Wrapping of bales, particularly round bales for silage have strongly increased during the last years particularly due to the flexibility of the system and the low labour requirement needed. The working method used most frequently today is used is such, that one man and one tractor first drive the round baler and produce the bales to be wrapped. After this an other man and an other tractor to which a bale wrapper is connected follow. This unit wraps the ready bales into plastic to disclose penetration of air into the bale and in this way make the process of lactic acid fermentation possible. To keep the oxygen of the outside air effectively away from the bale, it is most important that the film is evenly pre-stretched before it is tensioned around the bale, and that the film is applied as evenly as possible over the surface of the bale.

Until now all of the pre stretchers used on bale wrappers in general follow the principles shown in for ex. US 4,302,920 where the film is stretched between two rollers (1 & 2) which rotate at different speed. See Fig 1. The pre stretch is here determined by the peripheral speed difference between the first and the second roller. An almost similar stretcher is shown in the US 4,841,716.

See Fig 2. The difference is, that in this the film is stretched between the film roll 4 and the stretch roller 2, whereby the film pre stretch rate is determined by the ratio between peripheral speed of the stretch roller 2 and the film roller 4.

Other known solutions are shown in for example GB-A 968 594 and US-A 050 221, which both of them shows how the film can be stretched between two rollers which are rotating at different speeds.

By all of the above-mentioned types of pre stretchers, the film is stretched in one stage and the stretching zone is limited to the zone I strack (Fig 1 & 2) between the two rollers, respectively between the roller and the film roll.

To better be able to understand what is happening in the film pre stretcher it is important to know how the elongation of the film is depended of the force and the temperature. The diagram in Fig 3 shows in principal the characteristic for a typical agricultural stretch film. F = the stretch force of the film, A s = the elongation. The continuous curve shows the properties of the film by

normal temperature, the dashed curve shows the properties of the film at increased temperature tz. We can from the curve t, see, that up to about 70% stretch, the stretch force will steeply increase. After this the curve is evening up, we arrive at the so-called yield point of the film.

Here the stretch of the film will increase up to about 160% with only small increase of the film force. After 160% stretch the film force will again steeply increase, this is the so called secondary stretch, where the film already has lost its"plastic memory"it is the capability to by time try to recover to its original length.

When the film is stretched, heat is all ways generated due to the deformation of the plastic material. This will cause an increase of the temperature in the film. Particularly the yield point of the polythene based plastic films are deeply depended of the temperature.

We follow a length unit of the film an its path trough the pre stretcher. As it loosen from the roller 1 Fig 4 and enter the stretch area Isirack will be elongated due to the difference in peripheral speed between the roller 1 and roller 2. We can imagine that it will reach the point 1 in the stretch diagram of Fig 3. At the same time the temperature of the film 1 o will increase, and the yield point will decrease, it means that the film will partially soften. We can imagine the increase of the temperature such that the film will arrive at the point 2 on the stretch curve t2.

This will lead to a decrease in the film force and the film tension in the zone Istack between the rollers will decrease. As the film is pulled forwards and the difference in covered way by the film on the roller 1 and the roller 2 increase the film I. will elongate more and we reach l o + A 1 at point 3. Here the pre stretch is so high that we will enter the zone of secondary stretch. Now the stretch force will again increase heavily and a new state of balance will occur when the stretch force for the already elongated part for the film at the point 3 is equally high as the stretch force for the not yet heated film loosening from the roller 1. It means that the stretch forces in point 3 and point 1 are about equal. At this time a new length unit 102 of the film will elongate, its temperature will increase, the film force will decrease, the film 102 will elongate more until we reach 102 + A I and the point 3 in the tension diagram etc.. The scheme will be repeated as above described.

In the praxis this will be noticed so that the stretched film will have more or less regularly repeated vertical stripes, because the pre stretch is not distributed evenly pro length unit if the stretch is measured on mm level. See among other the wrapper leaflets: Tanco Autowrap Eckig Ballenwickelgerate, and Parmiter TR 76 Bale Wrapper, where this phenomenon is clearly seen.

It is clear, that a film stretched in such an uneven way do not have the optimal possibility to recover and neither to optimal air tightness.

