CORRUGATED THERMOPLASTIC BLANK FOR A CONTAINER

Abstract: This invention relates to a corrugated thermoplastic blank for a container. The blank (20) has a base panel (24) with wall panels (26, 28, 30, 32) connected to the base panel (24) by fold lines (38). The fold lines (38) comprise weld lines which destroy the original memory of the thermoplastic material. The free edges of the blank are sealed by seals (38) which prevent water from entering the blank. The sealed free edges and/or the weld lines or line (38) surround and reinforce each panel (26, 28, 30, 32) and increase the strength of each panel and thereby the strength of a container erected from the blank.

Description: CORRUGATED THERMOPIASTIC BLANK FOR A CONTAINER This invention relates to a corrugated thermoplastic blank for producing a container. Corrugated thermoplastic sheet has proved to be unsuitable for the construction of boxes especially where relatively heavy loads are to be carried in such boxes. These relatively heavy loads may comprise fish or vegetables which weigh in the order of 4 to 15 kilograms. Boxes constructed from the corrugated thermoplastic sheet have been too weak for these loads. Such corrugated thermoplastic sheet has traditionally been 3 to 4 mm thick and has had a weight of approximately 600 grams per m 2 with a buckling load resistance index of about 100. This sheet could simply be made thicker and heavier to increase its strength. However by increasing the thickness and the weight of the sheet, more material is used and consequently it is not cost effective to increase the thickness and the weight of the sheet to increase the strength of the sheet. It is an object of this invention to provide a lightweight yet sufficiently strong blank made of corrugated thermoplastic sheet. The applicant has addressed the above objective by increasing the overall thickness of the sheet whilst using substantially the same amount of material as used in prior art sheets. The skins and flutes of this sheet are thus thinner than the prior art sheets. However such sheet is unsuitable for the construction of the required boxes since the flutes can collapse as the skins move relative to one another. It is accordingly a further object of this invention to provide a reinforced lightweight, corrugated thermoplastic blank which can be used to produce a box of sufficient strength. Corrugated thermoplastic boxes have suffered from the disadvantage that water can be trapped between the two skins of the corrugated sheet. Such water, besides adding to the weight of a box, can promote the growth of bacteria which is obviously undesirable especially in food applications. The thermoplastic material, especially polypropylene, has a memory which is not destroyed by the creation of fold lines which are created in the same manner in which fold lines are created in corrugated cardboard blanks. Thus even after a fold line has been created on a sheet of polypropylene in the traditional corrugated cardboard manner, me sheet is not easily folded along that line because it tends to return to its original unfolded planar shape because of its memory. It is important that the walls of a box erected from the blank be perpendicular to the base of the box. If the original memory of the thermoplastic sheet is not destroyed the sidewalls of the box will tend to move out of their perpendicular orientation and thus decrease the stacking strength of the box. It is accordingly a further object of this invention to provide a corrugated thermoplastic blank for a box in which water cannot become trapped witiiin the corrugated sheet and wherein me original memory of the thermoplastic material is substantially reduced or destroyed along the fold lines of the blank. According to die invention a container blank made of corrugated thermoplastic sheet having two skins spaced from one another by flutes includes a base panel connected to wall panels by fold lines each comprising a weld line welding die two skins together and substantially reducing or destroying the original memory of the thermoplastic along me length of the weld line, and wherein the free edges of the blank are sealed to prevent liquid from entering the blank with the weld lines and die sealed free edges and/or the weld lines surrounding and reinforcing each panel thereby increasing the strength of each panel of the blank and thereby the strength of a container erected from the blank. The sheet is preferably at least 5 mm thick but not tiiicker than 10 mm and may be between 6,0 mm and 7,5 mm thick. The sheet may weigh between 400 and 850 grams/m 2 . Preferably the sheet weighs between about 500 grams/m 2 and about 700 grams/m 2 . The panels of the blank preferably have a. buckling load resistance index of between 300 and 500. The blank may include a plurality of apertures in at least one of the panels witii the edges of the apertures being sealed. At least one of the panels may include at least one weld wiύiin its periphery which increases the strength of the panel. The thermoplastic is preferably polypropylene. The invention will now be described by way of a non-limiting example with reference to the accompanying drawings in which: figure 1 is an end view of part of a sheet of corrugated polypropylene sheet used for forming a blank; figure 2 is a plan view of a blank for forming a box; figure 3 is a perspective view of a box formed from the blank of figure 2; figure 4 is a cross-sectional view on line IV - IV of figure 2; figure 5 is a cross-sectional view on line V - V of figure 2; figure 6 is a cross-sectional view on line VI - VI of figure 2; and figure 7 is a cross-sectional view on line VII - VII of figure 2. Referring firstly to figure 1, an extruded sheet 8 of corrugated polypropylene consists of two skins 10 spaced from one another by flutes 12. The overall thickness of the sheet is 7 mm as indicated by reference numeral 14 and the weight of the sheet is in me order of 600 grams per m 2 . Referring now to figures 2 to 6, the sheet 8 is die cut by heat and pressure to form a blank 20 and two end sections 20.1 for a box 22. The blank 20 has a base panel 24, side panels 26, end panels 28, oblique panels 30 and flaps 32. The end sections 20.1 of the blank each include an end panel 34 and two flaps 36. When the blank 20 and the end sections 20.1 are die cut, their free edges are sealed by seals 38 as shown in figure 4. The sealed free edges prevent water from entering die blank and becoming trapped witi-in the blank. As shown in figure 5, fold lines 38 separate die panels from one another. The fold lines 38 are weld lines which weld die two skins 10 togemer. The original memory of the polypropylene is substantially destroyed along t ese weld lines. Each panel is surrounded by its sealed free edges and/or its weld line or lines which reinforce die panel and increase its strengtii especially with respect to its resistance to buckling, shear and torsional forces. Holes 40 are formed in the base panel 24. The edge of each hole is sealed by a seal 42 which connects the skins 10 together. The applicant has found that the holes with tiieir sealed free edges increase the strength of the panels with regard to resistance to buckling, shear and torsional forces acting across the flutes. The holes 40 also act as drainage holes. Internal circular welds 44 are provided on die base panel 24. These circular welds 44 weld the two skins 10 togetiier. The circular welds 44 could also be provided in the otiier panels of die blank. The applicant has found diat these circular welds strengdien die panels witii regard to buckling, shear and torsional forces acting across the flutes. These welds 44 need not be circular but may be of any suitable shape and can be arranged to dissipate stress dependent on their pattern, size and position. The blanks 20 and die end sections 20.1 are erected to form the box 22 which is shown in figure 2. The applicant has found diat a corrugated polypropylene blank with a diickness in the range of 5 mm to 10 mm with a weight in the range of 400 to 850 grams/nr provides a lightweight box which is stronger than prior art boxes for a given weight. The panels of the blank according to the invention have a buckling load resistance index of between 300 and 500. It will be appreciated that many modifications or variations of the invention are possible without departing from the spirit or scope of the invention. For example the corrugated sheet could be made from polyethylene or any other suitable olefm. The flutes may be vertical, sinusoidal, double sinusoidal or inclined relative to their skins.

