AIM Processing Small Plastic Parts Blog

What’s known in the plastic injection molding business as “gate blush” or simply “blush” is a cloudy or hazy discoloration of the plastic on a finished part. It is most often found near gate locations, where material enters the part. Not only is the defect in color and gloss visually unappealing, the plastic in that area can be weaker than in the rest of the part. Needless to say, it’s an issue that needs to be corrected as soon as it is discovered.

How to Beat the Blush

Here are some common causes of gate blush in plastic injection molding and how they can be addressed:

• Gate is too small or the wrong style for the material. The gate restricts flow as the part fills. But, if it is too restrictive for the required flow, the material will react and blush will appear. The fix: review the gate size against the nominal wall thickness and material.
• Injection fill speed is too high. The speed at which resin enters the mold has a big impact on finished parts. If the mold is filled too quickly, the molten plastic will react to the flow restriction at the gate. The fix: adjust the fill speed until the optimal rate is achieved to prevent blushing.  Careful—slowing the fill speed down too much may introduce other problems.
• Injection pressure is too low. In order for a part to be formed properly, the resin must enter the mold with enough pressure that a consistent speed is achieved. If the pressure limit is set too low, then the flow may hesitate and accelerate. The fix: increase the fill pressure limit.
• Melt temperature is too high or too low. Success in plastic injection molding relies, in large part, on achieving the right flow. Plastic that is either too hot or too cold will not flow properly, which means it won’t fill the mold properly. The fix: find the ideal barrel heat for the material and the mold you are using.
• Nozzle diameter is too small. When the nozzle diameter is too small, it compounds the flow restriction in downstream runners and gates. The fix: enlarge the nozzle diameter. Ideally, the nozzle tip should be the same size as, or slightly smaller than, the sprue bushing opening.
• Gates are not in the right locations. We’ve blogged about gate location in the past. If gates are positioned such that thin areas fill first, the plastic there will begin to harden before thicker areas receive material. The fix: ensure that gates are positioned so that thicker-walled areas are filled first.

The Key to Being Blush-Free? Experience.

The causes and solutions listed above are just some of the factors when it comes to gate blush in plastic injection molding. The takeaway is that for every issue that impacts the quality of a part, there is corrective action you can take. The key is knowing what to do. As a Denver-based leader in our industry for more than two decades, we have a tremendous amount of experience to draw on. If you’ve got questions about your next project, or need help resolving an issue with a current project, we’re happy to help.