GB2197600A - Electrostatic paint spray gun - Google Patents

Description

2197600 1 MS=C SPRAY GEN The invention concerns an electrostatic spray gun

e.g. a paint spray gun, with an insulating body containing pulverulent or liquid (hereafter: paint) and air channels, an insert or adapter secured to the insulating body for enabling the flow of paint, a needle valve with a valve rod extending in the paint channel through the insulating body, and an electrode arranged in the paint channel for contact with the paint stream and connected to a high voltage power source by way of a conductor.

In the paint shops of industrial undertakings it often happens that objects of large surface area, thin tubular constructions and wires are to be painted one after another. The economic performance of such different tasks has hitherto required two electrostatic paint spray devices based on different systems. In most cases large-area objects are sprayed with pneumatically atomised means, wherein electrostatic paint technology has come into most widespread use. The construction, actuation and operation of electrostatic devices scarcely differs from the traditional exclusively pneumatic systems; all in all the 'barrel' of the spray gun is longer and is made of plastics, whereby to make it possible electrically to charge the atomised paint mist and to assure the effect connected therewlth. For flat surfaces and workpieces which fill the spray space well and are of average complicatedness the mentioned system may be used successfully with a significant saving of paint.

When, however, workpieces of low space utilisation are to be painted, e.g. frames consisting of thin tubes arranged at a spacing in front of each other, meshes, wire, nets, the system can only solve the problem at a high paint loss, even though the back surfaces of the mentioned workpieces are also coated with paint. The reason for this lies in the system of atornisaton which employs compressed air. This is because in this case the air stream atomises the paint in such a manner that at the outflow position of the paint the light paint drops receive an amount of kinetic energy which attempts to drag the drops past the workpiece although the electrostatic attraction tries to attract the d_rops to Che surfaces of the workpiece. Howexrer, whan the 2 kinetic energy is too great or the paint drops find themselves at too - a distance from the object, at most the flight path of the drops great is modified but they are not deflected at the workpiece; they fly by it whereby a significant loss of paint may arise.

Special atomisation nozzles are also known which atomise with a low air stream and at low air velocity; although with such devices a higher amount of paint is attracted to the workpiece surface, the paint loss is still significant.

From the practice of electrostatic paint spraying it is well known that the smallest losses occur with electrostatic spray systems without air atomisation, such as for example spraying from a stationary edge (ABG-brush), rotary circular edges or a rotary bell. Such spray devices, in manual or automatic embodiments, have been known for a long time, in fact the introduction of electrostatic paint spraying even began with the utilisation of systems with rotary bells (Rarisburg). -'E:ltru-- tural embodiments of systems based on rotary bells formed independent complete units; as such systems my be mentioned the Ransburg No. 2 apparatus (USA), SAMES (France), COLORSTAT (GDR) and HANDSPRAY (Hungary). The mentioned devices operate with a high percentage of paint utilisation (80 to 95%) but in contrast, the sprayed- out amount of paint with a value of 300-800 cir.3/jrinute, significantly differs from that of pneumatic systems, where it is approximately 50-150 cm3/minute. As a result spray devices with a rotary ball are suitable for coating workpieces of small surface area; in this field they have been used very successfully.

In the beginning of the construction of spray devices with rotary -he bell was made of metal. This solution proved itself well and bel! s 1 the atomisation was satisfactory. Later, however, practical problems arose from the viewpoint of safety technology. The wide-spread use of these devices required the monitoring standards regarding safety technology and protecton against fire, having regard to the easily yst wh e flamirable paints and solvents sprayed in electrostal s ems er -remely the risk of spark formation existed. Thus arose the ext. important task frorr. the point of view of fire hazard protection of preve--itiri the formation of sparks and to restrict the threshold value for ignition. As a result of technical and practical re-sea-rch activity :1 3 a f ire protection norm or standard was established according to which for paints with solvents with an inflammation point below 210C the spark energy must not exceed the value of 0.25 mJ. This means that in the system, in the vicinity of the spray under the most dangerous and most unfavourable circumstances no spark discharge should arise with a magnitude which could cause ignition or burning of the materials errployed.

The strength of the spark depends on how great the electric discharge is, the magnitude of which can be determined by conpitation according to laws of physics. The electric charge present on a metal body is proportLonal to the capacity of the body and the square of the voltage, or, put more precisely, Q=1/2 U2.C, wherein the discharge Q is in joules the voltage U is in volts and the capacity C is in P the farads. According to practical measurements, the self-capacity o menlk-ione-d metallic spray bell is approximately 5 to 10 pP, which eve-". at an operational voltage of 60 kV gives Q = 1/2(60.103)2.5.10-12 = 9.10-3 = 9 no which represents a multiple of the permitted value. When metal bells are touched with the hand or an earthed object, a high-energy discharge results, wthich can lead to ignition of the solvent vapours.

For this reason rotary bells which might be touched by the hand or with an object have lately been made from plastics in order to prevent by virtue of their insulating capacity, the formation of high surface charges as vi-all as sudden discharges.

For a bell made of insulating material the charge required for atomisation s conveyed along by the paint itself. The electric voltage is introduced into the paint by a metal electrode by arranging the electrode in the paint duct or channel to contact the paint be-fore the entry of the latter into the bell. From this location the self - conductivity of the paint is sufficient for the further conveying of the charge. The length of flow path from the electrode to the atomisation edge of the bell may, however, amount to several cm and since the specific resistance of individual paints is high, e.g. 108 Ohm.cm, (100 Megohm), a significant voltage drop occurs along this flow path so that the voltage prevailing at the atomising, bell edge is insufficient for an efficient charging of the paint.

4 In order to eliminate this drawback. care is taken in practice to operate with an increased voltage, approximately 80-100 kV, or to reduce the resistance of the paint to be sprayed by the use of known chemical methods.

The invention aims to avoid the above-mentioned disadvantages of known apparatuses but at the same time to unite their advantageous properties.

Aon that the stated aim my The inve-ition is based on the recognit be achieved by combining electrostatic paint spray g-ins of the abovementioned type with a rota.-y bell.

Accordingly, the invention concerns an electrostatic spray gun wAhich includes a spray device provided with paint and air channels, a ne--,. 'lle valve for ope-ning and blocking the path of the paint arranged in an adapter permitting paint flow a-nd secured to a plastics body; a valve rod arranged in the paint duct and running through the insulating body; and a high-voltage electrode arranged in the paint duct in contact with the paint flow.

The essence of the invention manifests itself particularly in that at the spray device of the paint spray gun there is an adaptor device made from an insulating mate-rial with a rotary bell the drive of w!-iich is connected by way of a re-leasable securing ring.

Preferably, the rotary bell is made of an insulating material and at the inner surface of its outer contour one or more cylindrical If set relative aiLn,ular e-lectrode(s) are arranged in a conf iguration of to the atomising edge.

It is also regarded as advantageous that mounted in the outer ting s=rface or jacket of the rotary bell made of insulat g material there are closely adjacent wire electrodes extending in the direction of the product. One end of these electrodes is in the annular distribution chir,,ber of the bell while the other end, disposed at the internal face I" the bell in the vicinity of the atomising of the outer surface o.

edge, is in contact with the paint. Preferably, the electrodes a-rranged in the rotary bell or reducing the resistance are made of a se--r.i-,cory-.ictor material.

Accordincly, the invention combines in essence two spray systems while using an adaptor de-,,ice. The possibility also a-rises of using a 1 rotary bell instead of the spray cap of the already present pneumaticelectrostatic apparatuses, whereby the apparatus becomes suitable for the change-over to an economic painting of wires, meshes and extremely thin pipes. The adaptor device containing the rotary bell-and its drive can be secured by way of the very same spigot nut which originally held the spray cap. The bell is set into rotary motion with the aid of compressed air introduced into the nozzle insert of the spray device which originally served only for atomisation purposes. To this end any construction can be used which converts flow energy of the compressed air into rotary motion. In the case of an originally flat spray cap, the r.p.m. of the bell may also be regulated with the aid of a set screw assuring the flattening of the spray pattern of the paint mist. The single or several electrode(s) mounted into the rotary bell reduce(s) the resistance of the paint film from the position at which the paint flows into the bell to the atomisation edge to such an extent that no appreciable voltage drop occurs; simultaneously no energy discharge exceeding the desired value starts from the accessible or touchable surface of the bell.

m Lhe invention is described by way of an advantageous preferred embodiment with the aid of the drawings, wherein:

Figure 1 is a longitudinal section of the preferred embodiment of the paint spray pistol according to the invention; Figure 2 is a section of a rotary bell for use with the apparatus according to the invention, wtile Figure 3 represents another preferred embodiment of the rotary bell.

Fig-are 1 shows a preferred embodiment of a spray gun according to the invention. A metal handle 1 houses an air valve 2 which is opened or closed via a valve rod 3 by means of an actuating lever or trigger 4. The compressed air required for actuation passes over a junction 5 into a duct 6 in the handle 1. Prom here it passes via a duct 7 into a duct 9 formed in an insulating body 8 arranged downstream of the valve 2 as well as of a conical regulating valve 10 which is disposed at the downstream end of duct 7 on an extension of an adjusting screw 11.

The air flows from the duct 9 into a duct 14 which is formed in an adaptor body 13 secured by way of a securing ring (spigot nut) 12, 6 the adaptor being used in place of a spray cap 42 of the basic apparatus. The duct 14 conveys the air to the drive device 15 of a rotary bell 17 which, using a method well-known in the technology, converts the energy of the compressed air into rotary movement, i.e. via a shaft 16 sets the bell 17 of plastics material into rotation.

