Now aluminum tools can replace steel in high-volume molding

Feb. 21, 2008
A plasma electrolytic oxidation (PEO) process from Keronite International Ltd transforms aluminum tool surfaces into a chrome-free complex ceramic composite.

This lets aluminum tooling replace steel in molding processes ranging from less aggressive blow molding and vacuum forming, to injection and resin-bonded sand core molding. Tool wear can be a real problem in the latter process, particularly opposite injection points.

Conventional steel tools can wear along edges and on corners where conventional dip-plating or painting processes produce thinner layers of protection because of surface tension or “dogbone” effects. Edges or sharp corners only get limited protection from hard anodizing because its columnar growth results in wedge-shaped cracks on tight radii, creating points of weakness.

Keronite surface-treated aluminium tools, however, reportedly show no defects or thinning on corners and edges. The ceramic faithfully follows mold contours and can be made thicker on sharp corners to better protect edges. Depending on the alloy and the thickness of the ceramic layer, surface hardness ranges from 500 to 2,000 HV: well above the capabilities of hard anodizing.

Aluminum surfaces are harder than steel, glass, and many silicon-containing compounds. This hardness with compliance of the Keronite layer makes surfaces wear better. Research at the University of Cambridge in the U.K. showed the stiffness of Keronite layers can be as little as 30 GPa, making them far more strain-tolerant than most ceramic layers. Impregnation with PTFE further boosts wear resistance.

Keronite is a conversion coating which grows into the surface of the aluminum in a uniform, controlled manner to form a defect-free interface with the original metal. The layer has far better adhesion to the substrate than most deposited coatings such as plasma sprayed ceramics. As well as inward growth, the layer grows out from the original surface in a predictable manner. The extent of outward growth depends on the alloy and is typically between 10 and 40%. This can be allowed for in tool design or, if necessary, it is possible to polish the layer back to original surface dimensions.

The natural Keronite surface has surface roughness (Ra) of approximately 10% of the thickness of the applied layer but can be polished back to a smoother finish if necessary. On blow and vacuum forming molds, Keronite can give the plastic parts surface texture or be polished to produce parts with smooth finishes.

Where a non-stick, quick release finish is needed, as is often the case with injection molding, PTFE water-based emulsions or thin solvent-based solutions can penetrate the Keronite layer to produce an extremely hard-wearing PTFE/ceramic composite. This PTFE-impregnated composite structure can be polished smooth.

More Information:
Keronite International Ltd

A plastic mold for a packaging cap.

A tool used for blow molding.

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