The gate is already on the front face. Can it be saved?

Answer: Three routes, rising in cost. Enlarge the nameplate to 140 mm × 80 mm and cover the seam; cut a 0.3 mm groove along it to turn the defect into styling; or move the gate, needing a new insert and 9 days.

What weld line strength loss is acceptable?

Answer: Match it to the drop rating. A case rated at 1.2 m on six faces needs loss under 20%, meaning 135° or more; a tool case rated at 0.8 m passes at 31% with the seam 30 mm clear of corners.

How many drops does a case need?

Answer: Divide flow length by 260 mm and round up. A 520 mm case with a 560 mm path takes two or three drops; a 760 mm case takes three or four. Extra drops add gate marks and 3–5% more part weight in runner scrap.

Which is easier to hide, a gate mark or a gas mark?

Answer: The gate mark, a regular recess 4.5 mm across and 0.15 mm deep that a label fills. A gas mark is a 40 mm × 25 mm haze of silver streaks needing a full repaint, at three times the rework rate.

Case Gate Location and Weld Lines

Quick answer: Case gate location fixes the weld line before steel is cut, and one drop 40 mm off centre runs a 0.08 mm seam straight across a 120 mm nameplate.

When the Gate Sits on the Front Face
Where the Weld Line Lands
Weld Line Strength Loss in Percent
Balancing Multi-Point Gates
Gas Marks and Trapped Air
FAQ

1. When the Gate Sits on the Front Face

In April 2025 a run of 520 mm × 380 mm × 210 mm instrument cases failed trial moulding. The gate sat 40 mm above centre, two drops on the lid, and the melt split around a 120 mm × 90 mm recess before rejoining. The seam was 0.08 mm deep and 112 mm long, exactly where the nameplate goes. Three polishing rounds fixed nothing, and 200 lids were scrapped.

Nobody had calculated the seam position. Its path follows the drops and the geometry between them; process settings change depth, never address.

Mark the visible zones first, strike out nameplate, label and parting-line band, then hand the rest to the gate. A cosmetic gate may only sit where it is hidden or covered.

2. Where the Weld Line Lands

The meeting angle of two flow fronts controls depth. A 90° convergence is head-on: the chilled skin folds into the joint, depth passes 0.1 mm and tensile strength drops to 55–65% of bulk. At 135° the fronts slide past each other and depth falls to 0.04 mm.

case gate location mould frames stacked in a workshop aisle, a worker in a red helmet under the crane, blue racking at right

Square recesses, ribs and bosses make most 90° convergences. On a 460 mm lid a 120 mm recess splits the melt into two 170 mm streams; centre the drop and the seam sits on the centreline, shift it 30 mm and the seam moves 8–12 mm toward the longer side. A fill-time plot shows that before the tool is cut.

Angling the drop 12–18° makes one front lap over the other, cutting depth to 40% of a butt joint at 5 mm off centre. Vents must follow the meeting point: beyond 45 mm, gas enters the seam and leaves 0.3–0.5 mm silver streaks.

3. Weld Line Strength Loss in Percent

A weld line is a structural weak line, not a cosmetic defect. A 0.08 mm seam cuts wall from 3 mm to 2.92 mm, only 2.7% of the section, yet strength falls an order of magnitude faster than thickness.

Material	90° convergence	135° convergence	150° convergence	Fracture face
ABS	45 MPa → 31 MPa, −31%	37 MPa, −18%	40 MPa, −11%	Flat along seam
ABS/PC blend	48 MPa → 32 MPa, −33%	39 MPa, −19%	42 MPa, −12%	Local streaking
PC	62 MPa → 37 MPa, −40%	48 MPa, −23%	53 MPa, −15%	Brittle, bubbles
PP plus 20% glass fibre	38 MPa → 21 MPa, −45%	28 MPa, −26%	32 MPa, −16%	Fibres pulled out

Bars were 3 mm dumbbells cut along the seam and pulled at 50 mm/min, eight per cell. ABS loses least because the butadiene phase still entangles; PC loses most; glass-filled compound loses most of all, fibres aligning in the convergence plane.

A 1 kg tup from 300 mm cracked 3 of 8 specimens at 90° and 1 of 8 at 150°. At 1.2 m onto six faces the finished case broke 4 units, all from the lid seam; with the drop shifted 30 mm onto a rib, 1 broke.

4. Balancing Multi-Point Gates

Cases above 520 mm need two to four drops, all filling inside one window: early melt has skinned, late melt cannot push it, and the seam becomes a cold joint.

case gate location workshop row of injection machines, silver ducting across the roof, white resin sacks on yellow flooring beside a box marked 13

An 8 mm sprue with 6 mm runners holds pressure spread within 2 MPa and fill-time spread at 0.15 s across four drops. Shrinking a gate from 4.5 mm to 3.5 mm raises resistance 2.7 times, and 0.3 mm between drops alone opens 0.4 s.

Four drops on the long sides leave 12 mm of flow difference, natural balance, one 4.5 mm diameter everywhere. Three drops leave the middle path 40 mm short, so that gate comes down to 3.8 mm.

Each drop adds a gate mark, 4.5 mm across and 0.15 mm deep: hidden inside the lid, a defect outside it.

5. Gas Marks and Trapped Air

Gas marks appear where the melt arrives last, twins of the weld line. Gate position sets flow direction, flow decides where the air goes, and the vent only receives. Move a gate 30 mm and the mark often moves 40 mm.

Vent depth has a floor and a ceiling: 0.02 mm by 6 mm works for ABS, 0.04 mm flashes, 0.01 mm quadruples resistance. Spacing along the parting line stays under 60 mm, with a vent at every corner.

Air concentrates at recess floors, boss roots and rib ends. Moving a gate 60 mm off a recess vents that floor through the parting line and shrinks the mark from 40 mm × 25 mm to 8 mm × 5 mm.

Gate plan	Weld line position	Tensile loss	Gas mark risk	Tool change
Single, front centre	Over the nameplate	31%	High, recess traps air	None, cosmetic scrap
Single, 30 mm offset	On a vertical rib	31%	Medium, streaking	One electrode
Two, inside lid	Inside cavity	18%	Low	New insert, 9 days
Four, inside rear	Inside cavity	12%	Low, plus 4 vents	Runner recut, 14 days

The plans span 19 points of strength loss and 14 days of tooling.

Conclusion

Case gate location can be written into three drawing notes. First, the exclusion zone on cosmetic faces: 15 mm beyond the nameplate outline, 10 mm either side of the parting line, and the whole handle recess, with no gate coordinate inside. Second, the weld line acceptance zone: the seam lands outside functional areas and at least 30 mm from any corner, with the convergence angle called out at 135° or more and a 0.3 mm groove where that cannot be met. Third, balance and venting: pressure spread within 2 MPa, and a vent within 45 mm of every convergence point, 0.02 mm deep and 6 mm wide. Written into the 2D notes, these three lines make the moulder return a fill-time plot before steel is cut, so a seam across the nameplate is caught at the drawing stage.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.