Welded plastic panels and method of making same

Abstract: An injection moulded panel (10) composed of two half-panels (12,14) each comprising a skin (18) having matching integral webs (20) disposed perpendicular to said skin (18) wherein the half-panels (12,14) are bonded together along the webs (20).

Description: The present invention relates to injection molded thermoplastic panels and a method of making the same. In particular, this invention relates to a panel formed from two half-panels which are injection molded in a stack mold or more preferably by tandem-molding and thereafter, as soon as practicable, are (immediately is preferable) hot plate welded or otherwise bonded together. Many products such as containers, vehicles, buildings, etc. utilise panels in their construction. Such panels, depending on the end use, are required to possess some of the following properties: bend, shear and compression strength in order to resist forces applied perpendicular to the plane of the panel; tensile, compression and shear strength and columnar buckling resistance in order to withstand forces applied parallel to the plane of the panel; stiffness; resistance to creep under long-term loading; impact resistance; ability to withstand temperature extremes and thermal shock; resistance to deterioration by moisture, chemicals, fungus, insects and ultraviolet radiation; capability for strong attachments; lightness of weight; fire resistance; thermal insulation; air and water tightness; buoyancy; smooth continuous or textured surfaces; capability of compound curvature; etc. In much of the known panel construction, the requirements noted above have traditionally been filled by wooden boards, Wooden boards, however, are limited to providing bending and columnar strength in the direction of the grain of the wood. In an effort to overcome this limitation, plywood, in which the grain direction of each ply is at 90 degrees to its adjoining plies, creates a panel which has bending and columnar strength in all directions in the plane of the panel, although the properties are greater in the grain direction of the outer plies than in the cross grain direction of the outer plies. Greater panel strength and uniformity of strength in all directions are obtained by laminating metal or plastic "skins" to a plywood core. To attain lightweight, less dense cores are used such as balsa wood on end grain, honeycomb material, cellular plastics and truss structures of metal or plastic material. In these configurations, both the core of the lamination and its attachment to the skins need to have sufficient shear strength to allow the skins to resist with their full strength when the panel is subjected to bending or edgewise compression. Another type of panel is known as the corrugated sheet. Said panels are efficient in providing bending strength and columnar strength in the direction of the corrugation, but exhibit very little of said strength in the crosswise direction. Still another type of panel is comprised of a pair of parallel sheets of material, wherein each sheet has indented depressions which are welded to corresponding depressions on the other sheet of the panel. If the depressions are in the form of parallel corrugations, thereby creating a panel which is a row of connected parallel tubes, the panel will be efficient in providing bending strength in the direction of the corrugations, but not crosswise to them. If the depressions are conical or pyramidal in form and distributed over the faces of the panels, the bending strength developed will be low for the amount of material used. Moreover, to provide bending strength in all directions, indentations of various shapes and arrangements have been utilized. However, when the panel is subjected to bending, a complex stress pattern results in which some areas of the skins are more highly stressed than others, with a resulting loss of efficiency. This type of panel does not have uninterrupted flat surfaces which are desirable in many applications. A panel is known from FR-A-2227409 consisting of two half-panels of expanded plastic material such as polystyrene bonded together. Each half-panel is formed on the inside with transversal and longitudinal ribs by means of which the two halves are connected together. It is an object of the present invention to provide a panel that has bending strength about all axes parallel to the plane of the panel and also about axes perpendicular to the plane of the panel. It is another object of the present invention to provide a panel that has compression and tensile strength in all directions in its plane and also perpendicular to its plane. According to the present invention there is provided a panel, preferably a thermoplastics panel, comprising two distinct panel portions each comprising a skin and integrally moulded supporting webs extending generally perpendicularly to the skin from a first of two surfaces thereof, the spacing of the webs being substantially greater than the thickness of the webs, and the webs being arranged in two groups with the webs in each group being uniformly spaced apart and parallel to each other and perpendicular to the webs in the other group to form a regular repeating geometric pattern of uniform rectangular shape defining a bonding surface spaced from said first surface of the skin by the extent of the webs with the panel portions being bonded at the bonding surface of each panel portion