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Tooling costs and quality of high volume packaging improved by Keronite process

Keronite International : 13 February, 2009  (Technical Article)
Applied to tooling for plastic packaging, the Keronite Plasma Electrolytic Oxidation (PEO) advanced surface treatment process is enabling manufacturers of consumer products and their plastic packaging to get a rapid return on their investment in tooling. At the same time, productivity is increased and the quality of moulded parts is improved.
Tooling costs and quality of high volume packaging improved by Keronite process
The Keronite patented, chrome-free process holds tight tolerances when moulding the highest volumes. It extends the lifetime of aluminium moulds far beyond that of their expensive steel counterparts whilst at the same time, improving heat transfer and the associated throughput rates. It enables moulding companies to reduce tool weight and handling costs as well as the time and expense involved in tool manufacturing, helping packaging companies to get products to market in record time.

Keronite surfaces are considerably harder than those of hardened tool steel and most of the as hard as titanium nitride or diamond chrome, it does have other performance advantages in terms of dimensional control and superior adhesion to the substrate.

The hardness of Keronite surfaces is dependent on the aluminium alloy used and the thickness of the ceramic layer created, but it can reach as much as 2000HV. This is well beyond the capabilities of more conventional hard anodising and even harder than steel, glass and most silicon-containing compounds. This characteristic in itself goes a long way towards creating a wear-resistant surface, but when combined with the substrate adhesion and the surprising compliance of the Keronite layer, the results are outstanding.

Up to seven times more wear resistant than hard anodising, Keronite on aluminium moulds is extremely durable and therefore able to minimise costly downtime for maintenance and repairs in high volume production.

Research at the University of Cambridge demonstrates that the stiffness of Keronite on aluminium can be as low as 30GPa - very unusual for such a hard surface. It is this peculiar combination of extreme hardness and surprising flexibility that makes Keronite more mechanically tolerant than other ceramic surfaces, enabling moulds to resist the strains imposed by differential thermal expansion in thermal cycling and more importantly, last that much longer.

Flexibility gives the coating resilience against small impacts or deformation of the substrate.

During the Keronite process, the exposed aluminium surface is progressively converted into a thin ceramic layer, creating a perfect interface with the base metal, free from any defects.

Because of the way the ceramic layer is developed, it has much better adhesion to the substrate than any deposited coatings such as plasma sprayed ceramics, reducing the risk of potentially costly surface chipping or flaking.

In addition to inward growth, the Keronite layer also grows outwards from the original surface, again in an extremely controlled and predictable way, ensuring good repeatability in high volume production. The extent of this outward growth depends upon the alloy selected, but it will typically reach between 10% and 40% of the total ceramic layer thickness.

Working with Keronite, tool designers can allow for the very slight surface growth when designing a new mould, or if a smoother surface is preferred, it is possible to polish the outer layer back to the original mould dimensions using conventional methods or by wet blasting with corundum, plastic media or nut shells in a liquid medium. Either way, tolerances remain tight and dimensions repeatable making the process ideal for duplicate parts such as interchangeable cavity inserts.

Keronite is suitable for use on complex ribbed or textured moulds, protecting against wear along edges and on vulnerable corners where conventional dip-plating or painting processes fail due to surface tension or “dog-bone” effects producing thinner layers and weak spots.

Hard anodising also offers limited protection in these critical areas because the nature of its columnar growth results in wedge-shaped cracks on tight radii, again creating vulnerability.

Images showing the surface protection of vulnerable corners Keronite is different in that it shows no thinning or points of weakness on corners or edges, faithfully following the contours of the mould surface.

Bare Keronite has a surface roughness (Ra value) of approximately 10% of the thickness of the applied layer, together with a range of very fine-scale surface-connected porosity. This is ideal for impregnation with a variety of topcoats to produce an even more wear resistant duplex system with the required release properties and/or friction characteristics for a given mould, ensuring perfect results, time after time.

The Keronite process transforms the surface of aluminium moulds into a layer of extremely hard ceramic. This thin but protective layer grows in a controllable fashion, providing good dimensional accuracy and enabling mould-makers to work within tight tolerances for greater repeatability and reduced risk of flash. The Keronite surface can be polished to achieve the desired level of friction and anti-galling properties for the particular plastic material in question, or the unique pore structure of unfinished Keronite can be used as the base for a duplex system, incorporating lubricants for excellent release characteristics.

A further advantage is that Keronite surfaces can be re-processed should the tool need to be altered, machined or welded at any time.

Keronite on aluminium is particularly effective in protecting mould surfaces from pitting or corrosion caused by gas burn, acid attack or by the chlorides and sulphides generated when certain types of plastic or rubber are heated. Naturally, this protective layer will also prevent corrosion caused by condensation and it enables the use of water based resins without the usual problems associated with them. With a reduced risk of corrosion, there are unlikely to be any oxidation by-products contaminating the mould surface and causing problems in the more sensitive medical or electronics applications.

Keronite is suitable for use on a wide range of mould types, from vacuum forming, blow moulding and rotary moulding to the more aggressive forming tools and resin-bonded sand core moulding. In the case of plastic injection moulding, tool wear can be an expensive problem in high volume production, particularly in the areas directly opposite the injection points. Keronite surfaces are not only resistant to hot or abrasive plastics and additives such as glass or halogenated flame retardants, but also remain stable under temperature cycling.
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