Injection molding control system is a cyclic process, including a set of preset programs, and will run next cycle when (and only if) certain conditions are met. Checking, monitoring of these conditions and giving start, end or continuation commands of each cycle are all done by sequence controller. Sequence controller provides a "safety guarantee" for injection molding machine. For example, when front door of injection molding machine is opened, controller does not allow mold to move.

 

Injection molding control system has following three functions:
Set sequence of actions of injection molding machine
Adjust and control main process and injection molding machine parameters
Inspection process status and performance
Injection molding machine open loop, semi-closed loop, 3/4 closed loop, full closed loop control
Speed and pressure control method of injection molding machine is very important for injection molding. When we manufacture and use injection molding machine, we always mention a term that is "precision". Accuracy of injection molding machine includes repeatability of weight of product, repeatability of size of product, etc. Accuracy of injection molded products is not only related to injection molding machine, but also molds, raw materials, technology, environment, and personnel.
These six aspects have an impact on accuracy of product, and there is no distinction between them. We must know this. We put aside other aspects, we only discuss impact of hydraulic control part of injection molding machine on accuracy of product. Injection molding machine is a key item in production of products. There is a problem that we must understand that accuracy of injection molding machine is not equal to electrical hydraulic control accuracy of injection molding machine. Strength and clearance fit of mechanical part of injection molding machine are also key to accuracy of injection molding.
In industry, injection molding machine control has open-loop, semi-closed-loop, 3/4 closed-loop, and fully-closed-loop terms. Here we will discuss it briefly.
Open-loop control of injection molding machine is main controller computer of injection molding machine. It only issues work instructions to solenoid coils of proportional flow valve and proportional pressure valve according to set process parameters (this command is generally measured by size of current). Coil generates a magnetic force after receiving this current signal, which pushes proportional valve core to move forward or generates a thrust, thereby generating hydraulic system flow and pressure set on our process parameters. As an open-loop control injection molding machine, computer and proportional pressure flow valve of injection molding machine do not detect commands or actions issued by themselves. Understand same, there must be a certain deviation, where is this deviation? In fact, there is a deviation in proportional pressure flow valve. This is because movement of spool is affected by friction, and this friction is affected by oil temperature, so there will be errors in the movement of spool, that is, hysteresis loss caused by temperature effect. On the other hand, there are wear and leakage of valve core, etc. Generally, repeatability of proportional pressure and force flow valve of open loop is between 2-3%. In addition to proportional pressure and flow error, there are pipe resistance loss, circuit leakage (oil plate internal leakage, valve core leakage, oil cylinder leakage), and effect of oil temperature difference. These parts will affect repeatability of injection molding machine. Since open-loop injection molding machine cannot measure actual speed and pressure in system, they cannot be accurately controlled. Generally, repeatability of open-loop injection molding machine is less than 1%.
Semi-closed loop control of injection molding machine is realized by proportional pressure and proportional flow valve of self-closing loop realized by proportional pressure and proportional flow valve of injection molding machine. Proportional valve of self-closing loop has its own controller. Injection molding machine computer sends pressure and flow signal to self-closing-loop proportional valve controller first, then controller sends instructions to proportional valve. Self-closing-loop proportional valve has its own valve position detection device. Through this detection device, spool position signal feedback signal is compared, then corrected signal is output to proportional valve, so that output pressure and flow are infinitely close to setting value of injection molding machine computer. This method is used to improve repeatability of flow and pressure. Generally, repeatability of a self-closing proportional pressure flow valve is 0.2%. This accuracy is very high. Ordinary open-loop injection molding machines will be extremely unstable when flow rate is less than 10%. This situation will disappear when a semi-closed-loop system composed of a self-closing proportional valve is used. Although control precision of self-closing-loop proportional valve itself is very high, it is also self-sweeping. There is no way to control factors that affect accuracy of circuit leakage (oil plate leakage, valve core leakage, oil cylinder leakage) and oil temperature difference effects. Proportional valve can only detect pressure and flow of system by measuring movement position of its spool, but cannot actually detect actual pressure and flow of system, so it is a semi-closed-loop proportional valve.  Precision of semi-closed-loop system is higher and more stable than that of open-loop system, and response speed is relatively fast. 3/4 closed-loop control is realized by a closed-loop variable pump. Because pressure is detected and controlled, it is a fully closed loop, and flow rate is detected indirectly through angle of swash plate, so it is a semi-closed loop. As a closed-loop variable pump, it is also controlled by its own controller. Injection molding machine transmits pressure and flow signals required by process to controller of oil pump. Controller sends instructions to proportional valve, then drives swash plate control cylinder and swash plate control cylinder to control angle of swash plate through proportional valve. Angle of swash plate controls output pressure and flow of system. On the other hand, oil pump itself has a pressure sensor for detecting system pressure and a swash plate position sensor for flow. These sensors transmit detected signals to controller of oil pump. Controller of oil pump compares pressure and flow signal given by computer of injection molding machine with pressure and flow signal detected by sensor, then controller gives a corrected signal to proportional valve to control angle of swash plate, so that actual value of system pressure and flow is close to set value, thereby realizing closed-loop control. Generally, control accuracy of closed-loop variable pump is between 0.2-1%. Since flow is semi-closed-loop control, repeatability of injection molding machine using closed-loop variable pump is between 0.6-0.7%. Control accuracy is higher than that of a semi-closed loop system, stability and response speed are relatively fast.
Closed-loop control of injection molding machine is divided into two types: hardware and software closed-loop. Generally, hinged injection molding machine does closed loop of injection speed, holding pressure, sol speed and back pressure. For the more exquisite molds, control of ejection and injection station also uses closed loop. Direct press, two-platen injection molding machine, and all motors must be closed loop for clamping force. Servo valve for hardware closed loop can achieve the best effect. Software closed-loop uses a general proportional valve, and uses powerful computer computing power to do closed-loop control. Above control is called full closed loop of injection molding machine. If there is cavity pressure conversion, even if injection molding machine is open loop, it is also called "full closed loop of the product". Let’s first explain full closed-loop control of injection molding machine with software closed-loop control. Software closed-loop control is completed by injection molding machine controller computer with closed-loop function. This computer has powerful calculation functions. CPU system response time is less than 0.5ms. Software closed loop is to directly detect system pressure through pressure sensor installed in motion actuator (cylinder, etc.), monitor system flow by detecting motion speed of motion actuator. Because pressure and speed of actuator are directly detected, oil pump error, proportional valve error, pipe resistance error, system unloading, oil temperature effect, etc., these errors that affect accuracy of injection molding machine can be controlled, so its control accuracy is high, repetition accuracy of software closed loop is more than ±0.5%, accuracy of fully closed loop is much higher than that of 3/4 closed loop. When making general products, you can choose an open-loop injection molding machine. If product requires a higher qualified product rate, you can choose semi-closed-loop control. If you have requirements for repeatability (weight, size) and apparent quality of product, you can choose 3/4 closed-loop control. If precision requirements of product are relatively high, it is recommended to use closed-loop control when producing products that belong to ranks of precision injection molding or ultra-high qualified product rate (external quality and internal quality).