The invention relates to a device for heat-sealing film material. Such a device is known from Dutch patent NL1020273 of applicant, where it is used for filling bags formed in film material with air and then sealing them tight. The thus obtained air bags can be used as filling material in packages, to protect products received therein. To that end, the known device comprises an air supply unit, a sealing unit and conveying means for carrying the film along these processing units. The conveying means comprise two circulating belts, which are each guided along a guiding block and are driven by suitable driven rollers in opposite direction of circulation, so that film material can be carried along therebetween. Here, at least one of the guiding blocks is provided with heating means in the form of a sealing thread extending under the circulating belt. With this, the guiding block also functions as a sealing block, in cooperation with the other guiding block, which can then function as a counter block or may itself be provided with a second heating element, so that the film material carried therealong can be heated two-sidedly. An advantage of this known device is that in it, the conveying means and the sealing unit both utilize the same part, i.e. the guiding block which also functions as sealing block and therefore has a double function, resulting in a relatively simple, compact construction. However, a drawback is that the conveying means, in particular the guiding block, is sensitive to wear and that, as a result of this double function, this wear directly affects the sealing quality. Wear occurs in particular at the running surfaces of the guiding blocks and the driven rollers along which the circulating belts are guided. Due to this wear, the grip on these belts is reduced, so that they can drift sideways and run off the guiding block, resulting in deformed or derailed sealing seams. Moreover, this sideways drifting leads to additional wear to the belts which, as a result thereof, may for instance start fraying which, in turn, leads to additional wear of the driven rollers. This worsens the grip on the belts even more and one is caught in a vicious circle. What is more, the circulating belts drifting sideways leads to irregular pulling forces on the film during sealing, which weakens the sealing seams. Also, due to such irregular pulling forces, the film may start to crease. As the sealing unit is not designed for such a doubling of material thickness, this also results in a weaker sealing seam which, furthermore, does not look attractive. The wear of the circulating belts furthermore results in a shortened lifespan thereof, so that he belts have to be replaced frequently, with all associated inconvenience and costs. The object of the invention is to provide a device of the type described hereinabove, wherein at least a part of the drawbacks of the known device is eliminated while maintaining the advantages thereof. In particular, the object of the invention is to provide a device wherein wear of the conveying provision and the negative effects to the feed-through of the film and the sealing quality ensuing therefrom are reduced. A further object of the invention is to provide a device with which the lifespan of the circulating belts is lengthened. The invention further contemplates providing a device with which formation of creases during transport of the film can be prevented. These and further objects are at least partly achieved with a device according to the invention, characterized by the features of claim 1. By subdividing the supporting block not only functionally but also physically in at least two parts, each part can be manufactured from a material most suited to the function of the respective part. Thus, for the different parts, a greater freedom of choice is obtained as to the choice in material to be used, and the choice of material can be geared to the different functions better, simply because the materials for each part have to meet fewer requirements. For instance, the part functioning as sealing block, with the heating means included therein, can be manufactured from a heat-resistant and heat insulating material. If the heating means comprise a resistance element, the material is preferably also electrically insulating. Suitable materials are, for instance, plastic, in particular Teflon. The part functioning as guiding block, in particular a circulating surface of the supporting block, or at least a part thereof, can be manufactured from, for instance, a material with a low frictional coefficient, in order to facilitate movement of the conveying means therealong. Additionally, the material is preferably wear-resistant and easy to process. Suitable materials are, for instance, metals, in particular aluminum, which preferably has undergone a hardening treatment. It is preferred that the parts functioning as guiding block are detachable so that they can be easily replaced without replacing the entire supporting block, for instance in case of wear (despite the use of wear-resistant material) or in order to adjust the material to specific conditions of use, such as the material of the conveying means, the conveying velocity, sealing temperature, etc. In a further advantageous embodiment, a device according to the invention is characterized by the features of claim 6. By designing the conveying means as circulating belts which extend on both sides of the film material and which carry the film material between them, direct contact between this film material and the heating means can be prevented and in this manner it can be