Decide 1: Schematic of a variable-depth thermocouple positioned in a flange of a swap line.
Provide: Mark Spalding
To be worthwhile, a single-screw extruder ought to operate on essentially the most payment whereas discharging at a specified stress and temperature. Measuring the pace and discharge stress from an working extruder is simple and straightforward. Measuring the discharge temperature, nonetheless, should not be really easy, notably using thermocouples positioned inside the swap line merely upstream of the die. This configuration is used extensively for industrial extrusions. The problem occurs due to the extreme thermal conductivity of the encircling metallic and the low thermal conductivity of molten resins.
As an example, a variable-depth thermocouple positioned in a swap line and via a flange is confirmed in Decide 1. The thermocouple measures the temperature on the junction on the tip of the probe. The temperature on the tip of the probe will depend upon thermal conduction and convection inside the native space. The sheath of the probe is usually produced from stainless-steel whereas the swap line is constructed from carbon steel. The thermal conductivity for stainless-steel is 17 W/(m°C) and for carbon steel it is 52 W/(m°C). The molten plastic flowing inside the swap line, nonetheless, has a thermal conductivity of about 0.25 W/(m°C).
Thus, the thermal conductivity for the encircling metallic is between 70 and 200 events elevated than that for the molten resin. As a consequence of this big distinction in thermal conductivities, the junction of the thermocouple could be very influenced by the swap line temperature and to a lesser stage from the molten resin.
This measurement downside is clearly acknowledged with a set of experiments. These experiments had been carried out using a 1.25-inch diameter single-screw extruder associated to a 25-mm diameter swap line. The circumstances of the extruder had been held fastened with a payment of 15 lbs/hr. of LDPE at a screw tempo of 60 rpm. The swap line pipe was maintained at each 183°C or 220°C.
The temperature profile of the flowing resin inside the swap line was measured using a plastic bridge constructed using a high-temperature resin (not confirmed). The bridge was positioned all through the flow into stream, and it was designed to eliminate energy conduction via the thermocouple gadget. It was positioned inside the heart flange in Decide 1. Thus, the bridge gadget contained plenty of thermocouples and measured the exact temperature of the flow into, and almost eradicated the thermal conduction downside.
Decide 2: Exact temperature profile and the profile measured using a variable depth thermocouple positioned in a swap line. The swap line pipe was managed to a temperature of 183°C.
The bridge was too fragile to be used in industrial operations. The swap line carbon steel pipe was managed at a temperature of 183°C. The radial temperature profile from the bridge gadget is confirmed by the “exact profile” line in Decide 2. Proper right here the profile is parabolic with the underside temperature being on the wall at 198°C, and the temperature on the center of the pipe at 233°C. This profile occurred because of the extruder was discharging at a temperature near 233°C, and the swap line was in a cooling mode with the pipe temperature at 183°C. The flow into velocity inside the downstream course was parabolic and symmetric to the pipe axis.
Subsequent, a variable-depth thermocouple was positioned inside the swap line as confirmed in Decide 1. The temperature on the junction was measured as a carry out of the insertion depth, as confirmed by Decide 2. Proper right here, the measured temperature elevated as a result of the probe was inserted deeper into the swap line. The utmost temperature was at an insertion depth of twenty-two mm at 232°C. Commercially, soften temperature measurements are obtained using probes which is perhaps flush mount to the wall. For this experiment, the temperature near the wall was measured at 198°C. This measurement is clearly in error as nearly all of the material is at a temperature near 233°C.
Moreover, the exact temperature from the bridge gadget at 22 mm into the stream was measured at 213°C. A extreme stage of thermal conduction via the sheath of the variable-depth thermocouple, nonetheless, provides an incorrect measurement at 232°C on the probe junction. At 22 mm into the stream, the conduction via the sheath, nonetheless, supplied an ideal estimate of the bulk temperature. Insertion of a thermocouple 90% all through a flow into stream is commercially impractical, notably for greater diameter pipes. The forces from the viscous flow into could possibly be extreme ample to bend the probe.
Decide 3: Exact temperature profile and the profile measured using a variable depth thermocouple positioned in a swap line. The swap line pipe was managed to a temperature of 220°C
The swap line pipe temperature was then elevated and managed at 220°C. The operation of the extruder was unchanged and thus the extrudate must be on the equivalent temperature as sooner than. The exact temperature profile was measured using the bridge gadget and it is confirmed in Decide 3. The inside wall temperature was 229°C, and the exact temperature profile was parabolic with the utmost temperature of 234°C on the center. The profile is considerably flatter than that confirmed in Decide 2 with a pipe temperature of 183°C. It is obvious that the thermal gradients are very small. A pipe temperature of 220°C did not induce the extreme stage of cooling that was seen for a pipe temperature at 183°C.
The swap line should not be used as a way to decrease the discharge temperature from an extruder.
The variable-depth thermocouple measurements had been nearly linear from 229°C on the wall to a most temperature of 235°C at a depth of twenty-two mm. For a industrial swap line with a flush-mount thermocouple, the temperature could possibly be reported at 229°C, a temperature close to the bulk temperature of 233°C. A variable-depth thermocouple that is perhaps inserted 4 mm into the flow into would report a temperature of 230°C. The probe experiences a lot much less thermal conduction from the swap line pipe, and it provides a higher measurement of the resin flow into.
The swap line should not be used as a way to decrease the discharge temperature from an extruder. As confirmed in Decide 2, a considerable stage of thermal gradients was developed inside the flowing resin all through cooling. The gradients will affect the viscosity of the resin and doubtless distort the shape or thickness of the product coming out of the die. In its place, the swap line must be managed near the bulk temperature of the extrudate, minimizing thermal gradients on the die. On account of the bulk temperature of the extrudate is usually not acknowledged, it must be generally measured using a handheld thermocouple inside the extrudate exiting the die. The swap line pipe temperature must be managed near this temperature.
Measuring the soften temperature using a thermocouple positioned in a swap line is regular inside the commerce. Even though the technique can current measurement errors, it is used because of it is simple, low worth and guarded. Many events, the soften temperature could possibly be measured using a handheld thermocouple probe by sticking it into the extrudate stream. It could nicely take a minute or further to get the probe on the temperature of the extrudate. Usually the downstream instruments much like rolls can cease the protected measurement of the extrudate. On this case, the extrudate temperature must be measured using in infrared (IR) gun.
Regarding the Author: Mark A. Spalding is a fellow in Packaging & Specialty Plastics and Hydrocarbons R&D at Dow Inc. in Midland, Michigan. All through his 39 years at Dow, he has centered on progress, design and troubleshooting of polymer processes, notably in single-screw extrusion. He co-authored Analyzing and Troubleshooting Single-Screw Extruders with Gregory Campbell. Contact: 989-636-9849; maspalding@dow.com; dow.com.