EP3790665B1

EP3790665B1 - Automatic airbrush coating apparatus and processing method using said apparatus

Info

Publication number: EP3790665B1
Application number: EP19727913.6A
Authority: EP
Inventors: Giuliano Dal Ceredo
Original assignee: Dal Ceredo Technology Srl
Current assignee: Dal Ceredo Technology Srl
Priority date: 2018-05-10
Filing date: 2019-05-09
Publication date: 2022-11-09
Anticipated expiration: 2039-05-09
Also published as: WO2019215653A1; CN112384305A; BR112020022814A2; EP3790665A1

Description

FIELD OF APPLICATION

The present invention relates to a coating apparatus applicable to coating booths mainly although not exclusively used in the sector of treating leathers or other materials using machines of the rotary turntable type or alternative, to which numerous automatic airbrushes are applied.
According to the invention it is envisaged that the actuated machine allows the introduction of a single flow of hot air with various airbrushes connected to the distribution system of the air called the working fluid.
Such heated working fluid is introduced through an insulated conduit which is located on the supporting part of the rotating turntable, which then continues towards the airbrushes through a tube that is also insulated, having dimensions adapted to the necessary flow of working air, ending directly at the airbrush, obtaining rather higher performance results with respect to traditional applications that use unheated working air, with important improvements in the application of the coatings on the objects to be treated.
The system according to the invention is applied in the sector of coating leathers but can also be advantageously used in the sector of wood or ceramic, through relevant similar machines in which numerous airbrushes are installed, the number of which will vary according to whether they are leather or wood or ceramic coating processes.

PRIOR ART

Automatic spray coating systems are known and have been used for some time, both of the electrostatic and traditional type, in which liquid coatings are dispensed by spraying towards objects of various kinds.
Automatic spray coating systems use compressed air as the working fluid for atomising the coating so that it can be deposited onto different types of surfaces uniformly, e.g. on leather, wooden objects, or made of ceramic or other materials.
Traditional pneumatic spray coating systems for leathers but, more generally, also other products, comprise a coating booth inside which a conveyor belt with a horizontal arrangement runs on which the leathers are placed so that they can be moved along an advancement direction and according to a predefined conveying speed.
Above said conveyor belt a rotating turntable is arranged, placed on a circular axis of rotation synchronised with the advancement of the leathers. A plurality of spray guns is applied to said turntable, which are placed at regular distances from each other along the peripheral area of the turntable itself and facing towards the conveyor belt, so as to be able to apply the coating onto the leathers or other parts that are advancing while the turntable rotates at a predefined rotation speed.
The turntable, while rotating, allows the vertically installed airbrushes to spray the coating onto the parts to be coated, dispensing the product in a fan-like way onto said parts.
The airbrushes are managed automatically and separately from each other by an electronic control, whereas the compressed air used during atomisation is taken from the dedicated circuit.
Compressed air represents the working fluid coming from a general compressed air line and passing through a variable pressure regulator, and conveyed air filtration.
The traditional type of filters that are used for filtering the compressed air use separating cartridges made of fibre, to be able to dehumidify the air through pressure absorption, and remove any oily particles and/or residues of hydrocarbons obtaining compressed, filtered and dehumidified air.
The flow rate of traditional compressed air systems for coating systems, can also be very variable and in the event of only one airbrush the consumption of working air can reach about 900 Nl/minute, i.e. Normal litre per minute (i.e. about 0,015 Nm³/s [Normal cubic metre per second]), considered an optimal consumption, but only by traditional standards.
Such flow rate is sufficient in the industry for a single airbrush, whereas for systems with airbrushes with a rotary turntable or alternative, this value must be multiplied by the number of airbrushes, therefore for example in the case of 12 airbrushes the total absorbed flow rate is 10800 Nl/minute (i.e. about 0,18 Nm³/s).
An important problem encountered during the use of coating systems with traditional type airbrushes is that related to the temperature of the compressed air which exits from the atomisers which, normally being taken directly from the dedicated feed line, is indicatively 2°/3° C.
The use of compressed air at low temperatures during the coating spraying steps creates a work condition that has been shown to be not optimal for the purposes of the final result as, being deposited on the object being processed, it tends to form a not perfectly uniform layer of coating, typically called the "orange peel effect" which compromises the quality of the product once the coating has been finished.
Therefore, to prevent this anomaly, the pressure of the working air is dramatically increased which leads to the dispersion of the coating used, and an equal loss thereof to that deposited on the part.
In this way the so-called "over spray" effect is also increased, also known as the transfer index (100% exits and 50% is deposited) with coating product waste levels that easily reach even 50%.
From tests performed, it has been found that increasing the temperature of the compressed air in spraying systems produces numerous beneficial effects on the quality of the finished product, but the use of heated air is currently limited to low flow rate systems which, however, have the problem of only being usable with a single spray gun, with the impossibility of using a single machine to feed the atomisation air for various airbrushes as in turntable or alternative type systems.
Therefore, in order to be able to use various airbrushes through heated compressed air through current technology, it would be necessary to associate a respective heating machine with each airbrush, making the construction of such a system impractical and the management thereof excessively expensive.
The documents EP2189544 and US4147923 are known in this sense, which envisage the use of partial air heating elements that are however of limited efficiency and do not allow the spraying nozzles to be heated according to the desired parameters. Method and apparatus for spraying articles are also stated in document GB791862 .
More in details, document EP2189544A1 , in accordance with its abstract, states an equipment for working flexible laminar surfaces, such as, notably, tannery leathers, comprising a spray booth, within which the working of the aforesaid leathers takes place, which are placed on at least one conveyor belt sliding below a series of devices, suitable to apply one or more cover products, according to predetermined modes, on a prefixed portions of the leathers; in particular, it is provided to perform the cleaning of the aforesaid devices used for applying the cover products and the relative plants in hidden time, i.e. in time intervals during which at least one of these devices is at rest.

