WO1998018614A1

WO1998018614A1 - Production of corrugated board structures

Info

Publication number: WO1998018614A1
Application number: PCT/IL1996/000136
Authority: WO
Inventors: Moshe Stoler
Original assignee: Best Carton, Ltd.
Priority date: 1996-10-30
Filing date: 1996-10-30
Publication date: 1998-05-07
Also published as: AU7330796A; EP0869865A1

Abstract

A corrugator for producing double and single wall corrugated board structures including two pairs of interlocking corrugating (31, 32, 33, 34) rolls. The spacing between the flutes of the corrugating rolls of one pair of interlocking corrugating rolls is an integral multiple of the spacing between those of the other pair. The adjacent center rolls are synchronized so that when rotating about their respective axes the flute crests of one roll are aligned with flute crests of the other. A pressure roll is disposed to apply pressure to one of the corrugating rolls for bonding a center liner to the flutes of one corrugating medium, whilst the flutes of the other corrugating medium are bonded to the center liner as it passes between the two center rolls. Also disclosed is a liner application station for bonding outer liners to the two corrugating media.

Description

PRODUCTION OF CORRUGATED BOARD STRUCTURES

FIELD OF THE INVENTION

This invention relates to a corrugator for producing double and single wall corrugated board structures.

BACKGROUND OF THE INVENTION

The wet end of conventional corrugators for producing double wall corrugated board structures comprises basically of two single facers and a double facer. The single facers produce single face corrugated board structures which are in general of different flute size. The two single face corrugated board structures are brought together at the double facer to be adhered to each other and to an additional outer liner in order to form the double wall corrugated board structure.

A noticeable disadvantage of conventional double facers is the unequal bonding of the outer liners. In the production process, one of the outer liners of the double wall corrugated board structure is the liner of one of the single face corrugated board structures, which in the following description will be termed the "first single face". This outer liner is bonded to the corrugated medium of the first single face by means of a pressure means adjacent to the corrugating roll of the single facer. As a result, the liner and the corrugated medium are bonded together when the corrugated medium is partially wrapped around the corrugating roll and the liner is partially wrapped around the pressure roll so that, in the region of bonding, the corrugated medium is curved taking on the curvature of the corrugating roll and the liner is curved taking on the curvature of the pressure roll. Consequently, when the first single face is straightened out on exiting the single facer, concave impressions, or indentations, are formed on the liner between the crests of the flutes of the corrugated medium that it is adhered thereto.

On the other hand, the other outer liner of the double wall corrugated board structure is bonded to the crests of the corrugated medium of the other single face corrugated board structure when both are in a flat configuration as they pass over hot plate sections together with the first single face. Hence, one outer liner is bonded to a corrugated medium whilst both are in a curved disposition, giving rise to indentations in its surface, whereas the other is bonded to a corrugated medium whilst both are in a planar disposition, so that no indentations are formed in its surface. The result is a non-symmetrical structure as far as the formation of the outer liners is concerned. Consequently, the strength of the double wall corrugated board structures thus constructed is weaker than a double wall corrugated board structure having flat outer liners (i.e., without indentations).

A further disadvantage of conventional double facers originates during the bonding of the two single face corrugated board structures and concerns the bonding of the flute crests of one single face corrugated board structure (again for the sake of clarity this will be termed the "first single face") to the liner of the other single face corrugated board structure (this will be termed the "second single face"). The liner of the second single face forms the center liner of the double wall corrugated board structure. During the forming of the first and second single faces by their single facers, each single face has a pressure roll which assists the bonding of the liner to each of the crests of the flutes of the corrugated medium. However, when the first and second single faces are brought together over the hot plate sections of the double facer, together with a further liner for forming the double wall corrugated board structure, the crests of the flutes of the corrugated. medium of the first single face and the center liner do not have the same bonding force and heat applied to them as did the crests of the flutes and the liners of the two single faces when they were formed by the single facers. As a result, the bonding of the center liner to the crests of the flutes of the first single face is not always effective, unless the two single faces are preheated prior to being brought together over the hot plate sections. During the preheating process, the liners of the two single faces are partially wrapped around preheaters of cylindrical cross section thereby taking on the curvature of the preheaters. On exiting the preheaters the liners straighten out and the indentations on the liner between the crests of the flutes of the corrugated medium that it is adhered thereto are further accentuated. Another disadvantage of conventional corrugators for producing double wall corrugated board structures concerns their size. The two single facers are arranged in tandem and the two single face corrugated board structures formed by them have to be led over an overhead bridge construction and then to preheaters and glue units after which they are adhered together and to an outer liner at a hot plate section. Hence, conventional corrugators for producing double wall corrugated board structures are relatively long.

