WO2002074525A1

WO2002074525A1 - Continuos press, particularly for manufacturing products with joined parts as well as synthetic adhesive

Info

Publication number: WO2002074525A1
Application number: PCT/IB2001/001369
Authority: WO
Inventors: Giovanni Gino Cattelan
Original assignee: C.G. Di Cattelan Gianni & C. Snc
Priority date: 2001-03-21
Filing date: 2001-07-31
Publication date: 2002-09-26
Also published as: EP1372942A1

Abstract

A continuos press, particularly for attaching products (M) with coupled parts, comprises a supporting structure (2) laid on the ground, a pair of annular conveyor belts (3, 4) motorised mounted on said supporting structure (2), with external surfaces (3, 4) facing each other along at least a common length (9) so as to define a passage (10) for guiding products (M) previously treated with heat setting adhesive, upon advancement thereof from an inlet door (11) to an outlet door (12) along an advancement direction (A), compression means (13) mounted on said supporting structure (2), operatively acting on said conveyor belts (3, 4) to exert through them a pressure on the products (M) advancing along said direction (A) within said passage (10), heating means (14) associated with said passage (10) to dry products (M). The compression means (13) comprise at least one first plate (15) carrying on a first active face (16) thereof a plurality of first shaped teeth (17) sliding on the internal surface (3'') of at least one of said conveyor belts (3, 4) to compress the products (M). A synthetic adhesive for adhesion of wooden assemblies, particularly adapted for use with the above press comprises a dispersion of resins based of polyvinyl acetate in water with acid pH, a pre-condensed in powder of amino formaldehyde resins added to the dispersion, in which said amino formaldehyde resins are of the type urea-formaldehyde, melamine-formaldehyde or combinations thereof, wherein the pre-condensed is comprised between 2% and 10% in weight of the dispersion.

Description

CONTINUOUS PRESS, PARTICULARLY FOR MANUFACTURING PRODUCTS WITH JOINED PARTS AS WELL AS SYNTHETIC ADHESIVE

Field of Application

The present invention relates to a continuous press, particularly for manufacturing products with joined parts.

The press according to the invention can be advantageously employed in industrial processes e.g. for the continuous manufacturing of wood or cardboard panels, of wood chip boards, plastic laminates, plywood panels, rubber ribbons, cut panels of wood and other semi-finished articles.

In operation, a continuous press of the above mentioned type accomplishes the continuous manufacturing of the products by means of hot setting synthetic adhesives and through the combined pressure-heat action.

The invention further relates to a synthetic hot setting adhesive for the coupling of wooden manufactured articles with a continuous press of the above mentioned type.

Background Art

As is known, the known continuous presses are conventionally provided with at least two motorised annular conveyors wound on end rollers, which conveyors extend parallel facing each other between the pressing planes to advance the coupled parts to be assembled.

These presses are further provided with one or more sources of heat to dry the adhesive during or after the pressing action. Currently, in accordance with the known technique, there are different types of presses as a function of the products to be assembled which may be introduced in the press with continuous or discontinuous flow. These presses differ from each other either for the different heating means or for the mechanism used to compress the products between the conveyor belts.

The efficiency of the product heating influences mainly the efficiency of the press insofar as it changes the advancement speed of the conveyor belts.

On the other hand, the same mechanism for exerting the pressure on the products to be joined not only influences the quality of the finished products but also the operating speed of the press.

In fact, a known problem of such press machines, that is still to be fully solved, resides in the need of reducing the friction between the conveyor belts and the compressed surfaces in spite of the high pressures actually exerted upon them.

In order to overcome such drawback, many presses have been provided with a plurality of pressure rollers which are normally free to rotate on the compression surfaces which are advanced by the movement of the conveyor belts. Similar machines are for example disclosed in US-A-3,850,21 3, US-A-3,883,285, US-A-3,795,470, US-A-3,993,426, DEA-1 9828822, DE-A-1 98581 52, DE-A-1 98581 25.

These known machines have the disadvantage not to provide a sufficiently even distribution of the pressure both in transverse and longitudinal directions. This can be a consequence of the deformation and misalignment of the rollers or of the variable dimensions of the products to be pressed, that provide areas of the products undergoing an excessively high pressure in comparison with other areas where the pressure is excessively low.

