---
title: "Safety gate of an injection molding machine"
url: "https://www.imspares.com/injection-molding-machine-parts/electrical/safety-gate/"
---

# Safety gate of an injection molding machine

How the safety gate (safety door) of an injection molding machine and its interlocks stop the mold closing, the signs of a faulty interlock, and how it is tested.

The safety gate of an injection molding machine, also called the safety door, is the sliding guard over the mold area. Its interlock, a set of switches worked by a cam on the gate, tells the controller whether the gate is closed. While the gate is open the machine must not close the mold, so the operator can reach into the mold area to remove parts or clear a jam without being trapped between the mold halves.

## How the safety gate interlock works

The drawing shows the main parts of a gate interlock:

- The **safety gate** slides across the front of the mold area on rollers or runners.
- The **gate cam** is a ramp on the gate. As the gate closes, it pushes the roller of the switch.
- The **roller lever** turns and works the contacts of the **limit switch**, which sends the gate-closed signal to the controller.

Safety gates normally have more than one interlock, so that a single failure cannot let the mold close with the gate open. The electrical interlock usually uses two switches, or one switch with both NO and NC contacts, whose signals the controller or a safety relay compares; if they disagree, the machine stops. On many hydraulic machines the gate also works a hydraulic safety valve that blocks the oil to the clamp, and larger machines often have a mechanical safety device that physically blocks the platen. The [limit switch guide](https://www.imspares.com/injection-molding-machine-parts/electrical/limit-switch/) explains the switches themselves.

## Front, rear and other guards

- The **front gate** on the operator side is opened whenever parts are removed by hand, and it wears the most.
- The **rear gate** on the other side is opened for mold changes and maintenance, and is interlocked in the same way.
- **Fixed guards and covers** around the mold area, the nozzle and the clamp keep hands out of the other danger zones. Some of them are interlocked too.

Opening the front gate during a cycle stops the mold movement. Depending on the machine and its safety design, opening the rear gate may also switch off the pump motor.

## Signs of a faulty safety gate

- A "safety gate open" alarm with the gate closed, so the mold will not close.
- The alarm comes and goes when the gate is pushed or the machine vibrates.
- The gate is hard to slide, or does not close fully.
- The machine moves with the gate open. Stop at once, lock out the machine, and do not use it until it has been repaired and tested.

## Causes

- Worn gate rollers, runners or guides let the gate drop or sit out of line, so the cam no longer presses the roller fully.
- A loose or bent cam, roller lever or switch bracket.
- A failed switch, or a broken return spring in the lever.
- A damaged cable where it flexes, or a loose connector or terminal.
- Oil and dirt in the switch or in the hydraulic safety valve.

## Testing and repairing the safety gate

Test the gate interlocks as often and in the way the machine manual describes, which is often at the start of every shift: with the gate open, the mold must not close. Repairs are for qualified maintenance staff, and electrical work on the switches is for qualified electricians.

1. Watch the gate signals on the controller's input page while slowly closing and opening the gate.
2. Check the gate rollers, guides and stops, and that the gate closes fully and squarely.
3. Check that the cam presses the roller fully, and that the switch and cam mountings are tight.
4. Before changing a switch, switch the machine off at the main isolator and lock it out. Fit a switch of the same type and wire it exactly as before.
5. Adjust the switch so that it changes over only when the gate is fully closed.
6. Test every interlock, electrical, hydraulic and mechanical, before production. Never bridge, tape or bypass a gate switch.

## What to send for a replacement quote

- Photos of the switch with its label, the roller lever and the cam.
- The contact arrangement and the voltage of the circuit.
- Which gate it is on (front or rear), and the machine brand, model and serial number.

See the [safety switch / limit switch](https://www.imspares.com/parts/ec-sf-01/) we supply for gate interlocks and travel-end detection, the [proximity sensor guide](https://www.imspares.com/injection-molding-machine-parts/electrical/proximity-sensor/) for the other sensors around the mold area, and the [electrical system guide](https://www.imspares.com/injection-molding-machine-parts/electrical/) for how the gate signal fits in the cycle.

## Questions

### What does the safety gate interlock do on an injection molding machine?

It tells the controller whether the safety gate is closed. While the gate is open, the interlock stops the mold from closing, so the operator cannot be trapped between the mold halves.

### Why does the machine not close the mold with the safety gate shut?

The controller is not receiving the gate-closed signal. The usual causes are worn gate rollers or a moved cam that no longer press the switch fully, a failed switch, or a damaged cable.

### How often should the safety gate be tested?

As often as the machine manual and your safety rules require, which is often at the start of every shift: with the gate open, the mold must not close. Test it again after any repair to the gate or its switches.

### Is it safe to bypass a safety gate switch to keep the machine running?

No. Bypassing a gate switch removes the protection that keeps the mold from closing on the operator. Repair the gate or the switch and test the interlock before running.

## Replacement parts

- [EC-SF-01 Safety Switch / Limit Switch](https://www.imspares.com/parts/ec-sf-01/): Safety and limit switch for injection molding machine guard door and travel detection.