The vacuum system is the core part of a vacuum plating machine, and operators need to be familiar with the startup order and coordination of each vacuum pump to shorten pumping time and protect the equipment.
When starting up daily, first turn on the cooling water circulation system. The cooling water is crucial for the diffusion pump, molecular pump, and coating sources, so the water pressure and flow should stay within the specified range. Check for any leaks in the water circuit and make sure all inlet valves are open. Then start the mechanical pump, open the roughing valve, and pull the vacuum chamber from atmospheric pressure to low vacuum. The mechanical pump will cause vibration and noise while running, so it should be placed on a vibration-damping base, with the exhaust pipe venting outside.
Once the vacuum reaches around 5 Pa, you can turn on the Roots pump. The Roots pump can't be started at atmospheric pressure, or it might overheat or even get damaged due to overload. After starting the Roots pump, the pumping speed increases significantly and the vacuum drops quickly. At this point, you should keep an eye on the Roots pump's temperature, and if it gets too high, check if the upstream pipes are clear. For systems equipped with a molecular pump, you also need to wait until the Roots pump has been running for a while and the vacuum reaches the Pa range before starting the molecular pump. Since the molecular pump is a high-speed rotating device, listen carefully during startup to make sure there are no unusual vibrations.
Starting a diffusion pump takes a longer preheating time. It takes about 30 minutes for the pump oil to heat up to the working temperature, during which the foreline mechanical pump should keep running. Operators can check if it's ready by looking at the thermometer or thermocouple readings on top of the diffusion pump. While the diffusion pump is preheating, you can close the roughing valve and maintain a low vacuum only through the bypass line. Once the diffusion pump is ready, opening the high-vacuum valve will quickly bring the vacuum level to around 10^-3 Pa or even higher.
During the coating process, process gases are introduced through a mass flow meter, which can cause the vacuum level to rise a bit. Operators need to closely monitor the vacuum gauge readings and adjust the gas flow according to process requirements. For reactive coating, the ratio of reactive gas to working gas needs to be precisely controlled, as fluctuations in vacuum can directly affect the composition and color of the coating. Once the coating is finished, first turn off the process gases, then close the high-vacuum valve, and finally switch off the heating of the diffusion or molecular pump, letting them cool down naturally. The mechanical pump must not be turned off until the diffusion pump has cooled below 80 degrees, to prevent pump oil from backing up and contaminating the vacuum chamber.
The shutdown sequence is the opposite of the startup. After closing the high vacuum valve, the mechanical pump keeps running for a while until the diffusion pump cools down, and then the mechanical pump and cooling water are turned off. During the whole pumping and venting process, operators should avoid opening the vacuum chamber door frequently to prevent moisture and dust from getting in. Every time the vacuum chamber is opened, the seals should be checked and cleaned, and a little vacuum grease applied to maintain good sealing performance.
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