Why is melt temperature control critical in twin screw extruders?
Jiangsu Xinda Tech Limited
PLASTIC COMPOUNDING EXTRUDER EXPERT SINCE 1996
Co-rotating intermeshing twin-screw extruders (TSEs) are the most common equipment used in the compounding industry for the continuous mixing of polymers with additives and fillers. Many formulations using atypical active ingredients are also processed on this type of machine.
But we must be aware that materials exposed to high shear and high temperatures are very susceptible to degradation, so almost every product benefits from strategically managing of how to transfer shear (and energy) to the material being processed, and measured by the resulting melt temperature.
There are many factors that can affect and control the melt temperature. In this article, the focus will be on the OD/ID ratio, melt zone in the screw, and front end design.
1.?Melting theory and design basis of twin-screw extruder
The TSE will use a segmented screw assembled with a high-torque splined shaft, and the barrel is also modular and utilizes liquid cooling. The motor inputs energy by rotating the screw to feed the metered material into the twin-screw process section, the speed of the screw is independent and set to a mode that optimizes processing efficiency. The segmented screw and barrel combine the controlled pumping and self-friction characteristics of co-rotating screws to match the screw/barrel geometry to the process task. Solids conveying and melting takes place in the first part of the process section. Next is the threaded element for mixing and devolatilization.
Free volume in a process section is related to the OD/ID ratio, which is defined as the outside diameter (OD) divided by the inside diameter (ID) of each screw. Deeper flights result in more free volume and lower average shear rates, but less torque because the screw shaft diameter will be smaller.
2.?Experimental conclusion
Comparing the 1.5/1OD/ID and 1.66/1 model TSE to generate experimental data (as shown below). The craft sections are interchangeable and work with the same gearbox. Initial testing was performed using neat resin with a 40:1 L/D process section and a 40 hp motor.
In ZSE-27 HP (27 mm diameter screw, 1.5/1 OD/ID ratio) and ZSE-27MAXX (28.3 mm, 1.66/1 OD/ID). In each case, the rate limiting factor was the volumetric feed capacity. For the 1.66/1 OD/ID ratio (even at higher throughput rates), the melt temperature is lower due to the lower specific energy input (kWh) per kg processed and the blending associated with deep flight 1.66/1 OD The effect is milder/ID screw geometry.
LDPE powder feedstock with an MFI of 12 was processed on ZSE-27 HP (27 mm diameter screw, 1.5/1 OD/ID ratio) and ZSE-27 MAXX (28.3 mm screw, 1.66/1 OD/ID ratio). In each case, the rate limiting factor was the volumetric feed capacity. The 1.66/1 OD/ID ratio allows more material to be fed into the feed throat before feed restriction is encountered. The achievable increase in feed rate was about 20%, comparable to the increase in free volume associated with higher OD/ID ratios. At higher screw speeds (greater than 800), the percentage increase is not as pronounced, as higher screw tip speeds seem to have a "propeller" effect that somewhat inhibits the feed.
For the 1.66/1 OD/ID ratio (even at the higher throughput rate), the corresponding melt temperature is lower due to the lower specific energy input (kWh) per kg processed, and compared to Shenfei 1.66/1 Milder mixing effects associated with OD/ID helical geometry.
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An additional series of experiments were performed on a ZSE-27 MAXX (1.66 OD/ID) to compare the melting temperature of different melt zone screw configurations (see figure below) with 2MFI PP pellet resin. Compare the "aggressive" melting zone, where melting is complete at barrel 3 position (12 L/D), to the "extended" melting zone, where melting is complete at barrel 4 position (16 L/D). Using a single set of kneading blocks after melting, an attempt was made to isolate and compare different melting zone configurations and resulting melting temperatures. In the experiment, we used a low-pressure discharge die, which can minimize the effect of pressure on the melt temperature, and also used flush and immersed melt temperature probes. Tests were performed at different speeds and screw speeds.
Experimental data were collected on a ZSE 27 MAXX twin-screw extruder (28.3 mm screw, 1.66/1 OD/ID ratio). 2 MFI PE pellet resins were processed, the temperature profile was optimized, and various screw speeds were tested. In each case, the melt temperature of the aggressive design was much higher than the extended zone design.
The aggressive melting zone design utilizes neutral/wide pan kneading block elements and reverse elements to achieve complete melting of the polymer through barrel zone 3. The goal of an aggressive melt zone might be to specify a shorter L/D, or to make room for additional unit operations (i.e. side feeding, mixing or devolatilization) in the second half of the process.
In contrast, elongated screw designs with narrow-disk kneader block elements provide a lower level of shear stress input to the polymer, which causes the polymer to melt more slowly. The purpose of extending the melting zone is to reduce the melting temperature and shear stress exposure of the material being processed. After melting, the individual kneading blocks are partially integrated into the screw design to minimize the heat rise inherent in the mixing process.
The experiment also optimized the temperature profile and tested various screw speeds. Data in the accompanying melting temperature chart was obtained using a hand-held immersion probe.
In each case, the melt temperature for the aggressive design (10°C to 30°C) was much higher than for the extended zone design. It is worth noting that the temperatures measured by immersion probes (sometimes 20°C to over 40°C) are significantly higher than those measured by flushed melt probes. Apparently, when the melt probe is not fully immersed in the polymer melt, the melt temperature reading is affected by the metal adapter set point - lower than actual and not accurate.
The achievable rate for both designs is also maximized by targeting 85% of the operating torque and increasing the rate until this threshold is reached. The extended melting zone design results in higher production rates and lower melt temperatures than aggressive screw designs. Comparing the two melting zones (active and extended) shows that the active melting zone leads to a significant temperature increase and lower achievable productivity compared to the extended melting zone. The higher temperatures inherent to aggressive screw designs can also lead to significant degradation, as evidenced by smoking and discoloration at increased screw speeds.
Source of content: From ptonline, author Charlie Martin (Leistritz), Xindatec Translation.
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