In use, paint passes from a container 18 via a hose 19, a junction 20, a short plastics pipe 21 as well as a junction 22 into a paint duct 23 which is closed in the outflow direction by the conical end cf the plastics valve rod 25 co-operating with a valve seat in the form of a narrowed bore of an insert 24.

When now the actuting trigger 4 is pulled, the air valve 2 is opened and the rod 26 (which is coupled to rod 25) is pulled with the aid of a spring plate 27 against the bias of a spring 28 out of the flow of paint in a forward direction is enabled.

valve seat, whereby.

The movement of the rod 26 is pressure-tightly sea_led by a stuffingbox 29 and a gasket 30. On opening the paint valve (24, 25) the paint flows into a bore 31 formed in the adaptor device body 13 as a continuation of the duct 23, and then via an outlet tube 32 into an -ion chamber 33 at the back of a rotary bell 17, from ann,L-,lar distribut where the paint passes through gaps 34 into the interior of the bell 17 to the annular edge 35 where it is atomised and sprayed out.

A high voltage generator 36 delivers the voltage required for the via a high voltage cable 37 to an electrostatic charging of the paint electrode 38 arranged in the paint duct for contact with the paint stream.

Figure 2 shows on an enlarged sca-le a possible embodiment of the atomisation bell of the spray gun according to the invention. The electrodes 39 are spaced apart so as not to be in metallic contact with each other, they are disposed on the inner wall of the plastics bell 17. Thus the electrodes 39 cannot be touched either by hand or by the object to be painted during operation. The electric resistance of the paint film is reduced, this film extending in the section between the distribution chamber 33 via the gaps 34 a-nd the internal surface 41 of the bell to the aton-iisation edge 35.

Figure 3 shows ancther possible embodiment of the atomisation bell of the spray g-un ac=rding to the invention. In this embodiment is J 1 7 there are tightly adjacently arranged metal wires 40 in the outer surface of the plastics bell 17. They are effective to reduce the resistance of the paint film between the annular distribution chamber 33 and the circular atomisation edge 35 in that one end of the wires is in contact with the radially outer surface of the annular distribution chamber 33 and the other end is in contact with the film at the conical internal surface 41 of the bell in the vicinity of the circular atomisation edge 35. These metal wires 40 are also arranged in such a way that they are not accessible either by hand or by the object to be sprayed, and in addition their individual capacity is so low that they are not capable of forming the minimall charge required for causing sparks which would carry fire risk at the given operational voltage.

Thus, the invention concerns a versatile electrostatic paint spray gun which applies paint (a) in the case of large-surfaced objects which f ill out the operating space well by means of a pneumatic atomisation system, and (b) in the case of wire or tube constructions which fill out that space with large gaps, by way of a rotary atomising bell. The essential characteristic of the invention lies in that, in contrast to the known spray guns, the two spray systems are united or combined in a single apparatus. The pneumaticelectrostatic system is suitable for painting large-sized objects, while for objects with small surfaces, instead of the cap (42) operating with air atomisation an adaptor device (13) is nc)unted which contains a plastics rotary bell (17), the compressed air operated drive (15) as well as a mechanism (18-24) for delivering the paint into the bell (17). In the bell (17) semi-conductor electrodes (39 or 40) are arranged which on the one hand assure the maintenance of electrical charging of the paint while on the other hand they satisfy al-1 demands regarding safety requirements.

8

Claims (6)

CIAIMS

1. Electrostatic spray gun, comprising an insulating body provided with a spray liquid duct and an air duct, an extension secured to the insulating body for enabling the flow of spray liquid, a valve with a valve rod made of plastics material in the liquid duct extending through the insulating body, an electrode arranged in the spray liquid duct for connection to a source of high voltage source by way of a conductor and for contact with the spray liquid, and an adaptor device made of insulating material and secured to the said insulating body, the said adaptor device being provided with a rotary bell and with a drive device for the bell.

2. Electrostatic spray gun according to claim 1, wherein the -or device is connected to said insulating body by way of a adapt releasable securing ring.

3. Electrostatic spray gun according to claim 1, wherein the rotary bell is made of an insulating material and at its inner surface there is an electrode, or there is a plurality of mutually offset annulax electrodes, offset towards the atomisation edge.

4. Electrostatic spray gun according to claim 1, wherein in the surface of the rotary bell, which is made of insulating material, there are closely adjacently lying wire electrodes extending in the directon of the atomisation edge, one end of the wires being in contact with the liquid in an annular distribution chamber of the rotary bell and the other end being in contact with the liquid at the inner surface of the bell in the vicinity of the atomisation edge.

5. Electrostatic spray gun accordIng to any preceding claim, wherein the electrodes arranged in the rotary bell for reducing the resistance of the liquid are made frorr, a semiconductor material.

6. Electrostatic spray gun substantially as herein described with reference to Figures 1 and 2 or Figures 1 and 3 of the ac2a-,.anying drawings.