Fluid Tip Selection
The Air Atomized Tip
The function of the fluid tip is to meter and direct the
material into the air stream. As previously stated, the fluid
tip is also the seat for the fluid needle. This fluid seat
and needle shuts off or restricts the flow of material.
The fluid tip and needle are a matched set and must be purchased
as such. The user selects an appropriate sized (orifice opening)
fluid tip based on the material viscosity.
High viscosity materials flow slowly and require a large
orifice and high fluid pressure to obtain the necessary flow
rates. On the other hand, lower viscosity fluids will require
smaller openings and less pressure for optimum performance.
Pressure feed and siphon feed guns use fluid tips of slightly
different designs.
Pressure feed guns require a tip which is tapered to guide
the fluid needle into place and allow it to fit snugly for
positive shut-off.
Siphon feed guns require a fluid tip in which the inside
walls are constructed at a wider, more open angle. This extra
opening allows the easy passage of material necessary in a
siphon gun. Since the fluid needle does not create a positive
shut-off in siphon guns, the needle need only seat itself
tightly enough to prevent dripping from the tip.
The selection of the diameter of the fluid tip and needle
will vary with the material being sprayed. The following table
designates tip sizes for materials of commonly used viscosities.
Fluid Tip Size Chart
Fluid tip orifice size
(in inches)
|
Viscosity Range
|
Viscosity Range |
.020 - .030
|
very thin
|
solvents (acetone) |
.040 - .050
|
thin
|
water |
.055 - .070
|
medium
|
S.A.E. 10 |
.085 - .110
|
heavy
|
S.A.E 50 |
.125 - .300
|
very heavy
|
adhesives |
The two materials most commonly used in the manufacture of
fluid tips are hardened steel for solvent based fluids and
stainless steel for water based paints and chemicals.
Delivery in Ounces Per Minute Chart
Orifice Size
|
At 8 P.S.I.
|
At 12 P.S.I.
|
At 18 P.S.I. |
.040
|
12.5
|
15.0
|
19.0 |
.046
|
17.0
|
21.0
|
27.0 |
.052
|
20.5
|
25.5
|
33.0 |
.059
|
24.0
|
30.0
|
40.0 |
.070
|
28.0
|
36.0
|
48.0 |
A rule of thumb that can be followed with standard materials
when setting pressures is that with the air supply cut off,
the paint will flow in a solid stream.
This fluid stream should begin to bend down about three feet
from the spray gun. Anything in excess of three feet means
too high a fluid velocity. (See figure below).
Airless Tip
Tips are selected by their orifice size (.007 to .072 inches)
and fan angle (10 to 80 degrees). The proper selection is
determined by the fan width required for the specific job
and the orifice size that will supply the desired amount of
fluid and accomplish proper atomization of the material.
For low viscosity fluids, smaller orifice tips generally
are desired. For higher viscosity fluids, larger orifice tips
are preferred. The quantity of fluid sprayed is determined
by the size of the orifice; the thickness is determined by
the orifice and the fan angle. Note that two nozzle tips having
the same orifice but different spray angles, deposit the same
amount of paint over a different area. Note that orifices
are not circular but are elliptical in shape. The diameters
referred to are equivalent to a circular diameter. A good
rule is to determine the largest fan angle and the smallest
orifice that is practical for your specific fluid and application
method.
How It Works
- Three basic methods used in finishing application to atomize
materials: air spray, airless spray (hydrostatic) and centrifugal.
- Air assisted airless concept uses the air and airless
methods.
- A pump is used to force material through a small orifice
or tip at low hydrostatic pressure.
- Airless pressures are typically 2000-3500 psi. Air assisted
airless operates at pressures generally under 2000 psi.
- Usually, low airless spray pressures form "tails."
- To complete the atomization and eliminate tails, low pressure
(10-30psi) compressed air is added to the airless spray
by an air cap.
With the air assisted airless concept, materials can be atomized
with full spray patterns at low pressures. This provides a
low turbulence of the material on the part surface and overspray
conditions are minimal, increasing the transfer efficiency
of the material.
Fluid Tip Identification
- The orifice size of all Graco airless tips is determined
by the last two digits of the six digit part number.
- Example: 182-211 = 0.011 Orifice
182-215 = 0.015 Orifice
- The minimum fan pattern width and degree of the angle
of the fan is designated by the fourth digit
- Double the fourth digit for minimum fan width
- Example: 182-211 = 4" minimum
- Add two inches for the maximum fan width:
182-211 = 4"+2" =6" maximum
All dimensions are based on 12" from work surface.
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