DE10228312A1 - Hot press method for joining a thermoplastic component to a second component involves heating the first component in an initial contact position then compression of the first against second component - Google Patents

Abstract

A first component(10) with a thermoplastic joining area(16) and a second component(12) are brought into an initial joining position. The joining area is heated to a temperature in the thermoforming range of the plastic. In a second stage the joining area of (10) and (12) are pressed together by a compression tool then the heated area is cooled below the thermoforming temperature. The second component(12) has a recess(14) and the compression tool has a face complementary to the recess. The heated area of the first component and the second component are pressed together by relative movement of the tool and second component. During compression the heated area of the first component is pressed at least partly into the recess. In the initial joining position the joining area(16) forms a sleeve around the second component. Material thickness in the joining area prior to heating is 0.5-3mm. Initial heating can be either by hot gas or infra-red. Gas temperature lies between the lower limit of the thermoforming region and 400[deg]C. Heating temperature are monitored. Compression tool temperature is held below the thermoforming temperature of the plastic. Compression duration is 0.5-10 seconds. For preventing adhesion the compression tool face may have an anti-adhesion coating and/or be polished and/or be coated in a release agent.

Description

The present invention relates to a method of hot caulking of two workpieces and especially for hot caulking of a first workpiece with a connection area made of a plastic material and one second workpiece.

When assembling devices, which have plastic and / or metal workpieces, it is necessary to connect them together.

A possible connection procedure this is hot caulking. It is a thermoplastic plastic workpiece in one Recess of another workpiece caulked by positioning the plastic workpiece on the other workpiece in a connection area corresponding to the recess by means of one is called Press tool until exceeded heated a predetermined deformation temperature and thereby in the Recess is pressed. After the plastic workpiece its Has reached its final shape, the plastic workpiece is below the deformation temperature cooled, so that it maintains its final shape.

However, this procedure has some Disadvantage.

So the press tool usually has a high heat capacity, which makes the cooling of the plastic workpiece slows down at the end of the caulking with the pressing tool attached becomes. In particular, it may be necessary to close the press tool cool. To make the next connection the press tool must then be heated again. This is either elaborate cooling or heating devices necessary or result from cooling Ambient air has comparatively long cycle times when the Bonding process.

Furthermore, the deformation of the plastic workpiece the temperature of the pressing tool above a glass temperature of the plastic of the plastic workpiece, but below a liquefaction temperature stay.

exceeds the temperature of the pressing tool the condensing temperature, the plastic melt at least partially, so that plastic from the press tool flows away and it penetrates too far into the plastic workpiece.

Falls below the temperature of the Press tool's glass temperature is a continuous deformation of the plastic is not possible and it can due to the high stresses occurring during the deformation in the Plastic microcracks or even cracks are created, which are the connection weaknesses. In particular, a white break occur. That through the glass temperature and the condensing temperature However, the temperature window formed is very narrow, making it difficult is the temperature of the pressing tool or the temperature of the deforming plastic within this temperature window hold.

It is also possible that The actual pressing process is started before the plastic material the glass temperature exceeded has, which in turn creates cracks in the plastic workpiece can.

Another disadvantage is that the pressing tool is usually shaped so that the plastic workpiece when reached the final shape rests on the pressing tool. However, this can be the case bring that the shape of the press tool not necessarily to the Shape of the plastic workpiece is adjusted at the beginning of the caulking and therefore at the beginning of the Caulking only a line or point contact between the press tool and the plastic workpiece comes about, which leads to poor heat transfer to the plastic workpiece. Furthermore with such a line or point contact is a very inhomogeneous one Temperature distribution to be expected.

The heat transfer to the plastic workpiece can improved by pressing the pressing tool against the plastic workpiece very precise process management is necessary for this, to the problem already mentioned avoid cracking.

The present invention lies hence the task of a method for connecting a first workpiece with a connecting area made of plastic with a second workpiece, with which a reliable connection can be established quickly and easily is.

The task is solved by a method with the features of claim 1.

The method according to the invention for connecting a first workpiece, which has a connection area formed from a thermoplastic material, to a second workpiece by hot caulking, provides that in a first step the first workpiece and the second workpiece are brought into an initial connection position and without use a pressing tool, at least one predetermined heating area of the first workpiece, which comprises the connection area, is brought to a temperature within a deformability temperature range of the plastic material, in a second step the first workpiece is pressed against one another with at least one pressing section of the connecting area and the second workpiece by means of a pressing tool acting on the first workpiece are pressed and at least the heating area of the first workpiece on a Temperature is cooled below the deformability temperature range, and in a third step, the press tool and the first workpiece are removed from each other.

