Operating procedures for small extruders - Operation inspection

Small extruders are widely used in China, and their deceleration principle is that in most extruders, the screw speed is changed by adjusting the motor speed. The motor typically rotates at full speed of approximately 1750rpm, but this is too fast for the extruder screw. If it rotates at such a fast speed, it will generate too much frictional heat, and the residence time of the plastic will be too short to prepare a uniform and well stirred melt. The typical reduction ratio is between 10:1 and 20:1.

Stage one can use gears or pulley systems, but stage two can use gears and the screw is located at the center of the next large gear. In some slow running machines (such as twin-screw used for UPVC), there may be three deceleration stages, with the maximum speed possibly as low as 30rpm or lower (ratio 60:1). On the other end, some long twin-screw used for mixing can operate at speeds of 600rpm or faster, requiring low deceleration rates and significant cooling. Sometimes, the deceleration speed does not match the task - it consumes too much energy - and it is possible to add a pulley system between the motor and the deceleration phase, thereby changing the high speed.

The extrusion of material as a coolant transfers energy to unheated plastic to melt it. The temperature of the input material is lower than the temperature of the material cylinder and screw surface in the feeding area. The surface temperature of the cylinder in the feed zone is at the melting point of the plastic and is cooled by contact with the feed particles, but the heat is maintained by the heat transferred from the thermal front to the back and controlled heating. Even if the front end heat is maintained through viscous friction and there is no need to input barrel heat, it may still be necessary to turn on the rear heater.

Small extruders are strictly operated according to the operating procedures during use, and inspections during operation include:

1, Check if there are any abnormal sounds in the gear reduction distribution transmission box and the main engine cylinder. If there are any abnormal phenomena, stop the machine immediately for troubleshooting.

2, Is the current of the main motor stable? If there are large fluctuations or rapid increases, the feeding amount should be temporarily reduced and gradually increased after the main current stabilizes. The speed should be adjusted smoothly. The screw is within the specified speed range.

3, Check if the temperature control, heating, and cooling systems are working properly.

4, There should be no abnormal vibration, jolting or other phenomena during machine operation, and the fastening parts should not be loose.

5, The water cooling system pipeline is unobstructed and there is no leakage.

6, Check if the feeding machine is feeding normally and if the screw is feeding evenly.

7, Regularly check the stability and uniformity of the machine head strip, whether there are any broken or blocked strips, poor plasticization, or discoloration due to overheating.

Small extruders have their basic principles, and mechanical principles belong to one of them. Its specific knowledge is that a screw rotates in the cylinder and pushes the plastic forward. The screw is actually a sloping surface or ramp that wraps around the central layer. Its purpose is to increase pressure in order to overcome large resistance. As far as an extruder is concerned, there are three types of resistance that need to be overcome: the frictional force of solid particles (feed) on the cylinder wall and the mutual frictional force between them during the first few rotations of the screw (feed zone); The adhesion of the melt on the cylinder wall; The internal flow resistance of the melt when it is pushed forward.

If an object does not move in a given direction, then the forces on the object are balanced in that direction. The screw does not move axially, although it may rotate rapidly laterally near the circumference. Therefore, the axial force on the screw is balanced, and if it applies a large forward thrust to the plastic melt, it also applies the same backward thrust to an object. Here, the thrust it applies is acting on the thrust bearing behind the feed inlet.

Most single screws have right-hand threads, such as screws and bolts used in woodworking and machinery. If viewed from the back, they are rotating in reverse because they have to try their best to spin out of the cylinder backwards. In some twin-screw extruders, two screws rotate in opposite directions and intersect with each other in two cylinders, so one needs to be right-handed and the other needs to be left-handed. In other interlocking twin-screw systems, the two screws rotate in the same direction and therefore require the same orientation. However, regardless of the situation, there are thrust bearings that absorb backward force, and Newton's principle still applies.

The thermal principle of small extruders is that the plastic that can be extruded is thermoplastic - they melt when heated and solidify again when cooled. Where does the heat from melting plastic come from?

Feed preheating and barrel/mold heaters may be effective and important during start-up, but motor input energy - the frictional heat generated in the barrel when the motor overcomes the resistance of viscous melt to rotate the screw - is an important heat source for all plastics, except for small systems, low-speed screws, high melt temperature plastics, and extrusion coating applications.

For all other operations, it is important to recognize that the cylinder heater is not the main heat source in the operation, and therefore its effect on extrusion may be smaller than we expected. The temperature of the rear cylinder may still be important as it affects the speed of solid material transport in the teeth or feed. The mold head and mold temperature should usually be the desired melt temperature or close to this temperature, unless they are used for a specific purpose such as polishing, fluid distribution, or pressure control.