Weld Line: Its Causes, Defects and How to Avoid It

Having any defect or blemish on a finished part will definitely 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.

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 it causes, and how to prevent it.

How do Weld Lines Appear?

weld lines in finished products

Weld line (also known as a knit line) is the line where two flow fronts meet when there is the inability of two or more flow fronts to “knit” together, or “weld,” during the molding process. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part.”

Another terminology that machinists popularly use in relation to 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.

weld or knit lines

To better understand how this phenomenon forms, it is necessary to know what happens when plastic flows. When plastic flows, they form 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.

How weld lines form

Causes of 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


If the pressure is not enough to push the flow and meld back together, this can create a broad weld line. This can happen if the machine is faulty or its setting is inappropriately done. It can also be a result of issues with the mold design.


If the temperature is not high enough, you may begin to have premature solidifying. Some parts might solidify while the other part still flows. With this, you may eventually have weld lines.

The temperature required for continuous resin flow might drop in different places. Some of such places are the mold, the runners to the mold, and the injection molding machine.

Mold design

example of an injection molding process workflow

Mold with a poor design might cause weld lines in different areas of your design. Some common errors in mold design that can cause knit lines are improper wall thickness and improperly placed gates.


With a low speed, the resin will travel through the mold slowly. This means the fronts may not cool at equal times. If one cools before it touches the other, this could cause knit lines in injection molding.


If the resin contains impurities, there’ll be no smooth flow through the mold. This may mean that one part of the flow will be faster than the other.

Excess mold release

If there’s too much mold release, you may require a higher pressure to push it through the machine. If not, the speed will drop, and it could create weld lines.

Why You Should Avoid Weld Lines

Knit lines in injection molding are undesirable, especially when surface appearance and part strength are significant concerns. Therefore, you should avoid them for the following reasons:


weld line at holes could cause fragility of parts

Weld lines are usually the weakest area on your part. Thus, the material can easily break off from such an area. If the part is designed for a purpose that requires good strength, the fragility caused by weld lines can hinder it from effectively serving such a purpose.

For instance, if you have a knit line around a screw hole in your part, the line might break when you drive and tighten a screw through the hole.

Deforms surface appearance

Knit lines might deform the appearance of your design. No matter how much effort you put into the design to look attractive, a single knit line might be the point of attention that deforms the design.

Tips for Eliminating Knit Line

You can eliminate weld lines in plastic injection molding by ensuring a single front throughout the molding process. You can also eliminate the knit by ensuring that the line is well covered. The following tips can help you to achieve this:

  • Alter the part design
  • Alter the molding design
  • Adjust the molding conditions

Alter the part design

part design of a plastic product

You can do this by increasing the wall thickness. This will help to facilitate pressure transmission and also ensure that there’s a higher melt temperature. Thickening part walls can help to slow down the resin cooling speed. This gives the resin more time to spread to the uncovered part before it eventually cools and solidifies.

However, you must be careful not to make the part walls too thick. If you do, you may end up having sink marks.

You can as well reduce the part thickness ratio. With a lighter flow, the liquid will cover a broader range faster. Thus, it should close up the weld line.

You can also adjust the gate dimension and position. Keep the knit-causing part as far as possible away from the edge of your design. If you keep it too close to the edge, it may easily break off.

Alter the mold design

You can alter the mold design by increasing the size of the gate and runners. You should also eliminate entrapped air in the weld line injection molding. If you don’t eliminate the air, it will create unfilled portions in your part design which would further cause more weakening. You can remove the entrapped air by placing a vent in the weld line area.

Another way to alter the mold design is to change the gate design. This will help eliminate the weld line injection molding or ensure that they form closer to the gate under high packing pressure and at a high temperature.

Adjust the molding conditions

Another way to eliminate weld lines in plastic injection molding is to adjust the molding conditions. The conditions are temperature, injection speed, and injection pressure.

If the melt temperature is low or the injection speed and pressure are low, you may end up with a more pronounced weld line. Thus, you can eliminate the weld line by increasing these conditions.


rapidcirect injection molded services

Having weld lines on any finished plastic part is a manufacturing defect to avoid at all costs. Apart from leaving an ugly scar on the surface of your product, it also has functional disadvantages such as increased fragility.

To ensure your finished plastic product is of the highest quality and devoid of any weld lines, RapidDirect is the best choice for you. We deliver parts of the highest manufacturing and functional quality which always leaves our clients satisfied. Within 12 hours of contacting us, we would send you a quotation estimating the costs for your parts production.

Also, we provide professional DFM analysis and mold flow analysis and send over design feedback for FREE for every order placed with us. This way, we can optimize your design to ensure the product has no defects.

With us, you get injection molded plastics of the highest quality.

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