Advantages of conical twin screw extruder

          Advantages of conical twin screw extruder

 For users, it is important to the choose and buy of twin screw extruder, the different types of twin screw extruder with different properties and applications, therefore, must want to understand all kinds of performance and the application of the twin screw extruder.For example, the mesh type synthetic rotating twin screw extruder for its high speed, shear rate, combination of screw, it widely applies to the thermal decomposition of the polymer modification, blending, filling, fiber reinforcement and material of reactive extrusion.For example, the mesh type differential rotating twin screw extruder, because of its good mixing plasticization functions, its biggest characteristics is a direct forming PVC powder.Such as change the geometric structure of the screw, can also be used for other materials processing, but its strength is still the PVC processing.According to the size of the section, plastic extrusion, by extrusion amount again to choose the specifications of the twin screw extruder.In the plastic processing and molding process conditions under the condition of basically the same, conical double screw extrusion mechanism to adapt to the larger head pressure, parallel twin-screw extrusion function adapted to small head pressure.

Two root conical screw horizontal arrangement, two axis in an Angle into the barrel, the center distance of two axis from the small to the big end increases gradually, make the transmission gearbox output shaft have two larger center distance, the transmission system of gear and gear shaft and bearing of the gear shaft radial and thrust bearings have larger installation space, it can device of the radial and thrust bearings of large size, the shaft is enough to meet the diameter of axle torque, so big torque, load bearing ability is an important characteristic of conical twin screw extruder.This parallel twin-screw extruder is incomparable.

Conical double screw extruder with the arrangement of two screw Angle, so the transmission gearbox output shaft have two larger center distance, in the gear box unit before and after the two staggered larger thrust self-aligning ball bearings, enough to stop by the axial force formed by the pressure of the nose, the characteristics of bearing capacity is big, the gearbox manufacturing cost is low, maintenance is convenient.

Design of PVC pipe extrusion

       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.

2-Twin screw extrusion

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.

3-Single screw extrusion

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.

Advantages of Single Screw Extruder

     Advantages of Single Screw Extruder

The significant advantages the Single Screw Extruder with barrier according to the present invention has the advantage that the pressure build-up capability is improved. Compared to the known Twin Screw Extruder it is not necessary any more to build up a very high pressure in the so-called feed zone as to enable a predetermined lower pressure at the end of thegravimetric feeder With the according to the present invention it is possible to substantially reduce the pressure between the feed zone and the melting zone. This in turn results in a reduction of wear of the gear pump in the transition area between the feed zone and the melting zone since it is operated with lower pressures.

(1)In a preferred embodiment the barrel comprises several grooves being equally spaced apart in circumferential direction which grooves extend preferably parallel to the longitudinal axis of the barrel. This has the advantage that the process of matching the grooves is simplified compared with the groove helically arranged.

(2)Due to the pressure reduction in the area of the feed zone it does not have to be constructed any more as complicated as in prior solutions and does not have to be provided with cooling means and a “heat separation” towards the heated melting zone. Advantageously, the feed zone and the melting zone may therefore be formed individually.

(3)In a further preferred embodiment the width and/or the depth of the groove varies in a longitudinal direction, preferably the grooves depth decreases towards the downstream end of the melting zone section, preferably to zero.

This has the advantage sheet production extruderthat the groove extends continuously without Single Screw Extruderany break along the feed zone and melting zone thereby further improving the output and the pressure build-up. 

In a further preferred embodiment of the invention at least one groove is provided also in the barrel inner surface (innerwall) in the area of the feed zone section, the groove extending parallel or helically relative to the longitudinal axis. Preferably, the groove in the area of the feed zone section leads into the groove in the area of the melting zone without transition. Preferably both grooves have the same lead angle.

What are the key differences between PVC and PE pipes and fittings?

      What are differences between PVC and PE pipes ?

PE  benefits

• Chemically inert, no off gassing or leaching.
• Flexible, can be pulled to around corners to eliminate fittings.  This give a better water flow, and saves time and money.
• Can freeze without bursting
• Is rated to last at least 200 years
• No glue to add poisonous vapors or soldering to reduce risk of fire.

Negatives

• Pipe and fittings are reduced in diameter, may need to up size for good flow.
• Pipe has no rigidity.  Must be supported it's entire length.
• Need special tools to install.

PVC and CPVC "for hot" benefits

• Any can do it, all you need is a cutter to assemble.
• Very cheap and you can get it anywhere.
• PVC is full bore, but only for cold water.

Negatives

• All sorts of leaching flavors and chemicals into water.
• Rated for 20 years, this is an over statement.
• Glue needs to cure for 24 hours before charging with pressure.  Most do not do this, but that is what is instructed.
• Freezes and bursts quite quickly.
• Some plumbers will not attach to it for it has a high failure rate.

How to Make Plastic Pipes

             How to Make Plastic Pipes

Plastic has largely become the material of choice for pipes. Its flexibility, ease of bonding, lighter weight than iron and steel and lower cost than copper has made it a common choice. Pipe is generally produced by an extrusion process. Although this process may vary in actual operating conditions (temperatures, pressures, extrusion rate) based on the type of plastic used, diameter of pipe and wall thickness, the fundamental process of extrusion is reasonably consistent throughout the industry. 

• Add the thermoplastic resin to the feed hopper. Although this can be done manually, it is generally transferred to the hopper via a vacuum feeder due to the steady state nature of the process. This helps minimize the chance of running out of resin during the process.
• Turn on machine screw and barrel heaters per specific resin requirements. The heaters add heat to the plastic while it is in the barrel to melt the plastic. The rotating screw adds shear heat to the plastic for melting as well as building up the pressure to force the plastic through the die.
• Extrude plastic through the die. The die is designed and built based on the dimensions desired in the pipe and the shrink rate of the type of plastic being used.

