Optimization of injection molding process based on

Optimization of injection molding process based on cavity pressure measurement

measuring cavity pressure sensor has been proved to be the only effective way to quickly optimize injection molding process. Recently, a large number of successful applications show that this technology still has great potential in cost saving and optimization of injection molding process

improve productivity and reduce cost based on cavity pressure measurement

improve productivity and reduce cost is the main purpose of automatic monitoring and optimization of injection molding process. In the past, people mainly focused on process control based on machine parameters, but in recent 20 years, injection molding control system based on mold cavity pressure has proved to be the only way to ensure high consistency of production process and optimize quality. This system can reduce costs and improve productivity through the following ways:

reduce defective product rate

reduce raw material consumption

optimize production cycle time

prevent mold damage

shorten equipment setup time

reduce energy cost

save labor cost

in order to meet the basic requirements of injection molding production for highly automated and simple control system operation, the system solution needs to form a closed loop, That is, quartz sensor for sensing mold cavity pressure, intelligent electronic instrument, user software with friendly interface, communication with injection molding machine, and automatic optimization of setting parameters

revolution in sensor technology

as the quality requirements for injection molded products continue to improve, the measurement system also needs to be more accurate. Even at a pressure of 2000bar, small mass fluctuations must be detected. The general sensor design in the market can not provide satisfactory measurement results. The sensor (Fig. 1) specially designed and developed by Swiss Qishile company for injection molding control can meet the requirements of injection molding process applications. These sensors have long service life, high linearity, wide frequency range (100kHz) and are insensitive to temperature

the linearity and hysteresis of the above two different sensors indicate that only Qishile quartz sensor with special quartz chip is suitable for injection molding process control. The hysteresis and linearity errors of Qishile sensor are only 0.13% respectively. The hysteresis error of the traditional design sensor is ± 13%, and the linear error is 8.2%

Qishile replaces the two-wire technology of the sensor cable with the single wire technology to meet the customized requirements of the cable length depending on the mold. With the single wire technology, the mold manufacturer can cut the cable according to the required length. The clip on plug specially developed by Qishile for this purpose is very easy to assemble. The diameter of single wire technology cable is less than 1mm, which is not only easy to operate, but also does not need pre tightening, so the manufacturing cost of the mold is significantly reduced

reducing the size of the sensor is also a key to the research of this application technology. In recent years, the industrial standard of front-end diameter of cavity pressure sensor is 2.5mm ~ 4 mm. Qishile has developed a pressure sensor with a front-end diameter of 1 mm. Due to the reduction of the sensor size, even the mold with a large number of cavities can be equipped with sensors. This technique has been used to measure the cavity pressure of micro parts

optimization of injection molding process based on cavity pressure

cavity pressure sensor has been successfully applied to:

optimize injection speed, switching point and maintain pressure level and duration

optimize cycle time

reduce setting time

obtain ideal cavity pressure curve can reduce equipment setting time and better protect the mold. Reducing the pressure spike inside the mold during the injection process can prolong the service life of the mold. In addition, it can make the parts obtain good surface characteristics and correct dimensions

the intake pipe of the four cylinder Audi produced by filterwerk mann+hummel company of Germany will sense the mold cavity pressure 2 The screw driver on the Rotating Zigzag rod has been used in the machining control of complex parts and achieved good results. The main quality parameters of the intake pipe are the size and the strength of the parts produced by the shell technology. The overall strength is determined by the strength of a single shell and the weld strength of vibration welding. The performance of the air inlet pipe depends on the setting of the injection molding machine and the processing process characterized by the cavity pressure

process record and part performance

many studies have shown that only mold cavity pressure is closely related to part quality and can be used as process feature record. All other process parameters are not measured near the injection molded parts, so there are certain uncertainty errors

the fluctuation of each production cycle leads to the change of curve in PVT diagram. Through the pressure curve, the change of part size can be detected. Monitoring this parameter can fully identify various fluctuations in the process, without considering the temperature

in addition to monitoring purposes, the measurement data of the cavity pressure sensor can be used for the initial information of the second processing stage. Weidmann plastic technology company of Switzerland uses mold cavity pressure to monitor the production of DaimlerChrysler C-class underbody trim panel, and ensures the quality through threshold monitoring to prevent problems in the next process

reduce the defective rate

the continuous monitoring of mold cavity pressure integrates quality assurance into the whole production process, and there is no need to carry out end product quality inspection. This reduces the cost of quality assurance. At the same time, the inspection of individual parts in mass production is extended to the complete monitoring of all parts

for nonconforming parts beyond the specified tolerance range, the monitoring system will react immediately at the end of the cycle to prevent successive defective products. The traditional quality assurance can only distinguish the defective products after the terminal quality inspection. The modern quality assurance system will archive the monitoring data for a long time for future use, provide quality basis and prove that the production process conforms to the specification

