Nine key elements in plastic extrusion process

2022-08-16
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Abstract: Nowadays, manufacturers mainly producing color printing packaging plastic extruded composite films must master the key elements of the extrusion process if they want to save production costs, improve product quality and give full play to the performance of plastic extrusion equipment effectively

key words: save production costs, improve product quality, master key elements

every plastic product production and processing enterprise, especially the plastic manufacturer that mainly produces color printing plastic extrusion composite packaging film, and every senior technical manager must have a good understanding of the characteristics of raw materials, processing technology, performance of extrusion equipment, product quality control, and productivity improvement of products produced by their own enterprises, The reduction of production costs and other related important factors, but among these many factors, one is the most important. Namely; Nine important factors in extrusion production process. This is what every extrusion composite plastic manufacturer and technical management personnel at all levels must firmly grasp. For this point, through years of exploration and research, we have finally mastered some shallow knowledge in the extrusion production process for your reference

I. fully understand the design requirements of the extruder screw

when using their own extruder, each manufacturer of extruded composite film must first fully understand all the important relevant parameters of the screw. We know that the screw design of the extruder is very important. Even if you buy a new screw, you should make sure that the screw design of the extruder you buy meets the shape, function, material and screw required for high-speed operation. For example, just buy an extruded PE screw for extruding film, you should know its material, heat treatment, thread and direction, and some functions that the screw must complete, such as continuous addition of resin, uniform solution resin, stable injection of liquid homogeneous mixed resin, and work cycle, material volume, pellet geometry, back pressure, and screw geometry. There must be a set of data and parameters that show the functions of the screw used. Because in today's computer world, simulation technology is easy and should simulate all screws, unless relevant data of the same material applicable to the actual situation can be obtained

because the wear of each screw is related to the problem of screw design, a worn screw is not necessarily a bad screw. Sometimes the wear is even beneficial to the mixture in the cylinder, or it is beneficial to increase the output when there is a pressure peak, which is beneficial to determine the location and cause of wear. However, what is actually needed is to prove that wear is causing trouble, which may mean a higher melt temperature for a given input interface

at the same time, we must master the development of screw design today, because at present, the most popular screw among extrusion manufacturers is general screw, which can be used by each manufacturer when producing several raw materials. However, we must master and understand that the design of this general screw is a thread compression screw, which has a feeding section, a compression section and a metering section, accounting for 50%, 25% and 25% of the total length of the thread respectively. The thread spacing of the screw is equal to its diameter, and the thread tooth width of the screw is equal to 1/10 of its diameter. The taper machining of the compression section is involute, and the compression ratio can be 2:1 to 3; 1. The calculation method is to divide the thread depth of the feeding section by the thread depth of the metering section. The ratio of screw thread length to screw diameter, i.e. l/d ratio, is 16:1~24:1

there is no feeding depth and metering depth in this description. Without these depths, the compression ratio is meaningless. It is easy to see that the screws provided by different extruder manufacturers are 60mm, 20:1, l/d, GP (general-purpose screws) from the screws with a feeding depth range of 6mm and 10mm. These depth values affect the filling and conveying of materials, the amount of existing plastic in the screw cavity, the throughput of materials, the shear ratio and the overall melt quality. No matter the exact depth, length and width of the screw used, it still brings effective value to the industry by using the screw. The key is the universal screw used in this extruder. The two elastic characteristics set the direction for its future application. The first feature is that under the condition of not damaging the performance of polymer, the function of screw design can process all kinds of resins according to various cycles and material quantities. The second feature is that the manufacturing of general screw design is the cheapest screw design

the basic principle of extruding plastic - each screw rotates in the cylinder and pushes the plastic forward. The screw is actually an inclined plane or slope, which is wound on the central layer. Its purpose is to increase the pressure in order to overcome the greater resistance, because for each extruder, resistance needs to be overcome. The friction force of solid particles on the cylinder wall is the mutual friction force between them when the screw rotates for the first few rounds. The screw does not move axially. Although it may turn to fast rotation near the circumference, the axial force on the screw is balanced. The single screw in the extruder is a right-hand thread. From the back, it has reverse rotation, because they have to try their best to rotate out of the barrel, so that a great forward thrust can be exerted on the plastic melt in the barrel. At the same time, it can also test various properties such as strength, wear resistance, permeability and aging, and also exert a thrust in the same direction on an object. This force eventually falls on the thrust bearing behind the feed inlet

