WO1997028943A1

WO1997028943A1 - Forming tool for producing open containers by thermoforming foam sheeting

Info

Publication number: WO1997028943A1
Application number: PCT/EP1997/000418
Authority: WO
Inventors: Jean-Claude Rolle
Original assignee: Convenience Food Systems B.V.
Priority date: 1996-02-07
Filing date: 1997-01-31
Publication date: 1997-08-14
Also published as: CH691447A5; AU1720297A

Abstract

Open containers of thermoformable sheet material are produced by drawing or deep drawing the preheated sheeting. If the sheeting consists of or contains at least one layer of foamed plastic material, the container walls may be expanded also by applying a reduced pressure. The inventive tool (1.2, 2.2) serves for a drawing process in which the sheeting is drawn with the help of a pair of forming tools which are moved relative to each other. During this movement the pressure on at least one side of the sheeting is reduced such that it reaches a value below ambient pressure which is low enough for expansion of the foam material when the male tool reaches its end position in the recess of the female tool or immediately afterwards. For preventing faulty containers due to air tight contacts between sheeting and tool surface combined with air cushions between tool surface and sheeting, the tool shows on the surfaces to come into contact with the sheeting a pattern of surface openings which openings are smaller than 0.5 mm and are spaced from each other by at the most 20 mm. Through these openings air is removed from between the tool surface and the sheeting for establishing the reduced pressure necessary for expanding the foam material. The inventive tool is e.g. made from a sintered material which has open pores connected to each other and forming fine channels in all directions through the material.

Description

FORMING TOOL FOR PRODUCING OPEN CONTAINERS BY THERMOFORMING FOAM SHEETING

The invention is in the field of the packaging industry and relates to a forming tool according to the generic part of the independent claim, which forming tool serves for producing open containers by thermoforming a flat piece of sheeting which sheeting consists of a foamed material or contains at least one layer of such a material.

Open containers, such as e.g. cups or trays often used for consuming and/or for packaging food stuffs are usually produced by drawing or deep drawing a piece of flat sheeting made of a thermoformable plastic material. Such open containers have a bottom wall and a side wall or side walls. AJI apparatus for drawing or deep drawing thermoformable plastic sheeting usually works with a pair of forming tools: a female tool with a recess over which the preheated sheeting is positioned and a male tool movable into this recess thereby drawing the sheeting. When sufficiently cooled down, the formed container is demolded, e.g. by ejection from the female tool by mechanical ejection means or by pressurized air supplied to the tool surface through corresponding nozzles. The use of a sheeting consisting of a foamed material or containing at least one layer of such a material is advantageous for making containers of the above mentioned sort, because a foamed material having the same weight per area unit is usually stiffer than the corresponding solid material.

If the sheeting for producing the open containers consists of a foamed material or contains at least one layer of such a material, specific areas of the container walls may be made thicker and therefore stiffer by applying in these areas a reduced pressure on at least one side of the sheeting when the foam material is still in a thermoformable state, such expanding the air trapped in the cells of the foam material. Such a process is described e.g. in the publication US-3846526. The aim of the described expansion is to expand the side wall of thermoformed cups for increasing their insulation capacity. For this purpose, nozzles connected to a vacuum source are arranged in the female tool in the area of the edge between side wall and bottom wall and in the male tool near the opening of the cup, i.e. around the upper rim of the cup. A process of the same kind is described in the publication JP-60192615, where the principle aim of the expansion is to produce a thickened upper rim on the side wall of a cup, which rim is to facilitate stacking of such cups inside each other and a thickened bottom wall. Therefore, nozzles are arranged on the female and on the male tool in the area of this rim and in the area of the edge between side wall and bottom wall and in the center of the bottom wall.

The nozzle position is determined primarily by esthetic reasons which make it desireable that the traces which are left on the finished container by the nozzles are in a position where they are not easily visible. In both cases described above, the container is first formed by drawing the sheeting and then the pressure is reduced for expansion, i.e. the two forming steps follow each other and are clearly separated from each other.

A further method and apparatus for drawing and expanding foam sheeting to form open containers is described in a co-pending application (same filing date as the present application). The method is a combination of drawing and expansion, whereby already during the movement of the male tool into the recess of the female tool, i.e during the drawing of the preheated sheeting, the pressure on at least one side of the sheeting is reduced such that it reaches a value (expansion pressure) below ambient pressure which is low enough for expanding the foam of the sheeting when the male tool reaches its end position or immediately afterwards. The tools are then kept in the same position and the expansion pressure is maintained until the sheeting is cooled down to a temperature below its plastication temperature. Before drawing the sheeting with the aid of the male tool, it can be prestretched with the aid of a pressure difference (lower pressure on the female tool side).

