As we all know, extrusion molding process is a molding processing method in the field of polymer processing, which has many varieties, many changes, high productivity, strong adaptability, wide application and a large proportion of output. Extrusion molding means that the polymer melt (or viscous fluid) is molded through a certain shape of die under the extrusion action of Yantai extruder screw, and the product is a continuous profile with a constant cross-sectional shape.

                                                                                                                                        Yantai extruder equipment

The extrusion process is suitable for all polymer materials. It can be used for molding almost all thermoplastics and thermosetting plastics, but it is only limited to a few thermosetting plastics such as phenolic aldehyde. Plastic extruded products include pipes, plates, bars, sheets, films, monofilament, cable cladding, various profiles and composites of plastics and other materials. At present, about 50% of thermoplastic products are formed by extrusion. In addition, extrusion technology is also often used for coloring, mixing, plasticizing, granulation and blending modification of plastics. Based on extrusion molding, combined with inflation, stretching and other technologies, it has developed into extrusion-blow molding and extrusion tentering molding to manufacture hollow blow molding and biaxially stretched films. It can be seen that extrusion molding is an important method in polymer molding.

There are two kinds of extrusion equipment: screw extruder and plunger extruder. The former is continuous extrusion, while the latter is intermittent extrusion, which is mainly used for forming high viscosity materials such as PTFE and high molecular weight polyethylene. Screw extruder can be divided into single screw extruder and multi-screw extruder. Single screw extruder is the basic extruder in production. Twin-screw extruder among multi-screw extruders has developed rapidly in recent years, and its application is increasingly extensive. At present, in the processing of PVC plastic door and window profiles, the twin-screw extruder has become the main production equipment, and the single-screw extruder will be phased out. However, in the extrusion processing of other polymers in Yantai extruder, the single screw extruder still dominates. 

Ultra-fine powder coatings have opened up a new field for the application of powder coatings. This paper summarizes the characteristics, properties and research progress of ultra-fine powder coatings.

1. Preface

Powder coating and painting is a rapidly developing powder coating with the outstanding advantages of environmental protection, economy, high efficiency and energy saving. The powder coating is 100% solid, unlike liquid paint, which contains a large amount of solvent emissions, and is an environmentally friendly coating product without public hazards. The unused powder of powder coating can be recycled for reuse. The spraying process is simple and stable, and the energy consumption is low. Compared with liquid coating, it has the characteristics of low cost and good performance. The development speed of powder coatings is very fast, and it is much faster in China than in other countries. The main reason is that China's economic development is very fast, and a large number of newly-built coating lines prefer to use powder coatings. China has become a big country in the use and production of powder coatings, but there is still a certain gap between China and developed countries in the production of high-grade raw materials and the research and development of high-grade products. How to aim at the cutting-edge topics with great practical significance, use domestic and foreign high-tech to tackle key problems and realize industrialization is an important decision faced by Chinese scientists and professionals.

Powder coatings have shown very strong competitiveness in the development of the last ten years, but there are also shortcomings and limitations that need to be overcome in the development process. The four recognized shortcomings of powder coating are that the coating is too thick, the apparent effect of the coating is poor, and it is difficult to change color without curing at low temperature. In view of these limitations and shortcomings, scientists and engineers in various countries have done a lot of research and achieved remarkable results in many fields. For example, in the research and development of low-temperature curing powder coatings, there are commercial products that can be baked and cured at 1201C, which are used for coating wood and composite boards; The research on UV curing of powder coatings has also made a breakthrough and has been applied industrially. Shanghai Xinxing Electrostatic Spraying Equipment Co., Ltd., the old factory of powder coating equipment in China, has successfully developed ultraviolet curing equipment for powder coating in cooperation with Belgian UCB company, which has made China gain beneficial development in this emerging field, and

Bring great potential. At present, in order to make industrial users accept powder coatings, it can be considered that the role of high quality and economy makes the greatest contribution to complying with environmental protection regulations. Compared with other types of coatings, the market share of powder coatings is not large. However, in the relatively mature field of industrial coatings, powder coatings are one of the few products that can maintain a high growth rate for many years.

