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COATING OF CAST IRON

Cast iron substrates have a porous surface microstructure and the surface will often include uncombined carbon (graphite) and silicates from the casting process.  This necessarily means that a thorough and tightly controlled pre-treatment process is required when applying protective coatings to cast iron substrates.

From our experience we would recommend the following procedure:

PRETREATMENT

1.         Degassing -   At a minimum of 20°C above the coating process  temperature for at least as long as the heating period used in the coating process.

2.         Grit/Shot blasting -    This serves two purposes.

(a) Removal of surface contaminants

(b) Provision of a mechanical key for the coating to adhere to.

The surface profile should be 30 to 50 micron to obtain the best adhesion.

The cleanliness of the substrate on removal from the blasting cabinet will be dependent on the dust extraction in the cabinet, the cleanliness of the blast media used and also the type of blast media used.  The harder types of steel grit/shot create more dust and are therefore likely to leave a less clean surface.

In order to ensure a clean surface prior to coating we would advise that items be degreased with a solvent or aqueous cleaner after grit/shot blasting. This may not be necessary if the blasting process consistently produces clean substrate surfaces and the blast media is regularly cleaned and/or changed.

3.         Fill of voids -  Any voids in the substrate surface will cause pinholes in the coating To avoid this, these voids should be filled with fire-cement.

COATING

Plascoat materials can be applied by the fluidised-bed process, by electrostatic deposition or by 'flock-spraying'.  Only PPA 571ES is suitable for electrostatic application.

By 'flock-spraying' we mean that the plastic powder is sprayed onto a pre-heated substrate.

Because cast iron substrates tend to be heavy items with large heat capacities we would recommend application by the fluidised bed process or by flock-spraying as thicker and therefore more protective coatings can be obtained than by electrostatic deposition.

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