Discussion on High Speed Extrusion Production Technology of PE-RT Floor Heating Pipe

1.      Summary

Polyethylene of heated temperature resistance, abbreviated as PE-RT. As a new type of plastic pipe, PE-RT pipe has become the main force in building hot water system and floor heating system with its excellent long-term hydrostatic strength, temperature resistance, processing flexibility, long life, hot melt and recyclability, which is better than traditional PE pipe. Especially, its market share in floor heating pipe system is rising steadily. In order to meet the needs of the rapid growth of market, the enterprise adopts the method of improving labor production efficiency, which is increasing the production line speed of extrusion equipment to improve the output per unit time. The general production speed of high-speed production line is more than twice that of ordinary line (5-6m / min). Especially for large-scale pipe production enterprises, reducing the number of equipment means that the plant can be used more effectively and the labor cost can be reduced. The reduction of the number of equipment can also make the production process easy to control, which is very important to ensure the uniformity of pipe quality. Moreover, the actual power consumption of high-speed production line is much smaller than that of ordinary production line at high speed, which is more energy-saving. The efficiency of single equipment is doubled, which is conducive to pipe enterprises to adapt to the ever-changing requirements of the market, complete customer orders in the shortest time, reduce the inventory of finished products and speed up the turnover of funds. This paper will discuss how to carry out the technical control of the production process and realize the high-speed production of PE-RT floor heating pipes, so as to improve the production efficiency and meet the growing market demand.

2.      Process control

process flow: granular raw materials → drying → extruder heating → special die for PE-RT pipe → vacuum sizing → cooling and setting → pipe printing → high-speed tractor → chipless cutting machine →coiler → inspection of appearance and size → simple packaging → pressure test → packaging after passing the test → warehousing

During high-speed extrusion, there are obvious differences in processing parameters due to the differences of raw materials. Therefore, strictly controlling the quality of raw materials is also an important guarantee to ensure the quality and service life of PE-RT pipes produced at high speed.

The processing temperature of PE-RT raw material is between 180-210 ℃, and the output is the highest. During high-speed extrusion, if the temperature is low, the plasticization is not good, the appearance of the pipe is not smooth, there is no brightness or melt breakage; when the temperature is too high, the export blank is too soft and small marks are easy to appear on the pipe surface. Therefore, the fluctuation range of temperature is controlled at about 5 ℃. When the difference is too large, the thickness and performance of the pipe will change.

3.  PE-RT pipe extrusion equipment

Because the floor heating pipe is a coil, a bundle of 200-300m, and the deviation of the middle pipe is not easy to find, there will be potential quality hazards. PE-RT pipe production requires to "copy" the good performance of raw materials to the pipe, and hope to realize continuous and rapid extrusion, so as to produce high-quality products and better economic benefits. It must require advanced equipment and high accuracy of control instruments. Therefore, the host design is very important, otherwise the output and quality are difficult to meet the requirements. The production line host usually adopts Φ 63 screw, the output can reach 300kg / h, and the stable production speed can reach more than 15-25m / min. The above design strives to perfectly combine all processes from the perspective of technology and economy, so as to achieve the best performance and cost performance. Due to the high viscosity of PE-RT melt, the extruder adopts high torque output, imported thrust bearing, and the gear is made of special alloy and special heat treatment, which reduces noise and heat loss; the extrusion operation adopts advanced control system to ensure the stable, safe and high-speed extrusion of PE-RT.

In the process of PE-RT pipe processing, if the control is poor in the process of raw material melting, pipe blank forming, cooling sizing, etc., the mechanical properties of the pipe will be reduced due to the decomposition of raw materials, the orientation of molecular chains or the crystallization during cooling. The plasticization of raw materials and the cooling sizing of tube blank are the key to control the extrusion quality. For extrusion equipment, the following points should be met:

(1) Low melt temperature should be adopted to minimize the decomposition of raw materials in the process of melting and plasticization.

(2) The melt has excellent and uniform properties, and the polymer molecular chain is not oriented along the longitudinal direction in the tube blank forming process.

