Heating automotive paint finishes speeds up
dry times by driving out solvents into the air, and heating catalyzed
paint products will also speed "cure times." (I’ll define
cured as a finish that you can sand and not gum up the sandpaper
and one that can be parked outside in the rain or dew and not
water spot easily, though, it’s possible to make almost any air-dry
paint water spot with enough hard water and bright sunlight.)
Using heat to speed production isn’t anything
new, but when the Environmental Protection Agency (EPA) passes
the proposed National Rule for automotive coatings, heating a
finish will become even more popular with body shops.
The undercoats and topcoats that will comply
with the new rule will be higher in solids (more resin, less solvent)
and will be predominately 2K (two component, i.e. paint plus hardener/catalyst).
These finishes will not only comply with the government’s desire
to reduce the air pollution from auto painting, but they’re also
more durable than paints without catalysts.
This is good news for consumers, who will
have products that will retain their gloss better, flex more in
cold weather and stand up longer to the sun’s ultraviolet rays
applied to their vehicles. To the body shop that isn’t currently
using 2K undercoats, however, these sound like more expensive
and slower-drying paints. They are.
But it’s not all bad. A more expensive cup
of primer-surfacer will cover much more area and won’t shrink
and swell like lacquer undercoats – but it will take longer to
air dry because there isn’t a big whoosh of solvent leaving the
paint film. The 2K products won’t reach dust free in seconds like
lacquer products do, but they’ll stay flat when topcoated and
can be repaired with no lifting. More good news is that these
2K finishes can be speeded up dramatically with the addition of
heat.
For this reason – along with cleaner paint
work – heating finishes is worth investigating.
In the autobody shop, there are two main types
of heat: a forced-air furnace attached to the paint booth or an
infrared curing light of one kind or another.
Air-Replacement Furnaces
Most automotive spraybooths have no method
to replace the 10,000 cfm that’s exhausted out the stack when
the fan is running. These shops are simply sucking the air from
inside the rest of the body shop through the booth doors until
that air is depleted. Then, the still-running fan pulls air from
outside the shop through open garage doors or cracks around the
windows. Not only does this make for dirty paint work (all the
dust from the neighborhood blows through the booth), it doesn’t
help dry and cure the paint very much either.
Paint dry speed depends mostly on air movement
and temperature. More moving air past the paint sucks the solvent
out of the paint film faster, and higher temperatures drive the
solvent off into the air and facilitate the chemical crosslink
that takes place between the paint and the catalyst.
To accomplish these objectives, a gas-fired
heater can be used.
Gas-fired heaters for paint booths are simply
great big furnaces capable of taking outside air and heating it
90 degrees F or more. No big deal, right? Except that the furnace
is heating 10,000 to 12,000 cubic feet of air every minute. Therefore,
the heater’s size should be matched to the speed of the exhaust
fan. The object is to have slightly more air pushed into the spraybooth
by the furnace than is pulled out by the exhaust fan, which creates
a slight positive pressure inside the booth, preventing dirt from
being sucked in.
To use the unit, the painter sets the furnace
to supply air heated to a comfortable painting temperature, like
75 degrees F. Once the vehicle is painted, the booth needs to
purge the last of the paint and solvent vapors that are still
inside. This "purge cycle" not only clears the air in
the booth, but also gives the bulk of the solvent inside the paint
film a chance to flash off. It only takes about one to three minutes
to evacuate the typical automotive booth.
The painter then sets the heater to the "cure"
mode, and the furnace brings up the incoming air temperature to
about 160 degrees F at the discharge (which may be limited by
the local fire department). This will gradually raise the air
temperature inside the booth to around 140 degrees F. The actual
panel temperature will be slightly cooler on the flats (hood,
top and deck lid) and as much as 40 degrees cooler on the lower
sides. This "force cure" can be accomplished in 15 to
30 minutes, depending on the paint finish.
A heated paint finish must cool down to the
touch before any work can be done to it. Total time for a typical
purge/cook/cool cycle is about 30 minutes. At this time, the car
can be safely reassembled, parked outside or (heaven forbid) sanded
and repainted.
