Infrared melt temperature measurement

IR probes that can be mounted in an extruder barrel or die are commercially available [6]. These probes are used to measure a more or less average stock temperature over a certain depth of the polymer, about 1 to 5 mm for most unfilled polymers. The actual depth of the measurement is determined by the optical properties of the polymer melt, in particular the transmittance. The measurement is affected by variations in the consistency of the polymer melt. Thus, when fillers, additives, or other polymeric components are added, the temperature readings will be affected. 

An important advantage of the IR stock temperature measurement is the rapid response time, which is about ten milliseconds. The response of conventional melt thermocouples is several orders of magnitude slower. This means that rapid temperature fluctuations can be made visible with IR allowing a more detailed study of the dynamic behavior of extruders and injection molding machines [83, 84]. 

The elements of an infrared melt temperature sensor are a sapphire window, an optical fiber, and a radiation sensor with associated signal-conditioning electronics as shown in Fig. 4.17. IR melt temperature probes are commercially available [85, 86] and fit in standard pressure transducer mounting holes. Because the sapphire window is flush with the barrel or die, the sensor does not protrude into the polymer melt. As a result, the sensor is less susceptible to damage, there is no chance of dead spots behind the sensor, and the melt velocities are not altered around the sensor. When melt velocities are changed, the melt temperatures will change as well. Therefore, the melt temperatures measured with an IR sensor are less affected by the actual measurement than with an immersion sensor. 

The window material is usually sapphire, because it provides good abrasion resistance and can withstand high pressures. Obviously, there is a potential problem of build-up of material on the window, which could partially block the sensor window. 

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In reality, the temperature distribution is dynamic in other words, melt temperature changes with time. These changes can be significant, but short-term (0-10 seconds) temperature changes cannot be measured with a conventional melt temperature sensor because the thermal mass of the probe is too large. Infrared melt temperature measurement allows detection of rapid (millisecond range) melt temperature fluctuation [114-118]. 

Methods for determining the presence, kind, and amount of configurational base units can be classified as relative or absolute. Absolute methods do not require calibration with polymers of known tacticity. Relative methods, on the other hand, require comparison with standard substances. X-ray crystallography, nuclear magnetic resonance, infrared spectroscopy, and optical activity measurements are all absolute methods. Relative methods include crystallinity, solubility, glass transition temperature, and melting temperature measurements as well as chemical reactions (Table 3-2). 

Figure 10.36 shows the infrared spectra of the liner. It matches the spectra of pol5winylidene fluoride which is a partially fluorinated fluoropol5mier and is susceptible to chemical attack by HF. DSC analysis measured a melting temperature of 165°C for the liner sample. Follow-up discussions with the plant personnel revealed that the wrong spool piece had been installed instead of a PTFE-lined pipe. 

Melt temperature n. The temperature of molten or softened plastic at any point within the material being processed. In extrusion and injection molding, melt temperature is an important indicator of the state of the material and the process. Many types of instruments, most of them based on thermocouples or resistance thermometers, have been employed in extruders, where melt temperature is usually measured in the head and sometimes in the die. In thermoforming, temperatures of softened sheets are measured with infrared pyrometers. 

For temperature measurements on the emerging extrudate, contacting-t)rpe measurements are not suitable because of damage to the extrudate surface. For non-contacting temperature measurements, infrared (IR) detectors can be used. The intensity of the radiation depends on the wavelength and the temperature of a body. Non-contact IR thermometers can be used to determine the temperature of the plastic after it leaves the die. IR sensors can also be used to measure the melt temperature inside the extruder or die see Section 4.3.3.2. 

Previous research has characterized the temperature variations observed in the nozzle by using specialized thermocouple arrays, costly infrared temperature probes, or intricate fixturing. While past studies have developed novel methods for measuring melt temperature, the present work focuses on interpreting data from commercially available temperature probes used for measuring melt temperature in the injeetion molding process. 

Cox and Macosko (19) have reported experimental results on measurements of the melt-surface temperature upon exit from the capillary using infrared pyrometry, which senses the radiation emitted by the hot polymer melt surface. Their work also included the numerical simulation of viscous heating in a capillary, a slit, and an annular die, using a method resembling that of Gerrard et al. (13). They used a boundary condition at the die wall in between the isothermal and adiabatic case, —k(dT/dr) = h T — To) at the wall, where 7o is the die temperature far from the melt-die interface as well as the inlet melt... 

Fig. 12.9 Melt surface temperature rise at the capillary exit, calculated for ABS Cycolac T and measured ( ) with an infrared pyrometer Tq = 505 K, Dq = 0.319 cm, L/Dq — 30. The relationships Nu = C(GZ) 3 are used to estimate h. [Reprinted by permission from H. W. Cox and C. W. Macosko, Viscous Dissipation in Die Flow, AIChE J., 20, 785 (1974).]...

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