Parting lines, and what they cost you
Where the tool splits, and why that decides the shape you can have.
A parting line costs a designer real freedom in how a part can be shaped, because a mould has to open into two or more separate halves to release a solid part, and wherever those halves meet leaves a faint seam on the finished surface while also ruling out any feature that would need to be undercut by more than one of those halves to exist at all.
Why the tool has to split
An injection moulding cavity is machined as a negative of the part, but it cannot simply be lifted off a finished part the way a lid lifts off a box, because a part with any depth or curvature would be gripped by the cavity walls from more than one direction at once, the same jamming problem draft angles exist to solve on a smaller scale. The mould is instead built as two or more blocks that come together to form the complete cavity while the plastic fills and cools, then pull apart once the part has set, each moving in a single straight direction to clear it.
Wherever those blocks meet is the parting line. Because the two halves can never align with mirror-perfect precision, a thin ridge or step of leaked material almost always forms along that seam, faint enough on a well-maintained tool to barely be felt, but present on every part the mould ever produces.
The seam round a chocolate egg
A chocolate Easter egg makes this seam impossible to miss. A hollow egg is cast in a two-part mould, leaving a fine, faintly raised ridge running all the way round the egg's equator where the mould halves met and a whisker of melted chocolate found its way into the gap. Nobody making the egg wanted that ridge, and a more precisely aligned mould would make it fainter, but as long as the egg is made in a mould whose halves have to open around a shape neither could release on its own, some trace of the seam survives onto the finished chocolate.
The egg also shows why the seam sits where it does. Its widest point is the equator, so splitting the mould there lets each half pull straight off a dome that only narrows away from the join. Put the split anywhere else and one half would have to pull past the widest part of the egg, which it could not do without breaking it.
Undercuts, slides and the price of a hook
Because the parting line is fixed by where the tool has to split, a designer has to decide early where it will fall, and that decision constrains which features can exist. A feature that wraps around and behind another part of the shape from the mould's point of view, an undercut, cannot be released by either straight-pulling half without tearing through solid plastic. In a standard two-piece tool it has to be redesigned away, or produced with a more complex, more expensive tool that has extra sliding sections moving sideways to clear that one feature before the main halves open.
Those slides cost money twice. The tool costs more to build, and each cycle takes longer, since the slide has to move clear before the halves open and return before the next shot. A plain two-piece mould is among the cheapest and fastest tools to run, so a designer choosing between a slightly less ambitious shape it can release and a marginally better one that forces a slide into the tool is choosing between a cost paid once at the design stage and a cost paid on every cycle for the life of the tool. This is why moulded parts so often look slightly simpler and more rounded than a purely functional design might call for.
Undercuts are sometimes worth the price. A tool built with side actions or collapsible cores can release shapes a two-piece mould never could, and where the feature is essential, a snap-fit clip that must hook back underneath an edge to hold two halves together, for instance, the extra tool complexity is simply what the function costs. The question to ask of each undercut is whether it earns its cost in real function, or crept in because a shape looked more pleasing on screen and nobody asked what it would cost the tool to produce.
Hiding the seam on an edge
The parting line's position also decides how the part looks. A seam running along a natural edge or a hidden face costs almost nothing in appearance, while the same seam running across a prominent visible surface, left to fall wherever the simplest tool geometry happened to put it, becomes a permanent flaw on every part the tool will ever make. Designers often add a deliberate edge, a small step or a crisp corner, just so the parting line has somewhere discreet to run, in the same way that the ridge round a chocolate egg is far less noticeable than it would be running across its face.