DESCRIPTION OF THE INVENTION

The present invention sets out to provide a machine for a booth that uses various airbrushes connected by means of a dedicated hot air feed conduit to guarantee high flow rates, so as to be able to serve various spray guns with a single heating system, in order to eliminate or at least dramatically reduce the drawbacks highlighted above.
According to the invention, the air introduced through the feed circuit crosses a heating system that can increase the temperature thereof and improve the application of the coatings to parts, thus creating an optimal condition for the purpose of the quality of the result obtained.
Such working air flow heating system, conveying in the insulated conduit related to the atomisation of the airbrushes, crosses a heating device which, as well as comprising a filtering and dehumidifying system, is adapted to dispense hot air through the distribution conduit positioned above the rotary turntable.
The air treated through filtering and heating offers the ideal working conditions for the coating airbrushes, by placing the installed heating system in the condition to be able to also withstand the load of various airbrushes activated simultaneously.
The invention further sets out to lower the viscosity of the coating through the exchange that takes place when the product is split up with the heated atomisation working air, with a consequent reduction of the quantity of air used and therefore lower waste of dispensed coating and a significant increase in the transfer index, whose value can even reach 90%.
This is obtained through a coating system, whose characteristics are defined in claim 1.
The dependent claims of the present solution outline advantageous embodiments of the invention.
The main advantage of this solution relates to the fact that the coating system according to the invention makes it possible on one hand to perform coating operations reducing the waste of materials and means to a minimum, and on the other to reach rather higher qualitative results than those that can be obtained with traditional cold air systems.

ILLUSTRATION OF THE DRAWINGS

Further characteristics and advantages of the invention will become apparent from reading the following description of an embodiment of the invention provided by way of non-limiting example with the aid of the figures illustrated in the appended tables of drawings, in which:

figure 1 represents a schematic and perspective view highlighting the conformation of a coating system comprising the treatment means for the optimisation of the working fluid relative to the compressed air;
figure 2 illustrates a schematic view highlighting a plan view of a rotary turntable;
figure 3 represents an exploded schematic view that highlights a device for the treatment of air according to the invention;
figures 4, 4a and 5 illustrate front, plan and profile views, respectively, of the same device for treating air according to the invention;
figure 6 is a schematic sectional view of the air treatment device with the lines of the path of the air internally thereto.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

