EP0365068B1 - Process and apparatus for stretching leather - Google Patents

Description

The invention relates to a method for stretching leather and similar flat materials, e.g. Skins, furs, but possibly also film cuts and the like.

From DE-OS 30 10 003 it is known to clamp leather pieces - usually called "skins" for short - in a gap between two elastically stretchable membranes pressed against each other by negative pressure and sealed along their circumference, and the stretching process by simultaneously stretching the two membranes to carry out frictional entrainment of the membrane clamped between them. In this way, it is possible to achieve a considerable increase in the size of the surface, which is distributed evenly over the surface of the leather and is free of wrinkles. For this purpose, however, a stretching device acting over the membrane circumference is required for both membranes. Because of the frictional connection between the two membranes and the leather, an essentially corresponding expansion of the membranes must be ensured.

• a) wenigstens ein Teil einer ersten Oberfläche eines Lederstückes auf einer bezüglich des Leders gleitfreundlichen Auflagefläche zur Anlage gebracht wird,
• b) das Lederstück mit über wenigstens einen Teil der Auflagefläche verteilt in Richtung gegen die Auflagefläche wirkenden Andruckkräften beaufschlagt wird;
• c) zur Streckung des Leders unter Gleitung relativ zu der Auflagefläche auf mindestens einem Teil der zweiten Oberfläche des Lederstückes wenigstens annähernd tangential bezüglich einer Lederoberfläche sowie wenigstens annähernd allseitig zum Rand des Lederstückes hin gerichtete Streckkräfte zur Einwirkung gebracht werden.

In contrast, the object of the invention is first of all to create a stretching process which enables equivalent stretching results to be achieved without the comparatively complex expansion of both leather surfaces. The inventive solution to this problem is determined by a method for inserting leather and similar flat materials, wherein

• a) at least part of a first surface of a piece of leather is brought into contact on a support surface which is easy on the leather to slide on,
• b) the leather piece is subjected to pressure forces which act over at least part of the support surface and act in the direction of the support surface;
• c) for stretching the leather under sliding relative to the bearing surface on at least a part of the second surface of the leather piece, at least approximately tangentially with respect to a leather surface and at least approximately all-round stretching forces acting towards the edge of the leather piece.

In-depth investigations have shown that excellent results can be achieved with regard to the size and uniformity of the surface enlargement by such one-sided surface application of the leather.

It generally depends on the simultaneous and essentially uniform gliding of the leather relative to the contact surface. The required ease of sliding of the contact surface can generally be ensured without difficulty taking into account the respective properties, in particular also the moisture content, of the leather by selection from commercially available materials. Surprisingly, it has been found that the pressure forces can also be generated here - in a manner known per se - by applying a vacuum to the leather with respect to the ambient atmosphere. This negative pressure is advantageously brought into effect in a gap space surrounding the leather.

Furthermore, the invention is concerned with the realization of further processing functions on the leather in connection with the stretch processing. This includes the combination of the stretching process with heating, which can also be used to improve the stretching results. In this context, the invention comprises as a further essential object the combination of the stretching process with drying. Surprisingly, this multiple treatment has proven to be fundamentally easy to carry out. In view of the great importance of setting certain moisture values in the course of leather production and leather processing, this results in an extremely significant progress in rationalization. In practice it has proven to be proved to be advantageous to apply suction vacuum to the leather on its second surface during the stretching process.

• a) eine Tragvorrichtung zur Aufnahme des ausgebreiteten Leders mit einer bezüglich der ersten Lederoberfläche gleitfähigen Auflagefläche vorgesehen ist;
• b) eine Streckvorrichtung mit wenigstens einem der Auflagefläche gegenüberliegend angeordneten, am Leder angreifenden Andruck- und Streckwerkzeug vorgesehen ist.

Another object of the invention is to provide a stretching device for leather and the like which is particularly suitable for carrying out the method according to the invention. The solution to this problem is determined by such a device in which

• a) a carrying device is provided for receiving the spread leather with a bearing surface slidable with respect to the first leather surface;
• b) a stretching device is provided with at least one pressing and stretching tool arranged opposite the support surface and engaging the leather.

Such a stretching device is distinguished from the known by advantageously simplified construction and reduced construction costs as well as increased robustness and operational safety. The slidability of the contact surface is advantageously achieved with the aid of a structure or a coating made of a plastic with a self-lubricating surface, e.g. Tetrafluoroethylene.