characterised in that the skin and webs of each panel portion are of the same thickness and each panel portion has gussets formed integrally with the skin and webs. The webs run in two directions to form a rectangular pattern, preferably squares, that is advantageous in the use of the panel. In the simplest case, two half-panels are brought together with matching webs facing each other, and are bonded along the matching surfaces of the webs. The bonding may be made by any means that results in a joint having shear and tensile strength that is adequate to enable the panel to resist the forces that it must support. The above and other objects and advantages of the present invention and comparisons with the prior art are considered in more detail with reference to the accompanying drawings in which: FIGURE 1 is a top plan view of a section of a panel not in accordance with the present invention; FIGURE 1A is a section view taken along the line 1-1 of Figure 1; FIGURE 2 is a top plan view of a half-panel embodying the invention; FIGURE 2A is a section view taken along the line 2-2 of Figure 2; FIGURE 3 is a top plan view of a section of a panel similar to Figure 1 having attachment bosses integrally molded into each half-panel at the junction of each web of said half-panel. FIGURE 3A is a section view taken along the line 3-3 of Figure 3; FIGURE 4 is a top plan view of a section of a panel similar to Figure 1 having attachment bosses integrally molded into the webs of each half-panel; and FIGURE 4A is a section view taken along the line 4-4 of Figure 4. With reference to Figures 1 and 1A, a panel construction not according to the present invention is shown at 10. As depicted, the panel is made up of an upper half-panel 12 and a lower half-panel 14 which are bonded together at a mid-plane location 16 by known means such as welding or adhesives. As shown in Figure 1A, each half-panel 12 and 14 comprises an outer surface or skin 18 and a plurality of webs or ribs 20 disposed perpendicular to the inner surface of the skin 18 and molded integrally therewith. The half-panels 12 and 14 are joined together at a plane 16 disposed between the skins 18 via the webs 20. With reference to the panel construction of Figure 1, a panel strip 76.2 cm (30 inches) long, 6.4 cm (2.5 inches) wide and 2.5 cm (1.0 inch) overall thickness, having skins 0.25 cm (0.10 inches) thick and webs 0.25 cm (0.10 inches) thick running longitudinally and transversely on 5.1 cm (2 inch) centers, was computer-simulated using Ideas® finite element software provided by the Structural Dynamics Research Corporation. Said panel was supported at each end so as to provide a free span of 76.2 cm (30 inches). A 22.5 Kg (50 pound) load was applied over a 10.2 cm (4 inch) long area spanning the center and resulting panel stresses were calculated. The lower skin of the panel strip was in tension and displayed a peak value of about 101 Kg/cm 2 (1440 Ib./in. 2 ) at the center of the panel. The upper skin was in compression with a peak value of about 114 Kg/cm 2 (1620 Ib./in. 2 ) at the center of the panel. The shear stress in the web areas in the mid-plane of the panel was calculated to be less than about 10 Kg/cm 2 (144 Ib./in. 2 ). As demonstrated, the highest stresses are in the panel skins and in the areas of the webs that are adjacent to the panel skins. Since the webs are integrally molded to the skins, the full strength of the material is available in these areas. As the weld areas of the panels are not highly stressed, the bending and columnar strength of the panels do not require the weld areas to be as strong as the parent material. For panels made of thermoplastic materials, mid-plane welds of adhesive bonds can be made whose strength in tension, compression and shear approximates that of the parent material. Some of the joints obtained, however, are not as capable of flexure as the materials being bonded. The joints at the mid-plane of the panel shown in Figures 1 and 1A are chiefly loaded in shear. They are not subjected to flexure during bending or columnar compression of the panel. Also, the bonded areas, being at mid-plane, are protected from flexure during impacts on the outer surface of the panel. Only under a severe overload, such as an impact perpendicular to the face of the panel sufficient to buckle a web, would a joint area to be subjected to flexure. The mid-plane joint location of the panel of Figures 1 and 1A is advantageous over panel constructions in which skins are welded or adhered to a lightweight core. In those structure, the joined areas, being just under the skin, are susceptible to damage from external impacts which may cause delamination of the panel and failure of the structure. For the welded panel loaded as described above to achieve its full potential bending strength, the skins on the upper side which are under compressive stress must not buckle before the compression yield strength is reached on that side or before the tensile yield strength is reached on the other side of the panel. To avoid buckling, the ratio of the web spacing to the skin thickness must not be too great. For example, in the case of a panel having equally-spaced ribs or webs running in the X and in the Y directions as shown in Figures 1 and 1A, the skin is divided into small squares and the stress In the skin at which buckling would start may be approximated by the following equation: σcr = [ 8.6 ] E 1 - V2 h b 2 where σcr = internal stress at which buckling begins, Kg/cm 2 (lbs./in 2 ) E = Young's Modulus for the skin material, Kg/cm 2 (lbs./in 2 ) V = Poisson's ratio for the skin material h = thickness of skin, cm (in). b = length of the side of an elemental skin square, cm (in). For the panel construction shown in Figures 1 and 1A, made in high-density polyethylene having a value of E of 12,680 Kg/cm 2 (180,000 lbs./in 2 ), a yield strength of 253 Kg/cm 2 (3600 the./in 2 ) and where V=0.45, h=0.25 cm (0.10 in) and b=4.8 cm (1.90 in), σcr will equal 380 Kg/cm 2 (5400 lb./in 2 ). Since σcr in this case considerably exceeds the yield strength of the material, when such a panel is subjected to bending, the compression side would not be expected to buckle and the tension side would be expected to resist up to the material's yield stress. As shown in Figures 3, 3A, 4 and 4A, attachment bosses 22 may be provided in a regular pattern or almost any required pattern. In Figures 3 and 3A, said bosses 22 are located at the junction of each web 20 of each half-panel 12 and 14, while in Figures 4 and 4A, said bosses 22 are located in the webs 20 of each half-panel 12 and 14. The bosses 22 which are integrally molded with the skin 18 and the webs 20 of each half-panel 12 and 14 may be cored for self-tapping or thread forming screws. As shown, the bosses 22 and each half-panel 12 and 14 may be in alignment and, therefore, bonded together concurrently with the webs. When an attachment is to be made, the skin 18 over one or more of the bosses 22 may be drilled through and attachment made by screwing and the like with or without the use of adhesive. Additionally, the bosses 22 may be provided with cored holes of a diameter suitable for receiving threaded inserts which are ultrasonically driven. When bosses 22 are located at the junction of webs 20 as shown in Figure 3, a tensile force applied perpendicular to the plane of the panel 10 at a boss 22 is transmitted by shear to the webs 20 which form, with the adjacent skins 18, a beam structure that resists the bending movements caused by the force. A force applied parallel to the plane of the panel 10 at a boss 22 is resisted by compression, shear and tensile forces in the skins 18 and the webs 20. While attachment bosses may also be located in the skins 18, but not the webs 20, of the panels 10, loads perpendicular to the plane of the panel applied to bosses in the skins cause local bending and displacement of the skin which is generally undesirable. In order to avoid buckling of the welded panels above-described in long-span bending, the thickness of the webs or ribs 20 could be increased, their spacing could be reduced and/or the thickness of the skin could be increased. As an alternative to any such measures which significantly increase the amount of material used, it has been found according to the present invention that the addition of gussets to the panel-halves, such as shown by way of example at 24 in Figures 2 and 2A, is an efficient way to increase buckling strength while adding little additional material. The gussets 24, which may be integrally molded with the panel-halves, are typically centered between the ribs 20, have a height of about 1/2 the height of said ribs 20 and a length of about 1/3 the distance between ribs 20. The panels of the present invention, i.e. molded and welded half-panels, may be produced from a number of materials such as polyethylene, polypropylene, acrylonitrile-butadiene-styrene (ABS), polycarbonate, acrylic, etc. Such materials are used in a unique manufacturing process which combines three known technologies in a new and novel way. Hot plate weldable materials are the preferred materials. The manufacture of the plastic panels of the present invention produce a number of problems which must be minimised in order to achieve maximum strength and stiffness in a panel structure. Said problems primarily stem from shrinkage of the half-panels during welding. It is known that post-molding shrinkage of engineering plastics is on the order of 0.5%. For polyolefins, it is of the order on 1.0%. Said shrinkage is made up of three components: (1) Thermal shrinkage; (2) Phase change of the plastic transitioning from liquid to solid; and (3) Molecular rearrangement. While the first two areas of shrinkage happen simultaneously as a part cools, the third area, i.e. molecular rearrangement, may take hours to several days. It is also known that shrinkage is a function of the following factors: (1) Material makeup (2) Processing temperatures/history (3) Packing pressure and time (4) In mold cooling time (5) Out-of-mold cooling condition (6) Internal stresses due to (i) section thickness variations, (ii) plastic flow variations in the mold, and (iii) packing pressure in the mold. In order to minimise the shrinkage problems noted above and produce panels as described in this disclosure, it has been found that it is preferable to mold two matching half-panels sequentially in an injection molding machine designed for "tandem molding". Tandem molding machines are different than standard injection molding machines in that they employ a center platen disposed between the normal stationary and moving platens. Two standard molds are mounted on each side of the center platen and, during operation, the two molds are opened alternatively for part removal. The injection unit is displaced towards the rear of the machine and injects each mold alternatively, then, the