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Weld Lines in Injection Molding: Identification, Causes, and Prevention
Having defects or marks on a finished part can leave anyone disappointed. This is especially true for plastic parts, as these defects can lead to functional failures. One common issue is the weld line, which can significantly impact the quality of your plastic product and may jeopardize the entire production process. Such problems could result in significant financial losses, as you would need to restart the process from the beginning.
What Is a Weld Line?
A weld line, also known as a knit line, is the junction where two flow fronts meet during injection molding. This line appears when the flow fronts cannot merge seamlessly, or “weld,” together. Weld lines often occur around holes or obstructions and indicate areas of reduced strength in the molded component.
Machinists also refer to these as meld lines. While both terms are similar in appearance, they differ slightly in terms of the meeting angle of the two flow fronts. For weld lines, this angle is always less than 135 degrees. If the meeting angle exceeds 135 degrees, it is classified as a meld line.
To understand how these lines form, it’s important to know how plastic flows during molding. Initially, plastic creates a smooth, continuous flow front. This flow remains uninterrupted as long as it doesn’t encounter any obstructions.
However, when the flow meets an obstacle, such as a pin, it splits into two parts to navigate around the object. As a result, you have one flow on one side of the obstacle and another on the opposite side. When the two flows reunite, they leave behind a slight depression at the surface, which is called a weld line or knit line. It is possible for the weld line to fade as the plastic continues to flow; the two flow fronts may gradually rejoin to form a single, uninterrupted flow front, causing the weld lines to disappear in the process of plastic injection molding.
Why Are Weld Lines a Concern?
Weld lines are more than just cosmetic plastic flaws; they are often barely visible. When there is a separation in the plastic—whether that gap is filled with air or a plastic additive—it creates a weak point. This is because the polymers do not effectively meet and bond in that area, leaving a gap that compromises the material’s strength.
Causes of Injection Molding Weld Lines
In plastic injection molding, raw material is fed into the injection molding press and pushed toward the mold by the injection unit. The molten plastic then cools and solidifies into the desired shape.
When a part has features like holes or notches, the plastic flow can split into two separate streams, or “flow fronts,” which flow around the feature and eventually meet.
When these flows join, they form a weld line, which may appear as a faint mark on the part. This occurs due to insufficient plastic fusion, often because of cooling or because the molecules are not aligned in the same direction.
Weld lines may occur due to the cooling and solidification of the streams or misalignment of the plastic molecules. Some common causes of weld lines include:
Temperature
If the temperature is insufficient, the molten plastic can solidify prematurely. Certain areas may solidify more quickly than others, ultimately leading to the formation of weld lines.
Additionally, the temperature necessary for a continuous flow of resin may decrease in different locations, such as within the mold, the runners leading to the mold, and the injection molding machine itself.
Pressure
If the pressure is insufficient to push the flow together and create a proper bond, it can lead to the formation of a broad weld line. This issue may arise due to a faulty machine or improper settings. Additionally, problems with the mold design can also contribute to this occurrence.
Mold design
Mold designs with flaws can lead to weld lines in various parts of your design. Common errors in mold design that may cause knit lines include improper wall thickness and mispositioned gates.
Impurity
If the resin has impurities, the flow through the mold will be uneven. This can cause one part of the flow to be faster than another.
Excess mold release
If there is excessive mold release, you may need to use a higher pressure to push it through the machine. Otherwise, the speed will decrease, potentially creating weld lines.
Speed
One key factor that can lead to the formation of weld lines in your finished plastic part is a low injection molding speed. When the speed is slow, the molten plastic moves through the mold at a reduced rate, causing the fronts to cool unevenly. If one front cools before the molten plastic reaches the other front, it can result in the development of weld lines in the final product.
How to Avoid Weld Lines in Plastic Injection Molding
There are several strategies for minimizing weld lines in plastic injection molding or repositioning them for better aesthetics or strength. However, addressing a weld line issue can sometimes lead to other, more significant problems. It is essential to consult with your plastic injection molding contract manufacturer before implementing any of these modifications to tackle weld lines.
Increasing the Temperature
If the temperature of the plastic isn’t sufficiently high, it can solidify too quickly. Even if the plastic is still flowing, if it’s too cool, it will not combine properly with the flow of other plastic. Maintaining the plastic at the optimal temperature during the injection and forming processes is crucial to preventing weld lines.
Changing the Product Wall Thicknesses
The thickness of the wall directly affects the time required to fill the mold. If the fill time is shorter or longer than expected, the flow fronts may meet at different points. This alteration can change where the weld lines form.
Reducing the Size of the Runner System
This allows the manufacturer to raise the molten plastic’s temperature at the front of each flow, preventing the formation of weld lines.
Changing the Raw Material
Switching to a plastic with lower viscosity or melting point can enhance flow properties and help reduce or relocate weld lines.
Increasing Injection Speed
Increasing the speed of material flow will fill the mold more rapidly and help prevent the material from cooling down too soon, which can avoid the formation of weld lines. Moreover, by adjusting the flow speed, the points where the flow fronts meet may change, potentially shifting the location of the weld line.
Changing the Gate Position
Molten plastic is injected into each cavity of the mold through an opening known as a ‘gate.’ The positioning of the gate can influence the location of weld lines on the finished plastic part. Therefore, it is important to position the gates in the mold carefully, ensuring that the weld lines are placed in less visible areas.
Conclusion
Weld lines on finished plastic parts are a manufacturing defect that should be avoided at all costs. In addition to creating unsightly scars on the product’s surface, they can also lead to functional issues, such as increased fragility. However, what if you could transform this challenge into an opportunity for excellence? Contact us for top-notch injection molding services.