(3) Achieve uniform cooling and reduce the internal stress of the pipe.

4.      The screw barrel is designed

According to the characteristics of PE-RT pipe raw materials, and the length diameter ratio of the screw is relatively large, which is usually designed as 36:1. The double-stage mixing screw is adopted in the screw structure, and there is a mixing element in the middle and head of the screw to avoid excessive shearing of the material; the barrel of the feeding section adopts an independent bushing structure, the cooling medium is cooled, and the inner surface is axially slotted. These structural designs help to improve the conveying efficiency of the feeding section; the screw barrel is made of high-quality alloy and the surface is specially treated. The barrel screw shall ensure high accuracy. If the accuracy is poor, according to the time-temperature equivalence principle, if there is residue adhesion in unsmooth places, the temperature will be greatly increased after a long time, degradation or decomposition products will appear after a long continuous production time, and there will be intermittent small black spots on the pipe surface, affecting the product quality.

5.  Design of PE-RT special die

Ordinary extrusion die support mandrel die is difficult to ensure the production of PE-RT pipes due to the fusion defect of material flow. At present, spiral dies are mostly used at home and abroad, and the tensile expansion ratio is between 1.3-1.6. The smoothness of the die and mandrel directly affects the appearance of the pipe. For example, due to the unstable output and quality, the pipe is prone to scratch and false crack and melt accumulation caused by water-cooling molding. After long-term use of high temperature and pressure, the pipe will cause blasting points from the marks and affect the service life of the product. In practice, a die suitable for PE-RT extrusion is designed, which is called PE-RT special die. When the material flow from the extruder enters the die head, it is first divided into multiple strands through the quincunx split shuttle, and then converged to form an annular tubular material flow through the spiral split fluid. After converging through the annular filter device, the material is gradually compressed, and finally enters the forming channel composed of mandrel and die to form pipe blank extrusion. The material flow is divided and converged for many times, the practice of high homogenization and plasticization shows that the die can realize PE-RT "low temperature and low pressure" extrusion advocated by foreign advanced production lines, which ensures that the material will not degrade due to excessive shear, and is more suitable for high-speed production.

6.  High efficiency sizing cooling system

PE-RT pipe production line is basically the same as general polyolefin pipe production line, and the main difference is that the structure of cooling sizing part is different.

Due to the high viscosity of PE-RT melt, in order to make the pipe blank enter the sizing sleeve smoothly, a precooling ring is generally added before the sizing sleeve. In addition, due to the high enthalpy of PE-RT, in order to speed up the cooling speed, the combined cooling and shaping device of sleeve type and disc type can be used. The structure, machining accuracy and cooling mode of sizing sleeve are very necessary for the quality of PE-RT pipe. The disc water ring sizing sleeve (the cooling water circulates the sizing water sleeve, and the cooling water does not contact the pipe) can produce thin-walled (cold water) pipes in a short time. However, due to the high temperature and high viscosity when the materials enter the sizing sleeve, the traditional water film sizing sleeve will gradually rough or even break the appearance of the pipe in long-term production. Therefore, the production of PE-RT pipe, the water curtain disc sizing sleeve is adopted to pre cool the pipe before entering the sizing sleeve. In addition, it also prevents the material from adhering to the inlet of the sizing sleeve and causing blockage. The enthalpy value of PE-RT material is large, especially when making pipes at high speed. In order to meet the faster extrusion speed, it is necessary to configure a longer sizing cooling length. Generally, the total cooling length of the production line is more than 23 meters, including one vacuum sizing machine and two cooling water tanks. The purpose of cooling length configuration is to meet the cooling capacity required when producing 20x2.0mm pipe at a linear speed of more than 15m / min.

7.  Conclusion

In order to ensure the output and quality of PE-RT pipe high-speed production, we must adopt more advanced and high-precision equipment and molds, adopt more reasonable and applicable sizing methods and good process control, and adopt strict quality control means and high-quality raw materials to produce high-quality products at high speed. By improving labor productivity, we can obtain lower production and processing costs and higher output, make the price more reasonable, and meet the expanding demand of the market.