Specifications

have fluid tip openings ranging
from .062" to.086", and are
designed to handle viscosities up
to 28 seconds in a No. 2 Zahn
Viscosity Cup.
For a DeVilbiss pressure feed gun,
the amount of material discharged
depends upon material viscosity,
inside diameter of the fluid tip,
length and size of hose, and
pressure on the material container
or pump.
If the fluid tip opening is too small,
the paint stream velocity will be too
high. If the fluid tip opening is too
large, you will lose control over the
material discharging from the gun.
For most HVLP guns, the paint
flow shouldn't exceed 16 oz. per
minute. For higher flow rates,
consult the DeVilbiss HVLP Spray
Gun catalog.
(5) Available air supply is the last
factor to consider.
Pressure feed air caps consume
between 7.0 and 25.0 CFM,
depending on design. If your air
supply is limited, because of an
undersize compressor, or many
other air tools are in use at once,
the gun will be starved for air,
producing incomplete atomization
and a poor finish.
20. What are the criteria for
selecting a pressure feed nozzle?
While the fluid discharge in ounces
per minute from a suction feed gun
is relatively stable (largely because
it is determined by atmospheric
pressure), the fluid discharge from
a pressure feed gun depends more
upon the size of the inside
diameter of the fluid tip and the
pressure on the paint container or
pump. The larger the opening, the
more fluid is discharged at a given
pressure.
If the fluid tip ID is too small for the
amount of material flowing from the
gun, the discharge velocity will be
too high. The air, coming from the
air cap, will not be able to atomize
it properly causing a center-heavy
pattern.
If the fluid tip opening is too large,
material discharge control will
be lost.
The fluid tip/air cap combination
must be matched to each other
and to the job at hand. Spray gun
catalogs include charts to help
you match them properly.
21. Of what metals are fluid tips
made?
Tips are made of the following
metals:
a) 300-400 grade stainless steel for
both non-corrosive and corrosive
materials
c) Carboloy inserts for extremely
abrasive materials
22. What is viscosity?
The viscosity of a liquid is its body,
or thickness, and it is a measure
of its internal resistance to flow.
Viscosity varies with the type and
temperature of the liquid. Any
reference to a specific viscosity
measurement must be
accompanied by a corresponding
temperature specification.
The most common measurement
used to determine viscosity in
finishing is flow rate (measured in
seconds from a Zahn, Ford, Fisher
or other viscosity cup).
Different viscosity cup sizes are
available. Each cup has a hole at
the bottom, specified to an exact
size. Use a viscosity cup that is
designed to handle the time range
of the materials in use. Viscosity
control is an extremely important
and effective method to maintain
application efficiency and quality
consistency. Always measure
viscosity after each batch of
material is mixed and make sure
material temperature is the same,
normally 70° to 80° F.
Viscosity recommendations may
be given in poise and centipoise
(1 poise=100 centipoise). Viscosity
conversion may be accomplished
by consulting a viscosity
conversion chart.
23. What is the spreader adjust-
ment valve?
A valve for controlling the air to the
horn holes which regulate the spray
pattern from maximum width down
to a narrow or round pattern
(see figure 8).
24. What is the fluid needle
adjustment?
This adjustment controls the
distance the fluid needle is allowed
to retract from its seat in the fluid
tip, which allows more or less
material through the fluid tip
(see figure 14).
With pressure feed systems, the
fluid delivery rate should be
adjusted by varying the fluid
pressure at the pressure pot. Use
the fluid adjustment knob for minor
and/or temporary flow control. This
will extend the life of the fluid
needle and tip.
25. What are the components of
suction and gravity feed
systems?
Typical suction and gravity feed
systems consist of: a suction feed
or gravity feed spray gun with cup,
an air compressor (not shown), a
combination filter/air regulator and
air hoses (see figure 12).
2. Air Atomizing Spray Guns (Cont’d)
8