04,01,2026 16 views
1. In the feeding area, stick to the cylinder and slide onto the screw
In order to maximize the solid particle conveying capacity in the smooth barrel feeding area of a single screw extruder, the particles should stick to the barrel and slide onto the screw. If the particles stick to the root of the screw, nothing can pull them off; The volume of the channel and the inlet volume of solids are reduced. Another reason for poor adhesion at the root is that plastic may be heated here and produce gel and similar pollution particles, or intermittently adhere and break with the change of output speed.
Most plastics naturally slide at the roots because they are cold when they enter, and friction has not yet heated the roots to the same level of heat as the cylinder wall. Some materials are more likely to adhere than others: highly plasticized PVC, amorphous PET, and certain desired polyolefin copolymers with adhesive properties for final use.
ForNanjing twin-screw extruderThe cylinder and plastic need to adhere here so that it can be scraped off and pushed forward by the screw threads. There should be a high coefficient of friction between the particles and the cylinder, which in turn is strongly influenced by the temperature of the rear cylinder. If the particles do not adhere, they only rotate in place and do not move forward - this is why smooth feeding is not good.
Surface friction is not a factor that affects the feeding process. Many particles never come into contact with the cylinder or screw root, so there must be friction and mechanical viscosity linkage inside the particles.
A slotted cylinder is a special case. The trough is located in the feeding area, which is thermally insulated from the rest of the cylinder and deeply water-cooled. The thread pushes the particles into the groove and creates a high pressure within a relatively short distance. This increases the bite allowance for the same output at lower screw speeds, thereby reducing the frictional heat generated at the front end and lowering the melt temperature. This may mean that cooling limits faster production in blown film production lines. The groove is particularly suitable for HDPE, which is the most smooth ordinary plastic besides perfluorinated plastic.
2.Nanjing twin-screw extruderThe pressure at the end of the screw is very important
This pressure reflects the resistance of all objects downstream of the screw: the filter screen and contamination crusher plate, adapter conveying pipe, fixed agitator (if any), and the mold itself. It not only depends on the geometric shape of these components, but also on the temperature in the system, which in turn affects the resin viscosity and passage speed. It does not rely on screw design, except when it affects temperature, viscosity, and throughput. For safety reasons, measuring temperature is important - if it is too high, the mold head and mold may explode and harm nearby personnel or machines.
Pressure is beneficial for stirring, especially in the rear area (metering zone) of a single screw system. However, high pressure also means that the motor needs to output more energy - and therefore the melt temperature is higher - which can set the pressure limit. In twin-screw, the interlocking of two screws is a more effective agitator, so no pressure is required for this purpose.
When manufacturing hollow components, such as pipes made with spider molds that use brackets to position the core, high pressure must be generated inside the mold to assist in the reassembly of separated logistics. Otherwise, products along the welding line may be weaker and may encounter problems during use.
3.Nanjing twin-screw extruderMechanical principles
The basic mechanism of extrusion is simple - 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 greater 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.
Newton once explained that if an object does not move in a given direction, then the forces on that 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 must be right-handed and the other must be left-handed. In other interlocking twin-screw systems, the two screws rotate in the same direction and therefore must have the same orientation. However, regardless of the situation, there are thrust bearings that absorb backward force, and Newton's principle still applies.
