Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Starting material is nonhollow planar finite length preform...
Reexamination Certificate
1998-11-04
2001-03-20
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Starting material is nonhollow planar finite length preform...
C264S321000, C425S387100, C425S388000, C425S405100, C425S412000
Reexamination Certificate
active
06203751
ABSTRACT:
The invention is in the field of the packaging industry and relates to a method according to the generic part of the first independent claim and to an apparatus according to the generic part of the corresponding independent claim for carrying out the inventive method. Method and apparatus serve for producing open containers from a thermoformable flat sheeting which consists of a foam material or contains at least one foam layer.
Cups, trays and similarly shaped containers with a bottom wall and a side wall (or side walls) are produced according to the state of the art by drawing or deep drawing flat thermoformable sheeting which may consist of a foamed material or may contain at least one layer of such a material. The sheeting is heated to a temperature at which it is thermoformable and at this temperature it is formed usually by a cooperating pair of a female and a male forming tool which tools are moved relative to each other. This drawing process may be assisted by a difference of pressure between the two sides of the sheeting.
A container produced by drawing or deep drawing of a sheeting with a uniform thickness, will have a wall thickness which is not uniform but varies between a thickness which is substantially the same as the original thickness of the sheeting in areas where the sheeting was drawn only little and a substantially reduced thickness in areas where the sheeting was strongly drawn. Depending on which element (male tool, female tool, pressure difference) is the primary forming element, the areas with a reduced wall thickness are located differently on the finished container. In a forming process in which the male tool is the principal actor, the side wall of the container is thin, the bottom wall having more or less the original thickness of the sheeting. This is due to the fact that the face of the male tool comes into contact with the sheeting when it is not deformed yet, and impedes sliding of the sheeting in the contacting area and therefore impedes drawing in this area also. If the main actor is a pressure difference (lower pressure on the female tool side) the side wall of the produced container shows a higher thickness than the bottom wall.
Furthermore, it is known to the one skilled in the art that flat foam sheeting or items made of such sheeting can be expanded in order to increase their wall thickness. Such expansion is achieved by again heating the sheet material to a temperature at which it is thermoformable and then applying a reduced pressure to at least one of its surfaces. Caused by the vacuum surrounding the sheeting and by the plasticated state of the material, the gas filled pores of the foam expand and with this the thickness of the sheeting increases. For stabilizing the foam in its expanded state, the material is cooled down before ambient pressure is established.
Expanded areas of a flat sheeting or of a container wall respectively have, compared with their not expanded state, an increased thickness and a reduced density. Therefore these areas usually have a higher buckling strength i.e. are stiffer and harder to bend or to buckle compared with the original (not expanded) state.
The publications U.S. Pat. No. 3,846,526 and JP-60192615 show each a process and an apparatus for producing open containers in a drawing or deep drawing step, followed by an expansion step in which expansion step the side wall is expanded for more insulating capacity (U.S. Pat. No. 3,846,526) or the upper edge of the side wall is expanded for forming a thickened collar region for facilitating stacking of such containers by positioning them inside each other. According to both named publications, the expanding step is introduced between the drawing step and the cooling step such that the container material does not need reheating for the expansion step but such that the expansion step adds to the total cycle time necessary for producing the container.
It is now the object of the invention to create a method for producing open containers with a bottom wall and a side wall (or side walls) from a sheeting of a thermoformable material usually having a uniform thickness which sheeting consist of an expandable foam material or contains at least one layer of such a material, in which method drawing and expanding are combined and controlled, such that the container walls regarding mechanical properties such as e.g. buckling strength are sufficient for a specific container function, such that the weight of the container is at a minimum and such that the cycle time necessary for producing the container is at a minimum too. Furthermore, it is an object of the invention to create an apparatus for carrying out the inventive method.
This object is achieved by the method and by the apparatus for carrying out the method as defined by the claims.
The inventive method consists of a combination of three techniques: drawing by applying a pressure difference, drawing by applying a tool and expanding foam material by applying vacuum, whereby for producing containers with a small drawing ratio and/or with a form causing little problems the application of the pressure difference may be omitted.
The three (or two) techniques are combined in a specific sequence to be described further on. The inventive method is carried out with the help of a male tool movable into the recess of a female tool. The male tool in its end position and the recess of the female tool form together a cavity which substantially corresponds to the container to be produced.
The inventive method is usually carried out in cycles whereby in each cycle one area of a quasi endless web of sheeting is processed to be formed into one container or into a plurality of containers. An exemplified action sequence of one cycle of the inventive method is substantially the following:
positioning a web of sheeting between male tool and female tool;
clamping the sheeting around the opening of the recess of the female tool;
moving the male tool towards the female tool and into the recess of the female tool and at the same time prestretching the sheeting by subjecting it to a pressure difference (lower pressure on the female tool side), such that the face of the male tool gets in contact with the prestretched sheeting inside the recess;
drawing the sheeting by further moving the male tool with its face in contact with the sheeting to its end position in the recess of the female tool and, during this further movement of the male tool, starting to reduce the pressure on at least one side of the sheeting such that a pressure which is sufficiently low for expanding the foam material is achieved when the male tool reaches its end position or very shortly afterwards;
leaving the formed container to cool to a temperature below the temperature range for thermoforming whilst keeping the tools in the same position and keeping the pressure on the at least one side of the sheeting at its value reduced for expansion of the foam material;
equalizing the pressure, demolding and declamping the formed container by moving the tools away from each other;
advancing the web in order to remove the formed container from between the tools and to position a further area of sheeting between the tools.
The clamping of the sheeting before thermoforming is not obligatory. If the sheeting is not clamped it might be drawn from the areas around the recess into the recess. If this is not acceptable, the sheeting must be clamped around the recess before prestretching or drawing respectively starts. Clamping of the sheeting may be effected with corresponding tools either around each recess or around a plurality of recesses.
For forming containers with a low drawing ratio (depth divided by top diameter is considerably below one) and/or with a shape that causes little drawing problems (round, not very steep side walls), good results can also be achieved without prestretching, i.e. the male tool contacts the unstretched sheeting and serves as only, drawing actor. In such a case, the same as in a process with prestretching, the reduction of the pressure on at least one si
Rolle Jean-Claude
Sjödin Kurt
Convenience Food Systems B.V. of the Netherlands
Kuhns Allan R.
Oppedahl & Larson LLP
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