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Practical use of energy-efficient extrusion technology in sheet and plate production lines

Author:孙锋林 ComeFrom: Date:2019/11/8 9:50:25 Hits:1783

This article introduces the actual use of high-efficiency and energy-saving extrusion technology in the sheet and plate production line in the past ten years in detail, and compares the advantages and disadvantages of various technologies. It has certain reference significance for manufacturers or users of this type of production line. .
Extrusion technology has been applied to thermoplastics since 1935. It has a history of more than 80 years. With the rapid advancement of the chemical industry and the continuous emergence of new thermoplastic materials, extrusion technology has also experienced multiple technical iterations, and its products are more extensive. It is used in many fields such as daily life, national defense industry, aerospace and other fields. With the large-scale rise of the plastics industry, its energy consumption efficiency has become more and more concerned. High efficiency, energy saving, large output, and automation are the three major concerns of the plastic extrusion processing industry today, especially energy efficiency, which is in line with national energy conservation. The emission reduction policy is of particular concern to the plastic processing industry. This article focuses on the practical application of high-efficiency and energy-saving extrusion technology in sheet production lines, and compares the advantages and disadvantages of various technologies. Reference meaning.

One. Extruder drive system

During the extrusion plasticizing process of the extruder, 10% -25% of the energy comes from the external heating coil (or heat transfer oil) heating, and the remaining energy mainly comes from the drive system of the extruder, which is the conversion of the mechanical energy of the motor It becomes the heat energy for plasticizing (it may be frictional heat or shear heat). At present, the mainstream structure is that a reduction gear is driven by an AC (direct) flow motor, and the screw is rotated after deceleration through the reduction gear. In this subsystem, the transmission efficiency of the motor and the reduction box is the focus of our attention, but we often only pay attention to whether the speed ratio selection is appropriate, and ignore the efficiency of the motor and the efficiency of the reduction box. The efficiency of domestic small and medium-sized AC motors (three-phase asynchronous) is 87%, while the frequency conversion motors can reach 90%, and the efficiency of foreign advanced motors can reach 92%. The transmission efficiency of reduction gearboxes is generally overlooked. The main reason for this is that most people do not seem to have other better replacement parts to replace the reduction gearbox. Different transmission ratios have slightly different transmission efficiency, and the general transmission efficiency can reach more than 95%. After reading the above data, we immediately realized that many common parts and components actually have a lot of room for efficiency improvement, but the improvement of efficiency means the increase of procurement costs, but the bigger problem is that equipment suppliers are likely to For the competitiveness of the equipment, this knowledge is not introduced to customers or expensive but energy-saving components are used. The emergence of direct-drive motors has changed the replacement problem of this subsystem. In addition to high prices, direct-drive motors have greatly improved their efficiency, reaching about 95%. But if it is a conventional three-phase asynchronous motor equipped with a reduction gearbox, its transmission efficiency is 87% X 95% ≈ 82.6%, which is very large compared with the direct drive system. Many users have no intuitive understanding of this difference. Let us take a conventional two-machine co-extrusion PP PS blister production line to illustrate it, which is very visual. The general domestic production line of this type uses a φ120 single-screw extruder and a φ65 single-screw extruder. Their motor power is 132KW and 55KW respectively. Based on the average load of 70% during production, direct drive systems and traditional systems The difference in energy consumption per hour is (132KW + 55KW) X 70% X (95% -82.6%) = 16.23KW. Since the extrusion production line is produced 24 hours a day, this is already a very large energy saving data. This means that by changing the drive system, the production line saves 16.23KW X 24 X300 = 116867KW ( One year is calculated according to 300 working days), and the annual energy saving cost exceeds 100,000 yuan. Although changing direct drive will increase the cost by about 200,000 yuan, but this transformation is obviously cost-effective. The key is how the equipment manufacturer communicates with the customer, so as to finally get the customer's approval.

