Introduction of injection molding process for hard polyvinyl chloride (UPVC)
UPVC, also known as hard PVC, is an amorphous thermoplastic resin made by polymerizing vinyl chloride monomer with certain additives (such as stabilizers, lubricants, fillers, etc.).
In addition to the use of additives, the method of blending and modifying with other resins has been adopted, which has obvious practical value. These resins are CPVC, PE, aBS, EVA, MBS, and the like.
UPVC has a high melt viscosity and poor activity, and even if the injection pressure and melt temperature are improved, the change in activity is not large. 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.
1. After the plastic is treated at room temperature for 24 hours, the water absorption is less than 0.02%, so there is no need to dry. If it must be dried, it can be placed in a hot gas oven at 60-70 ° C for 3 hours, or a dryer at 80 ° C for 1-1.5 hours.
Recycled materials can be mixed with up to 20% of the new material, depending on the ultimate use of the product. Abuse of recycled materials can cause injection molding to fail, not only because there are too many recycled components, but also because the materials are reprocessed many times. The more the quality of the recycled material is recovered, the more it is recycled.
2, injection molding machine selection
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.
Each type of clamping force model covered by Ramada has a special UPVC plastic injection system, and a special safety door device is required for the core pulling of UPVC pipe products. Under normal circumstances, 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, mold and gate design mold temperature can be set to 40 ° C. The length of the flow path is short and the diameter is large to reduce pressure loss and pressure holding energy to the cavity. The shorter the gate, the better, the die-cut surface should be round, the diameter of the nozzle mouth is at least 6 mm, and 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 can move smoothly on the flow path, and it will not stay in the traces of sharp corners, debris and metal, so as to avoid the situation of 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 then spray an oily spray or a siliceous spray.
4, the melting temperature can be used in the airborne method, ranging from 185-205 °C. The correct melt temperature of the UPVC can be known from the smoothness of the injected material. If the material is rough after the shot, it is confirmed that the material is not uniform (insufficient plasticization), indicating that the 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.
5, the injection speed of the injection rate should be slow, otherwise excessive shear will degrade the material, UPVC production of extremely smooth thick-walled products, should use multi-stage injection mold filling speed.
6, the screw speed should be matched with the molding cycle. The screw surface speed should not exceed 0.15-0.2m/s
7, back pressure up to 150bar, the lower the better, usually 5bar.
8, the residence time at 200 ° C temperature, the barrel retention time can not exceed 5 minutes. When the temperature is 210 ° C, the cylinder residence time can not exceed 3 minutes.
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