Extrusion molding is a molding processing method with many changes, high productivity, strong adaptability, wide application and large proportion in the field of plastic material processing.
Extrusion molding is to continuously shape the polymer melt or viscous fluid through a certain shape die under the extrusion action of the screw or plunger of the extruder. The resulting parts are continuous profiles with constant section shape.
Extrusion of rubber is usually called extrusion. Rubber extrusion molding has been used earlier, and the equipment and technology are relatively mature. Extrusion is a process that enables the rubber material to be continuously made into semi-finished products of various shapes through the extruder. It is widely used to manufacture tire tread, inner tube, rubber hose and semi-finished products with complex section shape or hollow and solid. It can also be used for rubber wrapping operation. It is an important process in rubber industrial production.
Extrusion molding is applicable to plastic film, mesh, products with coating, pipes, plates, holding materials, sheets, films, monofilaments, cable wrapping layers, various special-shaped materials with certain cross-section and continuous length, as well as powder granulation, dyeing, resin blending, etc; It is also widely used in the production of various reinforced plastic pipes, bars, special-shaped section profiles, etc.
The extrusion process parameters include temperature, pressure, extrusion rate and traction speed.
Temperature is one of the important conditions for smooth extrusion. Dalian extrusion molding starts from powdery or granular solid materials, and high-temperature products are extruded from the die, which has experienced a complex temperature change process. Strictly speaking, the extrusion molding temperature should refer to the temperature of the plastic melt, but this temperature largely depends on the temperature of the barrel and screw, and a small part comes from the friction heat generated when mixing in the barrel. Therefore, the barrel temperature is often used to approximate the molding temperature.
Because the temperature of the barrel and plastic is different in each section of the screw, in order to make the process of plastic transportation, melting, homogenization and extrusion in the barrel smoothly, so as to produce high-quality parts efficiently, the key problem is to control the temperature of each section of the barrel. The adjustment of the barrel temperature is realized by the heating and cooling system and temperature control system of the extruder.
The temperature of the die must be controlled below the thermal decomposition temperature of the plastic, and the temperature at the die can be slightly lower than the temperature of the die, but the plastic melt should have good fluidity.
In addition, the temperature fluctuation and temperature difference in the molding process will cause defects such as residual stress, uneven strength at each point and dull surface. There are many factors causing this fluctuation and temperature difference, such as instability of heating and cooling system, change of screw speed, etc., but the quality of screw design and selection has a great impact.
Dalian extrusion molding
Temperature parameters of several thermoplastic extrusion pipes, sheets, plates and films
In the extrusion process, due to the resistance of material flow, the change of screw groove depth, and the obstruction of filter screen, filter plate and die, a certain pressure is generated in the plastic along the axis of the barrel. This pressure is one of the important conditions for plastic to become a uniform melt and obtain dense plastic parts.
Increasing the head pressure can improve the mixing uniformity and stability of the extrusion melt and improve the density of the product, but too much head pressure will affect the output.
Like temperature, the change of pressure with time will also produce periodic fluctuation, which also has an adverse impact on the quality of plastic parts. The change of screw speed and the instability of heating and cooling system are the reasons for pressure fluctuation. In order to reduce pressure fluctuation, the screw speed should be reasonably controlled to ensure the temperature control accuracy of heating and cooling devices.
3. Extrusion rate
Extrusion rate (also known as extrusion speed) is the mass (in kg / h) or length (in M / min) of plastic extruded by the die of the extruder in unit time. The extrusion speed represents the extrusion capacity.
There are many factors affecting the extrusion speed, such as the structure of head, screw and barrel, screw speed, heating and cooling system structure and plastic characteristics. Both theory and practice have proved that the extrusion rate increases with the increase of screw diameter, screw groove depth, homogenization section length and screw speed, and with the increase of melt pressure at the end of screw and the gap between screw and barrel. When the structure of extruder, plastic variety and plastic part type have been determined, the extrusion rate is only related to the screw speed. Therefore, adjusting the screw speed is the main measure to control the extrusion rate.
The extrusion rate also fluctuates in the production process, which will affect the geometric shape and dimensional accuracy of plastic parts. Therefore, in addition to correctly determining the screw structure and size parameters, the screw speed and extrusion temperature should be strictly controlled to prevent the change of extrusion pressure and melt viscosity caused by temperature change, resulting in the fluctuation of extrusion speed.
4. Traction speed
Extrusion mainly produces continuous plastic parts, so traction device must be set. The plastic parts extruded from the head and die will have tensile orientation under the action of traction. The higher the degree of tensile orientation, the greater the tensile strength along the orientation direction, but the greater the length shrinkage after cooling. Generally, the traction speed can be equivalent to the extrusion speed. The ratio of traction speed to extrusion speed is called traction ratio, and its value must be greater than 1.