The method according to the invention is for any first workpieces applicable, insofar as this comprises at least one connection area have a thermoplastic plastic material for connection with the second workpiece on a designated section of the second workpiece.

The connection area can in principle be formed from any thermoplastic, in particular can amorphous or semi-crystalline plastics are used, too May include polymer blends. The plastics can about that further filling materials and in particular also a fiber reinforcement, for example a Short fiber reinforcement with glass fibers.

The second workpiece can be in the area in which it with the first workpiece is connected, basically be made of any material that is stable enough to act as an abutment when pressing the pressing tool and the second workpiece against each other for the To be able to serve pressing tool. The material of the second workpiece is therefore also preferred at the temperatures to which the heating area is heated, essentially not deformable by the forces occurring in the pressing step. In particular, it can be in the material of the second workpiece in the Area where it is connected to the first workpiece to plastic, Trade metal or ceramic.

The first step is before, during or preferably after a relative positioning of the first and second workpiece to each other at least one predetermined one, which encompasses the connection area heating area of the first workpiece heated. This is done without using a press tool, including what understand is that warming not by contact with a suitably heated press tool is achieved. As a rule, the heating area becomes that of the following Pressing deformed area include.

The heating takes place up to a temperature within a deformability temperature range of the plastic material of the connection area. Below the deformability temperature range of the plastic material in the sense of the invention becomes one for the plastic material understood specific temperature range in which the plastic material within one for an implementation of the inventive method relevant period of time, preferably within up to 10 minutes, is plastically deformable by applying pressure. An upper limit temperature the deformability temperature range is below or at condensing temperature of the plastic material, in the sense of the invention one for the plastic material specific temperature is understood below which within one for an implementation relevant period of time, preferably shorter than 10 minutes can be a significant flow of the plastic material solely due to gravity that make a connection difficult or would prevent cannot be done. The deformability range can in particular include a softening temperature of the plastic material.

For plastic materials, the amorphous Include polymers, the deformability range is above that Glass temperature and below a temperature at which a significant Flow of the plastic material occurs solely due to gravity. For plastic materials based on partially crystalline polymers the deformability range is above the glass temperature and below a melting temperature or a melting range of the plastic.

The temperature at which the heating area of the first workpiece heated is needed in the heating area not necessarily to be constant, but can also be within, for example the limits of the deformability temperature range in the heating range spatial vary.

Then in the second step by means of at least one pressing section of the connection area acting pressing tool the pressing section, which is particularly within of the heating area can lie, and the second workpiece pressed against each other. Here, an at least partial deformation of the heating area takes place instead, the particular larger than that actual press section can be. Basically, the pressing tool can do this or the second workpiece be moved. It can however also the pressing tool and the second workpiece at the same time to be moved towards each other.

While pressing and / or thereafter the heating area is at temperatures below the lower limit of the deformability temperature range cooled. The cooling down can in particular by heat transfer to the second workpiece and / or the pressing tool and / or the surrounding air take place.

After cooling down the heating area the pressing tool can then be removed from the pressing area in the third step be removed by moving the press tool and / or of the now connected with the second, first workpiece can.

Due to the contactless and thus direct heating of the heating area, a good heat transfer to the heating area is achieved, and with favorable heating it can in particular be achieved that the entire heating mation range is largely homogeneously heated to a temperature within the deformability temperature range. The occurrence of cracks or micro-cracks during the subsequent deformation of the pressing section or at least parts of the heating area by the pressing tool can be largely prevented, so that a very reliable connection is achieved.

In addition, there is no heating of the pressing tool to temperatures above the deformability temperature range heating may be necessary as well as cooling of the heating area occur very quickly, so that the method according to the invention allows short cycle times. In particular can cool down with a second workpiece, the one with the first workpiece area to be joined made of a highly thermally conductive material such as metal or a corresponding ceramic material is formed, very quickly second workpiece respectively.

Because the warming doesn't have that Press tool done, the method can be compared with a simple device can be performed where the press tool is independent from the heat source for warming of the heating area can be trained.

