CA1161618A

CA1161618A - Method of forming a net-like structure

Info

Publication number: CA1161618A
Application number: CA000398744A
Authority: CA
Inventors: Rudolf Parnigoni
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-03-18
Filing date: 1982-03-18
Publication date: 1984-02-07

Abstract

ABSTRACT OF THE DISCLOSURE

A method is disclosed for forming a net-like structure from a suitable material such as metal or a synthetic polymer.
The structure avoids the problems of low tear strength at the joints in the mesh and enables novel shapes and designs to be formed that are not possible to make by other methods. The method comprises the steps of forming a planar web from a suitable material in a plastic state, the forming providing for the planar web to have a substantially flat parting plane, the planar web having an expandible pattern of strands and strand joints with apertures therebetween, and expanding the planar web by forcing the exterior and interior of the web apart in a direction generally perpendicular to the planar web, to form a three dimensional net-like structure.

Description

This invention relates to a method of for]ni]lg a three dimensional net`-like structure from a material such as a suit-able synthetic polymer or a suitable malleable metal. The synthetic polymers include thermoplastic ancl thermosetting resins and compounds of the resins with or without additives provided they may be formed in a plastic state.
Net-like structures are used for baskets, sacks 7 cages and the like. In some cases they are made from an expanded mesh, wherein slits are cut or stamped in a plastic or metal 1~ sheet, which is then expanded to form a mesh. Such methods are limited by available sheet stock and have additional pro-duction costs of handling the sheets in the storage and cutting rooms, plus the costsof die cutting. The products of such methods are closely related to the paper cutting art o origami but share the inherent problems of low tear strength at the corners of the slits when the pattern is expanded. Further-more, cutting or stamping out a pattern in a flat disc does not allow one to shape the strands nor to design the strand joints for expansion.
; 20 One method of making a net-like structure, such as a basket, from a synthetic polymer material, is to use thre~
dimensional moulds or dies and injection mould or cast the basket.
However, three dimensional moulds or dies are very costly to manu-facture because they have complex parting planes, and these types of moulds or dies have a considerable mass of material which has to be moved backwards and forwards at every forming stroke. Hence the energy requirements for such a process are high. There are also a number of limitations in the depth of the basket and in the shape of ~he strands and the pattern of strands and strand joints to allow the moulds or the dies to part, otherwise the basket canno~ be released.
I have found that by forming a planar web from a ~ 61 ~
suitable material in the plastic state 9 I can provide an expandible pat~ern o-f strands and strand joints with apertures therebetween, wherein the strand joints are designed for maximum strength after expansion, and the shape of the strands can be selected to suit decorative or strength requirements. This process allows me to produce a net-like structure which cannot be made even by three dimensional moulding, or any other type of moulding, nor can they be made by cutting or stamping out of a flat disc of material. My three dimensional net-like structures can be formed to shapes such as cones, cylinders, spheres 7 tori, cornucopia, tubes, prisms, pyramids, vaulted structures, double walled structures, etc.
Another advantage of my method of forming a planar web is, I can ensure that strand joints are thicker at their center, having a larger cross section and thus more material than the strands. This allows expansion of the planar web to the three dimensional shape with the strands set, but the center of the strand joints still in plastic state, thus the final heat setting of the strand joints occurs with the web in the expanded configuration. This allows the web to be heat se~ in the expanded configuration. I can also make flexible hlnges at the strand joints which allows the three dimensional net-like structure to be collapsed to the planar web form for ease of shipping when empty.
The expandible pattern can be made in a number of different designs depending upon the re~uirements. Patterns may be in the form of spiral strands, concentric circles or hoops, vaulting design, concentric perimeters with parallel lines, round, oval or polygonal in shape.
The pattern may be formed so that apertures at the exterior of the web are greater than those at the interior, and changes may be made in the pattern from the interior to the _ ~ _ exterior o the web as desired. Some patterns have solid hoops and may be expa~ded ~nly in a tapered configuration, increasing in size from the center. Other patterns have interrupted hoops and are limited to form cylindrical articles. Yet others may be expanded to spheres, cornucopias, toris, vaulted structures or other configurations. The pattern has a number of strands which may have varying widths and thicknesses along the length of the strandA Strand joints are provided formed in a flat plane which generally have maximum strength after they have been expanded. The apertures in the pattern may be of varying shapes~
The planar web can be dimensioned to suit particular requirements when expanded such as loading, mesh size, basket shape, dimensions, etc. The web can be expanded into a great variety of shapes. This expanding step may be carried out cold if the material is sufficiently flexible and has cold working properties, or in the case of a synthetic polymer may be heated to a ~emperature whlch destroys the ma~erial's heat history so it retains its three dimensional shape. Uses of the three dimensional net-like structures are many and varied9 examples include, lampshades, Christmas decors, warning cones, macrame frames for handicrafts, bushel baskets, fruit baskets, vegetable nets with the top edges having drawstrings or ties, salad wrappers, nets for fishing and submarine farming, protective mesh for bottles and flasks, shipping containers, protective nets for plant roots, plant supporters, sports fishing nets, flower pot hangers, garbage baskets, laundry hampers, animal traps and cages, and carrying nets.
The present invention provides a method of forming a three dimensional net-like structure comprising the steps of, forming a planar web from a suitable material in a plastic state, the forming providing for the planar web to have a substantially flat parting plane, the planar web having an ~ 6~ ~

