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METHODS
FOR CURING
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AIR DRYING
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PRINCIPLE
OF OPERATION
Paints get
cured under room temperature as
1. volatile
thinner evaporates and paints become dried
2. dry thinner
oxides as a film by oxidizing polymers
3. hardening
paints get hardened by hardeners in paints
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DRAWBACK
a.
high humidity makes violate thinners harder to evaporate and poor paint film
quality is resulted (discoloured or dusty paint film). Air-conditioners and dust filtering system are required to improve paint
film quality.
b.
high room temperature can make
paints easier to
evaporate, less dust on paint film but poor flash-off
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CURING BY HEATING |
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Curing oven
can be divided into 3 types
1. Low
temperature oven
oven
temperature:
<100¢J,
it is used to speed up the curing time of self-cured
paints
2.
Medium temperature oven
oven
temperature:
100-150¢J,
it is used to cure paints that require condensation
reaction or polymerization
oven
temperature should not be
>150¢J,
otherwise, coating will get yellowing and brittle
3. High
temperature oven
oven
temperature:
>150¢J,
it is used for powder coating or electrophoretic
coating
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HEATING
METHODS FOR CURING OVEN
------------------------------------------------------------------------
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CONDUCTION &
CONVENTION HEATING (HOT AIR OVEN) |
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PRINCIPLE
OF OPERATION
Heating with gas or
liquid to:
1. by conduction:
transfer heat from the heating tubes to the paint
substrates with gas or diesel
2.
by conduction &
convention: transfer heated gas to the paint
substrates
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ADVANTAGE
Hot gas circulates evenly
in the oven to
a.
make objects evenly
cured, even for objects with recessed areas
b. speed up paints volatile
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DISADVANTAGE
a. Hot gas oven heats up
objects from outside to inside, which results in: low rising temperature,
low heat transfer efficiency, high power consumption, long oven size, high
dust prevention requirement
b.
(outside) coating cures
first than the inside part so pimples or bubble marks will easily deposit on
the coating surface
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STATIC OVEN & TUNNEL OVEN
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¡@ 1. Heating
by convention
"horizontal hot air circulation"
& equipped with
"interlock
hot air supply"
to avoid the risk of sudden temperature rise
2.
Oven
temperature: ambient to 220¢J,
thermostatic, self-timing with explosion proof
window
3.
Perfect insulation: oven outside
temperature is less than 40¢J, so
heat loss can be minimized
4.
Control panel composed of: master
switch, thermostatic, automatic drying timer with
alarm, indication lights and
explosion-proof window
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RADIATION HEATING (INFRARED OVEN )
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3 WAVELENGTHS OF INFRARED OVEN
1. Near infrared (short wave):
wavelength:
0.75-2.5£gm
radiator temperature:
2000-2200¢J
radiation efficiency: high
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2. Medium infrared (medium wave):
wavelength:
2.6-4.0£gm
radiator temperature:
800-900¢J
radiation efficiency: medium
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3. Far infrared (long wave):
wavelength:
>4.0£gm
radiator temperature:
400-600¢J
radiation efficiency: low
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CURING
PRINCIPLE
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1. when the wavelengths of infrared match with that
of the vibration of paint molecules, paint molecules
can absorb the infrared and the paint will be cured,
otherwise, the objected can only be heated and cured
by the evaporation of thinner inside
2. as the
wavelengths of the
vibration of
paint or water molecules is normally
>4.0£gm, we need to heat up the inner part of the
object by near infrared (short wave) and cure the
coating by far infrared (long wave)
3. distance
between the infrared reflector and the object should
be
120-300mm, otherwise radiation efficiency will be
adversely affected
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ADVANTAGE |
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As object is cured from inner to
outside coating by infrared oven which means the object and
coating is heated at the same time which will result in a
a.
