Plastic Pipe Extrusion Process Analysis

1,Process

At present, the largest domestic and foreign output is PVC and PO pipe. Figure 1 is the flow chart of polyolefin (PO) pipe production process. The plastic is squeezed evenly plasticized melt into the annular gap mold 2 by the screw, and the tube is extruded. Then the tube is cooled and finalized by the sizing device 3, and then fully cooled in the cooling groove 4. The tube is pulled out at a uniform speed by the adjustable tractor 6, and finally cut off by the cutting machine 7 according to the specified length. Thin-wall pipe with diameter below 160mm can be rewound into disk by winding machine 9.

1—Extruder；2—Pipe mold；3—Calibrator；
4—Vacuum tank；5—Cooling tank；
6—Haul off unit；7—Cutter；
8—Laser printer；9—Winder

2,Process and equipment

The production of PO pipe should be selected for high-efficiency single-screw extruder, the feeding area using axial slotted barrel, screw length-diameter ratio of 30 to 33D, screw structure for the two-stage variable depth variable pitch screw, feeding section pitch less than the screw diameter, melt section and homogenization section pitch greater than the diameter of the screw, in order to strengthen the mixing effect, some screws are equipped with additional mixing head components. These single-screw extruders have higher output and better plasticization, for example, the output of a single-screw extruder with a screw diameter of 90 mm can reach more than 600 kg per table. In addition, the production of filled calcium carbonate, barium sulfate, wood flour, glass fiber PO pipe is generally used in the same direction twin-screw extruder.
The production of PVC pipe, basically the same as PO pipe, is commonly used with anisotropic conical or parallel twin-screw extruders, and dry mixed powders can be extruded directly. Their output ranges from 10kg/h for small pipe diameters of 1100kg/h for large diameter pipes, with a length-to-diameter ratio of 25D replacing the old 18D screw. For example, a twin-screw extruder with a screw diameter of 90mm can produce more than 300kg per hour.
As can be seen from Figure 1, the extrusion auxiliary machine is mainly composed of cooling water tank,haul off unit, cutting machine, winding machine or stacker, etc.

Cooling tank

When the pipe comes out of the sizing and cooling device, it is not completely cooled to room temperature. If it does not continue cooling, the temperature gradient existing in the radial direction of its wall thickness will make the original hardened surface layer soften due to temperature rise and cause deformation. On the other hand, the pipe has to bear the stress of traction, cutting, winding and other devices, therefore, the residual heat must be excluded and the tube cooled to room temperature. Cooling methods are water bath method and spray method. The water level of the water bath cooling water tank should be completely submerged in the tube, the tank is separated by a number of gears, the cooling water inlet is set in the last gear, so that the water flow direction is opposite to the pipe extrusion direction, in order to form a cooling temperature gradient, the pipe cooling is more gentle, the internal stress is relatively small. The distance between the cooling water bath and the shaping cooling device should not exceed one-tenth of the total length of the cooling, otherwise it will easily lead to an increase in the temperature of the outer wall of the pipe and the cooling water. Although the water bath method is a simple method, the cooling water tank is different due to the upper and lower water temperature, resulting in uneven cooling and bending of the pipe. In addition, the pipe is easily deformed by buoyancy, especially not suitable for cooling of large diameter pipes.

