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. Overview
Polyethylene of Raised Temperature Resistance (abbreviated as PE-RT) is a new type of plastic piping material. Owing to its superior long-term hydrostatic strength, temperature resistance, processing flexibility, long service life, heat-fusibility, and recyclability compared to traditional PE pipes, PE-RT pipes have become a mainstay in building hot water systems and floor heating systems. In particular, their market share in floor heating piping systems is steadily increasing. To meet the demands of rapid market growth, manufacturers are adopting methods to enhance labor productivity, namely by increasing the production line speed of extrusion equipment to raise output per unit time. High-speed production lines typically operate at speeds more than double those of conventional lines (5–6 m/min). For larger pipe manufacturers in particular, reducing the number of equipment units allows for more efficient use of plant space and lowers labor costs. Fewer equipment units also facilitate easier process control, which is crucial for ensuring consistent and uniform pipe quality. Moreover, high-speed production lines consume significantly less actual power than conventional lines at high operating speeds, thus offering greater energy efficiency. The doubling of efficiency per individual machine enables pipe manufacturers to respond flexibly to ever-changing market demands, complete customer orders in the shortest possible time, reduce finished goods inventory, and accelerate capital turnover. This paper discusses how to achieve high-speed production of PE-RT floor heating pipes through technical process control, thereby improving production efficiency to meet growing market demand.
II. Process Control
Process flow:
Raw material pellets → Drying → Extruder heating → PE-RT pipe-specific die → Vacuum sizing → Cooling and shaping → Pipe printing → High-speed haul-off unit → Chipless cutter → Take-up unit → Visual inspection and dimensional check → Simple packaging → Pressure testing → Packaging upon qualification → Warehousing
Materials used for PE-RT pipes include DOWLEX 2344/2388E from Dow Chemical (USA), YUCLAIR DX800 and DX900 from SK Corporation (South Korea), SP980 from LG Chem, XP 9000 from Daelim Industrial ect. In high-speed extrusion, processing parameters vary significantly depending on the raw material used. Therefore, strict control of raw material quality is also an important guarantee for ensuring the quality and service life of high-speed produced PE-RT pipes.
For PE-RT raw materials, the highest output is achieved within a processing temperature range of 180–210 °C. In high-speed extrusion, if the temperature is too low, poor plasticization may occur, resulting in a dull, non-glossy pipe surface or melt fracture. If the temperature is too high, the extruded parison becomes too soft, and the pipe surface tends to develop fine ripple marks. Therefore, temperature fluctuations should be controlled within approximately ±5 °C. Larger deviations may cause variations in pipe wall thickness and performance.
III. PE-RT Pipe Extrusion Equipment
Since floor heating pipes are supplied in coils (typically 200–300 meters per coil), deviations in the middle sections of the pipe are not easily detectable, which may pose quality risks. PE-RT pipe production requires that the excellent properties of the raw material be "replicated" onto the finished pipe, while also achieving continuous and high-speed extrusion to produce high-quality products with favorable economic returns. This necessitates advanced equipment and high-precision control instruments. Therefore, the design of the main extruder is critically important; otherwise, both output and quality will be difficult to attain. The production line typically employs a Φ65 screw extruder, capable of achieving an output of up to 250 kg/h and stable production speeds of 15–25 m/min or more. The above design aims to optimally integrate each process from both technical and economic perspectives to achieve the best performance and cost-effectiveness. Due to the relatively high melt viscosity of PE-RT, the extruder is designed with high torque output, imported thrust bearings, and gears made of special alloys with specialized heat treatment to reduce noise and heat loss. Extrusion operations are controlled by advanced control systems to ensure stable, safe, and high-speed extrusion of PE-RT.
During PE-RT pipe processing, if the stages of melt melting, parison forming, and cooling/sizing are not properly controlled, the mechanical properties of the pipe may decline due to material degradation, molecular chain orientation, or crystallization during cooling. The plasticization of the raw material and the cooling/sizing of the parison are key factors in controlling extrusion quality. For extrusion equipment, the following points should be met:
1. Adopt low melt temperatures to minimize material degradation during melting and plasticization.
2. Ensure excellent and uniform melt properties, preventing longitudinal orientation of polymer molecular chains during parison formation.
