1. Field of The Invention
This invention relates to a method and apparatus for forming and sealing bags employed for packaging food or other material and, more particularly, to improved method and apparatus for applying an adhesive material to edges to be sealed.
2. Description of The Prior Art
In a typical vertical form and fill bag-making operation for packaging potato chips and similar snacks, apparatus is frequently employed in which film material in sheet or web form is supplied with a continuous or intermittent motion to a forming apparatus which forms the web into a tubular shape with either overlapping or fin-type edges. Glue or other adhesive is often pre-applied to one or more edges of the film. The film, after being formed into the tubular shape, is passed through a sealing apparatus in which heat and pressure are applied to seal the edges together. In overlapping edges the adhesive is applied to edges on opposite faces of the web; in fin-type seals the adhesive is applied to edges on the same face of the web.
In one form of such apparatus, a transverse seal is made to close the top of one bag and form the bottom of the succeeding bag. A downward pull is applied to the transversely sealed area and the overlapping edges of the tubular film are drawn along a device to heat-seal the longitudinal edges of the bag. The bag is filled and a transverse seal is formed at the top of the bag. The formed bag is severed, and the process is then repeated by the apparatus.
The film material employed should be one which adequately protects the food material or other product being packaged, is pleasing in appearance, is strong enough to withstand shipping and handling environments, and is amenable to forming and sealing, among other characteristics. The film material often used in the apparatus for forming bags may include a laminate having for example as one component a layer of polyvinylidene chloride class (e.g., that material sold commercially under the trademark "Saran") and as the other component a layer of uncoated polypropylene. For a lap seal, i.e., where one edge of the sheet material overlaps the other, this results in a "Saran" surface facing the uncoated polypropylene surface in the area of the overlap. These materials are not compatible for heat sealing to one another. Consequently, where heat sealing is involved, it is necessary to employ a separate material secured to one or more of the overlapping edges to insure that the surfaces facing one another for bonding have similar or compatible characteristics.
Where a thermal, or heat, seal is desired using dissimilar materials a separate strip of thermal seal material is bonded along one edge of the web by the manufacturer of the film to assure thermal bonding compatibility. This separate strip is oriented in the machine direction along the edge of the web so that as the forming apparatus creates the tube the strip applied, for example, to outer face of the polypropylene will provide a more compatible material with respect to thermal bonding capability facing the "Saran" layer side of the web in the vicinity of the overlap. This facilitates forming a thermal seal when the edges are pressed together and heated.
In addition to the time and expense required in applying this separate strip, the thickness of the strip causes a number of other problems in handling and forming of the sheet material into filled bags. Buckling can occur in the vicinity of the web material where the strip has been applied, causing the material to pucker when sealed. When the web material is rolled, one end of the roll, where the strip is applied, tends to be thicker than the other, causing the web to stretch. As a result uniformity and reliability in forming and sealing the bags is lost or at least substantially impaired. There is also a problem with air bubbles getting into adhesive seals.