What are the difficulties in vacuuming with low-temperature coating vacuum chamber?
The following special factors cause the difficulty of vacuuming the vacuum chamber for low-temperature coating:
1. There is the influence of oil vapor reflux. All PVD vacuum systems that use mechanical pumps and diffusion pumps to pump air have backflow pollution of oil vapor.
2. It is difficult to reactivate the adsorbent. Before the adsorbent is put into the vacuum chamber, it is generally activated by heating and drying treatment or low vacuum heating and drying treatment.
3. Water vapor is not easy to extract. When the material is outgassed, the gas adsorbed on the surface is first released. The main components of these gases and the gas desorbed by the adsorbent are water vapor. The adsorption heat of water vapor is high, the average residence time of collision on the surface of the material is long, and it is not easy to desorb.
4. Low heating efficiency when pumping air. Multi-layer insulation has good thermal insulation performance under vacuum. If the container is heated and degassed during the pumping process, it will cause difficulty in raising the temperature of the insulating material and adsorbent, uneven temperature distribution, and low heating efficiency.
5. Allow small deflation. In the case of using low-temperature adsorbent alone, the usually required allowable outgassing rate is 10⁻²Pa·L/s~10⁻⁷Pa·L/s. It is equivalent to the allowable deflation of the dynamic ultra-high vacuum system of the same volume.
6. Low degassing temperature. In the vacuum chamber, commonly used heat-insulating materials such as aluminized polyester film and nylon mesh, supporting heat-insulating materials such as epoxy glass fiber reinforced plastics and plastics, and sealing materials such as vacuum rubbers are generally allowed to be at 50°C~150°C The working temperature makes it impossible for the vacuum container to increase the degassing temperature during pumping.
7. Small conductance. The conductance of the vacuum chamber is limited by the conductance of the insulation gap and the suction nozzle.
8. Gas desorbed by adsorbent. Some thermal insulation materials such as glass fiber cloth and aluminized polyester film can cause a large amount of outgassing.
9. Large gas load. When pumping, the gas load in the container comes from the outgassing of the container wall, support, insulation material and adsorbent, and the leakage of the container. When the air leakage is limited to the allowable range, the gas load mainly comes from the outgassing of the insulation material.
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