EP0630690B1 - Air assisted flat jet spraying device for spraying coating material - Google Patents

Description

The invention relates to a pneumatic sprayer for a flat jet coating product. More specifically, it relates to a sprayer with a high flow rate and a low pressure of atomizing air, usually referred to as "H.V.L.P.".

When using a flat jet sprayer, an essential problem is to obtain a spray having a sufficiently flat shape so that the sweeping movement of the sprayer allows the most uniform deposit possible on the objects to be covered. To do this, horns are provided projecting from the front face of the spray cap: they make it possible to subject the spray of sprayed paint leaving the sprayer to the action of several air jets which crush it. We are talking about conformation of the jet.

However, in the flat jet sprayers of the prior art, whether manual guns or automatic projectors, the adjustment of the shaping air or horn air is delicate. Indeed, if it is not powerful enough, the paint spray is not wide enough and it is necessary to print a very rapid sweeping movement with the sprayer to obtain a satisfactory surface condition; this is not always possible in the case of an automatic sprayer; this is very tiring for the operator in the case of a manual sprayer. The advantages of the flat spray are then nonexistent.

On the other hand, if the shaping air is too strong, the paint spray is cut in half and there is no more paint in the central part of the spray: the spray obtained is no longer a flat spray but the combination of two more or less ovoid jets. The thickness of the coating obtained on the object to be covered is then irregular, which is unacceptable.

A conventional pneumatic sprayer uses a source of compressed air to drive the paint stream into an area where it is sprayed. We are talking about training air. A second source of compressed air is used to spray, that is to say divide into a multitude of droplets, the paint stream at the outlet of the spray orifice. We are talking about spray air. The drive air and the spray air may have a common source; they are then separated by channels inside the spray cap.

In the particular case of flat jet sprayers of the HVLP type, the drive air also constitutes the spray air. Its pressure at the rear of the spray cap is relatively low, preferably less than 1 bar. However, to be sprayed, the paint stream must be entrained and sheared by this air jet: the paint particles located in the center of the stream are not subjected to the shear action of the spray air. It is therefore sometimes necessary to increase the pressure of the atomizing air, which limits the advantages of the HVLP configuration. Low pollution, low paint loss and high deposition efficiency obtained with a sprayer of this type come essentially from the low pressure of the spray air and of the shaping air.

American patent US 5074466 describes a gun with high flow rate and low pressure of atomizing air in which the needle of the valve is used to make the vein of coating product hollow. In this way, the hollow jet which leaves the sprayer is much easier to spray and to crush by the conformation air. Indeed, in the case of a hollow vein, the ratio of the external perimeter to the section of the paint vein is greater than that of a full vein of the same diameter. In a hollow paint stream, the particles are on average closer to the outside of the stream and are therefore more subject to the action of external aerodynamic forces than in the case of a solid stream, whether the action of atomizing air or that of conforming air.

However, this technique is only valid if the needle is placed very close to the outlet of the paint channel, that is to say practically less than 5mm in the open position. In fact, if the needle is too far from the paint outlet, the vein closes and the spray of coating product obtained is a full spray. This phenomenon will occur more or less depending on the physical characteristics of the product such as its viscosity and its surface tension. The formation of the hollow vein will not be reproducible if paints of different chemical compositions are used.

In addition, the more or less hollow character of the vein depends on the degree of opening of the valve, which is prohibitive because the efficiency of the conformation then also depends on the opening of the valve, that is to say on the spray rate. The adjustment of such a sprayer is particularly delicate.

Finally, the position of the needle relative to the seat of the valve alone conditions the profile of the vein of coating product: if the needle is not exactly centered on the seat, the hollow vein obtained is not cylindrical. This asymmetry is necessarily found in the flat jet finally obtained.

The invention solves all of these problems.

It relates to a pneumatic sprayer of coating product with flat jet comprising a valve for supplying coating product delivering a paint stream and a spray nozzle characterized in that an insert comprising an annular channel is housed in the nozzle downstream of the valve needle and transforms said vein into a hollow spray of paint.

The presence of the insert immediately before the outlet of the paint channel and downstream of the valve needle guarantees that the vein retains its hollow form until it is ejected from the sprayer, that is to say until 'it undergoes the action of spray and shaping air.

Thanks to this insert, the hollow character of the vein is obtained independently of the adjustment of the flow rate of coating product and of its physical characteristics. The centering of the needle on the valve seat does not condition the symmetry of the coating product. Finally, in the case of an electrostatic sprayer, the relatively large electrical capacity that constitutes the metal needle can be housed deep enough in the body of the sprayer to avoid any risk of sparks.

It is easy to transform a flat jet sprayer known to benefit from the invention. It suffices to place the insert characterizing the invention in the sprayer nozzle in place of the conventional injector which is removable from the front of the sprayer.

• la figure 1 est une vue schématique en coupe d'un pulvérisateur pneumatique selon l'invention,
• la figure 2 est une vue à plus grande échelle de l'insert du pulvérisateur de la figure 1 et
• la figure 3 est une vue à plus grande échelle d'une variante de l'insert du pulvérisateur de la figure 1.