2. Brief introduction of superfine powder coating technology

Powder coating has developed rapidly with its outstanding advantages of environmental protection, economic efficiency and energy saving. However, powder coating also has shortcomings that need to be overcome urgently, and the two recognized shortcomings that limit powder coating are: too thick coating and poor coating surface flatness. The reason is that the particle size of powder coating is larger, far exceeding the thickness of ordinary paint film. It not only wastes materials, but also in many cases, too thick coating will cause the performance of paint film to decline. For example, the coating is easy to fall off and the hardness of paint film is reduced. In order to overcome the above defects, scientists in various countries have done a lot of research and developed ultrafine powder coatings. The particle size of this powder becomes smaller, and the coating surface effect is very good, which can realize thin coating.

Scientists of Canada Huixu Micro-powder Technology Co., Ltd. successfully overcome the molecular attraction between ultra-fine powder particles with special technology. The agglomeration phenomenon is avoided, and a powder coating with a particle size of 10~20pm and good fluidization performance is obtained. This kind of coating can not only form a very flat coating surface, but also be applied with a thin coating. The surface effect of ultra-fine powder coating is greatly improved. Salt spray experiment proves that the ultra-fine powder coating with very thin thickness has excellent corrosion resistance, mainly because the thickness of ultra-fine powder coating is higher than that of coarse powder coating at the thinnest part of the coating.

3. Fine powder of powder coatings and its development

Among the four main shortcomings of powder coatings, the most important ones are too thick coating and poor apparent effect of coating. The coating thickness of powder coating is usually 60~100pm, far exceeding that of ordinary paint film, which not only brings unnecessary waste, but also causes the performance of coating film to drop if the coating is too thick in some cases.

The poor apparent performance of the coating reduces the decoration of the powder coating, thus limiting the application and development of the powder coating, especially for some high-grade products (such as cars, etc.), it is impossible to apply the powder coating. The coating of powder coating is too thick and the apparent effect is not good, which is mainly caused by the larger particle size of powder coating. The particle size of ordinary powder coatings is usually 30~40mm, which is difficult to achieve smooth surface and good surface effect after electrostatic spraying. If the particle size of powder coating can be reduced, a coating with good surface effect can be obtained, and thin coating can be applied. Fineness of powder coating can not only save a great deal of cost of thin coating, but also get a very good coating surface, which can be said to be one of the most important topics in the research and development of powder coating. Scientists from all over the world and major powder coating companies around the world have given great investment to this topic, and at the same time, it has attracted the attention of other industries and some governments. For example, in 1997, the three major American automobile companies jointly invested to set up a research institute in Detroit, which specialized in the research of powder coating's fine powder, and tried to use superfine powder coating instead of paint for automobile body; In 1998, the Canadian government also allocated special funds for the research of powder coating micronization. Due to the market demand and people's attention, many breakthroughs have been made in the research of powder coating micronization. The most common way is to add some lubricants to the powder coating, which makes it difficult for the powder coating to agglomerate, thus achieving the purpose of appropriately reducing the particle size of the powder; Some large companies also adopt products with narrow particle size distribution, and select products with high leveling property and lubricant for adjustment, so that the particle size of powder coating can be further reduced and thin coating can be initially achieved. Ferro Company of the United States has developed a new process of preparing powder coatings with supercritical carbon dioxide, which can obtain powder coatings with even particle size distribution. In China, many companies produce products with high leveling property and claim to have achieved thin coating. In fact, the particle size of powder has not decreased, but it is really very popular in the market because it can get thin coating.

