Extrusion molding can be processed by many types of polymers, products are more diverse, molding process also has many differences, but the basic process is more or less the same, the more common is to solid state feeding extrusion products process. This extrusion molding process is: granular or powdered solid material added to the extruder hopper, extruder barrel outside the heater, through heat conduction will be the heat generated by the heater to the material within the material brief, the temperature rises, reaching the melting temperature. Machine operation, the barrel of the screw rotation, the material will be transported forward, the material in the movement process and the material, screw and material and material friction between each other, shear, generate a lot of heat, and heat conduction **** with the role of the material to join the continuous melting, molten material is continuous, stable transportation to a certain shape of the head (or called mouth mold) in. Through the mouth mold, in the flow state of the material to take the shape of the approximate mouth, and then into the cooling and shaping device, so that the material on one side of the curing, on the other hand, to maintain the established shape, in the traction device, so that the products are continuously forward, and to obtain the final size of the product. Finally, the product is truncated by cutting for storage and transportation.
The more representative processes of extrusion molding are: polymer melting, molding, shaping, cooling, haul-off, cutting, and stacking.
Other extrusion molding products, with the material characteristics, product size and output requirements, extruder structure, type and specifications can be different; head structure, shape, size according to the specific products and design and manufacture; cooling and shaping according to product varieties and material properties; the rest of the auxiliary machinery will also have many different points. However, the above process links are basically the same. Completion of an extruded product line usually consists of the main engine, auxiliary components, these components are collectively referred to as the extrusion unit.
1, host: a host has the following three parts.
①Extrusion system. It is the key part of the extruder, mainly composed of screw and barrel. For general thermoplastics, through the extrusion system, the material is plasticized into a uniform melt: for melt feeding and extrusion molding with chemical reaction, the main thing is to make the material evenly mixed into a fluid. These homogeneous fluids are continuously extruded from the die at the front of the extruder under the action of the screw thrust.
②Transmission system. Its role is to drive the screw to ensure that the screw in the working process of the required torque and speed.
③Heating and cooling system. It ensures the temperature control requirements of the material and extrusion system in the molding process.
2. Auxiliary
The composition of the auxiliary machine of the extrusion unit depends on the type of products, the following parts.
1) head (mouth mold). It is the main part of the product molding, when the head of the mouth of the mold cross-section shape of the material is different at the same time, you can get different products.
②Setting device. Its role is to refine the shape and size of the material extruded from the mouth mold, and fix them, so as to obtain a more accurate shape of the cross-section and bright surface products.
③Cooling device. The products coming out of the molding unit are cooled and cured sufficiently in the cooling unit to obtain the final shape.
④Drawing device. It is used to draw out the product uniformly so that the extrusion process is carried out stably. The speed of haul-off, to a certain extent, can regulate the cross-section size of the product, also has a certain effect on the productivity of the extruder.
5 cutting device. Its function is to cut the continuous extruded products into a certain length according to the requirements.
6 stacking or winding device. Used to cut into a certain length of hard products neatly stacked, or soft products into a coil.
3. Control system
The control system of the extruder is mainly composed of electrical instruments and actuators, and its main role is to: control the main and auxiliary drive motors, so that it operates according to the required speed and power, and to ensure that the main and auxiliary coordinated operation; control of the main and auxiliary humidity, pressure, flow rate and the quality of the products; to realize the whole unit of automatic control. Extrusion operation, the main process control factors are as follows
1. screw speed
Screw speed in the extrusion line host control device to adjust. The size of the screw speed directly affects the amount of material output from the extruder, but also determines the heat generated by friction, affecting the flow of molten material pieces. The adjustment of screw speed varies with the screw structure and the material being processed, depending on the shape of the product, the output and the cooling rate in the auxiliary machine.
2. Screw back pressure
The extruder before the porous plate, screen and head on the adjustable resistance elements of the melt flow control effect can produce different screw back pressure. The adjustment of the back pressure makes the material get different degree of mixing and shearing, changing the quality of plasticization and the smoothness of the material supply.
3. Barrel, screw and head temperatures
Thermoplastic polymer solids melt at certain temperatures and are converted to melt. Melt viscosity has an inverse relationship with temperature. Therefore, the extruder's extrusion volume is affected by changes in material temperature. When the material is added to the extruder feeder, it is subjected to the combined effect of the heat provided by the external heating device and the frictional heat generated by the work done. When the material is in the head, the heat is provided by the heating unit outside the head.
If the temperature of the extruded material is not high enough to melt the solid material during the operation, the line will flow poorly and the product will not be of the required quality; if the temperature is too high, the polymer will overheat or decompose. Temperature control is a very important control factor in the extrusion operation.
Temperature control of the screw is related to the material delivery rate, material plasticization, melt quality, many extruders will make the screw into a temperature-controlled structure. The temperature of each section of the barrel is set according to the needs of the changing state of the material. Larger heads also divide the heating unit into individual sections. The temperature of the extruder is set and controlled separately for the screw, each section of the barrel, and each section of the head.
4. Temperature of the molding device and cooling device.
Extrusion of different products, the use of shaping and cooling methods are different, the relevant equipment is a variety. But **** the same need to control the temperature. Cooling medium can be air, water or other liquids, the temperature relationship between the cooling moderate, production efficiency, product stress, if the crystalline polymer, but also related to the crystallinity of products, grain size related to some physical properties. The temperature and flow rate of the cooling medium are adjustable in operation.
5. haul-off speed
Extruder continuously extruded material into the head, the material from the head out of the haul-off, into the shaping device, cooling device, haul-off speed should be matched with the extrusion speed. Drawing speed also determines the product cross-section size, cooling effect. Traction effect produces longitudinal stretching of the product, affecting the mechanical properties of the product and the stability of the longitudinal size, sometimes some processes rely on the traction speed adjustment to obtain the required performance. The traction speed in the extrusion operation is very important.