In injection production process, products often have various quality defects, such as cracking, warping deformation, poor size, shrink marks, weld lines, color difference, flash, insufficient strength, gas traps and black spots. For a long time, designers have lacked scientific system analysis for causes of these problems, resulting in long blind adjustments and large waste of raw materials, but they cannot solve these problems, affecting quality and delivery time of products. In severe cases, it will cause large-volume returns and reduce competitiveness of enterprise.
To solve defects of product molding, it is not enough to rely on past experience, root cause of defect needs to be found. Now injection molding principle and process are briefly explained first, then causes of weld line of injection products are analyzed. Defects are solved by adjusting injection moulding process parameters and mold design optimization, and actual case is combined with simulation analysis.
Figure 1 Complete injection molding cycle
Dried granular resin is sent to barrel equipped with heating device through hopper of injection molding machine to be heated and melted. Under push of screw, melt is injected into injection mold cavity by high speed and high pressure, melt is gradually solidified and shaped after a set time of pressure-holding cooling; mold is opened and molded products is extruded to complete a molding cycle. A complete injection molding cycle is shown in Figure 1.
Weld line is commonly referred to as a bond line, which means that when two or more streams of molten metal meet when molten plastic is filled into injection mold cavity, front portion of flowing melt has been cooled, so that it does not fuse well at the confluence to cause defects, as shown in picture 2.
Weld line on the part is like a scratch or a groove, especially at deep or transparent locations. Weld mark affects appearance quality and structural strength of product, and is liable to cause product to break.
When injecting a cavity, main conditions for molten plastic to merge and form a weld line are:
Difference in product structure and different molding conditions in production process, mold making, form of weld lines are also different.
Figure 3 Stream front angle