Mold Cavity Pressure Sensors

Tbst procedures Mold at least five specimens, using a two-cavity ISO 294-3 type D2 mold, equipped with cavity pressure sensor. Mold at least five specimens. No mold is specified and no cavity sensor is required. 

Cavity pressure sensors are a contradiction in themselves on the one hand, they are highly sensitive measuring instruments on the other, they are in practice often treated no more respectfully than bolts, pins, or other mold standards, although a significant investment is made in them. 

Piezoelectric cavity pressure sensors are suitable for direct use in the cavity due to their physical properties. They are usually installed flush with the mold wall and can be adjusted to the surface by spark erosion or grinding of the sensor front. If installed correctly while maintaining the required drilling tolerances, an error-free measurement of the cavity pressure can be assumed. 

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. 

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. 

Quite similar is the switchover to holding pressure above a pressure threshold value with using a cavity pressure sensor. Once the optimal change is determined, it cannot react to viscosity variations. Both methods have the disadvantage in that the optimal switch over point is determined with difficulty, first using of a mold-filling study, and the switch over point then changes in the course of production. 

Groleau M, Groleau R. Comparing cavity pressure sensor technologies using in mold data. SPE ANTEC 2002. 

The uniformity of injection molding process of different compounds was characterized by melt pressure measurement inside the mold cavity. A piezo-electric sensor (9221 AA0.6, Kistler, Switzerland) incorporated inside the tensile mold cavity and located behind and in contact to the ejector pin to measure the force exerted to the pin by melt pressure during the cyclic molding process against the time. The influence of nano-CaC03 inclusion in the PP... 

Pressure sensors measure the pressure in the mold cavity which is the static compaction pressure initially. When the resin flow front reaches a sensor location, the reading changes to static compaction pressure plus the resin pressure. Thus, the increase in the pressure is evaluated as the resin arrival to the sensor location. These data are used to monitor the flow front position by embedding many pressure sensors in the mold cavity. 

Depending on the cross section of the molded part and the corresponding flow resistance of the melt, a pressure increase is measured during mold filling. After switch over to holding pressure, the melt is compressed, and there is a rapid pressure increase. Because of the pressure transfer in the polymer melt from the flow path end up to the sensor position near the gate, it takes some time for the increase in pressure to be measured, and that is why a cavity pressure-dependent switchover to holding pressure tends to be too late. 

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. 

While the cooling phase lasts until the mold opens, the time at which the cavity pressure again reaches atmospheric pressure is of great importance. At this moment, the plastic is released from the sensor front, and the shrinkage of the molded part starts. This time is not only significant for explaining the p-V-T-diagram but is also used in practical situations. 

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]. 

Tooling. Tooling is relatively expensive for injection molds. Production molds are normally machined and burned (electro-discharge machined) from tool steels (P20) and must be sfructured to withstand high pressures. Precision is required to incorporate cooling lines, ejector pins, and cavity pressure transducers or temperature monitoring sensors. The use of hot runner manifolds adds further complexity and cost, but may be justified because cold runners and their associated costs are eliminated in manufacturing. 

It was determined in this case that both in-cavity (behind ejector pin) pressure transducers and temperature sensors can be utilized to monitor micro-cellular injection molding and should be able to be developed as a basis of a process control model. For the pressure transducer, the gate location appeared to track part weight and dimensions more closely than at the end of fill, across the range of process changes. 

Apart from the visnafization of the flow, the mold also allows qnantitative measnrement of pressure (P) and temperatnre (T) in 5 locations along the flow path. A PT sensor from Kistler is nsed in this work to obtain pressure and temperatnre data from the cavity, located at the position shown on Fignre 4. 

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