first thing a toolmaker looks for and the most common thing missing from a first design.
How much draft a face needs
The usual starting point is a small angle per side on smooth surfaces, more on deeper walls because the contact area is larger, and considerably more on textured surfaces because the texture itself is an undercut. Shrink onto a core needs more than shrink away from a cavity. The exact numbers come from the resin, the depth and the finish, so agree them with the toolmaker rather than applying one figure everywhere.
Texture is the multiplier nobody budgets for
A moulded texture is a field of tiny undercuts, and the deeper the texture the more draft is needed to clear it without dragging. A part designed with the smooth surface allowance and then textured late in the programme is a common and expensive surprise, because the fix is either more draft, which changes the part, or a shallower texture, which changes how it looks.
What happens without it
Drag marks and scuffing down the walls. Parts that stick and need ejector force high enough to leave witness marks or stress whitening. Cycle times that creep up while the moulder babies each shot out. And a tool that wears faster, because every ejection scrapes steel that was not designed to be scraped.
Where you can spend less
Draft on internal ribs and bosses matters just as much as on external walls and is frequently forgotten. Faces parallel to the direction of draw need none. If a face genuinely cannot be drafted, that is what a side action or a lifter is for, and the cost of that mechanism is the price of the zero draft face, which is a useful thing to know before choosing it.
Questions people ask about draft angles
How much draft do I need?
It depends on depth, resin and surface finish. Smooth shallow walls need a little, deep walls need more, and textured surfaces need considerably more than either. Agree the figure with the toolmaker against your specific texture.
Can a part be moulded with no draft?
Only if something in the tool moves to release it, such as a side action or a lifter, and that mechanism costs money and adds maintenance. It is a deliberate trade rather than an oversight.
Does draft apply to the inside of the part too?
Yes. Ribs, bosses and internal walls shrink onto the core and need draft just as much as the outside faces do, and they are the ones most often forgotten.
When should draft be added to the model?
At the start. Adding draft late changes wall thicknesses, mating features and often the appearance of the part, so it is far cheaper to design with it than to retrofit it.