Method and device for producing a hard-foam film

Abstract

A method for producing a hard-foam film, for use as a core layer in sandwich composite elements, having an increased compressive strength along a thickness extension extending perpendicularly to its surface extension, from a foam block (3) of a thermoplastic, extruded, in particular partially crystalline and/or closed-cell, plastic rigid foam material, PET.

Claims

1. A method for producing a rigid-foam film, the method comprising the steps of: providing a foam block (3) made of a thermoplastic, extruded PET-plastic rigid-foam material selected from a partially crystalline PET-plastic rigid-foam material, a closed-cell PET-plastic rigid-foam material or combinations thereof, wherein the PET-plastic rigid-foam material has a higher compressive strength along a thickness extension axis (V.sub.s) than along a surface extension axis perpendicular to the thickness extension axis (V.sub.s), cutting a film section (6) from the foam block (3) by cutting the foam block (3) by using a knife (2), which has a cutting edge (7), along a feed axis (V.sub.b) extending perpendicular to the thickness extension axis (V.sub.s), wherein the film section (6) is cut along the feed axis (V.sub.b), lifting and separating the film section (6) from the knife (2) by elastic bending of the film section (6) to reduce friction forces acting on the knife (2) during the cutting process, wherein the PET-plastic rigid-foam material is at least partially heated by a heating means to a processing temperature above 40? C. and below the melting temperature of the PET-plastic rigid-foam material in such a manner that the PET-plastic rigid-foam material becomes se elastically bendable that the film section (6) can be elastic bent, lifted and separated from the cutting edge of the knife (2), and wherein the PET-plastic rigid-foam material of the film section returns to being rigid to form the rigid-foam film.

2. The method according to claim 1, wherein the heating means comprises an infrared heating means, the infrared heating means being adapted to the PET-plastic rigid-foam material in such a manner that a penetration depth of the infrared radiation is at least equal to a thickness of the film section (6) to be separated.

3. The method according to claim 2, wherein a surface temperature of the foam block is measured at a position disposed in front of the cutting edge and in front of a position of the infrared heating means in the feed axis (V.sub.b) and the measured surface temperature serves as an input variable for controlling radiant power of the infrared heat source.

4. The method according to claim 1, wherein the PET-plastic rigid-foam material is at least partially heated to a processing temperature above the glass transition temperature of the PET-plastic rigid-foam material minus 20? C.; wherein the PET-plastic rigid-foam material is at least partially heated to a processing temperature below 180? C.; and combinations thereof.

5. The method according to claim 4, wherein the processing temperature is below 120? C.

6. The method according to claim 1, wherein the cutting process is performed in such a manner that the rigid-foam film resulting from the cutting process has a minimum thickness extension (d) of 0.5 mm and/or of less than 10 cm measured perpendicular to the surface extension.

7. The method according to claim 1, wherein for cutting, the knife (2) is moved relative to the foam block (3) along a knife movement axis (M.sub.b) extending perpendicular to the thickness extension axis (V.sub.s) and perpendicular to the feed axis (V.sub.b), the knife (2) rotating in a rotation direction (U), or by a back-and-forth movement.

8. The method according to claim 1, wherein the knife (2) is free of saw teeth at the cutting edge (7) and/or comprises a metal band having a maximal material thickness from a value range between 1 mm and 5 mm.

9. The method according to claim 1, wherein the elastic bending is performed in such a manner that no plastic dimensional changes of the rigid-foam film results from the cutting process, except for interactions with the cutting edge (7).

10. The method according to claim 1, wherein the foam block (3) and/or the PET-plastic rigid-foam material and/or the rigid-foam film has a density from a value range between 40 kg/m.sup.3 and 250 kg/m.sup.3.

11. The method according to claim 1, wherein, when measured according to DIN 7726, the rigid-foam material has a compressive stress of more than 80 kPa at a compression of 10%.

12. The method according to claim 1, wherein the the heating means is an oven or wherein the heating means is an IR radiator in front of and/or in a contact area to the cutting edge (7) of the knife (2) along the feed axis (V.sub.b).

13. The method according to claim 1, wherein the foam block (3) is composed of a plurality of extruded body segments (4), the body segments (4) being welded and/or bonded to one another at their contacting surface sides, parallel or crossing stiffening lines being formed as a result when viewing a surface side of the rigid-foam film along the thickness extension axis (V.sub.s).

