Process for enamelling objects electrostatically by means of a gun

In this process comprising fluidizing the enamel powder in a fluidization vessel by blowing air in the bottom of the latter, steam is also introduced in the vessel in the course of the fluidization. The relative humidity is adjusted in the vessel to a value between about 15 and 45% in respect of a temperature in the vessel of between about 20.degree. and 30.degree. C. A steam boiler (12) is connected to the fluidization vessel (1) for this purpose.

DESCRIPTION 
The present invention relates to the enamelling of generally metallic parts 
by a process electrostatically applying enamel in the form of a powder. 
Those skilled in the art know that the carrying out of processes which 
apply powdered enamel electrostatically are highly dependent, as to their 
good operation, on the hygrometric state of the air surrounding the gun. 
This hygrometric state in particular affects the thickness of the deposit 
and the adherence of the powder to the parts. 
In order to satisfy these requirements, it is already known to impart to 
the air surrounding the application zone temperature and humidity 
characteristics which are defined with precision. 
In order to achieve this object, it has been proposed to put the paint 
application unit in an air-conditioned enclosure and to dry the air 
supplied to this enclosure to a variable extent. 
However, such a unit has in particular two drawbacks. 
An air-conditioned enclosure markedly complicates the unit since there must 
be constructed in the enamelling shop a cabin which is completely closed 
and communicates with the exterior solely by way of passages provided 
therein for conveying the parts. 
Further, it has been found that, notwithstanding the precise regulation of 
the atmosphere in the cabin, the results of the application of the enamel 
powder depend on exterior climatic conditions and in particular on the 
humidity of the air. Thus, in accordance with the atmospheric conditions, 
the deposits of enamel on the parts vary in thickness. This is due to the 
fact that the air which supplies and carries the powdered enamel to the 
parts, may have characteristics which are very different from the optimum 
conditions of application. Indeed, this air is very often produced by 
compressors located outside the air-conditioned enclosure and thereafter 
dried. The characteristics of this air consequently depend on the exterior 
climatic conditions. In the zone of application, this air expels the 
surrounding conditioned air and determines practically alone the 
conditions of application of the powder. 
In order to avoid these problems, it is also known to introduce water in a 
liquid phase at various points of a unit such as, for example, on the 
periphery of the mouth pieces of the guns, in the application cabin or in 
the conveying air. 
However, such an introduction of water does not give favourable results. 
An object of the invention is therefore to provide a process for treating 
the mixture of air and powdered enamel for the electrostatic application, 
by means of a gun, on objects which must be enamelled which avoids the 
aforementioned drawbacks. 
According to the invention, there is provided such a process comprising 
fluidizing the powdered enamel in a fluidization vessel by blowing air in 
the bottom of the vessel, extracting the fluidized powder from the vessel 
by entraining the powder by means of a stream of conveying air which 
carries the powder to an electrostatic application gun, wherein steam is 
also introduced in the fluidization vessel. 
By means of this process, it becomes possible, with the use of simple 
devices, to determine with a very high precision, the humidity content of 
the mixture of air and powder conveyed to the application guns. In view of 
the fact that the humidity of the mixture of air and powder in the 
fluidization vessel is mastered and that this mixture is determinant as 
concerns the conditions of application, as mentioned hereinbefore, it is 
possible to dispense with an air-conditioned cabin and other measures for 
regulating the surrounding atmosphere of the place where the powder is 
applied. Further, the introduction of steam in the fluidization vessel may 
be achieved by means of a servo or feedback control as a function of the 
real value of the humidity prevailing in this vessel.