expandible pattern of strands and strand joints with apertures therebetween, and expanding the planar web by forcing the exterior and interior of the web apart in a direction generally perpendicular to the planar web to form a three dimensional net-like structure.
The material may be a suitable synthetic polymer or a metal, preferably an alloy, in the molten state.
The forming of the planar web may occur under heat and pressure, and generally comprises a process such as injection moulding, compression moulding, casting, forging or calendering.
In one embodiment the planar web is formed by single gate, multiple gate or sequential injection moulding, and includes at least one cold tip ripper which breaks up the cold tips of advancing streams of the synthetic polymer in a cavity before the streams join in the cavities to prevent a cold weld.
Examples of suitable polymers include polyethylenes, high density molecular weight polyethylenes, polycarbonates, silicone rubbers, acrylics, polyvinyl chloride, nylon, polypropylene, ~ urethanes, fluoroplastics and polyesters or copolyesters. Other suitable polymers may also be used. Additives may be included with the polymers as desired. Small batches of material may be prepared ~or specific customer orders. Suitable metals include alloys of copper, lead, zinc, nickel, aluminum, gold and silver.
Certain types of steel may also be suitable.
The strand joints are preferably designed for maximum strength after expansion. These joints may be flexible hinge-type joints to allow flexing when the planar web is expanded, or they may be in the form of nodes having more material than the strands and the expanding step occurring while at least some of the material in the nodes is still in the plastic state.
In the node type strand jointl the cross section of the strand may increase towards the node.

In drawings which illustrate embodiments of the invention, Fig. 1 is a plan view showing part of one pattern for a planar web.
; Fig. 2 is an isometric view of the planar web shown in Fig. 1 expanded into a three dimensional net-like structure.
Fig. 3 is a plan view showing part of another pattern for a planar web according to the present invention.
Fig. 4 is an isometric view of an expanded strand joint of the planar web shown in Fig. 3.
Fig. 5 is an isometric view of another design of pattern in the expanded condition.
Fig. 6 is an isometric view of two strand joints from the pattern of Fig. 5.
Fig. 7 is an isometric view of a sector of a planar web having a pattern thereon according to another embodiment of the invention.
Fig. 8 is an isometric view of a strand joint from the pattern of Flg. 7 in the expanded condition.
Fig. 9 is a plan view showing another pattern of the present invention in a planar web.
Fig. 10 is a plan view of a complete planar web having the pattern shown in Fig. ~.
Fig. 11 is an elevational view showing the pattern of Fig. 9 in the expanded conditlon.
Figs. 12 and 13 are cross sectional views through a web in the planar and expanded conditions.
Figs. 14 and lS are cross sectional views through another web in the planar and expanded conditions.
Fig. 16 is an isometric view of a net-like structuTe in the form of a rectangular shaped basket.
Fig. 17 is a plan view showing part of another pattern q~