A continuation of the task of the invention is directed towards the creation of a combined stretching and heat treatment device for flat materials such as leather and the like. The solution to this problem according to the invention is determined in a stretching device of the aforementioned type in that a heating device which is operatively connected to the leather being processed is provided. This not only enables combined treatment tasks to be solved efficiently in one machine and saves on handling and work steps, but in some cases also improves the permanent stretching of the leather. Such a treatment device is preferably carried out in such a way that the support member forming the support surface consists at least partially of highly heat-conducting material, preferably metal, and is operatively connected to a conduction or convection heating device.

A particularly important continuation of the task of the invention also aims at a combination of different work functions, namely that of stretching with drying. The associated solution is determined by the fact that a gap space accommodating the leather piece is formed between the support surface and the stretching tool and is connected to a suction device. In general, it is expedient to equip this embodiment with the features of a heating device already mentioned.

• Fig.1 eine Gesamt-Seitenansicht einer erfindungsgemässen Leder-Streckmaschine,
• Fig.2 eine Prinzipdarstellung der erfindungsgemässen Streckfunktion,
• Fig.3 einen Teil-Vertikalschnitt in einem Randbereich eines Membran-Streckwerkzeuges mit Absaugvorrichtung sowie eines Tragorgans mit Heizvorrichtung, innerhalb einer Streck-Trocknungsmaschine in grösserem Massstab,
• Fig.4 einen Teil-Vertikalschnitt entsprechend Fig.3, jedoch mit abgewandelter Ausbildung des Streckwerkzeugs und des Tragorgans,
• Fig.5 wiederum einen Teil-Vertikalschnitt entsprechend Fig.3 für ein mit besonderer Arbeitsflächenstruktur versehenes Membran-Streckwerkzeug und
• Fig.6 eine Gesamt-Seitenansicht einer erfindungsgemässen Streck-Trocknungsmaschine mit abgewandelter Tragvorrichtung.

Further features of the invention are the subject of the remaining dependent claims. Advantages of the invention are explained with reference to the exemplary embodiments shown schematically in the drawings. Herein shows:

• 1 shows an overall side view of a leather stretching machine according to the invention,
• 2 shows a schematic diagram of the stretching function according to the invention,
• 3 shows a partial vertical section in an edge region of a membrane stretching tool with a suction device and a support member with a heating device, within a stretch-drying machine on a larger scale,
• 4 shows a partial vertical section corresponding to FIG. 3, but with a modified design of the stretching tool and the support member,
• 5 again shows a partial vertical section corresponding to FIG. 3 for a membrane stretching tool and provided with a special work surface structure
• 6 shows an overall side view of a stretch-drying machine according to the invention with a modified carrying device.

In the stretching machine according to FIG. 1, an overall plate-shaped stretching device STV, which is arranged along a horizontal plane, is mounted on the cantilever arm A of a stationary machine frame G in a vertically adjustable manner. A lifting and lowering device HS, indicated for the sake of simplicity as a piston-cylinder arrangement, allows the stretching device to be adjusted between the raised rest position shown in FIG. 1 and a lowered working position.

A piece of leather (hereinafter referred to as "skin") L is spread with its first (lower) surface L1 on the support surface AF of a carrying device TRV1, which is highly slidable relative to the leather. In the lowered working position of the stretching device STV, the underlying working surface AO of a pressure and stretching tool ASW comes into contact with the second (upper) surface L2 of the skin. As indicated in a simplified manner in FIG. 2, the latter is firstly applied with uniformly distributed pressure forces Pa acting in the direction against the bearing surface AF and then for stretching the skin under sliding relative to the bearing surface with essentially tangential to the leather surface and on all sides to the edge of the skin directed stretching forces Ps applied.

An essential component of the pressing and stretching tool ASW is an elastic stretchable membrane M1 covering the skin, which is maintained by means of a stretching drive device DAV of the pressure forces Pa is extended in the normal direction to their circumferential line and the tangential stretching forces Ps are generated by frictional engagement. The Dehn drive device DAV comprises a plurality of piston-cylinder units DZ arranged parallel to the membrane plane and distributed over the membrane circumference, the piston rods of which are attached to the edge of the membrane with rod heads SK. Such an arrangement is known per se from DE-OS 30 10 003 and therefore requires no further illustration and explanation here.

The pressure forces Pa are - as is also known per se from the aforementioned DE-OS - generated by means of negative pressure in a gap SP receiving the skin. This gap is formed between the contact surface AF on the one hand and the working surface AO of the membrane M1 on the other. To seal the gap space, a bead-like soft body seal DR running continuously along the circumference of the membrane M1 is provided, which rests sealingly on the contact surface AF. These relationships are shown in detail in Fig.3.

