Weld lines are weak points of a component. In plastics technology, we know static and dynamic weld lines. Static weld lines are formed, for example, during the welding of thermoplastic molded parts. Dynamic weld lines, on the other hand, are formed during injection molding by the confluence of mass flows (behind cavities, due to changing wall thicknesses or gates of the mold). Here, flow fronts collide and weld. The lower the temperature and pressure, the lower the strength of the weld line. If complete welding of the material fronts is not possible because the melt has already cooled down considerably, the weld line will show a groove or indentation.
What problems does a weld line cause?
If tensile stresses are present at the positioning of the weld line during subsequent application, this will result in excessive stress and possibly breakage. In addition, weld lines can produce undesirable color defects and dull halos. The mere presence of weld lines is also problematic when it comes to visible components, such as the high-class cockpit in a passenger car, the control button on a washing machine or that of a dishwasher. For reasons of strength, which can minimize the stability of the component by up to 50% depending on the material, and surface quality, the design of plastic components should be such that weld lines are avoided.
Early detection of weld lines
Special simulation software for injection-molded components can be used to identify and prevent weak points, such as filling problems, free-steel formation or weld lines, in the planned mold as early as the development and design phase.