on a planar web.
Fig. 18 is 'an isometric view showing part of another pattern on a planar web having raised fins thereon.
Fig. 19 is an isometric view showing an expanded strand joint in the pattern of Fig. 18.
Fig. 20 is an isometric view of a net-like structure around a flask.
Figs. 21 - 23 are isometric views showing different strand configurations.
Figs. 24 and 25 are views of hinged-type strand joints in the planar and expanded conditions.
Figs. 26 and 27 are views of another pattern on a web in the planar and expanded conditions forming a cornucopia shape.
Fig. 28 is an isometric view of a -further pattern on a web in the expanded condition having vaults and non-expandible radial groin strips.
Fig. 29 is a plan view of a further embodiment of a pattern in a planar ~eb formed in a spiral.
Flg. 30 is a plan view of a further embodiment of a pattern in a planar web formed in a series of rings.
Fig. 31 is a plan view of a portion of a pattern in a web having a fish scale design.
~igs. 32 and 33 are isometric views showing different designs of strands and strand joints.
Figs. 34, 35 and 36 are plan views of portions of further configurations of patterns in planar webs.
Fig. 37 is a plan view of multiple planar webs joined at their exteriors.
The planar web is formed from a suitable material in the plastic state. In one embodiment the material is a synthetic polymer, preferably thermoplastic or thermosetting resin~ The majority of materials are first heated so the resins 6~

are polymerized or ~luxed to a plastic state suitable ~or ~ormin~. The ~orming is by any of the known types of plastic processes such as injection moulding or compression moulding, casting in a mould, ~orging between flat dies, or calendering between engraved calender rolls. In all cases, the parting plane to separate moulds, dies or rolls is a flat plane.
Casting the planar web in a level mould permits degassing ~herein a vacuum is applied to release entrapped gases from the material into the mould before setting or curing occurs. Ater forming into a planar web, the material must be suficiently malleable or flexible to be able to expand into the three di-mensional shape. Polyethylenes, hi~h density molecular weight polyethylenes, polycarbonates, silicone rubbers, acrylics, polyvinyl chloride, nylon, polypropylene, urethane, Eluoro-plastics, and polyesters or copolyesters are all examples of suitable materials. Although all must have sufficient p-asti-cizer present so the material is not brittle.
Metals may be die cast, for~ed, or open cast. Jewelry may be formed of silver or gold.
In one embodiment the pattern is formed in the planar web with strand joints having a greater cross section and thus more material in them than the strands. With more material in the strand joints the cooling or hardening of the material in the planar web after forming takes longer for the strand joints than or the strand. The planar web may be expanded to the ex-panded condition before the material in the strand joints has hardened. The strand joints are then allowed to cool in the expanded condition and thus they are not under stress and provide the maximum strength in the expanded condition. In another em-bodiment the planar web is expanded while all the material in both strands and strand joints is still plastic, and then allowe~
to cool or harden in the expanded condition. This provides or the heat history of recovery of a flexible plastic ~aterial to be such that it' returns to the expanded condition rather than to the planar condition. In yet a further cmbodiment ~he complete planar web is allowed to cool or harden in the planar condition, thus the web collapses to the planar condition if not supported in the expanded condition.
In the forming step the planar web as it is produced has a substantially flat parting plane, whether the forming-takes place by moulding, casting, forging or calendering.
This substantially flat parting plane reduces the costs of dies and moulds. In some cases one of the two halves of the dies or moulds may be completely fla-t, and the shape of the strands and strand joints is formed in only one half of the dies or moulds.
In another case, both halves of the dies or moulds are engraved so that the strands are formed between the dies or moulds. The flat parting plane generally passes through the pattern o~
strands and strand joints in this configuration.
Injection moulding is made easier by having one or more cold tip rippers positioned strategically in the moulds so that as the streams of material in a plastic state pass along the cavities in the mould, the hardened advancing tip of the advancing material is broken or ripped by the blade or ripper releasing the hot plastic material underneath, thus when the streams join cold welds do not occur in the s~rands or strand joints. SpeciEic compounding of batches of material is possible even in small quantities. Colour and additives can be changed from one batch to another, furthermore, special additives may be included as required. In some cases, it may be preferred that the material be soft or flexible, in another case it may be more rigid and have a high tensile streng~h. In the forming step wherein the planar web is made, the material is in a plastic state sufficient for it to be formed into an expandible pattern.

