Typewriting machines – Typing by other than type-face or type-die – Thermal
Reexamination Certificate
1999-03-24
2001-07-24
Hilten, John S. (Department: 2854)
Typewriting machines
Typing by other than type-face or type-die
Thermal
C400S120090, C400S120110, C347S188000, C347S191000
Reexamination Certificate
active
06264382
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates to a method of printing and more particularly to a method of printing which utilises a print head comprising a plurality of printing elements arranged generally in a row each of which may selected, and energised and de-energised, under the control of a control means to transfer a pixel of marking medium from a carrier onto a substrate or to activate pixels of a sensitive substrate, and which, during a printing operation, the print head and the substrate are relatively moved in a direction generally transverse to the row of printing elements.
DESCRIPTION OF THE PRIOR ART
Conventionally, the relative movement between the print head and the substrate is stepped, and the selected printing elements are energised and de-energised for each step of relative movement. Thus an image is printed in each printing operation having a resolution of R times C where R is the number of printing elements in a row per unit length and C the number of columns of pixels per unit length which conventionally is the same as the number of steps of relative movement. R and C are usually expressed in dots per inch (dpi).
The maximum printing speed i.e. the speed at which the printing operation may be carried out, is restricted by the ability of the print head to undergo thermal cycles each involving energisation (heating), for a sufficient time for a pixel of marking medium to be melted or at least softened or a pixel of a sensitive substrate to be activated, and de-energisation (cooling). High speed thermal print heads are able to undergo a thermal cycle down to about {fraction (1/6000)} second so that a 300 dpi image can be printed at a speed of about 500 mm length per second.
Such thermal cycle time can be reduced by improved print head design, or improvements in the transfer medium characteristics, but there is a finite thermal cycle time.
Thermal print heaters could alternatively be driven by a synchronous motor which moves the print head continuously, not in stepped fashion. Again though, the printing speed is conventionally limited by the ability of the thermal print head to undergo thermal cycles, in order to produce an image to a particular resolution.
Another problem with conventional thermal printers occurs where such printers are used, for example in a production line environment where substrates such as articles which are to be printed upon, pass through a printing station whilst printing is effected by a stationary print head, or where the articles arriving at the printing station are held stationary whilst printing is performed by a moving print head. In the former case, constancy of speed of the articles cannot be guaranteed such that one article may pass through the printing station at one speed and a second article at a different speed. In the latter case, less time may be available for one article to dwell at the printing station than for a different article.
Thus in some instances insufficient time may be available for a print head to perform the necessary number of thermal cycles to print the image to a desired resolution, whilst in other instances too much time may be available for the print head, which is designed for high speed operation, to undergo the necessary number of thermal cycles without affecting the performance of the print head. For example if too much time is available for printing one image the thermal cycle time for future printing operations may be affected.
SUMMARY OF THE INVENTION
According to one aspect of the invention we provide a method of printing using a thermal print head comprising a plurality of printing elements arranged generally in an array each of which may selectively be energised and de-energised under the control of a control means to transfer pixels of marking medium from a carrier onto a substrate or to activate pixels on a sensitive substrate, the method includes providing to the control means data dependent on the nature of the image to be printed which would enable the image having an image length L to be printed as a matrix of pixels at a linear resolution of C where C is the number of image rows per unit length in the direction of printing, wherein the method comprises causing relative movement between the print head and the substrate such that the print head relatively traverses the substrate the image length L in an available time T whilst the print head performs E thermal cycles to print the image, and the control means manipulating the data so that the image is printed with the print head omitting or repeat printing at least some of the columns of pixels so that the image has R rows per unit length and Z columns per unit length.
Thus the thermal print head may be operated to print the image faster than would otherwise be possible albeit at the expense of resolution, when this is necessary because the time available to print the image is insufficient to enable the print head to perform the necessary number of thermal cycles to print the image to a full resolution. Where the time available for printing the image is long, the thermal print head may be operated to perform a thermal cycle more often than is necessary to print a full resolution image, to avoid the problem of affecting the performance of the print head during subsequent printing operations.
If the print head is thermally cycled twice in one column position, the image produced may have a linear resolution of C but usually the number of columns of pixels printed per unit length Z is not equal to the number of columns C per unit length contained in the data provided to the control means.
In one arrangement relative movement between the print head and the substrate is performed in stepped manner, e.g. by a stepper motor or the like, the control means relatively stepping the print head A times to move the print head the image length L in the time available to print the image, where A is not equal to the number of thermal cycles E performed by the print head in the time available T.
For example where the number of steps of movement of the print head A is greater than the number of thermal cycles E performed by the print head in the available time T to print the image the control means manipulates the data to omit columns of pixels from the image so that the resultant image has a reduced image linear resolution of Z. However where the number of steps of movement of the print head A is less than the number of thermal cycles E performed by the print head in the available time T to print the image the control means manipulates the data to repeat print at least some of the columns of pixels so that the resultant image may have a linear resolution of C.
In another embodiment the relative movement between the print head and the substrate is constant during the available time T to print the image. This may achieved by moving the print head relative to the substrate by a synchronous motor, or by moving the substrate past the print head.
In each case where the available time T to print the image is greater than that required by the print head to perform sufficient thermal cycles to print an image to linear resolution C, the control means may manipulate the data to cause columns of pixels to be repeat printed along the image length L so that Z is greater than C. Alternatively where the available time T to print the image is less than that required for the print head to perform C thermal cycles per unit length, the control means may manipulate the data to omit at least some of the, columns of pixels so that a reduced resolution image is produced.
According to a second aspect of the invention we provide a printing apparatus comprising a thermal print head having a plurality of printing elements arranged in an array each of which may selectively be energised and de-energised under the control of a control means to transfer pixels of marking medium from a carrier onto a substrate or to activate pixels on a sensitive substrate, means to cause relative movement between the print head and substrate in a printing direction whilst the thermal print head
Buxton Keith
Hart Philip
Chau Mina
Fish & Richardson P.C.
Hilten John S.
Markem Technologies Limited
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