Design of PVC pipe extrusion
1-Introduction
This article aims at giving a bird's eye view on PVC extrusion with particular emphasis on screw design.
PVC compounds are extruded in single as well as in twin screw extruders.
Whereas co rotating twin screws are mainly used for compounding operations, counter rotating screws are used for the extrusion of profiles, tubes and rods.
Twin screws are divided in parallel and conical configurations.
PVC is of low thermal stability and high melt viscosity. Therefore, it is combined with a number of additives to vary properties to suit different end-use applications. PVC formulation is key to processing success.
Extruded PVC can be 100% recycled. However
hot spots in the extruder lead to burnt polymer which has to be rejected
otherwise it will contribute to the degradation of the virgin polymer.
A co-rotating extruder with interchangeable elements is extremely versatile and can be used for most PVC compounds. Conical twins are best for rigid PVC. The production of rigid PVC pipes is dominated by counter-rotating twin screw extruders. This plasticizing system matches perfectly the demands of processing powder shaped, shear sensitive polymers. Compared to single screw extruders the twin screw extruders provide a constant feeding of powder as well as a gentle and uniform plasticizing at low screw speeds allowing for high output rates.
The counter-rotating twin screw extruders can be divided into two types with conical and parallel screw systems.
The lower the required output rate the more
the conical twin screw extruder is preferred. The application range of this
system goes up to 150 kg/h but as outputs increase so do the benefits of the
parallel system. At output rates above 300 kg/h parallel twin screw extruders
are dominant.
Conical extruders have been successful at
the lower throughput end of the extruder market. In the feed zone, the surface
area in contact with the material is large this improves the heat transfer to
the material.
The conical shape of the screw and the continuous reduction of the channel volume result in very gentle plasticizing. Shear friction in the metering zone is very low, due to the very small screw diameter at this point.
Conical screw convey the melt smoothly and steadily even at high die resistance, although pellet rigid PVC compound can be run in twin screw extruders (starve feeding required to avoid high amps & torque), twins are ideally suited for powder dry blend.
Twin-screw extruders were designed to
extrude powder by gravity feed. They normally have smaller horsepower motors,
but are able to achieve higher production rates than an equivalent size
single-screw extruder. The recipe should be designed to allow the powder to run
flood feed. This takes advantage of the twin screw extruder's constant pump
machine and achieves better dimensional control of the finished parts.
Single screw extruders have been traditionally utilized for sheet extrusion because of their low initial cost.
A general-purpose screw design for rigid PVC extrusion consists of a constant flight and pitch (lead). The pitch is equal to the diameter, and the length is equal to 24D. It should be bored for screw cooling. Screw flights should be hardened. Regardless of the screw size, a 2.4:1 compression ratio is recommended for rigid PVC. Higher compression screws, such as those designed for flexible PVC, can cause over-heating and degradation of the material. (ref: PolyOne technical report). Around 14 metering flights and a metering depth of 0.3inches are required.
Four rows of pins in the metering zone at 3, 6, 9 and 12 flights from the screw tip contribute to the homogenization of the melt
Screw designs, such as double flighted screws, can also offer improved performance such as a higher rate at the same melt quality and temperature.
At recommended processing temperatures (190ºC to 216ºC for a smooth extruding), rigid PVC is typically higher in viscosity than many other materials. To prevent overloading the motor, a 150 to 250 horsepower motor for a 4 1/2" extruder is used. Screw speed ranges of 40 to 80 RPM are used. This means a gear ratio should be used to give a maximum speed of about 80 but not more than 100 RPM. Higher viscosities also mean higher temperatures are reached by shear heating or friction. Thus, screw cooling and efficient barrel cooling (i.e. water cooling), are critical to rigid PVC extrusion. An extruder length of 24 to 1 should be considered minimum while the longer 30 to 1 or 32 to 1 is desirable, especially if a vented barrel and two stage screw is used.
Typically pelletized compound is used with single screw extruders. Powder compounds are available for single screw machines but they require special handling and are more prone to air and moisture entrapment in the melt. One of two methods is used to address this issue. For pelletized or powder compounds, a two stage screw can be used with either an open vent or an applied vacuum at the vent. A two stage screw and vent combination must be carefully balanced for specific rates, dies and compounds.
A second technique used for powder extrusion is the vacuum hopper. This allows the use of single stage screws with a special vacuum seal at the screw shank. A two hopper system is used to maintain a uniform vacuum on the feed hopper. With either system, a crammer feed is recommended to assure uniform feed of the powder.
Screw cooling is mandatory for PVC sheet extrusion. The system should be set up so that the oil flows to the screw tip first so as to cool the tip and thus prevent hang up and burning on the tip. The oil should then return between the screw and the feed tube. The oil temperature should be controlled, with a recommended starting point of about 93-121°C.
The two popular methods of feeding an extruder are starve-feeding and force-feeding. During force-feeding, a reserve of material is maintained in the hopper of the extruder and material is forced in the extruder.
In starve-feeding the extruder is fed at a rate less than the capacity of the screw. The hopper remains empty and functions as a conduit to avoid material from spilling. Starve feeding is the more popular method for feeding.
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