The aim of the method according to the co-pending application is not to expand the foam material for producing specific areas with a greater wall thickness but for producing a container whose mechanical properties are sufficient for the application of the container and whose weight is at a minimum and which container is produceable in a minimum cycle time. In order to achieve this aim, the step of drawing and the step of expanding are at least partly carried out simultaneously (drawing and reduction of pressure) and areas of the container wall are expanded to a varying degree. If for the method according to the co-pending application tools are used with nozzles connected to a source of reduced pressure as they are described above (nozzles positioned in places in which traces of nozzles are not an esthetic drawback), the containers produced tend not to be satisfactory regarding mechanical and esthetic quality. The main fault which occurs are irregularities in the side wall of the containers, which probably stem from the fact that the pressure reduction leads to immediate adherence of the sheeting to the tool surface in the area of the nozzle and to stationary air cushions between sheeting and tool in areas away from the nozzles. This does not only impede regular expansion but, if it happens when the sheeting is still being drawn, it leads to differences in the sliding of the sheeting on the tool and therefore to drawing differences between areas near nozzles and areas away from nozzles.

It is therefore the object of the invention to create a forming tool for application in the method according to the co-pending application, wherein a flat sheeting consisting of a thermoformable foam material or containing at least one layer of such a material is formed into open containers whereby the sheeting is drawn using a cooperating pair of a male and a female tool and in which during the movement of the male tool into the recess of the female tool the pressure on at least one side of the sheeting is reduced such that a pressure low enough for expanding the foam material is achieved when the male tool reaches its end position or immediately afterwards. By using the inventive tool in this method it is to be possible to produce containers with a good quality regarding optical appearance and mechanical properties, i.e. with no optical irregularities at least on their side walls although density and thickness may vary from area to area of this side wall.

This object is achieved by the forming tool as defined by the claims. The inventive tool shows at least on the one surface area which is to serve for forming the container side wall, a pattern of openings in the form of holes or slots, which openings are connected to a system of channels inside the tool. This system of channels is connectable to at least one source of reduced pressure, such that through the openings air can be removed from between the tool surface and the sheeting in a controlled manner. Good results are achieved with surface openings smaller than 0.5 mm (diameter of holes or width of slots < 0.5mm) and spaced from each other by not more than 20mm. Such openings do not leave inconvenient traces on the foam surface and their distance from each other is small enough for preventing the build up of pressure differences over the tool surface which pressure differences will lead to the named disadvantages.

A preferred embodiment of the inventive tool is at least partly made of a porous material. The porous material forms at least those portions of the tool which will be in contact with the container side wall or with both side and bottom wall. Such a porous material contains open pores connected with each other and such forming channels in all directions through the material which channels are open at the surface and form a very fine pattern of a quasi infinite number of very small openings (below 0.1 mm).

It can be shown that forming tools made of a sintered bronze material or having surfaces made of such a material are highly suitable for producing containers of e.g. polypropylene foam material. This is the case because very fine bronze powder is easily sintered to obtain an open-cell material with open pores forming channels in all directions through the material. The surface of this material is preferably ground or electroeroded, not polished as by polishing the pores may be closed. The surfaces may also be coated. If the containers to be produced with the tool are to be used in connection with food stuffs, coating with a food compatible material is obligatory.

Bronze is just an example for a material from which to produce the porous parts of an inventive tool. Any material with similar qualities is usable also, as e.g. aluminum, alloys of aluminum, other metals or ceramic materials.

The inventive forming tool is described in further detail in connection with the following Figures, wherein:

Figure 1 shows an exemplified pair of inventive forming tools, both tools having patterns of surface openings;

Figure 2 shows a further exemplified pair of inventive forming tools, both tools having bodies made from a porous material;

Figure 3 shows a further exemplified pair of inventive forming tools, both tools having a surface layer consisting of a porous material.

Figure 1 shows a pair of forming tools 1.1 and 2.1 for use e.g. in the above described method as claimed in the co-pending application. The pair of tools is shown in their end position. Between the tools their is a cavity 3 which corresponds substantially to the container to be formed. Both tools 1.1 and 2.1 have a tool body 10.1, 20.1 with a system of channels 6, which channels end in openings 7 at the cavity surface (11 and 21) of the tools. These openings have the form of holes and/or slots with a diameter or width respectively of less than 0.5mm and they are spread out over the tool surface in a pattern in which the holes or slots are distanced from each other by at the most 20mm. The male tool 1.1 and the female tool 2.1 shown in Figure 1 have both such a pattern of openings on the whole part of their surface which is to come into contact with the sheeting to be formed into the container (cavity surface 11 and 21). As mentioned above, for specific application it is sufficient to provide the pattern of surface openings only on one tool (preferably the female tool) and/or only on the surface part which is to come into contact with the container side wall.

The system of channels 6 is connectable selectively to a source of reduced pressure 71 or to an opening to ambient pressure 72 (for pressure equalization). If, after thermoforming, the formed container is to be ejected by pressurized air, the system of channels 6 may be connectable also to a source of pressurized air 73. For controlling the selectable connection, e.g. a corresponding control valve 7 (shown in a schematic manner) is provided.