In order to overcome such inconvenience, several types of press have rollers supported not only at their ends but also along their extension and provide the possibility to remove the pressure rollers from the outlet door to further introduce them at the inlet door. However, the mechanism employed to accomplish such operation is extremely expensive and has a low reliability and, in practice, does not overcome the problems of misalignment and uneven pressure distribution.

As previously said, in such machines generally the compression action is combined with a heating action adapted to dry the adhesive previously applied to the products. In the presses equipped with rollers the heat is transmitted to the products by the rollers themselves and with extremely low efficiency because the heat transmission substantially takes place through generating lines of the rollers.

In an attempt to uniformly distribute the compression force and to reduce the friction forces, different configurations have been conceived which provide for example a number of superposed rollers or pads (see IT-A-1 246728) and/or the arrangement of rollers in different positions to cover the whole area to be pressed as described in DE-A-2853285. However, also these embodiments are too complicate and expensive.

US-A-4,466,857 discloses an embodiment of press which provide a supporting plate acting upon a plurality of juxtaposed pressure rollers and having holes for circulating a heating fluid for heating the plates and the products to be treated. The same considerations laid down for the machines with pressure rollers also apply to this latter solution.

There are known machines in which a pressure plane supports a conveyor belt providing the advancement of the products to be pressed. Such known machines suffer numerous inconveniences. A too high friction force between the conveyor belt and pressure plane may cause not only a quick wear of the belt but also excessive local temperatures thereby determining expansions and other operation problems. In several presses of this type such problem has been overcome by using a layer of oil between the conveyor belt and the pressure plane so as to improve their mutual sliding.

However, such provision does not fully solve the problem but also cause leaks of oil and dirt the products to be pressed.

It is further observed that the use of a single pressure plane constituted by a unique continuous element necessarily involves an uneven pressure of the product areas located at different heights both in the longitudinal advancement direction and in the transverse direction of the conveyor belt. An exemplary press of this type is described in EP-A-1 03 396.

The different heights of the products in the same group or between different portions of the same products cannot be evidently compensated by adjusting the pressure by means of electric or hydraulic devices.

The choice of the pressure system influences greatly also the heating means employed in the same manner in presses with pressure rollers.

By way of example, the choice of an adjustment arrangement using hydraulic cylinders extending from the pressure plane on the products prevents the use of heating radiating means because they cause also overheating of the fluid medium contained in the hydraulic cylinders, thereby compromising their normal operation. Consequently, sometimes the heating by RF, high frequency or microwaves is accomplished outside the pressing area, thus providing an evident limitation of the functioning of the machine.

On the other side, also the solution proposed by WO-A-96/27482, that provides radiating means with a fixed electrode on which the products to be joined are forced and caused to move, is fully unsatisfactory for the same problems mentioned above, particularly due to high friction and wear.

Moreover, in such a solution the movable electrode is formed by single elements spaced from each other adapted to move with a product so as to compress it, by means of a complex and expensive operating mechanism. Such a solution further allows to treat exclusively non-continuos articles and to operate simultaneously only on a limited number of articles.

In general, the difficulties related to the choice of the compression mechanism and of the heating system, as well as the problems deriving from coupling thereof, generally prevent the known continuos presses to achieve a high production efficiency.

A satisfactory operation of the press mainly depends on the adhesive used.

Numerous adhesive mixtures for the mutual attachment of parts of assemblies manufactured in wooden material are known.

Generally, such mixtures are uniformly spread as a viscous fluid over one or both the surfaces to be joined and are further subjected to a compression and heating phase to evaporate the solvent and to cure the adhesive. The type and quantity of adhesive, the compression time, the gluing temperature depend on different factors such as the materials to be joined, the woody essence employed, the conditions of the surfaces, the moisture of the wood.

Several expressions shall be used hereinafter with the meanings indicated below.

Pressure Time: it is the time during which a pressure is exerted on the surfaces to be joined, expressed in seconds per millimetre of the adhesive film thickness up to the middle line of the attachment depth.

Pot Life: it is the maximum time interval, at the definite temperature and moisture conditions, that may pass between mixing the components of an adhesive and application of the adhesive mixture.

Open Time: it is the maximum time interval at a definite temperature and moisture conditions existing from the spreading thereof and the starting of the adhesive curing.

Application Weight: it is the weight mass of adhesive including the solvent per surface unit to be glued.

