Recently, some customers of the sample appeared the phenomenon of yellowing when using the company's white hard injection plastic for injection molding. After analysis by the company's technical staff, some points of attention were summarized, and I hope to help everyone.
During the injection of PVC, the temperature of the set shot barrel rises and the PVC material turns yellow. The reasons for the decomposition of the raw material PVC injection molding process PVC is a heat-sensitive material. Its injection molding processability is poor due to the excessively high melt temperature. Or too long heating time can easily decompose PVC, so controlling the melting temperature is the key to injection PVC products. The heat source for melting the PVC raw material comes from two aspects: the screw movement generates shear heat to the plastic and the resistance wire heating of the outer wall of the barrel, and mainly the shear heat of the screw movement. The external heating of the barrel is mainly to provide a heat source when the machine starts. Due to the poor thermal conductivity of PVC, although foreign plastics have been heated during the heating process, 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. All kinds of reasons must be paid full attention to in product design, mold design, injection molding process, etc., especially the PVC special injection molding machine of excellent manufacturer must be selected to make the ideal product.

The following is an introduction to the matters needing attention when injecting PVC, hoping to help you.
(A), the product design and mold design should pay attention to the following points:
1. The product should not have sharp corners or abrupt changes as much as possible, and the thickness should not change much to prevent the degradation of PVC.
2. The mold should have a draft angle of more than 10, leaving a shrinkage of about 0.5%.
3． Attention should be paid to the design of the flow path of the mold. A) The injection port of the mold should be slightly larger than the nozzle hole and larger in diameter than the intersection of the main flow channels, so that the PVC material flows into the mold cavity without stagnation and pressure is balanced. b) Try to use cutting gates so that the slag will not flow into the product and the temperature in the runner will be lowered and easy to form. C) The gate should be designed at the thickest part of the product, and the width should be sufficient, and the length should preferably be between 6-8mm to make the PVC material flow in smoothly. d) In order to reduce the feeding pressure and ease the demoulding, the flow channel is preferably circular, and it can be selected between 6-10mm according to the size and weight of the product.
4． The mold temperature should have a cooling water control device to make the mold temperature controllable between 30 ° C and 60 ° C.
5． The surface of the mold should be smooth and chrome-plated to prevent corrosion.
(B), the injection molding process should pay attention to the following matters:
1. Back pressure should be controlled at about 1Mpa. Excessive back pressure will generate 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 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 reduction in heat. A short period before the end of returning materials, the back pressure should be reduced to prevent salivation.
2. Screw speed should be different for different diameters. Generally, if the diameter is less than 60mm, the speed will be 50-70rpm; if the diameter is more than 70mm, the speed will be 20-50rpm to prevent the degradation of PVC due to excessive shear force.
3． In order to make the plastic injection into the inspection cavity, the speed is uniform at each place. Generally, the injection speed should be controlled in multiple stages. The principle is to slowly start the injection. As the molding area gradually increases, the injection rate must be increased. Do not inject too quickly to prevent the product surface from becoming eroded.
4． The temperature of the barrel during injection should be controlled between 170-190 ° C. When the barrel temperature reaches the set temperature, the blower should be started to cool down. When the temperature needs to be increased, the blower heating should be stopped to prevent PVC from overheating and deterioration.

Due to the special performance of PVC, a special PVC injection molding machine is selected for production. Among them, the following points should be noted:
1. The length-to-diameter ratio of the screw should be better than 20: 1, and the compression ratio should be between 1: 1.6-1: 1.2.
2. Screws and nozzles should be made exclusively for PVC and chrome plated.
3． The injection pressure, speed, back pressure, and temperature can be controlled in multiple stages and precisely.

Finally, pay attention to the following requirements in raw material preparation:
1. It is necessary to avoid the deterioration of PVC raw materials in other plastic PVC raw materials. Especially when injection molding other raw materials, the plastic in the barrel and screw must be cleaned before injection of PVC products. (Use PE \ PP \ PS material to clean the screw and barrel)
2. The PVC material must be completely cleaned before shutdown. If the machine is stopped for a long time, in order to prevent PVC from corroding the machine, the screw and barrel should be cleaned with cleaning materials.
3． When the PVC material is left for more than 6 months and the environment is hot and humid, it should be dried at 60-80 ° C for 2-5 hours before use.

Wuxi Jiahong Plastic Technology Co., Ltd. has nearly 30 years of experience in research and development, granulation production of modified PVC pellets, and research and development and production of modified engineering plastics. The professional technical service team can provide customers with one-stop modified plastic system solutions. . If you want to know more about the products, please visit our official website: www.safsedu.com, consult online customer service or call the hotline. Fixed: 0510-68755207 Mobile: 15190220696, we will serve you wholeheartedly.

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