It is known to produce thermoformed articles, for example plastics inserts for locating individual items in containers, by feeding a plastics web into an oven to pre-heat the web, thermoforming the heated web in a thermoforming station where the heated web is formed into a desired shape in a cooled mold, and thereafter cutting the formed articles from the web using a heated cutter blade in a cutting station.

A particular problem with such apparatus is that the drive mechanism which feeds the plastics web through the various stations can fail to maintain accurate registration of the web in both the thermoforming station and the cutting station, in particular due to expansion and contraction of the web as a result of the temperature variations to which the web is subjected. The result is that the distance between thermoformed parts of the finished articles produced in the thermoforming station and the cut edge of the articles produced in the cutting station can vary. Such variations are not only unsightly but they can lead to feed problems when the articles are both stacked in a stacking station after they have been cut from the web in the cutting station and also when they are destacked when used to package objects.

In order to overcome such registration problems it has been proposed hitherto to use a floating cutter blade in the cutting station, the cutter blade being guided into its cutting position using detail of the thermoformed web itself. In order to achieve the necessary floating action, the cutter blade is mounted on a support which is suspended from a heated back plate which is in turn in thermal contact with a heated platten. The desired floating action is then achieved using blots passing through holes through the cutter blade support and screwing them into threads in the heated back plate, the holes in the cutter blade support being of a larger diameter than the shanks of the bolts.

Although such floating cutter blades can overcome the registration problems experienced using fixed cutter blades, they suffer from problems with variations in the temperature of the cutting edge of the blade during certain parts of the web cutting cycle leading to poor cutting. This occurs because although good heat flow occurs from the heated platten and the associated back plate into the bolts screwed into the latter, heat flow into the cutter blade support, and therefore into the cutter blade itself, is relatively poor when the cutter blade support is suspended from the bolts screwed into the cutter blade support. Higher heat flow into the cutter blade support from the cutter back plate, and therefore into the cutter blade itself, occurs during the cutting operation. However, since this represents only a fraction of the overall cutting cycle, adequate heating of the cutter blade to effect good cutting of the thermoformed web is difficult if not impossible to achieve, particularly during start-up of the thermoforming machine.

According to the present invention there is provided a floating cutter for use in producing thermoformed articles from a web of thermoplastics material, the cutter comprising a floating cutter blade which in use is aligned with and by thermoformed portions of the thermoformed web, heater means for heating the floating cutter blade, cutter support means for supporting the cutter blade, and a retaining plate which retains the cutter support means between it and the heater means when the cutter is not in contact with the web and is thermally coupled to the heater means by at least one thermally conductive land, there being clearances between the retaining plate the cutter support and the heater means to allow the cutter blade to float, characterised in that the cross sectional area of the land is from 20 to 40 % of the area of contact between the retaining plate and the cutter support when the cutter support and the retaining plate are in contact with each other.

The invention further provides apparatus for producing thermoformed articles from a web of thermoplastics material, the apparatus comprising an oven for heating the web, a thermoforming station wherein the heated web is thermoformed, and a cutting station wherein the thermoformed web is cut to form articles from the thermoformed web, the cutting station coomprising a floating cutter in accordance with the present invention.

The use of floating cutters in accordance with the present invention enables particularly good heat transfer to be achieved between the heater means and the cutter blade when the cutter support is resting on the retaining plate.

The cross sectional area of the land is more preferably from 25 to 35, and especially about 30, percent of the area of contact between the retaining plate and the cutter support when the cutter support and the retaining plate are in contact with each other.

It is also preferred that the retaining plate contacts a high percentage of the surface of the cutter support when the cutter support and the retaining plate are in contact with each other, in order to increase heat flow from the retaining plate to the cutter support, and thence to the cutter blade. The area of contact between the retaining plate and the cutter support is preferably at least 35% and more preferably at least 45 percent of the area of the cutter support within the cutter blade.

It is generally preferred to have a single land between the retaining plate and the heater means, but two or more individual lands can be used. It is also preferred that the land or lands is/are integral with the retaining plate.

Although the land or lands can have a variety of sections taken parallel to the surface of the retaining plate, for example circular or polygonal, e. g. triangular, square, rectangular, etc, it is generally preferred that they have one or more arms which extend from a central region outwardly towards the cutter blade. Such arms preferably form part of a larger land, but they can themselves be individual lands.

An embodiment of apparatus according to the present invention will now be described with reference to the accompanying drawings in which:- Fig. 1 is a side view of a thermoforming machine including a floating cutter in accordance with the present invention; Fig. 2 is a vertical section on line A-A of Fig. 5; Fig. 3 corresponds to Fig. 2 with certain removed parts in position and prior to cutting a thermoformed article from a thermoformed plastics web; Fig. 4 corresponds to Fig. 3 with the floating cutter aligned by the thermoformed plastics web immediately prior to cutting the article from the web; Fig. 5 is an exploded isometric view of the floating cutter assembly of Figs. 2-4 seen from above; and Fig. 6 is an exploded isometric view of the floating cutter assembly of Figs. 2-4 seen from below.

The thermoforming machine shown in Fig. 1 consists of an oven 1, a thermoforming station 2, a cutting station 3, and a stacker station 4.