Pressure Temperature: it is the temperature at which the spread adhesive is maintained to effect the curing.

Wood Moisture: it is the percentual fraction w/w of water present in the wooden material. Specific Power: it is the power required to dry the adhesive by means of electromagnetic RF waves, expressed in W per gram of adhesive including the diluent.

A drawback of the known adhesives consists in that they need a temperature that is relatively high, namely ranging from 80° C to 1 30°C, and excessively prolonged in the time. Moreover, the joined assembly always requires a stabilisation after compression under the press.

Generally, the heating is accomplished by means of pressure surfaces heated with heating media such as steam, overheated vapour, diatherm oil.

It is known that adhesive containing polar components, such as water, may be heated much more efficiently by means of oscillating electromagnetic fields in the range of RF, namely with frequency higher than 4 MHz.

In such a way, energy is transferred directly and instantly inside the adhesive layers without dispersing in the outside environment or in the wooden material. Since such energy transfer is not influenced from variables such as dimensions and density of the support, the drying and curing phase take place in a much more uniform way with no residual stress, in an advantageous manner for the mechanical resistance and stability of the finished product.

Moreover, when heated with RF the quantity of glue or adhesive needed to obtain a predetermined mechanical resistance is lower than that employed with an indirect heating, for example with presses embodying heating plates.

Finally, heating with RF on one side reduces the time and pressure value as well as deformation, and the other side increases the productivity of the attachment process.

Of course, the desired results may be only obtained with adhesives which are capable to absorb in an optimum way energy from the electromagnetic field in the RF range.

As is known, the range of RF cannot be entirely used for industrial processes, but it is limited to several sub-ranges and values allowed according to national and international regulations.

Generally, the most frequently used frequency is 27.1 2 MHz and accordingly in correspondence of such value must be optimised the glues and the adhesives to obtain assemblies with high mechanical characteristics, as well as press machines with high velocity and production rate.

The known adhesive are normally deposited on both surfaces to be joined, are substantially chosen from the groups of resins base on polyvinyl esters, urea resins, melamine-formaldehyde resins and polyurethanes. Among these groups, the most diffused and common are polyvinyl esters compounds and urea resins.

The polyvinyl adhesives are compounds comprising resins based on polyvinyl acetate dispersed in an aqueous medium.

Table 1 indicates the operating conditions generally applied for polyvinyl adhesives. TABLE 1

The urea formaldehyde resin adhesive are compounds comprising resins based on precondensed urea-formaldehyde at solid state, generally dry powder, or dispersed in an aqueous medium, generally added with hardener or acid catalyst.

In case of product furnished in powder, this latter is dissolved in water before use. In Table 2 there are indicating the parameters generally used for an adhesive based on urea formaldehyde resins dispersed in water in 1 :2 ratio.

TABLE 2

With reference to the above mentioned methods of use, and particularly with reference to RF heating, the known adhesives have shown not to be exempt from drawbacks.

A first drawback resides in that in order to obtain high mechanical features of the joined product the attaching pressure must be comprised between 0.1 and 0.9 MPa, according to the types of adhesive employed, the materials to join and temperature.

The pressure force exerted by the press on the products may involve excessive stress on the components of the press itself.

A further drawback consists in that the mechanical resistance of the glued product highly depends on the quantity of the adhesive employed.

In other words, in order to obtain the highest mechanical resistance of the joined assembly it is necessary to select specific attachment pressure for each type of material. This means that the production lots must be homogeneous thereby notably reducing the flexibility of the plant. This is particularly disadvantageous for continuous presses operating on different types of material. Another drawback of such known adhesive is that they must be radiated with electromagnetic waves with RF of very high energy.

This involves a high cost of electric energy and may possibly cause electric sparks between the metallic components immersed in the electromagnetic field with consequent danger of fire.

Disclosure of the Invention

The main object of the present invention is to eliminate the drawbacks of the foregoing state of the art by providing a continuous press, particularly for products made of attachable parts, capable to improve the distribution of the pressure on the same products.

Another object of the present invention is to provide a press capable to work in a continuous manner on products of different dimensions and materials within a satisfactory range of dimensional tolerances.

Another object of the present invention is to provide a press with a high production efficiency.

Another object of the present invention is to conceive a press with a simple structure and a high reliability.

