The PVC material is a non-crystalline material. PVC materials are often added with stabilizers, lubricants, auxiliary processing agents, colorants, impact agents and other additives in actual use. PVC materials are non-flammable, high strength, weather resistant and excellent geometric stability.
Polyvinyl chloride (PVC) has poor thermal stability, the molding temperature is close to the decomposition temperature, the fluidity is not good, the appearance is easy to form poor defects, the heat resistance of PVC material is not good, the most easy to burn, the acid gas is generated and the mold is corroded, and the processing is performed. Plasticizers can be added to increase their fluidity.
PVC injection molding process
1, plastic processing
After 24 hours at room temperature, the amount of 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 end 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, mold and gate design
The 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 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.
3, Injection Molding Machine selection
Due to its high melt viscosity and easy decomposition, PVC has a corrosive effect on iron. Its injection molding requires special plasticized parts and temperature control system.
4, barrel retention time:
Under the temperature control of 2000C (rubber) barrel retention time can be more than 5 minutes.
5, injection speed:
The injection speed should be slow, otherwise excessive shear will degrade the material. When UPVC is used to produce extremely smooth thick-walled products, multi-stage injection speed should be used. If light brown stripes are emitted from the gate, it means the speed of the shot is too high. fast.
6, back pressure: the commonly used surface value is 5bar, increasing back pressure is conducive to color mixing and exhaust, but the back pressure should be as low as possible.
7. Shutdown: Due to the low thermal stability of PVC, the shutdown part is very important. The barrel should be clean and free of PVC. It can be used with other thermoplastics with high stability and resistance to PVC (such as PMMa, PP, LDPE, or GPPS), should not mix POMT UPVC in the same barrel, otherwise it will lead to excessive chemical reaction and cause serious damage to the machine.
8. Typical products: water supply pipes, household pipes, house wall panels, commercial machine casings, electronic product packaging, medical equipment, food packaging, etc.
