Wurster Coating (Fluid Bed Coating)
The Wurster process has been used for years to coat
particles, spheres, granules, and tablets. Systems have been
developed for use with a variety of coating formulations,
including aqueous/organic solvents, hot saturated solutions
and hot melts.
The basic concept in Wurster coating is to separate the
particles in the fluid bed from one another in an air (gas)
stream. While the particles are suspended, a coating
formulation is sprayed from the bottom of the bed up onto the
particles (bottom-up spray).
To get a better idea of how the Wurster process works,
click on the buttons below to cycle the animation slides.
Ultrasonic coating is a unique, relatively new
approach to coating fine, particulate substances
in a fluid bed using ultrasonic spray technology.
Ultrasonic spray devices use very high frequency
vibration instead of high pressure or compressed
air to produce extremely small and uniform droplets
which are ideal for high-precision coatings (such as Wurster
coating) as well as vaporization, spray drying and humidification.
When a liquid binder, coating material or other liquid inside an
Ultrasonic spray device vibrates at a specific frequency, capillary
waves are produced on the surface of the liquid. During atomization,
the capillary waves are transformed into droplets by increasing the
amplitude until the peak of the wave forms droplets.
The wavelength and, subsequently the
droplet size, are determined by vibration frequency.
High vibration frequency generates fine droplets
and low vibration frequency generates coarse droplets.
The distribution graph below illustrates the average droplet size, in microns, at
Dv 0.1, Dv 0.5, Dv 0.9, Dv 0.99 and the Dmax value at various pressures.