PVC is a thermal sensitive material, and its injection molding process is poor, because too high melting temperature or too long heating time is easy to make PVC decomposition, so the control of melting temperature is the key to PVC injection products.
The heat source of melting raw PVC comes from two aspects, the screw movement produces shear heat to the plastic and the resistance wire heating on the outer wall of the machine barrel. The main is the shear heat of screw movement, the external heating of the barrel is mainly to provide heat source when the machine starts.
As PVC heat conduction performance is poor, so in the heating process, although external plastic has been heated, but the intermediate material is still in the state of condensation, and even the external part is overheating to decomposition, the internal may be still unmelted. As a result, it must pay sufficient attention to product design, mold design, injection molding craft, etc. It is important to select PVC special injection molding machine from outstanding manufacturers in order to produce an ideal product.
Product design and mold design should pay attention to the following:
Products should without sharp angle or break as much as possible, and the thickness should not change much in order to prevent degradation and deterioration of PVC.
The draft angle of the mold 10°min., leaving about 0.5% shrinkage.
Concerns of designing mold runner
The injection gate of the die should be slightly larger than that of the nozzle hole and larger than the diameter of the intersection of the main runner, so that the flow of PVC material into the die cavity is not detained and the pressure can be balanced.
Try to use intercepting gate to prevent the molten slag from flowing into the product and the runner, so as to reduce the temperature and form easily.
Gate should be designed at the thickest wall of the product, and the width is enough, the length should be between 6-8 mm, so as to make the flow of PVC smoothly.
In order to reduce the feeding pressure and easy demoulding, the runner should be circular, which should be selected between 6-10mm according to the size and weight diameter of the product.
The temperature of the die should be controlled by cooling water control device, so that the temperature of the die can be controlled between 30 ~60.
The surface of the die should be smooth, chrome-plated and corrosion-resistant.
The following points should be paid attention during injection moulding process:
1. Back pressure should be controlled at about 1 Mpa, too high back pressure will produce excessive shear force, which will cause PVC superheating decomposition. 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 should be longer and the back pressure should be lower. As the effective length of the screw decreases, the back pressure should be increased to compensate for the decrease of heat. The back pressure should also be lowered to prevent salivation before the end of reclaiming.
The rotational speed of the screw should vary with its diameter. When the diameter is less than 60 mm, the rotational speed is 50-70 rpm; when the diameter is more than 70 mm, the rotational speed is 20-50 rpm to prevent the degradation of PVC due to excessive shear stress.
In order to inject the plastic into the cavity, the injection speed should be controlled in many stages. The principle is slow start injection. With the increasing of the forming area, the injection rate should be
increased, so that no cracks or shrinkage marks. Don’t inject fast to prevent the surface of the product from denudation.
In the process of the injection, the barrel temperature should be controlled between 170-190 ℃. When the barrel temperature reaches the setting temperature, the blower should be started to cool down. When the temperature needs to increase, the blower heating should be stopped to prevent the PVC from overheating and deteriorating.
we should pay attention to the following requirements in the preparation of raw materials:.
Must avoid other plastic or it will cause PVC raw material to deteriorate. Especially after other raw materials have been injected, the barrel and screw must be cleaned up before the PVC injection molding.
PVC material must be completely cleaned before shutdown. If it is long shutdown, the screw and barrel should be cleaned by PS material in order to prevent the corrosion of machine.
When the PVC material is stored for more than 6 months in high temperature and humid environment, it can be used only after drying for 1-2 hours at 60-80 ℃.
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