In modern vacuum coating equipment, the roughing stage is a critical foundation for achieving stable high-vacuum performance. A widely used configuration is the DLT.2V80 two-stage rotary vane pump combined with a Roots pump. This pairing forms an efficient pre-vacuum unit capable of rapidly reducing chamber pressure from atmospheric level to the low-vacuum range, creating the necessary conditions for the subsequent high-vacuum pumping stage. Understanding the correct operation process and technical logic of this system is essential for maintaining coating quality, protecting equipment, and extending pump service life.
The roughing system works on a staged pumping principle. The two-stage rotary vane pump acts as the backing pump, while the Roots pump provides high pumping speed in the medium-to-low vacuum range. During start-up, the rotary vane pump first reduces chamber pressure. Once the pressure reaches the allowable start pressure of the Roots pump, the Roots pump begins operation, greatly accelerating gas removal. This coordinated pumping allows the chamber to transition quickly from atmospheric pressure to the low-vacuum range required before high-vacuum pumps (such as diffusion or turbomolecular pumps) are engaged.
A key process requirement occurs after the rough pumping stage is completed. Before switching to high vacuum or entering the vacuum holding stage, the roughing pipeline must be evacuated and pressure-balanced. This step is often overlooked but is vital for preventing vacuum backflow. If pressure differences remain between the roughing line and the chamber, vacuum oil from the rotary vane pump may migrate backward into the Roots pump cavity, pipelines, or even the coating chamber. Oil contamination can severely damage thin film quality, block gas channels, and increase maintenance costs.
Proper operating procedure begins immediately after the chamber reaches the target rough vacuum level. The main roughing valve connecting the chamber to the roughing line should be closed first. This isolates the chamber from the backing pump system and prevents uncontrolled pressure fluctuations. Next, dry nitrogen or clean, dry compressed air must be introduced into the roughing pipeline for a controlled micro-pressure venting process. This step slowly equalizes the pressure within the pipeline to near atmospheric level without introducing moisture or contaminants.
Dry nitrogen is preferred because it is inert and contains no water vapor or oxygen, both of which can degrade vacuum oil and contaminate internal pump components. If nitrogen is unavailable, filtered and dried compressed air may be used, provided that oil and moisture traps are installed. The goal of this step is not to fully vent the chamber but to stabilize the pressure inside the roughing pipeline. Only when the pipeline pressure returns to a safe level should the system transition to vacuum holding mode or shutdown.
The importance of this pipeline pressure recovery process extends beyond cleanliness. It also protects the mechanical integrity of the pumping system. Rotary vane pumps rely on vacuum oil for sealing, lubrication, and cooling. Sudden pressure differentials can cause oil migration, foaming, or backstreaming. Roots pumps, which operate without internal lubrication in the compression chamber, are especially vulnerable to contamination from backing pump oil. By stabilizing pressure before isolation, operators reduce wear, prevent pump seizure, and minimize maintenance frequency.
Compared with single-pump roughing systems, the rotary vane plus Roots pump combination offers higher pumping speed, shorter pump-down time, and improved stability in industrial coating cycles. However, this higher performance also requires stricter operating discipline. Pipeline venting, valve sequencing, and dry gas protection are not optional steps; they are integral parts of the process design.
From an industry perspective, contamination control has become increasingly important as coating technologies advance toward higher precision and thinner films. Even trace oil vapor can affect adhesion, optical performance, or electrical properties of deposited layers. Therefore, standardized roughing procedures are now considered part of quality control in vacuum coating production.
In summary, the roughing stage using a DLT.2V80 two-stage pump and Roots pump combination is more than a simple pre-vacuum step. It is a carefully controlled process involving staged pumping, valve isolation, dry gas venting, and pressure stabilization. Following these operational guidelines ensures clean vacuum conditions, protects critical pump components, and guarantees consistent coating performance across long production cycles.