PE Pipe Extrusion Process (6)

Pullers

The puller must provide the necessary force to pull the pipe through the entire cooling operation. It also maintains the proper wall thickness control by providing a constant pulling rate. The rate at which the pipe is pulled, in combination with the extruder screw speed, determines the wall thickness of the finished pipe. Increasing the puller speed at a constant screw speed reduces the wall thickness, while reducing the puller speed at the same screw speed increases the wall thickness.

Standards of ASTM International and other specifications require that the pipe be marked at frequent intervals. The markings include nominal pipe size, type of plastic, SDR and/or pressure rating, and manufacturer’s name or trademark and manufacturing code. The marking is usually ink, applied to the pipe surface by an offset roller. Other marking techniques include hot stamp, ink jet and indent printing. If indent printing is used, the mark should not reduce the wall thickness to less than the minimum value for the pipe or tubing, and the long-term strength of the pipe or tubing must not be affected. The mark should also not allow leakage channels when gasket or compression fittings are used to join the pipe or tubing.

Take-off Equipment

Most pipe four inches or smaller can be coiled for handling and shipping convenience. Some manufacturers have coiled pipe as large as 6 inch. Equipment allows the pipe to be coiled in various lengths. Depending upon the pipe diameter, lengths of up to 10,000 feet are possible. This is advantageous when long uninterrupted lengths of pipe are required - for example, when installing gas and water pipes.

Saw Equipment and Bundling

Pipe four inches or more in diameter is usually cut into specified lengths for storage and shipping. Typical lengths are 40 to 50 feet, which can be shipped easily by rail or truck. The pipe is usually bundled before it is placed on the truck or railcar. Bundling provides ease of handling and safety during loading and unloading.

PE Pipe Extrusion Process (5)

Pipe Sizing

The dimensions and tolerances of the pipe are determined and set during the sizing and cooling operation. The sizing operation holds the pipe in its proper dimensions during the cooling of the molten material. For solid wall pipe, the process is accomplished by drawing the hot material from the die through a sizing sleeve and into a cooling tank. Sizing may be accomplished by using either vacuum or pressure techniques. Vacuum sizing is generally the preferred method.

In the vacuum sizing system, molten extrudate is drawn through a sizing tube or rings while its surface is cooled enough to maintain proper dimensions and a circular form. The outside surface of the pipe is held against the sizing sleeve by vacuum. After the pipe exits the vacuum sizing tank, it is moved through a second vacuum tank or a series of spray or immersion cooling tanks.

In the pressure sizing system, a positive pressure is maintained on the inside of the pipe by the use of a plug attached to the die face by a cable or, on very small bore pipe, by closing or pinching off the end of the pipe. The pressure on the outside of the pipe remains at ambient and the melt is forced against the inside of the calibration sleeve with the same results as in the vacuum system.

The production of very large diameter profile pipe, up to 10 feet in diameter, uses mandrel sizing. In one form of this process, the extruded profile is wrapped around a mandrel. As the mandrel rotates, the extruded profile is wrapped such that each turn overlaps the previous turn. In some other techniques, the turns are not overlapped. A typical profile wall PE pipe is shown in Figure 7.

Cooling

For either the vacuum or pressure sizing technique, the pipe must be cool enough so that it maintains its circularity before it exits the cooling tank. Various methods of cooling are utilized to remove the residual heat out of the PE pipe. Depending upon the pipe size, the system may use either total immersion or spray cooling. Spray cooling is usually applied to large diameter pipe where total immersion would be inconvenient. Smaller diameter pipe is usually immersed in a water bath. Cooling water temperatures are typically in the optimum range of 40° to 50°F (4° to 10°C). The total length of the cooling baths must be adequate to cool the pipe below 160°F (71°C) in order to withstand subsequent handling operations.

Residual stresses generated by the cooling process within the pipe wall are minimized by providing annealing zones.(4) These zones are spaces between the cooling baths which allow the heat contained within the inner pipe wall to radiate outward and anneal the entire pipe wall. Proper cooling bath spacing is important in controlling pipe wall stresses. Long-term pipe performance is improved when the internal pipe wall stresses are minimized.

