with a surface melting section The present invention relates to a three-dimensional printing system for preparing a three-dimensional object made at least partially of an expanded polymer, such as of polystyrene foam, as well as to a method for preparing a three-dimensional object made at least partially of an expanded polymer. Expanded polymers, i.e. polymer foams, are cellular structures, which are characterized by a low density. Foams are divided in closed-cell foams, open-cell foams, mixed-cellular foams and integral foams. While closed-cell foams comprise cells, which are completely surrounded by the solid polymer material and which are filled with gas, the cells of open-cell foams are not completely surrounded by the solid polymer material and thus interconnected with each other. Due to this, open-cell foams may absorb liquid, such as water, whereas closed-cell foams do not. Mixed-cellular foams comprise open-cells as well as closed-cells, whereas integral foams have thick, non-cellular or at least essentially non-cellular outer walls and therebetween a cellular core, wherein the density reduces from the outer walls to the inner core essentially continuously.

Foams are easily formable, have a low tensile strength, have a high acoustic insulation property and are furthermore characterized by a low thermal

conductivity. On account of these properties, foams are easily workable and are applied in various commercial areas. For instance, closed-cell foams, such as those made of polystyrene or polyurethane, are used as thermal insulating materials in a plurality of sectors of industry, such as e.g. as building insulation materials. Other examples for the commercial application of foams are acoustic insulating materials, cushionings, mattresses, mats and sponges. Foams may be made of nearly all commercially available polymers, such as of ethylene-vinyl acetate, of polyethylene, of nitrile rubber, of a copolymer of acrylonitrile and butadiene, of polychloroprene, of polyimide, of polyester, of polypropylene, of polystyrene, of polyurethane, of polylactic acid and of polyvinyl chloride.

Several methods for producing foam articles are known. One example therefore is the direct injection expanded foam molding process, in which a pressurized polymer melt including a blowing agent is injected through nozzles into a mold. In the mold, in which a lower pressure is present than the pressure of the pressurized polymer melt, the blowing agent expands, thus forming the polymer foam in the desired shape. Another example is to incubate polymer granulates in an autoclave at an elevated temperature and under pressure with a blowing agent, such as carbon dioxide, before the pressure is released and the temperature lowered so as to foam the granulates to foam beads. These foam beads may then be injected into a mold, before the foam beads are heat fused therein into the desired shape by the application of pressure and steam. Still another example therefore is to form expandable polymer beads by extruding a pressurized, blowing agent including polymer melt through the dies of a die plate and by granulating the polymer melt strands directly behind the dies in an underwater granulator, in which the polymer melt is cooled under pressure so as to avoid an expansion of the polymer strand. The expandable polymer beads may then be foamed and fused in a mold into an article having the desired shape.

Recently, it has been proposed to produce foamed articles making use of three- dimensional (3D) printing. This has the advantage that no moldings, which are laborious and expensive to produce, are required. Moreover, 3D printing is fast, allows to change the material during the process and generates only very small amounts of waste. CN 106493968 A discloses a method and an apparatus for producing a foamed product based on 3D printing. The apparatus comprises a 3D printer as molding unit, a supercritical infiltration unit and a foaming unit. While the supercritical infiltration unit comprises a preheater, a booster pump, a carbon dioxide storage tank and an infiltration vessel, the foaming unit is mainly composed of a steam generator, a foam box and a cover plate. The method comprises the following steps: firstly, printing a three-dimensional model of a polymer melt via the 3D printer; secondly, then putting the formed three-dimensional model into the infiltration vessel of the supercritical infiltration unit and infiltrating supercritical carbon dioxide and thirdly, carrying out steam foaming of the three-dimensional model in the foam box so as to obtain the foamed product. However, this process has several drawbacks. First of all, it does not allow to produce hybrid articles comprising foamed sections and non-foamed sections. Rather, this method only allows to produce articles, which are completely und uniformly foamed. In addition, the foam structure and the density of the foamed product produced with this method cannot be satisfyingly controlled. Furthermore, the polymer strands printed with the aforementioned method to not firmly stick together so that the formed article is not stable enough.

In view of this, the object underlying the present invention is to provide a 3D printing system and a method for preparing a three-dimensional object made at least partially of an expanded polymer, which is more flexible, which leads to a very stable object, and which particularly allows to control the foam structure and the density of the foamed product and allows to produce hybrid articles comprising foamed sections and non-foamed sections.