Claims: CLAIMS 1. A container blank made of corrugated thermoplastic sheet having two skins spaced from one anotiier by flutes includes a base panel connected to wall panels by fold lines each comprising a weld line welding me two skins togetiier and substantially reducing or destroying die original memory of the thermoplastic along die length of the weld line, and wherein the free edges of the blank are sealed to prevent liquid from entering the blank, with me sealed free edges and/or me weld lines surrounding and reinforcing each panel thereby increasing the strength of each panel of the blank and thereby the strength of a container erected from the blank. 2. The blank of claim wherein the sheet is at least 5 mm thick. 3. The blank of claim 1 wherein the sheet is not diicker than 10 mm. 4. The blank of claim 1 wherein the sheet is between 6,0 mm and 7,5mm thick. 5. The blank of claim 1 wherein the sheet weighs between 400 and 850 grams/πr. 6. The blank of claim 1 wherein the panels of d e blank have a buckling load resistance index of between 300 and 500. 7. The blank of claim 1 including a plurality of apertures in at least one of the panels witii die edges of die apertures being sealed. 8. The blank of claim 1 wherein at least one of the panels includes at least one weld within its periphery which increases the strengdi of the panel. 9. The blank of claim 1 wherein die thermoplastic is polypropylene. 10. A container erected from die blank as claimed in claims 1 to 9.

Inventors: GROBLER RONALD, MACKINNON MALCOLM ROBIN, GROBLER, RONALD, MACKINNON, MALCOLM, ROBIN
Applicants: GROBLER RONALD, KORPAK LTD, MACKINNON MALCOLM ROBIN, KORPAK LIMITED
Application year: 1993
Main classification: B65D5
IPC codes: B29C37, B29C53, B29C67, B65D5, B65D65