Injection Moulding Headaches Part 2: Flash (and How to Fix It) // Alliance Tooling Blog

What’s your biggest annoyance when it comes to injection moulded parts? Let’s talk about another common frustration. This week’s culprit is flash.

This is the next instalment in Alliance Tooling’s weekly series, where the company breaks down common moulding issues and looks at the engineering reality behind fixing them. This time, focusing on a defect that is easy to spot but often misunderstood.

The Problem: Flash

Flash is the thin, unwanted layer of plastic that creeps along parting lines, ejector pins, or shut-offs. It might seem minor at first glance, but it can quickly become a serious production issue.

• Ruins part aesthetics: Particularly problematic for visible surfaces.
• Interferes with assembly: Can affect tolerances and part fit.
• Adds post-processing time: Trimming increases cost and slows production.

Why does flash happen?

Flash occurs when molten plastic escapes from the mould cavity and solidifies where it should not.

Typical causes include:

• Worn or poorly fitting tool surfaces
• Insufficient clamping force
• Excessive injection pressure
• Poor venting or tool design

How to design it out (before it becomes a problem)

A well-designed tool should prevent flash, not fight it. Addressing the issue early in the design stage reduces the need for ongoing adjustments later.

Here’s what we focus on:

• Ensure robust parting line design with proper shut-offs.
• Specify tight tolerances on critical sealing surfaces.
• Design for balanced filling to avoid overpacking.
• Incorporate effective venting without compromising sealing.

Already seeing flash? Here’s how to tackle it

If flash is showing up in production, there are still practical steps that can bring it under control:

• Reduce injection pressure and speed where possible.
• Check and increase clamping force if needed.
• Inspect for tool wear or damage, especially on shut-offs.
• Optimise processing parameters to reduce overpacking.
• Consider tool refurbishment or modification if required.

Where we come in

Flash is often treated as a processing issue, but in many cases the root cause sits within the tool itself.

At Alliance Tooling, we focus on:

• Designing tools that seal correctly from day one
• Identifying wear points before they become costly problems
• Help you eliminate recurring defects, not just manage them

Dealing with flash issues or planning a new tool? Book a quick call with Chris Rossell, Technical Director, and we can review it with you.

Next week: Burn marks, what causes them and how to avoid costly cosmetic rejects.

Read more news from Alliance Tooling here.

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