In accordance with the present invention, this object is satisfied by providing a three-dimensional printing system for preparing a three-dimensional object made at least partially of an expanded polymer comprising: i) a printing device for transporting and depositing an expanded strand of polymer including a blowing agent onto a surface and

ii) a three-dimensional movement device for adjusting the position of the

printing device in a predefined three-dimensional matrix so as to allow to deposit the strand of expanded polymer at a predetermined time at a precise position within the three-dimensional matrix,

wherein the printing device comprises:

a) a feed section,

b) a transporting section,

c) a surface melting section and

d) a terminal printing head section for depositing the expanded polymer strand onto the surface,

wherein all of the feed section a), the transporting section b), the surface melting section c) and the printing head section d) are tubular sections having the same inner diameter, and wherein the surface melting section c) comprises a solid-state welding element, a laser beam, a generator of hot gas or liquid and/or a generator of heat by means of an exothermal reaction.

The 3D printing system in accordance with the present invention does not deposit onto the target surface a polymer strand, which has subsequently to be injected with blowing agent and then foamed. Rather, the 3D printing system in accordance with the present invention deposits onto the target surface a strand of already foamed polymer. More specifically, an expanded polymer strand, such as one with an at least partially crystalline foamed core and a substantially amorphous outer layer, may be prepared for instance extrusion or coextrusion, before this expanded polymer strand may be transferred into the feed section of the printing device, transported in the printing device into the surface melting section, where the surface portion of the expanded polymer strand is molten, but not the interior thereof, and then deposited by discharging it from the printing device. The 3D printing system in accordance with the present invention allows to change the concentration of blowing agent in the expanded polymer strand and allows to change the kind of polymer fed into the printing device over the time. On account of these reasons, the 3D printing system in accordance with the present invention allows to control the foam structure and the density of the foamed product at discretion. Moreover, it allows to produce hybrid articles comprising foamed sections and non-foamed sections, by temporarily stopping to feed into the printing device an expanded polymer strand and by replacing it with a non-expanded polymer not including any blowing agent. All in all, the present invention provides a 3D printing system and a method for preparing a three-dimensional object made at least partially of an expanded polymer, which is more flexible and which

particularly allows to control the foam structure and the density of the foamed product and allows to produce hybrid articles comprising foamed sections and non-foamed sections. In addition, since the surface area of the polymer strand is selectively molten immediately before depositing the expanded polymer strand through the printing head section onto the target surface, thus rendering the surface of the expanded polymer strand sticky, the deposited expanded polymer strand firmly adheres to the surface, which is for instance polymer already deposited some minutes before. Therefore, the present invention allows to produce very stable objects made at least partially of an expanded polymer.

In accordance with the present invention, , all of the feed section a), the

transporting section b), the surface melting section c) and the printing head section d) are tubular sections having the same inner diameter, which is preferably between 1 and 10 mm and more preferably between 2 and 4 mm.

Also, the present invention is in principle not particularly limited concerning the order of the transporting section b) and the surface melting section c), as long as both are between the upstream feed section a) and the terminal downstream printing head section d). Section denotes in this context a longitudinal segment, i.e. a segment extending in the longitudinal direction of the printing device. In accordance with one particular preferred embodiment of the present invention, the feed section a), the transporting section b), the surface melting section c) and the printing head section d) are arranged in this order from an upstream end to a downstream end of the printing device. However, it is possible to change the order of sections b) and c) so that alternatively the feed section a), the surface melting section c), the transporting section b) and the printing head section d) may be arranged in this order from an upstream end to a downstream end of the printing device.

In accordance with an alternative embodiment of the present invention, the transporting section b) and the surface melting section c) are combined to one section, i.e. the combined section is embodied so as to be the transporting section as well as the surface melting section. Downstream of this combined section b), c) follows then the terminal printing head section d).

In accordance with another alternative embodiment of the present invention, the feed section a), the transporting section b) and the surface melting section c) are combined to one section, which is followed by the printing head section d).

The purpose of the surface melting section c) of the printing device of the 3D printing system in accordance with the present invention is to selectively melt the surface area or surface layer, respectively, of the expanded polymer strand transported through the printing device so as to render it sticky, without melting the interior of the expanded polymer strand.

