Cavity Temperature Sensors

In principle, it should be noted that the functional principle of cavity pressure sensors and cavity temperature sensors is completely different, so the positioning of the sensors should be chosen differently. 

A cavity temperature signal, however, is precisely measured when the melt reaches the sensor. In this way, the position of the melt is always known and can be used for control purposes as automatic switch over to holding pressure. In contrast to the cavity pressure sensor, the cavity temperature sensor can be placed where it is needed. 

A combined pressure/temperature sensor for industrial application is for exactly these reasons not very useful. Figure 5.13 shows typical cavity temperature sensors. 

Often, only small series are produced, so the standard mold is often left on the injection molding machine while the mold inserts are exchanged for other small series. In these cases, the cavity pressure and cavity temperature sensors have to be automatically decoupled with the mold insert otherwise, the entire mold must be disassembled to replace the sensors. 

For this purpose, quick connectors are used, which connect and disconnect the measuring leads by simply sliding on [4]. The advantage of this principle is that the position of the connection always has to be at the same spot but not the position of the sensors, which can be freely selected in the mold insert. Figure 5.14 shows both a quick connector for the cavity pressure sensors, as well as a quick connector for cavity temperature sensors. Both quick connectors are automatically disconnected when the mold insert is removed or replaced. 

Today, for both the cavity pressure sensors and cavity temperature sensors, fail-safe, industrial multi-channel connector concepts are available to simplify the wiring of the sensors and significantly reduce the costs. 

Cavity temperature sensors are usually placed near the flow path end or where they are needed for regulating and control functions such as the sequential control. A cavity temperature sensor near the gate or in combination with a pressure sensor, as already mentioned, makes little sense in the industrial environment, since most procedures are not based on an absolute measurement of temperature, but only evaluate and control or monitor relative information such as temperature changes. 

The delivery of defect-free parts can therefore only be ensured by using cavity pressure and cavity temperature sensors in the mold. 

Especially cavity temperature sensors are increasingly used to control the injection molding process. Here, the arrival of the melt front on the sensor is detected in real time and used for switch over operations in real time. In contrast to the cavity pressure measurement, the position of the melt is always known this way and can he optimized with the help of programmable delay times. This allows moving weld lines in a certain direction, and the meeting of the melt (e.g., in sequential molding) can he optimized [8]. 

Figure 5.23 shows a door sill from the automotive sector that is produced in the sequential molding process. After opening the first nozzle (1), the melt reaches the first cavity temperature sensor, which automatically initializes the opening of the second nozzle (2). Following the same principle, the following nozzles (3 and 4) are automatically opened and the melt flow can be optimized with the help of programmable delay times. 

The detection of the melt front using cavity temperature sensors is used in practice to automate a variety of applications. 

On the other hand, cavity temperature sensors near the end of the flow path deliver a set of information that is ideal for the automatic control of the process. 

Differently filled cavities are the result of viscosity-related filling time differences in the individual cavities. Temperature sensors close to the end of the flow path recognize, however, exactly when the melt reaches this position. An intelligent control system evaluates the information and controls the individual nozzle temperatures until the individual filling times and thus the viscosities are identical [9]. 

Routine temperature measurement within the Discover series is achieved by means of an IR sensor positioned beneath the cavity below the vessel. This allows accurate temperature control of the reaction even when using minimal volumes of materials (0.2 mL). The platform also accepts an optional fiber-optic temperature sensor system that addresses the need for temperature measurement where IR technology is not suitable, such as with sub-zero temperature reactions or with specialized reaction vessels. Pressure regulation is achieved by means of the IntelliVent pressure management technology. If the pressure in the vial exceeds 20 bar, the... 

The MARS-S is constituted of a multimode cavity very close to domestic oven with safety precautions (15 mL vessels up to 0.5 L round-bottomed flasks, magnetic stirring, temperature control). The magnitude of microwave power available is 300 W. The optical temperature sensor is immersed in the reaction vessel for quick response up to 250 °C. A ceiling mounted is available in order to make connection with a conventional reflux system located outside the cavity or to ensure addition of reactants. These ports are provided with a ground choke to prevent microwave leakage. It is also possible to use a turntable for small vessels with volumes close to 0.1 mL to 15 mL vessels (120 positions for 15 mL vessels). Pressure vessels are available (33 bar monitored, 20 controlled). 