In the tests carried out in different agricultural institutes this phenomenon has not until now been taken in account. The stretch has always been measured on longer distances and so an average

stretch has been received. See among other the tests carried out during the summer 1996 at Ulvange forskningssenter in Norway. There the pre stretch was measured by means of an measuring wheel, which registered the length of film used to wrap a standard bale, it means the measuring distance was approximately 80 meter. This is a measuring of the stretch at macro level.

An other much used way of measuring the stretch is to measure a 10 cm distance on the surface of the film roll before the stretcher, and after to control the distance between these points as the film is already pre stretched on the bale.

To judge the evenness of pre stretch in the film and in this way its capability of keeping an even stretch in best possible way keeping the tightness these methods are too un-exact, but they clearly show us the level of today and the demands on the film pre stretchers of agricultural bale wrappers, set of the test institutes today.

To reach optimal air tightness for a wrapped bale and so optimal fodder quality a pre stretcher has been developed which is specified by means of the protecting claims below.

The most important future of the invention is that the film can get a more even pre stretch on the micro level. The film will then also better keep its width and the plastic molecules in the film will be more unidirected, as the pre stretch will occur at lower temperature.

Below an advantageous sample of the invention with reference to the following drawings: Fig 5 shows in principle wrapping of a bale seen from above.

Fig 6 shows the pre stretcher of the wrapper seen from above.

Fig 5 shows from above the principle at the wrapping of a bale 5 with a film from a roll 4.

Between the bale 5 and the film roll 6 is applied a pre stretcher 7, which stretches the film to obtain better tightness for the wraps around the bale 5.

Fig 6 shows from above the pre stretcher 7 of the wrapping unit formed by an support for the film roll 4, stretch rollers 1,2 & 3 for the film, a transmission unit 8, in the figure formed by the gears 9,10 & 11, which determinates the internal rotating speed differences between the rollers 1,2 & 3.

The film pre stretcher according the invention will work as follow: As the length unit 1 of the film loosen from the roller 1 it arrive in the pre stretch zone l stiack I where it will be stretched to about half its optimal pre stretch for the film quality in question. As the peripheral speed difference between the roller 1 & 2, it means the degree of pre stretch at this first stage is low the

film will all time stay on the linear part of the pre stretch curve where the recovering capability of the film is at its best. As the pre stretch in this first stage is low, only a little amount of heat will be created and the characteristic film pre stretch points 1,2 & 3 for that part of the film in the diagram of the Fig 7 will all time stay below the zone where the secondary stretch starts.

As the film is pulled further trough the stretcher the film part 1 a + A I will bee cooled down especially as it will round the roller 2.

As the fill loosens from the roller 2 and enter the second pre stretch zone 1 strack 2 it start a new pre stretch phase, where the now down cooled film part 1. + A I has reached an tension strength at least of the level of the adjacent part of the film. A new pre stretch phase will start where a new part of the film 1 02 start to pre stretch due to the peripheral speed difference between the rollers 2 & 3. As also this speed difference is relatively low, the pre stretch for the length unit 102 will also in this secondary phase now be kept on the linear part of the pre stretch curve. Therefore the recovering capability of the film will remain on an optimal level on the same time as the air tightness of the film will remain as high as possible.

An other benefit of the film pre stretcher according to the present invention is that the film width will remain bigger as for conventional pre stretchers. The width will remain better partly because the film stretch force will stay more even in a pre stretcher according the invention. Therefore the film will be kept better tensioned to the stretch rollers, where the friction between the roller and the film will prevent the film from decreasing in width. An other explanation why the film width will remain better is that the film temperature in the stretch phase will stay constant lower than in a pre stretcher of conventional type.

An additional benefit reached by use of a film pre stretcher according to the invention is that film molecules will be better unidirected due to the lower film temperature during the pre stretch.

This will result in bigger density and lower oxygen permeability for the film.

The invention is not limited to the above-described shape but a numerous variations of it are possible within the scope of the patent claims below.

So for example the film can flow directly from the roller 4 to the stretch roller 2 as in the shape of Fig 8. The film pre stretcher can also be equipped with one or more guide rollers for the film, where the guide rollers are not connected to the transmission unit 8. Such a sample is shown in the Fig 9.

Further one or more of the stretch rollers can be equipped with cooling devises to reduce the film temperature between the different stretch stages.