OVERVIEW
Definition of pipe
There are different definitions that are usually associated with the different industries (plumbing, gas transmission lines, beverages, medical, mining, and so on). A popular definition of a pipe is that it is rigid, hollow,long and larger in diameter than tubes. Tubings are basically the same except they are flexible and smaller in diameter, up to 0.13mm (0.5in) or with thin walls up to 15cm(6in).
Main devices of pipe extrusion lines
The basic in-line process (or train) for the production of pipe starts with material handling up-stream from the extruder, the extruder and die, devices for the outside and inside calibration of the pipes cross-sectional area, if required use of air or nitrogen pressure and/or vacuum, wall thickness measuring device, cooling tank, and automatic cutting with pallet equipment for rigid pipe or windup unit for flexible pipe. The line could include a marking device, testing device, etc. An important requirement is to cool the extrudate rather fast near the die while keeping control of dimensions and properties.
Important down-stream equipment used in pipe and profile extrusion
Example of a complete extrusion pipe line
calibration system for extrusion
- The above picture shows a calibration system for a pipe or tube extrusion line.The top left schematic shows a vacuum tank calibration with cascading,temperature controlled water baths where (a) is the pipe die, (b) vacuum with disks, (c) cascaded heated zone water baths, and (d) caterpillar take off puller. The top right schematic shows pressure calibration using a plug insert with water spray cooling where (a) is the pipe die, 03) water cooled calibrator (Fig. 13.101, (c) water spray cooling, (d) caterpillar take-off puller, and (e) plug insert to retain internal pressure that helps to control the pipes OD and ID. The lower schematic shows an example of differential pressure sizing for flexible tubing.
- If there is enough production demand and the extruder has the output capacity, it is common to use multiple dies, each die produces a pipe that is usually of different sizes.
Simplified schematic of tube extrusion
Plastic Materials For Extrusion
The largest and major consumption is of PVCs that is almost double to the next which is HDPEs. Others, going from the highest to the lowest consumption, are ABSs, PSs, TPEs, PCs, PPO/PPEs, PAs, ABS alloys, POMs, PBs, and PVDFs. CPVCs are included in the PVC group and XLPE in HDPEs.
Extrusion for prosessing PVC
In regard to PVC compounds, the single screw extruder predominantly uses granular form but can also process powders and flakes. The twin screw extruders principally handle powders. PVC dry blend mixes allow the processor to take advantage of using their own compounding formuations which should provide cost advantages.
Extrusion for prosessing HDPE
HDPE is used in various applications, such as pressurized gas and water pipe lines that require high mechanical strengths. When processing PE, it requires operating at the lowest possible melt temperatures while maintaining a high level of thermal-mechanical homogeneity in the melt.
Extrusion for prosessing NYLON
General nylon is extruded at as low a melt temperature as possible to provide sufficient melt viscosity and to minimize thermal degradation.Care should be taken to start-up the extruder with its output end at temperatures at -4-10°C (25-50°F) above its melting point.
Extrusion for prosessing TPUR
The toughness, flexibility, and low-temperature properties of TI'poyurethanes (TPUR) elastomers make them suitable for reinforced and unreinforced tubing and hose. Uses include lightweight fire hose,
unreinforced tubing for low- pressure pneumatic system, etc.
When processing TP polyester plastic, control of the melt temperature is important. If too high, it can reduce the melt strength significantly.
The result would be irregular wall thickness. Too low a melt temperature can result in a poor tube finish, uneven dimensions, and weld lines. Melt
and water bath temperatures are most important because polyester plastic is a crystalline material with a relatively narrow melt-freeze range.
Extrusion for Adding fillers
Fillers and reinforcements provide a means to change and improve the performance of plastics during processing and also in service. Perhaps
the most important reason for their use concerns savings in the cost of plastic compound. The most popular fillers used are calcium carbonate,talcum, barium sulfate, and wood dust. The number one in reinforcing fillers is short glass fibers 13, 431. Also, adding fillers and reinforcements to the different TI' materials provides similar gains.
Processing Characteristics
Extrusion of process route
Most pipe is made by straight through extrusion, in-line with the direction of extrusion, die, sizing or calibrating device or tank (Fig. 1.1), water
cooling tank (Fig. 1.2), conveyor (Fig. 1.3), cutter, if needed, and take off equipment at the end of the line.
