Common problems and solutions of PVC pipe fittings

The correct choice of injection molding machine

It is also very important to choose the right injection molding machine

• Choose a screw-type injection molding machine, not a plunger-type injection molding machine;

• Prevent corrosion and aging of equipment;

• Where it is in contact with the PVC plastic, there should be no sticking residue on the plasticized melt;

• The indicators of the equipment temperature control system must be correct;

• The length-diameter ratio of the screw is slightly smaller, and the head of the screw should be pointed;

• The screw compression ratio is 2 to 2.5, and the length of the three sections can be set to about 40%, 40%, and 20% respectively;

• Use general-purpose nozzles with apertures, equipped with heating and temperature-controlling equipment;

• Need to master the cleaning barrel technology;

• Observe the temperature rise at any time, and pay attention to the temperature rise at any time during the heating process of the barrel.

• When shutting down, all the materials in the barrel should be drained first, and the barrel should be cleaned in time with plastics such as PS or PE, or a special PVC packing material should be added to clear the original RPVC of the barrel before shutdown.

• Immediately after shutdown, apply oil to the cavity and runner surface of the mold to prevent rust.

Common problems and solutions of injection molding of PVC pipe fittings

1. In the process of injection molding pipe fittings, there is often a phenomenon that the mold cannot be filled. When the injection molding machine just started 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 mold cavity is large, and the material cannot fill the cavity. This phenomenon is normal, but it will disappear automatically after a temporary continuous injection of the digital model.

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

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

2): The set pressure holding time is not enough, and the pressure holding time can be appropriately increased.

3): The set cooling time is not enough, and the cooling time can be appropriately increased.

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

5): The water transport of the mold is uneven, and the cooling circuit can be adjusted to make all parts of the mold cool evenly.

6): The structure size of the mold gating system is too small, and the gate or the main, sub, and runner cross-sectional dimensions can be increased.

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

• Due to the low temperature of the molten material, the fluidity of the material is poor, and the heating temperature of the material can be appropriately increased, especially the temperature at the nozzle.

• The formula is unreasonable so that the plasticization of the material cannot be in place or there are too many fillers, the formula should be adjusted, and the plasticization quality and fluidity of the material should be improved through the reasonable coordination of processing aids, and the number of fillers should be controlled.

• The cooling of the mold is not enough, and the cooling effect of the mold is improved.

• The gate size is too small or the runner section is too small, and the resistance is too large. The runner section can be appropriately increased, the gate can be enlarged, and the resistance can be reduced.

• The content of moisture or other volatiles in the raw materials is high, the raw materials can be fully dried, or the moisture or volatiles can be removed by making the raw materials.

• If the exhaust is not good, an exhaust slot can be added or the gate position can be changed.

• The back pressure should be controlled at about 1Mpa. If the back pressure is too high, it will generate excessive shearing force and cause the PVC to be decomposed by overheating. At the same time, multi-stage back pressure control should be adopted in the melting process. In 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 reduction. A short period before the return of the material is terminated, the back pressure should be reduced to prevent saliva leakage.

5. The screw speed should vary according to the diameter. Generally, if the diameter is less than 60mm, the rotation speed is 50-70rpm; if the diameter is more than 70mm, the rotation speed is 20-50rpm, so as to prevent the PVC from being degraded due to excessive shearing force.

6. In order to make the plastic injection into the inspection cavity, the speed of each place is uniform, and the injection speed should be controlled in multiple stages. Do not inject too fast to prevent the surface of the product from being eroded.

7. The temperature of the barrel during the injection process should be controlled between 170-190℃. When the temperature of the barrel reaches the set temperature, the blower should be started to cool down, and when the temperature needs to be raised, the blower should be stopped to prevent the PVC from overheating and deteriorating.

8. The phenomenon of delamination and peeling is a common phenomenon in injection molding PVC pipe fittings, especially on thick products. This phenomenon mostly occurs due to the poor injection flow of the material. Therefore, only by adopting a reasonable formula to improve the material flow performance, reduce the material viscosity, and reduce the plasticizing temperature and gel time, can the occurrence of delamination be prevented. The internal and external lubrication in the formula should be relatively balanced, and the number of fillers and lubricants should not be too much. In addition, the low process temperature is also the cause of product delamination. The temperature can be appropriately increased, especially the nozzle temperature is beneficial to improve this phenomenon.

9. Silver wires or silver streaks appear on PVC-U injection molded products, which is also a common phenomenon in the production process, especially when processing dark thick-walled pipe fittings, this problem is more likely to occur. It is generally believed that this phenomenon occurs due to the formation of moisture in the raw material. But there are actually other reasons for this to happen.