However, the elimination of an underlying stretching device with its own membrane and associated expansion drive is significantly different from the known arrangement. This advantageous simplification results from the use of a slidable contact surface AF in the area of the support device below.

The use of a highly slidable leather pad is advantageous.

In addition, this use of a rigid but not adversely affecting the stretching process on the leather enables the design of the machine as a combined stretching heat treatment device in a particularly simple manner. For this purpose, as shown in FIG. 3, a conduction heating device HV is arranged on the underside of the support member TO1, for example in the indicated form of a hollow plate filled with a circulating heating medium, which has a good heat-conducting connection with the support member TO1. The support element itself is also made of highly heat-conducting material. The appropriate choice of material is made considerably easier or even made possible by the rigid nature of the support member because, on the one hand, in the area of rigid solids - in contrast to the soft-flexible materials required for a membrane, suitable thermal conductivity values and heat transfer values are available - and on the other hand, there is no need to take expansion properties into account. Overall, an intensive thermal connection between the heating device and the leather can be achieved.

Instead of or in combination with the conduction heating indicated in the example, a convection heating can optionally also be used. For this purpose, a gaseous heating medium is circulated through the gap accommodating the leather. In principle, it is also possible to equip the carrying device or the carrying member with radiant heating which acts on the leather.

Another step in the invention leads - as also shown in FIG. 3 - to a stretch-drying device. For this purpose, the gap SP is connected to a suction device SV, which enables the evaporated leather moisture to be removed quickly. The connection between the suction device SV and the gap SP is established by a manifold SR with a plurality of tubular, flexible branch lines ZL and via associated connections AS and a channel system KS of the membrane M1. In the example according to FIG. 3, this channel system is designed as a relief structure RS which is open in the region of the working surface AO of the membrane and has a planar network of channel-shaped depressions KV — indicated here in cross section. This results in an evenly distributed suction and steam discharge.

In the embodiment according to FIG. 4, the support member TO) is provided with a surface layer OST which forms the contact surface AF and is made of material which is highly slidable with respect to the leather. This enables an optimal combination of thermal conductivity of the support member with slidability of the support surface, the thickness of the surface layer being kept very small. The surface layer preferably consists of a highly slidable plastic, in particular tetrafluoroethylene. In the interest of high strength and wear resistance, the surface layer can also be provided with a fiber reinforcement. This favors an essential further development of the invention, according to which the support element TO1 has a base body GKM, to which the surface layer OST is detachably connected. This enables simple wear replacement. In the embodiment according to FIG. 4, a pressure-sensitive adhesive layer is provided for this releasable connection of the surface layer OSM.

A permeable membrane M2 with connection to a suction device similar to FIG. 3 is also provided in the embodiment according to FIG. However, in the area of its working surface AO, the membrane here has a pore structure PS which is open and permeable to the leather surface and which is connected to a suction device via passages DL and connections AS. Furthermore, the pore structure is contained in an elastically stretchable membrane surface layer OSM, which is detachably connected to a membrane base body GKM, to be precise - as with the support organ surface layer OST - by means of an adhesive layer HK. Instead, a Velcro fastener arrangement KV may also be used, as shown in FIG. 5 - there for example without a permeable membrane structure and suction device - for the detachable fastening of a surface layer provided with a toothing-like structure Z is indicated in the area of the membrane working surface AO. Such a surface structure enables a form-fitting transfer of the stretching forces to the leather and thus a particularly intensive stretching effect.

6 shows a variant of the stretching or stretching / drying machine according to the invention with a carrying device TRV2, which has a band-like carrying element TO2, which is movably mounted with respect to the pressing and stretching tool ASW in the direction parallel to the bearing surface AF and is connected to a feed drive device VA . The carrying device and thus the bearing surface are enlarged here - indicated in the example on one side - beyond the floor plan of the stretching device STV. This allows the application of a skin in a freely accessible area next to the stretching device and while processing another skin, as well as the subsequent, rapid transport of the skin placed on the work area. This means a considerable advance in rationalization.

