What Is a Gate in Injection Molding?

A gate is the small opening through which molten plastic flows from the runner into the mold cavity. It is usually the narrowest point in the feed system, so it freezes soon after packing and stops backflow. When the part is cut from the runner, the gate leaves a small mark called the gate vestige.

In short: the gate is the doorway into the part, and where it goes affects how the part fills, where defects appear and what the finished surface looks like.

What Does a Gate Do?

The gate has three jobs. It delivers the melt into the cavity. It freezes quickly once the cavity is full, sealing the plastic in so it cannot flow back into the runner when injection pressure drops. And it gives a clean, weak point where the part can be separated from the runner.

Because the gate is the only way into the cavity, it also controls packing. While it is open, the machine can keep pushing plastic in to make up for shrinkage; once it seals, packing stops. That is why a gate placed at a thin section can leave sink marks in thicker areas further along.

Gate location decides how plastic flows through the part. Where two flow fronts meet, around a hole or from two gates, a weld line forms, and moving the gate is one of the standard ways to move or prevent it. Gate position also sets flow direction: a center gate spreads plastic radially, while an edge or end gate sends it in a line, which tends to warp less.

Common Gate Types

Gate type	Where it enters the part	Vestige and degating	Typical use
Edge (side) gate	At the parting line, on the part's edge	Visible mark on the edge; trimmed after molding	The general-purpose default
Submarine (tunnel) gate	Below the parting line, through an angled tunnel	Sheared off automatically on ejection; small mark	Small parts where automatic degating matters
Pin gate	From above, in a three-plate mold	Small point mark; separates as the mold opens	Parts gated on a face rather than an edge
Fan gate	Widens along a straight edge	Wide mark to trim	Flat parts that need even, low-stress filling
Film gate	A thin slot along the full edge	Long mark to trim	Thin, flat parts where warpage must be minimal
Direct (sprue) gate	The sprue feeds the part directly	Large mark to cut off	Large single-cavity parts

Where Should the Gate Go?

Gate into the thickest section. Flowing from thick to thin lets the gate keep packing the heavy areas and avoids sink marks.
Keep the vestige off cosmetic faces. Put it on an edge, an inside surface or somewhere it can be easily trimmed.
Fill evenly. In multi-cavity or multi-gate molds, position gates so every cavity or region finishes filling at the same time.
Think about weld lines and orientation. Move the gate so weld lines fall in low-stress, non-visible areas, and so flow runs along the part rather than radially across it where warpage matters.

Gate location is checked during DFM for every LSR molding project at NICE Rapid, alongside wall thickness, parting-line placement and draft.

Frequently Asked Questions