Worldwide Largest Vps Coating System With Dry Vacuum Technology From Busch

The Medicoat AG company has now delivered the worldwide largest Vacuum Plasma Spraying (VPS) coating system to a US research institute. VPS is a coating process in which surfaces can be coated using a plasma-enhanced procedure in a vacuum. This gives the surfaces certain properties that the carrier material does not have. With a vacuum chamber volume of 19,000 litres, this is not only the largest VPS system, but also the first system in which Medicoat has worked with the completely dry vacuum technology from Busch by using a COBRA screw vacuum pump for the first time.
Fig. 1: The plasma burner works with a current of 3,000 amperes
Fig. 1: The plasma burner works with a current of 3,000 amperes

Medicoat AG has dealt with various coating processes in Switzerland for 30 years. At the site in Magenwil, Switzerland, Medicoat acts as a contract coater to offer its customers different coatings using thermal spraying. They mainly specialize in VPS coating, APS coating (Atmospheric Plasma Spraying) and HVOF coating (High Velocity Oxygen Fuel).

Medicoat uses the expertise they have gained over many years in the area of complex coating processes when building their own coating systems. "Thermal Spray Systems" plant manufacturing is located in Wohlen, Switzerland. VPS coating systems as well as APS and HVOF systems are designed and built there. Medicoat's great strength is that it responds to specific requirements of their respective customers and build coating systems in individual manufacture. In the process, the size of the process chamber, gas flow, vacuum supply as well as control and cooling are individually designed. Initially, Medicoat was mainly active in the field of medical technology and used the VPS process to create biocompatible coatings for implants that provide the best conditions for bone cell growth. In the meantime, Medicoat offers coating systems for many fields through to the coating of turbine blades, where the process provides high resistance to gas corrosion at high temperatures. Medicoat AG currently employs 70 staff in Magenwil and Wohlen. 95 percent of all manufactured systems are exported.

The technologically demanding VPS-system coating process requires a double-walled, water cooled vacuum chamber. The parts to be coated are put inside and then the chamber is closed. The vacuum system evacuates the chamber to a pressure of 0.08 millibars. This ensures that the oxygen in the atmosphere and the water vapour adherent to the parts is completely evacuated and cannot negatively influence the parts. This also creates the conditions for generating an extremely pure gas atmosphere. Then the chamber is flushed with argon to a pressure of 100 millibars and then further evacuated to 1 millibar.

It is only now that the actual coating process begins when the process gas mixture is let in and the plasma burner is switched on. This is customized in each case for the individual coating and the material. The coating can mainly be influenced by energy input, the gases and gas mixtures used, temperature, coating time, and by the pressure level of the individual process steps.

With a high current, the plasma creates an electric arc that transfers the energy to the part to be coated. The spray powder is mixed into the gas jet, where liquid droplets form in a temperature range above 20,000° Celsius and are accelerated, compressed and sprayed onto the surface of the work piece. A handling robot in the vacuum chamber moves the plasma burner during this process (fig. 1) and ensures that all parts are sprayed evenly. The entire coating process takes approximately 30 minutes.

The largest coating system that Medicoat has now built consists of a vacuum chamber with a 2.2 metre diameter and a length of four metres. The plasma burner works with a current of 3,000 amperes instead of the 1,700 to 2,000 amperes usually used in smaller systems.

Medicoat is now breaking new ground with the vacuum technology used. Because exact maintenance of certain pressure levels is extremely important for the customer's coating process, Project Manager Christian Specht decided to use frequency-controlled COBRA screw vacuum pumps as backing pumps for the vacuum system (fig. 2). The vacuum system is set up in three stages. In each case, it consists of two Puma vacuum boosters that pre-compress the evacuated air and the flushing or process gas and then convey it to two additional Puma vacuum boosters working in parallel. As a result, additional compression takes place before the gas mixture or gases are further compressed in both of the downstream COBRA screw vacuum pumps to thus achieve the ultimate pressure in the system. The sizes of the boosters used are coordinated in such a way as to achieve a pumping speed of 12,000 cubic metres per hour. The vacuum system is able to quickly achieve the required pressure for the initial evacuation of 0.08 millibars in the vacuum chamber, but is able to work in all pressure ranges up to 900 millibars and exactly maintain the required set point for the process.

Fig. 2: The vacuum system from Busch with two COBRA screw vacuum pumps as backing pumps and a pumping speed of 12,000 cubic metres/hour

In addition to the frequency-controlled drive, COBRA screw vacuum pumps also have the advantage that they are water cooled. This means that the operating temperature can be set or taken to a temperature range that prevents process gas from condensing in the vacuum pump. Both COBRA screw vacuum pumps are connected to the VPS coating system cooling water system. This supplies the entire system with ionised cooling water. Thanks to water cooling, there is no heat emission whatsoever. COBRA screw vacuum pumps operate with a completely dry process. That means that they do not require operating fluid within the compression chamber that could mix with the process gas.

Medicoat has worked with Busch Vacuum Pumps and Systems in the field of vacuum technology for 25 years and they design each vacuum system together. During the process, it is important to Project Manager Christian Specht that Busch can ensure competent, quick and uncomplicated consultation on site with its Swiss Busch AG company in Magden in the canton of Aargau. It is important to him that Busch provides a global service network with 60 of its own companies in 42 countries. They therefore recommend that all of their customers work together with and enter into a maintenance contract with the Busch company responsible in their country when they install a new coating system. This ensures fast reliable service around the world.

Until now, Medicoat mainly used vacuum systems that worked with R 5 rotary vane vacuum pumps as backing pumps for its coating systems. Christian Specht considers these oil-lubricated vacuum pumps to be extremely reliable and robust components in his systems. Due to the design, R 5 rotary vane vacuum pumps can only use frequency control within a limited range, so it was not possible to use this vacuum technology for the largest VPS coating system.

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