Discussion on UPVC plastic injection molding process

UPVC is also known as hard PVC. in addition to additives, UPVC is mixed with other resins to make it more practical. These resins are PE, ABS, EVA, CPVC, MBS, etc. UPVC has high melt viscosity and poor flowability, and even if the injection pressure and melt temperature are increased, the change in flowability is small. In addition, the molding temperature of the resin is almost close to the thermal decomposition temperature, and the temperature range in which molding can be performed is narrow, which is a difficult material to mold.
1、Plastic processing
If the water absorption is less than 0.02% after 24 hours at room temperature, the plastic does not need to be dried. If it must be dried, it can be placed in a hot gas oven at 60-70°C for 3 hours or in a dryer at 80°C for about 1-1.5 hours.
The recycled material can be mixed with up to 20% of the new material. The amount of recycled material depends on the end use of the product. Misuse of recycled material can lead to failure of injection molding jobs, not only because of over-recycling of components, but also because the material is processed multiple times. The quality of recycled material will gradually decrease with each recycling.
2、The choice of injection molding machine
The melt viscosity of UPVC is relatively high, so it is easy to decompose. The product of decomposition has a great corrosive effect on iron. Special plasticizing parts and temperature control system must be used for injection molding.
Each clamping force model covered by HMS has a special UPVC plastic injection system, and a special safety gate device is required for the pull core of UPVC pipe products. Under normal circumstances, as long as the injection volume can meet the weight requirement, this series of UPVC special injection molding equipment can form high quality products.
3、Mold and gate design
The mold temperature can be set to 40℃. In addition, the length of the whole runner must be short and the diameter is relatively large, which is used to gradually reduce the pressure loss and the pressure-holding energy can be transferred to the cavity. The shorter the gate, the better. The die-cut surface must be circular. The nozzle opening has a minimum diameter of 6 mm, is tapered and has an internal angle of 5.
The die-cut surface of the main channel is also circular, combined with the cold well, connected to the main channel and the gate by the radius of the die-cut surface, which can be 7 mm in diameter.
The sprue is connected to the plastic part by the radius of the circular surface. The shorter the better, the smoother the die-cut surface.
The cold well prevents the semi-solidified material from entering the mold cavity and its importance is often overlooked.
The gate should be correctly positioned so that the material flows smoothly over the runner without resting on sharp corners, debris and traces of metal to avoid compression or decompression, and the runner should be kept smooth and flat.
The molds are 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 chromium plated to create a protective effect.
After the production, the mold surface is carefully cleaned with mild alkali solution and sprayed with oil spray or silicon spray.
4、Temperature of the melt
It can be measured by air emission method and the temperature is between 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 the temperature is too low; if the injection sends out foam and a lot of smoke is discharged, it means the temperature is too high.
5、Injection speed
The injection speed should be slow, otherwise excessive shearing will deteriorate the material. When UPVC is used to produce extremely smooth thick-walled products, multi-stage injection mold filling speed should be used.
6、Screw speed
Screw speed should be further determined in conjunction with the molding cycle. Screw surface speed should not exceed 0.15-0.2m / s
7, back pressure
Back pressure can reach 150bar, of course, the data as low as possible, the common value of 5bar.
8、Staying time
In the temperature of 200 ° C, the cylinder dwell time should not exceed 5 minutes. When the temperature is 210°C, the cylinder dwell time can not exceed 3 minutes.

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