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Modern Flexible Coating Lines - Optimized for Demanding Coatings
by John Lowens B.Sc. M.InstMC SPRIMAG Spritzmaschinenbau Gmbh & Co KG, Kirchheim-Teck
Flexible Spray Cabins
Knowing what is required and getting the pallets to the
correct place is only half the battle. A spray cabin also is required to achieve
It is generally accepted that paint robots provide the most
flexible solution. Paint robots have been available for many years but
basically, were large units best suited to coating lines for automobile bodies,
etc. Their integration into more modest modular systems has been limited. With
the advent of the more compact "mini" painting robots, their integration into a
modular spray cabin has been simplified significantly.
Not only has the size of the new robots made their
integration more simple but also, their mounting possibilities. These new robots
may be ceiling- or sidewall-mounted, greatly reducing the congestion within the
cabin. The spray target may now be positioned in the middle of the cabin,
allowing unrestricted access from all sides (see Figure 1).
Ceiling-mounted robots also give the additional advantages of
better water flow across the basin (fewer obstructions) and more uniform airflow
across the pallet. The airflow may be so directed as to drop down from the
ceiling and pass uniformly over the pallet without encountering the disrupting
presence of the robot’s body. Ceiling-mounted robots do have the disadvantage
that the robot’s body can cast shadows within the spray cabin.
To complete such a flexible spray cabin requires its own
loading/unloading device (object’s defined interfaces). Robots are inherently
slow. The time is takes for a robot to ‘fly’ to its start position is time lost
for spraying. But on the other hand, robot spraying, when correctly programmed,
is very accurate with minimal over-spray, thereby allowing the load/unload
device to carry the new pallet in, exchange it with the coated pallet, and carry
it out while the robot starts spraying the new pallet (see Figure 2).
Drying and Curing
With the exception of UV coatings, drying and curing usually
takes place at elevated temperatures within electrically- (heater battery or
infrared) or gas-heated driers or ovens. These devices do not need to be
flexible as the temperatures that can be achieved are primarily defined by the
substrate being coated.
However, a flexible line must be able to handle a wide range
of substrates. Therefore, it is the temperature control of a drier or oven that
must be flexible. Precise electronic control using accurate temperature
measurement and final positioning devices are necessary. For gas-heated systems,
suitably-sized control valves with repeatable positioning and the correct
characteristics are an absolute must. For electrically-heated systems,
solid-state thyristor control of the heater battery provides the most stable,
accurate control. Infrared heating is very good for bringing parts up to
temperature; but for some coatings, this heating method is too intense - curing
the coating too quickly and resulting in a very poor finish.
To create a uniform temperature within a drier or oven,
careful consideration must be made of the air circulation. It is the air
circulation that provides the primary mechanism for the heat distribution.
Temperature measurements must be made as close as possible to the parts within
the drier or oven. It is the temperature that the parts are exposed to which is
important, not the upper or average temperature. To ensure that the parts are
being exposed to the required temperature for the specified time, the
temperature should be measured at a number of points within the drier or oven.
From these measurements a temperature profile may be created.
An important part of creating a flexible line fit for the
future is to have a good idea of the direction that the coatings are taking.
Previously, single component solvent-based coatings were the norm. However,
particularly for the coating of automotive plastic parts, two- and
three-component coating systems have virtually taken over. A flexible line must
be able to cope with all these variations.
Single component coatings represent no problem. All they
require is a supply system, a stirrer, and perhaps a timer to alarm just before
their pot life expires. Two- and three-component coatings on the other hand are
much more problematic. Not only do they require precise mixing but some also
have such short pot lives (measured in a matter of minutes) that they must be
mixed on demand and applied immediately. Mixing based purely on flow
measurements is too empirical. The best way to achieve the required accuracy on
a consistent basis is dosing. Dosing valves dose precise quantities into fixed
quantity mixing chambers. Mass flow measurement by means of coriolis mass flow
meters provides reportable supervision of the mixing process.
At the moment, solvent-based coatings hold sway. However,
with the tightening of the volatile organic compounds (VOC) emission regulations
on water-based coatings, single and 2/3-components will increase in popularity.
Due to the slightly superior finish quality achievable with solvent-based
coatings in comparison to the water-based coatings available today, it is not
expected that water-based coatings will supersede solvent-based coatings
A truly flexible coating line must be able to operate as well
with water-based as with solvent-based coatings. To accomplish this,
consideration must be made of their different chemistry in regard to their
separation in wet washout systems. With a mixed operation, a combined surface
and bottom extraction system is the best compromise. Such systems consist of a
tank, which continuously is scrapped by scrappers suspended between two
continuous chains. The scrappers first scrap the surface, pushing the flotsam to
the extraction ramp, before diving down to scrap the bottom. The scrappers
surface from the bottom of the tank pushing the sludge before them, collect the
flotsam that has been pushed forward previously, and then push everything
together up the extraction ramp and out of the tank. The extraction ramp allows
the water that is trapped in the sludge to drain out and return to the tank,
minimizing water loss.
Flexible coating lines are a reality. They are being
implemented by most of the Tier 1 automotive plastic part suppliers today. Such
flexibility already is paying off; and who knows, with such flexibility the
industry just might see automobile face lifts every year!
For more than 80 years, Sprimag has enjoyed a leading position in the field
of surface coating. Sprimag offers individual coating machines as well as
complete finishing systems – offering a competent, experienced, single source of
supply with worldwide service. The head office of Sprimag is in Kirchheim/Teck,
Germany, with subsidiaries in the USA and in Brazil. Sprimag specializes in
robotic coating machines for plastic parts and also manufactures a complete line
of spray guns, pumps, and 2K/3K mixing equipment. For information in North
America, call toll free (800) 942-0115, e-mail
or visit www.sprimag.com.