Common problems and solutions for PVC plumbing?

- Jul 04, 2019-

With the development of construction industry in our country, the market demand for hard PVC pipes and their accessories is increasing day by day. Many township enterprises have launched hard PVC injection molding production lines. However, due to weak technical power, the quality of products produced is poor, and the waste product rate is high, which seriously affects the efficiency of enterprises. To this end, this paper puts forward the common problems and solutions of hard PVC pipe parts for readers 'reference.
First, the correct choice of injection molding machine
It's also important to choose the right injection molding machine.
1. Select screw type injection molding machine, can not choose plunger type injection molding machine;
2. Prevention of corrosion and aging of equipment;
3. In contact with PVC plastics, the plastic melt can not produce paste residue;
4. The indicators of the equipment temperature control system must be correct;
5. The length ratio of the screw rod is slightly smaller, and the head of the screw has a pointed shape;
The screw compression ratio is 2 to 2.5, and the three-section length can be set to 40 %, 40 %, and 20 % respectively;
6. Universal nozzles with aperture, equipped with heating control temperature equipment;
7. Need to master cleaning cartridge technology;
8. Observe the temperature rise at any time. When the tube warms up, notice the temperature rise at any time.
9. When stopping, all the materials in the cartridge should be discharged first, and the cartridge should be cleaned in a timely manner with plastics such as PS or PE, or the original RPVC of the cartridge should be basically overcleared before stopping.
10. Oil and prevent rust at the mold cavity and the surface of the flow channel immediately after shutdown.
Frequently Asked Problems and Solutions for PVC Pipe Injection
1. Injection pipe parts in the processing process often have mold can not fill the phenomenon. When the injection molding machine has just started operation, due to the low temperature of the mold, the heat loss of the PVC molten material is large, it is easy to produce solidification, the resistance of the mold cavity is large, and the material can not be filled with the cavity. This phenomenon is normal, but it is only a temporary continuous injection of several modules will automatically disappear.
2. The causes of the indentation problem and the reasons for the insufficient injection have common features. Specifically:
1: The injection pressure is set to be low, which can appropriately increase the injection pressure.
2: Set the holding time is not enough, can increase the holding time appropriately.
3: Set the cooling time is not enough, can increase the cooling time appropriately.
4: Not enough SOLS, appropriately increase SOLS.
5: The mold transport water is not uniform, can adjust the cooling loop, so that all parts of the mold cooling evenly.
6: The mold pouring system has a small structural size, which can increase the size of the gate or increase the section of the main, division and flow channel.
3. The difference in surface gloss of PVC injection molding products is largely related to the fluidity of PVC materials. Therefore, improving material mobility is an important measure to improve the product. The specific manifestations are:
1: Due to the low temperature of the molten material and the resulting poor fluidity of the material, the heating temperature of the material can be appropriately increased, especially the temperature at the nozzle.
2: The formula is unreasonable, so that the plasticization of the material can not be in place or there are too many fillers. The formula should be adjusted to improve the plasticization quality and fluidity of the material through the reasonable cooperation of the processing Auxiliaries, and control the amount of fillers.
3: Die cooling is not enough to improve the die cooling effect.
4: The size of the gate is small or the section of the runner is small, and the resistance is too large. It can appropriately increase the section of the runner, increase the gate, and reduce the resistance.
5: The content of moisture or other volatiles in the raw material is high, and the raw material can be fully dried, or water or volatiles can be eliminated by the raw material.
6: Bad exhaust, can add exhaust tank or change gate position.
4. Backpressure should be controlled at about 1Mpa. Excessive backpressure will produce excessive shear force, which will cause PVC to overheat and decompose. At the same time, multi-stage back pressure control should be used in the melt process. At the beginning, the effective length of the screw is long, and the back pressure should be low. With the reduction of the effective length of the screw, the back pressure should be appropriately increased to compensate for the decrease in heat. Back to the end of a small section, and should reduce back pressure, prevent salivation.
5. The speed of the screw should vary depending on the diameter. General diameter <UNK> 60mm, then the speed is 50-70rpm; With a diameter of > 70mm, the speed is 20-50rpm to prevent PVC degradation due to excessive shear force.
