Laser markers, for marking an article formed from ABS plastics material with, for example, a serial number and other manufacturing data, have been proposed. Such markers cause a laser beam to strike the article in a focusing zone and cause a thermal change in the plastics material, effectively burning the information into the material. A suitable laser for such a technique is a 1064nm wavelength Nd: YAG laser with a suitable galvo scanner providing mirror-based laser direction.

It is a disadvantage of this laser marking technique that it is inappropriate for use with other plastics materials, in particular POM plastics material, since the thermal discolouration which occurs with ABS does not occur with POM which simply melts and then solidifies without providing a noticeable mark.

It is an object of the invention to provide a laser marker . which alleviates the above described disadvantage of the prior art.

SUMMARY OF THE INVENTION According to the invention in a first aspect, there is provided a laser marker for marking a plastics material coloured with a pigment, the marker comprising a laser, means for directing the laser in a marking zone for receiving an object to be marked and wherein the laser is of a frequency to effect a photo- chemical colour change in the pigment, to provide the marking.

Preferably, the laser is a frequency tripled Nd: YAG laser of a wavelength of 355 nanometres. When used with POM plastics material having a pigment such as titanium oxide, the laser light photo-chemically changes the pigment to cause a marking discolouration. The invention is also suitable for use with ABS plastics and in contrast to the laser marking technique of the prior art, the ABS plastics material is not thermally changed, the marking being effected by means of photo-chemical change of the pigment contained therein.

According to the invention in a second aspect, there is provided a laser marker for marking a plastics material coloured with a pigment comprising a frequency tripled Nd: YAG laser and means for directing a laser in a marking zone for receiving an object to be marked.

. According to the invention in a third aspect, there is provided a laser marker for marking a plastics material coloured with a pigment comprising a laser having a wavelength of substantially 355nm and means for directing a laser in a marking zone for receiving an object to be marked.

The invention extends to a product so marked.

BRIEF DESCRIPTION OF THE DRAWING An embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which a view of a laser marker, being an embodiment of the invention, is shown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment of Figure 1 includes a laser 100 which is mounted on a support frame 110 which supports, also, a controller 120 and laser power supply 130. The laser head 100 is connected to a galvo scanner 140 for directing the laser in a marking zone 150. A rotary table 160 is provided in the marking zone, the table being divided into two by a rotating partition 170. On each half of the rotating table, a rotary head 180 is provided, the rotary head bearing a jig 190 on which an article 200 to be marked is mounted.

The frame 110 is covered to form, together with the rotating . partition 170, a safety cabinet which is sealed whenever the laser is in use. After marking one article 200, the rotary table is turned through 180 degrees to allow another article 200 on the other half of the table to be so marked, the previously marked article being replaced by a further article to be marked and so on.

The laser 100 is, specifically, a Nd: YAG frequency tripled laser having a wavelength of 351nm, with components of the galvo scanner 140 being appropriately chosen, in particular the directing mirrors which are required to have compatible mirror coatings as is known in the art.

In use, the laser 100, galvo scanner 140 and rotary head 180 are software controlled by controller 120, in the manner known in the art. Specifically, the laser is directed by the galvo scanner 140 to provide a chosen marking, for example manufacturing information and/or a serial number, on the article 200. In order to obtain the maximum marking effect, the surface of the article 200 to be marked should be positioned at the focal point, or within a narrow range thereof, of the laser 100 within the marking zone 150, the position of the article 200 being controlled by the rotary head 180. In practice, the inventor has found that the satisfactory marking can be obtained provided the surface to be marked is within the range 3mm of the focal point.

. The surface of the article to be marked is formed from plastics material containing a pigment. The inventor has found that using a laser of such a high frequency causes marking by an effect different from that of the prior art, by discolouration of the pigment in the plastics material rather than by thermal change of the plastics itself.

The applicant has found that titanium oxide pigment is suitable for this purpose. The plastics material is preferably POM plastics but the invention works equally well with other plastics materials such as ABS.