Eddy sintering
Fluidized bed sintering is a widely used powder coating process to apply plastic coatings to metal surfaces . Plastic powder is evenly applied to a heated metal surface through a fluidized bed . Melting the plastic creates a dense coating.
To generate a fluidized bed, air is blown from below into a dry layer of plastic powder, the powder layer being in a basin with a sieve bottom. The plastic particles are held in suspension by the air blown in. The grain size distribution must be kept narrow because the fine fraction is carried out of the fluidized bed and the coarse fraction cannot be kept in suspension.
However, almost all fluidizable or micronizable polymers can be used if the grain size is in the range of about 50-300 μm.
The coating powder used includes polyamide , polyethylene , polyester , epoxy and poly (ethylene-co-chlorotrifluoroethylene) (ECTFE). Depending on the plastic powder, the resulting layer has different properties. A distinction can be made here between thermoplastically melting powders (plastics) and cross-linking systems (rather lacquer-like coatings).
The parts to be coated are preheated in an oven to a temperature of 100-200 ° C above the melting point of the polymer. With HALAR ( ECTFE ) the preheating temperature is 350–390 ° C.
The preheated parts are immersed in the fluidized bed while rotating and remain there for a defined time (1–3 sec). The coating powder melts on the hot metal as long as it is above the melting temperature or the surface temperature does not fall below the melting temperature of the polymer. The part can be immersed as often as long as the above temperature condition is met. The achievable layer thickness depends on the preheating temperature and the immersion time.
After the polymer powder has melted, the part is cooled while rotating on a track or in a basin. If there is no subsequent treatment, the surface characteristics are significantly influenced by the cooling process. If the powder is not completely melted, this must finally be done in an oven in order to achieve a suitable surface quality. This is called re-sintering.
application
An example of the application of the fluidized bed process are cutlery baskets in dishwashers. The resilience of these coatings should not be underestimated. By dipping, extremely high application efficiencies are achieved, which would not be possible with conventional spray painting.
literature
- Otto Schwarz, Friedrich-Wolfhard Ebeling, Brigitte Furth: Plastics processing. 7th edition. Vogel, Würzburg 1997, ISBN 3-8023-1717-3 .