The above-mentioned methods are only some ways to realize the powder coating micronization. They did bring a lot of benefits to the powder coating industry, but they did not really realize the powder coating micronization. Fine powder coating means that the particle size of powder coating is less than 20 mm. Generally, when the coating thickness is 2.5 times of the particle size of powder, the surface effect is better, while ultrafine powder has its own characteristics, that is, the gas fluidization performance is poor. This is due to the fact that with the decrease of the grain size of arrowroot, the quality of powder will be reduced exponentially, and the surface area of powder will be increased exponentially. As a result, the molecular force is greatly enhanced, which makes the ultrafine powder agglomerate and unable to carry out normal fluidization. Normal fluidization is the premise of electrostatic spraying of powder coatings. Therefore, constant fluidization becomes the main technical reason why the micronization of powder coatings is difficult to achieve. Agglomeration of fine powder is the natural characteristic of ultrafine powder. In order to achieve fine powder, it is necessary to overcome the molecular force between ultrafine powders.

3.1 Progress of superfine powder preparation technology

3.1.1 Mechanical method and chemical method

The preparation of ultrafine powder mainly includes mechanical crushing and chemical synthesis. Mechanical pulverization is the superfine grinding of conventional block or powder materials by mechanical force. The chemical synthesis method is to generate the basic particles of matter-molecules, atoms and ions, etc. through the chemical reaction of matter, and grow into ultrafine powder through nucleation growth and coagulation. This method has three advantages:

First, versatility. It can prepare ultrafine powders with various compositions, morphologies and particle sizes.

Second, the product quality can be controlled on the molecular or atomic scale.

Third, the process can realize precise control and adjustment, and it is easy to realize industrial production. From the perspective of preparation and application of ultrafine powder, the chemical synthesis method of ultrafine powder represents the development direction of ultrafine powder preparation technology, and has become the research and development focus of various countries.

3.2 Engineering problems of superfine powder preparation

The production process of ultrafine powder materials has its own special industrial reaction process. Compared with the main difference, the proportion of raw material cost of materials is relatively reduced. The function of materials determines that the high added value of products largely depends on the shape of products (shape and attitude distribution, crystal composition and shape, etc.). Therefore, in the development and research of the final production process, the main control indexes of production should be adjusted through the process parameters. The shape of powder material is the key to industrial production. The solution of engineering problems in material preparation is the premise of engineering control and process amplification. Mastering the law of ultra-fine powder process is the basis for solving engineering problems.

4. Requirements of superfine powder coatings

Ultra-fine powder coatings can realize flat and thin coatings, that is to say, powder coatings characterized by ultra-fine powder thin coatings may have some special technical requirements for coatings and painting. Usually, powder coatings require that the melt viscosity of resin is low, but the glass transition temperature of resin cannot be low. The pigment has better dispersibility and strong hiding power; The particle size of powder should be small and the distribution should be narrow.

This requires the crushing and grading effect of crushing equipment to be better; Powder coatings also need to have good fluidity and electrostatic performance of dry powder. To comprehensively solve these problems, we need the joint efforts of raw material manufacturers, powder mills, equipment manufacturers and users.

In this paper, the ultra-fine powder coating with good fluidity is studied. The focus of the research is that users can paint normally without changing any equipment. Below, we will list a series of technical problems encountered in the actual development and application process.

4.1 speed cover force

It is difficult for ordinary powder to be coated with thin coating, the film thickness is generally 60~90mm, and there is usually no hiding power problem. In application, it was soon found that the coating of superfine powder obtained by ordinary formula did not have enough hiding power when the coating was less than 50mm, especially for white products, which could not meet the requirements in actual coating. Therefore, we appropriately increased the content of pigment to make it have a high hiding power like liquid paint, among which, the white product is quite special, so we must choose rutile titanium dioxide with the strongest hiding power and increase the dosage at the same time, otherwise it will not meet the requirements. With the decrease of the film thickness, the sensitivity of the coating to hiding power increases exponentially. During the development, we found that a series of measures must be taken to prevent the uneven melting of products with the increase of pigment content. Firstly, resin with better melting property is used; Secondly, titanium dioxide with good melting property or titanium dioxide treated by wrapping is used. In addition, it is necessary to enhance the kneading effect during extrusion. Therefore, in order to achieve very good hiding power, the powder formula should be improved accordingly.

