In the process of using Pvc granules for injection molding, many people have encountered product black spots, black lines, and even product yellowing. So what is the reason? Please see below.
PVC is a heat-sensitive material, and its injection molding processability is poor. The reason is that too high melting temperature or too long heating time can easily decompose PVC. Therefore, controlling the melting temperature is the key to injection molding PVC products. The heat source for melting PVC raw materials comes from two aspects: the shear heat generated by the screw movement to the plastic and the resistance wire heating on the outer wall of the barrel, and the shear heat of the screw movement is the main source. The outer heating of the barrel is mainly the heat source when the machine starts. Due to the poor thermal conductivity of PVC, during the heating process, although the plastic inside and outside the barrel has been heated, the intermediate material is still in a condensed state, and even the outer part has been overheated and decomposed, and there may be unmelted parts inside (therefore PVC injection molding often The appearance of black streaks and black spots is a headache).
All kinds of reasons must be fully paid attention to in the screw barrel product design, mold design, injection molding process, etc. The selection of production equipment is also very important. The following is an introduction to the points that should be paid attention to when injecting PVC, hoping to be helpful to readers.

The following points should be paid attention to in product design and mold design:
1. The product should not have sharp corners or sudden changes, and the thickness should not change much to prevent the degradation of PVC.
2. The mold should have a draft angle of 10 or more, and allow about 0.5% shrinkage.
3. Several points should be paid attention to in mold runner design
a) The injection port of the mold should be slightly larger than the nozzle hole, and larger than the diameter of the staggered part of the main runner, so that the PVC flows into the cavity without stagnation and the pressure is balanced.
b) Use intercept gates as much as possible to prevent molten slag from flowing into the product and the flow channel of the product and reduce the temperature and make it easy to form.
c) The gate should be designed at the thickest wall of the product, and the width is sufficient, the length is preferably between 6-8mm, so that the PVC material flows smoothly.
d). In order to support the pressure drop and easy demoulding, the runner should be round, and the diameter should be selected between 6-10mm according to the product size and weight.
4. The mold temperature should have a cooling water control device, so that the mold temperature can be controlled between 30 ℃ and 60 ℃.
5. The surface of the mold should be smooth and chrome-plated to prevent corrosion.

The following items should be noted in the injection molding process:
1. The back pressure should be controlled at about 1Mpa. Excessive back pressure will produce excessive shearing force and cause the PVC to overheat and decompose. At the same time, multi-stage back pressure control should be used in the melting process. At the beginning, the effective length of the screw is long and the back pressure should be lower. As the effective length of the screw decreases, the back pressure should be appropriately increased to compensate for the decrease in heat. A short period before the end of the return, the back pressure should be reduced to prevent salivation leakage.
2. The screw speed should vary depending on the diameter. Generally, if the diameter is less than 60mm, the speed is 50-70rpm; if the diameter is more than 70mm, the speed is 20-50rpm to prevent PVC degradation due to excessive shearing force.
3. In order to make the plastic injection into the inspection cavity, the speed of each place is uniform. Generally, the injection speed should be controlled in multiple stages. The principle is to start the injection slowly. With the gradual increase of the molding area, the injection rate must be increased until there are no cracks and shrinkage marks. , Do not inject too fast to prevent the product surface from becoming eroded.
4. The temperature of the barrel during the injection 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 blowing and heating should be stopped to prevent the PVC from overheating and deterioration.

Due to the special properties of PVC, special PVC injection molding machines are used for production. The following points should be noted:
1. The screw length to diameter ratio should be 20:1, and the compression ratio should be between 1:1.6-1:1.2.
2. The screw and nozzle should be specially made of PVC with chrome-plated surface.
3. The injection pressure, speed, back pressure and temperature can be controlled in multiple stages and precisely.

Finally, the following requirements should be paid attention to in the preparation of raw materials:
1. It must be avoided that other plastic PVC raw materials cause deterioration of PVC raw materials. Especially when other raw materials have been injected, the plastic in the barrel and screw must be cleaned before injection of PVC products. (Clean the screw and barrel with PS material)
2. Before stopping the machine, the Pvc granules must be completely cleaned. If the machine is stopped for a long time, to prevent the PVC from corroding the machine, use PS material to clean the screw and barrel.
3. When the Pvc granules are stored for more than 6 months and the environment is hot and humid, they should be dried at 60-80℃ for 1-2 hours before use.