Limit switch of an injection molding machine

A limit switch on an injection molding machine is a mechanical switch whose contacts change over when a moving part presses its actuator. The most important limit switches are on the safety gate: a cam on the gate presses the switch's roller lever as the gate closes, and the switch tells the controller that the gate is shut. Without that signal the mold will not close. Limit switches are also used to detect the ends of travel of moving parts.

How a limit switch works

The drawing shows a typical gate switch:

• The gate cam is a ramp on the gate that pushes the roller as the gate closes.
• The roller lever turns as its roller rides up the cam, and works the switch contacts.
• The limit switch body holds the contacts and the terminals for the wiring to the controller.

Many limit switches have both a normally closed (NC) and a normally open (NO) contact. When the lever is pressed, the NO contact closes and the NC contact opens. Our safety switch / limit switch has NC + NO contacts and is listed for 24 V DC and 240 V AC circuits.

Switches for safety functions have positive-opening NC contacts: the actuator forces the contacts apart mechanically, so the circuit opens even if a contact has welded. Using both an NO and an NC signal, or two switches, lets the controller or a safety relay detect a switch that has stuck.

Where limit switches are used

• Safety gates: the front and rear gates, often with more than one switch per gate. See the safety gate guide.
• Travel ends: the end positions of moving parts. Which positions use switches depends on the machine; the electrical diagram in the machine manual shows them.
• Guards and covers that must be closed before the machine runs.

Many positions that older machines detected with limit switches are now detected by proximity sensors, which have no lever to wear, or measured by linear transducers.

Signs of a faulty limit switch

• A "safety gate open" alarm while the gate is closed, so the mold will not close.
• Intermittent alarms that stop the semi-automatic cycle.
• The signal on the controller's input page flickers as the gate settles.
• The switch still signals the gate closed while the gate is open. This is dangerous: stop the machine, lock it out and have it repaired before it is used again.

Causes

• Worn gate rollers or guides let the gate sit lower or further out, so the cam no longer presses the roller far enough.
• A loose cam or switch mounting, or a bent lever.
• A worn roller or a broken return spring in the lever.
• Contacts worn, burnt or contaminated by oil and dust inside the switch.
• A damaged cable where it flexes, or a loose terminal.

Checking, adjusting and replacing a limit switch

Electrical work on safety switches is for qualified electricians. Replace a switch only with the machine switched off at the main isolator and locked out.

1. Open the controller's input page and watch the signal while slowly closing and opening the gate.
2. Check that the cam presses the roller fully and squarely, and that the mountings and the gate guides are tight and not worn.
3. Isolate and lock out the machine. Note or photograph the wiring before disconnecting it.
4. Fit a switch with the same contact arrangement, actuator, mounting and rating, and wire it exactly as before.
5. Adjust the switch so that it changes over only when the gate is fully closed.
6. Test the safety function as the machine manual describes: with the gate open, the mold must not close. Never bridge or bypass a safety switch.

What to send for a replacement quote

• A photo of the switch with its label, the lever and the cam.
• The contact arrangement (NO, NC or both) and the voltage of the circuit.
• The mounting: hole spacing or bracket.
• The machine brand, model and serial number, and where the switch is fitted.

The electrical system guide shows how the switch signals are used in the cycle, and the safety switches and sensors category lists what we supply.