Another embodiment of the process includes casting the material in a plastic state into an open mould. The material is allowed to harden and is then removed.
The planar web is expanded either manually or by means of a mechanical pusher or similar. The exterior of the web is held and the interior is pushed in a direction substantially perpendicular to the planar web. A second forming step may be added to further contract a section of the axially expanded shape into inver~ed shapes and configurations. Contracting the exterior of a disc to a completely closed top is useful for sealing a receptac]e. The maximum extent of expansion is depend-ent upon the design of pattern. In some cases the expanding step occurs when the web is heated so that it retains the expanded condition when cooled. In some flexible materials, the :

expansion step only occurs when the net is to be filled with a product. Af~er use the net collapses to the planar conditi~n-.
In another embodiment the center of the web is held or restrained and the exterior is moved away from the center. This is particularly useful if one is fillin~ a nct-like structure.
Handles on the exterior merely have to be lifted to expand the web and contain the contents.
An example of a planar web 10 is illustrated in Fig. 1 showing the expandible pattern of strands 11 and strand joints 12 with apertures 13 therebetween. Fig. 1 illustrates the pattern in the flat or planar condition as formed. Each strand 11 is in the shape of an ogee connecting to an X-shaped strand joint 12.
The strands 11 are substantially perpendicular to acljacent strands 11 at the strand joints 12. As can be seen in Fig. 2 the expanded three dimensional net-like structure is shown practically cylindrical, although the apertures 13 at the exterior of the planar web are larger than the apertures at the interior of the web. The strands 11 become substantially straight when the web is in the expanded condition. As can be seen in Fig. 1, the apertures 13 are arranged in concentric perimeters 14 and the strand joints 12 in every other concentric perimeter 14 occur at the center of the apertures 13 in intermediate perimeters 14 forming a type of brick and mortar configuration.
Figs. 3 and 4 show another expandible pattern with thin slit like apertures 13 ln concentric perimeters 14. The aper~ures 13 have strands 11 in the form of wide ribs between the apertures.
The radial width of the apertures 13 is considerably less than the radial width of the strands 11. Furthermore~ as can be seen in Fig. 4 the radial width of the strands 11 is considerably greater than the thickness of the web, thus the strand joints 12 when in the expanded ondition are not flush g _ with the side wall of the expanded web, ~ut are at an angle to the side wall ta give a rippled effect. Figs. 5 and 6 illu-strate a different pattern design wherein the strands ll ha~e a radial width considerably less than the thickness of the web.
Strand joints 12 are X-shaped and as illustrated in Fig. 6 when in the expanded condition are generally in line with the side wall of the expanded web to produce a smooth side wallO This design has a minimum increase in mesh size towards the rim.
Figs. 7 and 8 show another configuration of an ex-pandible pattern wherein the strand joints 12 are H-shaped. The strands 11 have a radial width considerably less than the thick-ness of the web and considerably less than the radial width of the apertures 13. An interior base 20 is provided having a thickness less than the thickness of the patterned portion of the web.
A different configuration of an expandible pattern is shown in Figs. 9, 10 and 11, wherein the strand joints are in the form of nodes 30 having a greater cross section and more material than the strands. As seen in Fig. 9, long strands 31 extend from a node 30 in one concentric perimeter to nodes 30 in adjacent perimeters above and below the first node 30. Ogee strands 32 are in the form of an ogee or sinuous curve when in the planar condition and extend in a generally radial line from node to node 30 between the long strands 31. The complete pattern of the planar web is illustrated in Fig. 10, the ogee strands 32 forming a sinuous curved radial sector wi~h the long strands 31 being in substantially circumferential lines between the nodes 30. ~landles 33 are provided at the outside edges of the web for the basket or net-like structure. Fig. 11 shows the web in the expanded condition. The ogee strands 32 straighten and provide a limit to the maximum expansion that can be obtained with this pattern.