Yet another object of the present invention is to provide a synthetic heat curable adhesive for joining wooden assemblies, particularly applying electromagnetic waves in the RF range. A further object of the present invention is to conceive a heat curable adhesive allowing the use of low quantities of adhesive in comparison with those needed for conventional adhesives.

Another object is to provide a heat curable adhesive that can be efficiently employed for different wood materials.

Yet another object of the present invention is to provide a heat curable adhesive for gluing wooden assemblies by means of electromagnetic waves with reduced radiating power in the RF range.

These object as well as other are accomplished with a continuous press for products with attachable parts, which comprises a supporting structure laying on the ground, a pair of motorised annular conveyor belts mounted on said supporting structure with external surfaces facing each other along at least one common length to define therebetween a passage for guiding the products previously treated with heat curable adhesive, upon advancement thereof from an inlet door to an outlet door along an advancement direction, compression means mounted on said supporting structure, acting on said conveyor belts substantially stationary with respect to the supporting structure and adapted to exert through said conveyor belts a pressure on the products advancing along said direction through said passage, heating means operatively associated with said passage to dry the products advancing along said direction, characterised in that said compression means comprise at least one first plate carrying on a first active face a plurality of first shaped teeth adapted to slide on the internal surface of at least one first conveyor belt of said pair so as to compress the products advancing through said passage. By virtue of this arrangement it is possible to attach any type of semi-finished product having very large tolerances while ensuring an optimum pressure distribution.

In a second aspect, it is provided an adhesive for adhesion of wood assembled products, particularly adapted for use with a press according to the invention, comprising a dispersion of resins based of polyvinyl acetate in water with acid pH, a powdered pre-condensed in powder of amino formaldehyde resins added to said dispersion, in which said amino formaldehyde resins are of the type urea-formaldehyde, melamine-formaldehyde or combinations thereof, characterized in that said pre-condensed is comprise between 2% and 10% in weight of the dispersion.

Brief Description of the Figures

Further features and advantages of the invention will be more clearly understood by means of the following detailed description of a continuos press and of a synthetic adhesive, furnished by way of non limiting example with the aid of the attached drawing sheets, in which: FIG. 1 shows a lateral view of the continuous press according to the invention with several removed parts to better see;

FIG. 2 shows a lateral enlarged view of a detail of Fig .1 concerning a first configuration of the compression plates;

FIG. 3 shows a lateral enlarged view of a detail of Fig.1 concerning a second configuration of the compression plates;

FIG. 4 shows a detail of the compression plates of the press in enlarged scale.

Detailed Description of a Preferred Embodiment With reference to the attached drawings, a continuous press for the attachment of products with attachable parts is generally indicated with reference number 1 .

Press 1 enables to firmly attach parts of assemblies previously treated with heat curable adhesive by means of the combined heat and pressure action.

The press is particularly suitable for use in industrial process for assembling plates of wood and other materials, multi-layer products, boxes, cases and other assemblies with attachable parts, which products may be introduced in the press in a continuous manner (e.g. products assemblies constituted by ribbon-like products unwound from reels for coupling over each other within the press) or in discontinuous manner with products of the same kind but separated from each other.

In use, if press 1 is continuously fed it is capable to treat the products in line exerting on them a suitable pressure while drying them, which press 1 can be advantageously inserted in an automatic production line having upstream and downstream thereof robotised machines for assembling semi-finished products charged and discharged by means of suitable manipulators.

At first, the products to treat with press 1 are previously spread with a heat curable adhesive that may have a special composition in order to improve the efficiency of the press, as it will be clarified hereinafter.

In accordance with the embodiment shown in Fig. 1 , press 1 conventionally comprises a support structure 2 laying on the ground. The structure 2 supports a pair of conveyor belts 3, 4; the first upper conveyor belt 3 is wound in annular fashion on a first idle roller 5 and on a first motorised roller 6; a second lower conveyor belt 4 is wound in an annular fashion on a second idle roller 7 and on a second motorised roller 8.

The external surfaces 3', 4' of the two conveyor belts 3, 4 are arranged and parallel facing each other for at least one portion 9, in such a manner to define a passage 10 through which products M advance after being previously treated with heat curable adhesive.

