In the molding process of automotive injection molding parts, weld lines are a common and critical issue affecting performance. Weld lines are linear marks formed when two or more streams of molten plastic merge within the mold cavity, due to insufficient temperature at the weld tip or incomplete fusion. They not only affect the appearance of the part but also reduce its mechanical strength, such as impact and tensile strength, becoming a potential risk point for breakage. Therefore, avoiding weld lines is crucial for improving the performance of automotive injection molding parts.
To prevent weld lines at the source, attention must be paid to material selection and pretreatment. Plastic materials with good flowability should be selected, as these materials can better fill the mold cavity in the molten state, reducing weld lines caused by poor flow. At the same time, the raw material should be thoroughly dried to prevent moisture or volatiles from vaporizing at high temperatures, forming bubbles at the weld joint or interfering with fusion, thus affecting weld quality.
Mold design is a key aspect in avoiding weld lines. A reasonable selection of gate location and type can reduce the number of molten material splitting and merging, lowering the probability of weld lines. For example, using a single-point gate can prevent the convergence of multiple material flows, effectively preventing weld lines. Furthermore, optimizing the mold's venting system is crucial to ensure timely exhaust of gas within the mold cavity, preventing high temperatures caused by gas compression that could lead to material scorching or hinder complete melt fusion. Venting grooves should be added or enlarged at locations where weld lines are expected. For deep cavities with trapped air, venting inserts or vacuum venting technology can be considered.
Adjusting molding process parameters also significantly impacts weld line avoidance. Appropriately increasing the barrel and nozzle temperatures, as well as the mold temperature, can improve melt flowability, resulting in higher temperatures and activity during melt fusion, thus increasing weld line strength. Simultaneously, increasing injection speed and pressure allows the melt to fuse rapidly before cooling, reducing incomplete fusion due to temperature drops. Employing multi-stage injection technology, using medium speed as the melt flows through the gate, switching to high speed near the fusion zone, and finally holding at low speed, can further optimize the melt filling and fusion process.
During mold use, regular cleaning and maintenance are essential to prevent oil stains, rust, and other contaminants from affecting melt flow and fusion. Inspect and maintain injection molding equipment to ensure the plasticizing and injection systems are functioning properly, preventing melt temperature and pressure instability caused by equipment malfunctions, which can negatively impact weld quality.
For existing weld lines, their performance can be improved through subsequent treatments. For example, localized heating of the weld line area can increase the temperature, helping to enhance molecular entanglement and diffusion, thereby improving weld strength. Furthermore, surface treatment techniques such as painting, adding micro-grooves or textures can conceal weld lines and reduce their impact on appearance.
Avoiding weld lines during the molding process of automotive injection molding parts requires comprehensive consideration of multiple aspects, including material selection, mold design, process parameter adjustment, equipment maintenance, and subsequent treatments.