FIG. 1 shows a fluidization vessel which is in the form of a tank whose 
lower part is downwardly convergent. Placed in this lower part is a grate 
2 below which there is placed a nozzle 3 connected to a fluidization air 
pipe 4. 
The vessel 1 is closed in its upper part and communicates with a powdered 
enamel supply device by way of a vertical pipe 5. 
Mounted on the cover is a device 6 for taking off fluidized powder so that 
the latter is conveyed to an application gun 7. It will be understood that 
the same fluidization vessel may supply powdered enamel to a plurality of 
guns if required. 
The taking-off device comprises a pipe 8a which descends vertically in the 
vessel 1. A Tee union 8 comprises a horizontal branch in which a stream of 
conveying air passes which is applied to the taking-off device 6 by way of 
a conveying air pipe 9. Just upstream of the Tee union 8, a supply pipe 10 
for dosage air is connected to the pipe 9. 
The various streams of air introduced respectively in the pipes 4, 9 and 10 
are obtained in a distribution box 11 which is connected to a source of 
air under pressure (not shown) provided with the usual purifying devices 
such as a drier and an oil-separator. 
According to the invention, the fluidization vessel 1 is associated with a 
steam boiler 12 which is connected, on one hand, by its inlet to a dosage 
pump 13 supplied with water by way of a pipe 14 and, on the other hand, by 
its outlet, to an outlet pipe 15 leading to the fluidization vessel 1 
through the cover of the latter. 
A humidity and temperature sensor 16 is disposed in the vessel and 
connected to a regulating device 17 whereby the signal detected by the 
sensor 16 and a set or reference humidity signal may be sent, on one hand, 
to the dosage pump 13 by way of a regulating device 18 and, on the other 
hand, to an air heater 19 by way of a regulating device 20. 
An embodiment of the construction of the steam boiler is shown in FIG. 2. 
It comprises a metal unit 21 defining a chamber 22 which produces steam 
and has a generally conical shape the apex of which communicates with the 
pipe 15. The pump 13 has its delivery side connected to a pipe 23 which 
opens onto the periphery of the chamber 22. The unit 21 is heated by means 
of an electric heating plate 24 which may remain permanently connected to 
the supply or be regulated in accordance with needs. The steam boiler 12 
is completed by a lower protecting cover 25. 
The unit just described operates in the following manner: 
The powdered enamel is introduced in the fluidization vessel by way of the 
pipe 5. The fluidization air introduced in the bottom of the vessel by way 
of the pipe 3 puts this powder in suspension while the conveying air 
travelling through the pipe 9 constantly takes off powder in suspension 
and conveys it to the guns 7. The fine dosage of the amount of powder 
conveyed per unit time is achieved by acting on the stream of the air in 
the pipe 10, in the known manner. The regulating device 17 permits the 
setting, for a given temperature, of a desired degree of humidity within 
the vessel 1. If this set value corresponds to the value detected by the 
sensor 16, the servo control is at rest. On the other hand, when a 
deviation occurs between the two values, the servo control can act either 
on the amount of steam produced by modifying the speed of the pump 13, or 
on the temperature of the fluidization air by modifying the supply of 
power to the heater 19. 
Thus it can be seen that the process according to the invention permits the 
precise adjustment by simple means of the relative humidity of the 
powdered enamel conveyed to the guns 7. It has been found in practice that 
the introduction of steam in the vessel 1 is sufficient to obtain very 
good results, even on hollow parts such as, for example, the muffles 
employed for the construction of household cooking ovens which have up to 
the present time always presented difficulties as concerns the enamelling 
of the interior walls. The process according to the invention gives 
excellent results with these hollow objects without the use of an 
air-conditioned cabin which was the usual practice in the prior art. 
The value of the relative humidity in the fluidization vessel 1 may be 
between 15 and 45% for a temperature of the air in this vessel of between 
20.degree. and 30.degree. C. Preferred values of the relative humidity are 
between 27 and 30%, the best results having been obtained for a value of 
25%. Within these ranges of relative humidity and temperature, it has been 
found that it was possible to introduce 3 to 12 grams of water in the form 
of steam in the powder in suspension in the vessel per cubic metre of air 
introduced. 
By way of example, enamel powders of the type manufactured by the firm 
Ferro and designated by the reference ME 471 or manufactured by the firm 
Bayer and designated by the reference AC 8799/MX 5016, gave excellent 
results with the process according to the invention.