Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with over 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.
Twin-screw extrusion is one of the key processes in polymer processing. Its stable operation and the quality of the final product heavily depend on precise process control, with temperature control being at its core.
Due to the complexity of equipment models, screw configurations, and material diversity, the optimal process parameters are often derived from long-term experimentation and accumulated experience. The following provides a brief introduction to several key temperature-related factors in the twin-screw extrusion process.
01 Extrusion Process Temperature
Many factors must be considered when designing the extrusion process temperature. The primary consideration is the highest melting point among the material components. For example, in a polypropylene masterbatch, the polypropylene carrier has the highest melting point at 170°C, so the process temperatures for each zone are set around 170°C. Of course, this can vary depending on the heating method; temperatures differ between resistance heating and electromagnetic induction heating, with the latter typically being lower.
Secondly, process temperature is related to equipment size and production speed. Higher output requires more heat to soften and melt the material, necessitating a higher process temperature. Temperature is also designed segmentally based on conveying and shear needs. Higher temperature lowers material viscosity and shear force, while lower temperature increases viscosity and shear force. Adjustments to process temperature should also consider other physical properties of the material, such as viscosity.
02 Feed Throat Temperature
The key to setting the feed throat temperature is to prevent material from sticking to the screw, which would hinder normal feeding. To allow the material to melt early for shear dispersion, the feed throat temperature should be as close to the carrier's melting point as possible without causing sticking. In some formulations, low-melting-point additives are present in very small amounts. Even if they melt, they do not significantly affect overall material conveying, so their impact on process temperature is minimal.
However, some formulations contain many low-molecular-weight materials. Slightly elevated temperatures, combined with heat transferred from downstream heating zones via the screw, can cause these materials to melt prematurely at the feed throat, leading to material adhesion and feeding failure. Therefore, during pre-startup heating, the feed zone must be kept at a low temperature. Cooling may need to be activated to maintain this low temperature. Otherwise, screw slippage and feeding issues may occur after startup. To avoid startup abnormalities, it is often better to initially set temperatures conventionally and then lower the feed zone temperature after startup.
03 Vent Port Temperature
The vent port temperature generally needs to be appropriately reduced. Theoretically, to prevent melt from easily flowing out and causing vent bleed, the temperatures of both the zone before and after the vent should be adjusted. This adjustment ensures the material flows forward easily but has difficulty flowing upward and out of the vent port. However, under conditions of stable, rapid flow, good dispersion, and low melt pressure, special adjustments to the vent port temperature may not be necessary. Consequently, many operators do not pay close attention to this parameter.
04 Mixing Section Temperature
The mixing section is a critical area in twin-screw masterbatch production. Its temperature control is related to shear force requirements. Its key function is the shear dispersion of pigments, and the shear force is closely tied to temperature: excessively high temperature reduces melt viscosity and shear force. Appropriately lower temperature increases viscosity, resulting in better shear dispersion. The magnitude of shear force often directly affects the main motor current. Therefore, experienced operators adjust the process temperature in this zone based on changes in the main motor current.
05 Die Head Temperature
Die head temperature design: As the melt enters the die head and is about to be extruded for pelletizing (whether by strand pelletizing, water ring, or underwater pelletizing), the temperature generally needs to be appropriately reduced. Testing can determine the extrudate temperature, noting its difference from the melt temperature inside the barrel. Furthermore, if equipped with an on-the-fly screen changer, the duration and success of the screen changing process are often related to viscosity and melt flow rate, which can be managed by adjusting the die head temperature.
Other Influencing Factors Beyond Temperature
06 Feeder Speed Control
Feeder speed control directly affects output. During stable production, the extrusion rate equals the feed rate. Changing the feeder speed changes the output and simultaneously affects the process. Increasing the feeder speed adds more material into the screw, effectively lowering the process temperature; conversely, decreasing the feeder speed effectively raises the process temperature. Changes in feeder speed also affect product dispersion quality. Therefore, adjustments to feeder speed must be considered holistically, aiming for both a stable masterbatch production process and ensuring final product quality.
07 Main Screw Speed
Main screw speed is the rotational speed of the screws. With constant feed speed, a change in main screw speed only momentarily affects the extrusion rate before it gradually returns to normal. The key role of screw speed lies in shear dispersion, which is another critical factor for controlling product quality. This requires coordination between temperature and shear rate. Some products require high shear, necessitating higher screw speeds. Others require low shear, requiring lower speeds—of course, achieving low shear may also involve adjustments to process temperature. Every machine has a maximum speed limit, which must be respected with an appropriate safety margin.
08 Melt Pressure
Melt pressure is generally kept below 1 MPa. It is related to screen pack mesh size, pigment dispersion effectiveness, melt temperature, and viscosity. Smaller screen mesh, poorer pigment dispersion, and lower melt viscosity lead to higher pressure; conversely, pressure is lower. Melt pressure is a comprehensive reflection of multiple factors; avoid simplistic or arbitrary judgments based on it alone. However, it can serve as a useful reference for adjusting the process and monitoring the state of product dispersion.
09 Screen Pack Configuration and Replacement
Screen packs serve functions such as filtration and increasing shear by causing melt backflow. They should be configured and replaced reasonably according to specific product and quality requirements.
10 Environmental Focus for Twin-Screw Extruders
The key environmental concerns for twin-screw extruders are: first, dust at the feed throat; second, gases from vent ports and the die head; and third, cooling water treatment. Efforts should be made to comprehensively capture, filter, and collect these for proper disposal.
In summary, temperature is the core variable that runs through the entire twin-screw extrusion process. It is tightly coupled with parameters such as feed rate, screw speed, and pressure, collectively determining the melting, conveying, dispersion, venting, and final shaping of the material. A stable, high-quality extrusion process relies on precise and holistic control of the temperature system.
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