Injection moulding tool, and method for manufacturing a pedal crank for a bicycle

Abstract

The invention relates to an injection moulding tool for manufacturing a pedal crank for a bicycle from a thermoplastic material, the injection moulding tool comprising: at least two mould parts which, when closed, form a shaping cavity; an injection nozzle through which plasticised plastic melt can be injected into the cavity to form a workpiece; and an injector via which a fluid can be introduced into the cavity filled with plastic in order to displace liquid plastic melt from the interior of the solidifying workpiece and thereby produce an internal cavity in the workpiece. According to the invention, the injection moulding tool comprises a first mould part for forming a first axis receptacle in the workpiece, and the injection nozzle is connected to the first mould part such that plastic melt can be introduced into the cavity via at least one injection opening of the first mould part in the region of the first axis receptacle to be produced. The invention also relates to a corresponding manufacturing method.

Description

[0001] Injection molding tool and method for producing a pedal crank for a bicycle

[0002] The present invention relates to an injection molding tool for producing a pedal crank for a bicycle according to the preamble of claim 1 and a corresponding manufacturing method.

[0003] Cranks for bicycles usually have an elongated shape, with a first axle mount for a crankshaft at one end and a second axle mount for a pedal axle at the opposite end.

[0004] Such pedal cranks are often made of steel or a light metal such as aluminum. It is also known to produce pedal cranks from a thermoplastic material using injection molding to reduce weight. Production can be carried out using the gas injection process. Here, an inert gas (e.g. nitrogen) is introduced into the not yet fully solidified plastic mass of the workpiece to be produced via an injector. This displaces the still liquid plastic melt (plastic plastic core), creating an internal cavity. The displaced plastic melt can be channeled through specially provided openings into an overflow cavity or via an overflow channel back into the injection molding machine.When manufacturing such workpieces using generic injection molding tools, weld lines, which can represent weak points, are created, particularly at the connection points of the injection nozzle and, if present, the overflow cavity.

[0005] The present invention is based on the object of avoiding the occurrence of such weak points and thereby increasing the stability of the manufactured cranks.

[0006] According to the invention, this object is achieved by an injection molding tool having the features of claim 1 and by a method having the features of claim 9. Advantageous embodiments of the invention emerge from the subclaims and the following description.

[0007] Accordingly, an injection molding tool for producing a pedal crank for a bicycle from a thermoplastic material is proposed, which comprises at least two tool parts that form a molding cavity when closed. These can be two tool halves. The injection molding tool further comprises an injection nozzle through which plasticized plastic melt can be injected into the cavity to form the workpiece forming the pedal crank. Manufacturing the pedal crank from a thermoplastic material leads to a reduction in manufacturing costs and component weight. Furthermore, a more corrosion-resistant material than, for example, steel or aluminum can be used, which can also be recycled and reused.

[0008] Furthermore, the injection molding tool comprises an injector, through which a fluid can be introduced into the plastic-filled cavity by means of the fluid injection technology in order to displace liquid plastic melt from the interior of the solidifying workpiece and thereby create an internal cavity in the workpiece. Through the fluid injection, the cooling time and thus the cycle time in component production can be significantly reduced compared to the production of solid components. According to the invention, the injection molding tool comprises a first mold part for forming a first axle receptacle in the workpiece. The first mold part can be a section of a tool part or mold insert forming part of the cavity and can, for example, have a substantially cylindrical, star-shaped or cuboid shape. The first axle receptacle can serve to receive a crankshaft or a pedal axle and can be designed accordingly (ieIt is conceivable that the first molded part is directly overmolded with plastic melt, and its shape defines the shape of the first axle mount. Alternatively, it can be provided that a component such as a threaded sleeve is placed on the first molded part and the component is backmolded and / or overmolded with plastic melt, so that it defines or represents the final axle mount.

[0009] According to the invention, the injection nozzle is connected to the first molded part in such a way that plastic melt is introduced into the cavity via at least one injection opening which is formed in the first molded part in the region of the first axle mount to be produced. The connection of the injection nozzle is thus in the region of and in particular within the first axle mount. Due to this special geometric connection of the injection nozzle to the first molded part, no weld line is created on the outer area of ​​the final workpiece, which would represent a weak point. This applies in particular in the case that the actual axle mount is formed by an injected component. The injection nozzle is in particular located at least partially within the first axle mount of the finished workpiece. Preferably, a plurality of injection openings are formed on the first molded part.