14. A sandwich composite element having a core layer made of the rigid-foam film according to claim 13 sandwiched between at least two cover layers disposed on its surface sides.

15. The rigid-foam film, produced by means of a method according to claim 1, made of a thermoplastic, extruded, partially crystalline and/or closed-cell, PET-plastic rigid-foam material having a polymer structure stretched perpendicular to the rigid-foam film surface extension, the rigid-foam film having at least one surface side obtained by a knife cutting process and a minimum thickness extension (d) of 2 mm measured perpendicular to the surface extension.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages, features and details of the invention are apparent from the following description of preferred exemplary embodiments and from the drawings.

(2) In the drawings,

(3) FIG. 1 shows a partial illustration of a rigid-foam film cutting device designed according to the idea of the invention for performing the method according to the invention, wherein, for the sake of clarity, the heating means are not shown,

(4) FIG. 2 shows a side view of the device according to FIG. 1 along a knife movement axis M.sub.b extending perpendicular to a feed axis V.sub.b and perpendicular to a thickness extension axis V.sub.s,

(5) FIG. 3 shows an enlarged illustration of detail X of FIG. 2,

(6) FIG. 4 shows a top view of the device according to FIG. 1 along thickness extension direction V.sub.s.

DETAILED DESCRIPTION

(7) In the figures, the same elements and elements having the same function are referenced with the same reference signs.

(8) FIGS. 1 to 4 show different views and partially enlarged details of a rigid-foam film cutting device 1 designed according to the idea of the invention. Rigid-foam film cutting device 1 comprises relative movement means to create a relative movement along shown feed axis Vb between a knife 2 and a foam block 3; in the shown exemplary embodiment, foam block 3 has a front surface 20 and side surfaces 22, 24, and consists exclusively of a plastic rigid-foam material, in particular PET. The plastic rigid-foam material is a thermoplastic, partially crystalline plastic material which has closed-cell pores and which is obtained by extrusion. In the extrusion process, plastic material is subjected to high shear forces by means of propellant gas in an extruder and is pressed through a nozzle at the end of the extruder-downstream of the nozzle, the intended foaming process takes place by expansion of the gas. A polymer structure stretched in the extrusion direction is obtained by the extrusion process, more precisely the pressing though a nozzle. In the shown exemplary embodiment, foam block 3 is composed of a plurality of body segments 4 which are welded (alternatively bonded) to one another over their entire surface, a stiffening structure of, for example, crossing stiffening webs 5 resulting in a top view along a thickness extension axis Vs to be explained below, stiffening webs 5 either consisting of plastic melted by the welding process or, alternatively, of adhesive. As an alternative to the shown stiffening structure of crossing stiffening webs, it is also possible to realize exclusively parallel stiffening webs or alternative geometries as shown in FIGS. 3 to 8 of EP 1 536 944 B2, for example. It is also possible to not realize stiffening webs to produce rigid-foam films having small surfaces, for example.

(9) Irrespective of the specific arrangement of the body segments or stiffening webs, foam block 3 or the plastic rigid-foam material of the foam block in any case has a stretched polymer structure along thickness extension axis V.sub.s which extends perpendicular to feed axis V.sub.b and perpendicular to a knife movement axis M.sub.b. In other words, the compressive strength along thickness extension axis V.sub.s is higher or greater than perpendicular thereto.

(10) In the shown exemplary embodiment, knife 2 is realized as a band knife which rotates in a rotation direction U and which is displaced relative to foam block 3 along knife movement axis M.sub.b mentioned above in the area of foam block 3. The rigid-foam film 16, FIG. 3, is created in this way during the cutting process, in which a respective already separated film section 6 becomes longer along feed axis V.sub.b after passing a smooth cutting edge 7 of knife 2 as a result of the relative movement of foam block 3 and knife 2 along feed axis V.sub.b. The cutting edge extends parallel to knife movement axis M.sub.b and perpendicular to feed axis V.sub.b and to thickness extension axis V.sub.s.

(11) The combined view of FIGS. 2 and 3 shows that separated film section 6 is bent away from knife 2 by elastic bending around a bending axis A in an area behind cutting edge 7 of knife 2. Bending axis A extends parallel to knife movement axis M.sub.b. The elastic bending prevents large-scale contact of separated film section 6 with knife surface side 8 facing away from remaining foam block 6.