The products M are driven from the two conveyor belts 3, 4 from an inlet door 1 1 and an outlet door 1 2 along an advancement direction A through the passage 1 0. This passage has an height d substantially equal to that of products M to be joined.

Compression means 1 3 are arranged on the support structure 2 to act on the conveyor belts 3, 4 thereby exerting a compression force on products M advancing along the passage 10; heating means 14 are operatively associated with passage 10 to dry the products M passing through it.

According to the invention, the compression means 14 comprise a first plate 1 5 carrying on a first active face 1 6 a plurality of first shaped teeth 1 7 adapted to shift on internal surface 3" of first conveyor belt 3 to urge the products passing through passage 1 0 between the external surfaces 3', 4' of conveyor belts 3, 4 delimiting the passage.

In the embodiment shown in Fig. 2, the heating means 14 are mounted on the first plate 1 5 in such a manner to allow the press 1 to exert at the same time on products M a compression and heating action finalised to optimise the curing of the adhesive and consequently the adhesion of the different parts of products M to be joined. Thanks to such an arrangement, it is possible to achieve a high quality of the attachment and thereby of the products finally obtained.

Advantageously, heating means 14 may be constituted by a wave generator in the RF range comprising a plurality of emitting electrodes 30 located in suitable seats 31 provided above the first plate 1 5. The RF radiation is preferably effected at frequency of 27, 1 2 Hz in accordance with the applicable regulations.

The RF technology allows to selectively heat the water molecules present in the adhesive, causing their evaporation and consequently the drying of the adhesive itself.

The drying of water diluted adhesives take place by direct transfer of the electromagnetic energy inside the adhesive layers with high efficiency a no dispersion in the outside environment and with no remarkable increase in the temperature of the products M.

This technique particularly combined with a specific composition of the adhesive and with a compression mechanism deprived of parts directly subjected to a RF radiation involves a number of other advantages in the operation of the press according to the present invention which will be clarified hereinafter.

The first plate 1 5 is preferably made of a material transparent to RF radiation to enable the waves emitted by electrodes 30 to reach the adhesive applied to products M; the first plate 1 5 further has a low friction factor to allow teeth 1 7 to easily slide on the internal surface 3" of the first conveyor belt 3 with no heat generation in spite of the remarkable pressure acting on products M. The low friction factor is obtained by virtue not only of the material employed but also thanks to the particular shape of teeth 1 7 which will be better described hereinafter.

The plate 1 5 is further characterized by a low thermal expansion factor to avoid that rollers 5, 6 have driving difficulties.

The compression force P exerted by teeth 1 7 on products M via conveyor belt 3 is countered by the reaction force exerted by a second plate 1 8 having a second active face 1 9 slidingly interacting with the internal surface 4" of the second conveyor belt 4.

In the embodiment of press shown in Fig. 3, the second plate 1 8 is formed by a metallic profile having its active face 1 9 substantially planar. In the alternative embodiment shown in Fig. 2 the second plate 1 8 is provided with a plurality of second shaped teeth 20 on its second active face 19. In such case, the second plate 1 8 is provided with a metallic profile 1 8' placed against the active face 1 9.

The use of teeth 1 7, 20 to compress products M within passage 1 0 between the two conveyor belts 3, 4 allows to even the pressure on the entire contact surface of product M. To this end, teeth 1 7, 20 are made of a material having a low friction factor and a low thermal expansion factor, and further allows the same teeth to elastically deform to compensate the dimensional tolerances of products M or the clearances and misalignments of the whole compression system.

In fact, in case that a product M has for example a slightly higher portion and a slightly lower portion and therefore it is not perfectly aligned with the active surfaces 1 6, 1 9 of the plates 1 5, 20, the elasticity of teeth 1 7, 20 will allow to compensate such a difference to compress both the higher and the lower portions of products M with the same amplitude.

The same will apply if the planarity error between the contact surfaces of product M is due to an imperfect mounting of the plates 1 5 on press 1 .

It is noticed that the metallic profile 18 or 1 8', present in both the embodiments shown, allows on one side to guide the electromagnetic waves generated by the emitting electrodes 30 and on the other side to dissipate the heat transferred from products M to the second plate 20 via the second conveyor belt 4.

