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Hard anodised parts for moulding tools

Thermoforming to perfection with the right surfaces

The term thermoforming refers to a process for forming thermoplastic materials. The company Marbach Werkzeugbau GmbH is a leader in the production of thermoforming tools for yoghurt pots, meal trays and covers. In the moulds, different aluminium components such as mould inserts, retaining devices, mould bases and adapter plates are used. The requirements for the surfaces of the components are improved anti-friction properties, wear protection and also corrosion resistance. In order to meet these requirements, AHC Oberflächentechnik hard-anodises these parts using the HART-COAT® process. Here, different process variants are used, namely with or without PTFE and with or without sealing. The layer thicknesses are generally 30 μm or 50 μm.

The hard-anodised layer HART-COAT® forms due to anodic oxidation in a cooled acid electrolyte. The aluminium base material is converted into aluminium oxide on its surface. In the process, the oxide layer grows inwards and outwards in equal parts, which results in a defined dimensional change of the component. The very good bond with the base material from which the layer is formed is particularly noteworthy, being based on atomic binding forces. The hardness of the aluminium oxide produces the layer's high wear resistance. To improve the anti-friction properties, the HART-COAT® coating is additionally provided with incorporated PTFE lubricants (HART-COAT®-PLUS). Cleaning of the surface and demoulding are also facilitated by their anti-adhesion behaviour.

The HART-COAT® layers grow in a columnar manner (perpendicular) and form a stable cell structure (honeycomb structure). Each cell has a pore channel which does not reach the base material but rather ends at a so-called barrier layer. The pore channels can be closed very effectively by sealing. This process is usually carried out in demineralised water, usually without sealing additives, between 96 °C and 100 °C. This further increases the already good corrosion resistance of the HART-COAT® coatings.

Caption: Thermoforming tool for the production of polypropylene cups. The olive-coloured HART-COAT® coating is clearly visible on the moulds.

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