Claims (22)

1. Processes for stretching leather and similar flat materials, whereby:

   a) at least one part of the initial surface (L1) of a piece of leather (L) is placed on a bearing surface (AF) along which the leather can be slid easily;

   b) the piece of leather is loaded with the pressure forces (Pa), which are spread over at least one part of the bearing surface, working in the direction of the bearing surface;

   c) stretching forces (Ps) for stretching the leather under strain relative to the bearing surface, are put into effect over at least one part of the second surface of the piece of leather and are at least approximately tangential with reference to a leather surface as well as at least approximately polydimensional in the direction of the edge of the leather piece.
2. Process according to Claim 1, characterised by the fact that the pressure forces working on the leather (Pa) are produced by loading the leather with negative pressure with reference to the ambient atmosphere.
3. Process according to Claim 2, characterised by the fact that the pressing-down negative pressure is produced in a gap (SP) surrounding the leather.
4. Process according to one of the previous claims characterised by the fact that the leather is subject to drying during the stretching process.
5. Process according to Claim 4, characterised by the fact that the leather is subject to heating during the stretching process and is loaded on its second surface at least with suction negative pressure.
6. Facility for stretching leather and similar flat materials, in particular for the execution of the process according to one of the preceding claims, in which:

   a) a bearing appliance (TRV1, TRV2) to receive the spread leather (L) is provided with a slidable bearing surface (AF) in relation to the initial leather surface (L1);

   b) a stretching device (STV) is provided with at least one stretching tool (ASW) which grips the leather and is arranged opposite the bearing surface (AF).
7. Facility according to Claim 6, characterised by the fact that the stretching tool (ASW) exhibits an already familiar elastically extendable membrane (M1, M2) with a extending drive unit (DAV) working in combination with one of these membranes.
8. Facility according to Claim 7, characterised by the fact that the membrane (M1) exhibits a relief-type structure of its working surface (AO) which comes into contact with the second leather surface (L2), which exhibits cavities (KV) which are at least partly continously connected to each other and a suction device (SV).
9. Facility according to Claim 7, characterised by the fact that the membrane (M2) exhibits an open and poreous structure (PS), at least in the area of its working surface (AO) which comes into contact with the second leather surface (L2), which is connected to a suction device (SV).
10. Facility according to Claim 7 or 8, characterised by the fact that the membrane (M1, M2) exhibits a backing material (GKM) and a surface layer (OSM) linked to the former which is also elastically extendable and that the relief or pore structure (RS, PS) is formed in the surface layer (OSM).
11. Facility according to Claim 10, characterised by the fact that the surface layer (OSM) is connected in such a way to the membrane backing material (GKM) that it can be disconnected again.
12. Facility according to Claims 7 to 11, characterised by the fact that the surface layer (OSM) is connected to the membrane backing material (GKM) by means of a velcro fastener arrangement.
13. Facility according to Claims 7 to 12, characterised by the fact that the membrane (M2) exhibits a tooth-formed structure (Z) which positively grips leather during the stretching process on the working surface (AO) which comes into contact with the second leather surface (L2).
14. Facility according to Claims 6 to 13, characterised by the fact that a heating device (HV) is provided which works in combination with the leather being processed.
15. Facility according to Claim 14, characterised by the fact that the bearing element (TO1, TO2) which forms the bearing surface (AF) is at least partly made of of highly heat-conducting material, preferably metal, and works in connection with a conduction or convection heating device.
16. Facility according to Claims 6 to 13, in particular according to Claim 14 or 15, characterised by the fact that a gap (SP) which accommodates the piece of leather (L) is formed between the bearing surface (AF) and the stretching tool (ASW), which is connected to a suction device (SV).
17. Facility according to one of the Claims 14 to 16, characterised by the fact that the bearing element (TO1) is provided with a surface layer (OST) which with regard to the leather is made of highly slidable material and which forms the bearing surface (AF).
18. Facility according to Claim 17, characterised by the fact that the surface layer (OST) of the bearing element (TO1) exhibits a very slidable synthetic material, in particular tetrafluorethylene.
19. Facility according to Claim 18, characterised by the fact that the surface layer (OST) of the bearing element (TO1) exhibits felt-reinforced synthetic material.
20. Facility according to Claims 17 to 19 characterised by the fact that the bearing element (TO1) exhibits backing material (GKM) which can be disconnected from the surface layer (OST).
21. Facility according to Claim 11 to 20, characterised by the fact that a contact adhesive layer is provided for the removable connection of the surface layer (OSM, OST).
22. Facility according to Claims 6 to 21, characterised by the fact that the bearing appliance (TRV2) contains a band-type, with reference to the pressing and stretching tool (ASW), freely-supported bearing element (TO2) connected to a feeding drive unit (VA) in a direction parallel to the bearing surface (AF).

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