Developments and preferred embodiments of the invention are in the description, the claims and the drawings.

It is preferred that the second Workpiece one Recess has that a pressing tool with one essentially complementary to the recess shaped press surface is used, and that at least the pressing section of the first workpiece and the second workpiece below Use of the pressing tool acting at least on the pressing section a relative movement of the pressing tool and the second workpiece against each other depressed be, the pressing section at least partially into the recess is pressed. At least part of the heating or connection area and in particular the pressing section in caulked the recess, creating a particularly stable connection between the first workpiece and the second workpiece results. The press area does not necessarily have to conform completely to the shape of the recess be adapted, but only a sufficient deformation of the press section and further areas of the connection area deformed during the pressing step cause. It is expedient the press area however shaped so that corresponding parts during the pressing step of the heating area preferably Completely on the press surface adjacent to walls the recess pressed become.

Basically, the first workpiece can start of the method according to the invention have any shape. So it is particularly possible that it is plate-like or film-like or film-like. Prefers however, it is the first workpiece in the initial Connection position at least in the connection area, the second workpiece sleeve-like surrounds. This creates a particularly stable connection between the first workpiece with the second workpiece reached.

The material thickness of the first workpiece in the Connection area is essentially limited in that the warming area heatable in a reasonable time interval and the connection area is at least partially deformable without the appearance of cracks. It is preferred that the first workpiece in the connection area before Heat a material thickness between about 0.5 mm and 3 mm.

In particular for applications in the motor vehicle sector, it is preferred that the plastic material contains a polymer that is selected from the group consisting of polyamide 6 , Polyamide 66 , Polyethylene terephthalate and polybutylene terephthalate. These very resistant materials that can withstand comparatively high temperatures can be used in amorphous or semicrystalline form and can be processed very well with the method according to the invention. The plastic material can in particular also contain mixtures of the polymers mentioned.

The temperature is preferably to which the heating area heated is, in a middle range between the limits of the deformability temperature range.

The heating of the heating area can in principle as desired, unless it is done by means of the pressing tool.

The heating of the heating area of the first workpiece can in a preferred embodiment before positioning in the first step using a Fluids, i.e. a gas or a liquid. In particular, can the first workpiece in an appropriate liquid bath are brought, whereby a particularly targeted and even predetermined temperature can be reached.

Preferably, the heating of the heating zone however contactless.

For this purpose, it is preferred that in the first step the heating area of the first workpiece is heated with a hot gas, for example hot air. However, the hot gas can particularly preferably be an inert gas, for example nitrogen or carbon dioxide, so that oxidation of the plastic material in the heating region can be largely avoided. This type of heating is characterized on the one hand by the fact that only very simple devices are required for this, when using heated air only a blower and a nozzle fed by the blower, and on the other hand a very homogeneous heating of the heating area is achieved at the same time. The nozzle is particularly preferred designed in such a way that the hot gas flow, if possible, can only be directed towards the heating area.

When using hot gas it is particularly preferred that the temperature of the hot gas be between the lower limit temperature of the deformability temperature range of the plastic material of the deformation range and 400 ° C. This causes damage the surface of the plastic material due to excessive temperatures, in particular partial combustion avoided. Preferably the temperature is of the hot gas in dependence on the heating time chosen so that the warming area is heated as evenly as possible. In particular in a direction perpendicular to the surface of the heating zone is the warming essentially from the thermal conductivity and the specific heat of the plastic material and its material thickness influenced, so that the Temperature and strength of the hot gas flow as well as the heating time dependent on selected from the properties of the connection area can. Uniform heating of this Direction enables in particular a particularly gentle deformation without cracking the following pressing step.

As an alternative to or in addition to the use of hot gas, the heating area of the first workpiece can preferably be heated with infrared radiation in the first step. In particular, a laser can be used for this, which operates in a corresponding wavelength range. For example, CO ₂ lasers or preferably diode lasers with a corresponding emission wavelength in the infrared range can be used as lasers. The latter are more efficient. Furthermore, they can be made more compact. Depending on the polymer on which the plastic material is based, it may be expedient for the plastic material to contain additives which absorb the infrared radiation from the laser and convert it into heat. The use of a laser not only enables a particularly precise heating of a small, delimited heating area, but also enables a particularly simple construction of a device used to carry out the method according to the invention, since there is no need to move a hot air nozzle near the heating area, which must then be moved away later to make room for the pressing tool. Rather, the laser can be arranged outside the stroke of the pressing tool, so that its movement is not hindered and a very short cycle time can be achieved for the connection method according to the invention. Furthermore, with a corresponding penetration depth of the infrared radiation, uniform heating of the heating area can be achieved more quickly.

