Weld Lines in Injection Moulding: Strength and Validation

Updated: 2026-10-22

A weld or knit line forms where separated melt fronts meet during filling. Flow may divide around a hole or insert, arrive from multiple gates, or take paths with different resistance. The meeting region can be visible, mechanically weaker, both, or acceptable for its application. Appearance alone cannot establish its strength or leak tightness.

Understand the mechanism without assuming a strength percentage

Front temperature, pressure, contact conditions, trapped gas and material behaviour affect the joint. Reinforcing fibres can align unfavourably across the meeting region. There is no single remaining-strength percentage that applies to every glass-filled or unfilled polymer, specimen and loading direction. Use grade-specific evidence where available and validate the actual part under its relevant service conditions.

Terminology also varies: simulation software may distinguish a head-on weld from a more oblique meld. Do not turn the meeting angle into a universal acceptance rule. The earlier shorthand that a head-on meeting is always stronger is unreliable; flow direction, continued flow and material orientation matter. Autodesk's weld/meld explanation describes that distinction.

Map the line to the function

Mark the observed line on the part and drawing, with cavity, material grade and shot identified. Connect it to the filling pattern: is it behind a core, between gates, or near a region where one flow path hesitates? A controlled short-shot sequence conducted by the setter can help reveal the progression of filling. A surface line may instead be a flow mark or another defect, so confirm the mechanism before relocating a gate.

Application	Acceptance evidence to discuss
Visible housing	Agreed lighting, viewing distance, colour and reference sample
Loaded snap or screw boss	Assembly and load tests in the relevant direction
Pressurised or sealed wall	Relevant pressure, leakage and environmental test
Reinforced structural part	Actual grade, orientation and representative part testing

Keep cosmetic and functional decisions separate. A less visible seam may still lie in a critical load path; a visible one may be acceptable on a hidden, unloaded face. Identify restricted areas before tool design and specify the load, environment and service-life requirement rather than simply writing “no weld lines” everywhere.

Investigate the local conditions first

Check the selected grade's drying and processing instructions, actual melt and mould conditions, fill consistency and vent condition at the meeting point. Trapped air can obstruct contact; an unsuitable thermal history can degrade the material. Raising temperature or speed indiscriminately can introduce degradation, burns or flash. Select the hypothesis and stay within the approved material and machine limits.

Compare all cavities and time periods. A line that changes visibility after restart may indicate thermal instability, but the visual change is not proof of a strength change. A line that always occurs behind one core may be geometrically expected while its quality still varies with processing. Record both its position and the outcome of relevant functional tests.

Decide whether to improve the joint or move it

The gate strategy, flow-path balance and local geometry influence where fronts meet. Moving a gate can move a line while changing packing, orientation, gate marks and warpage elsewhere. An overflow can promote continued flow beyond the meeting region, but requires space, material removal and tooling. Sequential valve gating may be appropriate for some tools; it is not a guarantee that every weld line disappears.

Use flow simulation to compare alternatives with the actual grade, wall distribution and process assumptions. Confirm the predicted location in trials. A simulation colour plot does not establish the allowable load of the finished component. Tooling, geometry or material changes also require the other dimensional and cosmetic characteristics to be reassessed.

Run a test that can support approval

An illustrative teaching scenario: a clip's weld line lies at its flexing root. A revised gate moves it onto a less loaded face and makes the surface look cleaner. Approval still requires the clip's agreed assembly, retention and cycling tests after the specified conditioning; the visual improvement alone is insufficient. These are invented circumstances, not Nordmould test results or a recommended test-load value.

Compare identified baseline and trial parts from every cavity under the same method. Define the sample quantity, conditioning, load rate, fixture and failure criteria with the responsible product engineer. A test on a generic material coupon is useful background but does not reproduce every stress concentration or fibre orientation in the moulded part. Recheck any environmental ageing requirement relevant to the application.

Provide CAD, marked critical zones, grade, photographs, process history and test results when requesting a review. Use the sample approval pack to connect the observed seam, action and final acceptance. For the wider investigation, see the defects guide.

Additional engineering background: Asahi Kasei on predicting moulding defects. Neither source establishes a Nordmould simulation service commitment or a guaranteed weld strength.