In flat moulds, there can be something similar even though a core isn’t involved, as it establishes where the appliance actually stops and the skin begins.
A cutting edge and overflow are critical in foam appliances, especially where a mould has foam latex added and a core is pushed into it. A gap between the core and the mould face would ensure the excess foam could escape, and the contact point where the mould meets the core would be decided carefully and precisely.
This principle has carried on with silicone, although usually excess waste is minimised owing to the fact silicone isn't mostly made of air, as is the case with foam latex.
Wherever the core meets or touches the mould - be it keys, the cutting edge or an unintentional, is known as a touchdown. Getting great edges is important in making pieces which will blend into the skin and appear as part of it, rather than exhibiting a clear boundary where the fake stops and the real begins.
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