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Troubleshooting Injection Mold Gate Blush

When you see a non-uniform fanning out from the gate, which appears as a different color or gloss level to the surrounding plastic, you may be dealing with gate blush.

What is Gate Blush?

Also known simply as blush, or gate shear or halos, gate blush is a dull or discolored portion usually found just inside the gate location of a part. Occasionally, gate blush will show up in areas where there’s a wall-stock transition.

Blush can be confused with jetting, flow lines and flow marks, so you need to be careful when troubleshooting. Since jetting gives a blush-like appearance, you’ll want to confirm you’re dealing with gate blush before troubleshooting.

Troubleshooting Gate Blush

If you’re experiencing blush, it could be due to the molding process, mold itself or the machine. See below for possible causes.

Molding Process Mold Machine Material
Injection velocity Gate geometry Nozzle
Nozzle temperature Hot runner tip temperature Machine performance
Mold temperature Cold slug well
Melt temperature Gate location

Table 18.1 Gate Blush Troubleshooting Chart, found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 142)

How to Eliminate Gate Blush in Injection Molding

Most often, the culprit is an injection velocity that’s set too high. However, the solution isn’t as simple as lowering the injection velocity. Not at first, anyway. Before you reduce the fill speed, ensure your gate size and design are adequate. If your blush is caused by something with the tooling design and you try to process around it, you risk other defects, particularly short shots.

Pay attention to all temperatures: nozzle, mold and melt. If the nozzle or mold temperatures are too high, it can impact the first plastic in the mold, resulting in blush. A melt temperature that’s too high can hurt the quality of the initial formation of plastic out of the gate.

As we mentioned, check the size and design of the gate before attempting to process around a potential tooling issue. If the gate isn’t flush to the mold surface, that needs to be adjusted before you change any of the settings with the molding process.

If the problem is with the machine itself, your first place to look should be the nozzle. Its length, style, tip type, tip orifice and heaters all need to match the documented process.

Although less common, there could be issues with the material. PC/ABS blends and TPOs tend to be the most prone to gate blush.

Want to learn more? Check out these other great articles that solve common injection mold issues:

Source: Injection Molding Advanced Troubleshooting Guide: The 4M Approach by Randy Kerkstra and Steve Brammer.