PVC Screw Barrel: What Are They And What Are Their Special Features?
Date:2022-7-19 Author:hcscrew
In this article, we will discuss some tips on screw barrels for the PVC industry. There are four main types of PVC screw barrels: extrusion screw barrels, injection screw barrels, pelletizing screw barrels, and rigid PVC screw barrels.
Extruders and injection moulding machines can be used to process and produce PVC materials. We manufacture a wide variety of PVC products, such as extruded parts, injection moulded parts, compound pellets, pipes, profiles, and other rigid PVC products.
In terms of PVC screw barrel design principles, PVC screw barrel structure, and design requirements, different PVC products exist.
01 PVC extruder types
1. A single screw extruder with equidistant unequal-depth tapering and a single screw extruder with equidistant unequal-depth tapering.
2. Two different types of twin screw extruders with counter-rotating screws: parallel twin screw extruders and conical twin screw extruders.
PVC extruder screw design principles
1. Maintain strict temperature control. The screw design should be as low shear as possible in order to prevent overheating.
2. Corrosion protection is necessary for both screws and barrels.
3. It is imperative to strictly control the injection moulding process.
02 PVC injection moulding screw type
PVC injection moulding plastics come in granular and powdered forms, which are temperature-sensitive, highly viscous and corrosive.
Screws for PVC injection moulding have two characteristics: the surface must be chrome-plated; there is no splitting ring or splitting meson. This screw has excellent plasticisation, low shear heat generation, and is acid corrosion resistant. As it lacks a split ring, it cannot be used for low viscosity plastics and for products with more precise injection speeds and pressures.
PVC injection moulding screws are available in two designs: one-piece design and separate.
For products that require high quality or injection moulding screws that produce only one type of PVC material, one-piece designs are best. Alternatively, a split screw can be used. It is imperative to note, however, that a split screw with a separate head can cause material buildup. In order to control the temperature precisely, we recommend adding a special temperature gauge. When making PVC products, remember that heat dissipation and cooling are necessary. PVC products should be made using forced air cooling of the melt barrel and the screw. But, soft PVC for toys is the exception.
03 PVC pelletizing screw type
Pelletizing soft PVC pellets with a twin screw extruder require a high-efficiency screw conveying compression section, which slowly compresses the pellets. To ensure high mixing efficiency, the screw’s dispersion section should minimize temperature differences and fluctuations. The vented section of the screw ensures full evaporation and discharge of the gasification material. The compression section must have the right length-to-diameter ratio (L/D ratio), while the metering section assures even extrusion. In general, for an L/D ratio 32 PVC pelletizing screw, the general conveying compression section L/D is 11 to 12; the mixing conveying section L/D is 8 to 9; the exhaust section L/D is 3 to 3.5; the compression section L/D is 5 to 8; the metering section L/D is 3 to 4.
04 PVC profiles, PVC pipes, and rigid PVC products screw design
Conical twin screw extruders are the most common processing machine for PVC profiles, pipes, and other rigid PVC products.
What are the advantages of conical twin screws? In the same screw prism structure, we compare parellel twin screws and conical twin screws. The parallel twin screw is 60% longer than the conical twin screw in the filling section, but the heat transfer area is 45% smaller. However, when conveying with the conical twin screw, the relative speed of movement between materials is greater than when conveying with the parallel screw, i.e. the conical twin screw generates more shear heat than parallel twin screw. In the charging section, a conical twin screw is more effective at plasticizing PVC material than a parallel screw.
Moreover, the conical twin screws provide a higher energy conversion and better preplasticization due to the increased squeezing effect caused by the shrinkage of the screw groove during conveying. This brings the material into full contact with the screw. Due to the smaller diameter of the conical twin screw in the metering section, the material is less stressed. This results in less shear heat to meet the PVC plasticisation requirements.
That’s all for today. There are many PVC screw types not covered here, we’ll discuss them next time. Screw barrel selection is still dependent on the experience of industry engineers. Huachang Screw Barrel Factory’s summary: choose cost-effective products that will be suitable for their own products!
Welcome to contact us at HUACHEN Screw Barrel Factory to get the latest PVC screw barrel offer.
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