In accordance with a first particularly preferred embodiment of the present invention, the surface melting section c) comprises a solid-state welding element so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. For instance, the solid-state welding element may be provided on the outer wall of the tube of the surface melting section c). Any solid- state welding element may be used, such as an ultrasound generator, a

microwave generator and/or an infrared generator.

In accordance with a second particularly preferred embodiment of the present invention, the surface melting section c) comprises a laser beam so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. The laser beam may be provided on or in a distance to the outer wall of the tube of the surface melting section c).

In accordance with a third particularly preferred embodiment of the present invention, the surface melting section c) comprises a generator of hot gas or liquid so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. The generator of hot gas or liquid may be provided on or in a distance to the outer wall of the tube of the surface melting section c) and may be for instance a generator of hot gas and more preferably a generator of hot air.

In accordance with a fourth particularly preferred embodiment of the present invention, the surface melting section c) comprises a generator of heat by means of an exothermal reaction so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. For example, the generator of heat by means of an exothermal reaction may be provided on or in a distance to the outer wall of the tube of the surface melting section c) and may be generator of a flame.

As set out above, the expanded polymer strand fed into the feed section of the printing device of the 3D printing system in accordance with the present invention may be prepared by extrusion or coextrusion, before the produced expanded polymer strand is transferred into the feed section of the printing device. For this purpose it is suggested in a further development of the idea of the present invention that the 3D printing system further comprises an extrusion device for extruding an expanded polymer strand, in particular for extruding an expanded polymer strand with a solid core and an outer layer having a lower melting point than the solid core so that it can be molten, without melting the solid core, and even more preferably for extruding an expanded polymer strand with an at least partially crystalline foamed core and a substantially amorphous outer layer, wherein the core and the outer layer are made of the same polymer.

In accordance with an alternative preferred embodiment of the present invention, the 3D printing system further comprises a coextrusion device for coextruding an expanded polymer strand, such as preferably an expanded polymer strand with a solid core and an outer layer having a lower melting point than the solid core so that it can be molten, without melting the solid core, and even more preferably for coextruding an expanded polymer strand with an at least partially crystalline foamed core of a first polymer and a substantially amorphous outer layer of a second polymer.

According to another aspect, the present invention relates to a method for preparing a three-dimensional object made at least partially of an expanded polymer, wherein the method is performed in the aforementioned three- dimensional printing system.

In addition, the present invention relates to a method for preparing a three- dimensional object made at least partially of an expanded polymer, wherein the method is performed in a three-dimensional printing system comprising:

i) a printing device for transporting and depositing an expanded strand of polymer including a blowing agent onto a surface and

ii) a three-dimensional movement device for adjusting the position of the printing device in a predefined three-dimensional matrix so as to allow to deposit the strand of expanded polymer at a predetermined time at a precise position within the three-dimensional matrix,

wherein the printing device comprises:

a) a feed section,

b) a transporting section,

c) a surface melting section and

d) a terminal printing head section for depositing the expanded polymer strand onto the surface,

wherein the method comprises the following steps:

a) providing an expanded polymer strand with a solid core and an outer layer having a lower melting point than the solid core so that it can be molten, without melting the solid core, and preferably with an at least partially crystalline foamed core and a substantially amorphous outer layer, b) transferring the expanded polymer strand into the feed section of the

printing device,

c) transporting the expanded polymer strand in the printing device into the surface melting section, where the surface portion of the expanded polymer strand is molten, but not the interior thereof, and

d) depositing the expanded, surface molten polymer strand by discharging it from the printing device.

Preferably, the expanded polymer strand provided in step a) is prepared by extruding a polymer including a blowing agent and optionally a nucleating agent into an expanded polymer strand with an at least partially crystalline foamed core and a substantially amorphous outer layer.

Alternatively, the expanded polymer strand provided in step a) may be prepared by coextruding a first polymer including a blowing agent and optionally a nucleating agent and a second polymer into an expanded polymer strand with an at least partially crystalline foamed core of the first polymer and a substantially amorphous outer layer of the second polymer.