(12) To allow such an elastic bending of separated film section 6 in the first place, the plastic rigid-foam material is heated to a processing temperature by means of heating means 9, in the present example in the form of an infrared radiator; at said processing temperature, the plastic rigid-foam material does not melt, but can be elastically deformed to be able to lift separated film section 6 from knife 2 as shown. In the present exemplary embodiment, the processing temperature is 80? C. Corresponding bending means 10 in the form of a ramp are provided for lifting and therefore bending separated film section 6 in the shown exemplary embodiments (see FIG. 2).

(13) Other heating means can be realized as an alternative to the infrared radiatorrelatively thick rigid-foam films, in particular rigid-foam films having a thickness extension d of more than 2 mm extending parallel to thickness extension axis V.sub.s, whose polymer structure is stretched in thickness extension direction d can only be produced by heating the rigid-foam material to the processing temperature.

(14) FIG. 4 shows a section of device 1 according to the invention which is located in front of knife 2 in feed axis V.sub.b. The top view of FIG. 4 along the thickness extension axis shows that heating means 9 are disposed above foam block 3, heating means 9 being designed as infrared heating means. The infrared heating means comprise individual heating devices 11 which are disposed at an angle to feed axis V.sub.b in order to ensure even warming or heating of foam block 3 over the entire width of foam block 3. Additionally, FIG. 4 shows that heating means 9 are wider than the width of foam block 3, which also contributes to even warming of foam block 3 over the entire width of foam block 3.

(15) In FIG. 4, a first temperature sensor 12 and a second temperature sensor 13 are provided, at least first temperature sensor 12 being connected to a control unit 14 via a data connection; control unit 14, in turn, is connected to heating means 9 and can influence or control a control of heating means 9, in particular a control of the heat output or the radiant power of heating means 9. In the example of FIG. 4, another connection of temperature sensor 13 to control unit 14 can be provided in addition to a corresponding connection of first temperature sensor 12 to control unit 14, so that the surface temperatures of foam block 3 measured by temperature sensor 13 can also be provided to control unit 14 as a control variable.

(16) In the example of FIG. 4, temperature sensors 12 and 13 are designed as pyrometers and perform a contactless measurement of the surface temperature of foam block 3 vertically downwards along thickness extension axis V.sub.s. However, other arrangements and measurement directions of temperature sensors 12 and/or 13 can also be provided. Advantageously, however, temperature sensor 12 is disposed or aligned accordingly in front of heating means 9 along the feed axis at least to the extent that the respective surface temperature measured in a section 15 of the foam block during the time interval required for passing on the measured surface temperature to control unit 14, for generating a control variable for heating means 9 and for adjusting heating means 9 to a corresponding new or adapted radiant power, the respective surface temperature measured in a section 15 of the foam block corresponds to the time interval in which the foam block has just covered the distance which corresponds to the distance between measuring position 15 and the position of heating means 9. In other words, this means that the feed rate, the measuring position of the surface temperature and the inertia of the system are adapted to one another in such a manner for setting a changed radiant power that an optimal adaptation of the radiant power to the measured surface temperature can take place.

(17) The controlled radiation of the heat output of heating means 9 ensures that no excessive thermal energy which can have a negative effect on a film section in the form of wave formation of the film section, for example, after the separation of the film section is introduced into the foam block. The heating means, in particular the infrared heating means, are adapted to the PET material or the PET-plastic rigid-foam material in such a manner that the penetration depth of the infrared rays and therefore the direct or primary heating of the PET material is at least equal to the thickness extension or thickness of the film to be separated or of the film section of the foam block to be separated. This ensures sufficient but not excessive heating over the entire layer thickness or thickness of the film section to be separated, which, on the one hand, allows advantageous lifting of the separated film section, but at the same time prevents excessive heating and thus a negative impact on the produced product, namely the separated film section.

REFERENCE SIGNS

(18) 1 rigid-foam film cutting device 2 knife 3 foam block 4 body segments 5 stiffening webs 6 separated film section 7 cutting edge 8 flat knife side 9 heating means 10 bending means 12 first temperature sensor 13 second temperature sensor 14 control unit 15 section/measuring position V.sub.s thickness extension axis V.sub.b feed axis M.sub.b knife movement axis A bending axis d thickness extension U circumferential direction