The configuration of teeth 1 7, 20 may be optimised in accordance with the dimensions and types of products M to be treated. However, the shape depicted in Fig. 4 has shown in practice to be particularly suitable for a large number of products. In accordance with an embodiment illustrated in Fig. 4, the teeth have a height H ranging from 5 to 25 mm, preferably equal to 20 mm, a longitudinal width L measured along the advancement direction A of the conveyor belts comprised between 2 and 6 mm, and preferably equal to 4 mm, a depth F measured along a direction substantially transverse to advancement direction A of the conveyor belts also comprised between 2 and 6 mm and a tooth pitch comprised between 3 and 10 mm, preferably equal to 8 mm in longitudinal direction and 3 mm in a transverse direction.

In another alternative embodiment, the teeth are plate shaped to extend parallel along the transverse direction and the longitudinal advancement direction A of the conveyor belts, for spaced lengths comprised between 20 and 60 mm and preferably of 50 mm. According to a further important feature of the present invention, teeth 1 7, 20 are inclined towards the advancement direction of the conveyor belts so as to define an inclination angle α relative to the advancement direction ranging between 50° and 70° and preferably equal to 60° .

The pressure on products M and/or height d of passage 10 formed by conveyor belts 3, 4 is adjusted by means of actuator capable to raise the first plate 1 5 and/or the first conveyor belt 3.

In accordance with a preferred embodiment of the present invention, the first actuators means 40 are mechanically coupled to the supporting structure of the first conveyor belt 3 and advantageously constituted by an adjustment device of the worm-and-pinion type driven by electric motors 41 and second actuators means 42 mechanically coupled to the first plate 1 5 and consisting of hydraulic cylinders.

In accordance with another aspect of the present invention, the hot setting adhesive mentioned above comprises a water dispersion with pH acid of resins containing polyvinyl acetate and pre-condensed amino-formaldehyde.

By virtue of such formulation, the adhesive spread over products M may quickly dry under irradiation with electromagnetic waves generated by RF electrodes 30. Further, such drying enables to operate with a reduced compression of the plates with advantageous effect with a higher advancement speed of products M and consequently with a higher production efficiency of the machine 1 .

In particular, the amino-formaldehyde resins are urea-formaldehyde resins, melamine-formaldehyde resins or combinations of same. The amino-formaldehyde precondensates are comprised between 2% and 10% in weight of dispersion. Preferably, the precondensate weight percentage of pre-condensed is comprised from 5% to 8%. The water dispersion with acid pH of resins based on polyvinyl acetate has at least 45% solids content.

Further, the amino-formaldehyde precondensates in powder contains a curing and/or hardening catalyst.

In order to improve the film formation, the dispersion of resins containing polyvinyl acetate may include plasticizers and/or solvents.

The mixing temperature of the adhesive may range from 10° C to 25 °C and is preferably comprised between 1 2°C and 1 5° C.

After application of the adhesive on one or both the parts to be joined, the adhesive is subjected to heating through radiation with RF.

It has been verified that a radiating specific power comprised between 60 and 75 W/g of adhesive is sufficient to achieve optimum results from a technical and qualitative point of view.

Preferably, the specific power of the electromagnetic field in RF may be equal to about 70 W/g of adhesive.

The adhesion pressure necessary to obtain high mechanical resistance is comprised between 0, 1 and 0,56 MPa with a pressure time equal to about Example 1

An amount of about 4 Kg of powder urea-formaldehyde precondensate SYNTEKO 1 360™ sold by Akzo Nobel^® containing acid catalyst are added to about 46 Kg of about 50% emulsion of solids dispersed in a water matrix with a acid pH of resins containing polyvinyl-acetate 6-230 Emmevil RH-4TM traded by Emmebi International Industries Chemical.

The properties and features of the adhesive obtained after homogenisation effected at ambient temperature comprise between 1 2° C and 1 5 °C are indicated in Table 3.

TABLE 3

The applied instructions of use are indicated in Table 4. TABLE 4

Using the adhesive according to the present invention in the conditions indicated above, the advancement speed of product M in the press machine achieves 0.25 m/s as compared with 0.08 m/s maximum achieved with heretofore known adhesive.

The wood moisture is the most critical parameter. Moisture values higher than those indicated above may cause a decay in mechanical performances in the attachment joints and higher adhesion times with consequent heavy reduction of productivity.

