The present invention relates to a method and a device for directly marking the surface of objects having a plane or curved form, by transfer of the ink which is present on a temperature-sensitive band, and by using a jet (current) of hot air, which pushes the temperature-sensitive band against the object.
In particular, the images transferred on the object may be a bar code, a logo (logotype), a text, etc.
The prior art uses different methods in order to transfer images on a sheath, a packing, a plastic object, etc.
There have been already produced kinds of printers which employ a pad in order to press the inked band or strap against the object, for transferring the images.
Other methods of direct transfer of images, i.e. those which do not make use of pressure sensitive (adhesive) labels, are the following:
"EP-A-511 605 discloses a machine for transferring images from an inked support on a cylindrical object, in particular a can. The object is preheated before the image transfer, so that this machine cannot be employed for the case of objects which are damaged by heat. Moreover, this machine uses an electrode to electrostatically charge the inked band, and the image is not instantly transferred since a slot type nozzle is used."
An object of the present invention is that of providing a method and a related device for carrying out such method, whereby the transfer occurs through a hot air jet (current) under controlled temperature and pressure conditions, said jet being distributed uniformly on the portion of the inked temperature sensitive band or strap which bears the positive or negative image, in order to push that portion of the band or strap against the flat or curved surface of the object.
The present invention will now be described for illustrative and non-limitative purposes by means of a preferred embodiment thereof, which is shown in the only figure annexed to this document, representing a front and partially sectional view of the device.
The device shown in Fig. 1 may be incorporated in a standard electronic thermal transfer printer.
The device in Fig. 1 comprises a temperature-sensitive printing band or strap associated to a respective pay-off roller or decoiler 2 and a take-up roller or coiler 3. The reference numbers 4a and 4b indicate band tightening rollers or briefly, tighteners.
The central part 5 of the device is the innovative part, which directs the hot air jet against the Central horizontal section 1' of the inked temperature sensitive band 1.
Said horizontal section 1' is located at a predetermined distance from the object 6, which, even if it has a flat surface in the figure, may have a curved form whatever.
In fact, the hot air which leaves the diffuser 7 of compressed air, pushes the band 1 against the object 6, conforming to the shape of the latter, as will be explained later on. The central part 5 comprises a compressed air inlet duct 8 (for instance connected to a compressor), by means of which cold air is introduced inside a chamber with variable volume 9. The cold air is fed into the variable volume chamber 9 at a predetermined pressure (preferably from about 0,5 to about 8 bar) adjusted by a flow control valve 10, which is set either manually or automatically, depending on the size of the image to be transferred.
The variable volume chamber 9 comprises an upper part 11a and a lower part 11b; the latter one may be vertically displaced in order to increase or decrease the extension of the inner space of the variable volume chamber 9. This operation is done automatically. The volume of the variable volume chamber 9 is adjusted as a function of the dimensions of the image to be transferred, and internally it includes an electric resistance 12 having the shape of a coil which heats the air inside the variable volume chamber.
Reference numeral 13 denotes a control circuit which closes the flow control valve 10, when a predetermined amount of air has entered the variable volume chamber 9, and the pressure of this air has reached the pressure value adjusted by the flow control valve 10.
At this moment, while the variable volume chamber 9 is hermetically sealed, the electric resistance 12 is heated up through the control circuit 13.
The temperature of the air inside the variable volume chamber 9 is adjusted and maintained constant by means of a thermostat (not shown), at a value preferably between 40 and 700°C, depending on the kind of band or strap being employed and suited for thermal transfer of images.
The air contained in the variable volume chamber 9 will be, as a whole, maintained at a constant pressure and temperature.
In consequence of the automatic control obtained through the control circuit 13, the solenoid valve 14 is operated, so as to let the air flow out of the chamber 9, introducing it in the diffuser 7, or air expansion device, or air sprayer.
The diffuser 7 forms an "equipotential plane", that is, the heat flow which passes through the unit of area on the plane located at the outlet 15 of the diffuser 7, is the same at every point on this plane. Furthermore, the pressure of the air which flows out of the diffuser 7, is the same at every point of said plane. The term "equipotential" means that both conditions are satisfied.
In this manner, the pressure exerted on the portion concerned of the temperature-sensitive band or strap and the heating produced, will be the same at every point, allowing an optimum transfer of the image on the object. In order to speed up the emptying operation of the chamber 9, the flow control valve 10 may be opened simultaneously to the solenoid valve 14, introducing (cold) air into the chamber 9, which pushes downwardly the hot air contained in the chamber 9.
As shown in the figure, the section 1' of the temperature-sensitive band 1 is located at a variable distance from the diffuser 7, depending on the chosen volume of the variable volume chamber 9. The hot air is "sprayed" by the diffuser 7 on the band or strap 1', and the latter thermically transfers the image on the surface of the object 6 conforming to the shape thereof.