avoided that the film material sticks to the heating means. To that end, the circulating belts can be manufactured from a suitable material which is heat-resistant and further, preferably, has good anti-stick properties such as, for instance, Teflon. In a particularly advantageous embodiment, a device according to the invention is characterized by the features of claim 7. By building the circulating belts up of at least two layers, the thickness of the belt and hence its lifespan can be considerably lengthened, since due to the greater thickness, the tensile stresses in the belt will decrease, at least if the tensile forces applied in use to the belt remain equal. Alternatively, the thickness of the belt can be increased by manufacturing it from one single, thicker layer of material. However, with the shape of the belt being closed upon itself, this leads to internal bending stresses. The fact is that with such a shape, the fibres located on the outside will have to stretch slightly, while, conversely, the fibres located on the inside will have to contract. This leads to internal stresses which threaten the lifespan. By building up the belt of several thin layers and by attaching these layers, in a condition closed upon themselves (bent) against each other, the differences in length mentioned will be "automatically", taken into account and the outer layer will "automatically" be somewhat longer than the inner layer. By having the ends of the respective layers attached onto themselves in an overlapping manner, to mutually stagger, it is provided that if these attachments were to form weak spots (which has not appeared in practice) these weak spots will, in any case, be distributed along the belt. Further, the overlapping ends are preferably thinned such, for instance by bevelling them locally, that together these ends, in overlapping condition, have the same thickness as the rest of the layer. Thus, local thickenings and any accelerated wear associated therewith are prevented. Preferably, the edges of the different layers include an angle deviating from 90° with a longitudinal central axis of the respective layer. Such edges can offer a better resistance against bending forces acting, during use, on the belt than edges extending at right angles to said central axis and will hence come loose less rapidly which is beneficial to the lifespan of the belt. It is noted that such circulating belts according to any one of claims 6 - 10 can also be advantageously used in a sealing device without the limiting features of the preceding claims. In a further advantageous elaboration, a device according to the invention is characterized by the features of claim 11. By providing the supporting blocks with a cover plate, the feed- through of the film material along the sealing unit can be improved even further. For instance, the cover plates can prevent the circulating belts from drifting sideways. Further, a slit formed between the facing edges of the cover plates can guide the film material even better when it is filled with air and sealed tight. As a result, the formation of, for instance, creases and high lateral forces on the seal to be formed, which can both be caused by a filling with air which is (too) high, can be suppressed. In a particularly advantageous embodiment, it is preferred that the cover plates are provided with a lengthened leg. With it, the above-mentioned slit between the cover plates is, as it were, lengthened in upstream direction, so that the film can already be guided earlier, with all associated, above- mentioned advantages. It is preferred that to that end, the legs extend in the direction of the air inlet unit, more preferably in the direction of a knife thereof, so that the slit can make its favorable action felt from the moment the film is cut open by this knife and then, cut-open longitudinal edges are formed. It is noted that such cover plates with lengthened legs can also be advantageously used in a sealing device without the limiting features of the preceding claims. According to a further aspect of the invention, the device is characterized by the features of claim 13. Owing to such an air outflow opening running to a point, this can be placed close to the supporting blocks so that air (or different injecting medium) can be blown into the film bags until just before sealing them, resulting in a good filling. The invention further relates to a method for manufacturing a circulating belt according to the features of claim 16, and to a circulating belt obtained by means of this method, according to the features of claim 17, which circulating belt has a great wear resistance and long lifespan and can therefore be advantageously used in a device according to the invention, but can naturally be utilized with the same advantage in other devices in which such circulating belts are used. In the further subclaims, further advantageous embodiments are described of a device and method according to the invention. In clarification, the device and methods according to the invention will be further elucidated with reference to the drawing. In the drawing: Fig. 1 shows in front view a sealing device according to the invention; Fig. 2 shows, in top plan view, preconfLgured film material, suitable to be filled with air and sealed tight, with a device according to Fig. 1; Fig. 3 shows a cross-section along the line III-III of the set up shown in Fig. 2 provided with a bag filled with air; Figs. 4A-C show in further detail a guiding block according to the invention, provided with two parts manufactured from different materials, in front, side