How to Solve Weld Lines in Injection Molding
Weld lines are a common defect in injection molding that can affect the surface quality and performance of a product.
Causes of Weld Lines
Low Melt Temperature: When the melt temperature is too low, the material has poor flowability, making it difficult for multiple streams of molten plastic to fully merge in the cavity, resulting in weld lines.
Insufficient Injection Pressure: Insufficient injection pressure prevents the melt from filling the cavity, causing multiple streams of melt to converge and form weld lines.
Slow Injection Speed: A slow injection speed allows the melt to dwell in the cavity for too long, increasing the likelihood of cooling and solidification, leading to weld lines.
Poor Gate Location: Poor gate location results in uneven melt flow, making it easy to form weld lines at the branch points.
Low Mold Temperature: A low mold temperature causes the melt to cool too quickly in the cavity, leading to weld lines.
Mold Design Defects: Mold design defects such as blocked vents or cavities with depressions can cause weld lines.
Material Issues: Poor flowability or impurities in the material can increase the likelihood of weld lines.
Solutions to Weld Lines
Increase Melt Temperature: Increase the melt temperature, within the limits of material degradation, to improve melt flowability.
Increase Injection Pressure: Increase injection pressure to improve the filling speed of the melt in the cavity, reducing the formation of weld lines.
Increase Injection Speed: Increase injection speed to shorten the residence time of the melt in the cavity, reducing the chance of cooling and solidification.
Optimize Gate Design: Adjust the gate location and size to ensure even melt flow in the cavity, reducing the formation of weld lines.
Increase Mold Temperature: Increase the mold temperature slightly to extend the cooling time of the melt and improve flowability.
Improve Mold Design: Inspect the mold design to ensure that vents are clear and the cavity surface is smooth.
Change Material: If the weld lines are caused by material issues, try using a material with better flowability.
Use Multi-Stage Injection: Multi-stage injection can improve melt flowability and reduce the formation of weld lines.
Key terms and explanations:
Weld line: A visible line or seam in a molded part where two or more streams of molten plastic have merged.
Melt Molten plastic material.
Cavity: The hollow space within a mold where the plastic part takes shape.
Gate: The opening in a mold through which the molten plastic enters the cavity.
Vent: A small hole in a mold that allows air to escape as the mold is filled.
