Injection moulding, and the taper on every part

How the process works, and the geometry it forces on the designer.

Every injection moulded part carries a slight taper on its vertical faces because the tool that makes it is rigid and has to open and release the finished part cleanly, and a wall that runs perfectly straight up and down grips a rigid mould exactly the way two perfectly parallel surfaces grip each other, refusing to separate however hard they are pulled apart.

Watching a mould open and close a few hundred times makes every plastic object slightly more legible. A tray, a housing, a clip, all of them start to look less like arbitrary shapes and more like the record of a tool that had to fill, cool and then let go of exactly that shape, over and over, without ever damaging either the part or itself.

Two glasses jammed one inside the other

Two drinking glasses of the same size, one pushed a little too firmly inside the other, show the failure without any plastic or heat involved. If the glasses have dead-straight, parallel sides, pushing them together too far can jam them solid, and pulling or twisting achieves almost nothing, because the whole length of contact grips evenly with nothing at either end offering a way to break the grip. Two glasses that taper gently, wider at the top than the bottom, never jam in the same way, because only a small band near the bottom is ever in tight contact, and separating them only has to overcome that band. A mould without draft behaves like the straight-sided glasses, jammed with the part on every cycle, and a mould with draft behaves like the tapered pair.

Shrinking plastic grips a straight wall

Injection moulding works by forcing molten plastic into a closed cavity shaped like the negative of the part, letting it cool and solidify, then opening the tool and pushing the solid part free. Releasing the part is where a straight, vertical wall becomes a problem. As the plastic cools it shrinks slightly, tightening onto any core it has formed around and pressing flat against any mould surface beside it, and a wall with no draft is pressed against the mould along its entire height, with nothing encouraging separation as the tool opens.

A small taper changes this completely. Once the tool has moved even a fraction of a millimetre, a tapered wall has already lifted clear of the mould all the way along, so the grip is broken almost as soon as the pull begins and the part releases with a gentle push instead of a fight. Draft is therefore close to a mechanical necessity on a moulded part, and leaving it out asks the tool to do something it cannot do without damaging the part or itself.

One degree on a fifty-millimetre wall

The angles involved are small. A typical draft on a plain, polished wall is a degree or two, and a degree on a wall fifty millimetres deep moves the top edge inward by less than a millimetre compared with the bottom, about the thickness of a bank card. That is barely visible on the finished part, yet it is the whole difference between a part that pops free and one that tears, whitens or scuffs on its way out of the tool.

How much draft a wall needs depends on how deep the feature is and how much its surface texture grips. A deeper wall has more length in contact, and a textured surface has tiny hills and valleys that interlock with the matching texture in the mould, so both need a larger angle than a shallow, polished face. A leather-grain finish on the side of a housing, for example, has to be dragged across its own negative as the part comes out unless the wall leans far enough for the texture to lift clear.

Taper as the default shape

Because draft is close to unavoidable on any wall a mould has to release, a designer working in plastic treats tapered walls as the default shape. Any feature that truly needs to be vertical, a bearing surface or a mating face against another precisely made part, becomes a deliberate, flagged exception that the tool and the process will need extra attention to achieve. A moulded part is rarely a perfect rectangular box even when a box would serve its function equally well, and the small slope on every visible wall is the signature each injection moulded part carries as the price of coming out of its tool in one piece.

Leaving draft off a drawing does not make it go away. The mould shop is left either to fight the part out of the tool on every cycle, risking damage to both, or to add draft quietly without telling anyone. The second option tends to surface later as a part that no longer matches its own drawing, found only when a mating part designed against the original drawing no longer fits the tapered reality of the part being produced, and an assembly that should have gone together easily refuses to.