Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with over 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.
I. Process Flow
Currently, PVC and PO pipes are the highest volume products both domestically and internationally. Figure 1 shows the production process flow diagram for polyolefin (PO) pipes. Plastic is fed into the annular gap die (2) by the screw as a uniformly plasticized melt, extruding a pipe parison. This immediately enters the calibration unit (3) for cooling and sizing, then undergoes further sufficient cooling in the cooling tank (4). The pipe is pulled out at a constant speed by the adjustable haul-off unit (6), and finally cut to specified lengths by the cutter (7). Thin-walled pipes with diameters below 160mm can be coiled into rolls by the winder (9).
Figure 1 Schematic Diagram of PO Pipe Production Process Flow
1---Extruder; 2---Pipe Die; 3---Calibration Sleeve;
4---Vacuum Calibration Cooling; 5---Cooling Tank;
6---Haul-off Unit; 7---Cutter;
8---Printer; 9---Coiler
II. Process and Equipment
For PO pipe production, a high-efficiency single-screw extruder should be selected. The feed zone employs an axially grooved barrel. The screw length-to-diameter (L/D) ratio is 30–33:1. The screw structure is a two-stage design with varying depth and pitch: the pitch in the feed section is less than the screw diameter, while the pitch in the melting and homogenizing sections is greater than the screw diameter. To enhance mixing efficiency, some screws are equipped with additional mixing head elements. This type of single-screw extruder offers relatively high output and good plasticization. For example, a single-screw extruder with a 90mm screw diameter can achieve an output exceeding 600 kg/h. Furthermore, production of PO pipes filled with calcium carbonate, barium sulfate, wood flour, or glass fibers typically employs co-rotating twin-screw extruders.
The production of PVC pipes is fundamentally similar to that of PO pipes. Commonly used are counter-rotating conical or parallel twin-screw extruders, allowing direct extrusion of dry blend powder. Their output ranges from 10 kg/h for small-diameter pipes to 1100 kg/h for large-diameter pipes. The screw L/D ratio has increased from the past 18:1 to 25:1. For instance, a twin-screw extruder with a 90mm screw diameter can achieve an output exceeding 300 kg/h.
As can be seen from Figure 1, the pipe extrusion downstream equipment mainly consists of the cooling water tank, haul-off unit, cutter, coiler, or dump table.
When the pipe exits the calibration and cooling unit, it is not fully cooled to room temperature. If not further cooled, the temperature gradient existing in the radial direction of the pipe wall can cause the already hardened outer layer to soften again due to temperature rise, leading to deformation. On the other hand, the pipe must also withstand stresses from the haul-off, cutting, and winding equipment. Therefore, residual heat must be removed, cooling the pipe to room temperature. Cooling methods include water bath and spray cooling. In a water bath cooling tank, the water level should completely submerge the pipe. The tank is partitioned into several sections. The cooling water inlet is set at the last section, causing the water flow direction to be opposite to the pipe extrusion direction, creating a cooling temperature gradient. This results in more gradual pipe cooling and lower internal stress. The distance between the cooling water tank and the calibration/cooling unit should not exceed one-tenth of the total cooling length; otherwise, the temperature difference between the pipe outer wall and the cooling water may increase excessively. Although the water bath method is simple, temperature differences between upper and lower water layers in the tank can cause uneven cooling and bending of the pipe. Additionally, buoyancy forces acting on the pipe can easily cause deformation, making this method particularly unsuitable for cooling large-diameter pipes.
The function of the pipe haul-off unit is to provide a certain haul-off speed and force to the already sized and cooled pipe, overcoming the friction force generated by the sizing device on the pipe, thereby drawing the pipe out at a constant speed to the winder or dump table. The haul-off unit is one of the key pieces of downstream equipment for pipe production and must meet the following requirements.