two. Extruder heating system
Compared with the energy-saving technology of the drive system above, few people know about it, but the energy-saving technology of the extruder heating system is widely known by the users (supplied by the supplier, the overall cost increase is not large), the current two mainstream technologies are electromagnetic heating Technology and infrared heating technology. The principle of electromagnetic heating is similar to the induction cooker in the home. The high-frequency alternating current in the coil heats the barrel of the extruder, thereby raising the temperature. The biggest advantages of electromagnetic heating are energy saving, fast heating, and large heating power per unit area. The disadvantage is that the hardware cost is high (in the three heating methods, the hardware cost price from high to low is: electromagnetic heating> infrared heating> traditional mica or cast aluminum heating), and the thermal insulation ability is poor. Electromagnetic heating is generally suitable for the heating of materials from solid to semi-plasticized state, and is not suitable for heating in the molten state. Infrared heating transfers thermal energy to the barrel by means of infrared radiation, thereby heating the barrel. Infrared heating is characterized by energy saving, good thermal insulation and thermal insulation properties, but the disadvantages are high hardware costs, fragile infrared heating tubes, and poor heating in some heating areas. In the current application practice, the cost of electromagnetic heating is about 5 times that of traditional heating, the cost of infrared heating is about 2 to 3 times that of traditional heating, and the energy saving effect is roughly 20% -40% that of traditional electric heating. However, it should be clarified that the energy saving of the heating coil is limited to the comparison of the heating system. Because the entire extruder heating system only accounts for 10% -25% of the energy consumption of the extruder, the proportion is small, and the energy saving effect of the heating system is overall It's not as obvious as the publicity.

three. Extrusion insulation system

Compared with the heating system of the extruder, the entire insulation system is relatively simple to implement, but the effect is very obvious. We know that in the sheet and plate production line, especially in the coextrusion system of multiple extruders, there are many and very long runners. During the equipment production process, the entire runner is basically directly cooled by air. In order to prevent the runner temperature Too low, the heating ring of the runner needs to be continuously heated to maintain the balance of the runner temperature. In the process, a lot of power is wasted. The original purpose of the thermal insulation system is to maintain heat so that it is not easy to lose, and at the same time, it can greatly reduce the temperature around the extruder, which greatly improves the operator's environmental comfort. This type of product is similar to the insulation layer in northern buildings, and plays a role in heat insulation and insulation. It has good energy-saving effect on the long runner and little increase in hardware cost, which is worthy of widespread promotion.

four. High-efficiency screw and multi-screw technology
In addition to the above-mentioned three points, in addition to the increase in cost, the energy-saving effect is relatively clear. After weighing the advantages and disadvantages, users can obtain more obvious energy-saving effects through transformation. However, in extrusion production, the core of plasticization and output is still reflected in the design of the screw. The energy-saving effect is often determined by comparison. The gap between domestic and foreign extrusion systems is often reflected in this. Taking a single screw sheet extrusion production line as an example, in order to increase the production capacity (from a single machine output of 400KG / H to 800KG / H or even higher), two different paths have been taken at home and abroad. The domestic route is the traditional route. In order to increase the output, the simplest method is to increase the diameter of the extruder. The diameter of the extruder is increased from the traditional φ120 to φ150, or even φ180. Increasing the diameter of the extruder has certainly increased the output, but the cost and volume of the extruder have increased significantly. The unit energy consumption (the unit energy consumption refers to the power consumption required to melt and plasticize a 1KG thermoplastic material) has not decreased. When it comes to maintenance and replacement of parts, the difficulty is greatly increased; when replacing raw materials or switching colors, the waste generated by large screw extrusion will also increase significantly. The route taken abroad is different from ours. Instead of increasing the diameter of the screw, they have reduced the diameter of the screw, but have significantly increased the screw speed (the maximum screw speed can reach 700-1000 rpm), and have also achieved high production results. The disadvantages of large screw extrusion are that these high-speed small extruders can often be avoided, and the unit energy consumption is also greatly reduced. The overall energy consumption is reduced to 30%. This effect is still amazing.
Since the advantages of high-speed small screws are so great, why hasn't anyone followed up? The survey found that this seemingly simple problem actually has deep problems, and is not as simple as it looks. First, high-speed reduction gearboxes used in domestic extruders are generally unsuccessful. For gearboxes exceeding 120 rpm, vibration, oil leakage, and long-term use stability are not unacceptable. Second, screw design is not unacceptable. The extrusion industry is not a high value-added industry and cannot attract high-end talents for long-term adherence. Most domestic screw designs rely on experience rather than rheological parameters of material flow or a large number of experimental results. For this type of disruptive Screw design is often impossible to start with; again, screw processing and materials are not enough. Although the domestic screw can be surveyed and mapped by reverse engineering in China, it is found that the effect is completely different. It is easy to break and not wear-resistant. The final conclusion is that our materials and heat treatment can not meet the requirements.
Does that mean we have nowhere else to go? Not always! The success of foreign high-speed small screws has given us a great inspiration: walking a different path may be able to get out of the traditional misunderstanding. First of all, the new screw structure is very helpful for us to improve the existing screw design. Although it can not achieve the effect completely consistent with foreign countries, many principles are conceived. The existing screw technology has been improved, and the output and energy efficiency ratio have been greatly improved. Secondly, in the field of single screw, it may not be able to reach the heights of foreign countries in the short term, but no customer mandates that we must use a single screw for production. Their requirements are only high yield, high efficiency, and quality standards. We know that for an extruder, the greater the number of built-in screws, the better the shearing effect and deashing effect, and the lower the overall energy consumption, which is extremely advantageous for the processing of certain plastic products. Based on this principle, twin-screw and multi-screw extruders have been widely used in sheet metal production lines in recent years. Taking PET sheet processing as an example, after using a twin-screw extruder instead of a single-screw extruder, very good performance can be obtained in raw material adaptability, feeding stability, and material dewatering and dewatering, especially in feeding stability In addition, it can cover almost all thickness of PET bottle flakes and recycled materials, which greatly improves the adaptability of the equipment. In addition, since the twin-screw exhaust dewatering performance is far superior to that of the single-screw, the viscosity drop of PET can be well controlled during the plasticizing process, thereby obtaining higher quality sheet products. At present, the industry generally believes that some PET products (0.2mm-0.8mm) produced by twin-screw crystallization-free equipment have reached or exceeded the quality of sheets produced by traditional single-screw extrusion production lines (with crystal drying system), and With excellent unit energy consumption and applicable range of raw materials, this type of equipment has been widely used. Similarly, in the PVC calendering industry, planetary multi-screw extruders have long been used in large quantities, and their comprehensive efficiency is unmatched by traditional single-screw and twin-screw.