and top view, respectively; Fig. 5 shows, in further detail, a driven roller according to the invention; Fig. 6 shows in perspective view a circulating belt according to the invention, formed from two layers; and Figs. 7 A, B show in front view an anti-crease provision according to the invention, in not-mounted and mounted condition. In this description, identical or corresponding parts have identical or corresponding reference numerals. The device shown and the operation thereof will merely be explained in as far as required for a proper understanding of the invention. For a more detailed explanation, reference is made to the earlier mentioned patent NL 1020273 which belongs to applicant, and which patent is understood to be included herein by reference. Fig. 1 shows, in front view, a sealing device 1 according to the invention, suitable for sealing film layers onto each other, for instance a film tube preconfigured with cross-seals as will be further explained on the basis of Fig. 2. The device 1 comprises a housing 2, provided with a shaft 3 for receiving a roll of preconfigured film material 4 (represented in interrupted lines in Fig. 1). The device 1 further comprises an air supply unit 5, a sealing unit 6 and conveying means 7, designed for carrying the film material 4 from the roll along these processing units 5, 6. The housing 1 is further provided with a guiding provision 8 for guiding the film material 4 according to a desired path from the roll to the conveying means 7 and to meanwhile tighten it such that formation of creases is prevented or, at least limited. The device 1 further comprises a guiding plate 9, adjustably suspended in the housing 2, along which film material 4' filled with air can be carried out of the device 1. In the example shown, the sealing unit 6 comprises two substantially rectangular supporting blocks 10, which are each provided with a rounded short side 11. The supporting blocks 10, which are shown in further detail in Figs. 4A-C, are adjustably suspended in the housing 2 by means of first biasing means 13, lying side by side by a long side 12 at a short distance from each other, while the rounded sides 11 face the provided film material 4. Both supporting blocks 10 are provided with heating means 15, in the form of a sealing thread. This extends along a part of the supporting block circumference, in particular along the above-mentioned long side 12 and the rounded short side 11, in a groove 14 provided to that end as is more clearly visible in Figs. 4A-C. Here, second biasing means 13' are provided for tightening the sealing thread 15. Alternatively, only one of the supporting blocks may be provided with such heating means 15. In that case, the other supporting block can function as stop or counter block with which a particular counter pressure can be given when a seal is formed. The rounded side 11 of the supporting blocks 10 serves for guiding, during use, a circulating belt 20 therealong and experiences considerable friction as a result thereof. That is why this side 11 is covered on both sides of the groove 14 with wear-resistant material 17, for instance aluminum, represented in hatching in Figs. 4A-G. The remaining part 18 of the supporting block 10, which supports the heating means 15 and hence serves mainly as a sealing block, is manufactured from a heat-resistant and preferably heat insulating material, for instance Teflon. It is further preferred that the longitudinal side 12 of the supporting block 10, at the front side in Fig. 1, is provided with a bevelled surface 19, as shown in Figs. 4A-C, for reasons to be clarified hereinafter. The conveying means 7 comprise two circulating belts 20 closed upon themselves which are guided around the supporting blocks 10 and the heating means 15, and around two driven rollers 16, arranged behind the supporting blocks in the film feed-through direction. These driven rollers 16, one of which is shown in further detail in Figs. 5A, B, may be manufactured from plastic, preferably a temperature-proof plastic, for instance Silicone. The driven rollers 16 are preferably provided, at the front side in Fig. 1, with a bevel 21 comparable to the bevel 19 of the supporting block 10. In use, the driven rollers 16 are driven via drive means (not further shown) and, optionally, transmission means such that the circulating belts are driven in opposite circulating direction, the upper circulating belt in Fig. 1 circulating clock-wise and the lower circulating belt circulating counter-clockwise. The air supply unit 5 comprises an air outflow opening 22 (see Fig. 1) which is connected to pumping means 31 (see Fig. 2) with which compressed air can be blown through the outflow opening 22. In film feed-through direction, the outflow opening 22 is situated just before the supporting blocks 10 and tapers slightly towards these supporting blocks 10 so that the opening 22 can approach the supporting blocks 10 closely. As a result, compressed air can be blown into the film bags up to the last moment, just before these are sealed tight between the supporting blocks 10. The air supply unit 5 further comprises a substantially rod-shaped guiding element 24 which is provided just before the outflow opening 22 with a knife 23 (see Fig. 2) the function of which will be further clarified. The operation of the apparatus 1 described above will now be explained with reference to Fig. 2. Here, in top plan view, film material 4 is shown, in particular tubular film material which, by means of cross seals 26 arranged at a regular mutual distance, is subdivided into bags 25 that, if desired, can be