[0010] In one possible embodiment, it is provided that the injection molding tool comprises a second molded part for forming a second axle receptacle in the workpiece. The second molded part can be a section of a tool part or mold insert forming part of the cavity and can have, for example, a substantially cylindrical, star-shaped or cuboid shape. The second axle receptacle can serve to receive a crankshaft or pedal axle and can be designed accordingly (i.e., for example, comprise a thread or internal gearing). It is conceivable that the second molded part is directly over-molded with plastic melt and that its shape defines the shape of the first axle receptacle. Alternatively, it can be provided that a component such as a threaded sleeve is placed on the second molded part and that the component is back-molded and / or over-molded with plastic melt so that it defines or represents the final axle receptacle.

[0011] In this case, the injection molding tool can comprise an overflow cavity or an overflow channel for receiving displaced plastic melt. The overflow cavity or overflow channel can be connected to the second molded part in such a way that displaced plastic melt can flow into the overflow cavity or overflow channel through at least one opening in the second molded part in the area of ​​the second axis mount.

[0012] This special geometric connection of the overflow cavity or overflow channel to the second molded part results in the same advantages as described with regard to the first molded part. The inlet area of ​​the overflow cavity or overflow channel is preferably located within the second axle mount. Preferably, several openings are formed on the second molded part. The first and / or second molded part can be designed as a mandrel.

[0013] In another possible embodiment, the injector is arranged in the region of the first mold part. This can mean, in particular, that the injector is located closer to the first mold part than at an opposite end of the cavity (at which the second mold part is preferably arranged). Preferably, the injector is arranged adjacent to the first mold part, so that the fluid injection takes place in the region of or next to the first axle mount.

[0014] Alternatively or additionally, the injector can be arranged at an end of the cavity opposite an overflow cavity or an overflow channel. In this case, the fluid is injected at one end of the solidifying workpiece, and the displaced plastic melt exits at the other end, so that one or more internal cavities form between them (i.e., in the longitudinal direction). In another possible embodiment, the injector and the injection nozzle are arranged on opposite sides of the cavity, i.e., on opposite sides of the workpiece to be produced.

[0015] In another possible embodiment, the injection molding tool comprises at least two injectors for introducing fluid into the plastic-filled cavity. Using two injectors for the fluid injection technique allows two separate cavities to be created, which is particularly advantageous for cranks with central recesses. In particular, the two injectors can be controlled or regulated independently of each other to optimize the creation of the internal cavities. Of course, more than two injectors can be present.

[0016] The injectors can be arranged on the same side of the cavity, i.e., on the same side of the workpiece to be produced, for example, adjacent to one another and / or symmetrically to the workpiece's longitudinal axis. The injectors can be arranged in the area of ​​the first molded part, i.e., in the area of ​​the first axis mount.

[0017] In another possible embodiment, the first molded part is designed such that a first component for forming the first axle mount can be inserted into the cavity and can be back-injected and / or overmolded with plastic melt. Said component can form the first axle mount.

[0018] The injection mold preferably has a second molded part as described above. This is preferably designed such that a second component for forming the second axle mount can be inserted into the cavity and can be back-injected and / or over-injected with plastic melt.

[0019] The first and / or second component can be made of metal, for example, steel or aluminum, and can be, for example, a threaded sleeve or a sleeve with internal teeth. In this case, the first molded part can function as a type of holder for the first component and as a connection to the injection nozzle, or the second molded part (if present) can function as a type of holder for the second component and as a connection to the overflow cavity or overflow channel.

[0020] Alternatively or additionally, the use of non-metallic components such as pultruded, braided or wound fiber tubes is conceivable.

[0021] If first and second axle mounts are provided, which are formed by first and second components, any combination of the aforementioned components and materials can be used. The components can be inserted into the injection mold individually or in groups.

[0022] In a further possible embodiment, the injection molding tool comprises mold inserts connected to the tool parts, which form the actual cavity when closed. The mold inserts can preferably be exchanged and / or modified in order to change the shape of the cavity and thus the shape of the workpiece. For this purpose, at least one mold insert can be detachably attached to the respective tool part and designed as a replacement part. To change the cavity, the mold insert can be replaced with another mold insert. By using a master tool with interchangeable inserts, any component geometries can be created using additional shaping component cavities, which leads to cost savings.

[0023] Alternatively or additionally, it may be possible to expand the existing cavity by adding further mold inserts.

