Pressure sensors can be installed in the nozzle, hot runner system, cold runner system, and mold cavity of injection molding machines. They can measure the plastic pressure between the nozzle and the mold cavity during the injection molding, filling, holding, and cooling processes. This data can be recorded in the monitoring system for real-time adjustment of molding pressure and for inspection or troubleshooting during the production process after molding.
It is worth mentioning that this collected pressure data can become a universal process parameter for this mold and material, in other words, this data can guide production on different injection molding machines (using the same mold). We will only discuss the installation of pressure sensors inside the mold cavity here.
Types of pressure sensors
At present, there are two types of pressure sensors used in mold cavities, namely flat mounted and indirect type. Flat mounted sensors are inserted into the mold cavity by drilling a mounting hole behind it, with its top flush with the surface of the mold cavity，The cable passes through the mold and is connected to the monitoring system interface located on the outer surface of the mold. The advantage of this sensor is that it is not affected by pressure interference during demolding, but it is easily damaged under high temperature conditions, making installation difficult. Indirect sensors are divided into two structures: sliding and button type. They can all transmit the pressure exerted by the plastic melt on the ejector or fixed pin to the sensor on the mold ejector plate or moving template. Sliding sensors are usually installed on the ejector plate under the existing push pin. When conducting high-temperature molding or using low-pressure sensors for small top pins, sliding sensors are generally installed on the moving template of the mold. At this time, the push pin works through the ejector sleeve or another transition pin is used. The transition pin has two functions. Firstly, it can protect the sliding sensor from the interference of demolding pressure when using the existing ejector. Another function is that when the production cycle is short and the demolding speed is fast, it can prevent the sensor from being affected by the rapid acceleration and deceleration of the ejector plate. The size of the push pin at the top of the sliding sensor determines the required size of the sensor. When multiple sensors need to be installed inside the mold cavity, it is best for mold designers to use top pins of the same size to avoid setting or tuning errors by the mold manufacturer. Due to the function of the top pin being to transmit the pressure of the plastic melt to the sensor, different products require different sizes of top pins. Generally speaking, button type sensors need to be fixed at a certain recess in the mold, so the installation position of the sensor must be the most interesting position for the processing personnel. To disassemble this type of sensor, it is necessary to open the template or make some special designs on the structure in advance.
Depending on the position of the button sensor inside the mold, it may be necessary to install a cable junction box on the template. Compared to sliding sensors, button sensors have more reliable pressure readings. This is because the button type sensor is always fixed in the recess of the mold, unlike the sliding type sensor which can move inside the borehole. Therefore, button type sensors should be used as much as possible.
Installation position of pressure sensor
If the installation position of the pressure sensor is correct, it can provide the maximum amount of useful information to the molding manufacturer. Except for some exceptions, sensors used for process monitoring should usually be installed in the rear third of the mold cavity, while sensors used for controlling molding pressure should be installed in the front third of the mold cavity. For extremely small products, pressure sensors are sometimes installed in the runner system, but this can prevent the sensor from monitoring the pressure of the sprue. It should be emphasized that when the injection is insufficient, the pressure at the bottom of the mold cavity is zero, so the sensor located at the bottom of the mold cavity becomes an important means of monitoring the injection shortage. With the use of digital sensors, sensors can be installed in every mold cavity, and the connection from the mold to the injection molding machine only requires one network cable. In this way, as long as the sensor is installed at the bottom of the mold cavity without any other process control interfaces, the occurrence of insufficient injection can be eliminated.
Under the above premise, the mold design and manufacturer also need to decide which recess in the mold cavity to place the pressure sensor in, as well as the position of the wire or cable outlet. The design principle is that wires or cables cannot move freely after being threaded out of the mold. The general practice is to fix a connector on the mold base, and then use another cable to connect the mold with the injection molding machine and auxiliary equipment.
The important role of pressure sensors
Mold manufacturers can use pressure sensors to conduct strict mold testing on the molds that are about to be delivered for use, in order to improve the design and processing of the molds. The molding process of the product can be set and optimized based on the first or second trial molding. This optimized process can be directly used in future trial molds, thereby reducing the number of trial molds. With the completion of the trial mold, not only did it meet the quality requirements, but it also provided the mold manufacturer with a validated set of process data. These data will be delivered to the mold manufacturer as part of the mold. In this way, the mold manufacturer provides the molder not only with a set of molds, but also with a solution that combines the mold and the process parameters suitable for the mold. Compared to simply providing molds, this approach has increased its intrinsic value. Not only does it greatly reduce the cost of trial molding, but it also shortens the time for trial molding.
In the past, when mold manufacturers were informed by their customers that molds often had problems such as poor filling and incorrect key dimensions, they had no way of knowing the state of the plastic in the mold. They could only speculate on the cause of the problem based on experience, which not only led them astray, but sometimes could not completely solve the problem. Now they can accurately determine the crux of the problem by analyzing the state information of the plastic in the mold collected from the pressure sensor by the mold manufacturer Although not every mold requires a pressure sensor, every mold can benefit from the information provided by the pressure sensor. Therefore, all mold manufacturers should be aware of the important role that pressure sensors play in optimizing injection molds. Mold manufacturers who believe that the use of pressure sensors plays a crucial role in manufacturing precision molds can enable their users to produce products that meet quality requirements more quickly, while also promoting the improvement of their mold design and manufacturing technology.
Post time: Feb-19-2025