The SmithSynthesizer and SmithCreator are systems devoted to laboratory synthesis. They are constituted by a closed rectangular waveguide section playing the role of cavity. They can use specific cylindrical tubes. Pressure and temperature sensors allow real-time monitoring and control of operating conditions. This system was a good solution for laboratory experiments. The SmithSynthesizer is described by Fig. 1.13. More details about the SmithSynthesizer could be found in Sect. 12.7 in Chapt. 12. 

Fig. 2.1 Schematic diagram of the CMR [22]. 1, reactants for processing 2, metering pump 3, pressure transducer 4, micro-wave cavity 5, reaction coil 6, temperature sensor 7, heat exchanger 8, pressure regulator 9, microprocessor controller ...

In a representative reaction, 1.0 mmol aniline derivatives, 1.1 mmol dihalides, and 1.1 mmol potassium carbonate in 2 mL of distilled water were placed in a 10 mL crimp-sealed thick-wall reaction tube equipped with a pressure sensor and a magnetic stirrer. Tlie reaction tube was placed in the MW cavity (CEM Discover Focused Microwave Synthesis System with a build-in infrared temperature sensor), operated at 120 + 5°C, power 80-100 Watt and pressure 65-70 psi, for 20 minutes. After completion of the reaction, the organic portion was extracted into ethyl acetate. Removal of the solvent... 

The Ethos MR contains a multimode cavity very similar to that of a domestic oven but with safety precautions. It can use standard glass (420 mL up to 2.5 bar) or polymer reactors (375 mL up to 200 °C and 30 bar) with magnetic stirring. The microwave power available is 1 kW. The optical temperature sensor is immersed in the reaction vessel for rapid response up to 250 °C. An infrared sensor is also available. A ceiling mounting is available to enable connection with a conventional reflux system located outside the cavity or for addition of reactants. The Ethos CFR is illustrated in Eig. 2.17. It is a continuous-flow variant of the Ethos MR . 

Rgure 26.4 A novel continuous microwave reactor system (redrawn from Strauss and Trainor, 1995) 1 reactants for processing 2 metering pump 3 pressure transducer, 4 microwave cavity 5 reaction coil 6 temperature sensor 7 heat exchanger 8 pressure regulator 9 microprocessor controller 10 product vessel... 

Specialized microwave reactors for chemical synthesis are commercially available from such companies as CEM [20], Lambda Technologies [21], Microwave Materials Technologies (MMT), Milestone [22], PersonalChemistry [23], and Plazmatronika [24] which are mostly adjusted from microwave systems for digestion and ashing of analytical samples [25]. They are equipped with built-in magnetic stirrers and direct temperature control by means of an IR pyrometer, shielded thermocouple or fiber-optical temperature sensor, and continuous power feedback control, which enable one to heat reaction mixture to a desired temperature without thermal runaways. In some cases, it is possible to work under reduced pressure or in pressurized conditions within cavity or reaction vessels. 

The microwave equipment consisted of a microwave generator (85 W) and a tunable cavity operating in the TE mode, while temperature was monitored using a fiber-optic temperature sensor. The samples were maintained in a Teflon vessel of 1.5 cm diameter hole and 1.5 cm deep. During a typical run, 25 W of micro-wave power was required to heat the sample to the desired temperature over 80-200 s. It was demonstrated that microwave irradiation increased the rate of solution imidization over that obtained for conventional treatment by a factor of 20-34, depending on the reaction temperature. The apparent activation energy for this imidization, determined from an Arrhenius analysis, was reduced from 105 to 55 kj/mol when microwave activation was utilized rather than conventional thermal processing. 

A constant cavity temperature can be best achieved by an external temperature control. For the temperature controllers used in practice, a temperature sensor (thermocouple) is necessary, which is placed in the right location of the mold. Strong temperature fluctuations close to the cavity occur through the injected plastic, especially at the cavity surface (Figure 2.80). These occur for physical reasons (mold material, shape, molding compound, temperature) and cannot be influenced by the cooling system. 

If the temperature amplitude in a dimensionless presentation, of any distance x from the cavity surface, via the quotient position of the probe is applied to the temperature oscillation/l, a universal context results, out of which the position of the temperature sensor can be determined at a desired temperature amplitude (Figure 2.107). 

As already mentioned, the industrial measurement of the cavity temperature has been systematically enhanced only in recent years. The basis for this advancement are specially designed thermocouples, which also are built into the cavity, just as the cavity pressure sensors, and touch the melt or the molded part later on in the process. In contrast to conventional thermocouples, some series have been optimized that on arrival of the plastic melt they can react in a very short time, and can be used for switching and control operations [3j. The application possibilities of these sensors are also very versatile and effective, and the costs are kept within limits, compared to the cavity pressure sensors. 

---