EP1055608A1

EP1055608A1 - Seal for a sack or bag

Info

Publication number: EP1055608A1
Application number: EP00304275A
Authority: EP
Inventors: Donald Charles Lack; David Wrightson
Original assignee: British Polythene Ltd
Current assignee: British Polythene Ltd
Priority date: 1999-05-21
Filing date: 2000-05-19
Publication date: 2000-11-29
Also published as: GB9911813D0

Abstract

A seal for a sack 10 has a first line of seal 18 with venting holes 20 in a second line of seal 22 with venting holes 24. Venting holes 20 are offset relative to venting holes 24 to enable a product in the sack 10 to be vented while minimising penetration of moisture into the sack.

Description

The present invention relates to a seal for sacks or bags, and relates particularly, but not exclusively to seals for plastic sacks and bags.
Some products which are packaged in sacks produce gases which need to be vented. If such gases are not vented the pressure build up in sacks can lead to damage of the product or rupture of the sack.
A previous attempt to overcome the above described problem is disclosed in Application No JP05255331, which describes a bag for the containment of fermented food. The bag is formed from two sheets of film bonded together along at least three sides, the seals of said bonded sides being provided with gas venting holes, that is, points along the line of the seal where the two sheets are not bonded.
A bag or sack with such vents allows gases produced by the product contained within the bag, to escape. However, such vents allow the product immediately adjacent to the vents to come into direct contact with the air outside the sack. This air may be relatively moist when compared to the air inside the sack. In these circumstances the product, particularly that adjacent to the vent, may absorb some of this moisture, potentially having a detrimental effect upon the stored product.
The above described vents may allow fines or very small particles of a particulate product a very short direct path, through the vent, through which they can easily escape.
Furthermore such vents create points of weakness in the seal. These points of weakness may lead to splitting of the sack or enlargement of the venting holes and to the product escaping or at least exacerbation of the above described problems.
For very hydroscopic products it may be necessary to maintain closure of the venting holes until a specific internal pressure has been created by the product. In the above described venting bag the venting holes remain in an open condition irrespective of whether an internal pressure has built up or not. In the absence of any internal pressure, moist air is able to enter the sack through the venting holes.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to an aspect of the present invention there is provided a seal for a bag comprising:
a first seal having at least one interruption therein;
adjacent a second seal having at least one interruption therein;
wherein the at least one interruption of the first seal is offset relative to the at least one interruption of the second seal.
The interruption in the first and second seals may comprise a hole in the seal. The interruptions are provided to, on occasion, vent excess pressure from the inside of the bag.
By providing two lines of seal both containing at least one interruption, or venting hole, and said venting holes being offset relative to one another, the distance between the inside of the sack and the outside of the sack through the venting system is increased. Thus the product contained within the sack adjacent a venting hole on the inner line of seal is maintained at a greater distance from the air outside the sack by the provision of a second line of seal containing offset venting holes. Therefore moisture on the outside of the sack will take longer to come into contact with the product inside.
The outer line of seal also provides structural support for the weakness created by the venting holes in the first seal. This support is further enhanced by the offsetting of the venting holes.
Furthermore, two lines of seal containing offset venting holes also removes the short and direct pathway for fine particulate product from within the sack to escape, therefore decreasing losses of the product through the venting holes.
A surprising advantage of the two adjacent lines of seal, is that for certain thicknesses of plastic sheet film, combined with certain distances between the adjacent first and second lines of seal and certain sizes of venting holes, when a product is contained within the bag, the pathway from the venting holes in the first line of seal to the venting holes in the second line of seal remains closed, even to the passage of air, until a sufficient internal pressure has been built. When such a pressure has been created the pathway between the venting holes can open, to vent the excess gas, and re-closes once the pressure is reduced. As a result the product contained within the bag can be almost completely protected from incursion of moisture and loss of product through the venting system, whilst the venting properties of the seal are maintained.
Furthermore, the pressure at which the pathway between the venting holes opens, can be controlled by varying the thickness and/or flexibility of the material from which the bag is made, the distance between the first and second lines of seal, the size of the venting holes and the distance between the venting holes.
According to another aspect of the invention there is provided a method for the forming of a seal for a bag comprising the steps of:-
forming a first seal having at least one interruption therein;