Basically, the process parameters of the method according to the invention for the second and / or third step depending of the workpieces used for this and the device used for this through optimization attempts be determined. This particularly affects the length of time during which contactless heating of the heating area is required.

To achieve one for carrying out the inventive method as cheap as possible Temperature of the heating area to be able to ensure it is preferred that the temperature of the heating area be monitored during the heating becomes. This can be particularly preferably contactless, for example under Use of pyrometers. It can be particularly preferred according to the temperature of the heating area, the heating of the heating zone be managed.

The speed at which that Press tool is moved relative to the pressing section is preferred chosen so that there are no cracks or deformations in the connection area Micro cracks develop. To be as possible achieve high pressing speeds and thus short cycle times to be able can Appropriate optimization attempts are carried out.

It is further preferred that the temperature of the press tool below the deformability temperature range of the plastic material is held. It doesn't just make one rapid cooling of the heating area to temperatures below the deformability temperature range reached, but it will also stick to the press tool avoided the pressing section, which could occur especially when the temperature of the press tool near the melting point of the plastic material would. Especially the temperature of the pressing tool is preferred, in particular during all Process steps, less than 100 ° C. Depending on the size of the press tool, thermal conductivity the material of the pressing tool, the cycle time and the temperature, to which the heating area heated the press tool is expediently cooled. Preferably cooling takes place however to a significant extent by the ambient air and / or Press tool holder.

The temperature of the second workpiece can in principle arbitrarily chosen his. In particular, it can be in or above the deformability temperature range lie, causing the plastic material to adhere to the second Workpiece is favored. However, this will cool down the heating area significantly more difficult. It is therefore preferred that the temperature of the second workpiece below the deformability temperature range and particularly preferred below 100 ° C is held. Expediently just used it at room temperature.

The pressing time in the second step is like this chosen that a permanent deformation in the connection area and thus a permanent connection with the second workpiece is obtained. The pressing time depends, among other things, on the temperature to which the heating area is heated and the type of plastic material. It is preferred that the pressing time in the second step is between 0.5 seconds and 10 seconds.

Depending on the shape of the to be connected workpieces the pressing tool can contain one or more pressing elements, which each have at least one pressing surface. So for staking a plastic sleeve on a section of a cylinder, preferably a pressing tool can be used, which has at least two pressing elements, the pressing surfaces the shape of a circular section in each case in a cross-sectional plane have and preferably at equal angular distances from each other are arranged. The angles, over which extend the circular sections can in particular be the same be great and / or add up to 360 °, so the sleeve Completely be enclosed is.

To make the press tool as accurate as possible to be able to have reproducible force acting on the pressing section it is preferred that the pressing section of the first workpiece and the second workpiece be pressed against each other by means of a spring-loaded punch. It becomes a very precisely controllable deformation in the connection area achieved, on the one hand a firm connection between the first and second workpiece allows and secondly the occurrence of cracks or microcracks largely prevented.

To reduce the risk of clinging to at least the To reduce the pressing section on the pressing tool, it is preferred that a pressing tool is used, the pressing surface of which Has adhesive coating. This non-stick coating can it is, for example, a Teflon coating or expediently a hardened Act non-stick coating.

To reduce the risk of sticking it is further preferred that a press tool is used whose Press area is polished. Such a polished press surface has no depressions or Burrs that favor adhesion of plastic material could.

Sticking of the pressing tool the pressing section can furthermore preferably be avoided by that before pressing the pressing tool and pressing section together in the second step a release agent on the press surface of the Press tool and / or the pressing section is applied.

To ensure the longest possible service life of the press tool and especially its press area To achieve this, it is preferred to use a press tool will, whose press area through a ceramic surface is formed. The ceramic surface can go through the surface a ceramic body of the pressing tool or only with an appropriate ceramic Coating may be given. In particular, ceramics can be used here be that a particularly large Exhibit hardness. The coating can preferably be titanium nitride or silicon nitride contain.