• Cut the pipe at the desired length. The extruded pipe will exit the die onto a roller system to allow the pipe to cool without deforming under its own weight. Once it has passed a certain length, it will trip a sensor (electric eye) triggering a cutting operation on the pipe. The cut is made by a cutter that moves forward at the rate of pipe extrusion to offset the motion of the pipe moving forward so that the end of the pipe will remain perpendicular to the pipe wall after it is cut.
• Continue the process in a steady-state environment until the desired quantity of plastic is produced.

                  How is PVC Pipe Made?

Chemical Reaction

• PVC has its origins in the chemical gas referred to as vinyl chloride. When vinyl chloride is exposed to sunlight a chemical reaction occurs. The reaction is known as polymerization, which transmutes into a whitish solid material. To achieve the shape and solidity of a PVC pipe, a series of chemicals are introduced to one another. Natural gas is heated to create ethylene. The process is referred to as cracking. Later, sodium chloride (found in the form of rock salt) is spliced using electrolysis. As a result chlorine and lye (sodium hydroxide) is produced.

Molecular bonding

• Chlorine and ethylene (natural gas heated under pressure) are introduced to make vinyl chloride monomer (VCM). The molecules are bonded from each molecule's end. The result is a long chain of polyvinyl chloride polymer. In essence, plastic is created. The polymerized plastic, called thermoplastic PVC powder (which is compounded), melted and molded into piping. The result is a tube of PVC plastic. As a result of the chemical process (PVC becomes very solid and rigid), PVC is less likely to break during earthquakes. It can withstand pressures that many metals (such as copper piping) cannot tolerate. This is why PVC is the preferred material for plumbing and underground wiring.

Manufacturing

• As small as 16 mm and as large as 630 mm tubing/pipes are manufactured using a machine called an extruder. PVC plastic is routed through a double screw stem extruder (conical twin screw) or a parallel double screw extruder. Molding determines the wall thickness of the PVC hose. The diameter of the pipe is made by the PVC pipe extruder (The standard is 1/2 to 24 inches in diameter). The production speed of the standard PVC pipe extruder is about 20 meters per minute. The PVC hose runs through a vacuum pump. A ring cutting machine is employed at the end of the assembly line to divide the PVC tubing into sections of individual pipes. The individual pipes are cooled and racked. After inspection, the completed PVC pipes are sent to the warehouse for final inspection, labeling and shipping.

The advantages of twin screw extrusion

      The advantages of twin screw extrusion

Twin screw extrusion has become the standard over time in several industries because it offers numerous advantages over single screw extrusion:

1.More consistency in production and control of product quality

2.Increased productivity due to continuous processing, faster start up and shut down between product changes, quick changeover and advanced automation

3.Greater flexibility, with the capability to process a wide range of raw materials

4.Optimized footprint thanks to energy and water savings

5.Simple and easy to maintain and clean

Twin Screw 

How does extrusion work?

                  How does extrusion work?

Extrusion processing aims to physico-chemically transform continuously viscous polymeric media and produce high quality structured products thanks to the accurate control of processing conditions.

Twin screw extruders consist of two intermeshing, co-rotating screws mounted on splined shafts in a closed barrel. Due to a wide range of screw and barrel designs, various screw profiles and process functions can be set up according to process requirements. Hence, a twin screw extruder is able to ensure transporting, compressing, mixing, cooking, shearing, heating, cooling, pumping, shaping, etc. with high level of flexibility. The major advantage of intermeshing co-rotating twin screw extruders is their remarkable mixing capability which confers exceptional characteristics to extruded products and adds significant value to processing units.

In twin screw extrusion processing, the raw materials may be solids (powders, granulates, flours), liquids, slurries, and possibly gases. Extruded products are plastics compounds, chemically modified polymers, textured food and feed products, cellulose pulps, etc.

Overview of Double Screew Extruder

     Overview of Double Screew Extruder 

Overview

Twin screw extrusion is used extensively for mixing, compounding, or reacting polymeric materials. The flexibility of twin screw extrusion equipment allows this operation to be designed specifically for the formulation being processed. For example, the two screws may be corotating or counterrotating, intermeshing or nonintermeshing. In addition, the configurations of the screws themselves may be varied using forward conveying elements, reverse conveying elements, kneading blocks, and other designs in order to achieve particular mixing characteristics.

Process Schematic

Difference between single screw and double screw Extruder

              Difference between single screw and double screw Extruder

Compared to single screw extruder, double screw extruder has below features:

1) Material flow is stable, uneasy happen cutoff or billow, productive process is reliable.2) Most heat of double screw extruder mainly come from mechanical transformation during operation, small amount of heat comes from the heating jacket. The single screw extruder often require additional equipment preheat quenched materials.3) Time distribution is relatively narrow range of material retained in the machine, which is easier to control the temperature of the material, sufficient energy utilization, yield and quality are very stable.4) Double screw extruder has a screw surface self-cleaning effect, so that the transportation of materials is stable, little residual material in cavity when finish working, and no need stop to cleaning if change the production material.5) Double screw extruder has large productivity, is appropriate for processing of materials containing high oil (> 17%) and high humidity of materials (more than 30% moisture content) Double screw extruder has complex structure, request high precision machining, therefore,  investment for production facilities is much greater if select this type, so whether select it according to the actual situation.