Avoiding Weld Lines in Plastic Injection Molding

Having any defect or blemish on a finished part will leave anyone dissatisfied. With parts made from plastic, it could mean even more problems as the blemish could lead to a functional failure of the part. An example of such a blemish on plastic parts is the weld line.

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These lines could severely affect your plastic product as it could lead to failure of the whole production process. This would normally mean the loss of a huge amount of money as the process would have to start all over. To help avoid this, this article will run through the causes of weld lines, the defects they cause, and how to prevent them.

How Do Weld Line Defects Appear?

A weld line represents the junction where two flow fronts converge during the molding process. This line typically emerges when these fronts are unable to seamlessly merge or “weld” together. Commonly found around holes or obstructions, weld lines are indicative of areas with reduced strength within the molded component.

Another terminology that machinists popularly use about weld lines is meld lines. While both are similar in appearance, they have one slight difference. The difference is the meeting angle of the two flow fronts. For weld lines, the meeting angle is always smaller than 135 degrees. Any meeting angle greater than this creates a meld line.

To better understand how this phenomenon forms, it is necessary to know what happens when plastic flows. When plastic flows, it forms a smooth continuous flow front. This flow remains smooth throughout if it doesn’t contact any obstruction.

However, if the flow comes in contact with an object or obstruction, such as a pin, it immediately splits into two different parts to go around the object. Now, you have two different sides—one flowing through one part of the object and the other on the opposite side of the object.

The flow rejoins in the front, leaving behind a slight depression at the surface. This slight depression is a tiny line called the weld line or knit line. However, the weld line may not continue throughout the flow. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. This continues until the weld lines in plastic injection molding disappear.

Causes of Injection Molding Weld Lines

Like many part defects, a couple of factors contribute to the formation of weld lines during part design. Some of these factors include:

Pressure

Temperature

Mold design

Speed

Impurity

Excess mold release

Pressure plays a significant role in creating broad weld lines. If the pressure is not enough to push the flow and meld back together, this can create a broad weld line. Similarly, temperature also affects the formation of weld lines. If the temperature is not high enough, you may begin to have premature solidifying.

Mold design errors, such as improper wall thickness and improperly placed gates, can also cause weld lines. Additionally, speed, impurities in the resin, and excess mold release can contribute to the formation of weld lines.

Why You Should Avoid Weld Lines

Weld lines are undesirable in the injection molding process due to several reasons. Firstly, they weaken the area where they appear, making it fragile and prone to breakage. Secondly, weld lines can deform the surface appearance of the product, detracting from its overall design.

Tips for Avoiding Weld/Knit Lines in Plastic Injection Molding

To avoid weld lines, there are steps you can take during the part and mold design process, as well as by adjusting the molding conditions. Suggestions include altering the part design by increasing wall thickness or reducing part thickness ratio, altering the mold design by increasing gate size and eliminating entrapped air, and adjusting molding conditions such as temperature, injection speed, and pressure.

Conclusion

Having weld lines on any finished plastic part is a manufacturing defect to avoid at all costs. However, with the right approach and expertise, it is possible to minimize or prevent the formation of weld lines in plastic injection molding. By implementing proper design techniques and adjusting molding conditions, you can craft flawless products that meet high standards of quality.

At , we offer efficient and reliable injection molding services. With quick quotes, free Design for Manufacturability (DFM) analyses, and mold flow analyses, we strive to deliver perfection without the blemish of weld lines. Transform the way you think about injection molding by partnering with us for your manufacturing needs.