II. Process operation stability

for the conventional extruder of various plastic manufacturers, the following problems usually exist in use: 1. Improper control of process conditions and motor load pulsation will cause the fluctuation of extrusion temperature and pressure, affecting the quality of extrusion. 2. With the increase of extrusion pressure, the reverse flow and leakage flow will inevitably increase, making the whole extrusion flow rate drop. At the same time, when the screw pushes the material, the power consumed is large, the shear force on the material is large, and the energy consumption increases. 3. In the blending modification of polymer, in order to make the extrudate have uniform temperature and pressure distribution, it is necessary to increase the length of the metering section, but this will increase the difficulty and cost of screw processing, and increase the residence time of materials, which is not conducive to the processing of heat sensitive materials. How to solve this problem? Through the analysis of the structure of the large extruder of the imported equipment of our company, we believe that it is best to use a high-temperature melt gear pump. The purpose of this gear pump is to separate the melt plasticizing section of the polymer from the metering, and to transport the melt to the machine head independently. This pump is actually a positive transfer device, its outlet pressure is not affected by the change of inlet pressure, and its volumetric efficiency and energy efficiency are very high. It can not only meet the operational stability of the equipment, and then improve the quality of products, but also meet the requirements of high efficiency and low energy consumption

at the same time, the melt pump equipped on the extruder has the greatest feature that it has a stable extrusion pressure, which can increase output, reduce wear, increase the flexibility of the production line, reduce melt temperature and reduce energy consumption. As a qualified plastic extrusion product manufacturer, the hierarchical permission use management on the extruder of the enterprise can be properly considered

III. heating elements

we know that extrudable plastic is hot plastic, which melts when heated and solidifies again when cooled. The heat of melt plastic is mainly the preheating of feed and the role of barrel/mold heater. But the motor can also input energy -- the motor can overcome the resistance of viscous melt, and the friction heat generated in the cylinder when rotating the screw is the most important heat source of all plastics. For all other operations, it should be known that the barrel heater is insufficient. As the main heat source in operation, the temperature of the rear cylinder may still be the most important, because it affects the transmission speed of solid materials in meshing or feeding, and "the temperature of the die head and die should usually be the melt temperature or close to this temperature. Therefore, we should keep in mind that when extruding plastic, the thermal degradation of thermoplastic is directly proportional to its heating time. Generally, in plastic extrusion molding, in order to improve the internal quality of extruded products, we should try to prevent the thermal degradation of the melt, that is, we should avoid the melt staying in the die head for too long

IV. elements of the feeding area

as we all know, the input feed of each extruder is lower than the surface temperature of the barrel and screw in the feeding area. However, the surface of the barrel in the feeding area is almost always above the plastic melting range. Through the cooling in contact with the feeding particles, the heat is maintained by the heat transferred backward from the heat front end and controllable heating, and even the heat at the front end is rubbed by viscosity, When the heat input of the cylinder is maintained and is not required, the rear heater needs to be turned on at this time. In particular, the general trough feeding cylinder is specially used for low-pressure polyethylene. In order to maximize the conveying capacity of solid particles in the feeding area of a smooth cylinder of a single screw extruder, plastic pellets should be on the cylinder and slide onto the screw. If these materials stick to the root of the screw, nothing will pull it down, and the channel volume and solid inlet volume will be reduced. Most plastics slide naturally at the root, so they are cold when entering, and the friction force has not heated the root as hot as the cylinder wall, such as highly plasticized PVC and some polyolefin copolymers with adhesive properties that are desired in end use

for the machine barrel, the plastic is adhered to the barrel so that it can be removed and pushed forward by the screw thread. At this time, there should be a high friction coefficient between the particles and the cylinder wall, and the friction coefficient is also strongly affected by the temperature of the rear cylinder. If the material particles do not adhere, they just rotate locally without pushing forward, which is the reason why smooth feeding is not good

surface friction is not the only factor affecting the feeding. Many particles always do not contact the cylinder or the root of the screw, so there must be friction and mechanical and viscosity interlocking inside the cylinder. Of course, the case with grooves in some barrels is another case. The trough is thermally insulated from the rest of the barrel in the feeding area and is also deeply water-cooled. The thread pushes the particles into the groove and forms a high pressure within a considerable distance, which increases the bite allowance of the same output and lower screw speed, thus reducing the friction heat generated at the front end and lowering the melt temperature. This may mean that the flow rate in the membrane production line with cooling restriction is accelerated. Therefore, slotting in the barrel is particularly suitable for polyethylene

v. pressure elements at the end of the screw

the pressure at the end of the screw is very important in extrusion. This pressure reflects the resistance of all objects downstream of the screw, the filter and adapter conveyor pipes, and the die itself. It depends not only on the geometry of these components, but also on the temperature in the system, which in turn affects the viscosity and moving speed of the resin. Although it will not depend on the screw design, except when it affects the temperature, viscosity and throughput. In terms of safety factors, the temperature measurement of each extruder is very important. If it is too high, the die head and die may crack and hurt the operator and the whole extruder. However, the pressure is beneficial to the mixing in the barrel, especially in the metering degree of the screw. However, too much pressure means that the motor has to output a large amount of energy, which causes the resin melt temperature to be too high, but the pressure limit can be specified

VI. let the car not only be convenient

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