TECHNICAL FIELD

The present invention relates to a device for heating tools in thermoforming, comprising a tool for fixedly retaining a thermoplastic blank.

BACKGROUND ART

The method of applying an end wall onto a packaging container body is described, for example, in Swedish Patent Specification No. SE 467097. The packaging container body may be manufactured from a sheet or from a material web which is cut off to suitable parts or lengths. The packaging container blank is sealed into tube form along a longitudinal joint or seam. The material from which the packaging container body is manufactured may be a laminate with a core of paperboard or paper to which are laminated outer layers of thermoplastic. The packaging container body may also consist entirely of a plastic material or may be manufactured from a metal with outer layers of thermoplastic. The end wall blank which consists of a thermoplastic is heat-sealed to the packaging container body. In those cases when use is made of end wall blanks consisting of pre- cut material pieces adapted in size to suit the purpose, these end wall blanks are fixedly retained by a tool. The end wall blank is heated by some form of heat source, for example IR heat, and is thermoformed and sealed to the packaging container body. The edge which is then held in this process will not be heated, and thus, not sealed against the packaging container body. This can be avoided by heating the tool to the softening temperature of the thermoplastic.

Heating the tool is normally carried into effect with the aid of heating elements and external regulators. When the tools consist of a plurality of interconnected tool units, this may involve difficulties given that the intention is to employ as few connections as possible. This is particularly important in aseptic applications.

OBJECTS OF THE INVENTION One object of the present invention is to realise a heating of the tool to such uniform temperature as possible and which is simple to install.

A further object of the present invention is to realise a heating method which is suitable for aseptic production of the packaging container.

SOLUTION These and other objects have been attained according to the present invention in that the device of the type disclosed by way of introduction has been given the characterizing feature that there are provided, in the tool, a number of resistors of the PTC type, so that both contact surfaces of the resistors are in contact with different parts of the tool, these parts being connnected to an external current source. Preferred embodiments of the present invention have further been given the characterizing features as set forth in the appended subclaims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One preferred embodiment of the present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings, in which:

Fig. 1 is a plan view of a tool;

Fig. 2 shows a partial section of the tool of Fig. 1 ;

Fig. 3 shows one embodiment of a construction of the tool, partly in section; Fig. 4 shows, in diagram form, the characteristics of a PTC resistor;

Fig. 5 shows another embodiment of a construction of the tool, partly in section;

The accompanying Drawings show only those parts essential to an understanding of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the Drawings, Figs. 1 and 2 are plan views of a part of a tool 1 in which a number of different packaging containers may simultaneously be provided with end walls from pre-cut end wall blanks of thermoplastic. Each circular recess in the tool 1 corresponds to an end wall applicator station. A number of resistors 2 of the PTC type (Positive Temperature Coefficient) are placed out on the surface of the tool 1. Ideally, the PTC resistors 2 are uniformly placed throughout the surface of the tool 1. A PTC resistor 2 is a resistor displaying a highly unlinear temperature coefficcient. A PTC resistor is chosen which displays a steep slope in the temperature range which is relevant to the practical application in question. As shown in Fig. 4, the selected PTC resistor has a final temperature of

between 126 and 132°C, implying that the thermoplastic is heated to softening temperature. This range is achieved by varying the voltage between 12 and 24 V. If the tool 1 is cooled, the temperature will drop, the resistance likewise and the power increase will counteract the drop. This takes place very quickly and mostly at that point where the drop is at its greatest. In the case when uneven temperature occurs in the tool 1, each PTC element or resistor 2 will react individually and re-establish a uniform temperature in the tool 1. Thus, each PTC resistor may be likened to a combined temperature indicator, regulator and heater. PTC resistor elements 2 as a commercial product consist of a ceramet

3 doped to certain temperature characteristics and placed between two contact plates 4. The contact plates 4 are intended to be applied under pressure.

The PTC resistor elements 2 may be placed in the tool 1 as shown in Fig. 2 in milled grooves 5 where the one contact surface 4 of the PTC resistor element 2 is in contact with the main portion 6 of the tool 1, and its other contact surface 4 is in contact with an inlay 7 in the groove 5 in such a manner that both of the parts 6 and 7 are electrically insulated from one another. As shown in Fig. 2, the parts 6 and 7 must be assembled with packing or gaskets 8 between them in order to avoid dirt and other impurities in the air gap which occurs between the two tool parts 6 and 7. When the tool 1 is intended to be used for aseptic packages, the method according to Figs. 1 and 2 of applying the PTC resistors 2 may be less appropriate. Another method for then applying the PTC resistors 2 is to drill channels in the tool 1 and lay parts in these channels so that both of the contact plates 4 are in then in contact with different parts of the tool 1.

As a result of this application of PTC resistors (which technically are connected in parallel in the tool 1), there will be obtained a number of PTC resistors 2 which are distributed so as to realise a uniform heating of the tool 1. The different parts 6, 7 of the tool 1 are connected to a current source (not shown). The current source is preferably a direct current source, which is easy to regulate. The two tool parts 6 and 7 may also be connected to an alternating current source.

Fig. 3 shoλvs how a tool 1 for the application of an end wall on a packaging container blank may be designed. The end wall blank 9 is retained in a retainer device 10 which fundamentally consists of two tubular tools 1 of

which one or both may be heated using the above-described method. In that one or both of the tools 1 included in the retainer device 10 are heated, the edge zone 11 which is retained by the end wall blank 9 will be heated to softening temperature, which entails that the thermoplastic is softened, but must not become tacky. In order to avoid tackiness, the tools 1 may be provided with Teflon® surfaces. Once the end wall blank 9 has been heated up in its entirety at the same time as the edge zone 1 1 is heated by the tool 1 , a packaging container body is moved on a mandrel (not shown) in towards the end wall blank 9 and the end wall biank 9 is sealed against the outer thermoplastic layer of the packaging container body. When the tool, after the sealing operation, releases the edge zone 11 which is now heated, the tool 1 will, in its movement towards the packaging container body, press in the edge zone 11 against the outer surface of the packaging container body and thereby create a certain seal and adhesion against the surface of the packaging container body. In Fig. 5 another embodiment of the invention is shown. Here the tool 1 consists of a mandrel 1 , arranged to bear a thermoplastic blank 9, in this embodiment consisting of a preformed tube. The thermoplastic blank is hold fixedly on to the mandrel 1. The mandrel 1 and the blank 9 is surrounded by an outer moulding tool 13, for the final forming of the blank 9 into a ready-made package in the form of a plastic bottle. Inside the mandrel between its two parts 6 and 7 are arranged a number of PCT resistors 2 as shown in Fig. 2. The blank 9 is heated to softening temperature by the heated tool 1 and then formed into the outer moulding tool 13.

As will have been apparent from the foregoing description, the present invention realises a method of heating a tool which entails that a number of heater elements may, in a simple manner, be connected in parallel, with few connections, and which further entails that a uniform temperature may be maintained in the tool.

The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being conceivable without departing from the spirit and scope of the appended Claims.