With reference to the appended figures, and initially in particular to figure 1, 10 generally indicates a coating system according to the invention as a whole.
The coating system 10 comprises a conveyor belt 11 adapted to move the articles undergoing coating along a substantially horizontal advancement direction, so that from the spraying area the coated articles can be conveyed towards a drying oven indicated overall with 12.
Furthermore, the system 10 comprises a rotary turntable 13 represented in a plan view in figure 2, which is located on a frame above the conveyor belt 11 and is rotatable according to an axis of rotation orthogonal to a plane of the conveyor 11 itself on which the leathers or other articles to be coated lie and are moved.
The rotary turntable 13 comprises a plurality of spray guns 14 for spraying the coating, said guns 14 being mounted along the outer circumference of the turntable 13, in the example illustrated in figure 2, each of them is positioned on the end of arms 15 extending radially with respect to the axis of rotation.
According to the embodiment represented in figure 1, the spray guns 14 are positioned on the end of conduits coming from a compressed air line 16, one of the hoses 17 thereof forms the conduit 18 that enters the turntable with the conduit 19 and branches off inside the turntable into the various channels for each gun.
According to the embodiment of figure 1 which represents the invention, upstream of the spraying booth with respect to the advancement direction along the conveyor belt 11, a device 20 is arranged which represents the main characteristic of the invention as it constitutes the element for heating and filtering the air introduced into the guns 14.
The device 20 comprises an external tube 21, an internal heating tube 22, which, in turn, internally comprises a group of electrical heating elements 23.
The device 20 is equipped, at its lower base, with a bottom cover 24, whereas at the top it is closed by a cap or head 25.
According to the embodiment illustrated in figures 3, 4 and 5, the cap 25 is provided with an air inlet hole 26, an air outlet hole 27, and a plurality of smaller holes 28 suitable for passage of cables and feed probes.
The device 20 comprises spaces afforded between the external tube 21 and the internal heating tube 22, the latter, in turn, being provided with windows 29 in the lower part thereof, for passage of the air entering from the inlet 26 and exiting from the outlet 27.
According to the embodiment represented in figure 6 the device 20 is equipped internally with electrical heating elements 23 located in proximity to the respective dissipaters that are licked by the air passing into the device and conveyed towards the guns 14.
The system thus described is commanded and controlled by a computerised control unit that controls all of the operating parameters that have been set and particularly the air temperature that has been set and is necessary for correct use of the system, by acting on the activation and deactivation of the heating elements.
The device 20 is newly conceived in this sector and is applied to the compressed air inlet that feeds the complete machine, and the total air consumption of such machine notably exceeds the flow rate of the machines currently on the market, as the indicative rated consumption for 12 guns is around 6500/7000 Nl/h (i.e. between about 0,0018 Nm³/s and about 0,0019 Nm³/s, where " Nm³ " is Normal cubic metre).
A further characteristic of the invention comes from the presence of insulated working air conduits. Such conduits start from the heating device 20, pass through the distribution manifold 30 shown in figures 1 and 2, to be connected with the distribution conduit 15, also insulated, which feeds each individual gun.
From the operating point of view, the air at the inlet enters the device 20 at room temperature.
As shown in figure 6, after travelling through a space 32 adherent to the internal wall of the external cylindrical body 21, the air flow reaches the bottom of the device 24, where it inverts its direction and enters the internal cylindrical body 22 where the heating elements 23 are positioned each of which, in the case in question, is packed between two finned dissipaters, which are licked by the air flow along the whole length thereof yielding heat to the air itself.
Based on the temperature measurement of the exiting air detected by the thermistor, the electronic control system compares this value with the value entered by the user and adjusts the activation and deactivation of the heating elements in a continuous modulation mode with an indicative absorption of around 16 A.
The elements are switched on and off by means of a feedback (or "tracking") control that constantly compares the value just read with the objective value, in an attempt to bring the magnitudes of interest towards such value, with a typically asymptotic trend or characterised by small oscillations.
The adjustment parameters are, in this case, all the possible ones: proportional, integral and derivative (PID type control).
The elements are not supplied or cut off according to the start-stop modes typical of common thermostats, as PID control allows continuous adjustment and with a small hysteresis range.
For this reason, the elements are supplied by a solid state static relay, controlled by the control board with PWM logic, which sends them high frequency voltage pulses with a variable duration of each pulse, according to the temperature difference measured between the outlet air and the set value.
The modulation takes place based on times of 2 msec with a variable work cycle from 0 to 100%, with minimum variations of 0.5%.
PID adjustment determines the activation percentage of the static relay (0-100%) according to the relationship: $%_{Staticrelay} = K_{p} (e + \frac{1}{T_{i}} \int edt + t_{d} \frac{de}{dt})$
as :

e = TSET - T ARIA is the difference between the set temperature and the temperature of the air to be controlled.
Kp = the proportional action constant, as the latter increases the action connected with an instantaneous difference between the measured temperature and the setpoint value increases.
td = derivative time, as the latter increases the action that tends to compensate for the variations increases, connected with the error signal derivative $\frac{de}{dt}$
ti = integral time, which is related to the accumulation of the error over time by the error integral and allows the error to be reset to standard.

The coating system according to the invention allows a sufficient flow rate to be obtained so that the booth and therefore also the guns for the air heated and filtered from all impurities, are treated so as to:

Remove oil particles;
Remove dust particles;
Removing humidity from the air itself allows the coating to be spread more evenly due to the reduced viscosity because of heating through the delivery of hot air;
The reduced viscosity allows a reduction of the air used for spraying "coating" products; the reduction in the air pressure in this case is 30% with respect to normal conditions;
The consequence derived from the reduction in the amount of air sprayed allows less wasted coating, with a transfer index that reaches about 90%;
The reduced drying times in turn lead to lower dilution of the sprayed coating, which is more dense, therefore offering a better final result in terms of quality and productivity.