[0024] In a further possible embodiment, the workpiece, ie the pedal crank, is designed as a two-component component, e.g. from hard and soft components, which are preferably produced by means of an index plate or rotary table injection molding tool or by means of a second cavity arranged in the injection molding tool (multi-cavity tool). The present invention further relates to a set which comprises an injection molding tool according to the invention and at least one further mold insert. The at least one tool part is designed such that, in order to change the shape of the cavity, a mold insert detachably fastened to the tool part can be removed and a further mold insert can be attached in its place, or a further mold part can be additionally attached, as described above.

[0025] The present invention further relates to a method for producing a pedal crank for a bicycle using a thermoplastic material by means of the injection molding tool. This results in the same properties and advantages as for the injection molding tool according to the invention. The latter can, in particular, be designed according to any of the previously described embodiments.

[0026] According to the invention, in the method, plasticized plastic melt is introduced into the cavity via the injection nozzle and at least one injection opening formed in the first molded part. Furthermore, (at least) one fluid is introduced into the cavity via an injector after the workpiece, i.e. the plastic melt previously introduced into the cavity, has partially solidified. In particular, the outer shell of the workpiece solidifies first, with the plastic melt inside initially still liquid, slowly cooling and solidifying in the process. The fluid is introduced at a time when the plastic melt inside is still liquid, so that it is displaced from the interior of the solidifying workpiece and an internal cavity is thereby created in the workpiece.Because the injection nozzle is connected to the first molded part and in particular is arranged at least partially within the first axle mount, the formation of weakening weld lines is avoided.

[0027] In one possible embodiment, by introducing fluid via the injector, liquid plastic melt is forced through at least one opening formed in a second mold part into an overflow channel and preferably further into a screw antechamber of an injection molding machine, or into an overflow cavity. The second mold part is intended, in particular, to form a second axis mount surrounded by plastic in the workpiece, as previously described.

[0028] In another possible embodiment, a gas, preferably an inert gas, in particular nitrogen, is used as the fluid. Alternatively or additionally, a liquid such as water can be used.

[0029] In an optional embodiment, at least two different fluids can be introduced into the cavity via one or more injectors and their fluid volume flows and / or pressure-time profiles can be controlled separately.

[0030] In another possible embodiment, at least one component, in particular a threaded sleeve or a sleeve with internal teeth, is inserted into the cavity and back- and / or overmolded with plasticized plastic melt, as described above. Preferably, a first component is placed on the first molded part and / or a second component is placed on a second molded part.

[0031] In another possible embodiment, at least one sensor is inserted into the cavity and back-molded and / or overmolded with plasticized melt. This can, for example, be a sensor for measuring cadence.

[0032] In another possible embodiment, the injection of the fluid is controlled based on a pressure-time profile and / or a volume flow. For this purpose, the injection molding tool or an injection molding machine connected to the injection molding tool comprises a corresponding control unit.

[0033] Preferably, a set fluid pressure is maintained inside the workpiece for a defined period of time, after which (after the workpiece has completely solidified) fluid is drained, blown out, squeezed out, and / or sucked out of the workpiece. In another possible embodiment, the cavity is only partially filled with plastic melt, and after partial solidification, liquid plastic melt is displaced by the introduction of fluid. The liquid plastic melt is inflated in such a way that it adheres to an inner wall of the cavity and completely covers it, creating a cavity inside the workpiece.

[0034] In another possible embodiment, the plastic used to manufacture the crank and introduced into the cavity is a fiber-reinforced plastic, which in particular contains glass fibers and / or carbon fibers and / or basalt fibers and / or natural fibers. The plastic can be made of PP (polypropylene), PE (polyethylene), PA (polyamide), PET (polyethylene terephthalate), POM (polyoxymethylene), PC (polycarbonate), PPA (polyphthalamide), PPS (polyphenylene sulfide), or PEEK (polyetheretherketone), or it can contain any combination of the aforementioned plastics.

[0035] In a further possible embodiment, it is provided that, before the injection of the plastic melt, unidirectional tapes and / or organosheets are inserted into partial areas of the injection molding tool or the cavity or over a large area in a cold and / or hot state into the injection molding tool or the cavity and back-injected, whereby in particular a material-locking connection with the plastic plastic melt is created.

[0036] In another possible embodiment, the manufactured workpiece has a star-shaped section (star) at one end. The cavity is preferably shaped accordingly so that the star is directly injected and formed integrally with the rest of the workpiece.

