High-volume low-pressure (HVLP)
Spray guns
Legislation which is part of the Environmental
Protection Act requires that spray painters must
reduce the amount of paint vapours being released
into the atmosphere as a result of their working
activities (see Chapter 15).
In conventional air-atomized spray painting,
about 35 per cent of the mixed paint actually
reaches the job surface. The remaining 65 per
cent is either extracted to the atmosphere or collects
on the workshop floor and walls. Of this
waste material, 30 per cent is solid whilst 70 per
cent is classified as volatile organic compounds
(VOCs). It is with these VOCs that the legislation
is concerned.
In an effort to conform to the EPA, spray gun
manufacturers have developed spray guns which
atomize the paint at low air pressure, that is
0.6 bar (10 psi) as opposed to the usual 4–5 bar
(60–75 psi). This reduced air pressure results in
greatly reduced overspray, and as a further bonus
65 per cent of the mixed paint reaches the surface.
Savings on paint wastage are obvious.
Though atomization is achieved with much lower
pressure, these spray guns still require large volumes
of compressed air to operate them. A typical air pressure
at the air transformer may be 4 bar (60 psi), but
this is reduced in the gun body by means of an air
restrictor which reduces the air velocity at the gun
outlet in the ratio of about 6:1. Other manufacturers
like DeVilbiss do not have restrictors in the guns and
only need 25 psi at the inlet to give 10 psi at the cap
(Figures 17.18 and 17.19).
A special air cap is provided with the gun. This
has a pressure gauge attached to it which enables
the spray painter to adjust the outlet pressure. When
the pressure has been adjusted to 10 psi, the air cap
is removed and a more conventional type fitted
(Figure 17.20). The fluid cup is pressurized and can
be either attached to the gun (Figure 17.18) or
remote from it (Figure 17.21). Gravity types are
also available (see Figure 17.22). The basic parts of
these types of spray guns are seen in Figure 17.23.
Figure 17.18High-volume low-pressure (HVLP)
cup gun (DeVilbiss Automotive Refinishing
Products)
Figure 17.19Sectional view of an HVLP cup gun
(DeVilbiss Automotive Refinishing Products)
588Repair of Vehicle Bodies
17.8 Basic parts of a standard spray gun
Air cap This is the nozzle at the front of the gun
that directs compressed air into the stream of paint,
thus atomizing it and forming the spray pattern.
Air caps are designed to give a wide variety of
spray patterns and sizes, and in addition ensure
perfect atomization over a wide range of paint viscosities.
The choice of an air cap (Figure 17.24)
depends on the following:
1 Volume of compressed air available
2 Type of paint feed system being used
3 Type and volume of paint to be sprayed
4 Size of the fluid tip (air caps are usually
designed to operate with a particular fluid tip)
5 Nature and size of the surface to be painted.
Figure 17.20Air cap with pressure gauge
attached (DeVilbiss Automotive Refinishing
Products)
Figure 17.22HVLP gravity-fed spray gun (DeVilbiss
Automotive Refinishing Products)
Figure 17.21HVLP spray gun using a remote fluid
cup (DeVilbiss Automotive Refinishing Products)
Automotive finishing and refinishing 589
Figure 17.23Basic parts of the HVLP spray guns (DeVilbiss Automotive Refinishing Products)
1 Nylon air cap retaining ring
2 Front air cap baffle (nylon)
3 Kit of 2 baffle seals
4 Rear baffle assembly
5 Spreader/fan adjustment ring
6 Fluid needle packing set
7 Fluid needle packing gland
8 Kit of 5 fluid needle springs
9 Kit of 5 gun body bushing gaskets
10 Fluid needle body bushing
11 Fluid needle adjusting screw
12 Complete air valve assembly
13 Air valve stem
14 Kit of 3 air valve (trigger) springs
15 Kit of 5 trigger pivot screws
16 Kit of 5 female trigger pivot studs
17 Chrome plated trigger
18 Male/male air connector in BSP
19 1 pint nylon gravity cup assembly
20 Gravity cup lid assembly
21 Kit of 5 drip check lids
22 Kit of 5 O-rings for gravity cup
23 Mini QD female valve and male stem
24 Adaptor to fit regulator to QD valve
25 Cup pressure regulator
26 Kit of 2 regulator elbow connectors
27 Kit of 5 regulator to cup tubes
28 Cup pressure tube connector
29 QD stem
30 QD valve
31 Complete cup lid assembly in BSP
32 Fluid tube in BSP female fitting
33 1 quart KR cup yoke
34 Kit of 5 washers
35 Cam lever
36 KR 1 quart pressure cup lid
37 Kit of 3 KR cup lid gaskets
38 Washer
39 Fluid tube retaining nut
40 Kit of 5 drip free diaphragms
41 1 quart (1.14 litre) PTFE lined pressure cup
42 KR cup and lid assembly (PTFE lined)
43 Fluid inlet connector in BSP
44 Fluid inlet locknut
590Repair of Vehicle Bodies
Fluid tip This is situated behind the air cap and
meters out the paint. The volume of paint passing
through the fluid tip and into the stream of compressed
air is governed by the diameter of the orifice
in the fluid tip. The choice of fluid tip
depends, in the main, on the type of material to be
sprayed. Heavy, coarse or fibrous materials require
large nozzle sizes to prevent clogging, whilst thin
materials, which are applied at low pressures,
require small nozzle sizes to prevent an excessive
flow of paint. The fluid tip provides a seating for
the fluid needle (Figure 17.24).
Fluid needle This seats in the fluid tip, its function
being to start and stop the flow of paint. For
the gun to operate efficiently, the fluid tip and fluid
needle should be selected as a pair, and should be
of the same size (Figure 17.24).
Fluid control screw This is an adjustment control
which limits the length of travel of the fluid needle,
governing the flow of paint from the fluid tip.
Trigger The function of the trigger is to operate
the air valve and also the fluid needle.
Air valve This is situated in the handle of the
spray gun (or gun body) directly behind the trigger
by which it is operated. Its function is to control
the passage of air through the gun.
Gun body This can be regarded as a basic frame
on to which the spray painter will mount a suitable
set-up to suit his requirements.
Spreader control This is of great importance in controlling
the volume of air passing to the horn holes of
the air cap. If air is cut off from the horn holes a narrow
jet of paint giving a spot pattern is ejected, but
when air is allowed to pass through the horn holes a
fan spread is obtained, the width of fan varying
according to the volume of air (Figure 17.25).
17.9 Spray gun maintenance and cleaning