Author: Site Editor Publish Time: 2023-10-04 Origin: Site
The co rotating intermeshing twin-screw extruder has become a core equipment in the polymer compounding modification process due to its excellent blending performance, flexible screw combinations with self-cleaning properties.
When selecting a compounding extruder, the production efficiency is a highly valued factor for modified enterprises because high production efficiency brings out maximized economic benefits
It is generally believed that extruders with larger diameters have higher production efficiency. However larger diameter is not the only factor to improve extruder's production efficiency.
Factors affecting extruder's production efficiency include screw speed, volume ratio, specific torque, screw combination design, material state, plasticization level, equipment wear, etc.
→Analysis of Factors Affecting The Production Efficiency Of A Twin Screw Compounding Extruder
◆Material State
Low-bulk density material implies more air in the material. For a twin-screw extruder, the volume of the conveying section is fixed. Within the same volume, lower density also means a decrease in conveying capacity. Lower conveying capacity decreases output.
During the melting process of the extruder, particles and powders are compressed and sheared into a melt, the remaining air is released. The only way for air to leave the extruder is to flow back. The backflowing air can hinder the continuous feeding of powders. When the backflow pressure/velocity exceeds a critical value, powder return can occur. As the screw cross-sectional area becomes smaller or the material filling level approaches 100%, the backflow velocity/pressure of air increases. At this point, increasing the screw speed may have a negative impact on the feeding amount. In other words, the higher the screw speed, the more the feeding becomes fluidized.
The melting process may seem unrelated to the production efficiency of extrusion in theory, but in reality, it is crucial. In actual compounding extrusion plants, materials with lower production efficiency often consist of a combination of resin and powder. Powder could flow only when thoroughly mixed with the melted resin that the screw in the extruder can push it forward for extrusion. If the process condition is set improperly and the resin cannot be adequately melted and plasticized, it can lead to the agglomeration of non-flowing powder, causing blockages in the extruder.
For formulations that include powder materials, most process conditions set up main feeder for a resin and side feeder for powder.In this way,resin and part of the powder are fed into first barrel section,after melting and plasticization in the following 2 to 3 barrel sections, and feed remaining powder through side feeder in the rear section of the extruder. If the resin in the main feed is not fully plasticized and remains in a granular solid state, the subsequently added powder in the rear section may not be properly enveloped and carried by the resin melt, potentially limiting the powder feed and affecting production efficiency.
Additionally, if the material is over-plasticized in the plasticization section, it can lead to a sharp decrease in the viscosity of the resin melt, making it too low in strength and viscosity to adhere to the powder. Simultaneously, the overall friction coefficient of the material decreases, directly impacting conveying efficiency.
The design of the twin-screw extruder screw combination is crucial in the conveying and plasticizing process of the extruder. Generally,considering economic benefit and easy operation,the length of twin screw extruder typically range from 40D to 56D .
For extruder with side-feeder, the plasticizing section length is often only 4D to 8D to ensure that the resin is adequately plasticized. To achieve better plasticization, reverse elements are incorporated into the screw design to increase the residence time. However it can also affect the conveying of the main feed material and increase the energy consumption of the extruder. Therefore, when design the screw combination,a comprehensive consideration is required to ensure that the material is adequately plasticized while avoiding excessive pressure in the extruder.
Regarding conveying function screw elements, it is advisable to choose elements with a pitch about 1D to 2D. According to solid conveying theory, in the non-intermeshing zone ,material is transported forward along the screw flights, and the axial length traveled by the screw flights in one rotation of the screw is equal to the pitch of the screw element. In other words, larger pitch conveying elements have higher conveying efficiency. However, it's importangt to note that an excessively large pitch can introduce excessive circumferential shear, which can hinder material transport.
◆Summary
The nature of extruder conveying is the friction between the material and the screw and the forward transporation by the intermeshing twin-screw intermeshing . If you are trying to improve the production efficiency of a compounding extruder ,you should take the above mentioned factors into accout.
