1.What is UPVC material?
UPVC is a mixture of an amorphous thermoplastic resin prepared by polymerization of vinyl chloride monomer with certain additives (such as stabilizers, lubricants, fillers, etc.). In addition to the use of additives, it is also possible to use a blending modification with other resins to make it have obvious practical value.
UPVC has a 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 very close to the thermal decomposition temperature, and the temperature range in which molding can be performed is narrow, which is a material that is difficult to mold.
UPVC features:
(1) Weather resistance: refers to the ability of plastics to withstand various climates. These include visible and ultraviolet light, moisture, temperature, atmospheric chlorination, and chemicals encountered during the use of the product. The most important weatherability, including non-fading, powdering resistance and durability of physical properties. The weather resistance of the modified UPVC is comparable to that of aSa and has better physicochemical properties. As we all know, among all polymer materials, aSa is known for its excellent weather resistance, and UPVC can completely replace aSa in weather resistance.
(2) Flame retardant performance: UPVC itself has good flame retardant properties. The oxygen index of general UPVC is above 27, and the oxygen index of UPVC after special modification can reach or exceed 42 or more. Comparable.
(3) Variety of performance: UPVC will exhibit different properties when adding different performance additives in UPVC. This makes UPVC look diverse. Make it more versatile. Make it one of the world's largest plastic products.
2.UPVC injection molding process
(1) Selection of injection molding machine
UPVC has a high viscosity and easy decomposition, and the decomposition products have a corrosive effect on iron. The plastic injection parts and temperature control system must be used for injection molding. UPVC special injection molding equipment can be used to form high quality products.
(2) Processing of plastics
The water absorption of UPVC is not high. After 24 hours at room temperature, the water absorption is less than 0.02%, and no drying is required before use.
If the raw material is dried as required by the processing, it can be placed in a hot gas oven at 60-70 ° C for 3 hours or a dryer at 80 ° C for 1 to 1.5 hours.
(3) Design of molds and gates
Mold temperature can be set to 40 ° C
The design of the runner needs to be short and thick, requiring a short length and a large diameter to reduce the pressure loss.
The shorter the gate, the better, the die-cut surface should be round, the diameter of the nozzle mouth is at least 6 mm, the shape is conical, and the inner angle is 5°.
The die-cut surface of the main channel is also round, combined with a cold well, connected to the main channel and gate by the radius of the cut surface, and the diameter can be 7mm.
The gate is connected to the plastic part by the radius of the rounded surface. The shorter the better, the smoother the die cut surface. Cold wells prevent semi-solidified materials 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 flow path and does not stay in the traces of sharp corners, debris and metal, so as to avoid compression or decompression, the flow path should be kept smooth and smooth.
The mold is cast in stainless steel with a chromium content of at least 13%, preferably 16%, and a Rockwell hardness of at least 55. The hard steel mold can be chrome-plated to form a protective effect. After production, use a mild lye to carefully clean the mold surface and spray an oily spray or a siliceous spray.
(5) Melt temperature
It can be measured by the air-launch method and varies from 185 to 205 °C. The exact melt temperature of the UPVC can be known from the smoothness of the injected material.
If the material is rough after the injection, it proves that the material is not uniform (insufficient plasticization), indicating that the set temperature is too low; if the foam is emitted after the injection and a large amount of smoke is emitted, the temperature is too high.
(6) Injection speed
The rate of fire is slow, otherwise excessive shearing will degrade the material. When UPVC is used to produce extremely smooth thick-walled products, multi-stage injection mold filling speed should be used.
(7) Screw speed
Should be in accordance with the molding cycle. The screw speed should not exceed 0.15-0.2 m/s.
(8) Back pressure
Up to 150 bar, but the lower the back pressure in actual production, the better, usually 5 bar.
