PVC,also known as hard PVC,is composed of 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 is also used to make it have obvious practical value.These resins are CPVC,PE,ABS,EVA,MBS and so on.
UPVC has high melt viscosity and poor fluidity.Even if the injection pressure and melt temperature are increased,the fluidity will not change much.In addition,the molding temperature of the resin is very close to the thermal decomposition temperature,and the temperature range that can be molded is very narrow,making it a difficult-to-mold material.
1.Treatment of plastics
After 24 hours at room temperature,the water absorption is less than 0.02%,so there is no need for drying.If it must be dried,it can be placed in a hot air oven at 60-70°C for 3 hours,or a dryer at 80°C for 1-1.5 hours.
Up to 20%of recycled materials can be mixed with new materials,and the amount of recycled materials depends on the end use of the product.Misuse of recycled materials can cause injection failure.Not only is there too much recycled content,but also because the materials have been reprocessed many times.The more the recycled material is recycled,the more its quality will decrease.
2.Selection of injection molding machine
UPVC has a high melt viscosity and is easy to decompose,and the decomposition products have a corrosive effect on iron.Its injection molding must have special plasticized parts and temperature control systems.
Some injection molding product companies have a special UPVC plastic injection system for each clamping force model covered by the product,and a special safety door device is equipped to meet the requirements of UPVC pipe product core pulling.Under normal circumstances,as long as the injection volume can meet the weight requirements,this series of UPVC special injection molding devices can be used to form high-quality products.
3.Mold and gate design
The mold temperature can be set to 40°C.The length of the runner should be short and the diameter should be large to reduce the pressure loss and the pressure holding energy to be transmitted to the cavity.The shorter the gate,the better,the die cutting surface should be round,the minimum diameter of the nozzle mouth is 6mm,it is conical,and the inner angle is 5°.
The die cutting surface of the sprue should also be round,combined with the cold slug well,and the sprue and gate are connected by the radius of the cut surface,and the diameter can be 7mm.
The gate should be connected to the plastic part with the radius of the tangential surface,the shorter the better,and the die-cut surface should be smooth.
The cold material well can prevent the 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 sharp corners,debris and metal traces,so as to avoid compression or decompression,and the runner should be kept smooth and smooth.
The mold should be cast in 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 chrome-plated to form a protective effect.
After production,use mild lye to carefully clean the mold surface before spraying with oil spray or silicon spray.
It can be measured by the air shot method,ranging from 185-205°C.From the smoothness of the injected material,the accurate melting temperature of UPVC can be known.If the material is rough and uneven after injection,it proves that the material is not uniform(insufficient plasticization),indicating that the set temperature is too low;if the material foams and emits a lot of smoke after injection,it indicates that the temperature is too high.
The shooting speed should be slow,otherwise excessive shearing will degrade the material.When using UPVC to produce extremely smooth thick-walled products,a multi-level injection mold filling speed should be used.
Should be matched with the molding cycle.The screw surface speed should not exceed 0.15-0.2m/s
It can reach 150bar,the lower the better,and it is usually 5bar.
At a temperature of 200°C,the residence time of the barrel cannot exceed 5 minutes at most.When the temperature is 210°C,the residence time of the barrel cannot exceed 3 minutes.