Energy costs to accomplish this speedy paint
production vary. It depends on your climate, your spray temperature,
the cure time and the local cost of natural or LP gas. If the
outside temperature was about 32 degrees F and the painter sprayed
at about 70 degrees F and cured at about 140 degrees F, the energy
cost should be around $2 to $3 per car.
What does it cost for the furnace itself?
It’ll depend on whether your unit of choice is direct fired (heats
the air directly with flame) or indirect fired (heats a core and
blows air across the hot core), on how many cfm can be heated
and on what the temperature rise is (some units raise the ambient
air 90 degrees F, some units can raise the outside air 140 degrees.)
As for air-makeup units, they cost anywhere between $8,000 and
$12,000. Additional costs might include duct work to connect to
your existing booth and adding a sufficient fuel/gas supply to
feed the giant furnace.
Note: If the wave of the future is really
higher-solids products and increased pressure from insurance companies
to produce the work faster, the cost for a spraybooth furnace
can be recovered pretty quickly.
It’s also important to note that a good furnace
control panel should be easy to use. Ideally, the panel should
have a temperature adjustment for spray temperature and another
adjustable control for force-dry temperature. Timers for the purge
cycle, the cure cycle and the cool down also are helpful. No timer
is required for the painting cycle because no one except the painter
knows how long that job will take!
During the cure cycle, when the temperature
rises so high in the booth, some systems disable the lights and
compressed air and will kick off completely if the door is opened.
The moral here is: Don’t be hangin’ around in the booth when the
incoming air is 160 degrees! A cool-down timer then allows the
painter back in the booth about 10 minutes after the cure cycle
ends.
Some systems also have a thermometer on the
control panel that shows the actual temperature inside the booth,
and most systems have an emergency-stop control for safety’s sake.
Simply flipping a switch or two after painting
the car is a pretty simple way to get the car done and out the
door. Besides simplicity, force curing the paint with a convection
furnace also cures the whole car – even the "blind spots"
under door handles or low on the rockers – and it happens while
the painter is productively doing something else.
Infrared Curing Lights
Most paint repairs in a typical collision
shop aren’t complete paint jobs, but are, instead, panel repairs
(as many as 75 percent by some estimates). So, rather than firing
up a furnace to cure just a panel or two, many shop owners choose
portable infrared curing lights.
Using this method, it’s possible – and desirable
– to wheel the light over to the car at several stages during
the repair. If the bare metal is slightly heated before applying
body filler, any moisture on the panel is driven off; if the technician
heats the body filler with infrared, it will cure faster and more
completely; and if the painter "cooks" the primer surfacer,
it, too, will dry from the bottom up and in the minimum time.
These units are all "line of sight,"
which means that the energy emitted by the bulb shines on the
surface like a flashlight beam. If the panel is too large to be
covered by a single positioning of the light (for example, the
flashlight beam only covers half the panel), the light must be
repositioned for another cycle.
Many infrared curing lights are hinged or
jointed to allow the energy from the bulbs to better cover the
painted panel on rounded cars. Most multiple-bulb units also offer
the option of turning on the lights one at a time. If the repainted
panel is small, the painter need not turn on all six bulbs (incidentally,
the terms bulb, tube and emitter are used here interchangeably).
When curing paint, how hot is hot? A dark
colored car in a parking lot on a sunny, summer day can reach
a metal temperature of 140 degree F, and at around 180 degrees
F, damage can occur to several parts of the vehicle. Therefore,
most manufacturers recommend a cure temperature of 140 to 160
degrees F.
Infrared energy will affect the paint finish
based on the distance the bulb is from the panel and the duration
of the exposure. Heat also promotes the crosslink between the
catalyst and the paint resin. How quickly this happens is a function
of several things. One very important variable is the distance
from the panel. As a general rule, the bulb shouldn’t be closer
than 20 inches from the painted surface or more than about 36
inches away. The closer the bulb, the faster the panel will heat
and the hotter the temperature on the panel. Metal panels will
reach and hold a temperature of 135 to 140 degrees if the surrounding
air is cooler than the panel. This emmisivity point is higher
for plastic parts; urethane parts will accept and hold heat up
to around 185 degrees, and at that temperature, the plastic can
become hard and brittle internally and deform its shape externally.
