What should I do if there is fusion line during injection molding?

Welding wire or braided wire may be the most common and difficult injection defect to eliminate. They occur when the melt flow front collides in the cavity. Poor braided wire will not only cause appearance defects, but also greatly weaken the structural integrity of parts. The strength of braided wire may be only 20% of the nominal strength of the part, or 100% of the nominal strength of the part, depending on many variables. What should I do if there is fusion line during injection molding? These methods to help
The origin of weak welding connection is material selection, part design, tools and processing. For welding lines, some materials are more or less "tolerant". The part design is very important because the non-uniform wall thickness will change the shear force and flow velocity at the front of the melt, resulting in the flow path splitting. The processing effect includes multiple gates entering the mold cavity and projections (such as bosses and ribs) and holes or depressions in the mold, all of which will interrupt the melt flow and divide the melt flow into independent leading edges. The temperature change in an area of the mold surface may also produce an uneven flow front.
The material properties will affect the entanglement of the melt front. When the polymer chain is only partially entangled on the fusion line, it will lead to weakening. Amorphous resin usually has better fusion line strength than semi-crystalline resin, while resin with higher flow rate can achieve better accumulation, thus forming a stronger fusion line. Adding glass fiber will also reduce the strength of the fusion line.
Sometimes, the volatiles emitted by the resin during processing will reduce the strength of the welding line. Unless properly discharged, the gas will separate the front of the airflow. In general, the flow mode of plastic entering the mold cavity is the most important for the strength of the welding line. Minimize flow interruption and carefully place them so that the flow front can meet and flow for a distance to merge correctly, which is the key to optimize the performance of parts.
The first rule is to place the gate so that the weld position is not in the part area that will bear high stress during use. Change the gate position to move the weld to a non-stressed area. If the part has multiple gates, try blocking some gates to reduce the number of potential flow fronts (but get permission first!). Or, try adding an overflow tab to promote air evacuation and molecular chain entanglement. What should I do if there is fusion line during injection molding? These methods to help
The welding line is optimized by selecting the gate position, which allows the polymer to continue flowing and merging after recombining at the flow front. Another key rule of part design is uniform nominal wall to provide consistent flow front and prevent flow change during filling. The type of resin and its shrinkage are of great significance here. The maximum wall thickness change of amorphous or low shrinkage resin is 25%, while the maximum nominal wall thickness change of semi-crystalline or high shrinkage resin is limited to 15%.
The performance of the welding line is optimized by selecting the gate position, which will allow the polymer to continue to flow and fuse after recombining at the flow front. It is also important to properly empty the area at and near the weld. It may be worthwhile to add a deflector, which can better weave the flow gap, and can also be used as a vent for trapped air when the flow front converges. The traffic tab must be cut off, which requires a second operation. Boss, rib, etc. shall be along the flow direction to facilitate filling and exhaust. Other methods to reduce or exhaust residual air are to use perforated steel inserts or ventilated core pins to improve ventilation. Vacuum exhaust is another method.
If you suspect that the mold has hot spots, please leave the mold idle until the temperature is uniform. Compare the first shot with the subsequent material shots. If the flow path is different, the reason is temperature and cooling problems related to tool steel. Check the mold for hot spots and try to achieve uniform cooling. Make sure that the two halves are at the same temperature.
Processing will affect the strength and appearance of the welding line, but it cannot eliminate the root cause in the design of materials or parts or tools. The low pressure at the flow front will not promote molecular chain entanglement, resulting in poor impact strength. The part may not be completely packed, and if the welding line is in the last area to be filled, you may not see too much packing pressure.
The trapped air (or volatile matter) will prevent the good weaving of the confluence front. Core pins, "blind holes" and special mold features can cause air retention. Injection may also be the cause of the uneven melt flow and weak fusion at the flow front.
It usually helps to improve the injection speed, reduce the filling time and increase the shear rate. The cold current front is not generally believed to be the culprit. The temperature of the flow front has little effect on the molecular chains that cross the boundary of the flow front and entangle with the chains in the converging flow. Although many processors like to increase the melt temperature to improve the fluidity and the strength of the fusion line, the volatiles escaping from the polymer are easy to reduce the strength of the fusion line. Adhering to this strategy is a last resort.
On the contrary, it usually helps to improve the injection speed, reduce the filling time and increase the shear rate, which can reduce the viscosity of the polymer during the filling process, thus achieving better chain entanglement and better stacking. It is also helpful to increase the holding pressure or pressure, and the holding time is longer. Increasing the holding pressure or the holding pressure can help eliminate the low pressure of the welding line. Another strategy to promote more chain entanglement at the weld is to increase the mold temperature by 10 ° C (20 ° F).