The invention will be better understood and other advantages thereof will appear more clearly in the light of the following description of an embodiment of a pneumatic sprayer of flat jet coating product in accordance with its given principle. by way of example only and with reference to the appended drawings in which:

• FIG. 1 is a schematic sectional view of a pneumatic sprayer according to the invention,
• FIG. 2 is an enlarged view of the sprayer insert of FIG. 1 and
• FIG. 3 is an enlarged view of a variant of the sprayer insert of FIG. 1.

The sprayer 1 of FIG. 1 is of the HVLP type. It includes a paint inlet channel 2 supplied by a conduit 3 penetrating into the sprayer through the butt 4. The trigger 5 makes it possible to control a valve 6 whose needle 7 rests on a seat 8 near the spray orifice 9 formed in a spray nozzle 10. An air supply duct 11 also enters the butt 4. It makes it possible to supply, through ducts 12 and 12 'The sprayer 1 in drive air and spray on the one hand and in horn or conformation air on the other hand. The horns 13a and 13b include orifices 14a, 15a, 14b and 15b of exit of conformation air directed towards the paint stream leaving the sprayer through the orifice 8. These horns form part of a spray cap 16 held in place place on the nozzle 10 by a clamping nut 17.

Downstream of the seat 8 of the paint supply valve is disposed an insert 20, for example of plastic material, transforming the paint stream into a hollow paint stream. This insert is housed in the nozzle 10 near the spray orifice. It in fact constitutes the terminal part of the paint channel in the nozzle 10.

The insert 20 is visible in Figure 2. It has four inlet channels 21 regularly distributed on the side of the seat 8 and the needle 7 of the paint supply valve and an annular outlet channel 22 constituting the orifice spray. Channels 21 and channel 22 are in communication.

In the case of an electrostatic type sprayer, the insert 20 has an axial bore 25 in which is housed a charging electrode 26 supplied in a manner known per se from a high-voltage generator not shown. The electrode 26 protrudes from the insert 20 over a sufficient length L so that it charges the vein of paint both by contact and by Corona discharge.

The operation is as follows: after passing through the pipe 3 and the pipe 2, the paint passes between the needle 7 and the seat 8 of the supply valve. It is then divided into four threads by the four conduits 21, which are joined in the channel 22 to form a single jet which is kept hollow up to the spray orifice 9 thanks to the central internal part 27. The presence of the central part 27 to the end of the insert 20 prevents the paint stream from becoming a full stream before the spray air begins to shear and entrain it and before the shaping air doesn't start to overwrite it. It is then possible to use spray air and conformation air at a relatively low pressure to entrain and flatten this paint stream.

If the charging electrode 26 is long enough, the crushing begins at the end of the latter and it is in direct contact with the paint stream, which leads to charging by contact. Otherwise, the electrode charges the paint stream with Corona discharge because its end is close to the paint stream being sprayed.

The variant of FIG. 3 relates to an insert 120 including a modification of the geometry of the central part 27 of FIG. 2: it is referenced 127. It has a frustoconical end 128 which deflects the paint stream. The paint therefore has a radial velocity component when it meets the spraying air which is substantially parallel to the axis of the insert 120. In this way, the penetration of air into the paint stream is further facilitated and the pressure of the spraying air can be kept at a minimum value.

In the case of an electrostatic type sprayer, the insert can also be metallic or made of a conductive material. The electrode 26 is then removed and the coating product is charged by contact when it passes through the channels 21 and 22 and / or by Corona discharge on the edges of the insert 20 or 120. This insert can also be produced by ceramic, which gives it a longer life because of the hardness of this material.

The description has been made with reference to a manual sprayer but the invention also obviously relates to an automatic sprayer. Likewise, the invention applies equally to an electrostatic or purely pneumatic pneumatic sprayer, with a high flow rate and low pressure of conventional or spraying air.

Claims (8)

1. Air-assisted flat-jet spray device for a coating product, comprising a coating-product feed valve (6) delivering a stream of paint, a spray nozzle (10) and shaping air orifices (14, 15) directed towards the stream of paint exiting the said nozzle, characterized in that an insert (20, 120) which includes an annular channel (22) is housed in the said nozzle downstream of the needle (7) of the said valve and converts the said stream into a hollow jet of paint.
2. Spray device according to Claim 1, characterized in that the said spray device is of the type having a high volume and low pressure of atomizing air.
3. Spray device according to either of the preceding claims, characterized in that the insert (20, 120) includes several inlet channels (21) on the side facing the needle (7) of the valve (6), the said channels communicating with the annular channel (22).
4. Spray device according to one of the preceding claims, characterized in that the hollow stream of paint is atomized at the exit of the insert and then shaped into a flat jet by jets of air emanating from horns (13a, 13b).
5. Spray device according to one of the preceding claims, characterized in that the hollow stream of paint has a radial velocity component on exiting the insert (120).
6. Spray device according to either of Claims 3 and 5, characterized in that the central internal part (127) of the channel of the insert includes a frustoconical end (128) forming a deflector.
7. Spray device according to one of the preceding claims, characterized in that the insert (20) includes an axial drill-hole (25) in which a charging electrode (26) is housed.
8. Spray device according to Claim 7, characterized in that the electrode (26) extends beyond the insert (20) by a length (L) sufficient for it to be in contact with the compressed stream of paint.

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