Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Utility Patent
1999-07-19
2001-01-02
Loney, Donald (Department: 1772)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S177120, C264S209100, C156S244130
Utility Patent
active
06168739
ABSTRACT:
The present invention relates to an improved extruded plastics sheet material having voids therein, having particularly useful application to the packaging of food products, especially fresh fruit and vegetables.
PCT patent Specification No WO 91/16243 describes such an extruded sheet material comprising a pair of opposed faces between which run a series of elongate voids arranged such that there are two series of interconnections between the two faces, these two series joining one another where they meet, to form a “criss-cross” structure of interconnecting beams. Other shapes of voids are also described, including circular and oval. The voids reduce the weight of the sheet whilst the interconnections enhance the strength against both compressive and buckling forces. An “I” beam structure is also commonly used, in which the opposed walls are separated by lateral support members.
This type of sheet is generally extruded through a suitably shaped die into the space between upper and lower plates. In order to cool the extruded sheet, the plates (known as a “calibrator”) are water cooled and air is also blown through air nozzles in the die, into the elongate voids, although the latter has a lesser effect due to the air being heated during its passage through the voids. In addition, to sustain the sheet during cooling, a vacuum is applied to the plates.
Any attempts to increase the strength of the sheet (which necessitates increase in the sheet weight and/or thickness) result in longer cooling times, hence slower overall production.
Strength tests show that the traditional “I” beam structure gives the best resistance to vertical compressive force applied at the point where a lateral support contacts the sheet face, but that resistance to buckling demonstrated by the “criss-cross” structure is about three times greater than the traditional “I” beam structure (as determined by the Euler Load Ratio).
The present invention therefore seeks to provide an improve structure of voids, which maximises mechanical performance, minimises sheet weight and is cooled faster during the extrusion process.
According to the present invention there is provided a rigid or semi-rigid plastics sheet material comprising a pair of opposed faces between which run elongate voids defined by interconnections between the two opposite faces, the cross sectional shape of at least some of the voids being a polygon with more then four sides.
The polygon may be regular or irregular, but the preferred embodiments utilise a repeating pattern of regular octagons or hexagons (hereinafter referred to as the primary voids).
Preferably, the hexagonal or octagonal voids are joined together at one side or at one apex.
The spaces between adjacent hexagonal or octagonal voids may be filled in with suitable plastics material, but it is preferred that these spaces comprise smaller polygonal voids (hereinafter referred to as the secondary voids) whose shape will be determined by the particular shape chosen for the primary voids (triangular secondary voids for hexagonal or octagonal primary voids).
These voids are easily extruded integrally with the sheet in one pass through a suitably shaped die, and suitable materials for the sheet include thermoplastics such as a polyolefin eg polyethylene or polypropylene.
The opposed faces may be external faces, or one or more may be an internal face if the sheet is part of a more complicated cross-sectional profile.
Preferably, each primary void and each secondary void has at least one associated air inlet point, with more than one air inlet per void giving improved cooling in relation to the air blown through the sheet during extrusion.
There are a number of advantages of hexagonal or octagonal primary voids over the generally square primary voids (i.e “criss-cross” interconnecting beam structure) illustrated in the preferred embodiment of PCT Patent Specification No WO 91/16243. One is that each hexagonal or octagonal void gives two contact points with each of the two opposed faces, whereas the “criss-cross” structure gives only one per face per square void, resulting in better resistance to compressive forces (particulary as determined by the flat crush test or FCT). Also, the hexagonal or octagonal void gives a much larger area in contact with the sheet faces resulting in much faster cooling by the calibrator plates. The distance between two adjacent cells is also greater, giving a lighter sheet. Finally, it is not necessary to take special steps to avoid sharp corners in contrast to WO 91/16243.
REFERENCES:
patent: 3641230 (1972-02-01), Jenks
patent: 4106588 (1978-08-01), Moore et al.
patent: 5360500 (1994-11-01), Evans et al.
patent: 2235785 (1975-01-01), None
patent: 2284444 (1976-04-01), None
patent: 2543055 (1984-09-01), None
patent: 0587282 (1947-04-01), None
PCT/GB91/00583—Publication GB 2197618 A—UK Patent Application.
FRA. Mo Snc di Franca Riva & C
Howard & Howard
Loney Donald
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