PVC, also known as hard PVC, is an amorphous thermoplastic resin made by polymerization of vinyl chloride monomer plus certain additives (such as stabilizers, lubricants, fillers, etc.).
In addition to additives, it also adopts the method of blending and modifying with other resins, making it have obvious practical value. These resins include CPVC, PE, ABS, EVA, MBS, etc.
UPVC has high melt viscosity and poor fluidity, and even if the injection pressure and melt temperature are increased, the fluidity does not change much. In addition, the molding temperature of the resin is close to the thermal decomposition temperature, and the temperature range at which molding can be performed is very narrow, making it a material that is difficult to mold.
1. Treatment of plastics
After 24 hours at room temperature, the water absorption is less than 0.02%, so there is no need to dry. If it must be dried, it can be put into a hot gas furnace at 60-70°C for 3 hours, or a dryer at 80°C for 1-1.5 hours.
Up to 20% of the recycled material can be mixed with virgin material, and the amount of recycled material depends on the end use of the product. Misuse of recycled material can lead to injection molding failure, not only because too much content is recycled, but also because the material has been reprocessed many times. The quality of recycled material decreases more with each additional time it is recycled.
2. Injection molding machine selection
UPVC due to the high viscosity of the melt, easy to decompose, the decomposition product has a corrosive effect on iron, its injection molding must be special plasticized parts and temperature control system.
Ramada has a special UPVC plasticizing injection system for each clamping force model covered by the product, and a special safety gate device is equipped with the core pulling requirements of UPVC pipe fittings. Under normal circumstances, as long as the injection volume can meet the weight requirements, the selection of this series of UPVC special injection molding equipment can form high-quality products.
3. Mold and gate design
The mold temperature can be set to 40°C. The length of the runner should be short and the diameter should be large to reduce pressure loss and transfer of pressure holding energy to the mold cavity. The shorter the gate, the better, the die cutting surface should be round, the diameter of the nozzle is at least 6mm, conical, and the inner corner is 5°.
The die-cut surface of the main channel should also be round, combined with the cold well, and connected to the main channel and the gate by the cut radius, and the diameter can be 7mm.
The gate should be connected to the plastic parts with a radius of cut round surfaces, the shorter the better, and the die cutting surface should be smooth.
Cold wells prevent semi-solidified material from entering the mold cavity, and their importance is often overlooked.
The position of the gate should be correct, so that the material flows smoothly on the runner, and will not stay in sharp corners, fragments and metal traces, so as to avoid compression or decompression, and the runner should be kept smooth and smooth.
The mold should be cast in stainless steel, the chromium content is at least 13%, preferably 16%, the Rockwell hardness is at least 55, and the hard steel mold can be chrome-plated to form a protective effect.
After production, the mold surface should be carefully cleaned with a mild lye solution before spraying with oil or siliceous spray.
4. Melting temperature
It can be measured by air-emission method, ranging from 185-205 °C. The exact melt temperature of UPVC can be known from the smoothness of the injected material. If the material is rough and uneven after injection, it proves that the material is not uniform (insufficient plasticization), indicating that the temperature set is too low; If it foams and emits a lot of smoke after the injection, it means that the temperature is too high.
5. Injection speed
The rate of fire should be slow, otherwise excessive shear will degrade the material, and when using UPVC to produce extremely smooth thick-walled products, multi-stage injection mold filling speed should be used.
6. Screw speed
It should be determined with the molding cycle. The screw surface speed should not exceed 0.15-0.2m/s
7. Back pressure
Up to 150 bar, the lower the better, commonly 5 bar.
8. Residence time
At a temperature of 200°C, the barrel residence time should not exceed 5 minutes at most. When the temperature is 210°C, the barrel residence time cannot exceed 3 minutes.
