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1、Application NotesPressure Sensor in Plastic Machinary By Michael SaliPlastic injection molding machinePlastic MachineryPlastic Injection molding machines are used to manufacture plastic parts. Injection molding machines typically include a reservoir of molten plastic and a nozzle through which the m
2、olten plastic is forced at high pressures into a mold. The plastic is thereafter packed and cooled to form the part. In the manufacture of such parts, it is important to precisely determine the pressure of the injected melt to maintain accuracy of the part because the part must be made at a constant
3、 pressure. This is so because otherwise the resulting part may include voids, ripples, poor dimensional accuracy or the like.Plastic Machinery Working Process CyclesCST3Pressure sensor mounted on plastic machineIn a plastic injection machine, pressure sensors are mounted in the injection molding noz
4、zle, hot runner system or cool runner system and inside the cavity of the mold. it can measure plastic pressure from the injection molding nozzle to cavity plastic pressure somewhere in between plastic injection molding, mold filling, pressure maintaining and cooling process. Those data can be recor
5、ded by the monitoring system, to be used for true time control of the molding pressure, combine inspect or exclude product process appear malfunction after molding.CST4Pressure sensors on plastic machinethere is a need for pressure transducer devices which can accurately measure the pressure of molt
6、en plastic within injection molding machines.Additionally, such pressure transducers are faced with the following constraints: 1. the pressure transducer must have low working stresses to avoid metal fatigue cracking under repetitive cyclic loading; 2. the pressure transducer, for economic practical
7、ity, must be physically arranged such that it can easily be integrated within injection molding machines shot chamber/nozzle area; and 3. the pressure transducer must not disrupt the plastic flow or have pockets where plastic will remain and eventually degrade and cause staining of the parts as the
8、degraded plastic moves into the flow.CST5Methods of switchover in a plastic machineThe available means to switch over in a modern Injection Molding machine, in increasing order of accuracy, are1. injection time2. screw position3. hydraulic pressure4. nozzle pressure5. cavity pressureCST6Injection ti
9、meTemperature effects the viscosity of the melt, which presents resistance to the advance of the screw. Increased resistance slows down the screw and prevents the cavity from filling in the given injection time. On the other hand, reduced resistance would lead to over packing. Injection time switcho
10、ver is the only means available in Injection Molding machines without screw position and pressure sensors.CST7Screw position switchoverScrew position switchover is not effected by temperature nor viscosity. This is the preferred method in machines with screw position potentiometer. Like injection ti
11、me switchover, screw position switchover could be considered open-loop as screw position is not a direct measure of volumetric filling. A leaky nozzle misleads the machine computer into switching over before the cavity is filled. So could a worn screw valve and a worn injection cylinder. Furthermore
12、, if the screw diameter is large relative to (the cube root of) the mold cavity volume, variation of 0.1 mm could give an over packed or under packed fill. Despite its deficiencies, this is the most widely used switchover method in a modern Injection Molding machine most probably because it is a sta
13、ndard (not optional) feature.CST8Hydraulic pressure switchoverThe packing of the melt in the mold cavity has to be balanced by the hydraulic pressure driving the screw forward. A rise in the hydraulic pressure during injection could be used to signal the switchover. Due to a roughly 10:1 ratio betwe
14、en the twin injection cylinders and the screw cross sectional areas, the injection cylinder hydraulic pressure is less than the screw tip pressure by the same ratio. The pressure drop at the runners and sprue gate separates the cavity pressure from the screw tip pressure. The compressibility of the
15、melt (between the cavity and the screw tip) delays the time the pressure is felt. As a result, hydraulic pressure is not an accurate detector of the volumetric filling point. However, hydraulic pressure switchover does have the advantage of the sensor working in a congenial environment (oil temperat
16、ure below 50oC, oil pressure at system pressure, (usually 140 bars) and the sensing is independent of the mold (not attached to the mold). Hydraulic pressure sensor is usually an option in a modern Injection Molding machine.CST9Nozzle pressure switchoverNozzle pressure is also called injection pressure, which is the pressure of the melt in the nozzle or at screw tip. Nozzle pressure switchover is improved ove