shorten the setting time

the setting of the injection molding machine is usually carried out by the equipment operator according to professional knowledge and experience. As the quality characteristic is the decisive factor for correct setting, the above method is slow and inaccurate, resulting in unreasonable production conditions, high raw material consumption cost and low equipment utilization

an effective solution is to optimize production based on mold cavity pressure. With this method, when the part is optimized, the corresponding cavity pressure curve is stored. When the equipment needs to be reset, the stored "fingerprint" of the mold cavity pressure is used as the reference curve to optimize the processing process so that it is the same as the process corresponding to the reference curve and ensure the consistent quality of parts

another important advantage of this approach is that it is device independent. Different from the parameter setting method of injection molding machine, injection molding machines produced by different injection molding equipment manufacturers can use the same reference curve as the equipment operation standard. In addition, even if the same model of injection molding machine shows different performance due to different wear. The direct monitoring of the cavity pressure in the cavity has nothing to do with the equipment, so the setting error can be eliminated and the defective products can be reduced

weidmann plastic technology company has achieved good benefits by setting up injection molding machine with mold cavity pressure sensor. Although they use compatible injection molding machines, the parameter settings need to be optimized when the mold is changed from one injection molding machine to another. The high requirements of the production process and the high quality of parts require accurate and rapid process optimization to optimize the utilization of equipment and meet the requirements of customers for product quality

automatic identification of switching point

switching point from injection stage to pressure maintaining stage is an important process parameter, which plays a key role in part quality and production cost. When the mold cavity is fully filled, it is necessary to switch from the injection stage to the pressure maintaining stage. Traditionally, injection time, screw position, hydraulic pressure or mold cavity pressure are used as switching parameters. The common feature of these methods is to specify a threshold, and control switching when the parameters reach the threshold. The threshold value is determined repeatedly through the injection process of manual control without maintaining pressure. This method is time-consuming and costly, and has the following two disadvantages:

when the injection pressure curve changes, the threshold value must be determined again

small differences in process and raw materials may lead to premature or late switching

cavity pressure curve can be used to accurately identify the cavity filling degree. Regardless of the geometry of the cavity, the cavity pressure will rise rapidly after the injection phase due to the compression of the melt. The "inflection point" in the cavity pressure curve occurs when the cavity is completely filled, which can be used to trigger the switching of the injection molding machine. Qishile invented a judgment method, that is, the important date: put it into the charge amplifier (smartamp), evaluate the mold cavity pressure of various parts, and calculate the switching point in real time. Smartamp generates a control signal to automatically switch the machine from the injection stage to the pressure maintaining stage

the application of this technology has greatly reduced the defective rate and raw material consumption. The influence caused by process fluctuation is compensated in the cycle. Because of the automatic identification of the switching point, it is no longer necessary to use part of the mold cavity filling to set the parameters repeatedly, which significantly simplifies the setting work

control according to "fingerprint"

only when the energy consumption of the produced parts can be reduced by 30% to meet the requirements, can the production cost be minimized. For this reason, Qishile invented the autoflow system, which can not only control the switching point, but also control other parameters that ensure the same mold cavity pressure curve, as well as the process "fingerprint" of each cycle

smartamp charge amplifier processes the signal of cavity pressure sensor and switches in real time. The pressure signal is used to calculate new setting parameters, such as injection speed, holding pressure value and holding time, as well as mold temperature. The purpose of control is to obtain the same cavity pressure curve, which is a sign to keep the quality of parts consistent in each machining cycle. The calculated setting parameters are transmitted to the injection molding machine through the host computer

reference curve (part "fingerprint") is determined and stored in the process of optimizing part processing, and then controlled by the system. Autoflow uses the current cavity pressure curve and the stored "fingerprint" to continuously calculate new setting parameters. When the die is replaced, call out the "fingerprint" of the part and control again. By using neural networks, untrained novices can also operate injection molding equipment

autoflow can significantly reduce the setup time. Even if there is a big difference between the current process and the reference process, it can be compensated through the entire setup phase through autoflow

automatic balancing

in recent years, multi cavity mold has become more and more important. Process control needs to adapt to this trend. The main entry point is the filling mode of each single cavity

balanced filling, i.e. good balance, can ensure consistent performance of the whole part passing through the mold. The traditional balance method is adjusted by studying the injection method, which is very complicated, and the cost will increase sharply with the increase of the number of mold cavities. For the mold with hot gate, the cost can be effectively saved by automatic balancing

each cavity in the mold is equipped with a cavity pressure sensor. The sensor signal is processed by the charge amplifier and then sent to the multiflow control system. The setting parameters calculated by the multiflow are sent to the hot gate control system or the injection molding machine through the interface

the control system can generate the same injection and compression phase functions in all mold cavities and obtain the same pressure curve. The measured pressure curve is analyzed by fuzzy logic after each cycle to calculate the new set temperature of the hot gate, which is transmitted to the injection molding machine or the hot gate control system. Cavity pressure curve of an unbalanced 8-cavity mold. Pressure curves of all cavities during injection and compression are not