Injection molding machines, also known as molding machines, primarily consist of four parts: electrical, mechanical, hydraulic, and auxiliary. While they come in a wide variety of types, brands, and functions, their fundamental operating principles remain same.
To complete this cycle, mechanical, electrical, and other components all work in coordination within this cycle, focusing on specific actions. Circuit design must follow motion diagram, as must mechanical movement and hydraulic transmission. After the entire cycle is completed and repeated, system achieves automatic operation. If a component is used to trigger operation, such as a safety door, it becomes semi-automatic. Electrical, mechanical, and hydraulic systems are closely intertwined, mutually controlling and restricting each other. Furthermore, they significantly impact molded product. Relationship between these components is shown in Figure 3-1.

 

Injection molding machine's electrical control system is a crucial component of the entire machine, and its performance is crucial to the overall machine. Application of microcomputer technology in injection molding machines has brought control technology of injection molding machines to a new level. Main functions of electrical part are: driving oil pump motor to supply power to hydraulic part; supplying electric heating power, and automatically controlling injection molding temperature, supplying power to mold adjustment motor, adjusting fixed mold position of injection molding machine, and supplying power to program control action; program control, providing various control signals according to action cycle diagram for combined operation, amplification, feedback and output control voltage to control solenoid valve coil and other loads. Electronic control system of injection molding machine can be divided into following three parts according to its composition: heating control part; motor and its control part; sequence controller part. Following introduces working principle of injection molding machine control.

(2) Temperature control part

In injection molding machine, temperature control of barrel is very important. If temperature is too high, plastic in barrel will decompose and deteriorate; if temperature is too low, plastic will not be plasticized well, fluidity will deteriorate, and product will not be well formed. At present, heating method adopted by injection molding machine is mostly to fasten several sections of heating rings outside barrel, and temperature of each section of heating ring can be controlled independently. Generally, medium-sized machines feature four or three heating stages. Number of heating stages varies depending on machine model, and temperature of each stage can be adjusted based on process requirements.
To control temperature, the first step is to properly detect temperature. Commonly used temperature sensing element is a thermocouple.
Currently, the most common method for temperature control is to use a temperature meter. There are many types of temperature meters. Display methods include pointer and LED displays; temperature settings include knobs and dials; control methods include two-position, PDI, and time-proportional. Choosing one of these meters requires comprehensive consideration of required temperature control accuracy, matching thermocouple, heating control mechanism, and price. Generally, for low-precision control requirements, a two-position control meter and a contactor circuit breaker for heating control can be used.
For higher temperature accuracy requirements, especially for small-shot-volume, precision injection molding machines, a meter with either a time-proportional or time-proportional control is recommended. At this time, actuator of heating control should also be changed accordingly. If output of control instrument is 4-20mA current, corresponding thyristor and corresponding control trigger circuit should be used; if control output of control instrument is pulse width modulation, a solid-state relay or thyristor trigger device should be used. In this case, it is not allowed to use a contact switch because switch is too frequent.
With promotion and application of microcomputer technology in injection molding machines, use of microcomputers to simultaneously control heating of several temperature sections has attracted more and more attention. Using a microcomputer to control heating temperature can achieve very high temperature control accuracy. At the same time, it can easily achieve function of over-temperature alarm and low-temperature alarm.
(2) Motor control part
① Selection of motor. Selection of motor refers to selection of type and capacity of motor, determination of housing and installation type according to working conditions, determination of motor speed and voltage according to load speed and power voltage, and determination of motor capacity according to load size. Motor of injection molding machine often uses a hydraulic pump as a load.
② Motor starting control. Motor starting is generally done by direct start or reduced pressure start. Small capacity motors can be started directly. Large capacity motors need to consider reduced pressure start to avoid fluctuations in power grid. For injection molding machine manufacturers, since it is difficult to determine transformer capacity of user, 11kw motors can be simply used as limit. There are many reduced pressure starting methods, such as resistor reduced pressure starting, reactor reduced pressure starting, autotransformer starting, and star-delta conversion starting. Star-delta conversion starting is more commonly used in injection molding machines.
(3) Sequential controller part
Sequential controller is core of injection molding machine's electronic control system. Injection molding machines mainly use hydraulic systems and controllers to enable machine's actuators (motors, valves, etc.) to complete process in a certain order. Sequential controllers include relay controllers. Many modern injection molding machines use proportional valve hydraulic systems. Process steps of mold closing, mold opening, injection, and pressure holding are divided into several levels. Each level uses different pressures and flows to meet needs of product quality and stable operation of the entire machine. Sequential transitions between process steps are sometimes conditional on position of actuator, while others are time-dependent.
Currently, limit switches, proximity switches, or displacement sensors are often used to detect when an action has been completed. In relay control, limit switches act as contacts to control corresponding relays; in microcomputer controllers, limit switches and proximity switches act as input switches, and displacement sensors act as analog signals.
Time control is often accomplished by timers. In relay control, time relays are often used, with contacts actuating when timer expires to drive relays. In microcomputer control, internal timers are used.
In development of injection molding machine sequence controllers, more and more controllers are adopting programmable controllers and microcomputer systems, using displacement sensors and pressure sensors instead of limit switches and proximity switches for feedback control.

