Injection Moulding Headaches Part 7: Weld Lines (and How to Fix Them)

Here’s another issue that can quietly weaken a moulded part.

This week’s issue is weld lines, sometimes referred to as weld joins.

This is the seventh instalment in our weekly series, where we break down common moulding defects and look at the tooling and design decisions that help prevent them.

The Problem: Weld Lines (Weld Joins)

Weld lines are visible lines that appear where two flow fronts of molten plastic meet inside the mould.

In some cases they are purely cosmetic. In others, they create structural weak points that affect part performance.

Weld lines commonly appear around:

• Holes
• Bosses
• Inserts
• Multiple gate locations
• Complex flow paths

If not properly controlled, weld lines can:

• Reduce part strength
• Create cosmetic defects
• Cause cracking or failure under load
• Lead to inconsistent product quality

Why do weld lines happen?

Weld lines form when two separate flows of molten plastic meet but fail to bond together properly.

This usually happens because the material cools too much before the flow fronts combine.

Common causes include:

• Low melt temperature
• Poor gate placement
• Slow injection speed
• Long or complex flow paths
• Insufficient venting
• Material cooling too quickly around holes or features

How to design them out (before tooling even starts)

Good part and tool design can significantly reduce weld line problems before production begins.

At Alliance Tooling, we focus on:

• Optimising gate location for balanced flow
• Designing flow paths to minimise opposing fronts
• Improving venting in weld line areas
• Controlling wall thickness transitions
• Designing tooling around material flow behaviour
• Identifying cosmetic and structural risk areas early

Already seeing weld lines? Here’s how to fix them

If weld lines are already appearing in production, several process and tooling adjustments can help improve the result:

• Increase melt temperature
• Increase injection speed
• Optimise mould temperature
• Improve venting near weld areas
• Modify gate location or gate size
• Adjust process settings to improve flow front bonding
• In some cases, tool modifications may be required

Where we come in

Weld lines are often accepted as an unavoidable part of moulding, but many can be reduced or eliminated through better tooling and flow management.

At Alliance Tooling, we:

• Analyse how material flows through the tool
• Identify weak or high-risk weld areas
• Design tooling for stronger, cleaner mouldings

Whether it’s:

• A new mould tool
• Troubleshooting an existing component
• Or improving part performance and appearance

We help you achieve more reliable, production-ready parts.

Dealing with weld line issues or weak mouldings? Book a quick call with Chris Rossell, Technical Director, and we can review it with you.

Next week: Voids, why internal air pockets form and how to prevent them.