Injection molding process analysis about UPVC plastic raw material
June 02, 2022
PVC, also known as hard PVC, is an amorphous thermoplastic resin made by polymerizing vinyl chloride monomers plus certain additives (such as stabilizers, lubricants, fillers, etc.).
In addition to additives, blends with other resins have been modified to give them significant practical value. These resins include CPVC, PE, ABS, EVA, MBS, and the like.
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 close to the thermal decomposition temperature, and the temperature range in which molding can be performed is very narrow, making it a difficult material to mold.
1. After the plastic is treated at room temperature for 24 hours, the water absorption is less than 0.02%, so no drying is required. If it must be dried, it can be placed in a hot air oven at 60-70°C for 3 hours or at 80°C for 1-1.5 hours.
Up to 20% of recycled material can be mixed with new material. The amount of recycled material depends on the end use of the product. Misuse of recycled materials can cause injection molding failures, not only because the recycled components are excessive, but more because the materials are reprocessed many times. The quality of the recovered material will decline more once it is recovered.
2, the use of UPVC injection molding machine due to melt viscosity, easy to decompose, decomposition products on the iron corrosion, the injection molding must be dedicated plastic parts and temperature control system.
Each type of clamping force that the company covers in the product has a special UPVC plastic injection system, which is equipped with a special safety gate device to meet the requirements of UPVC pipe core products. In general, as long as the injection volume can meet the weight requirements, this series of UPVC injection molding devices 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 runner must be short, and the diameter must be large in order to reduce the pressure loss and hold the pressure to the mold cavity. The shorter the gate, the better. The die cut surface should be round. The diameter of the nozzle mouth should be at least 6mm, it should be conical, and the internal angle should be 5.
The die cut surface of the main channel should also be circular, combined with cold material wells, and connected to the main channel and gate through the radius of the cut plane. The diameter can be 7mm.
The gate should be applied with a radius of a round face to connect the plastic parts. The shorter the better, the smoother the cutting 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 flow smoothly on the flow path, and it will not stay in sharp corners, debris and metal marks, so as to avoid compression or decompression, and the flow path should be kept smooth and smooth.
The mold is made of stainless steel. The content of chromium is at least 13%, preferably 16%. The Rockwell hardness is at least 55. The hard steel mold can be chrome-plated to form a protective effect.
After production, carefully wash the mold surface with a mild lye and spray with an oil spray or siliceous spray.
4. Melt temperature can be measured by air-ejection 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 after injection, it proves that the material is not uniform (insufficient plasticization), which means that the set temperature is too low; if the foam is ejected and a lot of smoke is emitted, it means that the temperature is too high.
5, the injection speed of the slower rate of fire, or excessive shear will degrade the material, the use of UPVC production of extremely smooth thick-walled products, multi-level injection mold filling speed should be used.
6, screw speed should be consistent with the molding cycle. Screw surface speed should not exceed 0.15-0.2m/s
7, back pressure up to 150bar, the lower the better, common 5bar.
8. Retention time At a temperature of 200°C, the barrel residence time cannot exceed 5 minutes. When the temperature is 210°C, the barrel residence time cannot exceed 3 minutes.
