The gate is the small opening through which molten plastic enters the cavity, and its type, size and position decide more about the finished part than almost any other tooling choice. It sets how the cavity fills, where the weld lines land, how well the part packs out, and what mark is left on the surface after it is separated from the runner.
The common gate types
An edge gate feeds the part at the parting line and is simple, robust and leaves a visible tab to trim. A submarine or tunnel gate cuts itself off during ejection, so the part arrives degated automatically. A pin or cashew gate feeds from a three plate tool onto a surface. A direct sprue gate feeds the part centrally and leaves a large mark. Fan and film gates spread the flow to fill wide thin parts evenly.
Gate vestige and how to control it
Vestige is the small remnant left where the gate was separated. A tunnel gate leaves a small scar on a side wall; a sprue gate leaves an obvious stub that must be machined or trimmed. Vestige cannot be eliminated, only placed. Deciding early which face is cosmetic lets the toolmaker put the gate somewhere the vestige is invisible or falls inside a recess.
Position changes the part, not just the mark
The melt fills from the gate outward, so the gate decides where flow fronts meet and therefore where weld lines fall. It also decides which sections are packed through which, so a gate feeding a thick section through a thin one guarantees sink. If a part must be strong at a particular point, or must not sink somewhere visible, say so before the gate is chosen.
Valve gating and what it buys
A valve gate is a hot runner nozzle with a mechanical pin that opens and closes the gate under control. It leaves a very clean vestige, allows sequential filling of large parts so weld lines can be positioned deliberately, prevents drool and gives independent control of each cavity. It costs considerably more and adds maintenance, so it pays on large cosmetic parts and high cavitation tools.
Questions people ask about injection molding gate
What is a gate in injection moulding?
The restricted opening through which molten plastic enters the cavity from the runner. Its type, size and position decide filling, packing, weld lines and the mark left behind.
What is gate vestige?
The small remnant left where the gate separated from the part. It cannot be removed entirely, so it is placed where it does not matter.
What is valve gating for?
Clean vestige, sequential filling that positions weld lines deliberately, and independent control of each drop on a multi cavity or large cosmetic part.
Can the gate be moved after the tool is cut?
Sometimes, and it is not trivial. Gate position is part of the tool design, so raise cosmetic and strength requirements before the design is frozen.