Share technical knowledge of plastic coextrusion process
Multi-component composite products can be produced by a variety of methods, and the co-extrusion process is the easiest and most convenient method. It has become one of the most advanced plastic molding methods in the modern era. The high polymer co-extrusion process is a process in which a plurality of extruders are separately supplied to different melt streams, and a multi-layer composite product is obtained by co-extrusion in a composite head.
It enables multiple layers of materials with different properties to be combined with each other during the extrusion process, so that the product has the excellent characteristics of several different materials and complements in characteristics to obtain special requirements of performance and appearance, such as anti-oxidation. And moisture barrier properties, colorability, thermal insulation, thermoforming and thermal bonding capabilities, and mechanical properties such as strength, stiffness, and hardness. These multilayer composites with comprehensive properties have extremely wide application value in many fields. In addition, it can significantly reduce the cost of the product, simplify the process, reduce equipment investment, the compound process does not use solvents, does not produce three waste materials. Co-extrusion technology is therefore widely used in the production of composite films, sheets, pipes, profiles and wire and cable.
The following discussion focuses on the composite pipe, composite film, flat film and cast film, PVC core foam composite pipe, plate and profile coextrusion technology which have been widely used in recent years.
Composite pipe coextrusion
The aluminum-plastic composite pipe combines the advantages of plastic and metal, and has the advantages of non-toxic, smooth, corrosion-resistant, light texture, high strength, good heat resistance, low embrittlement temperature, convenient installation, generous appearance and long service life. Transportation of corrosive and corrosive gases in hot and cold water and drinking water pipelines, ground and underground heating pipes, gas pipes, petrochemical industries, compressed air delivery, and transportation of liquids such as beverages, wine and milk in the food industry, etc. In the near future, it is possible to gradually replace galvanized pipes, copper pipes and plastic pipes.
In industrialized countries, the share of aluminum-plastic composite pipes in pipes is about 15%. The technology was applied for by the British engineer Itzhak Barnoach in 1974, and then Kitech, Germany, Unicor and Klaubo have improved the pipe structure, processing equipment and manufacturing technology to improve their performance. Commercial applications in Europe and Australia began in the early 1990s. In the mid-1990s, China began to introduce the technology of aluminum-plastic composite pipe production line, and began to produce and apply aluminum-plastic composite pipe.
The aluminum-plastic composite pipe is composed of 5 layers (polyethylene, hot melt adhesive, aluminum foil, hot melt adhesive, polyethylene), with cross-linked polyethylene (XLPE) as the inner and outer layers, and the middle layer is welded aluminum pipe to increase the strength of the pipe. The inner and outer surfaces of the aluminum tube are coated with an adhesive and a plastic layer, and formed by a co-extrusion process.
Flat film and cast film coextrusion
The casting film forming principle is that the plastic melt is extruded through a T-die in an extruder, directly into an aqueous solution or a chill roll, and after cooling and drawing, a cast film is obtained. This processing method can fully exert the performance of the material being processed while maintaining the best dimensional accuracy. Most thermoplastic films can be produced by casting. Especially suitable for semi-crystalline thermoplastics.
The forming principle of flat film extrusion is: extruding a plastic melt which has been plasticized uniformly in the extruder from a flat film head, cooling and solidifying by contact with a cooling roll, and finally cutting into a film of a certain width and winding into a film. volume. The structure of each layer of the coextruded film may be symmetrical or asymmetrical. When the adhesion between the two films is not good, it is necessary to add a thin bonding layer between the two layers to improve the heat. Sealing properties and boundary adhesion properties.
There are three types of co-extrusion heads for flat and cast film, namely multi-channel co-extrusion heads, co-extrusion heads with feed blocks, and a combination of multi-flow heads and feed blocks. Extruder head.
(1) Multi-channel co-extrusion head: The melt extruded by several extruders flows into the split channel of the set width and thickness from the feed end of the head with multiple channels, and the melt of each layer Composite molding in the nose of the machine. With this method, one can select a plastic material having a large difference in fluidity and melting point to obtain a composite product. However, the number of composite layers should not be too much, otherwise the co-extrusion head is too large.