It is an object of the present invention to provide a corrugator that significantly reduces or eliminates the aforementioned disadvantages. It is further an object of the present invention to provide a method for producing double wall corrugated board structures in which the aforementioned disadvantages that arise during their construction are significantly reduced or eliminated.

It is yet a further object of the present invention to provide double wall corrugated board structures in which the aforementioned disadvantages in the so above constructed double wall corrugated board structures are significantly reduced or eliminated. SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a corrugator for producing double wall corrugated board structures from two outer liners, two corrugating media and a center liner comprising: a first corrugating roll cooperating with a second corrugating roll, whereby a corrugation is formed on a first corrugating medium passed between said first and second corrugating rolls, said first and second corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced first fluted members with first flute crests and a third corrugating roll cooperating with a fourth corrugating roll, whereby a corrugation is formed on a second corrugating medium passed between said third and fourth corrugating rolls, said third and fourth corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced second fluted members with second flute crests, the spacing between each of said first fluted members being an integral multiple of the spacing between each of said second fluted members, said second and third corrugating rolls forming center rolls and being parallel and adjacent to each other and being synchronized so that when rotating about their respective axes each of the first flute crests is aligned with one of the second flute crests; pressure means disposed to apply pressure to said third corrugating roll; glue applicators adjacent to said second and third corrugating rolls to apply glue to the crests of said corrugating media prior to their contact with the center liner; and a double facer for bonding said outer liners to the corrugating media.

If desired, said pressure means is a pressure roll. Further if desired, said pressure means is a pressure belt. Still further if desired, said pressure means is a fluted cylinder. Preferably, said double facer comprises two glue applicators to apply glue to the crests of the corrugating media prior to their contact with the outer liners, two nonoverlapping hot plate sections disposed to allow the double wall corrugated board structure to pass between them whilst remaining in contact with them and two nonoverlapping suction conveyer belts disposed to withdraw corrugated board structures from between the center rolls.

The corrugator can be used to produce both double and single wall corrugated board structures. For producing single wall corrugated structures only one corrugating medium is used. Furthermore, the center liner becomes an outer liner, therefore only one additional outer liner is required. Two different types of single wall corrugated board structures can be produced depending on whether the corrugating medium is passed between the first and second corrugating rolls, or between the third and fourth corrugating rolls. The difference between the two types of corrugated board structures is in their flute size.

Therefore, in accordance with the present invention there is provided a corrugator for producing corrugated board structures comprising: a first corrugating roll cooperating with a second corrugating roll, said first and second corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced first fluted members with first flute crests and a third corrugating roll cooperating with a fourth corrugating roll, said third and fourth corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced second fluted members with second flute crests, the spacing between each of said first fluted members being an integral multiple of the spacing between each of said second fluted members, said second and third corrugating rolls forming center rolls and being parallel and adjacent to each other and being synchronized so that when rotating about their respective axes each of the first flute crests is aligned with one of the second flute crests; a pressure means disposed to apply pressure to said third corrugating roll; glue applicators adjacent to said second and third corrugating rolls; and a double facer, downstream to said corrugating rolls. If desired, said pressure means is a pressure roll. Further if desired, said pressure means is a pressure belt. Still further if desired, said pressure means is a fluted cylinder. Preferably, said double facer comprises two glue applicators to apply glue to crests of corrugating media prior to their contact with outer liners, two nonoverlapping hot plate sections disposed to allow corrugated board structures to pass between them whilst remaining in contact with them and two nonoverlapping suction conveyer belts disposed to withdraw corrugated board structures from between the center rolls.

Further in accordance with the present invention there is provided a method for producing double wall corrugated board structures, from two outer liners, two corrugating media and a center liner, comprising the steps of: forming a corrugation comprising regularly spaced first fluted members with first flute crests on a first corrugating medium by passing it between first and second interlocked corrugating rolls; forming a corrugation comprising regularly spaced second fluted members with second flute crests on a second corrugating medium by passing it between third and fourth interlocked corrugating rolls, the spacing between each of said first fluted members being an integral multiple of the spacing between each of said second fluted members; bonding a center liner to the crests of said first and second corrugating media so that each of the first flute crests is aligned with one of the second flute crests, with the center liner separating between them; and bonding outer liners to the crests of the flutes of the first and second corrugating media.

