Melt does not fill a cavity at random. Polymer chains stretch along the flow direction as melt squeezes through the gate. The cold mold wall freezes them in that stretched state within milliseconds. This frozen orientation is why a part shrinks differently along the flow path than across it.
The gap is small in unfilled resins. It turns large in glass-filled grades. Shrinkage across the flow can reach two to three times the along-flow value when fibers line up with the stream. That difference bends parts. It also decides where a part is strong and where it may fail.
Orientation Shows Up on the Inspection Table
Two-thirds of fracture lines pass through the gate area. Mold engineering handbooks record this number, and the physics behind it is simple. The gate region carries the highest molecular orientation in the part. Chains there stretch the most and freeze the fastest.
Weld lines are the other weak point. Their strength runs 20% to 80% of the base material, depending on melt temperature, packing pressure, and venting. A weld line sitting in a screw boss or a snap arm is a field failure waiting for a date. Gate position decides where weld lines land. That makes gate position a reliability decision, not a cosmetic one.
Stiffness follows orientation too. A part resists bending better along the flow than across it. Designers who need stiffness in a specific direction should know which way the melt traveled. Ask your molder for the fill pattern before you finalize the rib layout. Moving a rib 10 mm is cheap on paper and expensive in steel.
Gate Into the Thickest Wall
Packing pressure can only reach plastic that is still molten. Thick sections stay molten longest. They should sit closest to the gate. That is the standing rule.
Thin-to-thick filling is the pattern to avoid. Melt races through the thin area, the gate freezes early, and the thick section never gets packed. The result is sink on the thick side and sometimes a short shot at the far end of the part. When the gate must sit in a thin wall, add a thickened flow leader running from the gate toward the heavy section. The leader carries packing pressure where the part needs it without thickening the entire wall.
Flow leaders also help when the wall thickness varies across the part. A leader is a local thickened channel, not a global wall change. It guides hot melt to the last areas to fill. Use it where published flow data says the resin is near its limit.
Watch for Flow Hesitation
When melt meets a thin region beside a thick one, the flow front often stalls in the thin wall. The thick area keeps filling while the thin edge sits still and cools. If it freezes before filling completes, you get an unfilled edge or a visible hesitation mark.
One practical fix is to place the gate so the thinnest walls fill last. That shortens the time the front spends hesitating. Thin-walled containers and lids respond well to this approach. The hesitation window shrinks because the thin section meets hot melt for the shortest possible time.
Hesitation also causes gloss differences. The stalled front cools, then fresh melt pushes over it. The overlap line shows as a dull band. If your part has a cosmetic face, keep thin cosmetic walls near the end of fill, not near the gate.
Keep Gates Away From High-Stress Features
Gate removal leaves a mark. Trimming or degating can leave small notches or scrapes on the part edge. Each one works as a stress concentrator.
Never gate directly into a screw boss, a snap arm, or a mounting hole. These features already carry service loads. Adding gate-induced stress and a notch on top cuts fatigue life sharply. Move the gate to a low-stress wall and let the melt flow toward the feature instead. The extra flow length costs a little pressure. The longer fatigue life is worth it.
Check Flow Length Against Material Data
Every resin has a flow limit. Suppliers publish spiral flow data showing how far the melt travels in a standard spiral channel at set pressure and temperature. Measure your longest flow path from gate to last-fill point. Compare the two numbers.
If the path is marginal, run a mold-filling simulation before cutting steel. Moldflow and Cadmould both handle this check in a few hours. Guessing costs more than the analysis. A mold that cannot fill gets welded, re-cut, and re-tried. Each loop burns weeks.
Balance Flow Around Cores
Long, unsupported cores bend under uneven melt pressure. The high-pressure side pushes the core off center. Wall thickness grows on the gate side and shrinks on the far side.
In severe cases the far side never fills. In other cases the core springs back after filling and pinches the thick wall. That pinch can lock the part in the mold or crack it during ejection. The fix is symmetric gating around the core so pressure balances from both sides. For deep cores, two gates timed to meet evenly beat one gate every time.
Core shift also shows up as dimensional drift. A connector housing that measures fine on T1 can drift out of tolerance as the process settles. When a round hole measures oval, check core deflection before blaming shrinkage.
Quick Checklist Before Approving Gate Location
- Gate sits in the thickest wall section, not the thinnest.
- No thin-to-thick filling unless a flow leader is added.
- Thinnest walls positioned near the end of fill.
- No gate on screw bosses, snap arms, or load-bearing holes.
- Weld lines kept out of high-stress zones; check the weld line strength range for your resin.
- Longest flow path checked against spiral flow data.
- Flow balanced around long cores, with symmetric gates where needed.
- Fill pattern reviewed with the molder before steel is cut. See our defect root cause guide for gating failures on real parts.
- Cosmetic faces placed away from hesitation-prone thin edges; compare with flow mark causes if gloss bands appear.