For ejection of the formed container with the help of pressurized air, it is possible also to provide separate nozzles, connected to a separate system of channels in the tool. As ejection takes place when the sheeting is not in a thermoformable state any more, these nozzles do not need to be as numerous and as spread out over the cavity surface as the openings enabling the pressure reduction necessary for expanding the foam material.

Figure 2 shows a further embodiment of the inventive forming tool. Both tools (1.2 and 2.2) of the tool pair shown have a tool body 10.2, 20.2 made of a porous material, e.g. of sintered bronze. On the cavity surfaces 11 and 21, the porous material is preferably ground or electroeroded. On the other tool surfaces the pores of the material are closed by suitable means, e.g. a surface coating or a corresponding, tightly fitting tool container 12 or 22, which tool container has at least one opening 13 or 23, connectable to e.g. a control valve 7 as described in connection with Figure 1. As mentioned above, it is not a condition that both tools have a pattern of surface openings. Tool pairs with e.g only a female tool with a tool body of a porous material are feasible also.

Figure 3 shows a further embodiment of the inventive forming tool. The pair of forming tools 1.3 and 2.3 are similar to the ones shown in Figures 1 and 2 regarding their form. They differ from the tools according to Figures 1 and 2 in that they have tool bodies 10.3 and 20.3 made from a solid material and showing a system of channels 6 being on the one side in the described manner connectable to at least one control valve 7 and on the other side open to the surface of the tool body 10.3 and 20.3 which surface is covered with a surface layer 14 or 24 respectively made of a finely porous material preferably with a ground or electroeroded surface, which porous surface layer constitutes the cavity surfaces 1 and 21.

Depending on the mechanical properties of the surface layer 14 and 24, the tool body 10.3 or 20.3 may contain more or less hollow spaces or may be replaced by one hollow space.

As mentioned before, it is possible for pairs of male and female tools not to be similar. Depending on the quality of the container to be formed and also depending on the sheeting used, it is possible to have only one of the tools made in the shown manner, whilst the other tool has no openings or shows distinct nozzles for removing air from between tool and sheeting and/or for supplying pressurized air for ejection of the formed container. It is also possible to combine in one pair of cooperating tools, two tools according to any of Figures 1 to 3, whereby each tool of a pair can have a pattern of openings only on the surfaces for forming the container side walls or on more surfaces (container bottom wall or part thereof).

Claims

C L A I M S

1. Forming tool (1.1/2/3, 2.1/2/3) being one of a cooperating pair of a male and a female forming tool for producing open containers with a bottom wall and a side wall or side walls from flat sheeting consisting of a foamed thermoformable and expandable material or containing at least one layer of such a material, which female tool (2.1/2/3) has a recess and which male tool (1.1/2/3) is movable into the recess of the female tool, whereby the preheated sheeting is drawn and wherein during the movement of the male tool into the recess of the female tool the pressure on at least one side of the sheeting is reduced such that it reaches a value below ambient pressure which is low enough for expanding the foamed material when the male tool reaches its end position or immediately afterwards, characterized in that the forming tool (1.1/2/3, 2.1/2/3) has at least on the one surface part which serves for forming the container side wall a pattern of surface openings which surface openings are conne- ctable in a controlled manner to at least one source of reduced pressure.

2. Forming tool according to claim 1, characterized in that the pattern of surface openings consists of holes and/or slots which have a diameter or a width respectively which is less than 0.5mm.

Forming tool according to claim 1 or 2, characterized in that the surface openings of the pattern are spaced from each other by at the most 20mm.

4. Forming tool according to claim 1 to 3, characterized in that it has a tool body (10.1, 20.1) with a system of channels (6) which channels end on the one side in surface openings and are on the other end are connectable to the at least one source of reduced pressure.

5. Forming tool according to claims 1 to 3, characterized in that it has a tool body (10.2, 20.2) consisting of an open-celled material with open pores connected with each other and forming channels through the material in all directions and means (12/13, 22/23) for connecting these channels to the at least one source of reduced pressure.

6. Forming tool according to claims 1 to 3, characterized in that it has a tool body (1.3, 2.3) with a system of channels 6 and is covered on at least part of its surface with a surface layer (13, 24) of an open-celled material with open pores connected with each other and forming channels through the material in all directions, whereby the channels of the system of channels (6) end on the one side under the surface layer ( 14, 24) and are on the other end connectable to the at least one source of reduced pressure.

7. Forming tool according to claim 5 or 6, characterized in that the open- celled material is a sintered material.

8. Forming tool according to claim 7, characterized in that the sintered material is sintered bronze.

9. Forming tool according to claim 7 or 8, characterized in that the sintered material is coated with a further material.

10. Forming tool according to claims 7 or 8, characterized, in that the surface of the sintered material is ground or electroeroded.