The two pairs of corrugating rolls are interlocked in the sense that their fluted members are mutually intermeshed.

There is also provided in accordance with the present invention a double wall corrugated board structure comprising two outer liners, a first corrugating medium having a fluted surface comprising regularly spaced first fluted members with first flute crests, a second corrugating medium having a fluted surface comprising regularly spaced second fluted members with second flute crests, a center liner bonded to the crests of said first and second corrugating media so that each of the first flute crests is aligned with one of the second flute crests, with the center liner separating between them; said double wall corrugated board structure characterized in that: the outer liners are totally flat without any indentations; and the spacing between each of said first fluted members is an integral multiple of the spacing between each of said second fluted members.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding, the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 shows a typical conventional double wall corrugated board structure produced by a prior art corrugator; Fig. 2 shows a typical double wall corrugated board structure produced by the corrugator of the invention, for the case W = 2w;

Fig. 3 shows a typical double wall corrugated board structure produced by the corrugator of the invention, for the case W = 4w;

Figs. 4 and 5 show a schematic illustration of one embodiment of the corrugator of the invention;

Fig. 6 shows an enlargement of the contact region of the center corrugating rolls of the corrugator shown in Fig. 4;

Fig. 7 shows a schematic illustration of another embodiment of the corrugator of the invention; and Fig. 8 shows a schematic illustration of the corrugator shown in Fig.

7 with hydraulic cylinders for maintaining contact between the rolls.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Attention is first drawn to Fig. 1, showing a typical conventional double wall corrugated board structure 1 produced by a prior art corrugator.

Some of the flute crests 2 of the corrugated media 3 and 4 are aligned (i.e., directly opposite each other), others 5 are almost aligned, whereas others, 6 of corrugated medium 3 fall between two flutes of corrugated medium 4. As can clearly be seen liner 7 has indents between the crests of the flutes of corrugated medium 4 to which it is adhered, whereas liner 8 is flat.

Attention is now drawn to Fig. 2, showing a typical double wall corrugated board structure 10 produced by the corrugator of the invention. It comprises two corrugated media 11 and 12, center liner 13 and outer liners 14 and 15, which are totally flat. Every flute crest 16 of corrugated medium 12 is aligned with a flute crest 17 of corrugated medium 11. The width W of the flutes of corrugated medium 12 is twice the width w of the flutes of corrugated medium 11, i.e., W = 2w. In Fig. 2 there are no flute crests that are "almost" aligned as in Fig. 1.

In general, in accordance with the present invention, W = nw, where n is an integer. That is, in general, the width of the flutes of one corrugated medium is an integral number of times the width of the flutes of the other corrugated medium. The width of a flute is given by the distance between the crests of adjacent flutes and is hence also equal to the distance between the flutes. Corrugated medium 11 has a higher pitch (or frequency) than that of corrugated medium 12. In general, if W = nw then (n-1) flute crests of the high pitch corrugated medium will fall between two adjacent aligned crests of the low pitch corrugated medium. In Fig. 2, there is one such flute crest and in Fig. 3, showing the case W = 4w, there are three such crests 18.

Therefore, in double wall corrugated board structures produced by the corrugator of the invention, each flute crest of the low pitch corrugated medium is aligned with a flute crest of the high pitch corrugated medium, however, the converse is not true. In general, the term "crest" refers to the "upper peak" of a wave and the term "trough" refers to the "lower peak" of a wave. This distinction is not used here. Instead, the term "flute crests" has been used to refer to both the "upper" and "lower" peaks. In particular, with reference to Figs. 2 and 3 the term "flute crests" has been used to refer to those parts of the corrugated media that are in contact with the center liner of the double wall corrugated board structure. In general, this use of the term flute crests is unambiguous and its meaning should be clear both from the drawings and the associated explanations.