In step c), the surface portion of the expanded polymer strand is molten in the surface melting section, but not the interior thereof. This may be achieved for instance so that the surface melting section comprises a solid-state welding element so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. For instance, the solid-state welding element may be provided on the outer wall of the tube of the surface melting section c). Any solid-state welding element may be used, such as an ultrasound generator, a microwave generator and/or an infrared generator. Alternatively, the surface melting section c) may comprise a heating element so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. For instance, the heating element may be provided on the outer wall of the tube of the surface melting section c), wherein the heating element may be an active heating element, such as a Peltier element and/or an electric resistance heater, or a heat exchanger. Still alternatively, the surface melting section c) comprises a laser beam so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. The laser beam may be provided on or in a distance to the outer wall of the tube of the surface melting section c). Still alternatively, the surface melting section c) comprises a generator of hot gas or liquid so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. The generator of hot gas or liquid may be provided on or in a distance to the outer wall of the tube of the surface melting section c) and may be for instance a generator of hot gas and more preferably a generator of hot air. Still alternatively, the surface melting section c) comprises a generator of heat by means of an exothermal reaction so as to selectively melt the surface portion of the expanded polymer strand, but not the interior thereof. For example, the generator of heat by means of an exothermal reaction may be provided on or in a distance to the outer wall of the tube of the surface melting section c) and may be generator of a flame.

The present invention may be performed with any foamable polymer. Suitable examples therefore are polymers being selected from the group consisting of thermoplastic polyurethanes, polyolefins (such as polyethylenes or

polypropylenes), polyesters (such as polyethylene terephthalates), ethylene vinylacetate copolymers, ethylene butyl acrylate copolymers, polystyrenes, polylactic acids, thermoplastic elastomers, nitrile rubbers, copolymers of acrylonitrile and butadiene, polychloroprenes, polyimides, polyvinyl chlorides and arbitrary combinations of two or more of the aforementioned compounds.

Particular good results are obtained when the polymer is a polyethylene terephthalate.

Even if the present invention may be performed with one or more chemical blowing agents, it is particularly preferred that the blowing agent used in the method in accordance with the present invention is a physical blowing agent. Preferred examples for the physical blowing agent are those selected from the group consisting of carbon dioxide, nitrogen, water, cyclopentane, isobutane, pentane and arbitrary combinations of two or more of the aforementioned compounds.

In order to adjust the size of the bubbles formed by the blowing agent during the expansion, it is suggested in a further development of the idea of the present invention that the expanded polymer strand is prepared by making use of at least one nucleating agent. Good results are in particular obtained, when the nucleating agent is selected from the group consisting of talc, waxes, graphite, bentonites and arbitrary combinations of two or more of the aforementioned compounds. In accordance to another aspect, the present invention relates to a method for preparing a three-dimensional object made at least partially of an expanded polymer, wherein the method is preferably performed in an aforementioned printing system and comprises the following steps:

a) providing an expanded polymer strand with a solid core and an outer layer having a lower melting point than the solid core so that it can be molten, without melting the solid core and preferably with an at least partially crystalline foamed core and a substantially amorphous outer layer, b) transferring the expanded polymer strand into the feed section of a printing device,

c) transporting the expanded polymer strand in the printing device, d) melting the surface portion of the expanded polymer strand, but not the interior thereof,

e) depositing the expanded polymer strand by discharging it from the printing device and

f) applying a glue to the surface of the expanded polymer strand.

By applying a glue to the surface of the expanded polymer strand, its surface is rendered sticky, so that subsequently deposited strand firmly adheres thereto. On account of this reason, also this embodiment of the present invention allows to produce very stable objects made at least partially of an expanded polymer.

The glue or adhesive, respectively, may be selected from a wide variety of adhesives being available on the market depending on the type of the foamed polymer that is to be bonded (e.g. based on the polarities of the material and the adhesive). Examples for suitable adhesives are synthetic adhesives, hot melt adhesives based on ethylene-vinyl-acetate, polyolefines or other thermoplastic polymers and their mixtures, reaction adhesives based on cyanoacrylates, acrylates, epoxide, polyutrethanes or others and bioadhesives based on natural polymers like lignin, starch or others more. In accordance with a further aspect, the present invention relates to a three- dimensional object, which is obtainable with any of the two aforementioned methods. The three-dimensional object may be in particular an acoustic insulating material, a cushioning, a mattress, a mat, a sponge, a shoe sole, a sports shoe, a protective equipment, a support structure or a filling structure.