Precise statements about the strength of a weld line are difficult or impossible to make. New simulation models should make it possible to predict the weld line strength.
Basically, weld lines are considered to be areas of insufficient strength on the molded part. They occur where the melt flow in an injection mold splits into several streams at an obstacle and then flows together again. The weld line (weld line, flow line), which is visible as a line, forms at the flow fronts that meet. As soon as an injection mold has inserts for holes or similar, the weld line risk increases. If the melt cools too quickly at the mold wall of the mold, the merging mass flow will no longer bond optimally. Small notches are formed, which represent a weak point in terms of strength, and this can be as much as half the basic strength! Mechanical stresses at these points can cause premature failure. A fracture usually runs along the weld line. In order to prevent failure of the molded part during product development, predictions of the weld line strength are required.
Tensile tests and simulation model
Simulation tools for plastic injection molded parts are already very good at representing the positioning and dimensions of a weld line. Determining the weld line strength is only possible in the abstract because of the numerous factors involved. Tensile tests are used here. With these tests, the strength of the weld line under tensile stress can be measured and evaluated mechanically in several runs. The result is an approximation of the real strength of the weld line.
Avoiding the weld line
The occurrence of weld lines is often unavoidable in complex molded parts. Especially if the part does not allow much room for adjustments. In the development phase, mold designers can resort to common practices to avoid the weld line. However, these invoke other disadvantages in return, such as longer cycle times or increased energy consumption. The use of dynamic high-speed temperature control systems in the injection mold, which allow thermal energy management at specific points and close to the contour, makes it possible to avoid weld lines. And this can be done without lengthening the entire process and also without increasing the energy used. In terms of sustainability, this is an important point. Quality defects due to a lack of weld line strength can thus be ruled out in advance.
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