(2) Handpiece with coextruded feed block: the melt extruded by several extruders is fed through the feed block splitter, and the melt flow rate set therein is adjusted by the regulating valve and the thickness adjustment plug. Then merge into the hanger head extrusion molding. This method allows one to produce more layers of composite film, and the co-extrusion head is small and precise. The disadvantage is that only plastics with similar fluidity and processing temperature can be combined with each other, and the processing range is narrow.
(3) A co-extrusion head of a combination of a multi-flow head and a feed block. It is a co-extrusion head developed by Reifenhäser GmbH, Germany, which is used to process more than five layers of heat sensitive materials.
Profile extrusion
The purpose of coextrusion of plastic profiles is to squeeze different polymers of different properties into different parts of the same profile to give special features to the profiles or to obtain the best performance and price ratio, thus diversifying or multi-functionalizing the products. To improve product quality and reduce costs.
The profile coextrusion process can be divided into two types: pre-coextrusion and post coextrusion according to the molding state of the coextruded material. Pre-coextrusion means that two materials are composited in the process of not being fully formed; post-coexcitation means that one material has been completely formed and then composited with another material. The advantage of post coextrusion is that it can utilize waste materials and is economical.
According to different extrusion materials, it can be divided into organic coextrusion and inorganic coextrusion. Organic coextrusion includes co-extrusion of the same material (such as pre-coextrusion of fine materials and mixed materials) and co-extrusion before different materials (such as co-extrusion of PMMa and PVC) and post-coextrusion of soft and hard PVC; inorganic coextrusion Can be divided into aluminum-plastic composite co-extrusion and steel-plastic composite co-extrusion.
The focus is on co-extrusion, co-extrusion of aluminum-plastic composite profiles, co-extrusion of steel-plastic composite profiles, and two-color coextrusion.
Post-coextrusion (hereinafter referred to as PCE) technology is a revolutionary advanced molding technology developed by Austrians in the late 1980s and is the latest development in coextrusion technology. Compared with the traditional pre-coextrusion (hereinafter referred to as FCE) technology, it has the remarkable features of simple process, flexible application, low scrap rate, easy recycling, and controllable bond strength. At present, this technology is mainly used to manufacture profiles for doors and windows with sealing strips.
The traditional FCE technology is a molding technology. Molten materials having different rheological behaviors or different colors are extruded from the two or more extruders into the same molding die, and the melts flow in the respective flow paths in the molding die, and then merged and extruded at the die, and The set is vacuumed and cooled. In this process, due to the difference in melt viscosity and pressure and the difference in flow rate, when the materials of each layer meet in the die, unstable laminar flow is easily generated, resulting in irregular and uneven composite interface, and the layers are easily separated after the die is discharged. . In addition, the difference in melt viscosity also makes the extrusion melt difficult to set during vacuum cooling (such as easy clogging when entering the sizing sleeve), making the process more complicated and difficult to control. Therefore, if you want to ensure the quality of the molding, you need to design and manufacture complex molds and skilled operation techniques.
Alumina ceramics are ceramic materials based on alumina (Al2O3) for thick film integrated circuits. Alumina ceramics have good conductivity, mechanical strength and high temperature resistance. It should be noted that ultrasonic cleaning is required. Alumina ceramics is a versatile ceramic. Because of its superior performance, it has been widely used in modern society, satisfying the needs of daily use and special performance.The high-purity alumina ceramics have a Al2O3 content of 99.9% or more. Since the sintering temperature is as high as 1650-1990 ° C and the transmission wavelength is 1 to 6 μm, the molten glass is generally used to replace the platinum crucible; Sexual and alkali-resistant metal corrosion is used as a sodium lamp; it can be used as an integrated circuit substrate and high-frequency insulation material in the electronics industry.
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