higher rising
temperature, higher heat transfer efficiency, lower power consumption
b. faster thinner evaporation, better coating quality,
shorter
oven size, lower dust prevention
requirement
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DISADVANTAGE
|
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1. uneven oven temperature in infrared oven
means different curing speed for objects with different shapes; so infrared
lamp panel is more suitable for objects with different shapes
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2. as water molecules can absorb infrared,
blower is required to blow away water molecules in order to speed up thinner
evaporation
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DIFFERENT
INFRARED CERAMIC HEATERS
------------------------------------------------------------------------
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Infrared
heating plate |
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1. radiant ray is parallel to its horizontal ray and
diffusion ray, this plane is suitable to cure plane
objects because of even heat distribution
2. poor heat transfer on the back of the heating plate
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Infrared heating lamp |
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a. is less affected by radiant distance, smallest
temperature difference for radiant distance of
200-600mm, so it is suitable for complex objects
with different sizes ¡@ ¡@
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Infrared
heating tube
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a. made of ceramics, insulation layer in the
middle, outside coated with far infrared coating
b. as this tube has thin heating layer and does not
have resistance wire, heat distribution can be even
and there is no visual light loss which can
ultimately improve the heat transfer
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MIX OF HOT AIR & INFRARED OVEN
------------------------------------------------------------------------
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PRINCIPLE
OF OPERATION
|
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To let thinner out by infrared
heating and to keep even oven temperature by hot air
convection
heating
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a. as metal can absorb radiant heat
energy from infrared heating, therefore, in order to speed up the heating effect,
we can get higher oven temperature (above 300
¢J) and shorter oven tunnel at the
same time
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b. this oven can maintain a
stable oven temperature as there is an air insulation layer in the
middle to guarantee the oven outside temperature is less than
40
¢J - a good way to minimize heat loss and energy saving
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c. each section of our tunnel
oven is checked with
Oven Tracker
- to maintain
stable oven temperature before
delivery
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ADVANTAGE
|
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have advantages of both infrared heating and hot air
heating:
1.
advantage of infrared heating
a. fast oven rising temperature
b. good coating quality
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2.
advantage of hot air heating
a. even oven temperature: even curing
speed for objects of different size
b. fast thinner evaporation
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UV OVEN
------------------------------------------------------------------------
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ADVANTAGE
|
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to cure objects by UV
lamps
a. short curing time: ideal for paper, plastics or wood coating
b. low oven temperature for curing
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DISADVANTAGE
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a. short diffusion distances
b. only suitable for simple objects
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1.
²Ä¤@Ó¼o®ð¼Ë¥»©ó¥»¼t¼Ð·Ç¼QÏ@©â¨ú
2.
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COMPONENT
VOC
|
OES
Limit |
Sampling
with standard Water-Wash Spray
Booth
(Shenzhen: 7/1/98) |
Sampling with Specially made Water-Wash
Spray Booth
(Chek Lap Kok:30/7/98) |
Isopropyl alcohol
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400ppm |
<0.1ppm |
<0.1ppm |
n-butyl alcohol
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50ppm |
<0.1ppm |
<0.1ppm |
Ethanol
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200ppm
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0.5ppm
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<0.1ppm |
Methylethyl ketone |
1000ppm |
6.8ppm |
2.8ppm |
Phosphoric acid |
1mg/m3 |
0.2mg/m3 |
<0.05mg/m3 |
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Conclusion
The
concentrations of five different parameters in the paint
mist were measured at Shenzhen (7-1-98) and Chek Lap Kok
(30-7-98) respectively and the results are presented. It is
concluded that the VOC (including Isopropyl alcohol, n-butyl
alcohol, Ethanol, Methylethyl ketone) and Phosphoric acid
arepresent in an insignificant amount and within the OES
limit.
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A note on the quoted OES listed in the above table
The
full name of OES is Occupational Exposure Standard and it is a concentration
of an specific airborne Substance averaged over a reference
period at which there is currently no evidence that
repeated or day after day exposure by inhalation, will be
injurious to the Health of employees. The OES¡¦s
should not be exceeded but if this does occur,
effective measure should be taken to reduce the exposure as soon
as practicable.
The OES quoted here is based on the
EH40 which is a part of The Control of Substances
Hazardous to Health (COSHH) regulations established by
the Health and Safety Executive of UK.
The objective of this regulation is
to prevent disease at workplace due to exposure of
hazardous substances.
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OEL ¼Ð·Ç |
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GB/T 16157-1996 |
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