10. When injection molding white pipe fittings, red or blue markings are easy to appear. 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 for the melt to enter the mold, there is flow resistance and temperature here. It is easy to produce discoloration on the surface of the product. Therefore, resistance to flow and temperature variation must be considered in the gate design. This is why in the pouring design, the gate size should be large rather than small, the gate runner should be short rather than long, and thick rather than thin. There is also a very important mold cold silo design industry. In addition, too high a melt temperature, too fast an injection speed, and poor mold exhaust will also cause discoloration on the surface of the product, which can be solved by adjusting the process temperature and parameters.

11. Difficulty in demolding is caused by the mold and improper process, but in most cases, it is caused by the improper demoulding mechanism of the mold. Among the demolding mechanisms, there is a hook mechanism, which is responsible for hooking out the cold material at the main, runner, and gate: an ejector mechanism, which uses an ejector rod or a top plate to eject the product from the movable mold. If the demolding slope is not enough, demolding will be difficult. There must be enough pneumatic pressure when the pneumatic ejector is ejected. , otherwise, there will be difficulty in demolding. In addition, the core pulling device of the parting surface, the thread core pulling device, etc. are all important components in the demolding structure, and improper design will cause the molding difficulty. Therefore, in the mold design, the mold release mechanism is also a part that must be paid attention to. In terms of process control, too high temperature, too much material supply, too high injection pressure, too long cooling time, etc. will cause difficulty in demolding.

It can be seen from this that various problems will occur in the injection molding of PVC pipe fittings, but all the problems are caused by equipment, technology, molds, formulas, etc. As long as there are perfect equipment and molds, reasonable formulas, and processes to avoid problems. In the actual production process, such problems often occur, or the causes and solutions are unknown. Therefore, rich operating experience is also a condition for ensuring the perfect product.

The following requirements should be paid attention to in the preparation of raw materials:

• It is necessary to avoid other plastic PVC raw materials, which will cause the deterioration of PVC raw materials. Especially when other raw materials have been injected, the plastic in the barrel and screw must be cleaned before the injection of PVC products. (Clean the screw and barrel with PEPPPS material).

• The PVC material must be completely cleaned before shutdown. If the machine is shut down for a long time, to prevent PVC from corroding the machine, the screw and barrel should be cleaned with cleaning materials.

• When the PVC material has been placed for more than 6 months, and the environment is high temperature and humid, it should be dried at 60-80 ℃ for 2-5 hours before use.

Principles to follow for PVC pipe fittings lubricant dosage

A mature lubrication system, on the one hand, should help reduce the viscosity of high-temperature polyvinyl chloride melt, improve the fluidity of the material, and reduce the friction between the material and the contact surface of the equipment, to facilitate the processing of pipe fittings; on the other hand, excessive Lubricants cause material plasticization delay and low plasticization degree.

The PVC resin powder, fillers, stabilizers, and lubricants used by PVC pipe fittings and pipe manufacturers vary greatly. In addition, the specifications of the pipes and the differences between the extruder and the extrusion die also require appropriate adjustment of the lubricant. The amount of lubricant should follow the following principles:

(1) For resin powder with a high degree of polymerization, the internal lubricant should be appropriately increased;

(2) When using a composite stabilizer containing lubricating ingredients, the lubricant should be appropriately reduced;

(3) The internal and external lubrication in the formula should be balanced to avoid processing difficulties:

(4) For resin powder with poor stability, the number of stabilizers should be increased, and the lubricant should be appropriately reduced;

(5) When adding a lot of fillers, the lubricant should be appropriately increased;

(6) When the processing temperature is high, the polymer is easy to bond with the processing equipment, and the lubricant should be appropriately increased;

(7) When larger processing equipment is used, the surface to which the polymer is bonded will increase, and the lubricant should be increased;

(8) Excessive lubricant will reduce the degree of plasticization of the material and weaken the mechanical properties of the pipe fittings

How to improve the Vicat softening point of pipe fittings

Influencing factors

• Various lubricants reduce Vicat;

• Liquid additives reduce Vicat;

• CPE reduces Vicat;

• Fill to increase Vika within a certain range;

• ACR additives increase Vicat;

• The selection of suitable resin powder also has a great influence on Vicat;

PVC pipe fittings production method:

1. Add CPVC resin, the Vicat softening point of pure CPVC resin is about 128 degrees, the mixed CPVC COMPOUND, the Vicat softening point of injection molding grade is above 103 degrees, and the Vicat softening point of extrusion grade is above 110 degrees ( Added lubricants, organotin stabilizers will reduce Vicat). It is up to you to adjust how much CPVC resin to add to your CPVC to meet your requirements. The advantage is that it is cheap, and the disadvantage is that it is technically difficult.

2. Add PVDF resin. The Vicat softening point of pure PVDF resin is about 140 degrees. You can add it directly. It will not burn or decompose, and the weather resistance is also the best. The disadvantage is that the price is super expensive, and the advantage is that the technology is not difficult. You can consider looking for some secondary materials to reduce costs.