The double-gated tensile bars used in these tests are relatively thick. Weld lines in parts with thin walls, long flow lengths and/or difficult-to-fill geometries may be significantly weaker. In unfilled PC and PC blends, it is safer to limit applied stress at weld lines to no more than 75% of the published yield strength, less if the weld line area is particularly difficult to fill. For glass-filled versions of these materials, the allowable limit for weld lines drops to 50% of the published tensile strength-at-break values. These guidelines are for briefly applied static loads. Guidelines for long-term loads call for a maximum applied strain of about 0.5% for weld lines in most unfilled PC and PC blends. The actual stress or strain limits for a specific application need to be verified via thorough finished part testing.
Additional factors may affect weld lines in thin or difficult geometries. Some geometries may require a very fast filling speed to maintain a hot flow front and good bonding at the weld line. Undersized gates can limit the filling speed and reduce weld line strength. Inadequate venting at the weld line can trap air and impede bonding at the interface. Vents should be provided at critical weld lines, particularly if they are at the end of fill.
Gate placement and design are also important. When possible, position gates to avoid weld lines in areas subjected to impact or long-term loading, or in areas where a weld line would be cosmetically undesirable. Parts with center holes or openings, such as filter bowls, can be filled via a continuous diaphragm gate in the hole to avoid weld lines between gates.
Sequential valve gating can reduce or eliminate weld lines between gates in multi-gate applications. Hot runners for sequential valve gating are equipped with valves that can open and close the gates independently. Filling typically begins with one gate. Additional gates then open in sequence behind the advancing flow front, so weld lines are avoided. This process is most suited to parts with gates at intervals along their length.
Done properly, molding simulation can predict weld line locations, weld line severity, and the flow front temperature at the weld line. It can predict if flow stops at the weld line or if flow continues in layers beneath the surface, thereby enhancing weld line strength. Simulation can also indicate if the gate is large enough to satisfy the filling speed needs without causing shear-related defects.
Rapid heat cycle molding (RHCM) is a relatively new processing technique in which selected areas of the cavity surface are rapidly heated for filling using hot oil, steam or heaters, and then rapidly cooled with water or cooler oil to solidify the part. This technique has been used in items such as TV bezels or automotive trim to enhance surface replication and to minimize or eliminate the appearance of weld lines. While RHCM has proven it can improve cosmetics, it is still unclear how much this surface-modifying process enhances weld line strength.
Weld lines are a normal part of injection molding and will appear in nearly any part with holes, openings, or multiple gates. While seldom a problem, they can be a source of weakness, particularly in parts subjected to impact or long-term stress, and their appearance can be objectionable in certain applications. It is therefore important to understand the techniques to optimize weld line appearance and performance as well as the ways to account for weld lines in the part design.
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