What should we pay attention to when using hard UPVC compounds for injection production?
PVC is also called rigid PVC. It is an amorphous thermoplastic resin made by the polymerization of vinyl chloride monomer and certain additives (such as stabilizers, lubricants, fillers, etc.).
In addition to using additives, the method of blending with other resins has also been adopted, which has obvious compound value.
These resins include CPVC, PE, ABS, EVA, MBS, etc.
UPVC has high melt viscosity and poor fluidity. 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 very close to the thermal decomposition temperature, and the temperature range in which molding can be performed is very narrow. It is a difficult material to mold.
1. Handling of plastic
After 24 hours at room temperature, the water absorption is less than 0.02%, so there is no need to dry. If it is necessary to dry, put it in a hot gas oven at 60-70 ℃ for 3 hours, or a dryer at 80 ℃ for 1-1.5 hours.
Recycled materials can be mixed with new materials up to 20%. The amount of recycled materials depends on the end use of the product. Misuse of recycled materials can make injection molding fail, not only because too many components are recycled, but also because the materials are reprocessed multiple times. Every time the recycled material is recycled, the quality decreases.
2. Selection of injection molding machine
Because UPVC has a large melt viscosity and is easy to decompose, the decomposition products have a corrosive effect on iron. Its injection molding must have special plasticized parts and temperature control systems.
Some injection molding companies have a special UPVC plastic injection system for each clamping force model covered by the product, and they have a special safety door device to match the requirements of the core pulling of UPVC pipe products. In general, as long as the injection volume can meet the weight requirements, this series of UPVC special injection molding equipment can be used to form high-quality products.
3. Mould and gate design
The mold temperature can be set to 40 ° C. The length of the flow channel should be short, and the diameter should be large to reduce the pressure loss and the pressure can be transferred to the cavity. The shorter the gate, the better. The die cutting surface should be round. The diameter of the nozzle mouth should be at least 6mm. It should be conical and the inner angle should be 5 °.
The die cut surface of the main flow channel should also be round, combined with the cold material well. The main flow channel and the gate are connected by the cutting surface radius, and the diameter can be 7mm.
The gate should be connected to the plastic parts with a radius of a tangential surface. The shorter the better, the smoother the cutting surface.
Cold material wells prevent semi-solidified material from entering the mold cavity, and its 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 the corners, debris and metal traces, so as to avoid compression or decompression. The runner should be kept smooth and smooth.
The mold should be made of stainless steel, with a chromium content of at least 13%, preferably 16%, and a Rockwell hardness of at least 55. Hard steel molds can be treated with chrome plating to form a protective effect.
After production, the mold surface should be carefully cleaned with a mild lye before spraying with an oil spray or a silicon spray.
4, melt temperature
Can be measured by air shot method, ranging from 185-205 ° C. The precise melt temperature of UPVC can be obtained 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), which means that the set temperature is too low; if it foams and emits a lot of smoke after injection, the temperature is too high.
The firing rate should be slow, otherwise excessive shearing will degrade the material. When using UPVC to produce extremely smooth thick-walled products, multi-stage injection mold filling speed should be used.
Should match the molding cycle. Screw surface speed should not exceed 0.15-0.2m / s
It can reach 150 bar, the lower the better, 5 bar is common.
8. Detention time
At a temperature of 200 ° C, the cylinder residence time cannot exceed 5 minutes at most. When the temperature is 210 ° C, the cylinder residence time cannot exceed 3 minutes.