The invention is illustrated below by way of example of the drawings further explained. Show it:

1 1 shows a schematic partial sectional view of a connection area of a first workpiece and a second workpiece in an initial connection position,

2 is a schematic partial sectional view of the arrangement in 1 with a device for heating a heating area of the first workpiece with hot air, and

3 is a schematic partial sectional view of the first and the second workpiece in 1 with a press tool.

For connecting a first workpiece, a plastic sleeve 10 made of polyamide 6 , with a metal cylinder 12 with a circumferential ring groove 14 The plastic sleeve is the second workpiece by means of a method for hot caulking according to a preferred embodiment of the invention 10 and the metal cylinder 12 placed in an initial connection position, partially in 1 is shown and in which the plastic sleeve 10 the metal cylinder 12 encloses along part of its axial extent.

One through an upper ring section 16 the plastic sleeve 10 formed connection area of the plastic sleeve 10 is in the initial connection position on the metal cylinder 12 and covers the ring groove 14 from.

The wall thickness of the plastic sleeve 10 in the ring section 16 is 1 mm or 1.5 mm at the upper or lower end. The deformability temperature range of the polyamide used 6 lies between 210 ° C and 225 ° C. When using polyamide 6 With additives, the deformability temperature range can be shifted to higher temperatures depending on the additives used. For example, a range from 240 ° C to 250 ° C can be achieved.

Both the plastic sleeve 10 as well as the metal cylinder 12 are at room temperature at this time.

In a further sub-step of the first step (cf. 2 ) becomes an annular heating area 18 in the ring section 16 heated contactless with hot air. For this, an annular nozzle 20 used by a heat shield arranged concentrically to it 22 is surrounded. The nozzle 20 and the heat shield 22 are over the free end of the metal cylinder 12 and the plastic sleeve around it 10 moved and coaxial to the plastic sleeve 10 and positioned in the axial direction relative to this so that from the jet 20 escaping hot air along the circumference of the plastic sleeve 10 evenly essentially on the heating area 18 is directed.

It will then go through the nozzle 20 Hot air at a temperature of about 270 ° C on the heating area 18 steered until it reaches a temperature of about 220 ° C, which is within the deformability temperature range.

Thereupon the nozzle 20 with the heat shield 22 from the plastic sleeve 12 moved upwards.

In a subsequent second step, an annular punch serving as a pressing tool is used 24 part of the ring section 16 in the ring groove 14 caulked (cf. 3 ).

The ring-shaped stamp 24 is, not shown in the figures, constructed in three parts from three identical ring segment-shaped ring sections, so that the plastic sleeve is brought together in one pressing step by bringing the ring sections together 10 along their entire circumference with the metal cylinder 12 can be connected.

The Stamp 24 points one to the ring groove 14 complementary shaped press surface 26 which is provided with an appropriate ceramic coating based on titanium nitride to extend the service life and is polished to prevent plastic from sticking.

The Stamp 24 , which has a temperature of about 50 ° C when the process is carried out cyclically, with the pressing surface 26 against a press section 28 in the ring area 16 or the heating area 18 pressed (cf. 3 ). The press section 28 is thereby under plastic deformation in the ring groove 14 pressed. The pressing movement takes place within about a second.

After that the stamp 24 for another about two seconds in his in 3 leave the position shown, the heating area 18 through heat transfer to the metal cylinder 12 , the stamp 24 and the ambient air is cooled to a temperature below 200 ° C. and thus below the deformability temperature range.

In a third step, the stamp is then 24 from the second workpiece 12 away.

There is a connection of the plastic sleeve 10 with the second workpiece 12 that can be manufactured quickly and yet is free of cracks or microcracks.

Due to the low temperature of the stamp 24 will stick the heated plastic material of the heating area 18 on the pressing surface 28 of the stamp 24 avoided.

In a second embodiment, instead of the annular stamp 24 uses a pressing tool which has four segments spaced apart from one another at equal angular intervals and extending over an angle of approximately 45 °, which, when pressed together, form only sections of a ring. The plastic sleeve is then not along its entire circumference with the metal cylinder 12 connected, but only on the segments corresponding to the pressing tool circumferential sections

In a third embodiment, instead of the metal cylinder 12 uses a cylinder made of a plastic material whose deformability temperature range is above the temperature to which the heating range 18 the plastic sleeve 12 is heated.

