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When the plastic melt fills the mold cavity during injection molding, if the temperature of two or more melts is not exactly the same when they meet in the front section, these melts cannot be completely fused, and linear grooves will be formed at the joint, thus forming fusion marks.

The causes and solutions of weld lines in injection molding are as follows:

1. Melting temperature too low

The low-temperature melt has poor shunting and confluence performance, which is easy to form welding traces. When the inner and outer surfaces of plastic parts have welding fine lines at the same position, the poor welding is often caused by the low temperature of the material. In this regard, the temperature of the barrel and nozzle can be appropriately increased, or the injection cycle can be extended to promote the rise of material temperature. At the same time, the passing amount of cooling water in the mold should be controlled and the mold temperature should be appropriately increased. Under normal circumstances, the strength of the fusion mark of plastic parts is poor. If the corresponding part of the mold with fusion mark is locally heated, and the local temperature of the fusion part of the mold part increases, the strength of the fusion joint of plastic parts can usually be improved. If the low-temperature molding process is adopted due to special needs, the injection speed and pressure can be appropriately increased, the convergence performance of the melt can be improved, and a small amount of lubricant can be appropriately used in the raw material formula to improve the flow performance of the melt.

2. Unreasonable gate position

The gate with less diversion should be adopted, and the gate position should be reasonably selected to avoid inconsistent filling mold speed and interruption of filling mold material flow as far as possible. If possible, single point feeding should be selected. In order to prevent low-temperature melt from injecting into the mold cavity to produce weld marks, the mold temperature can be increased, and cold material cavities can be set in the mold at the same time.

3. Poor mold exhaust

At this time, first check whether the exhaust hole of the mold is blocked by the solidification of the melt or other objects, and whether there is foreign matter at the gate. If the carbonization point still appears after the blockage is removed, the exhaust hole should be added at the mold collecting point, or by repositioning the gate, appropriately reducing the clamping force and increasing the exhaust gap, so as to speed up the confluence of the collecting materials. In terms of injection molding process, auxiliary measures such as reducing material temperature and mold temperature, shortening high-pressure injection time and reducing injection pressure can be taken.

4. Improper use of release agent

In the process of injection molding, only a small amount of release agent is evenly applied to the threads and other parts that are not easy to be demoulded. In principle, the amount of release agent should be minimized.

5. Unreasonable structural design of plastic parts

If the wall thickness of plastic parts is designed too thin, or there are too many inserts, it will cause poor fusion of melt. When designing the shape and structure of plastic parts, it should be ensured that the thinner part of plastic parts must be greater than the smaller wall thickness allowed during molding. In addition, the use of inserts should be minimized and the wall thickness should be as consistent as possible.

6. Other reasons

For example, the content of water or volatile matter in the used plastic raw materials is too high, the oil stain in the mold is not cleaned up, the distribution of fiber filler in the cold material or melt in the mold cavity is uneven, the design of the mold cooling system is unreasonable, the melt cooling is too fast, the insert temperature is too low, the nozzle hole is too small, the plasticizing capacity of the injection machine is not enough, and the pressure loss in the plunger or injection machine barrel is too large, May lead to different degrees of poor melt convergence and welding traces. Therefore, in the production process, measures should be taken to solve the problems according to different conditions, such as drying raw materials, regularly cleaning the mold, changing the size and position of the mold cooling water channel, controlling the cooling water flow, increasing the insert temperature, replacing the nozzle with larger aperture and using the injection machine with larger specification.