Haul off unit

The role of the pipe haul-off machine is to provide a certain hauling speed and traction force to the shaped and cooled pipe, to overcome the friction generated by the shaping device on the pipe, and to lead the pipe out to the winder or turning table at a constant speed. The traction machine is one of the key equipment of the pipe auxiliary machine, and the traction machine must meet the following requirements.
（1）Traction speed must be able to stepless and smooth speed regulation, and to ensure constant speed traction, unstable traction speed will make the pipe change from big to small. The traction speed should be closely matched with the extrusion speed, by adjusting the traction speed to adjust the wall thickness of the pipe, the slower the traction speed, the thicker the pipe wall, the faster the traction speed, the thinner the pipe wall. Selecting the correct traction speed is an effective way to ensure the product size qualification rate. Now the maximum speed of traction speed is designed up to 30m/min.
（2）Should maintain a constant traction force, can not have a push and pull phenomenon, otherwise it will cause pipe surface corrugation defects. There should also be sufficient traction force, the size of the traction force increases with the increase in contact area and shaped radial pressure. Small and medium diameter pipe traction force is generally 100 ~ 600N, large diameter pipe is generally 800 ~ 10000N.
（3）The clamping force of the tractor gripper should be adjustable and can clamp a variety of pipe calibers, and will not lead to damage and deformation of the surface of the pipe, at present, the use of crawler gripper more. It is composed of 2 to 12 tracks, evenly set around the pipe, the tracks are embedded with a certain number of rubber clamping blocks, mostly made into a concave shape or with a certain angle to increase the area of radial pressure on the pipe. Clamping force adjustment is achieved by pneumatic, hydraulic or screw nut mechanism. The larger the diameter of the pipe, the greater the number of tracks.

Cutter

The pipe will be hauled to a certain length and then must be cut at a fixed length. There are various types of cutting machines, and the cutting type is selected according to the pipe diameter and wall thickness, type of raw material, cutting length, cutting quality, cutting form, etc. Gate automatic cutting machine and circular radial sawing machine are suitable for cutting small and medium diameter pipes; planetary automatic cutting machine is suitable for large pipe diameter pipes. When commanded to cut, the cutter is clamped by the clamps and moves in the direction of pipe traction while completing the action of cutting the pipe, and is pushed by the pneumatic mechanism to reset when the pipe is cut.
Winding machine and turning frame. In the bending does not produce deformation of the pipe before using the winding machine to wind into bundles, automatic cutting, automatic unloading. Winding line speed and extrusion speed synchronization, and can be stepless speed adjustment, extrusion speed of less than 2m min, the general use of single-station winding machine; when the extrusion speed is greater than 2m / min, should use double-station winding machine, or even multi-station winding machine.

3,Pipe defect control elements

The main points of the factors related to the exclusion of quality defects are listed in the order in the process flow diagram in Figure 1, respectively.
（1）The feeding part: raw material formula; shape and size of raw material; coloring method; drying method; blending edge recycling material; auxiliary types and their measurement; hopper seat cooling capacity.
（2）Extruder: screw diameter; screw L/D ratio; screw compression ratio; screw structure form; screw speed; exhaust performance; control of screw temperature; barrel heating and cooling control; temperature of each section of the extruder; extruder torque; power consumption; head splice; temperature selection and control of screen changer; type and specification of filter.
（3）Mold: mouth mold clearance; molding section length; mold structure form; flow channel shape; melt distribution state; temperature setting and control; head pressure; control of wall thickness.
（4）Sizing and cooling: sizing method; sizer size; vacuum box vacuum or internal pressure sizing tube air pressure and length; sealing of sizing system; sizing time; cooling water flow; cooling water temperature; cooling mode.
（5）Cooling water tank: cooling method; arrangement of water pipes; cooling water flow; length of cooling water tank; temperature of cooling water; cooling effect of pipes.
（6）Haul off unit: traction speed and control; calibration of traction force; clamping force and control; number of tracks and effective length; surface hardness of clamping block and its shape; traction contact surface.
（7）Cutting machine: cutting machine form; saw blade tooth shape and material; reset mechanism; chip collection and dust removal; noise control; setting of limit system; clamping mechanism; transmission system and power; automatic cutting action system.
（8）Winder and rewinder: tension control; fixed length command system; winder station selection; winding diameter; rewinding action indication; winding line speed.
（9）Automated production conditions: wall thickness measurement; outside diameter control; measurement of weight and its output statistics; working condition of gear melt pump; mold alignment system.

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