The main causes of weld lines are: when the molten plastic encounters inserts, holes, areas with discontinuous flow velocity or areas with interrupted filling flow in the mold cavity, the confluence of multiple melts; When the gate injection mold filling occurs, the materials cannot be completely fused. For example, electrical appliance shell, rice cooker shell, sandwich machine plastic shell, plastic shoe rack, automobile OEM front bumper, etc. Next, we will share the specific causes and corresponding solutions of weld lines.
1. The temperature is too low
The low-temperature melt has poor shunting and confluence performance and is easy to form weld lines. 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 the 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.
2. Mold defects
The structural parameters of the mold pouring system have a great influence on the fusion condition of the molten material, because the poor fusion is mainly caused by the diversion and confluence of the molten material. Therefore, the gate form with less diversion should be adopted as far as possible and the gate position should be reasonably selected to avoid inconsistent mold filling rate and interruption of mold filling material flow. If possible, one point gate should be selected, because this gate does not produce multiple streams of material, and the molten material will not converge from two directions, which is easy to avoid weld marks.
3. Poor mold exhaust
After this kind of fault occurs, first of all, check whether the exhaust hole of the mold is blocked by the solidified product of the molten material or other objects, and whether there is foreign matter at the gate. If the carbonation point still appears after the blockage is removed, an exhaust hole should be added at the die collecting point. It can also be accelerated by repositioning the gate or appropriately reducing the closing force and increasing the exhaust gap. In terms of process operation, auxiliary measures such as reducing material temperature and mold temperature, shortening high-pressure injection time and reducing injection pressure can also be taken.
4. Improper use of release agent
Too much mold release agent or incorrect variety will cause weld marks on the surface of plastic parts. In injection molding, a small amount of release agent is generally applied evenly only on the parts that are not easy to demould, such as threads（injection plastic custom PA6 nut）. In principle, the amount of release agent should be minimized. The selection of various release agents must be determined according to the molding conditions, the shape of plastic parts and the variety of raw materials.
5. Unreasonable plastic structure design
If the wall thickness of plastic parts is designed too thin, there may be great differences in thickness and too many inserts, which will cause poor fusion. Therefore, when designing the shape structure of plastic parts, it should be ensured that the thinnest part of plastic parts must be greater than the minimum wall thickness allowed during molding. In addition, the use of inserts should be minimized and the wall thickness should be as uniform as possible.
Post time: Jul-19-2022