What is weld lines
Weld lines are the most common defects in injection molded products. Except for a few injection molded parts with very simple geometry, weld lines occur in most injection molded parts (usually with one line or V-shaped groove), especially in large-scale complex products requiring multi gate mold and inserts.
Weld lines not only affect the appearance quality of plastic parts, but also the mechanical properties of plastic parts, such as impact strength, tensile strength, elongation at break, etc. In addition, the weld mark also has a serious impact on the product design and the life of the plastic part, so it should be avoided or improved as much as possible.
The main causes of the weld mark are: when the melt plastic meets the area of insert, hole, inconsistent flow rate or interrupted filling material flow in the mold cavity, the confluence of multiple melts; when the gate jet filling occurs, the material can not be fully integrated.
Causes and solutions of weld lines
1 temperature too low
The diffluence and confluence performance of low temperature melt is poor, and it is easy to form weld lines. If the internal and external surfaces of plastic parts have fine weld lines in the same position, it is often caused by the low material temperature. Therefore, the temperature of the barrel and nozzle can be increased properly or the injection cycle can be prolonged to promote the rise of the material temperature. At the same time, the amount of cooling water in the mold should be controlled and the mold temperature should be increased properly.
In general, the strength of the weld line of plastic parts is poor. If the corresponding parts of the mold that produce the weld line are locally heated, and the local temperature of the weld line of the molded parts is increased, the strength of the weld line of plastic parts can often be improved.
If it is necessary to adopt the low temperature molding process due to special needs, the injection speed can be increased properly, and the injection pressure can be increased, so as to improve the fusion performance of the melt. A small amount of lubricant can also be added to the raw material formula to improve the flowability of the melt.
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2 mold defects
The structural parameters of the mold pouring system have a great influence on the fusion condition of the material, because the poor fusion is mainly caused by the separation and confluence of the material. Therefore, the gate form with less diversion should be adopted as far as possible and the gate location should be selected reasonably, so as to avoid inconsistent filling rate and interruption of filling material flow as far as possible. If possible, a one-point gate should be selected, because this gate does not produce multi strand material flow, and the molten material will not converge from two directions, so it is easy to avoid weld marks.
If there are too many or too small gates in the pouring system of the mold, the location of multiple gates is not correct, or the distance between gates and the melt joint of the flow material is too large, the inlet part of the main flow channel and the cross section of the flow channel of the shunt channel of the pouring system are too small, resulting in the material flow resistance is too large, which will cause the poor fusion and make the surface of the plastic part have more obvious fusion marks. In this regard, the number of gates should be reduced as much as possible, the gate location should be set reasonably, the gate section should be increased, the auxiliary flow channel should be set, and the diameter of main flow channel and diversion channel should be expanded.
In order to prevent the weld marks produced by low temperature melt injection into the mold cavity, the cold hole should be set in the mold at the same time of increasing the mold temperature.
In addition, the part where the weld mark of the plastic part often occurs is often filled with high pressure, and the weld mark will not produce shrinkage cavity after this kind of flash is generated, so this kind of flash is often not used as troubleshooting, but a very shallow groove is opened at the part where the flash is generated on the mold, and the weld mark on the plastic part is transferred to the additional wing of the flash, and the wing is removed after the plastic part is formed This is also a common method to eliminate the weld line fault.
3 poor exhaust of mould
When the melting line of the molten material coincides with the closing line or caulking of the mold, the air driven by the multi stream materials in the mold cavity can be discharged from the closing gap or caulking; However, when the melting line does not coincide with the mold closing line or caulking line, and the vent hole is not set properly, the residual air in the mold cavity driven by the flow material cannot be discharged, the bubble is forced under high pressure, the body gradually becomes smaller, and finally compressed into a point. Because the molecular kinetic energy of the compressed air is converted into heat energy under high pressure, the temperature at the melting material collecting point increases, when the temperature, etc When the temperature is higher than or slightly higher than the decomposition temperature of the raw material, the yellow spot will appear at the melting point. If the temperature is much higher than the decomposition temperature of the raw material, the black spot will appear at the melting point.
In general, such spots near the weld mark on the surface of plastic parts always appear repeatedly at the same position, and the parts that appear always regularly appear at the collecting point. In the process of operation, such spots should not be mistaken as impurity spots. The main reason for this kind of spots is the poor exhaust of the die, which is the carbonization point formed after the high temperature decomposition of the melt.
In case of such failure, first check whether the die vent is blocked by the solidified material or other objects of the melt, and whether there is any foreign matter at the gate. If there is still carbonization point after the blockage is removed, the vent hole shall be added at the die collecting point. It can also speed up the confluence by repositioning the gate or reducing the binding force and increasing the exhaust clearance. 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 type of mold release agent will cause weld marks on the surface of plastic parts. In injection molding, a small amount of release agent should be evenly applied to the thread and other parts that are not easy to be demoulded. In principle, the amount of release agent should be minimized.
The selection of various mold release agents must be based on the molding conditions, the shape of plastic parts and the variety of raw materials
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