two molding cycles are offset. Tandem moulding machines of the type described are available from Husky Injection Molding Systems, Ltd., Bolton, Ontario, Canada. Another technique which may be used to reduce internal stress in the panel-halves and thus reduce asymmetric shrinkage and warpage involves gas-assist molding. Gas-assist molding realises lower residual stress in the molded panels by greatly reducing the pressures required for packing and filling. In addition to reducing the stress, the pressures permitted by gas-assist reduce the machine tonnage required by a factor of 4. This yields a much more capital-efficient process than would otherwise be possible. The panel-halves which are used to form the panels of the present invention may be produced by the tandem molding process noted above with or without gas-assist. In molding said panel-halves using the tandem molding process, the panel-halves are but one-half cycle different in age when joined together to form a full panel. As such is the case, shrinkage after molding and just prior to joining will be held to a minimum. Further, as each half-panel is molded, its length and width may be monitored and any necessary corrections to the molding parameters may be automatically made to achieve the desired dimensions in the on-coming half-panel. Joining the parts, i.e. the half-panels, shortly after removal from the mold, using hot plate welding, allows the least possible time for differential shrinkage to occur and provides self-fixturing of the two parts during cooling to prevent warpage. Hot plate welding is the preferred method of bonding and creates immediate bonds due to its very short setting-up time and overall short-cycle time. The setting-up times (the time between when the parts first come into contact to when full strength is achieved) is typically between 10 and 20 seconds. While other injection molding methods and apparatus, such as batch molding and stack molding, may be employed to form the panels of the present invention, said methods and apparatus create problems in shrinkage control and, thus, are more complex and costly.

Claims: A panel (10), preferably a thermoplastics panel (10), comprising two distinct panel portions (12,14) each comprising a skin (18) and integrally moulded supporting webs (20) extending generally perpendicularly to the skin (18) from a first of two surfaces thereof, the spacing of the webs (20) being substantially greater than the thickness of the webs (20), and the webs (20) being arranged in two groups with the webs (20) in each group being uniformly spaced apart and parallel to each other and perpendicular to the webs (20) in the other group to form a regular repeating geometric pattern of uniform rectangular shape defining a bonding surface spaced from said first surface of the skin (18) by the extent of the webs (20), with the panel portions (12,14) being bonded at the bonding surface of each panel portion (12,14) characterised in that the skin (18) and webs (20) of each panel portion (12,14) are of the same thickness, and each panel portion (12,14) has gussets (24) formed integrally with the skin (18) and webs (20). A panel according to Claim 1 characterised in that each web (20) is aligned with a web (20) on the other of the panel portions (12,14). A panel according to Claim 1 or Claim 2 characterised in that the gussets (24) are about half as tall as each web (20) and one third as long as each web (20). A panel according to any one of the preceding Claims characterised in that the gussets (24) include a hypotenuse extending from the web (20) to said first surface of the skin (18). A panel according to any one of the preceding Claims characterised in that the gussets (24) are located in a central location along each web (20). A panel according to any one of the preceding Claims characterised in that each gusset (24) extends towards a centre area of each web defined geometric pattern. A panel according to any one of the preceding Claims characterised in that the webs (20) define a regular repeating geometric pattern of square shape. A panel according to any one of the preceding Claims characterised in that each panel portion (12, 14) includes at least one boss (22) for receiving a projection from or through the other panel portion (12,14). A panel according to Claim 8 characterised in that a boss (22) is provided at the corners of each web defined geometric pattern. A panel according to Claim 8 characterised in that a boss (22) is provided between the corners of each web defined geometric pattern. A panel according to any one of the preceding Claims characterised in that the panel portions (12,14) are bonded together by hot plate bonding or by adhesive bonding or by friction welding. A panel according to any one of the preceding Claims characterised in that each panel portion (12,14) is comprised of hot plate weldable, or adhesive, or friction weldable material. A panel according to any one of the preceding Claims characterised in that each panel portion (12,14) is composed of polyethylene or polypropylene or acrylonitrile-butadiene-styrene or polycarbonate or acrylic or metal.

Inventors: GAYDOS MARK C, HARDIGG JAMES S, WELLS ROBERT L, GAYDOS, MARK C., HARDIGG, JAMES S., WELLS, ROBERT L.
Applicants: HARDIGG IND INC, HARDIGG INDUSTRIES, INC.
Application year: 1996
Main classification: B29D24
IPC codes: B32B3, E04C2, B29L31, B29K23, B29K55, B29C65, B29D24, B29C45, B29K33, B29K69