Attention is now drawn to Figs. 4 and 5 showing a schematic illustration of one embodiment of a corrugator 30, of the invention. Corrugator 30 produces corrugated board structures of the type shown in Figs. 2 and 3. Corrugating rolls 31 and 32 form an interlocking pair, each having identical fluted members. It should be noted that the diameters of the two corrugating rolls 31 and 32 do not have to be the same. Similarly, corrugating rolls 33 and 34 (which also do not have to have the same diameters) form an interlocking pair, each having identical fluted members but different in size (pitch and possibly amplitude) to those of corrugating rolls 31 and 32. In accordance with the invention, the spacing between each fluted member of corrugating rolls 33 and 34 is an integral multiple of the spacing between each fluted member of corrugating rolls 31 and 32. Clearly the interlocking pair of corrugating rolls 31 and 32 can be replaced by another interlocking pair of corrugating rolls having different pitch and/or amplitude. Similarly, the interlocking pair of corrugating rolls 33 and 34 can be also be replaced by another interlocking pair of corrugating rolls having different pitch and/or amplitude. The only constraint on the different interlocking pairs of corrugating rolls is that the fluted members of the corrugating rolls of one of the interlocking pairs of corrugating rolls be an integral multiple of the spacing between the fluted members of the corrugating rolls of the other interlocking pair of corrugating rolls. It is therefore useful to prepare modules of interlocking pairs of corrugating rolls of different flute size so that they can be easily changed, thereby enabling the production of many different corrugated board structures of different flute size. A corrugation is formed on corrugating medium 35 as it passes between corrugating rolls 31 and 32. Similarly, a corrugation is formed on corrugating medium 36 as it passes between corrugating rolls 33 and 34. Owing to the difference in spacing between the fluted members of corrugating rolls 31 and 32 and those of corrugating rolls 33 and 34, there will be a corresponding difference in the spacing between the fluted members of corrugating medium 35 and those of corrugating medium 36. Pressure roll 37 is disposed to apply pressure to corrugating roll 32 for bonding center liner 38 to the flute crests of corrugating medium 35 after the flute crests have had glued applied to them by glue applicator 39. The flute crests of corrugating medium 36 have glue applied to them by glue applicator 40 after which they are bonded to center liner 38 by pressure that exists between the corrugating rolls 32 and 33.

It should be noted that Figs. 4 and 5 are only a schematic illustration of the corrugator of the invention and that pressure roll 37 could be any pressure means for applying pressure to corrugating roll 32. For example, a pressure belt could just as well be used. Another possibility would be to use a fluted cylinder similar in structure to corrugating roll 32. The fluted cylinder could be of different diameter to corrugating roll 32 but its fluted members would have to have the same pitch and amplitude as the fluted members of corrugating roll 32. However, the fluted cylinder and corrugating roll 32 would have to be synchronized so that when rotating about their respective axes the flute crests of the fluted cylinder are aligned with the flute crests of corrugating roll 32. Fig. 6 shows an enlargement of the contact region of the center corrugating rolls 32 and 33. Center liner 38 is shown to be bonded to the flute crests of corrugating medium 35 after having passed between pressure roll 37 and corrugating roll 32 (as shown in Fig. 4). Corrugating medium 36, which has glue on its flute crests (applied by glue applicator 40, as shown in Fig. 4) is bonded to center liner 38 as the flute crests of corrugating roll 33 align with flute crests of corrugating roll 32. The two corrugating rolls are "pushed together" at the point of alignment of the flute crests, as shown by the two arrows 45 and 46 by known mechanisms for creating pressure between the rolls, for example, hydraulic cylinders could be used, as will be described briefly with respect to Fig. 8.

As the two corrugated media 35 and 36 and the center liner 38 pass between the point of alignment of the flute crests of the two corrugat- ing rolls 32 and 33, a corrugated cardboard structure that will be referred to herein as a "twin face " exits from between the corrugating rolls 32 and 33 in the direction of arrow 41. The twin face comprises center liner 38 with the two corrugated media 35 and 36 adhered to either side of it. On exiting from between the corrugating rolls 32 and 33, in the direction shown by arrow 41, it only remains to bond outer liners to the two corrugating media 35 and 36 of the twin face. This is achieved by using the double facer of the corrugator 30. The double facer is shown in Fig. 5 and comprises hot plate sections 50 and 51, counter pressure plates 50' and 51', glue applicators 52 and 53 and suction conveyor belts 56 and 57. Counter- pressure plates 50' and 51' serve to ensure good contact between the corrugated board structure being formed and the hot plate sections 50 and 51, respectively.

