Electric heating – Heating devices – Combined with pressure application means
Patent
1995-02-23
1998-11-24
Jeffery, John A.
Electric heating
Heating devices
Combined with pressure application means
156250, 156510, 264153, 425423, 425290, B65D 8126, B26F 124
Patent
active
058411070
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method and apparatus for perforating, or shaping the perforations of, thermoplastic sheets, for example plain or corrugated polyolefin sheets.
BACKGROUND OF THE INVENTION
Containers for the preservation and transport of food stuffs may be made from thermoplastic sheet material so as to be solid, light and economical. However, when used for the transport and storage of wares having a high moisture content, for example fruit or vegetables, deterioration of the product may occur owing to build up of moisture within the container. It has been proposed to provide such containers with a plurality of holes, for example by forming them from perforated sheets, and this reduces the problem considerably.
However, when the sheets are perforated with heated needles or punches alone, or similar techniques where direct heat is used to melt the material (including the use of lasers), the melted material forms a deposit, as indicated at 1 in FIG. 1 of the accompanying drawings. The deposit 1 is upstanding from the general plane of the face of the corrugated sheet and therefore provides a trap or barrier for retention of moisture 2 which can both lead to deterioration of the packed product, in particular fruit and vegetables, and unwanted variations of the tare (unloaded weight) of the package.
If perforation of the sheet is carried out using conventional ultra sonic equipment, as illustrated in FIG. 2 of the accompanying drawings, the moisture retention and stagnation problem is avoided but instead a different problem arises. This is the weakening of the whole structure due to the large sealing area 3 which is needed and the unbalanced deformation of the sheet as illustrated at 4, both of which affect the mechanical performance of the package. The present invention seeks to provide a method and apparatus for producing perforations in thermoplastic sheet materials which are improved in the above respects.
BRIEF SUMMARY OF THE INVENTION
According to the present invention there is provided an apparatus for shaping perforations of sheet material which comprises a first mold structure at one side of the sheet material and a complementary mold structure at the other side of the sheet material to cooperate with the first mold structure wherein the structures are arranged to be reciprocated and are capable of meeting approximately half way through the sheet material whereby to shape the perforations between them.
The apparatus can include a needle so the perforations are simultaneously formed and shaped, or it may be used to shape pre-formed perforations. If a needle is included it may be of the ultra-sonic type or it may be heated, e.g. electrically heated. If a needle is used, one of the mold structures may be defined by an appropriate formation on the needle, or the structure may be separate.
According to one example of the present invention, the needle may be an ultra-sonic electrode having one of the mold structures formed about its periphery spaced from its end and the method of perforating sheet material comprises placing a sheet of thermoplastic material beneath the ultra sonic electrode of the apparatus, bringing the electrode into contact with the sheet whereby to penetrate it until it contacts the structure, raising the structure until it and the electrode meet at a point approximately half way through the sheet, lowering the electrode once again and continuing to raise the cylinder until the upper peripheral portion of the cylinder contacts the complementary portion of the electrode comprising the mold structure approximately in the middle of the thermoplastic sheet whereby to finalise sealing thereof, and retracting the electrode and structure.
In another embodiment of the invention an electrically heated needle may be surrounded by a mold structure in relation to which it can reciprocate. A complementary mold structure is located on the other side of the sheet capable of receiving the needle. The needle perforates the sheet, immediately after which the complem
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