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13/03/2009
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The continued development of a hard coating process is set to benefit mould toolmakers, Tom Shelley reports
 The Plasma Electrolytic Oxidation process, Keronite, is being applied to aluminium tooling for plastic injection moulding and other plastic forming processes for the first time. Seven or eight companies are said to be using the process commercially with a number of others testing it in Europe, the United States and China.
The process, invented in Russia, applies an alternating pulsed voltage high enough to produce a plasma discharge in an aqueous solution on the surface of light metals such as aluminium, magnesium or titanium.
The result is an oxide ceramic surface with a hardness of up to 2000HV. It also has up to seven times the wear resistance of hard anodised coatings. At the same time, research conducted under Professor Bill Clyne, in the Gordon Laboratory in the Department of Materials Science and Metallurgy at Cambridge, shows that the stiffness of Keronite on aluminium can be as low as 30GPa. This prevents the surfaces of treated moulds from cracking during thermal cycling. Keronite treated aluminium moulds are said to have lifetimes far beyond steel tools and retain a much higher thermal conductivity, reducing both cycle times and plastic part costs.
It also adheres much better to substrates than deposited coatings such as plasma sprayed ceramics. This makes it very suitable for use on complex ribbed, or textured surfaces, as it protects along edges and corners, where conventional dip plating or painting processes fail due to surface tension or dog bone effects. Hard anodising offers limited protection in these areas because the columnar growth inherent in the process results in wedge shaped cracks on tight radii.
The Keronite coating grows both inwards and outwards as it is formed. The amount of outward growth depends on the alloy but is typically between 10% and 40% of the total coating layer thickness. Toolmakers can either allow for the surface growth when designing a mould, or polish it back to the original dimensions. The process produces a roughness that is about 10% the thickness of the applied layer.
There is also a range of nanometre scale porosity in the coating that can be used as a basis for impregnation of a wide variety of top coats. This can produce an even more wear resistant duplex system. Surfaces can be re-processed should a tool need to be altered, machined or welded.
Keronite on aluminium is said to be particularly effective in preventing mould surfaces from corrosion caused by gas burn, acid attack or by the chlorides and sulphides produced by certain types of plastics and rubbers as they are heated.
Although the main area of interest is proving a superior alternative to anodising aluminium, Professor Clyne's group has also been doing a lot of work on applying it to titanium, primarily for surgical implants.
The process is being applied to tools for plastic injection and blow moulding, but is also being tested on tools for rotary and vacuum moulding. Motorsport is said to be a key application area and the technology is also being applied in offshore and marine applications.
Pointers
* Keronite plasma electrolytic oxidation coatings are proving to be particularly efficacious on aluminium tooling for plastic processing.
* As well as reducing the cost of tooling, as compared hard steel, lifetimes are superior.
* Thermal conductivity is higher which reduces cycle time and part costs.
* Other applications include motorsport, commercial automotive, marine, offshore and medical implants.
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Author
Tom Shelley
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