Methods for size of pipe for extrusion
There are different techniques used to control and calibrate the size of the pipe. Basically, there are the free flow usually used for small pipe/tube and the others use sizing devices/dies that arebasically calibrating devices as summarized in Figs. 1.4-1.5.
1.1 Example of Gatto/Conair's vacuum sizing tank
1.2 Example of Gatto/ Conair's water and spray tanks .
The calibration methods are in most cases used in combination with a circulating water trough/ tank cooling system. The main advantage is that the still soft, moldable extruded profile is drawn down or pressed by vacuum and/or air pressure to the wall of the sizing/calibrating device.
Nitrogen can be used instead of air. Simultaneously, the hot melt is being cooled. Regardless of the approach used, the target is to use as little plastics as possible, only use what is needed.
1.3 Example of Gatto/Cnair's take -away conveyor
1.4 Pipe sizings
1.5 Sophisticated sizing systems
● Sizing Systems For Extrusion
Figure 1.5 shows different sizing systems with: (a) vacuum trough sizings/calibrations, using calibrated disks; (b) vacuum trough sizing/ calibration, using a perforated sleeve housing; (c) sizing/calibrating system that combines controlled water (temperature and pressure) withontrolled vacuum; and (d) sizing /calibrating .cusing just vacuum (profile shape not pipe).
There are different styles of plates, disks, or sleeves used to provide the sizing or calibration of the pipe. The plates and/or disks can be stacked.
Each should contain precisely drilled, usually tapered holes to meet the pipe OD configuration that should match the drawdown ratio. These holes will usually be made larger to compensate for the final shrinkage of the extrudate after it cools and probably added shrinkage after in torage. Spacing between them provides a simple smeans of adjusting the stacked system to meet the drawdown ratio. With the sleeve system, as shown in Fig.1.5, different designs are used that can include openings to permit the water and/or differential air pressure in making contact.
● Cut-off Unit For Extrusion
Another important aspect in the extrusion line is the precision cutting of the pipe when it is not going around a wrap-up reel. To meet the different requirements, since the plastic properties influence how the pipe is to be cut, different cutters are available that travel with the line speed during its cutting operation. After cutting, They quickly return to their original position to make the next cut with the pipe.
Example is a planetary fixed -length cutter
Process Optimization For Extrusion
It is essential that the length of this cooling-calibrating system is cor-rectly selected. If the length is too short and the extruded pipe is still soft,the pipe may be easily deformed. However, if the cooling system is longer than necessary, the friction between plastic and metal increases and pulling the pipe through the device will be more difficult or cause the line to stop.
The length of the cooling system's trough or sizing unit is determined arbitrarily in practice or based on experience. If the cooling is not satisfactory, the rate of the water required to flow will have to be increased (at a cost). If too long, water is being wasted.
Die For Extrusion
The important features of the die system are as follows: (1)the extruder adapter and die are designed to target for a smooth streamline melt flow;(2) the mandrel's supporting spiders/webs should be of smooth sym-metrical (airfoil/streamline shape) profile beginning and ending with near knife edges; (3) the holdup area of the spiders should be minimized;(4) the cross-sectionalarea between the supporting spiders of the mandrel should be at least three times greater than the cross-sectional area of the annulus/ring; (5) the approach channel to the final (relatively) parallel lands should taper gradually to maintain melt compression and assist in welding together the melt streams after passing the mandrel spiders; (6)the die lands must be long enough to exert back pressure on the melt and to help eliminate weld lines; (7a) the mandrel tip and the plane of the die face must be in-line; (7b)if the mandrel is projecting or recessed, drag marks may be apparent in the bore of the extruded tube; (7c) drag marks should be avoided since many plastics are highly notch sensitive; (8a)there must be provisions for applying air through the mandrel or alterna-tively a vacuum chamber for use with sizing devices;(8b) an easy acces- sible hole of about 6.4mm (0.25in) in diameter is usually sufficient; (812)during processing be careful to keep this hole clean, it is not to be partially or completely plugged; and (9) the die should be snug enough to prevent leaking, yet free enough to be moved for adjustments.
Applications For Extrusion Pipe Lines
There are many different ways pipe and tube products used in moving liquids, gases, solids, and so forth. They can be shaped to provide decorations, safety supports, and so on.
Round Dripper Inlet Irrigation Pipe Extrusion Line
