1. Anti-smashing

The working condition of coal mining is harsh. Some keyboards may be smashed. The protective cover can only protect the keyboard circuit, but cannot protect the exposed keys. Therefore, we should do something to protect the keyboard. Drop molding technology is a perfect solution.

Drop molding is based on the fact that thermoplastic polymer materials have viscous fluidity under certain conditions, and can return to a solid state at room temperature. People use appropriate methods and special tools to mold them into different shapes as required when they are in a non-fluid state. The method of dispersing plastic on the indicator film can effectively improve the anti-smash performance of the keyboard.

2. Moisture-proof

We are talking about membrane switches working in the harshest environments underground, where the humidity is particularly high. More than 50% of keyboards are damaged due to moisture or dust. Therefore, ordinary membrane switch cannot meet the requirements for underground working condition. It is necessary to place a protective cover over the membrane switch, and fasten the keyboard plate to the housing of the device with screws.

To prevent the keyboard from deforming when the screws are tightened, and ensure that the peripheral screws can compact the keyboard, a frame is added around the keyboard board. The raised steps around the keyboard can compact the double-sided tape, to ensure sealing performance.

3. High temperature resistance

The film and adhesive should have high temperature resistance, and be non-volatile at high temperatures. Therefore, you should try not to use adhesive without a base material to prevent it from sticking to the key circuit, causing increased contact resistance. The layers that make up the membrane switch will deform and expand under high temperature, resulting in decrease in performance of the membrane switch.

Therefore, when producing membrane switch for mining, the film should be shrunk. After heating, exhaust air and dry, then shrink.

4. Electromagnetic compatibility

Shearers, scraper conveyors and other equipment in coal mines are equipped with large motors. There will be electromagnetic interference during the production process. For this reason, electromagnetic compatibility measures should be taken for the keyboard. Otherwise, the display window may become a source of electromagnetic compatibility 'leakage window' of the entire machine. It makes the whole machine unable to pass the routine electromagnetic compatibility test.

Put a layer of copper foil above the membrane switch, or add a layer of passivated (or silver-plated) copper mesh, and connect it to the housing. It can improve the anti-interference ability of the bracket controller against spatial fields and electrostatic fields.

In addition, adding a metal protective cover to the keyboard can prevent electrostatic discharge from the human body. It can effectively protect the integrated circuit from being destroyed.

5. Tests

We have some tests on the mining membrane switch to ensure the reliability. Let's dive in.

a. Fatigue resistance test

When the membrane switch is continuously working, check whether the metal domes will not return to reset, and whether it will inhale moisture or dust in the air, and whether it reduces the contact conductivity of the keyboard.

In addition, the mechanical load force should meet the requirements. After applying an operating force of 100N to the center of membrane switch for (60±5)s, the membrane switch should still work normally.

b. Moisture resistance and waterproof tests

Check whether water will enter the keyboard under external pressure. Put the device with membrane switch below 1.2m underwater for testing. In addition, the membrane switch is also placed in a device with high-pressure water jet for testing.

c. Key operation strength test

Check the operating force and feel of the metal domes to determine whether the keyboards' operating comfort meets usage requirements.

d. High and low temperature tests

Test the double-sided tape of the membrane switch when high and low temperatures change alternately.

e. Insulation performance test

The membrane switch should pass the insulation performance test of 500MΩ and DC500V.