Extrusion molding process parameters include temperature, pressure, extrusion rate, and traction speed.
1. The temperature
Temperature is one of the important conditions for the smooth process of extrusion molding. Starting from powder or granular solid materials, high-temperature products are extruded from the machine head and undergo a complex process of temperature change. Strictly speaking, the extrusion molding temperature should refer to the temperature of the plastic melt, but the temperature is largely determined by the temperature of the barrel and the screw, a small part of which comes from the friction heat generated when mixing in the barrel, so the barrel temperature is often used to approximate the molding temperature.
Since the cylinder and plastic temperature in screw paragraphs have differences, in order to make the plastic conveyor in the cylinder, melting, homogenization and extrusion process smoothly, in order to efficiently produce a high-quality product, the key problem is to control the temperature of the cylinder paragraphs, the material is to rely on the regulation of extruder barrel temperature heating cooling system and temperature control system.
The head temperature must be controlled below the thermal decomposition temperature of the plastic, and the die temperature is slightly lower than the head temperature, but the plastic melt should be ensured to have good fluidity.
In addition, the temperature fluctuation and temperature difference in the molding process will cause residual stress, uneven strength of each point, and the surface of the defects such as dull and dull. There are many factors causing such fluctuations and temperature differences, such as the instability of heating and cooling systems and the variation of screw speed, but the screw design and selection have the greatest impact.
The pressure of 2.
In the extrusion process, due to the resistance of the material flow, the change of the depth of the screw slot, and the obstruction of the filter screen, filter plate and die, a certain pressure is generated inside the plastic along the axis of the barrel. This pressure is one of the important conditions for plastic to melt uniformly and obtain compact plastic parts.
Increasing the head pressure can improve the mixing uniformity and stability of the extrusion melt and increase the product density, but too much head pressure will affect the yield.
Like the temperature, the pressure will fluctuate periodically with the change of time, which will also have a negative impact on the quality of plastic parts. The change of screw rotation speed and the instability of heating and cooling systems are the reasons for the pressure fluctuations. In order to reduce the pressure fluctuation, the screw speed should be controlled reasonably to ensure the temperature control accuracy of heating and cooling devices.
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3. Extrusion rate
The extrusion rate (also known as the extrusion speed) is the plastic mass (kg/h) or length (m/min) of the extruder die extruded per unit time. The extrusion speed represents the extrusion capacity.
There are many factors that affect the extrusion speed, such as the structure of machine 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 the screw diameter, the depth of the spiral groove, the length of the homogenization section and the screw rotation speed, and increases with the increase of the melt pressure at the end of the screw and the gap between the screw and the barrel. In the case that the structure of the extruder and the type of plastic and plastic parts have been determined, the extruder rate is only related to the screw speed, therefore, adjusting the screw speed is the main measure to control the extruder rate.
The extrusion rate also fluctuates during the production process, which will affect the geometric shape and dimensional accuracy of the plastic parts. Therefore, in addition to the correct determination of screw structure and size parameters, the screw speed should be strictly controlled, the extrusion temperature should be strictly controlled, to prevent the temperature change caused by extrusion pressure and melt viscosity changes, resulting in the fluctuation of extrusion speed.
4. Traction speed
Extrusion molding mainly produces continuous plastic parts, so it is necessary to set up a traction device. The plastic extruded from the head and the die will be stretched under the action of traction. The higher the degree of tensile orientation, the greater the tensile strength of the plastic parts along the orientation direction, but the length shrinkage is also large after cooling. In general, the traction speed can be equivalent to the extrusion speed. The ratio of the traction speed to the extrusion speed is called the traction ratio, and its value must be greater than 1.
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