A weld line is the seam left where two melt fronts meet in the mold and fail to fully knit together. It can leave the part with anywhere from 10% to 90% of the base material's strength. The usual fix is a hotter process or a relocated gate.
How two flows become one weak seam
Watch a mold fill and the plastic rarely travels as one front. It splits around every hole and boss, then the two fronts meet on the far side. That meeting point is the weld line.
Multi-gate molds create the same problem on purpose. Each gate starts its own front, and the fronts knit wherever they collide. The designer chooses the gate layout, so the designer chooses where the weld lines land — whether they admit it or not.
Air makes it worse. The two fronts trap a thin cushion of air between them as they close in. Without a vent at that exact spot, the air has nowhere to go, and the fronts never fully touch.
What the line costs you
Published test data put weld line strength anywhere from 10% to 90% of the base material. That range is wide because the details matter. Hotter fronts knit better, and some resins forgive more than others.
Glass-filled grades sit at the bad end of that range. The fibers line up with the flow, so at the weld line they lie parallel to the seam instead of crossing it. The plastic has to carry the load alone, and it usually cannot.
Cosmetics suffer too. The line shows as a faint hairline, and on textured surfaces it can read as a gloss change. Buyers notice it on visible faces long before they test strength.
I've seen a housing pass every dimensional check and still snap along the weld line in a simple drop test.
Gate position decides where the line lands
Most buyers never ask where the weld line will sit. They should. The gate layout fixes where the fronts meet. Moving a gate a few centimeters can drag the line off a cosmetic face or out of a stress zone.
Recombination works best near the gate. The fronts are hottest and fastest there, so the knit is strongest. A good mold designer places gates so the fronts meet early, in thick sections, away from holes.
If the part must have holes, expect a line behind each one. The honest move is to put the line where it matters least. Prove the position with a fill simulation before cutting steel.
If your supplier can't show you where the weld lines fall, they haven't thought about it.
Process fixes that work on the press
When the mold is already cut, the process window is what you have. Raise the melt temperature first. Hotter fronts stay liquid longer and knit deeper. Then raise the mold temperature. It slows the freeze-off at the meeting point.
The trade-off is cycle time. A hotter mold always costs cycle time, and the buyer pays for those seconds in the part price.
Increase injection speed so the fronts meet while still hot. Pack harder to squeeze the knit together. Vent the weld area so trapped air has a way out. A shallow vent at the meeting point lets the air escape without flashing the part.
Each of these has a cost. Higher melt temperature risks material degradation and longer cooling. Faster filling can trap air elsewhere or jet. More packing raises clamp tonnage needs and internal stress.
When the line will not move
Sometimes the line cannot be eliminated. A part with holes on all sides will always have weld lines. Then the job changes from removal to management.
Put the line in a low-stress zone. Add a rib across it so load has another path. Or gate the part so the fronts meet in a thick section that forgives a weak knit.
Sequential valve gating can drag the weld line to a chosen spot by opening gates in sequence. It works, and it adds hot-runner cost and control complexity most parts do not need.
Before you approve the next mold design, ask where the weld line will sit and what carries the load across it.
FAQ
Can weld lines be completely eliminated?
It depends on the cause. If the line comes from multiple gates, a single gate or sequential valve gating can remove it. If holes or inserts force the flow to split, the line stays. The work then is moving it to a low-stress, hidden area. Anyone who promises zero weld lines on a part full of holes is guessing.
Do weld lines weaken all plastics equally?
No, the loss varies widely. Unfilled resins like PP or ABS usually keep most of their strength across a good weld line. Glass-filled grades lose far more because the fibers align with the flow and never cross the seam. For a loaded glass-filled part, treat every weld line as a likely failure point until testing proves otherwise.
Will faster injection always reduce weld lines?
Faster is not always better. Faster filling keeps the fronts hotter, which helps them knit. But push too far and the flow turns turbulent. It traps air or jets, and trapped air at the weld area makes the line worse. There is a window, not a direction. Find it with short-shot studies, not by maxing out the speed setting.
How do you check weld line strength on real parts?
Bend or flex the part at the line by hand. A good knit bends; a bad one snaps cleanly along the seam. For numbers, mold test bars with and without the weld line and pull them. Compare both sets of numbers. If the welded bar is far weaker, the process window or the gate layout needs work before production.
Should visible weld lines mean rejecting the parts?
A visible line alone is not a reject reason. A faint line on a non-loaded, hidden surface is often harmless. Reject when the line sits on a cosmetic face the customer sees. Reject it too when it crosses a snap-fit or the main load path. Write the rule on the drawing. State where weld lines are banned, and the supplier will mold to it instead of arguing later.