PVC material is inherently anti-inflammatory because of its low price, strong and chemical resistance, shrinkage rate is 0.2-0.6%. Products are increasingly used in electrical, mechanical, construction, daily necessities, toys, packaging. In view of the characteristics of PVC material, the injection molding process of products is analyzed as follows:
CHARACTERISTICS OF PVC MATERIALS
PVC has poor thermal stability, close to the decomposition temperature, poor fluidity, easy to form defects in appearance, poor heat resistance of PVC materials, the most easily burnt, acid gas and corrode molds, plasticizer can be added to increase its fluidity during processing, and additives are generally required to be used. Its strength, electrical insulation and resistance are good.
- Design of Moulds and Gates
In order to shorten the injection forming period, the shorter the injection hole is, the better, the transverse section should be circular, the diameter of the injection nozzle should be minimum 6 mm, the conical shape of the injection nozzle should be circular, and the inner angle should be 5 degrees. It is better to add a cooling well, which can prevent the semi-solid materials from melting into the mould cavity, and such materials will affect the surface decoration and the strength of the product.
The inclination of drawing die should be between 0.50 and 10 to ensure that there are enough exhaust equipment in the die cavity. The commonly used size of exhaust hole is 0.03-0.05 mm deep, 6 mm wide, or 0.03-0.05 mm peripheral clearance of each ejector pin. Dies are made of stainless steel or plated with hard chromium.
- Forming Technology of PVC
PVC is a heat-sensitive plastic. Overheating or excessive shearing can cause decomposition and rapid spread, because one of the decomposition products (such as acid or HCI) can produce catalytic effect, lead to further decomposition of the process, acidic substances will corrode the metal, make it become depression, and make the protective layer of metal peel off, lead to rust, which is more harmful to human body.
The common ratio of length to diameter of screw is 18 to 24:1, the ratio of three segments is 3:5:2, the compression ratio is 1.8 to 2, and the depth of groove in feed section is recommended as follows:
Screw diameter (mm) 3550 60 75 100
Groove depth (mm) 234 556
The tip of the screw should have an inner angle of 25-30 degrees. When the screw is in place, the distance between the tip and the nozzle should be 0.7-1.8 mm. The screw must be made of non-embroidered steel or chrome plated.
1) Screw cushion: Screw cushion between 2 and 3 mm, larger opportunities.
2) Injection volume: The actual tube retention time should not exceed 3 minutes.
3) Barrel temperature setting:
The temperature provided can only be used as a reference, and can be adjusted appropriately according to the actual situation of machines and raw materials, which may also exceed the recommended range.
Front and middle feeding section of position die nozzle
The injection volume of temperature (0C) 30-60 170-190 160-180 150-170 140-160 is 20-85% of the theoretical injection volume of the machine. The smaller the actual injection volume, the longer the residence time of the material, and the greater the risk of deterioration after heating.
4) Barrel retention time:
The maximum retention time of barrel can exceed 5 minutes under the temperature control of 2000 C.
5) Injection speed:
Injection speed should be slow, otherwise excessive shearing will degrade the material. When UPVC is used to produce extremely smooth thick-walled products, multi-stage injection speed should be adopted. If light brown stripes radiate from the gate, it means that the injection speed is too fast.
6) Back pressure: The commonly used surface value is 5bar. Increasing back pressure is beneficial to mixing and exhaust, but the lower the back pressure, the better.
7) Shut-down: Because of the low superheat stability of PVC, it is very important to stop the machine abruptly. To clean the barrel without leaving a little PVC, other thermoplastic plastics (such as PMMA, PP, LDPE, or GPPS) with high superheat stability and resistance to PVC can be used (for example, PMMA, PP, LDPE, or GPPS). Do not mix POMT UPVC in the same barrel. Otherwise, it will cause excessive chemical reaction and cause serious damage to the machine.
8) Mixing ratio: The maximum mixing ratio of nozzle material is 20%, otherwise, the quality of products will be affected.
9) Typical products: UPVC is mainly used to manufacture drainage pipes, or computer and television shells, filter pressure troughs, photocopier shells, watt-hour meters and gas shells, transparent cover of printers, ventilation grids and various electronic components.