4.2 Leveling

The leveling and sagging of ordinary powder coatings are a pair of contradictions. When the leveling is good, it is easy to sag. Ultra-fine powder coatings are not prone to sag due to Bo Tu, so the amount of leveling agent can be increased to achieve better leveling performance. The ultrafine powder has a very smooth coating surface because of its fine particle size and uniform spraying.

4.3 Chargeability of powder

Ultra-fine powder is small in quality and difficult to powder. Theoretically, some electrifying agents should be added to improve the powder rate. However, in practical application, it is found that the lower one-time powdering rate is actually an advantage. Because of the low powder coating rate, the selectivity of spraying is enhanced, that is, it is easy to get uniform coating thickness when spraying. Because superfine powder solves the fundamental problem of powder fluidization, there is no problem in recycling superfine powder.

4.4 Cost

As the film thickness is greatly reduced, the cost of superfine powder coating will be greatly reduced. But the percentage of cost reduction is not directly related to the percentage of powder saving. Because the selection of many high-grade raw materials increases the production cost, superfine powder is usually much more expensive than ordinary powder. However, the white superfine powder products use high-grade titanium dioxide because of its large size, and its cost is higher than other color products. The cost of dark superfine powder increases little. Generally speaking, superfine powder coating still has obvious advantages in comprehensive cost, and the more high-grade products, the more obvious the cost is reduced. The market experience in the past six months is that, due to insufficient awareness of new products, customers who take the lead in using superfine powder are not interested in the cost reduction factors of painting, but the driving force is to improve the product grade in order to get a smooth apparent effect. Of course, even if the cost saved now is obviously higher than the cost increase.

5. Coating of superfine powder

5.1 Spraying equipment and process

The starting point of overcoming the problem of superfine powder coating is to thoroughly solve the fluidization problem of superfine powder, so that theoretically, no coating process will be changed. In the process of practical application, ultra-fine powder can be completely fluidized like ordinary coarse powder, and there is no problem of poor fluidization such as gun blockage. However, the spraying of superfine powder still has its particularity. Ultra-fine powder has smaller mass and larger surface area. Although the charge of a single particle is less, the total charge increases a lot. After more than one year's test and application, we found that the spraying equipment basically didn't need to be changed, whether it was the manual spray gun in the simple spray room or the most advanced automatic spray gun in the modern spray room, it could spray freely. However, the spraying process needs to be slightly changed according to the specific situation, for example, the distance can be a little closer, and the voltage can be a little lower. The spraying process parameters of ultrafine powder are the same as those of ordinary coarse powder, and each production line has its own best coating conditions to meet its own requirements, which requires technicians to conduct some groping tests on the spot.

5.2 Powder rate and selectivity

There are many fine powders in the recycled powder of ordinary coarse powder, and the problems of poor fluidization, such as agglomeration and powder spitting, often occur in repeated use, which brings trouble to the recycling of powder coatings. Therefore, it is necessary to mix recycled powder with coarse powder in a certain proportion and recycle it. Ultra-fine powder is not as easy to get to the surface of workpiece as coarse powder because of its small mass, and the one-time powder feeding rate is worse than coarse powder, but this is not a bad thing. The poor powder feeding rate makes it easy to get a uniform and thin coating by spraying ultra-fine powder with better selectivity, which is difficult for coarse powder to achieve. On the other hand, because the superfine powder solves the fluidization problem of fine powder, there is no problem of poor fluidity in the recovered powder. As long as the equipment has a recovery device, there is no problem in coating.

