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