[0037] Further features, details, and advantages of the invention will become apparent from the exemplary embodiments explained below with reference to the figures. They show: Fig. 1: a perspective view of a workpiece produced with the injection molding tool according to the invention according to a first exemplary embodiment;

[0038] Fig. 2-3: perspective views of the workpiece according to Fig. 1 with injection molding tool components displayed;

[0039] Fig. 4: a longitudinal section through the workpiece according to Fig. 2-3 in a side view;

[0040] Fig. 5: a cross-section through the workpiece according to Fig. 2-3 in a perspective view;

[0041] Fig. 6: a longitudinal section through the workpiece according to Fig. 2-3 in a plan view; and

[0042] Fig. 7: a perspective view of the workpiece according to a second embodiment with injection molding tool components displayed.

[0043] Figure 1 shows a perspective view of a first embodiment of a workpiece 10 produced with the injection molding tool according to the invention or by means of the method according to the invention. The workpiece 10 represents a pedal crank for a bicycle and has an elongated shape. A first axle receptacle 11 is located at a first end of the workpiece 10. A second axle receptacle 12 is formed at the opposite end.

[0044] In the embodiment shown here, the first axle mount 11 is a mount for a bicycle crankshaft and is preferably formed by an injection-molded first component 1 in the form of a sleeve with internal toothing, into which a crankshaft formed with complementary toothing can be inserted. However, the exact shape or connecting cross-section of the first component 1 or the first axle mount 11 is arbitrary and can be adapted to the shaft to be accommodated. For example, the first axle mount 11 could have a square instead of internal toothing.

[0045] In the embodiment shown here, the second axle mount 12 is a mount for a pedal axle of a bicycle pedal and is preferably formed by an injection-molded second component 2 in the form of a sleeve with an internal thread, into which the pedal axle can be screwed. However, the shape or connection cross-section of the second component 2 or the second axle mount 12 is also arbitrary and can be adapted to the shaft / axle to be accommodated.

[0046] The workpiece 10 has in a central section a recess 16 extending from a front side (ie the side which can be seen in Figs. 1 and 2) to the rear side (see Fig. 3), which recess can for example be substantially rectangular in shape and / or widened or bevelled towards one side.

[0047] One end of the workpiece 10 (e.g., the end where the second axle mount 12 is located) can have a smaller width than the other end. However, different shapes of the workpiece 10 or the pedal crank are conceivable, and the design of the workpiece 10 shown here is merely an example.

[0048] The workpiece 10 is manufactured from a thermoplastic material by injection molding using the injection molding tool according to the invention. The injection molding tool, or rather the tool parts, and the molding cavity are not shown in the figures. However, some components 20, 30, 40 of the injection molding tool are shown in Figures 2-4 and 7 (this is intended only to illustrate the relative arrangement of these components 20, 30, 40 with respect to the final workpiece 10; these components 20, 30, 40 are not present in the finished workpiece 10).

[0049] Figure 2 again shows the workpiece 10 in a perspective view looking at the front, while Figure 3 shows the back of the workpiece 10. Figure 4 shows a central longitudinal section through the workpiece 10 (along the long side and through the center axes of the first and second axle mounts 11, 12), and Figure 5 shows a central cross-section through the workpiece 10 (along the short side). Finally, Figure 6 shows a longitudinal section through the workpiece 10 perpendicular to the section in Figure 4.

[0050] The injection molding tool has an injection nozzle 20, which is shown schematically and arranged in the area of ​​the first axle mount 11. Plasticized plastic melt is injected into the cavity via the injection nozzle 20 to form the workpiece 10. To prevent a weld seam from forming due to the connection of the injection nozzle 20 to the workpiece 10, which would represent a weak point in the final pedal crank, the injection nozzle 20 is connected to a first molded part of the cavity (not shown), which forms the boundary of the cavity in the area of ​​the first axle mount 11. The first molded part can be a mandrel, which can be formed integrally with a surrounding area of ​​the cavity. The first molded part has corresponding injection openings through which the plastic melt is injected into the interior of the cavity.

[0051] In the embodiment shown in Figure 4, the injection nozzle 20 has a widened section arranged within the first molded part or within the first axle mount 11. The widened section preferably has several openings on its circumferential surface, which correspond to corresponding injection openings of the first molded part distributed around the circumference. In principle, only one or two injection openings can be provided.

[0052] In the illustrated embodiment, the first axle mount 11 is formed by a first component 1 inserted into the cavity, which component is made, for example, of steel or aluminum (metallic insert) and is back-injected and / or over-injected with the introduced plastic melt. During demolding, the injection nozzle 20 and the first molded part are removed from the first axle mount 11. Since the first component 1 is located in this area, no weakening weld line is formed.