PE Pipe Extrusion Process (3)

Extruder 

An extruder is usually described by its bore size and barrel length. Pipe extruders typically have an inside diameter of 2 to 6 inches with barrel lengths of 20 to 32 times the bore diameter. The barrel length divided by the inside diameter is referred to as the L/D ratio. An extruder with an L/D ratio of 24:1 or greater provides adequate residence time to produce a homogeneous mixture.

The extruder is used to heat the raw material and then force the resulting melted polymer through the pipe extrusion die. The barrel of the machine has a series of four to six heater bands. The temperature of each band is individually controlled by an instrumented thermocouple. During the manufacturing process, the major portion of the heat supplied to the polymer is the shear energy generated by the screw and motor drive system. This supply of heat can be further controlled by applying cooling or heating to the various barrel zones on the extruder by a series of air or water cooling systems. This is important since the amount of heat that is absorbed by the polymer needs to be closely monitored. The temperature of the extruder melted polymer is usually between 390˚F and 450˚F, and it is also under high pressure (2000 to 4000 psi).

Breaker Plate/Screen Pack

The molten polymer leaves the extruder in the form of two ribbons. It then goes through a screen pack which consists of one or more wire mesh screens, positioned against the breaker plate. The breaker plate is a perforated solid steel plate. Screen packs prevent foreign contaminants from entering the pipe wall and assist in the development of a pressure gradient along the screw. This helps to homogenize the polymer. To assist in the changing of dirty screen packs, many extruders are equipped with an automatic screen changer device. It removes the old pack while it inserts the new pack without removing the die head from the extruder.

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.

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 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.

Maintenance Method Of Plastic Extruder in Daily

Today we know the plastic extrusion machine maintenance method!

Ventilation points • plastic extrusion equipment should be placed to ensure motor heat and prolong its life; maintain good grounding of the machine.

• Regularly check tool screws, new machine using 1 hours Hou, with tool tight solid moving knife, set knife of screws, strengthening blade and turret between of fixed sex; should regularly on bearing raises lubricants, guarantee bearing between of lubrication sex; for guarantee tool incision of sharp degrees, should often check tool, guarantee its sharp degrees, reduced due to blade blunt deficiency and caused other parts of not necessary damaged; regularly check belt whether relaxation, timely adjustable tight.

• Restart-the second start, should clear the remaining scrap, reduce the starting resistance. should periodically turn inertia cover and belt pulley cover, remove flange bottom ash, Crusher discharge powder into the axle bearing.

• Replacement parts-when replacing the cutters, knives and the gap between the fixed knife: 20HP Crusher 0.8MM, 20HP Crusher 0.5MM. recycled material is thinner, the clearance can be adjusted.

Extrusion System For Plastics Extrusion Machine

Extrusion system consists of screw and barrel, Hopper, die, and die, plastics by squeezing the system plastics melt evenly, and, in the process of establishing, under pressure from continuous by screw extruder.

1, screw: is the most important parts of extruder, it is directly related to applications and productivity of extruders, made from high-strength corrosion-resistant alloy steel.

2, barrel: is a metal cylinder, generally with higher heat resistance, compressive strength, strong wear-resistant, corrosion-resistant alloy steel composite pipe made of or lined with alloy steel. Barrel and screw fit, realize the plastic pieces, softening, melting and plasticizing, vent and compaction, and uniform conveyor rubber molding system. Barrel length is 15~30 times the diameter, so that the plastic is fully heated and fully plastic into principles.

3, Hopper: hopper with cut-off device at the bottom, so you can adjust and cut off the flow, side of the hopper with sight and calibration of metering devices.

4, die and mold: head made of alloy steel and carbon steel coat, and head is equipped with mold, heads are used to rotate the plastic melts into parallel straight line movement, steady import model Kit, and endows plastics with the necessary pressure. Plastic plastic in the barrel of the compaction, porous plate along a certain flow through head neck into head mold, die mandrel set appropriate, formation cross section decreases of the annular space, plastic melt around the wire to form a continuous solid tube coating. In the guarantees head plastic reasonable channel, elimination of accumulated plastic corner, often placed with split sleeves, for elimination of plastic extrusion pressure fluctuations, there is set of grading rings. Head is also equipped with mould correction and adjustment device, easy to adjust and align core and mould sleeve of concentricity.