A flexible thermoplastics web 5 is fed from a reel 6 into the oven 1 by a spiked chain drive mechanism (not shown). The web 5 is heated in the oven 1 and then passed hot into the thermoforming station 2 where it is shaped using cooled tooling of the shape required to produce the desired shape of thermoformed articles in the web. The thermoforming can, alternatively, be effected using heated rather than cooled tooling.

The thermoformed web is then passed to a cutting station 3 where a heated cutter blade cuts the web 5 around the thermoformed regions of the web 5. In order to prevent the resulting articles from parting from the web 5 at this stage, the cutter blade is usually designed to leave bridges between the articles themselves and the rest of the web 5 from which they have been formed.

The web 5 with cut articles held into it by bridges is then passed to a stacking station 4 where the articles are pushed out of the web 5 and formed into stacks. The remainder of the web 5 with the thermoformed articles removed is then wound up on a take up reel 7.

Within the cutting station 3 is a floating cutter which will now be described in greater detail.

Figs. 2 and 3 show exploded isometric views of a floating cutter assembly 20 in the cutting station 3, the assembly consisting of a cutter blade 21, a cutter support 22, a retaining plate 23, and an alignment plug 32, the latter having been omitted from these Figs. for reasons of clarity.

In use, the cutter blade 21 is a tight fit on the periphery of the cutter support 22 so that the blade 21 remains attached to the support 22 and the blade 21 and the support 22 thereby function as a single component.

As can be seen more clearly from Fig. 2, extending upwardly from the upper surface of the retaining plate 23 is a substantially "X"shaped land 24 which substantially corresponds in size and shape to a substantially"X"shaped aperture 25 in the support 22. The dimensions of the land 24 and the aperture 25 are such that a clearance is provided therebetween so that the support 22 can move within the aperture 25.

As can be seen more clearly from Fig. 3, the support 22 has a peripheral lip 26 defining a recess 27 which in use receives the retaining plate 23, a clearance being provided between the periphery 28 of the retaining plate 23 and the lip 26 so that the support 22 can move in two mutually perpendicular directions within the aperture 25.

A cutter back plate 29 is secured by blots to a heated platten 30, this providing good thermal contact therebetween. The retaining plate 23 is in turn secured to the cutter back plate 29 by bolts (not shown) which pass through holes in the retaining plate 23 and the lands 24 into threads in the back plate 29, thereby ensuring good thermal contact between the lands 24 and the cutter back plate 29. Heat from the heated platten 33 therefore flows through the cutter back plate 29 and into the retaining plate 23 via the land 24.

The height of the land 24 above the retaining plate 23 is greater than the thickness of the cutter support 22, and as a result the support 22 is free to move to a certain extent in a vertical direction in addition to its being able to move in mutually perpendicular horizontal axes, thereby providing the cutter blade 21 with an ability to be moved in x-, y-and z- directions relative to the cutter back plate.

The amount of free or floating movement of the cutter blade 21 and its support 22 relative to the retaining plate 23 is determined by the clearance provided by the aperture 25 in the support 22, and the thickness of the support 22, relative to the size of the land 24.

In order to position the cutter blade 21 correctly relative to the thermoformed portions 30 of the web 31, the blade 21 is guided into position by the alignment plug 32 (Figs 5 and 6) which is in contact with the support 22, and it is secured within the cutter blade 21. The alignment plug 32 is made of a polymeric material, for example polytetrafluroethylene, to reduce heat flow into the thermoformed portion 30 of the web 31.

Fig. 5 shows the cutter support 22 and its associated cutter blade 21 hanging from and in thermal contact with the retaining plate 23, heat from the cutter support 22 passing to the cutter blade 21 through the cutter support 22.

When cutting is effected using the cutter assembly 20, the assembly 20 is lowered on to the thermoformed portion 30 of the web 5 as shown in Fig. 6. Freedom of movement of the cutter assembly 20 relative to the heated platten 33 and the cutter back plate 29, as described above, enables the cutter blade itself to be positioned so that the resulting cut edge of the articles produced is correctly positioned relative to the thermoformed portions 30 of the web 5.

During the cutting operation the cutter support 23 is pressed upwardly against the cutter back plate 29, heat passing from the back plate 29 directly into the cutter support 22 and then to the cutter blade 21 to enable good cutting to be effected.

When the cutter assembly 20 is parted from the thermoformed portion 30 of the web 5, direct thermal contact of the cutter support 22 with cutter back plate 29 is lost because the cutter assembly falls under its own weight until the cutter support 22 contacts the retaining plate 23.

Unlike hitherto proposed floating cutters, the large contact area between the cutter support 22 and the retaining plate 23, and the large cross-sectional area of the land 24, results in the cutter blade 21 being maintained at a temperature which facilitates cutting of the thermoformed web 5 even when the cutter assembly 20 is lowered on to the thermoformed portions 30 of the web 5. More particularly, even if the temperature of the cutter blade 21 does fall when the cutter assembly is in its lowered position as shown in Fig. 6, the time taken to raise its temperature to that required to effect good cutting of the web 5 is short when compared with hitherto proposed floating cutters where the cutter support was merely held to a cutter back plate by bolts, the latter being positioned in over-sized holes in the back plate and with their heads in oversized recesses in the back plate to facilitate the desired floating movement.