Fives. Simplified material handling system with exhaust system

In the process of plastics processing, moisture and volatiles have a great impact on the quality of the final product. Generally speaking, the less the moisture, the more the volatile matter is detached (delimed), and the better the quality of the product. In order to achieve this effect, the raw materials are often dried and dehumidified before entering the extruder, but the energy consumption of this process is very large. Take PET as an example. For each ton of PET processed, the traditional crystal dehumidifier consumes 100-150 kWh of electricity, and the energy consumption accounts for a large proportion. The exhaust technology of the extruder can greatly simplify the raw material processing system and reduce the energy consumption of raw material drying (or dehumidification). Both single screw and multi-screw can realize exhaust. From the structural principle, the more the built-in screw, the better the deashing and exhaust effect. Twin screw is the most typical deashing extruder. In some special reactive extrusion, it can achieve the function of multiple deashing. From the perspective of energy saving, the stronger the exhaust ash removal ability, the lower the requirements for the front-end processing of raw materials, and the more energy saving.

six. One-step method instead of multi-step method
Some sheets require multiple steps to be processed by traditional processes, such as multi-formulation granulation and raw material processing (such as drying and dehumidification) before extrusion, and multi-layer compounding after sheet extrusion, Multiple processes such as gluing and printing have high process costs. If the process steps can be simplified, one-step molding can be used locally, which can greatly reduce production costs and increase product competitiveness. In the field of sheet materials, the use of twin-screw production of PET sheets, the integration of PET sheet molding and blister integration, the molding of PVC table strips, and the integration of gluing are all typical cases, which have achieved good economic and energy saving effects.
At present in China, the traditional sheet metal plate market has become saturated, the profit margin has been decreasing year by year, and the operating risks are increasing. With the continuous increase of domestic labor costs, it has become a problem that every industry must face, and it is especially important in the plastic sheet industry. More than 95% of sheet machinery is heated and driven by electricity. There is a joke in the industry that "affordable equipment can not afford electricity". It is a typical large consumer of electricity, increasing output, reducing unit energy consumption, and improving products. Competitiveness is of positive significance, which is our diligent goal of "crowding people out."
(Contributed by Sun Fenglin, Suzhou Kingwell Machinery Co., Ltd.)

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