Tell You The Main Points Of PVC Molding

PVC is a heat-sensitive plastic. Overheating or excessive shearing can cause decomposition and spread rapidly, because one of the decomposition products (such as acid or HCI) will have a catalytic effect, causing further decomposition of the process, and the acid will attack the metal and make it If it is dented, the protective layer of the metal will peel off, causing rust, which is more harmful to the human body.

PVC is a heat-sensitive material, and its injection molding process 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 of the outer wall of the barrel, but mainly the shear heat of the screw movement, and the external heating of the barrel is mainly to provide the heat source when the machine is started. Due to the poor thermal conductivity of PVC, during the heating process, although the peripheral plastic has been heated, the intermediate material is still in a condensed state, and even the peripheral part has been overheated and decomposed, and there may be unmelted parts inside. All kinds of reasons must be fully paid attention to in product design, mold design, injection molding process, etc.

An introduction to the matters that should be paid attention to in injection molding PVC molds:

(1) The mold temperature should have a cooling water control device, so that the mold temperature can be controlled between 30°C and 60°C. 5. The mold should have a smooth surface and be chrome-plated to prevent corrosion.

(2) 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. 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 reduction. A short period before the return of the material is terminated, the back pressure should be reduced to prevent saliva leakage.

(3) The screw speed should vary with different diameters. 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.

(4) In order to inject plastic 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 the shrinkage mark, do not inject too fast to prevent the surface of the product from becoming eroded.

(5) The temperature of the barrel during the injection process 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, and when the temperature needs to be raised, the blower should be stopped to prevent the PVC from overheating and deteriorating. The screw and nozzle shall be specially made of PVC with chrome plated surface. Injection pressure, speed, back pressure and temperature can be controlled in multiple stages and precisely.

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