- lQ -~ 6~

Figs. 12 and 13 show an expandible pattern similar to that sho~n in Figs. 3 and 4 wherein the apertures 13 have a radial width less than the radial width of the strands 11 and the thickness of the web is considerably less than the radial width o~ the strands 11. When expanded as shown in Fig. 13 the side wall has a rippled effect. A rim 40 is illustrated at the outer edge of the web and a dish shaped base 20 is provided in the center of the web. The rim 40 and the base ~0 are considered separate from the planar web, and may not be planar, but can be dish shaped or any three dimensional shape as required.
The thickness of the web for the portion of the pattern is greater at the extremity of the web and tapers slowly inwards towards the center. Pigs. 1~ and 15 illustrate an example of an expandible pattern similar to that shown in Figs. 5 and 6. The radial width of the strands 11 is considerably less than the radial width of the apertures 13, and also less than the thickness of the web. When the web is in the expanded condition, the side wall is smooth as the strand joints 12 remain fla~. Handles 33 are provided at the exterior of the web and a flat interior base 20 is provided in the center of the web. The thickness of the web ~or the portion of the pattern is greater at the extremity of the web and tapers slowly inwards towards the center.
A basket is illustrated in Fig. 16 which has a sub-stantially rectangular base. The corners are rounded and the pattern is designed to extend around the corners. Another design pattern is illustrated in Fig. 17 wherein the strand joints 50 are in the form of rings with holes in the center thereof. The strands 11 are formed in coneentric perimeters.
Figs. 18 and 19 i]lustrate another embodiment of a planar web, the expandible pattern is somewhat similar to that shown in ~ig. 3, but includes the addition of protrusions 60 extending from both surfaces of the planar web on the strands 11 betweeII the apertures 13. The pro-trusions 60 are formed sub-stantially perpendicular to the flat parting plane. Fig. 19 illustrates the pattern in the expanded condition showing the protrusions 60 in the shape of triangular protrusions which extend across apertures 13 between the strands 11. The pro-trusions 60 in this configuration extend in line with the side wall of the net-like structure. When protrusions 60 are incor-~porated in patterns such as those shown in Figs. 5 and 6, they extend substantially perpendicular to the side wall.
Fig. 20 illustrates a bottle or flask having a protec-tive net cover surrounding the article. A single solid hoop 70 is left at the center or largest diameter o~ the bottle 71.
As can be seen the vertical distance between the strand joints 72 increases towards the top of the bottle 71 as the outside diameter is reduced. Loops 73 are provided at the top of the bottle 71 which have hook and eye connectors 74 to hold the net around the bottle 71. The material of construction may be a foamed plastic to provide a cushion for the flask or ~ottle.
Protrusions may also be provided to cushion the bottle 71. Other shapes may be formed such that after the expansion step the edge of the pattern is inverted and turned in to form an enclosed structure or a contracted enclosure. Such shapes as spheres, toris, double walled structures and hour glass structures are but a few of the many possible configurations.
Examples of the cross sectional shape of strands are illustrated in Figs. 21 - 23. The flat parting plane 80 shown in the figures represents the separating plane of the dies or moulds.
Fig. 21 provides for the dies or moulds to have indentations made on eaeh half of the die or mouldO In Fig. 22 the grooved indentations are ~ade only on one ~alf of the die or mould, and in Fig. 23 the relief indentations are made on one half of the die or mould. The shape of the strands shown in Figs. 22 and 23 may be made by open casting with no top half to the mould or 9 alternatively if injection moulding is to be used to form the web, sne half of the die has a completely flat surface. Whereas these examples illustrate the strands having a substantially uniform cross section along the length, it will be appreciated that the cross section may be varied along the length of the strand between strand joints. This choice of shape for the strands allows the net-like structure to be formed with decorative and useful shapes and configurations 7 many of them impossible to make by any known technique or with any existing equipment.
Furthermore9 the strands themselves can be finished with a smooth or textured finish as preferred for ~he particular article.
Examples of cross sectional shapes of strands include square, rectangular, triangular, diamond, oval, round, key-hole, T-shaped, V-shaped, L-shaped, U-shaped, trapezium, hexagonal, octagonal, polygonal, mushroom and semi-circular. This list is in no way limiting as almost any shape that can be formed could be used.