It is to be noted that the traditional application system used with airbrushes with low pressure filtered air and without the use of hot air, does not exceed 55% of the transfer index, while with the new system according to the invention the transfer index is 80/90%, therefore the advantage of the system according to the invention is clear.
According to a further embodiment, it is envisaged that along said conduit 18 that enters the turntable 13 through the conduit 19 for branching off into the latter in the various channels of each spray gun 14 and downstream of said device 20, an auxiliary heating unit 33 is inserted.
Said auxiliary unit 33 comprises an insulated and protected tubular element that is introduced coaxially into the conduit in which it is inserted, which in turn comprises electrical heating elements 34 and a control probe for maintaining the temperature managed by the control unit of the system.
It is also envisaged that the control system through the control unit that manages the heating process can be interfaced with the different company management IT systems.
The invention has been described in the foregoing with reference to a preferential embodiment thereof. However it is clear that the invention is susceptible to numerous variants which fall within the scope of the appended claims.

Claims

A coating system (10) comprising:
a) a conveyor belt (11) adapted to move the articles undergoing coating along a substantially horizontal advancement direction towards a drying oven (12);

b) a rotary turntable (13) located on a frame inside a spraying booth above the conveyor belt (11) and that is rotatable according to an axis of rotation perpendicular to a plane surface of the conveyor (11);

c) said rotary turntable (13) comprising a plurality of spray guns (14) for spraying the coating, said guns (14) being mounted along the outer circumference of the turntable (13) and positioned on the end of conduits coming from a compressed air line (16) through at least one hose (17) that forms a conduit (18) that enters the turntable and branches off towards each gun for delivering compressed air;
wherein along said conduit (18) for channelling air and upstream of said turntable located in the spraying booth with respect to the direction of advancement of the conveyor belt (11), a device (20) equipped with means for heating, filtering and dehumidifying the working air that is introduced into the guns (14) is inserted, wherein said device (20) comprises an external cylindrical tube (21) and an internal cylindrical heating tube (22), which, in turn, internally comprises a group of electrical heating elements (23), between said external (21) and internal (22) tubes spaces being positioned for the passage of air entering said device (20) from an inlet (26) and exiting said device (20) from an outlet (27), said internal heating tube (22) being in turn provided, in the lower part, with windows (29) for the passage of the air entering from the inlet (26) and exiting from the outlet (27), and wherein compressed air is introduced through a mouthpiece (26) of said device (20), and after having travelled through a space (32) adherent to the inner wall of the external cylindrical tube (21), the flow of air reaches the bottom (24) of the device (20), where it reverses its direction and enters into the internal cylindrical heating tube (22) where the electrical heating elements (23) are positioned, to then be conveyed, heated, towards the outlet (27) and from the latter towards channelling (19, 30) and then towards the spray guns (14).
The coating system (10) according to claim 1, characterised in that working air conduits, which start from the device (20), pass through a distribution manifold (30) and then connect to a distribution conduit (15) which supplies each gun, are insulated.
The coating system (10) according to one of the preceding claims, characterised in that at its lower base, said device (20) is equipped with a bottom cover (24), whereas at the top it is closed by a cap or head (25).
The coating system (10) according to one of the preceding claims, characterised in that the cap (25) on said device (20) is provided with an air inlet hole (26), an air outlet hole (27), and a plurality of holes (28) suitable for passage of cables and feed probes.
The coating system (10) according to one of the preceding claims, characterised in that it is commanded and controlled by a computerised control unit that controls all of the operating parameters that have been set, and wherein the electronic control system, on the basis of measurement of a value of temperature of air exiting the device (20) detected by a thermistor, compares said measured value with a value of temperature entered by the user, the electronic control system adjusting activation and deactivation of the heating elements (23) in a continuous modulation mode.
The coating system (10) according to one of the preceding claims, characterised in that along said conduit (18) that enters the turntable (13) through a conduit (19) for branching off into the latter in the various channels of each spray gun (14) and downstream of said device (20), an auxiliary heating unit (33) is inserted.
The coating system (10) according to claim 6, characterised in that said auxiliary unit (33) comprises an insulated and protected tubular element that is introduced coaxially into the conduit in which it is inserted, which in turn comprises electrical heating elements (34) and a control probe for maintaining the temperature managed by the control unit of the system.