A filled packaging container used for milk, fruits drink, etc. is generally made from laminated packaging material. The packaging material has a comparatively rigid main supporting layer covered with thin layers of plastic materials. This material can also contain materials such as aluminum foil. The common feature of this type of packaging laminated material is that the thermoplastic material (usually polyethylene) layer provided on the outside and inside of the laminated material is used to form a seal in a liquid tight state with heat and pressure. In order for a seal to have a desired strength and a desired liquid tight performance, both thermoplastic layers that are to be sealed should be clean, and should not include impurities. With a clean layer, sufficient melting of each thermoplastic layer can be obtained and, as a result, the optimum seal can be achieved by strong high sealing performance. Since impurities of the thin oxide formed on the packaging laminated material during the extrusion steps of thermoplastic layers usually exist in the thermoplastic layer, a perfect melting between the thermoplastic layers is occasionally prevented. Therefore, a seal cannot acquire the necessary strength and sealing performance. Moreover, when sealing packaging materials under liquid food, impurities associated with residual substances of the contents (liquid food) which hinders the sealing further may also be generated or present on the surface of the thermoplastic layer. This is a problem peculiar to the packaging filling system in which sealing of the laminated material is performed under the liquid surface of the liquid food. That is, in this packaging filling system, the content food must be first pushed out from the crevice between the surfaces of the thermoplastic materials before sealing.
However, as a practical matter, a very small quantity of residual substance remains, without the content food being completely squeezed out, and this residual substance weakens the seal.