Method of laser marking a body of material having a thermal cond

Details Method of laser marking a body of material having a thermal cond Method of laser marking a body of material having a thermal cond

1996-07-01

1998-06-16

Walberg, Teresa J.

Electric heating

Metal heating

By arc

347225, 264482, B23K 2600

Patent

active

057674833

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a method of providing a body of material with a sub-surface mark that is invisible to the naked eye but which is capable of being rendered visible under polarized light.
Many products are packaged in containers of glass or plastics and there has been a desire for many years to provide a method of marking containers of this type so that once a mark has been applied, it cannot be removed. Clearly such a method of marking would have a wide range of applications, not least in combating parallel trading.
In the past, in order to produce an indelible mark, manufacturers have relied, almost exclusively, on surface marking. However, the problem with this type of mark is that it may be either destroyed by removing that part of the surface on which the mark is applied, or imitated by the application of an identical mark on a substitute container.
In order to overcome these problems, the Applicant developed a method and apparatus for providing a body of material with a sub-surface mark which are described in International Patent Publication No. WO 92/03297. The method described comprises the steps of directing, at a surface of the body, a high energy density beam to which the material is transparent and bringing the beam to a focus at a location spaced from the surface and within the body so as to cause localised ionization of the material and the creation of a mark in the form of an area of increased opacity to electromagnetic radiation substantially without any detectable change at the surface. This provided the advantage that the resulting mark was both difficult to imitate and near impossible to remove.
In order to provide a method of marking having further advantages, it can be desirable that the resulting mark is invisible to the naked eye. In this way, a potential counterfeiter will not only have difficulty in removing or imitating the mark, but will also run into problems in locating the mark in the first place.
U.S. Pat. No. 3,657,085 describes a method of proving a sub-surface mark using an electron beam but also mentions the possibility of using a laser beam as an alternative. The object of the U.S. patent is to provide a method of marking an article, such as a spectacle lens, with an identification mark which is normally invisible but which can be rendered visible when required. To this end, the electron, or laser beam, is directed onto a mask placed over the spectacle lens so that that part of the beam passing through the cut-out portions of the mask, impinges upon the material of the spectacle lens. The beam is scattered by collisions with the molecules of the material that makes up the lens with the result that the kinetic energy of the beam is absorbed as heat producing permanent stress patterns within the lens. These stress patterns are invisible to the naked eye but may be rendered visible by double refraction in polarized light.
When referring to the possible use of a laser beam, U.S. Pat. No. 3,657,085 does so in conjunction with the marking of mass coloured materials, i.e. materials having a chromophore throughout their bulk and not simply ones provided with a coloured surface layer. It is this chromophore that absorbs the laser radiation and, in doing so, generates sufficient localised heating to produce permanent stress patterns within the material. Since the resulting mark is spaced from the surface of the material, the material must be at least partially transparent to the laser radiation used in order to allow the laser radiation to penetrate the material to the required depth.


SUMMARY OF THE INVENTION

In contrast, according to a first aspect of the present invention, there is provided a method of providing a body of material with a sub-surface mark comprising the steps of directing at a surface of the body a beam of laser radiation to which the material is substantially opaque, the beam energy absorbed at the surface of the material being sufficient to produce localised stresses within the body at a location spaced from said s

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patent: 4092518 (1978-05-01), Merard
patent: 4744647 (1988-05-01), Meshel et al.
patent: 4822973 (1989-04-01), Fahner et al.
patent: 4990792 (1991-02-01), Frei
patent: 5206496 (1993-04-01), Clement et al.
patent: 5575936 (1996-11-01), Goldfarb

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