Injection Moulding Headaches Part 11: Gate Blush (and How to Fix It)

Here’s another defect that can tell us a lot about what’s happening as material enters the mould cavity.

This week’s issue is gate blush.

This is the eleventh instalment in our weekly series, where we explore common moulding defects and the practical steps that can prevent them through better design, tooling, and process control.

The Problem: Gate Blush

Gate blush is the cloudy, hazy, or discoloured area that appears around the gate of an injection moulded part.

Unlike silver streaks or splay, which can extend across the component, gate blush is normally concentrated at or immediately downstream of the gate.

Gate blush can:

• Ruin the cosmetic appearance of a component
• Make the gate area much more noticeable
• Reduce customer confidence
• Increase rejection rates on appearance-critical parts
• Indicate excessive shear or stress as material enters the cavity

Why does gate blush happen?

Gate blush is commonly caused by molten plastic experiencing excessive shear as it passes through the gate and rapidly enters the cavity.

If the gate is too restrictive, or the initial injection speed is too high, the material can be subjected to high shear rates and localised heating. This can create the characteristic cloudy or discoloured appearance around the gate.

Common causes include:

• Gate size being too small
• Excessive initial injection speed
• Poor gate geometry
• Restrictive flow immediately after the gate
• Incorrect melt or mould temperature
• High back pressure
• Material that is particularly sensitive to shear

How to design it out (before tooling even starts)

Good gate design is critical to reducing the risk of gate blush. At Alliance Tooling, we focus on:

• Selecting the correct gate type and size
• Positioning the gate to promote smooth material flow
• Avoiding unnecessary restrictions immediately after the gate
• Designing smooth transitions into the cavity
• Considering the flow and shear characteristics of the chosen material

The objective is simple: get the material into the cavity smoothly, rather than forcing it through an unnecessarily restrictive gate.

Already seeing gate blush? Here’s how to fix it

If gate blush is appearing during production, there are several areas worth investigating:

• Reduce the initial injection speed as material passes through the gate
• Increase gate size where appropriate
• Review gate geometry and how material enters the cavity
• Optimise melt and mould temperatures
• Check material condition and drying requirements
• Use a profiled injection speed to better control the initial filling phase

In some cases, a relatively small change to the gate or surrounding geometry can make a significant difference to the finished component.

Where we come in

Gate blush is a good example of how small details within a mould tool can have a significant effect on the finished part.

At Alliance Tooling, we:

• Design gates around the component and material
• Consider how the polymer will enter and fill the cavity
• Optimise tooling to create a stable, repeatable processing window
• Help troubleshoot existing tools where gate-related defects are causing problems

Whether it’s:

• A new mould tool
• An existing component with cosmetic issues
• Or a process that’s proving difficult to stabilise

We look beyond the visible defect to understand what’s actually causing it.

Got a gate-related defect that’s proving difficult to eliminate? Book a quick call with Chris Rossell, Technical Director, and we can review it with you.

Next week: Delamination, why layers can separate within a moulded part and what you can do about it.