US 3401883 A
Description (OCR text may contain errors)
Sept. 17, 1968 O GEBHARDT ET AL 7 3,401,883
SPRAY PISTOL Filed Aug. 23, 1966 INVE R6 0- e blar I M- L ierel A i i349.
United States Patent 1 Claim. (C1,. 239-15 ABSTRACT OF THE DISCLOSURE A spray gun nozzle having an air chamber housing with a pipe therein which narrows conically toward a spraying end and the pipe being coaxial with an air chamber in which there is provided a backwall having angularly arranged bores therein to Set up turbulence in the chamber.
The invention relates to spray pistols operated by compressed air for the production of coatings of extremely finely devided liquid and/or solid substances by Spraying with electrostatic charging of the sprayed material.
Very many spray pistols are known in which the atomisation of the sprayed material and the guiding of the atomised sprayed material to the workpiece which is to be coated is eifected by compressed air. An essential disadvantage of all these compressed air pistols is however a relatively high consumption of sprayed material, since according to the shape of the workpiece, always a more or less large quantity of said material misses the workpiece and is thus lost without serving any useful purpose. On the other hand, spray pistols are also known which operate purely electrostatically. With these spray pistols, the consumption of sprayed material is substantially smaller on account of the said material being attracted by the workpiece and in addiiion an embracing of the workpiece with the sprayed material is obtained. However, the electrostatic spraying method cannot be used in all cases. For example, it is impossible for the interior of a narrow container or vessel to be sprayed by purely electrostatic means, since in this case the charged droplets of sprayed material are already so strongly attracted by the rim of the container that only a small fraction of the sprayed material reaches the interior of the container. Consequently, attempts have also frequently been made to provide combination pistols which operate with both compressed air and with electrostatic charging. The most obvious procedure consists in applying high voltage to the spraying nozzle of a conventional compressed air pistol, that is to say, to use the spraying nozzle simultaneously as a spraying electrode. In such a case, the noz zle for the sprayed material generally consist of a nozzle tube which is connected to a supply device for the sprayed material and a high voltage generator, said tube being surrounded concentrically by a compressed air nozzle formed as an annular gap. However, it has been found that in this way only an incomplete charging of the sprayed material discharging at high velocity from the nozzle tube can be obtained, so that more especially substantially less satisfactory results are reached as regards the complete embracing with sprayed material than is achieved with purely electrostatic operation. In addition, the danger exists with these pistols that, more especially with certain spacings between the pistol head and the workpiece, a reduced pressure zone is set up directly in front of the spraying nozzle, whereby a satisfactory spraying becomes impossible.
The invention now has for its object to provide a compressed air pisfol with a spraying nozzle serving as an Patented Sept. 17, 1968 "ice electrostatic high voltage electrode, with which satisfactory and complete charging of the sprayed material is effected. According to the invention, this object is achieved by the fact that the spraying nozzle consists of a plurality of individual nozzles, which open substantially perpendicularly of the air stream issuing from the annular gap. The nozzle for the sprayed material preferably consists of a tube which tapers towards the spraying end and is covered at said end, six separate nozzles being provided in symmetrical arrangement in the conical part of the pipe wall. According to another development of the invention, the annular nozzle is supplied tangentially with compressed air.
By means of the invention, the advantage is obtained that an intensive charging of the spraying material issuing from the separate nozzles is achieved. The reason for this consists in that the material being sprayed remains substantially longer than hitherto in the discharge region of the nozzle electrode on account of the particular arrangement of the separate nozzles, this effect being further intensified by eddy formations of the compressed air on the nozzle body. There is obviously no danger of a backpressure of the spraying material at the nozzle Opening in accordance with the invention, but rather any air which possibly flows back in the centre of the round stream of sprayed material leads to a further improvement in the charging of said material.
Other features, advantages and details of the invention become apparent from the description, the drawing and the claim. One constructional form of the invention is shown by way of example in the drawing, wherein:
FIGURE 1 is a longitudinal section of a conventional spraying head, and
FIGURE 2 is a longitudinal section of a spraying head according to the invention.
