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The
quality and appearance of every part depends upon the gel
coat, the type of application equipment used and the skill
of the operator. It is critical that each of these components
be given careful consideration prior to application or the
resulting finish may be less than satisfactory. This section
will present the major type of spray equipment recommended
for standard gel coat applications. Because this guide cannot
provide precise specifications for every individual application,
it is intended to aid generally in the selection of proper
spray equipment. The exact spray equipment and settings for
a particular operation will depend on the plant lay-out, production
rate, configuration of the parts to be coated and the spray
operator.
DETERMINING
NEEDS AND CONDITIONS
Before
deciding on the appropriate spray system, attention must be
given to the nature and number of parts to be sprayed and
the location in the plant of the spray operation. Major consideration
should be given to the following:
- Determine
all safety procedures and equipment necessary before beginning
the operation.
- Allow
for increased production and adaptability to a variety of
operations
- Determine
time and labor involved for set-up and clean-up.
- Determine
if parts to be sprayed are simple, complex, large, small,
flat or curved.
- Determine
the number of colors to be sprayed.
- Determine
number of parts to be sprayed and whether they will be coated
at regular intervals or at random.
- Thoroughly
evaluate your maintenance program to determine if it should
be supplemented to sufficiently protect your investment.
By
a thorough analysis of the nature of the operation, you can
determine which spray equipment described below will best
meet your needs.
TYPES
OF SPRAY EQUIPMENT
Comparison
of spray equipment types can be based on three major considerations:
How the material is delivered to the gun, how the catalyst
is added and finally, how the gel coat is atomized.
ADDITION
OF THE CATALYST
"Hot
Pot" System
In
this system, a measured amount of catalyst is stirred by hand
directly into a container (pressure pot) which is then sprayed
from a pressure feed tank within the allowable gel time period.
This is currently the most accurate system but uses a large
quantity of clean-up solvent and any unforeseen delays can
lead to lost hoses and a difficult cleaning job. This system
should not be used for continuous production due to waste
and safety reasons.
Catalyst Injection System
Many Gel coat applications today use the catalyst injection
system, especially for high production fabrication. It permits
uninterrupted spraying. In this system, the liquid catalyst
is injected into the atomizing air supply by an adjustable
venturi device with a flow meter. The ball settings of the
catalyst flow meter are translatable into cubic centimeters
or grams per minute so the catalyst feed can be matched to
the measured weight output of Gel coat per minute and ratio
established. As the output of the gun changes, e.g., change
of tip or change of pump pressure, the catalyst delivery rate
must also be adjusted to maintain the proper ratio and assure
the scheduled performance of the gel coat in gel and cure
time.
ATOMIZATION
OF GEL COAT
Airless Spray System
Airless spraying equipment for gel coat application is
the most popular today. Advantages include better film leveling
at high delivery rates and elimination of blow-back when spraying
in closed areas. In a typical airless system, the gel coat
never comes into direct contact with compressed air and is
not broken up by air. Instead, an air-activated high ratio
pump (usually 33 to 1) puts the gel coat under high pressure
(1000-3000 psi.) by forcing it through a small orifice (.015-.026
in.). The high velocity fluid impacts with ambient air at
the orifice and breaks up due to the sudden and extreme pressure
differential. The typical airless system uses the air-activated
pump to feed the nozzle which delivers the gel coat spray.
A catalyst nozzle is located next to the fluid nozzle and
delivers a catalyst spray which mixes by implosion into the
gel coat fan. Airless systems are used primarily for coating
large, basically open or flat parts.
Air Atomized System
In this system, the gel coat is broken up by direct contact
with an air system pressure of 70 - 90 psi. The material break-up
and delivery can be accomplished by two methods.
1.
Internal Mix
The gel coat and initiator combine within the spray gun
head and are forced through a single orifice. The internal
mix tends to cause porosity and rougher finish, and requires
a flushing system to keep material from hardening in the
mixing head. This method is not suitable for most gel
coat operations and is, therefore, not recommended.
2.External
Mix-Atomized System
Stage
One
The annular ring around the fluid nozzle tip emits an
envelope of pressurized air which immediately surrounds
the fluid stream being sprayed through the tip. This instant
contact breaks up or coarsely atomizes the gel coat with
the air.
Stage
Two
Two converging streams of air forced from air holes located
on either side of the annular ring intersect the fluid
stream to further break it down resulting in a finer atomization.
Two straight contaminant holes indexed at 90 keep the
stream from fanning out.
Stage
Three
Right after second stage atomization, side port air jets
are emitted from passageways within the angular projections
on the nozzle (often referred to as wings, ears or horns).
Catalyst is injected into the above air stream by an independent
catalyst injection system.
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