Advantages of Nozzle Pressure Measurement The above has demonstrated the importance of pressure regulation during injection and holding pressure. Therefore, the accuracy and repeatability of pressure detection are critical factors. In closed-loop systems, pressure detection is very important. Only by ensuring accurate pressure detection can the regulator be able to bring the actual pressure close to or equal to the setpoint.
In an open-loop system, the accuracy and repeatability of pressure detection are even more important due to the direct connection to the driveline. Now, open-loop systems are still in use, and they are more widely used in high-tonnage models.
In general, the speed control based on the set value is performed during the injection process (that is, the speed change is measured by a potentiometer or a magnetostrictive sensor), and the measurement is converted to pressure regulation. Access can be initiated based on quotas (quota passages) or pressure. In any case, the pressure activation path must be used when the pressure activation path also serves as a “cut” to limit the filling pressure, prevent flash formation, and damage the mold. Once the pathway is established, the subsequent pressure maintenance process is regulated by pressure (profile is no exception). The pressure of the hydraulic press is generally detected in the hydraulic circuit and is rarely performed in the mold nozzle. For injection molding, the probe point must be as close to the mold cavity as possible. Therefore, the mold pressure measurement is preferably performed at the nozzle, and even if it is not directly, the pressure can be measured in the hydraulic circuit.
Unlike mold pressure detection, nozzle detection also controls the plasticization process by adjusting the back pressure. When the pressure close to the injection actually reaches the set point and this pressure is maintained for the time required for the injection of the material, the mold pressure detection can be switched. The measurement can be performed directly or through a probe (eg, a piezoelectric sensor). Direct detection within the mold is very effective, the only limitation being that it will leave traces under the molding. Indirect detection is often affected by probe structure and gaps. For example, excessive tolerance leads to material pours, resulting in insufficient detection accuracy.
Nozzle pressure detection is less effective than cavity pressure detection because the material has to pass through a length of flow path (either cold or hot). However, nozzle pressure detection has certain advantages, mainly including: detection on the material; no need to modify the mold; no trace is left on the molded part. By controlling the melt pressure (preferably in the mold cavity), the risk of overfilling (and subsequent flash formation) at the initial pressure can be avoided. As a result, the effectiveness of control can be improved, burnt material can be avoided, mold filling can be prevented, cycle time can be shortened, and repeatability can be enhanced.
There are indeed technical problems with producing sensors that ensure system reliability and ease of use. If uniform back-pressure is required, the process-related difficulty is indeed small.
The sensor used for nozzle pressure detection must meet the following requirements:
1, can not interfere with the molding process.
2. It can ensure the detection accuracy under high pressure (2500 bar) and high temperature (350-400).
3, small size, solid structure, easy to replace in the event of a failure.
4, when contacted with mold material, it has excellent wear resistance.
5. It can ensure the validity of detection for a long time (when there is friction and pollution after prolonged use, it can ensure that the measurement has no deviation, no error, no lag).
6, to provide high-speed sampling (2-5 microseconds) and standardized communication protocols, such as: CAN open version CANbus or DeviceNet.
Therefore, the problem is more complicated. It is not difficult to understand that, so far, the hydraulic machine is still equipped with sensors in the hydraulic circuit, all the motors are used for force detection, neither of which uses a melt sensor. For many years, melt sensors are widely used in extruders. However, extruders have lower requirements for detection range, accuracy, response time, and structural integrity. (Compared to the static stress on an extruder, installed on On the injection machine, the mechanical fatigue stress on the sensor film is much greater).
