Dr.P.L.Norman, CTA Ltd.

Originally Published in "Coatings" October 2000

For many years, the control of the thermal spray process has been one of the main goals for the industry as a pre-cursor to reproducibility of the coating itself.

The task is a daunting challenge and requires strict control of the parameters, gas quality, design of the thermal spray gun, the size, shape and quality of the powder and the control of the immediate environment in which the process is used (humidity, dust removal, cleanliness and so on).

It is obvious that total control of one of these items cannot guarantee control of the coating quality and reproducibility itself if the other factors are ignored.

An underlying consequence of this requirement for coating is to develop a process to produce the desired quality of coating and then refuse to move from those defined parameters. Understandably, this led to reluctance to change guns, powders, and other essential components, which make the accepted coating.

The typical thermal spray system is made up of:

1. an energy source including ignition.
2. a thermal spray "gun".
3. a control unit for electrical parameters.
4. a control unit for gas parameters.
5. a container and feeder for the wire or powder.
6. a cooling system for the "gun".

Also, included as an integral part of the system may be extraction systems, manipulation systems and safety devices.

These ancillary units are nowadays normally controlled from the main console unit of the plasma system. For example, the extraction will normally be required to maintain and record the extraction rate at the point of spraying to conform with the requirements relative to the hazardous area assessment calculations provided in conjunction with suppliers and users. If the rate deviates from a set point, the control console, and therefore the system, will not operate . Similar controls from the manipulator programme (X-Y scanner, robot, turntable etc.) are also commonly linked into the spray programme controlled by the console.

It is, however, generally agreed that the major parameters which are used by thermal spray engineers to define and maintain the quality of the coating will relate to the process, the gun and the powder feed.

Provided these remain constant, there would be normally no need to change parameters when renewing a system.

This is the concept behind the Universal Thermal Spray System. A console will, in addition to controlling the other ancillary parts for the system, control all other guns, all other powder hoppers, all other power sources, and all other fuel controls. In other words the user can choose the gun, choose the powder feeder and choose the process. The original parameters will remain unchanged for a specific coating.

This type of console can control plasma, thermal spray, HVOF gas thermal spray and HVOF liquid thermal spray processes. The "guns" which can be used include all conventional plasma, all HVOF gas fuelled guns and all liquid fuelled HVOF guns.

The console will supply all necessary parameters to each gun as required by the manufacturer or the user of the gun.

The Universal Thermal Spray System incorporates all the recommended and mandatory design and safety features to comply with all European Directives.

As reported in a recent paper by Dr. Ing Klaus Nassenstein of GTV mbH, problems involving closed loop gas control can be overcome by the use of mass flow controllers which can keep gas mass flows constant and independent of environmental influences such as pressure and temperature (Schweissen and Schneiden, 5 (1998) Heft 2, Page 86- Morderne Steuerungen für das Thermische Spritzen). The application of closed-loop controllers enables an output  signal to be compared continuously to a set valve and differences between set and actual vales are adjusted automatically.

The control system is centred around the microprocessor which acts as the intelligence, the memory and the signal receiver and transmitter. The microprocessor  can store and activate programs for the spray process, monitor the process and adjust the actual valves after comparison with set valves.

Not only can the parameter values be set by the keyboard digitally, it is possible to display set and actual values on a coloured screen.  Modern PC screens can indicate graphically gas flows by coloured columns and also to record these values continuously against time.

As mentioned previously the consistency of the powder is very important and data regarding the powder can be stored in the memory to remind the operator which specification to use.  The user of the equipment can therefore put into the program all necessary information to ensure  that the  coating material  is always as recommended or specified.

Then, in accordance with the requirements of ISO 9000 Quality Assurance procedures the user can develop and supply the  complete record of the coating component and moreover show all the parameter settings and control over the  complete spray time.

You choose the gun and the parameters and the modern Universal Thermal Spray control system will do the rest.

CTA Ltd., founded by Dr.P.L.Norman, A.D.Wiltshire and Eileen Stelfox are independent exclusive agents for GTVmbH, Lineage Alloys, WOKA and Industriekeramik Hochrein and for Genie Products.

Further details from:

http://www.ctaltd.co.uk

Contact CTA by e-mail on:

ctaeileen@ctaltd.co.uk

Tel: +44 (0) 61 929 7801
Fax: +44 (0)61 929 7802

For other articles presented at the TSSEA conferences or printed in "Coatings" see the publications page or contact TSSEA for further information.

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