6. In order for the plastic to be injected into the cavity at a uniform speed, the injection speed should generally be controlled by multiple sections. The principle is that the slow start injection must increase the injection rate with the gradual increase of the forming area, so as to have no cracks and shrinkage marks. Do not inject too quickly to prevent the surface of the product from being eroded.
7. The temperature of the barrel during injection should be controlled between 170-190 °C. When the temperature of the cartridge reaches a set temperature, the blower should be started to cool down. When it needs to be heated, the blowing and heating should be stopped to prevent PVC from overheating.
8. The common phenomenon in plastic PVC pipes during stratified stripping is particularly likely to appear in thick products. This phenomenon is mostly due to the poor fluidity of injection molding materials. Therefore, only adopting reasonable formula to improve material flow performance, reduce material viscosity, reduce plastic temperature and gel time, can prevent the generation of layered stripping. The relative balance of internal and external lubrication should be achieved in the formula, and the amount of filling agent and lubricant should not be excessive. In addition, low process temperature is also one of the reasons for the stratification of products. Suitable to increase the temperature, especially the nozzle temperature is conducive to improving this phenomenon.
9. Silver or silver lines appear on PVC-U injection molding products. This is also a common phenomenon in the production process. In particular, processing dark thick-walled pipes is more likely to occur. It is generally believed that this phenomenon is due to the formation of moisture in the raw materials. But in fact there are other reasons for this to happen.
10. Red or blue markings are likely to appear during injection molding and processing of white pipes. The appearance of this phenomenon is also related to the pouring system in the mold. The gate is a very important part of the mold. Since the gate is the portal for the melt to enter the mold, the material flow resistance is the largest and the temperature is the highest. It is easy to produce the discoloration of the surface of the product. Therefore, it is necessary to take into account the resistance and temperature changes that can reduce the feed flow in the gate design. This is why the pouring design should be followed, the size of the gate should not be small, the gate flow channel should be short should not be long, should not be thick thin principle. There is also the die cold well design industry is crucial. In addition, the melting temperature is too high, the injection molding speed is too fast and the mold exhaust is poor, etc. will also produce the surface discoloration of the product, which can be solved by adjusting the process temperature and parameters.
11. There are reasons for the difficulty of demoulding and there are also factors that are improper in the process, but in most cases it is due to the improper demoulding mechanism of the die. In the demolding mechanism, there is a hook material mechanism. It is responsible for hooks out the cold materials at the main, shunt and gate: the top out mechanism, using the top out bar or roof to top the product out of the moving mold. If the demodulation slope is not enough, demodulation difficulties will occur. There must be enough pneumatic pressure when the pneumatic top is demolded. Otherwise, it will also produce a difficult phenomenon of demodulation. In addition, the core extraction device and the thread core deactivation device on the sub-surface are all important components in the demolding structure, and improper design will cause the demolding phenomenon. Therefore, in the mold design, demoulding mechanism is also a must pay attention to the part. In terms of process control, high temperature, too much feed, high injection pressure, and long cooling time will cause difficulties in demoulding.
It can be seen from this that PVC pipe injection molding will produce various problems during the processing process, but all the problems are caused by equipment, processes, molds, formulas, etc.. As long as there are perfect equipment and molds, reasonable formulas and processes, You can avoid problems. In the actual production process, such problems will often arise, or problems will not know the reasons and solutions, so rich operating experience is also a condition for ensuring product perfection.
Finally, the following requirements should be noted in the preparation of raw materials:
1. It is necessary to avoid the deterioration of PVC raw materials in other plastic PVC raw materials. In particular, when molding other raw materials, must first clean the drum and screw plastic, in order to inject PVC products. (Clean screw and barrel with PE \ PP \ PS material).
2. Make sure to completely clean the PVC material before stopping, such as stopping for a long time. In order to prevent PVC from corroding the machine, clean the screw and barrel with cleaning material.
3. When PVC material is placed for more than 6 months and the environment is high temperature and humidity, 60-80 °C temperature should be used before use.