5.3 Recycling performance and recoating performance

At present, there are two commonly used recycling systems: cyclone recycling and filter bag recycling. Both methods can effectively recover all unused powders. Both coarse powder and superfine powder contain particles of various sizes, but the proportion of children is different. Although the average particle size of ultrafine powder is much smaller than that of coarse powder, its particle size is within the designed recovery range of existing recovery equipment. It should be said that there is no problem in normal recovery, and the recovered powder usually has a smaller particle size than the powder used for the first time. The technical key of superfine powder coating is to thoroughly solve the fluidization problem of superfine powder, so the recovered powder of superfine powder has the characteristics and application of superfine powder coating.

5.4 coating performance

The coating of superfine powder is actually a successful spraying process by solving the fluidization of fine powder. The coating of superfine powder has different coating properties from ordinary powder for the following reasons. First, the particles are fine, the coating is dense and the surface is smooth, so the scratch resistance and smoothness of the surface are enhanced; Secondly, the coating is thin, which avoids the disadvantages such as coating falling off caused by thick coating. In addition, it is difficult to get thin coating by coarse powder coating, and too thick coating actually brings great waste. In order to save costs, conventional powder coating must add a large amount of cheap fillers, and these moisture fillers. Although it does not affect the covering power of the coating, it affects the chemical properties and corrosion resistance of the coating to a certain extent. Ultra-fine powder has to use the best raw materials because of its thin coating and high covering power, and it can afford the best raw materials because of its thin coating. Therefore, the performance of superfine powder coating is obviously superior to that of ordinary powder coating in many aspects, such as corrosion resistance, weather resistance, softness, adhesion hardness and so on. Of course, if strong mechanical friction resistance is required, then thin coating is not as good as thick coating.

6. Application examples of superfine powder coatings

6.1 Black parts

Most of the interior parts of automobiles can be coated with powder coatings, the main function of which is protection. However, because ordinary powder can not achieve thin coating, the coating thickness is usually 60~100pn, so it is not widely used. However, black ultrafine powder has no problem of hiding power and surface flatness. In practical application, the film thickness has dropped to an average of 20pm, which saves a lot of costs. The corrosion resistance of the coating is equivalent to that of ordinary powder, and the hardness and cohesiveness have been increased.

6.2 Furniture, containers and other indoor products

Some customers choose superfine powder for producing export products with high surface requirements. In order to achieve the best apparent quality, the film thickness is reduced little, and as a result, the cost is not reduced. This shows that in the low-priced indoor product market, the application of superfine powder has no cost advantage for the time being.

6.3 Outdoor weathering products

The spraying of aluminum profiles, etc. firstly requires excellent weather resistance, and secondly, better apparent performance. Ultra-fine powder is made of high-grade weather-resistant polyester and inorganic weather-resistant pigment, which can obtain coating products with excellent performance in both aspects, and more importantly, it can save cost significantly.

6.4 Automotive field

For many years, automotive finishing varnish has been considered as an inaccessible area for powder coatings. However, due to the advantages of powder coatings in economy and environmental protection, recently, since the successful application of powder coatings with one bottom surface in the field of automobile coating, automobile manufacturers and coating manufacturers have carried out extensive research in this field. BMW is the first automobile manufacturer in the world to use powder finishing varnish in its standard products. By the end of 2000, powder finishing varnish had been put into commercial production in BMW's German factory, producing 500,000 cars in total.

6.5 Other application markets

Other markets of powder coatings include anti-corrosion powder coatings applied to pipelines and used to reinforce threaded steel bars. This kind of powder coatings is mainly pure epoxy system (sintered type). In Europe, the market of reinforced bar is almost neglected, but from the technical point of view, this field is considered to have great growth space. Because of the different statistical classification methods in different areas, it is very difficult to evaluate the consumption of powder coatings in different areas. When making economic comparison between powder coatings and some VOC (volatile organic compound) coatings that meet the selection principle, we must pay attention to the total cost including coating cost. Compared with other environmental protection coatings, in the actual coating process, the advantage of using powder coatings is that the utilization rate of raw materials is increased from 95% to 99%; Energy loss is reduced by 30% (compared with traditional carbon paint with low solid content); Labor costs will be reduced by 40%

50%; Discarded materials due to surface defects are reduced by about 46 times, so waste materials are reduced by almost 90%.