separated from each other by weakening lines 27 provided between successive cross seals 26. The cross seals 26 reach from a first longitudinal edge 28 of the tube film 4 to a point at a short distance from an opposite, second longitudinal edge 29, thereby leaving clear an opening 30. With this, the film can be slid over the guiding element 24 when the material 4 is fed through in the direction of arrow A. Here, longitudinal edge 29 is cut open by knife 23, whereupon, via the cut-open side, air is blown through the outflow opening 22 into the passing bag 25. This bag 25 will then puff up somewhat as is illustrated in Fig. 3. Due to the bevels 19 and 21 of the supporting block 10 and the guiding roller 16, respectively, the bag 25 can puff up more than would be the case without these bevels. Therefore, the bevels 19 and 21 contribute to achieving a higher degree of filling. Subsequently, the bag 25 filled with air passes the sealing unit 6 by which it is sealed tight. The orientation of the newly formed sealing seam 32 is then such that this seam and the cross seals 26 intersect so that a bag 25 closed on all sides is obtained, bounded by the longitudinal edge 28, the cross seals 26 and the newly formed sealing seam 32. If desired, when leaving the apparatus, the bags 25 can be separated from each other via guiding plate 9 by breaking the weakening lines 27, to then be used as filling material in packages. An advantage of such packaging material is that the preconfigured film webs 4 can be stored until used in relative compact form which takes up little space. Fig. 6 shows in perspective view a possible embodiment of a circulating belt 20 according to the invention, which is built up from several, in the case shown two, layers 40, 42. These layers 40, 42 are each provided with bevelled end faces 43 which include an angle β deviating from 90 degrees with their respective longitudinal central axes H. The layers 40, 42 are further provided with bevelled ends 4OA, B, 42A, B so that in the attached, overlapping condition, these ends together have a thickness which is substantially equal to the thickness d of the rest of the layer 40, 42 as shown in detail 6B for the outer layer 40. Upon realisation of the belt 20, the respectively layers 40, 42 are attached against each other and themselves only after they have been brought to the position closed upon themselves shown in Fig. 6. Thus, when attaching the layers, the different required lengths can be taken into account by regulating the degree of overlap. For instance, the outer layer 40 will have to be somewhat longer than the inner layer 42. Thus, an endless belt closed upon itself can be formed which is substantially free of internal bending stresses and which can still have a sufficiently great thickness, with associated strength and lifespan. In Figs. 7A and B, a further part according to the invention is shown, in particular a cover plate 33, which can optionally be arranged on a sealing device according to the invention, in order to improve the feed-through of the film material along the air supply and sealing unit. Fig. 7A shows the cover plate 33 in unmounted condition, Fig. 7B in mounted condition, while the plate 33 is attached against a supporting block 10, thereby covering this supporting block 10 and the adjacent driven roller 16. This offers a number of advantages. For instance, the cover plate 33 can help prevent the circulating belt 20 from running off the supporting block 10 and/or the driven roller 16. Additionally, the cover plate 33 can help prevent, when the bags are being filled with air, that a great (too great) sideways force (transverse to the feed- through direction, as indicated by arrow B in Fig. 3) is applied to the seal to be formed for instance as a result of too great a filling, causing the seal to weaken. It will be clear that in this case, the above-mentioned bevels 19, 21 of the supporting block and the driven roller, respectively, can be omitted. It is preferred that, at a side 36 upstream in feed-through direction A, each of the cover plates 33 is further provided with a sort of lengthened leg 34, which extend at a short distance from each other, thereby forming a guiding slit 35 with which the film can be guided even better and crease formation can be prevented even better. Preferably, the lengthened legs 34 reach adjacent the knife 23 or the rod-shaped guiding element 24, so that the slit 35 can actually assume the guiding function of the guiding element 24, which is particularly advantageous at the moment the film material 4 has passed the knife 23 and, therefore, is cut open. As a result, the cut open film edges can be effectively kept from, still, starting to undulate or crease. The above-described features according to the invention all contribute to the reduction of wear to parts, cooperating in the transport of the film material, in particular the supporting blocks, the circulating belts and driven rollers. As a result, these parts not only have a longer lifespan, but also, a better feed-through of the film material is obtained, i.e. with less crease formation and improved control of forces acting on the film material during sealing which, eventually, contributes to a sealing connection of better quality. The invention is not limited in any manner to the exemplary embodiments shown in the description and the drawing. All combinations of (parts of) described and/or shown embodiments are understood to fall within the inventive concept. Moreover, many variations thereon are possible within the framework of the invention as set forth in the following claims.