Gate size gets the attention, but gate location decides whether a molded part comes out straight, filled, and cosmetic. Two molds with identical gates in different spots can produce completely different parts — one flat within 0.1 mm, one bowed like a potato chip. Here is how experienced molders think about placement, and the rules we apply before cutting steel.
Why gate location drives warping
Molten plastic shrinks as it cools, and it does not shrink evenly. Material that packs longer against the gate stays denser than material at the end of flow, so differential shrinkage between regions builds internal stress. When the part ejects and those stresses relax, the part distorts. A glass-fiber filled grade makes this worse: fibers orient along the flow direction and shrink roughly half as much across flow as along it. Fill a rectangular panel from one edge and it will bow toward the gate side — often measurable in tenths of a millimeter on a 150 mm part.
The practical fix is usually not "more packing pressure." It is placing the gate so the flow pattern balances shrinkage instead of fighting it.
Five rules we use to place a gate
- Gate at the thickest section. Fill from thick to thin. Gating into a thin wall and pushing material into a thick boss guarantees sink marks opposite the gate — the thin section freezes first and you lose packing access to the thick one.
- Balance flow length. Aim for a flow-length-to-wall-thickness ratio inside the material's limit (roughly 250–350 for ABS at 2 mm, less for PC). A center gate on a round cover beats an edge gate that travels 300 mm one way.
- Put weld lines where nobody looks. Every obstruction — a hole, a boss, a window — splits the flow and creates a weld line downstream. Move the gate so welds land on non-cosmetic areas or, better, near a rib where the line gets structural support.
- Keep critical dimensions near the gate side. The gate half of the part is denser and dimensionally more stable. Tolerance-critical features (sealing surfaces, bore positions) should sit closer to the gate than to the end of flow.
- Never gate directly at a core pin or thin steel. The impinging jet erodes steel and leaves a jetting defect on the surface. Offset the gate so flow wraps around, or use a submarine gate angled into the wall.
Worked example: a 180 mm round housing cover
Customer's first version gated at the edge with a 1.2 mm direct sprue. Result: a 0.4 mm dish across the flange and a visible weld line right across the logo area, caused by flow splitting around a central boss. We moved to a three-pin-point gate on a runner on the B-side, feeding symmetrically around the boss. Flow became balanced, the weld line moved to an area hidden under the gasket, and flatness came in around 0.1 mm. Same material (PC/ABS), same machine, same cycle — only the gate layout changed.
Gate types and where they fit
- Edge gate: cheapest, easy to degate by hand; fine for brackets and non-cosmetic parts.
- Submarine gate: gates below the parting line and shears off automatically — good for cosmetic parts where the vestige must be tiny.
- Pin-point gate (3-plate or hot runner): lets you gate in the center of a cover for balanced fill, at the cost of a vestige bump and more tooling.
- Hot runner direct gate: zero runner waste and independent valve control per drop — the strongest tool for warping control on large parts, but overkill for most prototype volumes.
Prototype-stage advice
On an aluminum prototype mold, our habit is to cut the first gate conservatively small and wide-shallow, then adjust after seeing fill and warpage on real parts. Cutting steel wider or adding a second gate is cheap; removing an oversized gate is not. If your part has tight flatness callouts, ask your molder to run a short-shot study at T1 — it shows the flow pattern and weld positions before you commit to cosmetic tool polishing.
Gate placement interacts with everything else in the mold: cooling layout, ejection, and venting all follow the fill pattern. If you are planning a molding project, our plastic injection molding team reviews gate location as part of every DFM report, and our soft tooling service keeps gate revisions affordable during prototyping.
Quick checklist before you approve the mold design
- Gate feeds the thickest section?
- Flow lengths balanced within material limits?
- Weld lines mapped and acceptable?
- Critical features on the gate side?
- Gate vestige location agreed with the cosmetic requirements?
Get these five answers right at the design stage and T1 shots look like production parts. Get them wrong and you will spend your budget moving steel instead of shipping product.