Figs. 24 and 25 illustrate hinge joints 90 between strands 91. The joints 90 provide thin cross section portions 92 which are thinner than the strands 91, thus as the pattern expands bending occurs at the thinner portions 92, as can be seen in Fig. 25 ~nd the strands 91 remain straight. This con-struction of strand joint 90 is referred to as a flexible hinge joint, because the flexing occurs at the hinge rather than in the strand itself~
Figs. 26 and 27 illustrate a planar web with increas-ing strand length to one side and a series of off-set rings 100 with apertures 10] which are wider on one side of the web than on the other. A center disc 102 is offset in the planar web and expansion of the planar web produces a cornucopia shape as illu-strated in Fig. 27. Such a Shape may also be made by attaching at least one solid strip or stave to a web such as that shown in Fig. 1 used às a curved cage display shape or elbow chu~e.
Fig. 28 illustrates an expanded pattern which has a series of radial solid strips or staves 110 spaced at even dis-tances apart around the circumference of the planar web, thus pro-viding a vaulted expanded pattern. Such a pattern may be -formed either by forming ~he staves 110 at the same time of moulding or casting the pattern or, alternatively they may be added after the pa~tern on the planar web has been formed. Vaulted structures ; 10 may also be made from webs having a variation of mesh size in radial rows at several radian locations. In Fig. 29 the ex-pandible pattern on the planar web is a spiral with radial con-nections 120 located in every other coil 121 of the spiral. A
first series of radial connections 120 in a radial line connects at strand joints across every other coil or winding 121, and a second series of radial connections also in a radial line is spaced between the first series of radial connections 120 and-connects across windings not joined in the first series of radial connections 120. In Fig. 30 the expandible-pattern com-prises a plurality of overlapping rings 130 with strand joints 131 at the points of overlap. The overlapping rings 130 are equispaced apart about a circle and joined at the center of the circle. This design has the same length of s~rand throughout the pattern. Special hinge joints 132 are illustrated in the top portlon of the figure to allow for flexing when the pattern is expanded. A fish-scale style of design is illustrated in Fig. 31.
Figs. 32 and 33 illustrate examples of strands and strand joints. Fig. 32 shows the strands having a radial width smaller than the thickness of the web and Fig~ 33 shows the strands having a radial width greater than the thickness of the web. Different types of protrusions, decorative designs, flex-ible joints and node type joints are all illustrated in the drawings.
Another pattern or configuration is shown in Fig. 34 wherein the limit of expansion is restricted by a radial row of diamond-shaped patterns 1~0 radially extending from the center of the web. Circular strands 141 join the diamond-shaped patterns 140 together. These dia~ond-shaped patterns 140 re-strict the expansion of the web and thus control the amount of opening that can occur. In heat set materials they become staves and such structures are no* collapsible. Fig. 35 illustrates a pattern with a series of hoops 150 concentric about a center disc 20. Ogee strands 151 are arranged in opposite sloping directions between the concentric hoops 150 and strand joints 152 are provided with the ogee strands 151 connected substantially perpendicular to the hoops 150. When the pattern is expanded the hoops 150 cannot change in diameter but extend upwards and rotate slightly so that the ogee strand 151 forms a substantially straight line between the strand joints 152 and are in line with the adjacent ogee strand 151. Such designs cannot be inverted or collapsed after being heat set in the expanded configuration.
Fig. 36 shows a further embodiment of a pattern with eyelets 160. Concentric circular strands 161 are located about a center disc 20 with ogee strands 162 between the circular strands 160 and all sloped in the same direction. Flexible hinge type strand joints 163 are positioned between the ogee strands 162 and the circular strands 161~ with the strand joints 163 located in radial rows between every other circular strand 161. In such a configuration, expansion of the circular strands 161 cause them to bend at the strand joints 163 and are pulled out of shape unlike the pattern illustrated in Fig. 35. The pattern appears as a honeycomb design in the expanded configura-tion.
Figo 37 illustrates four circular webs shown joined ~ 15 ~

