least 12 -24 hours or the consistency of coating will not be
predictable and may peel or frost.
Dedust: Turn on the exhaust blower and pull the excess dust
off the tablets. Jog the pan at a very slow speed and make sure
that tablet flash or extrusions are removed with the dust.
Preheat: When it comes to film coating the tablets must be
preheated or the coating will not dry immediately. The sweet
spot is generally around 43-45°C. Dropping below 42 and
going over 48°C is asking for problems (there are always exceptions). Remember that when the spray cycle begins it will
have a cooling effect. If temperatures are too high the coating
may flash off and not impinge into the surface.
Avoid Black Marks: Sometimes it is helpful to preheat the
coating pan before loading tablets that get black marks from the
stainless steel. Many calcium based products mark very easily.
To avoid the marks there are two methods; preheat the pan or
coat the pan with the same solution used to coat the tablets.
Pan RPM: Initially the pan should be jogged to dedust, do
not run the pan continuously prior to turning the spray on
to avoid erosion. Tablet shape and bed depth have an impact on the flow of the cores within the pan. Generally start
up slow at around 4-5 rpm once the initial coating is applied
then speed can and should be increased. In general the objective is a tranquil bed of tablet cores, we do not want to
see tablets jumping all over the place.
Coating Solution: Prepare the coating solution ahead of
time to allow the suspension to adequately dissolve. This
means the solution should be prepared a minimum of 60 minutes before hand. Make certain the solution is free of lumps
and air bubbles. Most solutions require continuous mixing
throughout the process. Again the name of the game is reproducibility batch to batch. Establishing a solution hold time is
a good idea; and the ideal range is typically 2-24 hours. The
wider the range often times the less predictable the results.
Spray too soon and the guns clog easily, wait too long and the
viscosity changes due to evaporation. The hold time is dependent on the type of solution, water or solvent. The window for
solvents based coating solutions needs to be narrower.
Solution Spray Rate: The objective is to spray a thin film
across the surface of the tablet bed to get even coverage a
little at a time. We want small amounts of coverage multiple
times, not the other way around. The spray rate depends
on the solution and the core. Generally solution rates are
between 40-120 ml/minute per gun. The spray should dry
on contact. Too much air pressure will cause spray drying,
orange peel, and poor coverage. While not enough air will
cause over wetting resulting in sticking, picking, peeling and
worse. Again the key is consistency.
Drying Air: The coating pan has small holes to allow air to
dry the coating solution. The air supply and exhaust pressure
is balanced to minimize the rubbing of the tablets against the
perforated pan surface. If supply air was only blown into the
pan the spray would go everywhere. If exhaust air was only
pulled out of the pan the air pressure would damage the tablets. The proper way air is balanced is that the incoming air is
pushed along with the current of the sprayed solution while at
the same time the air is pulled through the tablets like pulling
air through a filter. The air flow needs to be fast to carry the
moisture away from the tablets. Solvent solutions are not as
reliant on air volume as aqueous based solutions.
Build Up: Most film coatings quantities are visual or a
thin 3% buildup by weight. Typically tablets are weighed
after they are preheated and then the weights are checked
throughout the process until the desire weight gain is
achieved or the appearance is achieved. Many companies
predetermine the quantity of solution to be added and simply spray until it is all gone.
Cool Down: Once the proper coating is on the core the spray
is turned off. When the spray is turned off the heat should be
turned off. Slow the pan rpm or put into a jog mode until the
tablets are cooled and dried. Unload and inspect carefully.
THINGS CAN GO WRONG:
Bearding: Dried solution occurs from the spray drying
around the nozzle. This can happen because of a poorly prepared solution, improper spray rates, too much atomization air
pressure, and inadequate negative
If a solution line
breaks shut off
the spray and
the heat supply,
keep the pan
tumbling to prevent sticking.
Make the repairs
and continue the
Poor Negative Pressure: Make certain the dust collector
can handle the entire run. If the exhaust air volume and
static pressure change this can cause over wetting and ultimately create many defects.
Solution adhesion can cause coating defects
Coating solution must be mixed thoroughly