forming a second seal adjacent said first seal having at least one interruption therein;
wherein the at least one interruption of the first seal is offset relative to the at least one interruption of the second seal.
By providing a method containing the above steps, a bag can be sealed to create a venting seal with the above described advantages.
According to a further aspect of the invention there is provided apparatus for forming a seal for a bag, the apparatus comprising:-
a first heated bar for forming a first seal, said bar having at least one gap to produce at least one interruption in said first seal;
a second heated bar for forming a second seal, said bar having at least one gap to produce at least one interruption in said second seal;
wherein the at least one gap of the first bar is offset relative to the at least one gap of the second bar.
By providing the above described apparatus a bag can be sealed to create a venting seal with the previously discussed advantages.
According to another aspect of the present invention there is provided a bag having at least one seal according to the first aspect of the present invention.
A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:-
Figure 1 is a schematic representation of a sack having a seal embodying an aspect of the present invention.
Figure 2 is a front view of part of seal forming apparatus embodying an aspect of the present invention.
Figure 3 is a bottom view of the part of the seal forming apparatus of Figure 2.
Figure 4 is a side view of the part of the seal forming apparatus of Figure 2.
Referring to Figure 1, a sack 10 comprising a first sheet of plastic 12 and a second sheet of plastic 14 (not shown). Three sides of said sack are sealed by non-venting seals 16. The seal for the fourth side of sack 10 comprises first line of seal 18 with venting holes 20 and second line of seal 22 with venting holes 24. Venting holes 20 in first line of seal 18 are offset relative to venting holes 24 of second line of seal 22.
The bonds between sheets of plastic 12 and 14 may be formed by the known technique of mechanically applying heated metal bars to the plastic films 12 and 14. The vent holes in first and second lines of seal 18 and 22 are produced by gaps in said heated metal bars. A typical plastic film for producing such a sack is a polyolefin film.
Referring to Figures 2, 3 and 4, a heated sealing bar 30 comprising a first sealing bar 32 having gaps 34, and second sealing bar 36 having gaps 38. Heated sealing bar 30 is constructed such that gaps 34 of first sealing bar 32 are offset relative to gaps 38 of second sealing bar 36. Heated sealing bar 30 works in cooperation with a second heated sealing bar (not shown) such that the first sealing bars 32 of the pair of heated sealing bars 30 meet to form the first seal, and the second sealing bars 36 of the pair of heated sealing bars 30 meet to form the second seal. The gaps 34 in the pair of first sealing bars 32 are aligned such that when plastic sheets 12 and 14 are placed between the pair of heated sealing bars 30 and pressure is applied between them they produce a seal with venting holes 20 in the first seal 18. Similarly the gaps 38 in the pair of sealing bars 36 are also aligned such that when plastic sheets 12 and 14 are placed between the pair of heated sealing bars 30 and pressure is applied between them they produce a seal with venting holes 24 in the second seal 22.
The temperature to which sealing bar 30 is heated, the pressure applied and duration of contact required to bond the plastic sheets are all dependent upon the plastic used in the film and the thickness of the film.
It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as described. For example the sack may be constructed from a tube of plastic film which is sealed at the ends. When supplying such sacks it is commonplace for a sack manufacturer to form a sack from such a tube and seal one end, for the purposes of this invention with a venting seal, so that the product supplier can fill the sack and apply a non-venting seal to the other end.
It will also be appreciated by the person skilled in the art that the venting seal maybe used along more than one edge of the bag

Claims

A seal for a bag comprising:

a first seal having at least one interruption therein;

adjacent a second seal having at least one interruption therein;

wherein the at least one interruption of the first seal is offset relative to the at least one interruption of the second seal.
A seal according to claim 1, wherein at least one said interruption in the first or second seals comprises a respective hole in the seal.
A seal for a bag substantially as hereinbefore described with reference to the accompanying drawings.
A method for the forming of a seal for a bag comprising the steps of:-

forming a first seal having at least one interruption therein;

forming a second seal adjacent said first seal having at least one interruption therein;

wherein the at least one interruption of the first seal is offset relative to the at least one interruption of the second seal.
An apparatus for forming a seal for a bag, the apparatus comprising:-

a first heated bar for forming a first seal, said bar having at least one gap to produce at least one interruption in said first seal;

a second heated bar for forming a second seal, said bar having at least one gap to produce at least one interruption in said second seal;

wherein the at least one gap of the first bar is offset relative to the at least one gap of the second bar.
An apparatus for forming a seal for a bag, the apparatus substantially as hereinbefore described with reference to the accompanying drawings.
A bag having at least one seal according to any one of claims 1 to 3.