In a further embodiment, instead of the nozzle 20 with the heat shield 22 an infrared diode laser, operating at a wavelength of 940 nm, for heating the heating area 18 used. With a laser output of 30 to 50 watts, it is then possible to quickly heat the material above the deformability temperature within a few seconds.

To ensure uniform heating of the heating area 18 along the entire circumference of the plastic sleeve 10 to be achieved during the heating process of the metal cylinder 12 and the plastic sleeve 10 rotated together around their longitudinal axis so that a laser beam of the infrared diode laser over the entire heating range 18 to be led. All other process steps are carried out as in the process described first.

10

Plastic sleeve

12

metal cylinder

14

ring groove

16

ring section

18

heating area

20

jet

22

heat shield

24

stamp

26

Press area

28

Press section

Claims (15)

Method for connecting a first workpiece ( 10 ) which has a connection area formed from a thermoplastic material ( 16 ) with a second workpiece ( 12 ) by caulking, in which in a first step the first workpiece ( 10 ) and the second workpiece ( 12 ) are brought into an initial connection position with respect to one another, and at least one predetermined connection region (without using a pressing tool) ( 16 ) comprehensive heating range ( 18 ) of the first workpiece ( 10 ) is brought to a temperature within a deformability temperature range of the plastic material, in a second step the first workpiece ( 10 ) with at least one press section ( 28 ) of the Ver binding area ( 16 ) and the second workpiece ( 12 ) on the first workpiece ( 10 ) acting press tool ( 24 ) are pressed against each other and at least the heating area ( 18 ) of the first workpiece ( 10 ) is cooled to a temperature below the deformability temperature range, and in a third step the pressing tool ( 24 ) and the first workpiece ( 10 ) from each other. A method according to claim 1, characterized in that the second workpiece ( 12 ) a recess ( 14 ) has a pressing tool as a pressing tool ( 24 ) with a substantially complementary to the recess ( 14 ) shaped press surface ( 26 ) and that at least the pressing section ( 28 ) of the first workpiece ( 10 ) and the second workpiece ( 12 ) using the at least on the press section ( 28 ) acting press tool ( 24 ) by a relative movement of the pressing tool ( 24 ) and second workpiece ( 12 ) are pressed against each other, the pressing section ( 28 ) at least partially into the recess ( 14 ) is pressed. A method according to claim 1 or 2, characterized in that the first workpiece ( 10 ) in the initial connection position at least in the connection area ( 16 ) the second workpiece ( 12 ) surrounds like a sleeve. Method according to one of the preceding claims, characterized in that the first workpiece ( 10 ) in the connection area ( 16 ) has a material thickness between about 0.5 mm and 3 mm before heating. Method according to one of the preceding claims, characterized in that the plastic material contains a polymer which is selected from the group consisting of polyamide 6 , Polyamide 66 and polybutylene terephthalate. Method according to one of the preceding claims, characterized in that in the first step the heating area ( 18 ) of the first workpiece ( 10 ) is heated with a hot gas. A method according to claim 6, characterized in that the temperature of the hot gas between the lower limit temperature of the deformability temperature range of the plastic material of the connecting area ( 16 ) and 400 ° C. Method according to one of the preceding claims, that in the first step the heating area ( 18 ) of the first workpiece ( 10 ) is heated with infrared radiation. Method according to one of the preceding claims, characterized in that the temperature of the heating region ( 18 ) is monitored during heating. Method according to one of the preceding claims, characterized in that the temperature of the pressing tool ( 24 ) is kept below the deformability temperature range of the plastic material. Method according to one of the preceding claims, characterized characterized in that the pressing time in the second step between 0.5 s and 10 s. Method according to one of the preceding claims, characterized in that the pressing section ( 28 ) of the first workpiece ( 10 ) and the second workpiece ( 12 ) are pressed against each other by means of a spring-loaded punch. Method according to one of the preceding claims, characterized characterized in that a pressing tool is used, the pressing surface of which Has non-stick coating. Method according to one of the preceding claims, characterized in that a pressing tool ( 24 ) is used, the press surface ( 26 ) is polished. Method according to one of the preceding claims, characterized in that before the pressing tool ( 24 ) and press section ( 28 ) in the second step, a release agent on the pressing surface of the pressing tool ( 24 ) and / or the press section ( 28 ) is applied.