Weld Lines in Plastic Injection Molding: What They Are and How to Avoid Them

Reliable and durable, plastics made using China injection molds are suitable for applications involving heavy use or exposure to extreme conditions. However, certain issues can affect the quality and longevity of these products, with weld lines being a notable concern.

Weld lines, also known as knit lines, are imperfections that occur in places like edges, corners, and angles within the mold. During injection molding, thermoplastic material is melted and injected into a double-sided custom mold. As it cools and solidifies, taking the shape of the mold, weld lines can form at the points where separate flows of melted material converge and knit together. This often happens when molten plastic encounters obstructions in the mold, causing it to split and then rejoin, potentially forming air pockets or weak points if not properly fused.

The importance of addressing weld lines

While weld lines in China injection molds may appear as mere cosmetic issues, they can create weak points in the product. This is because the polymers don’t fully bond to bridge gaps, affecting the overall integrity of the item.

Strategies to avoid weld lines

Preventing weld lines is crucial as they can’t be corrected post-manufacturing. Here are some effective ways to avoid them during the injection molding process:

• Maintain adequate pressure – Ensuring the injectors provide enough pressure allows the plastic to flow forcefully and evenly. Consistent flow rates increase the likelihood of a seamless melding of the two flows, reducing the chance of weld lines.
• Optimize mold design – Poorly designed molds, especially those with overly thick walls, can impede plastic flow. A well-thought-out mold design is essential to maintain consistent flow and prevent weld lines.
• Control plastic temperature: If the plastic isn’t hot enough, it may solidify prematurely or fail to combine effectively with another flow. Maintaining the optimal temperature during injection and formation is key to minimizing weld lines.
• Use high-quality plastic – Plastics of lower quality may contain impurities that disrupt flow and increase the likelihood of weld lines forming.

Our team at HLH Prototypes has extensive experience in producing China injection molds to avoid these issues. We focus on preventing costly mistakes and ensuring the production of high-quality, resilient items. In addition, we offer design tips to ensure your mold is optimally designed for injection molding. Contact us for a comprehensive project review or to get a quote.