DefinitionA gate is the small opening through which molten plastic flows from the runner into the mold cavity.
Explained
What Does a Gate Do?
The gate delivers molten plastic into the cavity, freezes quickly after packing to seal the cavity and stop backflow into the runner, and provides a clean, weak point where the part can be separated from the runner. Because the gate controls how long packing continues, its placement affects shrinkage, sink marks and overall flow pattern. Gate location also determines where weld lines form and whether plastic spreads radially or linearly, which influences warpage.
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 so the gate can keep packing heavy areas and avoid sink marks.
- Keep the vestige off cosmetic faces; place it on an edge, inside surface or somewhere easily trimmed.
- Position gates so every cavity or region finishes filling at the same time.
- Move weld lines into low-stress, non-visible areas and orient flow along the part where warpage matters.
Key takeaways
- A gate is the narrow opening that lets molten plastic flow from the runner into the mold cavity.
- It freezes soon after packing, sealing the cavity and preventing backflow into the runner.
- After the part is cut from the runner, the gate leaves a small mark called the gate vestige.
- Gate location controls packing time, flow direction, weld-line position and potential warpage.
- Common gate types include edge, submarine, pin, fan, film and direct gates.
- Best practice is to gate into the thickest section and keep gate vestiges off cosmetic surfaces.
Example
A submarine (tunnel) gate enters below the parting line through an angled tunnel, so it shears off automatically as the part ejects—ideal for small parts where automatic degating and a small gate mark are important.