3. Achieve uniform cooling to reduce internal stresses in the pipe.
IV. Screw and Barrel Design
Based on the characteristics of PE-RT raw materials, the screw requires a relatively large length-to-diameter (L/D) ratio, typically designed at 33:1. The screw structure adopts a two-stage mixing screw design, with mixing elements incorporated both in the middle and at the front end of the screw to avoid excessive shear on the material. The feed section of the barrel employs an independent bushing structure with cooling medium circulation and internal axial grooving. These structural designs help improve the conveying efficiency of the feed section. The screw and barrel materials are made of high-quality alloys with specialized surface treatments. The barrel and screw must be manufactured to very high precision. If precision is insufficient, based on the time-temperature equivalence principle, any stagnant material adhering to non-smooth surfaces will effectively undergo prolonged high-temperature exposure over time, potentially leading to degradation or decomposition products. Continuous production over extended periods may then result in intermittent black specks on the pipe surface, adversely affecting product quality.
V. PE-RT-Specific Die Head Design
Conventional pipe extrusion die heads, such as spider-type (mandrel-supported) dies, struggle to guarantee PE-RT pipe production due to weld-line defects in melt flow. Currently, spiral die heads are predominantly used both domestically and internationally, with a drawdown ratio typically between 1.3 and 1.6. The smoothness of the die lip and mandrel surface directly influences the pipe's surface appearance. Issues such as output and quality instability can lead to scratches, false cracks caused by water-cooling, or melt accumulation on the pipe surface. Under prolonged high-temperature and high-pressure service, these surface imperfections can become initiation points for burst failures, thereby shortening the product's service life.
In practice, a die head design more suitable for PE-RT extrusion has been developed, referred to as the PE-RT-specific die head. When the melt flow from the extruder enters the die head, it first passes through a star-shaped flow divider, splitting into multiple streams, which then recombine and pass through a spiral distributor to form an annular tubular melt flow. This flow then passes through an annular filtration device, undergoes gradual compression after recombination, and finally enters the forming channel composed of the mandrel and die lip to be extruded as a parison. Through multiple splitting and recombining steps, the melt achieves a high degree of homogeneous plasticization. Practical experience has demonstrated that this die head enables the "low-temperature, low-pressure" extrusion advocated for PE-RT by advanced international production lines, and its design inherently prevents material degradation due to excessive shear, making it particularly suitable for high-speed production.
VI. High-Efficiency Sizing and Cooling System
The basic configuration of a PE-RT pipe production line is similar to that of general polyolefin pipe lines, with the primary difference lying in the structure of the cooling and sizing section.
Due to the high viscosity of PE-RT melt, a pre-cooling ring is typically installed before the sizing sleeve to facilitate smooth entry of the parison. Additionally, because PE-RT has a high enthalpy, a combined sleeve-type and disc-type cooling and sizing unit can be employed to accelerate cooling. The structure, machining precision, and cooling method of the sizing sleeve are critical to the quality of PE-RT pipe processing. A disc-type water-ring sizing sleeve (where cooling water circulates around the sizing sleeve without directly contacting the pipe) enables rapid production of thin-walled (cold water) pipes in the short term. However, since the material enters the sizing sleeve at a relatively high temperature and with high viscosity, prolonged use of a conventional water-film-type sizing sleeve may result in gradual roughening of the pipe's outer surface or even pipe breakage. Therefore, for PE-RT pipe production, a water-curtain disc-type sizing sleeve is recommended. This design provides pre-cooling of the pipe before it enters the sizing sleeve and also prevents material adhesion and blockage at the sizing sleeve inlet. Given the relatively high enthalpy of PE-RT material, especially during high-speed pipe production, a longer total cooling length is required to meet the higher extrusion speeds. Typically, the total cooling length of the production line exceeds 23 meters, comprising one vacuum sizing tank and two cooling water troughs. This cooling length configuration is designed to provide sufficient cooling capacity for producing 20 x 2.0 mm pipe at linear speeds exceeding 15 m/min.
VII. Conclusion
To ensure both output and quality in high-speed PE-RT pipe production, it is essential to employ advanced, high-precision equipment and tooling, adopt rational and appropriate sizing methods and sound process controls, and enforce strict quality control measures in conjunction with high-quality raw materials. By improving labor productivity, manufacturers can achieve lower processing costs and higher output, thereby offering more competitively priced products to meet the continuously expanding market demand.
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