The composition of the adhesive as well as its substantially constant dosage of 1 00 g/m², does not change in accordance with a different types of wooden material; the spreading of adhesive on only one side permits to reduce the change in total thickness between the surfaces to be joined thereby producing a higher steadiness of the resistance and quality properties of product M.

The use of the above indicated adhesive may therefore provide numerous advantages, among other a remarkable reduction of the joining pressure (strength), increased productivity, safety, flexibility of the press, reduction of the adhesive quantity and of the requested radiating power.

Further, according to the invention, it is possible to notice that the higher quality of the joined products M is due to their lower exposition to the electromagnetic RF field thereby inducing a lower heating and lower possibility of dimensional change.

In operation, the press machine described above from a structural point of view operates as follows.

As shown in attached Fig. 1 , products M once deposited on first portion 50 of the second lower conveyor belt 4 are carried from this latter up to the inlet door 1 1 of press 1 so as to reach the passage 10 delimited by the two lengths 9 of conveyor belts 3, 4 parallel and facing each other. Plates 1 5, 20 mounted internally on the rings defined by conveyor belts 3, 4 restrict the height of d of the above mentioned passage 1 0 to a value slightly lower than the height of products M. Thus, upon advancement of products M through passage 1 0 these latter are compressed between the external surfaces 3', 4' of the conveyor belts 3, 4. The adjustment of height of the first plate 1 5 allows the pressure P acting upon M to be changed. At the same time, electrodes 30 radiate RF waves which heat the water molecules contained in the adhesive causing drying thereof while the different parts of the product are reciprocally compressed. This provides an optimum attachment of the different parts of products M to be joined.

Accordingly, by virtue of the reduced friction exerted by plates 1 5, 1 8 on the conveyor belts 3, 4 specifically caused by the shape of teeth 17, 20, and by virtue of the optimum attachment due to the better pressure distribution, to the combined and simultaneous of the heating with RF and pressure, and finally by virtue of the particular adhesive employed which dries quickly when irradiated with RF it has been possible to achieve high advancement speed of conveyor belts 3, 4 and to finally optimise the productions efficiency of press 1 .

The invention as described above achieves the intended objects. In practise, it may have any shapes and configurations different from those illustrate above while not departing from the scope of protection ad disclosed in the appended claims.

Further, all the details may be substituted by technical equivalents and the dimensions, shapes and material employed may be anyone according to the circumstances.

Claims

1 . A continuos press, particularly for manufacturing products (M) with joined parts, comprising: - a supporting structure (2) laying on the ground;

- a pair of motorised annular conveyor belts (3, 4) mounted on said supporting structure (2), with external surfaces (3, 4) facing each other along at least one portion (9) thereof to define a confining passage (10) for guiding products (M) previously treated with heat curable adhesive during advancement thereof from an inlet door (1 1 ) to an outlet door (12) along an advancement direction (A);

- compression means (13) mounted on said supporting structure (2), acting on said conveyor belts (3, 4) substantially stationary with respect to said supporting structure and adapted to exert through said conveyor belts (3, 4) a pressure on the products (M) advancing along said direction (A) through said passage (10);

- heating means (14) operatively associated with said passage (1 0) to dry products (M) advancing along said direction (A); characterised in that said compression means (1 3) comprise at least one first plate (1 5) having on a first active face (1 6) thereof a plurality of first shaped teeth (1 7) adapted to slide on the internal surface (3") of at least one first conveyor belt (3) of said pair (3, 4) so as to compress the products (M) advancing through said passage (1 0).

2. Press according to claim 1 , characterised in that said heating means

(14) are arranged on said first plate (1 5) to exert on products (M) a combined pressure and heating action.

3. Press according to claim 2, characterised in that said heating means (14) comprise a RF waves generator.

4. Press according to claim 3, characterised in that said RF waves generator (1 4) comprises a plurality of emitting electrodes (30) located in seats (31 ) provided on said first plate (1 5).

5. Press according to claim 1 , characterised by comprising a second plate (1 8) mounted on said structure of support (2), with a second active face

(1 9) adapted to slidingly contact the internal surface (4") of said second conveyor belt (4) to compress the products (M), advancing through said passage (10) with a reaction pressure (P) opposed to the pressure exerted by said first plate (1 5).

6. Press according to claim 5, characterised in that said second plate ( 1 8) it formed by a metallic profile with a substantially planar second active face (1 9).