7. The latest development of superfine powder coatings in various fields

The latest development in the painting equipment is to manufacture the painting production line which is easier to be cleaned and faster to change colors. The trend of developing low-temperature curing and highly reactive powder coatings can increase the production line speed, save energy and make the economic advantages of powder coatings more attractive.

MDF powder electrostatic spraying has the advantages of excellent coating performance, high construction efficiency, low energy consumption and so on. After the wood products are encapsulated by 100% solid powder coating, the volatilization of harmful substances inside the wood can be prevented, making it a veritable green product. Of course, this still needs to be improved, such as the chemical storage stability of the powder: latent catalyst or additives that can prevent the base material from reacting at the storage temperature without affecting the curing conditions; Physical stability: increase the glass temperature of the system, but not affect the reaction speed and viscosity of the system.

Coil coating is a kind of pre-coating, which is different from the traditional "post-coating" process. Because of a series of advantages, such as simplified production process, efficient construction, saving investment and operation cost, meeting the requirements of environmental protection laws and regulations, and better coating performance than traditional methods, coil coating has become one of the development directions of coating industry today. Color plate production technology was first developed in the United States in 1927, and coil coating and coating technology was introduced from 1980s in China. At the end of 1990s, the consumption and production of color plates began to rise in China, and its growth momentum was extremely rapid. By the end of 2003, 124 enterprises had built 169d coating units with a production capacity of 8.74 million tons. The powder coating device is in a strong electrostatic field, and the powder rotating brush generates a coating powder cloud. The solid coating particles of the powder cloud are charged with high charge, which fly to the substrate running at high speed, generating enough boundary penetration, and the powder particles are uniformly deposited on the surface of the strip steel. At present, there are five powder paint color coating lines in the world, with the coating strip width of 1300mm and the process speed of the unit of 100m/min. There is no real powder coil coating line in China. With the strict requirements of the state on environmental protection and the consideration of cost performance, the powdering of coil coating will gradually be recognized by people and become the development trend of coil coating.

1. In the mid-1980s, with China going out, foreign formulas, production technology, production equipment and coating equipment were gradually introduced into China. The work direction of the industry turned to digesting and absorbing foreign technology, striving for localization and making great progress. In 1993, China changed from a planned economy to a market economy, and China's accession to the WTO in 2002 was a major node of China's economic development. The powder coating industry is developing rapidly. With the rapid progress of powder coating formulation technology, the research and development and production of raw materials such as epoxy, polyester, curing agent, leveling agent, matting agent, pigments and fillers have also made rapid breakthroughs. Powder coating production equipment has gradually developed into a characteristic large-scale supply system since the introduction of imported equipment and the learning stage. The coating production line has also developed from completely relying on imported foreign equipment to complete localization of the whole set of design and production, greatly reducing the production and application cost of powder coatings. It has played an important role in promoting the development of powder coatings production and application in China.

2. The domestic output of thermosetting powder coatings increased from 300t tons in 1983 to 26,000 tons in 1993, from 340,000 tons in 2003 to 1.115 million tons in 2013. China's powder coating production and sales have maintained rapid growth for many years. Since surpassing the United States in 2002, China's powder coating production has accounted for 42.6% of the global total in 2012, greatly surpassing the second-ranked United States (6.8%) and becoming a veritable power in powder coating production and application. However, from the quality and technical content of powder coatings, it is not a powerful country.

