The Bonding Process can be divided into three distinct parts :
1. Attachment - Attachment of the metal to the powder coating is achieved simply by heat
softening the powder coating and then mixing in the metallic pigment until all of the
particles are literally 'stuck' to the powder surface. The method most commonly used to
achieve attachment is high speed dispersing, the powder coating and the metal pigment
are loaded into a high speed mixer (normally jacketed to allow some control of process
temperature) and then mixed for some minutes at high speed. The energy of mixing
provides the required temperature rise for the powder to reach it's softening point
(40-600C), softening allows the metal pigment to adhere to the powder surface.
Mixing continues until all of the metal particles have adhered to the softened powder. The
end point is critical and is controlled via temperature control and energy instrumentation.
Once the end point has been reached the mix must be quickly discharged into a cooler to
prevent premature curing / solidification.
2. Cooling / Batching - The warm discharged mix has to be cooled as quickly as possible
(150C) and this is achieved using a separate low intensity mixer that is jacket cooled or
have sufficient cooling to reduce this bulk temperature. In most cases the bonded quantity
is a small part of a larger batch so blending of a number of mixes is also achieved at this
stage. The size of the cooler mixer is governed by the batch size spread.
3. Sieving - Because the powder coating is subjected to heat during the bonding process,
and as this can be localised, it is necessary to refine the final product to remove any
agglomerates that may have been formed. Course sieving at 130 - 150 Microns is usually
The final requirements of bonded powder coating are optimum powder reclaim due to the
fine pigments bonding to the larger powder coating particles. No separation from the
powder coating during spray application, even application of effect pigment and the
powder coating to the work piece.