7. Press according to claims 3 and 6, characterized in that said metallic profile is adapted to receives the waves emitted by said RF waves generator (1 4).

8. Press according to claim 6, characterized in that said metallic profile is adapted to dissipate the heat transferred from said second conveyor belt (4).

9. Press according to claim 5, characterized in that said second plate

(1 8) has on said second active face (1 9) a plurality of second shaped teeth

(20) adapted to slide in contact with the inner surface (4") of said second conveyor belt (4).

1 0. Press according to claim 9, characterized in that said second plate (1 8) is coupled with a metallic profile (1 8') suitable to receive the waves emitted by said RF-waives generator (1 4).

1 1 . Press according to claims 1 or 9, characterized in that said teeth (1 7, 20) have a height (H) comprised between 5 and 25 mm and a longitudinal width (L) comprised between 2 and 6 mm.

1 2. Press according to claims 1 or 9, characterized in that said teeth (1 7, 20) are reciprocally spaced along the advancement direction (A) of the conveyor belts (3, 4) for a distance (X) comprised between 5 and 1 0 mm.

1 3. Press according to claims 1 or 9, characterized in that said teeth (1 7, 20) are parallel and extend along a substantially transverse direction relative to the advancement direction (A) of said conveyor belts (3, 4) with a depth (F) ranging from 2 to 6 mm.

1 4. Press according to claims 1 or 9, characterized in that said teeth (1 7, 20) are blade-shaped and extend parallel to each other along a substantially transverse direction relative to the advancement direction (A) of said conveyor belts (3, 4) for separate lengths comprised from 20 to 60 mm.

1 5. Press according to claims 1 or 9, characterized in that said teeth (1 7, 20) are inclined towards the advancement direction (A) of said conveyor belts (3, 4).

1 6. Press according to claims 1 or 9, characterized in that said teeth ( 1 7, 20) form an inclination angle (α) relative to the advancement direction of said conveyor belts (3, 4) comprised between 50 and 70° .

1 7. Press according to claim 1 , characterized in that first actuators means (40) are mechanically coupled to said first conveyor belt to adjust the height of said passage (10).

1 8. Press according to claim 1 , characterized in that said actuators means (42) are mechanically coupled to said first plate (3) to change the pressure exerted on products (M) advanced from said conveyor belts (3, 4).

1 9. Press according to claims 1 or 5, characterized in that said plate (1 5, 1 8) is made of a plastic material at least partially elastically deformable.

20. Press according to claims 1 or 5, characterized in that said plate (1 5, 1 8) is made of a material transparent to RF waives, having a low friction factor and a low thermal expansion factor.

21 . Press according to the claims 1 or 5, characterized in that said plate

(1 5, 1 8) is made of a plastic or elastomeric material.

22. A synthetic adhesive for joining wooden assembled products, particularly adapted for use with a press according to any preceding claims, comprising a dispersion of resins based on polyvinyl acetate in water with acid pH, a powder amino-formaldehyde precondensate resins added to said dispersion, in which said amino formaldehyde resins are of the type urea- formaldehyde, melamine-formaldehyde or combinations thereof, characterized in that said pre-condensed is comprise between 2% and 1 0% in weight of the dispersion.

23. Synthetic adhesive according to claim 22, characterized in that said pre-condensed is preferably comprised between 5% and 8% in weight of the dispersion.

24. Synthetic adhesive according to claim 22, characterized in that said water dispersion in water with acid pH of resins based on polyvinyl acetate contains at least 45% of solid.

25. Synthetic adhesive according to claim 22, characterized in that said powder amino formaldehyde precondensate comprises a curing and/or hardening catalyst.

26. Synthetic adhesive according to one or more of the preceding claims 22, characterized in that said dispersion in water with acid pH of resins based on polyvinyl acetate contains plasticizers and/or solvents to improve the film formation.

27. Use of a synthetic adhesive according to one or more of the preceding claims from 22 to 26, characterized by radiation heating of one or more surfaces spread with adhesive by means of electromagnetic oscillating field of RF frequency with a specific radiating power comprised between 60 and 75 W/g of adhesive.

28. Use of an adhesive according to claim 27, characterized by providing a adhesion pressure comprised between 0, 1 and 0,56 MPa.

29. Use of an adhesive according to claim 27, characterized by providing a pressure time of about 20 sec.