The twin face, after exiting from between the corrugating rolls 32 and 33 as shown in Fig. 4, first has glue applied to the crests of corrugated medium 35 by glue applicator 53. It should be noted that glue applicator 53 is equipped with contact roll 53' (or alternatively a contact bar) to ensure that the glue roll of the glue applicator makes contact with the flute crests of corrugated medium 35. The crests of corrugated medium 35 with glue applied to them then come into contact with outer liner 55 as corrugated medium 35 and outer liner 55 pass over the hot plate sections 51, which aids in the curing of the bonding between them.

Following the bonding of outer liner 55 to corrugated medium 35, outer liner 54 is bonded to corrugated medium 36 in a similar manner. Glue is applied to the flute crests of corrugated medium 36 by glue applicator 52 which is equipped with contact roll 52' (or alternatively a contact bar) to ensure contact between the glue roll of glue applicator 52 with the flute crests of corrugated medium 36. The crests of corrugated medium 36 with glue applied to them, then come into contact with outer liner 54 as outer liner 54 together with corrugated medium 36 passes under hot plate sections 50. With both outer liners bonded to the two corrugated media, the formation of the double wall corrugated board structure is complete. Suction conveyor belts 56 and 57 (which have vacuum pumps, not shown, suitably fitted to them) have their belt velocity adjusted to the rotational velocity of the corrugating rolls 31, 32, 33 and 34 to ensure that the twin face is drawn into the double facer without tearing it.

It should be noted that the bonding process of outer liner 54 could be performed before the bonding process of outer liner 55 by moving the glue unit 52 together with the hot plate sections 50 and the counter-pressure plates 50' closer to the corrugating rolls than the glue unit 53, the hot plate sections 51 and the counter-pressure plates 51'. Similarly, the positions of suction conveyer belts 56 and 57 can also be interchanged relative to the corrugating rolls. That is, suction conveyer belt 56 could be positioned closer to the corrugating rolls than suction conveyer 57. In accordance with one aspect of the invention the corrugator 30 can be used to produce a single wall corrugated board structure. Since the corrugator 30 can produce double wall corrugated board structures of the type shown in Figs. 2 and 3, that is with high pitch and low pitch flutes, it can accordingly produce two types of single wall corrugated board structures, one with a "high pitch" and one with a "low pitch". The terms low and high are, of course, relative and used only in the context of a comparison between the pitches of the two types of corrugated medium produced by the corrugator 30.

In accordance with a first specific application of the above described aspect of the invention a single wall corrugated board structure of "low pitch" can be obtained by using the corrugator 30 without feeding in corrugating medium 35 and outside liner 55. The resulting single wall corrugated board structure comprises a corrugated medium 36 and outer liners 38 and 54 (see Figs. 4 and 5). In this specific application pressure roll 37 is not required and can be moved so as not to apply pressure to corrugating roll 32. In accordance with a second specific application of the above described aspect of the invention a single wall corrugated board structure of "high pitch" can be obtained by using the corrugator 30 without feeding in corrugating medium 36 and outer liner 54. The resulting single wall corrugated board structure comprises corrugated medium 35 and outer liners 38 and 55 (see Figs. 4 and 5).

For both of the above so constructed single wall corrugated board structures, outer liner 38 would be the center liner of the double wall corrugated board structure that would have been formed had corrugating media 35 (36) and outer liner 55 (54) also been applied.

A representation of a "high pitch" single wall corrugated board structure can be obtained from Fig. 2 by eliminating corrugated medium 12 and outer liner 15, whereas a "low pitch" single wall corrugated board structure can be obtained by eliminating corrugated medium 11 and outer liner 14.

The thickness of the corrugated board structures produced by the corrugator (for a given set of corrugating rolls 31, 32, 33 and 34) can therefore vary depending on whether a double or single wall corrugated board structure is to be produced, and if the latter whether it be of "high pitch" or of "low pitch".

In order to accommodate the variable thickness of the corrugated board structure that can be produced by the corrugator, hot plate sections 50 and 51 (see Fig. 5) are set parallel but do not overlap (i.e, they are staggered). The vertical distance between the two faces of the hot plate sections that make contact with the outer liners is such that corrugated board structures of any thickness can pass smoothly between them. This arrangement allows both thin and thick corrugated board structures to pass between hot plate sections 50 and 51 whilst at the same time ensuring that good contact is made with them. Corrugated board structures of different thicknesses are able to pass between the hot plate sections 50 and 51 by slightly bending in the gap between the two hot plate sections. The aforementioned arrangement for accommodating corrugated board structures of various thicknesses is useful for a double facer with a fixed vertical distance between the two faces of the hot plate sections that make contact with the outer liners. However, this is not a binding arrangement and if desired corrugated board structures of various thicknesses can also be accommodated by suitably varying the vertical distance between the hot plate sections.

The suction conveyor belts 56 and 57 are also staggered in a similar manner, therefore also allowing corrugated board structures of different thicknesses to pass between them. Using two suction conveyor belts distributes the pulling force over both liners, which is of particular relevance when producing lightweight corrugated board structures, since it avoids possible shearing of the flutes. As with the case of the hot plate sections, corrugated board structures of various thicknesses can also be accommodated by the suction conveyer belts by suitably varying the vertical distance between them.

Although the corrugating rolls 31, 32, 33 and 34 are generally heated to assist bonding, preheaters (not shown) can also be used and would be applied to corrugating media 35 and 36 before they engage corrugating rolls 31 and 34, respectively. Similarly, a preheater could be applied to the center liner 38 before it engages pressure roll 37. In addition to preheaters, preconditioners can also be applied to corrugating media 35 and 36.

Although corrugating rolls 31, 32, 33 and 34 are shown to be arranged with their axes aligned in a plane, this is not the only possible arrangement for the disposition of the corrugating rolls of the corrugator of the invention. Attention is now drawn to Fig. 7 showing a schematic illustration of another embodiment of the corrugator of the invention, obtained by changing the disposition of the corrugating rolls and pressure roll 37. The axes of corrugating rolls 32, 33 and 34 are aligned in one plane whereas those of corrugating rolls 31 and 32 and pressure roll 37 are aligned in another plane. In fact, many different possible configurations for the disposition of the corrugating rolls are possible. The configuration shown in Fig. 7 is obtained from that shown in Fig. 4 by rotating corrugating roll 31 around corrugating roll 32 until the axes of corrugating rolls 31 and 32 and pressure roll 37 are in the same plane. Another configuration could be obtained by rotating corrugating roll 34 about corrugating roll 33.

Many intermediate configurations can be obtained by varying the angle of rotation of corrugating roll 31 and pressure roll 37 around corrugating roll 32 and also by varying the angle of rotation of corrugating roll 34 about corrugating roll 33. An important constraint on the possible configurations is that there be room for the glue applicators 39 and 40, and that motion of the corrugating media 35, 36,the liner 38 and the resulting twin face exiting from between rolls 32 and 33 not be hindered. It is emphasized that changing the disposition of the corrugating rolls and the pressure roll in the manner described does not bring about any fundamental change in the operation of the corrugator or in the resulting corrugated board structures produced by it. Apart from the above mentioned constraints on the disposition of the various rolls, the proper functioning of the corrugator depends on the existence of correct forces acting between the rolls. Whatever configuration is used, there should exist forces applied along the line joining the centers of two adjacent rolls tending to maintain contact between the two rolls. These forces will be termed "attractive radial forces". Hence, attractive radial forces should exist between the pairs of corrugating rolls 31 and 32, 32 and 33, 33 and 34 and between pressure roll 37 and corrugator roll 32.

Attention is now drawn to Fig. 8 showing a schematic illustration of the corrugator shown in Fig. 7 with one possible embodiment of hydraulic cylinders for maintaining contact between the rolls. Frames 70 and 72 have attached to them hydraulic cylinders 74, 76, 78 and 80. The axes of corrugating rolls 31, 32, 33 and 34 and of pressure roll 37 are mounted on bearings 82, 84, 86, 88 and 90, respectively, to allow rotation of the rolls about their respective axis. In accordance with the embodiment shown, hydraulic cylinder 74 maintains an attractive radial force between corrugating rolls 33 and 34 and hydraulic cylinders 76 and 77 maintain an attractive radial force between corrugating rolls 32 and 33. Hydraulic cylinders 78 and 80 maintain attractive radial forces between corrugating rolls 31 and 32 and between pressure roll 37 and corrugating roll 32.

In the foregoing description only those features pertinent to the present invention have been illustrated. Hence, for example, the corrugating rolls are provided with suitable means for ensuring that the corrugated media maintain contact with the corrugating rolls in the regions of contact between the two. These means are well known in the art and can be suction means or pressure means. The present invention has been described and illustrated with a certain degree of particularity. However, it should be understood that various alterations and modifications may be made without departing from the spirit or scope of the invention as hereinafter claimed. In particular, the shape of the flutes as illustrated should not be construed as limiting in any way.

Claims

CLAIMS:

1. A corrugator for producing double wall corrugated board structures from two outer liners, two corrugating media and a center liner comprising: a first corrugating roll cooperating with a second corrugating roll, whereby a corrugation is formed on a first corrugating medium passed between said first and second corrugating rolls, said first and second corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced first fluted members with first flute crests and a third corrugating roll cooperating with a fourth corrugating roll, whereby a corrugation is formed on a second corrugating medium passed between said third and fourth corrugating rolls, said third and fourth corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced second fluted members with second flute crests, the spacing between each of said first fluted members being an integral multiple of the spacing between each of said second fluted members, said second and third corrugating rolls forming center rolls and being parallel and adjacent to each other and being synchronized so that when rotating about their respective axes each of the first flute crests is aligned with one of the second flute crests; pressure means disposed to apply pressure to said third corrugating roll; glue applicators adjacent to said second and third corrugating rolls to apply glue to the crests of said corrugating media prior to their contact with the center liner; and a double facer for bonding said outer liners to the corrugating media.

2. A corrugator according to Claim 1, wherein said pressure means is a pressure roll.

3. A corrugator according to Claim 1, wherein said pressure means is a pressure belt.

4. A corrugator according to Claim 1, wherein said pressure means is a fluted cylinder. .

5. A corrugator according to any one of the above Claims, wherein said double facer comprises two glue applicators to apply glue to the crests of the corrugating media prior to their contact with the outer liners, two nonoverlapping hot plate sections disposed to allow the double wall corrugated board structure to pass between them whilst remaining in contact with them and two nonoverlapping suction conveyer belts disposed to withdraw corrugated board structures from between the center rolls.

6. A corrugator for producing corrugated board structures comprising: a first corrugating roll cooperating with a second corrugating roll, said first and second corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced first fluted members with first flute crests and a third corrugating roll cooperating with a fourth corrugating roll, said third and fourth corrugating rolls having mutually intermeshed fluted surfaces comprising regularly spaced second fluted members with second flute crests, the spacing between each of said first fluted members being an integral multiple of the spacing between each of said second fluted members, said second and third corrugating rolls forming center rolls and being parallel and adjacent to each other and being synchronized so that when rotating about their respective axes each of the first flute crests is aligned with one of the second flute crests; pressure means disposed to apply pressure to said third corrugating roll; glue applicators adjacent to said second and third corrugating rolls; and a double facer downstream to said corrugating rolls.

7. A corrugator according to Claim 6, wherein said pressure means is a pressure roll.

8. A corrugator according to Claim 6, wherein said pressure means is a pressure belt.

9. A corrugator according to Claim 6, wherein said pressure means is a fluted cylinder. .

10. A corrugator according to any one of Claims 6 to 9, wherein said double facer comprises two glue applicators to apply glue to crests of corrugating media prior to their contact with outer liners, two nonoverlapping hot plate sections disposed to allow corrugated board structures to pass between them whilst remaining in contact with them and two nonoverlapping suction conveyer belts disposed to withdraw corrugated board structures from between the center rolls.

11. A method for producing double wall corrugated board structures from two outer liners, two corrugating media and a center liner, comprising the steps of: forming a corrugation comprising regularly spaced first fluted members with first flute crests on a first corrugating medium by passing it between first and second interlocked corrugating rolls; forming a corrugation comprising regularly spaced second fluted members with second flute crests on a second corrugating medium by passing it between third and fourth interlocked corrugating rolls, the spacing between each of said first fluted members being an integral multiple of the spacing between each of said second fluted members; bonding a center liner to the crests of said first and second corrugating media so that each of the first flute crests is aligned with one of the second flute crests, with the center liner separating between them; and bonding outer liners to the crests of the flutes of the first and second corrugating media.

12. A double wall corrugated board structure comprising two outer liners, a first corrugating medium having a fluted surface comprising regularly spaced first fluted members with first flute crests, a second corrugating medium having a fluted surface comprising regularly spaced second fluted members with second flute crests, a center liner bonded to the crests of said first and second corrugating media so that each of the first flute crests is aligned with one of the second flute crests, with the center liner separating between them; said double wall corrugated board structure characterized in that: the outer liners are totally flat, without any indentations; and the spacing between each of said first fluted members is an integral multiple of the spacing between each of said second fluted members.