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PVC Pipe Fittings Common Problems And Solutions
- Nov 04, 2018 -

1. Injecting pipe fittings often have molds that cannot be filled during processing. When the injection molding machine is just starting to run, because the mold temperature is too low, the heat loss of the PVC molten material is large, and it is easy to cause solidification. The resistance of the cavity is large, and the material cannot fill the cavity. This phenomenon is normal, but it will disappear automatically after the temporary continuous injection of the digital model.


2. The cause of the sinking problem has something in common with the cause of insufficient injection, specifically:

A: The set injection pressure is low, and the injection pressure can be appropriately increased.

B: The set dwell time is not enough. The dwell time can be increased appropriately, and it is valid only before the gate freezes. Reducing the depth of the sink at the near gate requires more time.

C: The cooling time is set to be insufficient, and the cooling time can be appropriately increased.

D: The amount of the sol is insufficient, and the amount of the sol is appropriately increased.

E: The mold water is not uniform, the cooling circuit can be adjusted to make the mold parts cool evenly, the shrinkage of the far gate increases the mold temperature, and the shrinkage of the near gate reduces the mold temperature.

F: The mold casting system has a small structural size, which can increase the gate or increase the size of the main, minute and flow passage sections. The position of the gate should be close to where the shrinkage is likely to occur.


3. The difference in surface gloss of PVC injection molded products is largely related to the fluidity of PVC materials. Therefore, improving the fluidity of materials is an important measure to improve products.


4. The back pressure should be controlled at about 1Mpa. Excessive high back pressure will produce excessive shear force, which will cause the PVC to overheat and decompose. At the same time, multi-stage back pressure control should be adopted in the melting process. At the beginning, the effective length of the screw is long, and the back pressure should be low. As the effective length of the screw decreases, the back pressure should be appropriately increased to compensate for the heat loss. A short period before the return of the material, the back pressure should be reduced to prevent leakage. Excessive back pressure can cause the following:

A. The melt pressure at the front end of the barrel is too high, the material temperature is high, and the viscosity is lowered, which will reduce the plasticizing efficiency;

B. The screw is slow backwards, and the pre-plasticizing time is long, which will increase the cycle time, resulting in a decrease in production efficiency;

C. The nozzle leaks, which wastes the raw materials and causes the heating ring near the nozzle to burn out;

D. The mechanical wear of the pre-plasticizing mechanism and the screw barrel is increased.


The following problems occur when the back pressure is too low:

A. The screw retreats too fast, the density of the melt flowing into the front end of the barrel is small, and the air is trapped;

B. It will lead to poor plasticizing quality, unstable injection rubber, product weight and product size change;

C. The surface of the product may have undesirable phenomena such as shrinkage, air flower, cold streaks, uneven gloss;

D. The inside of the product is prone to bubbles, and the periphery of the product is easy to go through the glue.


5. The screw speed should vary depending on the diameter. Generally, the diameter is <60mm, the rotation speed is 50-70rpm; the diameter is >70mm, the rotation speed is 20-50rpm, to prevent the excessive shear force and PVC degradation.


6. In order to inject plastic into the cavity, each speed is uniform, and the general injection speed is controlled in multiple stages. The principle is slow start injection. As the forming area increases, the injection rate must be increased to avoid cracking and shrinkage. Until the mark, can not be injected too fast to prevent the surface of the product from being eroded.


7. The temperature of the barrel during the injection should be controlled between 170-190 °C. When the temperature of the barrel reaches the set temperature, the blower should be started to cool down. When the temperature is to be raised, the air blower should be stopped to prevent the PVC from overheating.


8. The phenomenon common in injection molded PVC pipe fittings during layer peeling is especially prone to thick products. This phenomenon is mostly caused by poor injection molding fluidity of the material. Therefore, only by using a reasonable formula to improve material flow properties, reduce material viscosity, reduce plasticizing temperature and gel time, can prevent delamination. The internal and external lubrication in the formulation should be relatively balanced, and the amount of filler and lubricant should not be too much. In addition, low process temperatures are also one of the reasons for product delamination. An increase in temperature, especially the nozzle temperature, is beneficial to improve this phenomenon.


9. Silver-silver or silver-colored on PVC-U injection molded products, which is also a common phenomenon in the production process, especially the processing of dark thick-walled pipe fittings, which is more prone to such problems. It is generally believed that this phenomenon is caused by moisture in the raw material. But there are actually other reasons that can cause this to happen. The holding time setting is too short; the setting time is too short; the molten material is not continuous in the cavity; the plastic temperature is too high or the back pressure is too high; improper plastic ingredients or uneven plastic particles may appear silver or silver Grain. Solution:

A. Strict plastic proportion formula, the mixture should be uniform in thickness;

B. Dry before plasticizing to avoid pollution;

C. Dry the mold surface moisture and dirt, and use the release agent reasonably;

D. Increase the mold temperature, increase the pressure and speed of the injection, extend the cooling time and the injection cycle;

E. Starting from the nozzle, reducing the temperature of the melt cylinder or reducing the screw speed, so that the back pressure of the screw is reduced;


10. Red or blue markings are prone to occur when injection molding white tube parts. The appearance of this phenomenon is also related to the gating system in the mold. The gate is a very important part of the mold. Since the gate is the gateway to the mold from the melt, the flow resistance is the highest and the temperature is highest. It is easy to cause discoloration on the surface of the product. Therefore, it must be considered in the gate design to reduce the resistance and temperature variation of the flow. This is why it should be followed in the casting design, the gate size should be large and should not be small, the gate flow path should be short and not suitable, and the thickness should not be thin. There is also a design industry for mold cold wells. In addition, the melt temperature is too high, the injection speed is too fast, and the mold is poorly ventilated, etc., which may also cause discoloration of the surface of the product, which can be solved by adjusting the process temperature and parameters.


11. The difficulty of demoulding has the reason of the mold, and there are also factors of improper process, but in most cases, the mold release mechanism is improperly caused. In the demolding mechanism, there is a hooking mechanism, which is responsible for hooking out the cold material at the main, branch and gate: the ejecting mechanism, using the ejector rod or the top plate to eject the product from the moving mold. If the draft angle is not enough, it will cause difficulty in demolding. There must be sufficient pneumatic pressure when pneumatically ejecting and demolding. Otherwise, it will also cause difficulty in demoulding. In addition, the core pulling device of the parting surface, the thread core removing device and the like are all important components in the demolding structure, and improper design may cause difficulty in demoulding. Therefore, in the mold design, the demolding mechanism is also a part that must be taken seriously. In terms of process control, over-temperature is too high, too much feed, excessive injection pressure, and too long cooling time can cause mold release difficulties. Mold:

A. The design has an unreasonable undercut, the demoulding structure is unreasonable or the position is improper, and needs to be corrected.

B. The mold is not enough in the mold, modify the mold and increase the angle.

C. The surface of the mold is scratched and scratched or the surface of the cavity is rough and needs to be polished.

D. The highly polished mold surface makes it difficult for the injection molded part to be molded under vacuum negative pressure.

E. The ejector mechanism is not appropriate and needs to be revised.


    It can be seen that PVC pipe fittings will produce various problems during the processing, but all the problems are caused by equipment, process, mold, formula, etc. As long as there are perfect equipment and molds, reasonable formula and Process can avoid problems. In the actual production process, such problems are often generated, or problems are generated and the causes and solutions are not known. Therefore, rich operating experience is also a condition for ensuring perfect products.