Controller of an injection molding machine (main CPU board)

The controller of an injection molding machine is the computer that runs the machine. Its main CPU board holds the machine program: it reads the switches and sensors, steps through the cycle, and sends commands to the valves, the heaters and the servo drive, in the same way as a PLC (programmable logic controller). Most machines use a dedicated injection molding controller rather than a general-purpose PLC, but the principle is the same.

Where the controller sits

The CPU board is mounted in the electrical cabinet with the I/O modules, the drive interface board and the power supply board; the drawing numbers each of them. The operator panel on the front of the machine is its screen and keyboard. Machines use controllers from various makers, and the controller model is printed on the boards and on the operator panel.

How the controller runs the machine

Like any PLC, the controller works in a fast repeating scan:

1. Read the inputs: gate switches, proximity sensors and buttons, and the analog values from position transducers, pressure transducers and thermocouples.
2. Run the program: decide which step of the cycle the machine is in and what must happen next, using the settings from the operator panel.
3. Write the outputs: switch the valve solenoids and contactors, set the commands for the proportional valves and the servo drive, and switch the heater zones.

On top of the sequence, the controller runs closed control loops: temperature control for each heater zone (see barrel temperature control), and on many machines closed-loop control of injection speed and pressure. It also handles the safety logic for the gate and the emergency stop, usually together with separate safety relays or hard-wired safety circuits.

Dedicated controller or general-purpose PLC

• Dedicated injection molding controllers combine the CPU, the I/O, the temperature inputs and the operator panel in one system with the molding software built in. This is the usual arrangement on injection molding machines.
• General-purpose PLCs with a separate touch panel (HMI) are used on some machines and in retrofits; the machine builder writes the program.

Either way, a replacement board must match the controller model and, in most cases, its hardware and software version.

Signs of a failing controller

• The machine does not start up, or the panel stays on the start-up screen.
• A CPU, watchdog or memory alarm, or the controller restarts during production.
• Communication alarms with the panel, the I/O modules or the drive.
• Settings or machine parameters lost after power-off.
• All outputs dead at once although the supply voltages are correct.

Many apparent CPU faults are caused elsewhere: a weak or noisy 24 V supply, a loose connector, a faulty I/O module disturbing the communication between boards, or a flat backup battery. Check these first. Real CPU board failures are often caused by heat and dust in the cabinet, moisture and condensation, voltage surges, or static discharge during handling.

Diagnosing and replacing the CPU board

Board replacement is work for qualified electricians, with the machine switched off at the main isolator and locked out.

1. Record the alarm messages and the status LEDs on the boards.
2. With the machine in a safe state, have the electrician measure the supply voltages at the board.
3. Back up the mold data and machine parameters if the controller still runs.
4. Isolate and lock out the machine. Wear a grounded wrist strap and handle the board by its edges.
5. Photograph the board and its connectors, label every cable, and note the position of any switches, jumpers or memory cards.
6. Fit the new board with the same settings, reconnect it, and restore the program and parameters as the controller manual describes.
7. Check the transducer calibration, then test every movement, the emergency stop and the safety gate before production.

What to send for a replacement quote

• The machine brand, model and serial number.
• The controller model printed on the operator panel and on the board.
• The board's own model number or label, and any version sticker.
• Photos of both sides of the board, with the connectors visible.
• The alarm or symptom.

See the main CPU board we supply, a 24 V DC board listed for Haitian MA and MA II series machines, and the other circuit boards. The electrical system guide shows how the controller fits with the rest of the machine.