According to FIGURE 1, a conventional spraying head comprises a supply pipe 10 for the material to be sprayed, said pipe terminating at its Spraying end in a central nozzle opening 11. The spraying nozzle 11 is dc fined by the marginal edge of an air chamber housing 13. A flange 14 on the spraying pipe 10, which is formed with holes 15, closes the air chamber 16 at the rearward end. The complete spraying head is mounted on the barrel end of a spray pistol (not shown), the pipe 10 being connected in the usual manner to the supply for the material to be sprayed and the openings 15 are connected to the supply for compressed air. The pipe 10 advantageously consists of metal and is in addition connected conductively to a high voltage source. If now the paint and air supply devices are uncovered, using conventional means, the compressed air issuing from the annual gap 12 withdraws the material to be sprayed from the nozzle 11 by virtue of an ejector action and conveys it to the workpiece. At the same time, the material being sprayed is charged at the rim edge of the nozzle 11 connected to the high voltage source. As already set out above, only a partial charging of the material being sprayed is achieved, firstly because only the peripheral region of the discharging jet of spraying material comes in direct contact with the electrode nozzle and secondly because the material being sprayed only remains a very short time within the corona region of the electrode nozzle, and instead flows away immediately in the direction of the work-piece. In addition, a vacuum can be produced centrally within the round stream consisting of a mixture of spraying material and air (region a in the drawing), whereby the further discharge of paint or colouring agent is prevented.
The arrangement according to the invention is Shown in FIGURE 2. In this figure, individual nozzles 17 are arranged laterally in the conical end portion of the supply pipe 10' for the material to be sprayed, and in fact advantageously six nozzles are provided, while the actual end of the pipe is closed. When air and spraying material are now released in accordance with the invention, the air issuing from the annular gap 12 carries with it the spraying material from the nozzles 17 along the sloping surface of the nozzle pipe 10, so that there is obtained a satisfactory charging of the spraying material, which remains for a relatively long time in the region of the discharge zone. In addition, the compressed air flowing along the nozzle body has a tendency to form eddys, and this likewise contributes to improving the charging. With the invention, a mist of spraying material which perhaps flows back n the centre of the round jet is obviously unable to influence the discharge of fresh material, but rather also improves the ionisation of the material being sprayed.
An essential feature of the invention now consists in that the holes 15 leading to the air chamber 16, by contrast with the prior art, do not extend parallel to the axis of the nozzle pipe 10, but with a slight inclination relatively thereto (at an inclination to the plane of the drawing in FIGURE 2). The setting of the holes or passages 15 is such that there is rotation of the compressed air concentrically of the nozzle pipe 10 in the chamber 16, so that the air is moved with a helical motion forwardly towards the annular gap 12 and emerges from the latter as a rotating round stream or :jet, whereby there is obtained a substantial increase in the spraying action, namely, a better ionisation of the sprayed material and avoidance of a return flow of said material.
According to a modification of the invention, the supply of air to the chamber 16 can also take place from the side and approximately tangentially of the annular gap 12, and in this case a rotating air jet also issues from the annular gap 12.
The arrangement according to the invention shows an extraordinary depth effect of the sprayed jet. Thus, even places which are of diificult access and even narrow cavities can be satisfactorily sprayed. Moreover, it has been found that it is possible with the arrangement according to the invention to obtain a better coverage with the sprayed material than is the case with the former pistols operating by means of air atomisation. Finally, it is also to be pointed out that the arrangement according to the invention can also be used without an electrostatic high voltage field, and excellent spraying results far superior to those obtained with the conventional compressed air pistols are then produced.
1. A spray gun operated by compressed air, for the production of coatings of finely dispersed, liquid and solid substances under an electrostatic charge of the spraying material, a nozzle comprising an air chamber housing, a nozzle in the housing for the spraying material and attached to a high voltage source and comprising a pipe which narrows down conically toward a spraying end and which is covered up and has exit openings emptying essentially perpendicularly in relation to an axis of the pipe in symmetrical arrangement and in a conical part of the pipe, a compressed air nozzle in the form of an annular slot arranged upstream of said openings and concentrically surrounding the nozzle pipe at the spraying end and with an air chamber between the pipe and housing to feed the annular slot with compressed air and surrounding the nozzle pipe concentrically, a back wall in the chamher with bores therein to supply compressed air therethrough, said bores being angularly arranged relative to the axis of the nozzle pipe so that a turbulence will be set up in the chamber which is concentric relative to the nozzle pipe.
References Cited UNITED STATES PATENTS 1,427,910 9/1922 Quinn 239405 1,432,528 10/1922 Burdett 239434.5 1,526,923 2/1925 Medn 239434.5 X 1,646,703 10/ 1927 Mulholland 239406 2,046,592 7/1936 Tracy 239-405 X 2,090,566 8/1937 Andler 239406 2,966,310 12/1960 Sedlacsik 23915 3,111,266 11/1963 Axelson et al. 239424 X FOREIGN PATENTS 998,642 9/1951 France.
M. HENSON WOOD, 111., Primary Examiner. V. M. WIGMAN, Assistant Examiner.