(1) The haul-off speed must be capable of stepless and smooth adjustment, ensuring constant speed haul-off. Unstable haul-off speed will cause the pipe diameter to fluctuate. The haul-off speed must be closely coordinated with the extrusion speed. Pipe wall thickness is adjusted by regulating the haul-off speed: slower haul-off speed results in thicker walls, faster speed results in thinner walls. Selecting the correct haul-off speed is an effective method to ensure product dimensional conformity. Modern designs can achieve maximum haul-off speeds up to 30 m/min.
(2) A constant haul-off force must be maintained, without any push-pull phenomenon, otherwise it can cause surface waviness defects on the pipe. Sufficient haul-off force is also required. The required haul-off force increases with contact area and sizing radial pressure. For small and medium-diameter pipes, the haul-off force is generally 100–600 N; for large-diameter pipes, it is generally 800–10,000 N.
(3) The clamping force of the haul-off unit gripper should be adjustable and capable of gripping pipes of various diameters without causing surface damage or deformation. Currently, caterpillar-type grippers are widely used. These consist of 2 to 12 tracks evenly arranged around the pipe. The tracks are embedded with a certain number of rubber/plastic clamping blocks, mostly concave or angled to increase the area applying radial pressure on the pipe. Clamping force adjustment is achieved via pneumatic, hydraulic, or lead screw-nut mechanisms. The number of tracks increases with pipe diameter.
After the pipe is hauled off to a certain length, it must be cut to length. Cutters come in various types, selected based on pipe diameter and wall thickness, material type, cut length, cut quality, cutting form, etc. Guillotine-type automatic cutters and circular radial saws are suitable for cutting small and medium-diameter pipes; planetary automatic cutters are suitable for large-diameter pipes. Upon receiving the cut command, the cutter grips the pipe with a clamp and moves in the pipe haul-off direction while completing the cutting action. After cutting, a pneumatic mechanism pushes it back to the reset position.
Coiler and Dump Table. Only pipes that do not deform during bending are wound into coils using a winder, with automatic cutting and unloading. The winding linear speed is synchronized with the extrusion speed and can be steplessly adjusted. When the extrusion speed is less than 2 m/min, a single-station winder is generally used; when the extrusion speed exceeds 2 m/min, a dual-station or even multi-station winder should be used.
III. Key Factors for Controlling Pipe Defects
Following the order of the process flow in Figure 1, the key related factors for eliminating quality defects are listed below.
(1) Feeding Section: Raw material formulation; shape and size of raw materials; coloring method; drying method; blending of regrind/recycled material; types and metering of additives; cooling capacity of the hopper throat.
(2) Extruder: Screw diameter; screw L/D ratio; screw compression ratio; screw structure type; screw speed; venting performance; screw temperature control; barrel heating and cooling control; temperature profiles along extruder zones; extruder torque; power consumption; adapter; temperature selection and control for screen changer; screen pack type and specification.
(3) Die: Die gap; land length; die structure type; flow channel shape; melt distribution; temperature setting and control; die head pressure; wall thickness control.
(4) Calibration & Cooling: Calibration method; calibrator dimensions; vacuum box vacuum level or internal pressure calibration tube air pressure and length; sealing of the calibration system; calibration time; cooling water flow rate; cooling water temperature; cooling method.
(5) Cooling Water Tank: Cooling method; water pipe layout; cooling water flow rate; cooling tank length; cooling water temperature; pipe cooling effectiveness.
(6) Haul-off Unit: Haul-off speed and control; haul-off force calibration; clamping force and control; number of tracks and effective length; surface hardness and shape of clamping blocks; haul-off contact surface.
(7) Cutter: Cutter type; saw blade tooth profile and material; reset mechanism; chip collection and dust removal; noise control; limit system setup; clamping mechanism; drive system and power; automatic cutting action system.
(8) Coiler & Dump Table: Tension control; length-cut command system; winder station selection; winding diameter; dump action indication; winding linear speed.
(9) Appendix: Conditions for Automated Production: Wall thickness measurement; outer diameter control; weight measurement and production statistics; gear melt pump operation status; die centering system.
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