Since plastic won’t reflect and radiate like metal, be very careful
using any type of heat on plastic parts. Several manufacturers
recommend covering thin plastic parts and trim with reflective
tape that shields them from the heat.
The "ramp up" time measures how
long it takes to raise the panel temperature to the desired degree,
and, if heat is applied too quickly to the fresh paint, solvent
popping or die back may result. What happens is the top of the
paint film skins over before the bulk of the solvent (trapped
inside the film) can evaporate. Allowing the fresh paint to flash
off for a few minutes before subjecting it to any heat is generally
a good idea. However, some new, very high-solids products respond
better to a quick application of the heat light because the air
movement past the panel tends to skin over the thick coating.
Follow your paint manufacturer’s directions for best results.
Note: I recommend a shop moving any light
further from the vehicle and letting it "cook" for a
few more minutes at a slightly lower temperature. The finish will
still powder when sanded, and the cooler temperature will eliminate
any chance of high-heat damage. Moving the bulbs too close in
either type of light can overheat the panel rapidly.
If you intend to use your infrared heat light
inside the paint booth, the law may require at least a three-
minute purge time. Some units use a gradual timer that runs the
bulb(s) at half power for a few minutes to warm the panel progressively.
Another point to consider: There’s lots of
talk about the best wavelength of infrared energy to use on auto
paints. Without taking sides, here’s an explanation of what the
difference is:
The visible light spectrum runs from violet,
to blue, to green, to yellow, to orange and to red. These visible
wavelengths are surrounded on both ends by energy we can’t see.
Below the color violet are energy waves that are "ultraviolet"
and below those are X-rays and gamma rays. Above the color red
are "infrared" energy waves. Short-wave infrared (closest
to the visible red) are at least .075 microns long but no more
than approximately 2 microns; medium-wave infrared energy is between
2 and 4 microns; and long-wave infrared waves reach from approximately
4 microns out to the energy that becomes short-wave radio broadcasts.
Long-wave infrared might be best described
as the heat bulb you have in your bathroom or the bulbs that keep
the chicken hot in the glass rotisserie at the grocery store.
This long-wave energy tends to dry paint from the top down, which
traps solvent inside the paint film. Both medium- and short-wave
will heat the substrate and drive the solvent up from the bottom
of the paint film and out into the air.
The difference between short-wave and medium-wave
infrared curing systems is in how quickly they penetrate the paint
and reflect back from the substrate. At the right distance, short-wave
energy can zip through the air and penetrate the paint like a
bullet. It can also heat the finish so quickly that it will sizzle
the paint – which is why some units have a half-power, heat ramp-up
setting to drive the heat into the finish gradually.
Medium-wave infrared doesn’t have quite as
precise a reflected area as short wave and isn’t quite as fast
as the short-wave bullet. However, it’s absorbed more uniformly
into the finish; medium-wave units don’t generally have a warm-up
setting, as the panel absorbs the heat more slowly and warms up
gradually.
Some short-wave vendors claim they can cure
specific paints in as little as six minutes – but to get these
eye-popping cure times, everything must be just right. There would
be detailed settings, depending on the substrate, the size of
the part, the thickness of the coating, the distance from the
emitter, etc. And changing one variable would change the cure
time by seconds or minutes. Medium-wave units could take twice
as long to reach cure for any particular finish on a metal substrate;
but, if you double a theoretical short-wave 10-minute cure, it
still only takes 20 minutes using medium wave.
The cost to power either style of curing light
with enough electricity to cure a typical one- or two-panel repaint
is somewhere between 10 to 50 cents. The cost of the infrared
units themselves ranges from $200 to $10,000.
Heat Helps
No matter which style of heat you use, from
the hand-held infrared units to the largest air-replacement furnace,
heating paint makes it dry faster – and faster dry times mean
cleaner work and more production.
Be proactive, not reactive. Don’t wait for
the new VOC rule to investigate something that can benefit your
shop now.
Mark Clark, owner of Clark Supply Corporation
in Waterloo, Iowa, is a contributing editor to BodyShop Business.