3. The initial development of China's powder coatings benefited from the imported main raw materials, milling equipment, coating equipment and coating technology. Since then, the main raw materials have been independently developed and applied, which has reduced the formulation cost of powder coatings and played an important role in expanding the application market of powder coatings. However, in the past, most Chinese patents for powder coatings were applied by overseas enterprises. Domestic enterprises rarely get involved. In recent years, there has been a breakthrough in patent application of powder coatings in China. By the end of 2011, there were 827 Chinese invention patent applications for powder coatings. There are 27 utility model applications, accounting for 4% and 15.8% of Chinese patent applications for coatings respectively. In addition, from the variety structure of powder coatings, there is still a big gap between the product technology of powder coatings in China and developed countries and regions in Europe and America.

4. The development of powder coating technology depends on the development of higher market. The development of powder coating technology is corresponding with the development of application market. The requirements for powder coatings in different market areas are very different. In the past 30 years, with the continuous expansion of domestic application market, professional manufacturers of household appliances powder coatings, pipeline anticorrosion powder coatings, aluminum profile powder coatings, radiator powder coatings, anti-theft door powder coatings, highway guardrail powder coatings, insulating powder coatings and other products have emerged continuously, laying a solid foundation for the advanced development of powder coatings technology in China.

5. The coating of powder coil has always been the focus of powder industry. Recently, after the state promulgated the regulations of levying consumption tax on solvent-based coatings, the voice of "replacing paint with powder" in color steel plate industry is growing. The painting speed of domestic color steel plate has reached 120m/min, and the current electrostatic powder spraying technology is difficult to meet this requirement.

RollCoater abroad innovated to pump a large amount of powder into powder coating room. After static electricity is loaded in the static electricity generating area, a large amount of coated powder can reach the substrate. This method can realize industrial production. However, it is difficult to control the film thickness in the production process, and it is difficult to obtain a 30-40μm thin coating.

A domestic foreign-funded enterprise has developed a powder coating with a particle size of 12μm(D50), coating a 23μm thick film on galvanized steel, which has good flatness and corrosion resistance, and has passed the salt spray test for 1000h hours, reaching the national standard index of color steel plates. The company has established a high-pressure slit powder spraying test line similar to that developed by Firstprecision Company, with a linear speed of 25m/min, and is in the experimental stage.

6. At present, the technology of producing this kind of fine powder coatings in China has become mature. Only when the research breakthrough is suitable for this kind of fine powder electrostatic coating equipment, this novel and advanced application market will gain explosive growth immediately. At present, the objective market demand is forcing the scientific and technical workers in the paint coating industry. Aiming at this application field, we should make great efforts to improve the gold content from various aspects such as theoretical application, process technology research and equipment research and development, and leap to a new platform.

7. Technical bottleneck MDF powder coating technology has been popularized and applied for more than 10 years, but the technical bottleneck of the coating process has not been broken in time, resulting in the situation of "three ups and three downs". The application of color steel powder coating technology will also encounter similar technical barriers. 1 meter wide color steel plate is measured according to the production speed of 100m/min, and the coating thickness is 30-40 μ m. It is required that 4-5kg of fine powder can be uniformly sprayed on the surface of the steel plate within 1 minute, and at the same time, it is also required to ensure the uniform charge of the powder on the surface of the steel plate, and the powder coating should be completely cured within 5-10 seconds. This is the technical bottleneck encountered in the application of color steel powder technology. Only by conquering it will this application market be started.

Recently, some colleagues in the industry are planning to adopt the mechanism of "polymer galvanic polarization electrification" to design and manufacture a brand-new powder coating model for color steel plates. This model combines the improvement of powder mist spraying amount and powder electrification to solve these difficulties in coating. Whatever the success or failure of this research. This is the only way for China's powder coating technology to advance. In order to realize the Chinese dream of powder coatings, the scientific and technological workers in our country must firmly take this step and dare to develop the market of novel and high technology, which not only gives us pressure, but also gives us the impetus of innovation and development.

