A METHOD OF PRINTING

The present invention relates to a method of printing and more especially to a method of printing on to thermoformed plastic packaging.

A well known method of producing packaging involves a thermo-forming process whereby three dimensional packaging is produced from a flat sheet/reel of plastic material. Examples of packaging formed using a thermo-plastic process are "blister" packs which can be laminated to a back card or other plastic component, "Clam" packs which have a formed hinge and fold into themselves to create a enclosed pack, "Punnets" which generally comprise a base and a lid component, or simply a component, which is then flow wrapped or sealed with a heat seal film, and "Pots" which are normally conical in design and consist of a base and lid component.

Once formed these packaging components typically have non-uniform surfaces. This makes it difficult to print an image on the packaging through known processes which involve a printing plate coming into contact with surface of the article being printed (often referred to as "pad printing" or "dry offset").

It is therefore desirable to be able to print an image prior to thermo- forming and for the image to be able to stretch with the surface of the plastic during the forming process. In this way it would be possible to print non-uniform surfaces (e.g coned or round packaging).

One difficultly with pre-printing prior to forming the packaging is that the forming process distorts the artwork printed, depending on the shape that it is formed into. Furthermore it is difficult to anticipate exactly how the forming will distort the artwork as the level of distortion changes depending on where the specific forming is in relation to the forming tool

(often tools comprise of multiple identical formings laid out in the same tool to maximise the web width of the forming machine and to maximise the index of the forming machine).

The invention seeks to provide a printing process that overcomes, or at least alleviates, the aforementioned disadvantages of printing on thermo- formed packaging by providing a method of modelling the movement of the surface of the plastic during the thermo-forming process and then using that model to create an artwork distortion mapping which reverse distorts the printed artwork so that, during forming of the packaging, the artwork is transformed back to its original design.

According to one aspect of the invention, there is provided a method of printing graphics onto thermo-formed plastic packaging comprising the steps of printing graphics prior to the thermo-forming process onto a flat surface, wherein the printed graphics includes at least one cavity; calculating the extent to which the flat surface of the material distorts during the thermo-forming process for the or each cavity in the forming; and calculating a reverse distortion such, that when a printed distorted image is thermo-formed, it returns to its non distorted image as a result of the thermo-forming process.

Preferably, the method utilises an ink that is resistant to heat and will stretch during the thermo-forming process.

Preferably still, the method includes the step of registering the printed material to the thermo-forming tool so that each index of the thermo- forming tool aligns with the next series of distorted images.

Preferably, the method utilises the printing technique of UV Flexographic printing, lithographic printing or gravure printing .

Preferably still, the method further comprises the step of conducting an initial test grid for each cavity, applying the test grid to the plastic thermo-forming material, thermo-forming the grid for each cavity and then calculating a reverse distortion for each cavity.

When a plastic packaging component is thermo-formed it will usually be manufactured by feeding a reel of plastic material into a thermo-forming machine, heating the material until it becomes malleable and then using a multi-cavity tool to form a number of identical packaging components in a single index of the machine.

These formed components are then fed further through the machine for punching/cutting into singles, and stacking.

The exact forming will vary very slightly from cavity to cavity depending on where the cavity is located in respect to the edges of the material (the centre, for example, will heat up more than the outside) and also with respect to the position within the forming tool (i.e the front leading edge of the die or back edge of the die).

When forming the reel of plastic raw material is stretched and deformed differently depending again on the position within the web width and the position in relation to the front or back of the leading edge of the forming tool.

The applicant has found that, although the plastic forms differently when comparing one cavity to another within the same forming tool, under repeatable machine set-up parameters, when comparing the forming of the same cavity from index to index, the plastic formation is uniform for certain chosen materials, for example, polyester and PVC.

Consequently, it is possible to create a model to illustrate the thermo- forming effects of each and every cavity within the tool. This model can then to be used to calculate the distortion for each cavity that would occur on an artwork printed on the surface of the plastic being formed.

An example of the process for the modelling of the distortion will now be described with reference to the accompanying Figures in which :

Figure 1 illustrates a typical tool layout;

Figure 2 illustrates a first step of creating a distortion grid;

Figure 3 illustrates a second step of the forming of a distortion grid;

Figure 4 illustrates a fourth step of mapping the distortion; and

Figure 5 is an illustration of the artwork after the distortion mapping has been applied.

Referring first to Figure 2, a grid 10 is printed on plain material which relates to the tool layout of the thermo-forming tool (as shown in Figure 1). The grid 10 comprises a number of cavities 12 is numbered within the grid 10 so that once formed, it is possible to work out which cavity 12 is which (each being unique once formed). An eye mark 14 is printed on the grid 10 to enable the thermo-forming machine to register its index repeat length to the repeat of the printed grid 10.

Referring now to Figure 3, the material with the printed grid 10 is formed through the thermo-forming machine (not shown) and the distortion of the flat sheet material is shown by the distortion of the uniform grid 10.

Referring now to Figure 4, for each cavity 12, the distorted grid 10b is scanned into a computer. If the surface of the material is not flat, then

pieces of the surface are cut out and pieced together on a flat piece of card to create a flat image version of the distortion.

Using a computer software package, and by laying the non distorted grid 10a over the top of the distorted grid 10b, each co-ordinate in the grid 10 is mapped from its non distorted position to its distorted position and hence a mapping is created which calculates the extent to which the grid 10 has moved during the thermo-forming process.

Again, using a computer software package, a new mapping is created which is the exact inverse of the distortion mapping, so that if a point A moves to a point B during the distortion process, then under the inverse mapping point B would move to point A.

The final step is to again use a software package to apply the reverse distortion mapping to an artwork 16 for each individual cavity 12 (see Figure 5). This distorted artwork will, under the thermo-forming process, distort back to create the original artwork image for each and every cavity 12.

This artwork 16 is the output of the process and when printed on reels of plastic material, can be thermo-formed to create the desired packaging components with printed graphics.

It is important that inks are used that are capable of stretching without cracking and that do not discolour or change properties during exposure to the high temperatures involved in the ^~ thermδ-foϊτning process.

The image may be surface or reverse printed on the clear plastic material. If reverse printed then the image is inverted so that when viewed through the material the image is correctly orientated.

It will be appreciated that the foregoing is merely exemplary of a printing method in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention.