CA1121258A

CA1121258A - Sandwich construction

Info

Publication number: CA1121258A
Application number: CA000306788A
Authority: CA
Inventors: Eric K. Tornow; Per Tornow
Original assignee: COPENCRAFT AS
Current assignee: COPENCRAFT AS
Priority date: 1977-07-06
Filing date: 1978-07-05
Publication date: 1982-04-06
Also published as: DK142403B; NZ187770A; IT7825408A0; FR2396647B1; FI782158A; CH625465A5; GB2000725B; DK142403C; DE2828734A1; GB2000725A; NO782348L; ES244599U; ES244599Y; JPS5433577A; NL7807253A; FR2396647A1; SE7807569L; IT1097504B; AU3782778A; DK304077A

Abstract

Abstract of the Disclosure:
A sandwich structure is described, wherein a core of foamed plastic is enclosed between surface layers of a hard, wear-re-sistant material. The core material consists of small units of a foamed plastic, each unit being totally enclosed by an inte-gral, moisture tight plastic skin. The units are in the form of flat blocks, and when placed edge against edge, they form a sub-stantially unbroken surface. The units may be fibre-reinforced.

Description

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The invention relates to a sandwich structure of the kind `-having the form of a laminate wich a core of foamed plastic material enclosed between surface-layers of a hard material resistant to pressure, tension, and wear.
Sandwich structures are used in several constructional fields, e.g. for the production of hulls of smaller and larger boats, for bulkheads in boats, in the production of containers, open as well as closed, and in the car industry for the pro-duction of platforms and truck bodies, e.g. for refrigerated vans, and generally for all purposes where the lightness, strength, particularly the large bending strength, and insulat-ing properties of the material is advantageous.
Thus, sandwich structures are known, in which the core ma- `~
terial consists of cross-cut balsa wood,`i.e. wood~which has been cut so that the fibers are substantially perpendlcular to;~ ~
the surfaces. The wood has been cut to form small flat blocks `
which are placed edge against edge in the structure, where they may be temporarily kept together by adhesively applied threads or web. Making up the core from such small blocks makes it pos-sible to produce more or less curved sandwich structures.
However, balsa wood is comparatively costlyj and the avail-ability thereof is limited, and~there~is thus a need for~a cheaper and more available core material.~As such, various types of foamed plastics have been used where, after foaming~
and curing the plastic in large blocks, the blocks are sliced, and the slices are possibly further cut into small blocks.
In several respects, however, this is inexpedient. Above~
all this relates to the compression strength, since this may vary up to 50~ along the surface of a slice cut from such a larger block. Moreover, the surface will have numerous pits originating from cut cells, which partly causes a reduction : : ~ ~ : -:: : : : :
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of the compression strength, and partly results in an addition-al consumption of adhesive in building-in of the material in the sandwich structure, since all of the small pits have to be filled with adhesive in order to get a proper bonding to the surface layers.
The object of the invention is to redress the said disad-vantages in the use of foamed plastic as core material, and this is attained by a core material as described in claim 1.
It is very surprising that the use of such core material, usually called integral foam, has not formerly been proposed, since it is advantageous in many ways in preference to the hitherto used blocks produced by cutting larger blocks.
Thus, the integral foam has a foamed interior surrounded by a hard and tight surface layer, and because the small blocks making up the core material are cast and foamed in a great number at a time, but each in its own mould, you get blocks with a hard and tight surface on all sides, which means that each block represents a kind of box-structure with good cor-ner and side rigidity. Besides contributing to a greater strength of the sandwich structure as a whole, the kight and hard surfaces result in a cushioning over the surface and in-to the block of local compressive actions, so that the struc-ture is not so easily damaged by such actions. Also the tight surfaces involve a minimal consumption of adhesive in khe building up of the sandwich structure. Finally, khe tight surfaces counteract or prevent accumulation of moiskure in the blocks, and even if khe surfaces of some few blocks should `
be damaged, so khat moisture can penetrate, such moisture can-not spread to the undamaged adjacent blocks.
In order to simplify and facilikate the production, the core material units in an appropriate embodiment of the sub-ject of the invention are in the form of hexagonal parallele-pipeds. They may, for example, be of khe dimensions 30 x 30 x 10 mm, the sides being ak righk angles to one another.
In anokher embodiment according to the inven-tion, two opposite sides have the form of parallelograms, by which is atkained that the individual block funckions as a ~kind of brace in khe sandwich skructure.

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According to the invention, it can further be expedient to have the core material units reinforced. This increases the strength and rigidity to a substantial degree, which is of special importance in sandwich structures of a thickness from 30 mm and upwards. The sturdiness and strength of a sandwich structure is namely not only a q~estion of being able to resist actions perpendicular to the surface thereof, but also of being able to resist actions parallel to the surface, that is to say that the core material should be re-sistant to such actions.
By way of example, the reinforcement can consist of fi-bres, particularly glass fibres, which are incorporated in the plastic prior to the foaming thereof, but also other kinds of reinforcements, such as paper,textiles,or plastic films in sheets or strips, can be built-in by pIacing the reinforcing material in the moulds, in which the foaming of ~ "
the plastic is to be carried out. The reinforcing material can also be continuing over partitions between the moulds, thus serving to lock together a number of units to make larger sheets.
Particularly in the production of larger sandwich struc-tures, it will often be expedient to have the core material units thus locked together or connected to a sheetlike pro-duct in other manner, e.g. by being attached to a plastic film, a supporting web, or supporting threads. Such attach-ment is easy to carry out in connection with the production of the units, and the resulting product is easily handled and can be rolled up and transported to the field of appli-cation, where it can also be built in as so large plates or sAeets as is practical at a time. The sandwich structures are namely mostly built up in situ, whereby first one surface layer, reinforced with fibres or a web, is built up in a -mould, then the core material is adhesively joined to said first surface layer in suitably great pieces at a time, and finally the other surface layer is applied.
It is of the greatest importance to the strength of the sandwich structure that no air is captured between the core material units and the surface layer during the glueing of ' ~12~L~5~

the former to the latter, whether the gluing is performed with a separate adhesive, or is performed simply by pressing the units into a surface layer, before the latter has been cured.
When using balsa blocks as core material this is no great problem, because of the balsa wood being porous to some ex-tent, which together with capillary action upon the adhesive causes the air to disappear and the adhesive to penetrate with the result that such blocks almost fasten themselves.
In the present core material units, the same possibili-ties do not exist, and when obtainment of the greatest pos-sible strength is aimed at, special precautions have to be taken to avoid the said inclusion of air. In an embodiment of the subject of the present invention this is attained by providing ventilating openings across the layer of core ma-terial units.
Thus, according to the invention, the units can have a central opening crossing the largest surfaces of the unit.
In the production, such openings can be brought about in known manner, using retractable core pins, but this makes the moulding apparatus fairly complicated. This can be avoided by placing the openings at the corners of the core material ~;
units when, according to the invention, the corners of a unit are bevelled. Thus, a throughgoing aperture will be formed where the corners of four units placed in a quadrangle meet one another, whether only one corner or all four corners are bevelled. Whethe~ the aperture has a cylindrical or an angu-lar surface is of no importance for the ~untion as an escape passage for air trapped behind the units.
Providing such apertures ma~es it also possible to fasten the units by injection moulding, in that adhesive is simply injected under pressure through some of the apertures. The in-jected adhesive will then chase possibly trapped air before it to other apertures or fissures between the units, where the air can escape. Also the adhesive can emerge at the same places to provide a control of the penetration of the adhe-sive which can be used to determine the number and distribu-tion of the injection sites.
In order to facilitate the laterial distribution of the adhesive, the units can also, according to the invention, have . .. . ...

gully-shaped recesses in either one or both of their largest surfaces. Further, by placing one such unit on top of another, a thicker unit with apertures lengthwise and crosswise in the core can be formed, whereby all interspaces are more easily filled for attaining great strenght of the structure. Possib-ly, filling of the interspaces may be supported by creating a vacuum therein.
If the sandwich structure is to be curved, as for example in parts of a boat's hull, the narrow sides of the individual core material units according to the invention can be formed as respectively convex and concave cylinder surfaces with the same radius of curvature. This results in that the units can be laid down along a curved surface without wedge-shaped fis-sures being formed between neighbouring units, because a con-vex side fits into and turns in a concave side. Thus, the core structure will become more compact, and the consumption of adhesive will be less, since the fissures do not expand in the curving.
Each unit can have two convex and two concave sides, or every second unit can have only convex sides, and the others only concave sides.
An embodiment of a sandwich structure according to the invention is illustrated by the accompanying drawings, in which Fig. 1 is a cross-sectional view of ths structure, Fig. 2 shows some of the core material units of the struc-ture, affixed to a wide-meshed web, Fig. 3 shows a core material unit with two inclined sides, Fig. 4 shows a core material unit with two convex sides `
and two concave sides, Fig. S shows a special embodiment of a core material unit, and Fig. 6 shows an arcuate sandwich structure.
As it will appear from the drawings, the sandwiah struc-ture consists of a core material composed of small parallele- -~
pipedic units 5 and 6 of a foamed plastic material, e.g. poly- -~
urethane of the socalled integral-skin type, where the foamed plastic has a firmly adhering, hard and tight surface on all sides. The production of a foamed plastic of this type belongs ~`

Z5~3 to the known technique and is, therefore, not described here.
The core material units meet one another edge against edge in a plate-like structure with surface layers 7 and 8, which may be similar or different to the two sides.
The surface layers can be prefabricated sheets of any suitable materiale, for example metal, plastic or plywood, which is glued to the core material units by means of a suit-able adhesive layer 9.
The surface layers can also be produced by a selfcuring plastic which, if desired, can be reinforced with fibres 10, for example glass wool fibres, being sprayed directly onto a moulding surface, and the core material units can be pressed into the plastic before it is fully cured. The core material units can also have reinforcement, e.g. be fibre-reinforced, as suggested in the unit 6.
The construction of the sandwich structures is greatly facilitated when the small core material units 5 (or 6) are fastened to a carrier material, for example a coarse woven material 11, as shown in Fig. 2. In this way,a large number of core material units at a time can be put where they be-long, and also transport and storing of the units is facili-tated, since the woven material with the fastened units is easily rolled or folded together.
The fastening can be performed, for example, by gluing the units to the woven material, and by a suitable choice of ~-the adhesive, the fastening to the web can be made temporary, e.g. when the web is intended to be torn off after the core material units have been permanently fastened to one of the surface layers 7 and 8 of the sandwich structure, or the web may serve as a reinforcement of the structure.
As shown in Fig. 3, the units can also be made as paral-lellepipedes which are no~ right-angled in one direction. In this way, a sort of bracing of the sandwich structure is at-tained, and also that the fissures be-tween the individual units do not open excessively in curved sandwich structures, since instead a mutual displacement of neighbouring units takes place.
As shown in Fig 4, a unit can also be of such shape that .. . . , . . i one of opposite sides forms a concave cylindrical surface 12 and the other a correspondingly convex cylindrical surface 13, allowing for the units to abut tightly also in curved structures. This is advantageous in respect of strength, since it makes the sandwich structure more compact, and at the same time adhesive is saved, since the fissures between the units do not open by laying out the units along an ar-cuated surface.
Another embodiment of a unit is shown in Fig. 5. Here, the corners 14 of the unit 5 are cylindrically recessed to form a through-going opening when four units are placed in a quadrangle.
Fig. 5 also illustrates that a central opening 15 can be provided across the unit 5 with gully-shaped recesses 16 extending from the opening 15 to the borders of the unit.
This embodiment makes it possible for the units to be fastened by injection of the adhesive, e.g. an adhesive in the shape of, or based upon a polyester or an epoxy compound.
The adhesive is then injected through some of the transverse openings and chases the air forward to and out through others of the openings so that a close connection is ensured between the core material units and the surface layers in the sandwich structure to give it optimal strength. During the injection of the adhesive, a vacuum can also be created to remove air pockets behind the core material units.
The transverse, gully-shaped recesses 16 can serve to facilitate the distribution of the adhesive, and to promote ;
the gluing if it is desired to give the sandwich structure a thicker core by placing more core material units atop of one another.
When thus two units are placed with the surfaces with the gully-shaped recesses 16 abutting, a thicker unit is ob- ;~
tained with distribution channels, which are parallel to the lengthwise direction of the sandwich structure and may serve for injection of an adhesive.
Fig. 6 illustrates the making of a curved sandwich struc-ture by means of core material units 5 (or 6). Here units are used having the form of re~tangular parallelepipeds with a .`.

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central venting or injection opening 15, the curved laying out creating larger spaces below and between the units so that it is important that the possibility exists of venting, possibly supported by suction (evacuation).