8. Other powder coating technology markets, which are described above, are two novel and advanced markets. Their successful application and promotion will undoubtedly greatly promote the development and promotion of powder coatings in China. In addition, the application range of powder coating technology is particularly wide. The development and application of many new industries will not only increase the usage of powder coatings, but also enhance the gold content of powder coatings technology from many aspects. For example, metal bonding powder coating, antistatic powder coating, conductive powder coating, high temperature resistant powder coating, antibacterial powder coating, anion powder coating, high reflectivity powder coating, reflective heat insulation powder coating, heat dissipation powder coating, etc. Their development and application not only expands the application range of powder coatings and serves the society. At the same time, it will enrich many high-tech and high-level powder coatings varieties for China's powder coatings library. Comprehensively improve the scientific and technological level of powder coatings in China.

Add water or appropriate liquid to the raw material powder, and keep stirring. The stirred material is extruded from a porous die or metal mesh with high extrusion pressure. Usually, after the material is put into a cylindrical container, the material is extruded by a screw. After using the frequency conversion technology, the pressure can be controlled, so that the appropriate linear speed can be selected. Principle of Single Screw Extruder Single screw is generally divided into three sections in effective length. The effective length of the three sections is determined according to the screw diameter, pitch and screw depth, which is generally divided into one third each. The last thread of the nozzle is called the conveying section at first: the material here should not be plasticized, but it should be preheated and compacted under pressure. In the past, the old extrusion theory thought that the material here was a loose body, and later it was proved that the material here was actually a solid plug, which means that the material here is a solid like a plug after being extruded, so its function is to complete the conveying task. The second section is called the compression section, at this time, the volume of the screw groove gradually decreases from large to small, and the temperature should reach the degree of material plasticization. The compression generated here is from the third conveying section to one, which is called the compression ratio of the screw (3: 1). Some machines also change, and the plasticized material enters the third section. The third section is the metering section, where the material is kept at the plasticizing temperature, and only the melt material is accurately and quantitatively transported like a metering pump to supply the die head. At this time, the temperature cannot be lower than the plasticizing temperature, which is generally slightly higher.

Pay attention to the following when using extruder:

1. The extruder should run forward to avoid inversion.

2. Avoid running on an empty stomach, and be sure to feed the heat engine, so as to avoid the phenomenon of stick bars (holding shafts).

3. It is strictly forbidden to enter the iron and other miscellaneous materials in the feed port and vent hole of the extruder, so as not to cause accidents and affect production.

4, safe electricity, grounding wire.

5. When the machine is running, it is forbidden to touch the rotating parts such as feed port, discharge port, belt and gear with hands.

6. Before using the machine, inject lubricating oil to avoid damage to the machine. The function of extruder is to transform solid plastic into uniform melt by heating, pressurizing and shearing, and send the melt to the next process. The production of melt involves the processes of mixing additives such as masterbatch, blending resin and re-grinding. The finished product must be uniform in concentration and temperature. The pressure should be large enough to squeeze out the viscous polymer.

Mixer is a machine that uses mechanical force and gravity to mix two or more materials evenly. Mixing machinery is widely used in chemical industry, agriculture, pharmacy, food, construction and other industrial production and daily life.

Mixers can mix the mixture of various materials evenly. For example, in the pharmaceutical industry, the raw materials of different components are evenly mixed together to form a compound. It can also increase the contact surface area of materials and promote chemical reaction; It can also accelerate the physical changes, such as adding granular solute into solvent, and accelerating the dissolution and mixing evenly through the action of mixing machinery.

Mixing machines commonly used in daily life are divided into four categories: gas and low viscosity liquid mixer, paste mixer, thermoplastic material mixer and granular solid material mixer.

Here, we mainly introduce the solid material mixer.

Most of the mixers for powdery and granular solid materials are intermittent operation. At present, there are three types of mixers on the market: V-mixer, 3D mixer and trough mixer.

Mixing requires that all materials to be mixed be evenly distributed. The degree of mixing can be divided into three states: ideal mixing, random mixing and completely immiscible. The mixing degree of different kinds of materials in the mixing machine depends on the proportion, physical state and characteristics of materials to be mixed, the type of mixing machine used and the duration of mixing operation.