In 2005 WRAP awarded funding to IFP Ltd to develop a commercial technology for reducing the amount of packaging material used to make the top and bottom seals (cross seals) on flexible bags. By replacing the conventional crimp seal by a cut and weld impulse bead seal technology (Integrity Seal), the amount of film used to make the seal could be reduced from 15 mm to 1mm, a saving of around 10% in the length of the bag. This project was successful and Marks & Spencer decided to implement the system for bagged salads at one of their key suppliers. IFP are now supplying systems across a number of food sectors, such a frozen chips, whole potatoes and with further fresh salad companies. See our case studies research for further details and reports.
It was recognised however that although the new cross seal technology was successful in reducing the film usage and also making the seals hermetic, this would be of little benefit if the bags were still leaking via the back seal. All companies that IFP approached stated that they still have a major problem with packs leaking through poor back seals and indicated that they would like to see the same improvements on a back seal system.
International Food Partners (IFP) Ltd were awarded funding from WRAP’s Innovation Fund in November 2006 to develop a superior sealing technology for making the back seal on flexible packaging, which would both reduce the amount of packaging material required to form the seal and ensure that the seals were hermetic. The project followed on from the successful cross seal project.
Traditional flat bar or band sealing which has been used since automatic packaging machines were invented cannot produce hermetic sealing with thin gauge monolayer polymers such as Oriented Polypropylene (OPP). Flat profile sealing relies on the thin layer of sealant on the face of the film, usually just a few microns thick, to make the heat seal. Such a thin layer is insufficient to melt, flow and make the seal hermetic. Furthermore, the flat profile bars and bands very often induce creases which a flat profile bar will not seal hermetically. Laminated films are sometimes used in an attempt to produce hermetic seals and they can be successful but cost more to manufacture and use more polymer. Typically the sealant layer on a laminate is 12 – 20 micron with a substrate layer of 15 – 20 micron, so a total thickness of 35 – 40 micron.
The width of the back seal bars are generally 8 ‐ 10 mm so a significant amount of film is used in an attempt to make the seal hermetic. In reality, the width does not contribute to making an hermetic seal so is an unnecessary use of film. The Integrity Seal systems use only a 4 – 6 mm wide seal so a 50 % reduction in the seal width is possible. It is not possible to reduce the width any further because of the film tracking which is always present on vertical‐form‐fill seal machines.
Whilst a single technological solution was possible in delivering the Integrity Seal for cross sealing, this was not possible for the back seal. Differences in existing machine platforms meant that two systems had to be developed and proved, one for continuous motion machines and one for intermittent motion machines.
The objective of this project was to make a hermetic seal using a rotary heated wheel system for continuous motion machines and an impulse or constant heating system for the intermittent motion machines. A main characteristic of all the systems was the development of an embossed feature to deliver more pressure and thereby produce hermetic seals.
For the continuous motion machines Ceetak developed an innovative heated embossed, low mass, pair of sealing rollers to deliver high pressure at a tangent to the heated wheel and hence deliver point loading. The pressure was supported by a double pair of rotary anvil wheels mounted inside the product tube.
For the intermittent motion machines an impulse sealing technique was developed, based on the same principle used for the successful cross seal project. A resistance heated system was also developed providing an alternative hot bar option.
Prototype designs were built and evaluated by Ceetak at their Bedford facility, and products tested for seal strength and seal integrity. Trials and production runs took place on fresh produce at Tilmanstone Salads (Sandiacre intermittent machine) and Soleco (Ilapak continuous motion machine).
The trials were successful and demonstrated that hermetic seals were produced by both systems under production conditions. The benefits of these new technologies can be summarised as follows:
- Hermetic seal
- Narrow seal profile
- Seal area reduced by 50%
- Film width reduced by 1%
- Potential film saving from using thinner gauge film of 10 - 15%
- Improved pack appearance
- Better temperature control
- Potential improvement in product quality and shelf life