Gate location and weld lines

The gate decides more than the fill

The gate is the small opening where melt enters the cavity. Its position sets the direction of flow, the length of the flow path, the point where pressure is highest and the place where the part is packed last. Those four things in turn set the shrinkage pattern, the warp, the weld line positions and the appearance of the part.

A gate in the wrong place can be compensated for with more pressure and a longer cycle, which is why some tools run acceptably and cost more than they should. It cannot be compensated for in every case. A weld line that lands on a sealing face or a hinge will reduce function no matter how the process is tuned.

Gate location is therefore a design decision, not a toolmaking detail. It should be agreed at the DFM stage with a flow study, and the part drawing should allow for the mark the gate leaves.

Gate types and the mark each one leaves

The gate type follows from the tool layout, the part geometry and whether the runner is removed automatically. Each type leaves a different vestige, and the vestige is often the reason a gate is rejected on a visible part.

Where to put the gate

Gate into the thickest section. Flow travels more easily through thick material, and packing works from the gate outwards, so a thick section near the gate packs the thin sections at the end of flow. A gate into a thin wall forces high pressure and often leaves the thick section at the far end under packed and sunk.

Keep the flow balanced. In a multi cavity tool, or in a part with several arms, the flow should reach the extremities at about the same time. Unbalanced flow means one cavity packs before another, and the parts differ from cavity to cavity in a way that no process setting can fix.

Avoid gating directly onto a cosmetic face or a sealing surface. If the gate must be on a visible face, agree the vestige height, the position and whether it can be machined or covered. A gate mark on a sealing face is a leak path, not a cosmetic issue.

Watch the flow front. A gate that fires melt into a free space produces jetting, a snake of frozen material that leaves a surface defect. A gate that is too small produces high shear, which can degrade the material and leave a splay mark. Both are visible in a flow study and both are cheaper to fix in the analysis than in steel.

• Gate into the thickest section, not the thinnest.
• Balance the flow so extremities fill together.
• Keep gates off sealing faces and visible show surfaces.
• Size the gate for shear, not just for fill.
• Allow for the vestige on the drawing.

How a weld line forms

When two flow fronts meet, they do not merge into one continuous material. The front surface has already cooled and oxidised slightly, and the pressure at the meeting point is lower than at the gate. The result is a line where the polymer chains are less entangled across the interface, and where any fibre or filler is aligned along the join rather than across it.

That is why a weld line is mechanically weaker than the surrounding material, and why it is more visible on a dark or glossy part. Where the fronts meet while still hot, the join is stronger and is sometimes called a meld line. The distinction is useful but it does not change the design rule: do not rely on a weld line to carry a load.

Weld lines appear in predictable places. In a single gated part with a hole or a pin, the flow splits around the obstruction and rejoins behind it. In a part with two gates, the fronts meet between the gates. In a ring shaped part, the fronts meet opposite the gate. A flow study shows all of these before the tool is cut.

Designing around a weld line

Move it first. Changing the gate position, adding a gate, or moving a hole relative to the gate will move the weld line. If the line must exist, put it where the part is least stressed: away from a hinge, a snap fit root, a sealing face and a screw boss that takes a bending load.

Then strengthen it. A higher melt temperature and a hotter tool improve the bond. So does faster injection, which keeps the fronts hot when they meet. In a glass filled material, the fibre orientation at the weld is the dominant effect, and no process change will recover the strength of the bulk material. Where the load is high, change the material or change the geometry so the weld is not in the load path.

An overflow well is a tool feature that lets the cold front flow into a small pocket beside the part and be trimmed off later. It carries the contaminated front away from the weld, which improves both appearance and strength. It costs a little material and a trimming step, and it is often the cheapest fix on a visible part.

Venting matters here too. Air trapped at the meeting point produces a burn or a short fill right at the weld, which makes a weak line weaker. Add a vent at the last point to fill, not just at the parting line.

Gate marks, flow marks and the cosmetic callout

A gate vestige is the small remnant left when the runner is separated. It can be flush, proud or recessed, and each option has a cost. A flush gate on a visible face usually needs a secondary operation or a gate designed to shear cleanly. A recessed gate hides the mark but leaves a small pit that can trap dirt.

Flow marks are different. They are the wavy pattern that appears where melt fronted against a cold wall, and they are common near a gate on a large flat face. They are a process and design issue together: gate position, gate size, wall thickness and injection speed all contribute. A texture often hides them; a high gloss surface does not.

Put the cosmetic requirement in writing. State which faces are class A, which can carry a gate mark and what vestige height is acceptable. A drawing that says no visible defects, without defining the faces, gives the moulder no target and the inspector no rule.

What to ask for in the DFM response

Ask for the gate position on a marked up model, the gate type and size, the predicted fill pattern, the weld line positions and the pressure at the end of fill. Those five items tell you whether the tool will make the part the drawing describes.

Then ask where the weld lines fall relative to the functional features. If a weld line crosses a seal, a hinge or a terminal pocket, the answer should be a change of gate, not a note that the process will be adjusted. The process cannot move a weld line more than a small distance.

Finally, ask what the gate will look like on the finished part. A photograph of a similar gate on a similar material is more useful than a description, and it settles the cosmetic argument before the tool is cut.

1. 01

   Mark the gate

   Get the position, type and size on a marked up model, not in a paragraph.

2. 02

   Review the fill

   Check fill time, pressure at the end of flow and any hesitation or jetting.

3. 03

   Place the weld lines

   Confirm they avoid seals, hinges, bosses under load and visible faces.

4. 04

   Agree the vestige

   State the allowed gate mark and the faces where it is acceptable.