Draft angles, and the taper on every plastic part
Why a moulded part has to be slightly conical in order to leave the tool.
A moulded plastic part has to be shaped as a very slight cone rather than a true cylinder or a perfectly straight-sided box, because a part with dead straight, parallel sides grips its mould by friction and suction along the entire depth of every wall, and pulling it free without a draft angle to release that grip can tear the part or jam it in the tool entirely.
Straight walls grip along their whole depth
A part moulded in a two-piece tool is formed against the tool's inner walls while the plastic is hot, and pulling the finished part free means sliding it out along the direction the tool opens. If the part's walls run dead straight and parallel to that direction of pull, the entire surface of every wall stays in contact with the tool right up until the moment it separates, gripping by friction and by the faint suction of two matching surfaces sliding apart, along the complete depth of the part. The plastic also shrinks a little as it cools, and on a core (the part of the tool that forms the inside of a box or cup) that shrinkage clamps the part tighter onto the steel. A slight taper angles every wall so that contact with the tool starts easing the instant the part begins to move, a small gap opening progressively all the way round, and that is the difference between a part that lifts out cleanly and one that has to be forced or damaged to remove.
Deeper parts feel this more than shallow ones, since a wall that grips along its entire depth has more surface holding it the deeper it goes, which is why a deep, straight-sided pocket is far more prone to sticking than a shallow one of the same width.
How little taper it takes
A draft angle of only a degree or two is usually enough to let a part release cleanly, a lean so slight it is barely visible to the eye or felt by a finger running along the wall. The arithmetic shows why it is so hard to spot: at one degree, a wall fifty millimetres deep moves inward by less than a millimetre between its open end and its closed end, about the thickness of a bank card. Because so small an angle costs almost nothing in the shape of the part, draft is added as standard practice to almost every wall on a moulded part, and not only to the ones that seem likely to stick.
The jelly mould that lets go
A jelly turned out of a mould with sides that lean slightly outward toward the top slips free with barely any effort, since the moment it starts to lift, a thin gap opens all around it and air gets in to break the suction holding it to the walls. The same jelly set in a mould with dead straight sides, or worse, one that narrows slightly toward the top, clings to the mould by suction along its entire surface until enough force is applied to tear it. A plastic part in a straight-sided tool is in the position of the jelly in the wrong mould, and the tiny outward lean built into almost every moulded plastic object exists for the same reason the good jelly mould has one.
What the drawing has to show
Because the taper runs the full depth of every wall, a moulded part is never quite the shape it appears to be at a glance. A box that looks rectangular is very slightly wider at the open end than at the closed one, and a designer working from a drawing that ignores this ends up specifying a shape the tool can never release, which is exactly the kind of gap a workshop's returned questions tend to catch. For a mould, a part with no draft anywhere on it is often unmakeable, since no amount of extra force reliably substitutes for the small, deliberate gap the taper was supposed to provide. A part that does eventually come free from a straight-sided tool through sheer force usually shows it, scuffed or slightly deformed along the very walls that were gripping it.