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A sandwich structure of laminated form with a core of foamed plastic material enclosed between surface layers of a hard wear-resistant material, the core material consisting of small units of a foamed plastic enclosed by a moisture-tight skin of the same plastic, with two opposite sides of each unit providing parallel surfaces, and the units being placed edge-to-edge in butting relationship.

2. A sandwich structure according to claim 1, in which a strength-giving fibre-reinforcement is incorporated in each unit.

3. A sandwich structure according to claim 1 or 2, in which the core material units are parallelpipeds.

4. A sandwich structure according to claim 1 or 2, in which two opposite sides of the core material units are in the form of parallelograms, whereas the other sides are rectangular.

5. A sandwich structure according to claim 1 or 2, in which the core material units are reinforced.

6. A sandwich structure according to claim 1 or 2, in which the core material units are joined together by being fastened to a plastic film, to paper in the form of sheets or strips, to a supporting web, or to supporting threads.

7. A sandwich structure according to claim l or 2, in which the core material units are made so as to have convex or concave sides or so that each unit has two convex sides and two concave sides.

8. A sandwich structure according to claim 1, in which the core material units have bevelled corners to form venting openings through the core material layer, or each unit has a central venting opening, or the units have both kinds of openings.

9. A sandwich structure according to claim 8, in which gully-shaped recesses are present in one or both of the surfaces of a core material unit which is to be joined with sur-face layers or other layers in the sandwich structure.
10. An improved sandwich structure comprising a core of small units in the form of rigid plastic foam blocks, enclosed by a moisture tight skin of the same plastic, each of said blocks being arranged in edge-to-edge relationship between surface lay-ers of a hard wear-resistant material, each of the core blocks being an integral polyurethane foam.
11. A sandwich structure as set forth in claim 10, wherein two opposite sides of each core block are in the form of parallo-grams, the other sides of the core blocks being rectangular.
12. A sandwich structure as set forth in claim 10 or 11, including a sheet-like backing, means for fastening each of said core blocks to said backing such that said core blocks are joined together to form a plate-like product.
13. A sandwich structure as set forth in claim 10, including a sheet-like backing, means for fastening each of said core blocks to said backing and wherein said core blocks include at least one venting opening means extending through the core material layer for venting the area between the backing and the applied core block.
14. A sandwich structure as set forth in claim 13, wherein said venting opening comprises a bevel surface in at least one corner of a block forming a through-going aperture between adjacent blocks.
15. A sandwich structure as set forth in claim 13, wherein said means for fastening the blocks includes adhesive and each block further includes gulley-shaped recesses in the surface of at least one of said opposite sides of the core block for facilitating distrubution of the adhesive.
16. A sandwich structure as set forth in claim 15, which gully-shaped recesses are present in one or both of the surfaces of a core material unit which is to be joined with sur-face layers or other layers in the sandwich structure.

10. An improved sandwich structure comprising a core of small units in the form of rigid plastic foam blocks, enclosed by a moisture tight skin of the same plastic, each of said blocks being arranged in edge-to-edge relationship between surface lay-ers of a hard wear-resistant material, each of the core blocks being an integral polyurethane foam.

11. A sandwich structure as set forth in claim 10, wherein two opposite sides of each core block are in the form of parallo-grams, the other sides of the core blocks being rectangular.

12. A sandwich structure as set forth in claim 10 or 11, including a sheet-like backing, means for fastening each of said core blocks to said backing such that said core blocks are joined together to form a plate-like product.

13. A sandwich structure as set forth in claim IO, including a sheet-like backing, means for fastening each of said core blocks to said backing and wherein said core blocks include at least one venting opening means extending through the core material layer for venting the area between the backing and the applied core block.

14. A sandwich structure as set forth in claim 13, wherein said venting opening comprises a bevel surface in at least one corner of a block forming a through-going aperture between adjacent blocks.

15. A sandwich structure as set forth in claim 13, wherein said means for fastening the blocks includes adhesive and each block further includes gulley-shaped recesses in the surface of at least one of said opposite sides of the core block for facilitating distrubution of the adhesive.

16. A sandwich structure as set forth in claim 15, wherein said gulley-shaped recesses are arranged to extend across the surface of the block adjacent the backing to opposite borders of the block and said venting opening comprises an aperture intersecting with said gulley-shaped recesses.

17. A sandwich structure as set forth in claim 16, wherein said gulley-shaped recesses extend centrally along both longitudinal and lateral axes of said blocks and said aperture is centrally located.