Mechanical control generally includes clamping unit, injection unit, other components such as couplings and safety gates. It also involves mechanical sensing components, such as safety door limiters, frames, and hinges, as well as electromechanical signal acquisition components such as optical encoders and racks.
Hydraulic system generally includes an oil pump, oil pipelines, and various types of solenoid valves. The most important components are proportional flow valves, proportional pressure valves, and combination unit valves.
Auxiliary system includes a cooling pump and pipelines, an oil filter and lubrication system, an automatic loading system, and mechanical electrical protection devices.
Main functions of mechanical system are: to return hydraulic pressure with pressure and speed to control valves to drive cylinders, thereby driving mechanical components according to motion cycle diagram. Controlled valve drives oil motor to rotate screw, generating injection pressure for injection molding. Position signals of various mechanical components are collected and fed back to electrical system for processing.
Hydraulic system's functions include: oil pump generates oil pressure to supply solenoid valves and pipelines, generating operating pressure and flow; proportional flow and proportional pressure valves drive other valves; hydraulic valve rings are all electrically controlled to operate, and they cooperate with mechanical system to complete injection molding process.

(1) Traditional relay type
Includes traditional relay control box, electric control cabinet composed of digital-voltage converter, related signal detection and output control circuits. Among them, digital-voltage converter connects logic switch control circuit and analog control valve, converts dial digital into analog voltage, and controls proportional valve through proportional amplifier. Signal input and numerical display unit can only be a digital dial potentiometer.
Before 1987, most of China's injection molding machines used this control system, which still has a certain market, while it has been completely eliminated abroad. Because system uses wire connection, it is only applicable to a certain fixed process flow. It has disadvantages of long development cycle and debugging time, inconvenient modification, short life, poor reliability, difficult fault detection and error finding, and low control accuracy. As a result, it restricts improvement of China's injection molding machine level and loses market competitiveness. This system has a simple structure and is relatively cheap.
(2) Programmable controller type
PLC is an industrial control device with a microprocessor as core. Over the past 30 years, it has gone through three stages of practicality, maturity and accelerated development, forming three generations of products. Modern PLCs offer following features:
① Small and large sizes, high speed, and large storage capacity;
② Diversified architectures. Small PLCs employ non-bus architectures, while large PLCs employ their own bus architectures. Standard bus-based products have also been developed. Network systems are available in both low-speed and high-speed configurations. These systems can be cascaded and compatible with different types of PLCs and computers, forming local networks with extensive control ranges. These systems can implement multi-layered PLC-based FA network control systems and adhere to Manufacturing Automation (MAP) protocol. These control systems are divided into four levels: a real-time control level for sequential control; a coordinated control level for coordinating coordination of various machines; PLC loading, managing data collection and scheduling; and data processing, where higher-level computers handle all data.
③ Intelligent peripheral modules are developing rapidly. Intelligent interface modules, as independent systems, have their own CPU, system program, memory, and interfaces connecting external processes to control system bus. These modules connect and exchange data with controller's CPU module via system bus, operating independently and in parallel under coordinated management of CPU module. These templates include temperature PID, high-speed counting, position control, network communication, remote I/O, fuzzy control, programming coprocessors, graphic display coprocessors, and network coprocessors. These help improve PLC's information processing capabilities and functionality, ensuring it meets requirements of process and batch control and broadening its application areas.
④ Enhanced programming and control capabilities, using multiple programming languages, enable users to implement process-oriented programming.
⑤ Integrated programmable controllers (IPLCs) can run DOS, including IPLC-Pyramid Integrator launched by A-B in the United States in 1998 and Programmable Computer Controller (PPC) launched by B&R in Austria in 1994. PPC combines functions of a PLC and an industrial control computer (IPC), employing a multi-tasking and modular structure, a time-sharing operating system, excellent openness, network communication capabilities, and high security. It utilizes PG200 graphical programming system and utilizes PL2000 control-oriented structured high-level language for programming.
PLCs can completely replace relays in electrical systems, offering excellent performance, low cost, a simple and easy-to-learn command system, and ease of operation, use, and maintenance. Combined with analog voltage output devices (such as digital-to-voltage converters and potentiometers), they can be used to control injection molding machines using fixed-value technology. Data processing and generation of buffer and ramp signals are handled by other auxiliary functions.
PLCs utilize program storage, offering easy modification, compact design, reliability, responsiveness, and high speed. They not only perform on-off logic control but also offer closed-loop control of analog quantities (such as pressure and flow). Using an LCD or digital display and keyboard, process parameters can be set and modified, and production processes monitored. This has significantly improved the overall performance of injection molding machines, bringing them up to par with similar foreign products from the early 1980s.