together at their exteriors. Other configurations may be included; a multiple number of webs may be joined together in different configurations. The design of the pattern, design of the shape of the strands, design of the outside shape of the planar web is all a matter of selection, provided in each case the pattern made can be expanded into a three dimensional shape. `
Various changes may be made to the design of the pattern on the planar web without departing from the scope of the present invention, which is limited only by the following claims.

Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method of forming a three-dimensional net-like structure comprising the steps of forming a preform from a material in plastic state, the preform being in the form of a substantially planar web having a pattern of strands and strand joints defining openings therebetween, the planar web including a central portion and an exterior portion surround-ing the central portion; forming hinges on the strands in the area of the strand joints; and erecting the structure from the planar web by forcing the exterior and central portions of the planar web apart in a direction generally perpendicular to the plane of the planar web by bending the strands about the hinges.

2. A method as defined in claim 1, wherein the step of forming said respective hinges comprises providing a rela-tively smaller cross-section of strana material in the direction of bending at the point on the strand where the hinge is to be formed.

3. A method as defined in claim 1, wherein the step of forming said respective hinges comprises applying heat to strands in the area of the strand joints in order to render the hinges to at least a partially plastic state and erecting said structure from said web by bending said strands at said respective hinges while the hinges retain their at least partially plastic state.

4. A method as defined in claim 3, wherein the hinges are allowed to cool when the strands are bent about the hinges and the structure is erect.

5. A method as defined in claim 1, wherein the strand joints are in the form of nodes having a section greater than the combined cross-sections of a pair of joining strands, and including the steps of applying heat and forming the planar web in a plastic state, cooling the so-formed planar web to a point in time wherein the strands are in non-plastic state but the greater cross-sectioned nodes are still partially in a plastic state and erecting a three-dimensional net-like structure from the planar web at said point in time by bending the strands at the nodes and cooling the nodes completely in the three-dimensional form.

6. A method as defined in claim 1, wherein the material is a synthetic polymer.

7. A method as defined in claim 6, wherein the synthetic polymer for forming the planar web is selected from the group consisting of polyethylenes, high density molecular weight polyethylenes, polycarbonates, silicone rubbers, acrylics, polyvinylchloride, nylon, polypropylene, urethanes, fluoro-plastics and polyesters or copolyesters.

8. A method as defined in claim 6, wherein the planar web is heated to a temperature which destroys plastic memory recovery whilst erected, so that the once erected structure retains the shape of the three-dimensional net-like structure.

9. A method as defined in claim 1, wherein the material is a metal provided in a molten state.

10. A method as defined in claim 6, wherein forming the planar web occurs under heat and pressure.

11. A method as defined in claim 6, wherein forming the planar web utilizes a process selected from the group consisting of injection moulding, compression moulding and calender moulding.

12. A method as defined in claims 6 and 9, wherein forming the planar web utilizes a process selected from the group consisting of casting and forging.

13. A method as defined in claim 6, wherein the planar web is formed by injection moulding and includes at least one cold tip ripper which breaks cold tips of advancing streams of the synthetic polymer in a plastic state in a mould before the advancing streams join in the mould, thus preventing cold welds in the strands and the strand joints.

14. A method as defined in claim 9, wherein the metal for forming the planar web is selected from the group consisting of steel, copper, lead, nickel, aluminum, zinc, gold, silver and their alloys.

15. A method of forming a three-dimensional net-like structure comprising the steps of injection moulding or casting in dies or casting moulds a preform from a material in plastic state, the preform being in the form of a substan-tially planar web having a central portion and an exterior portion surrounding the central portion and a pattern of strands forming a plurality of concentric circles interrupted by apertures in the web, strand joints formed between adjacent apertures in every other concentric circle; forming hinges on the strands in the area of the strand joints and erecting a three-dimensional structure having a bottom and an open top, from the planar web by forcing the exterior and central por-tions of the planar web apart in a direction generally perpendicular to the plane of the planar web.

16. A method as defined in claim 15, wherein the expandable pattern of strands comprises a plurality of overlapping rings in the web, equally spaced apart within a circle and being joined in the center of the circle, the strand joints formed by overlapping adjacent rings and other rings.

17. A method as defined in claim 15, wherein the expand-able pattern of strands comprises a plurality of radial rows of diamond-shaped patterns extending from the center of the web, the diamond-shaped patterns being joined one to another by part circular strands defining concentric perimeters in the web.

18. A method as defined in claim 14, wherein the pattern is such that when the web is expanded it forms a cornucopia shape or vaulted shape net-like structure.

19. A method as defined in claim 15, wherein the pattern of strands is formed in a spiral configuration with strands in first radial lines connecting at strand joints across every other winding of the spiral configuration, and in second radial lines spaced between the first radial lines connecting at strand joints across the windings of the spiral configuration not joined in the first radial lines.