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Plastic Injection Molded Parts: How to Control Weld Lines

Plastic Rapid Injection Molding is popular not only in low volume manufacturing but also in large quantities molding, it is the most common way to manufacture plastic parts. Weld lines are the result of fundamental physical effects taking place inside of the mold tool cavity, they are common imperfection to be found in most plastic injection molded parts. How can we minimize them and reduce their cosmetic effects by careful product design and manufacturing process control? Let’s share our opinion and experience here:

What Is A Weld Line?

We can observe a straight line or a curve radiate away from a hole, inclusion or other such part feature on the surface of a plastic injection molded part, this mark is a weld line.

How are They From?

Molten plastic resin is pressuized into a mold cavity through a injection gate, it fill the entire cavity. When molten plasitc contacts the wall of cavity, it cools down as the cooling channels around the wall and begin to form solid plastic shape that’s desired. However, if the molten plastic doesn’t move in one continuous unbroken path, but rather is forced to proceed around a hole or other such part feature, then the path diverges into two branches, which meet up again on the side away from the gate. These two branches are pressed together later, but they will be at unequal temperature as the plastic resin has already had time to contact the cooler tool walls and therefore has already begun to partially solidify. This solidification prevents the two branches from mixing completely and causes the characteristic weld line.

How Can We Control Weld Lines on plastic injection molded parts?

–  Optimizing the injection parameters through combination of adjustinig the molding temperature,varying the gate pressure, and increasing the dwell time. Meanwhile, Using more advanced cooling method such as cyclic cooling or conformal cooling

–  Analysis the gate location, select a best injection point for reducing weld lines exprose on cosmetic surfaces.

How Can We Design to reduce Weld Lines on plastic injection molded parts?

Weld lines can be partially hidden by masking through intergrating fins, lines, raised or recessed lettering. Also, applying the heavy texture and lgiht part color can partly reduce the visibility of weld lines. Please bear in mind that the weld line repesent an area of comparative weakness, part failures usually happen at this location under mechannical stress. A careful designer may want to therefore add reinforcements at these critical areas.

As a good manufacturing partner, TEAM Rapid always suggest corrective actions to our customer before building the Rapid Injection Molding tool. It is improtant for you to know the details and how to allow for them in your design. We have a dedicated team ready to handle any question or enquiry you may have, contact us at [email protected] today!

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