[0053] The workpiece 10 is manufactured using fluid injection technology. For this purpose, in the embodiment of Figures 1-6, an injector 30 of the injection mold is provided, through which a fluid (e.g., nitrogen or water) is fed into the cavity or into the workpiece 10 as it is just solidifying. Since the latter first solidifies on the outside and is still liquid on the inside for a while, the introduction of fluid at a certain point in time can displace the still-liquid inner plastic core, thereby creating a cavity 14 (cf. Figures 4-5). This significantly reduces the cooling time of the workpiece 10. The injector 30 can be arranged on the opposite side of the injection nozzle 20.

[0054] In the embodiment shown in Figures 2-6, a single injector 30 is arranged next to the first axle mount 11. At the opposite end is an overflow cavity 40 of the injection mold. This overflow cavity has openings through which displaced plastic melt can flow into the overflow cavity 40. The overflow cavity is removed from the workpiece 10 during demolding.

[0055] The second axle receptacle 12 is defined or delimited in particular by a second molded part of the cavity (not shown). This preferably has corresponding openings that correspond to the openings of the overflow cavity 40. Like the injection nozzle 20, the overflow cavity 40 can also be connected to the second molded part and arranged at least partially within the second axle receptacle 12 so that no weakening weld line remains. The second axle receptacle 12 is formed in particular by a second component 2 inserted into the cavity, which component is made of steel or aluminum (metallic insert), for example, and is back-molded and / or encapsulated by the introduced plastic melt.

[0056] In the embodiment shown, the first component 1 has an internal toothing, while the second component 2 has an internal thread.

[0057] Figures 5 and 6 show the shape of the cavity 14 created by the arrangement of the injector 30 and the shape of the cavity or the workpiece 10 due to the fluid injection. In the exemplary embodiment shown, the workpiece 10 has, in addition to the central recess 16 already described on the rear side, a further recess 17 which does not extend to the front side. Due to this central "barrier", two cavities are formed in the central section of the workpiece 10 which are separate and run more or less parallel to one another and which can be connected to one another at the end sections.

[0058] Figure 7 shows a further embodiment in which the injection molding tool has two injectors 30 arranged side by side in the area of ​​the first axle mount 11. The two injectors 30 can be used to control or regulate the fluid channels separately. This is particularly advantageous for workpieces with openings or recesses in the center, as a clean formation of cavities 14 can be achieved. In the embodiment of Figure 7, the workpiece 10 has a recess 18 extending from the front to the back and further recesses 19 that do not extend to the other side. The shape and number of recesses 18, 19 can, of course, vary.

[0059] List of reference symbols:

[0060] 1 First component

[0061] 2 Second component

[0062] 10 Workpiece

[0063] 11 First axle mount

[0064] 12 Second axle mount

[0065] 14 Cavity

[0066] 16 recess

[0067] 17 Recess

[0068] 18 recess

[0069] 19 Recess

[0070] 20 Injector nozzle

[0071] 30 injector

[0072] 40 Overflow cavity

Claims

Patent claims 1. Injection molding tool for producing a pedal crank for a bicycle from a thermoplastic material, comprising at least two tool parts which, in a closed state, form a molding cavity, an injection nozzle (20) through which plasticized plastic melt can be injected into the cavity to mold a workpiece (10), and an injector (30) via which a fluid can be introduced into the plastic-filled cavity in order to displace liquid plastic melt from the interior of the solidifying workpiece and thereby create an internal cavity (14) in the workpiece (10), characterized in that the injection molding tool comprises a first mold part for forming a first axle receptacle (11) in the workpiece (10), and the injection nozzle (20) is connected to the first mold part in such a way thatthat plastic melt can be introduced into the cavity via at least one injection opening of the first molded part in the region of the first axle holder (11) to be produced.

2. Injection molding tool according to claim 1, wherein the injection molding tool has a second mold part for forming a second axle receptacle (12) in the workpiece (10) and an overflow cavity (40) or an overflow channel for taking of displaced plastic melt, wherein the overflow cavity (40) / the overflow channel is preferably connected to the second molded part in such a way that displaced plastic melt can flow into the overflow cavity (40) / the overflow channel via at least one opening of the second molded part in the region of the second axle holder (12) to be produced.

3. Injection molding tool according to claim 1 or 2, wherein the injector (30) is arranged in the region of the first mold part and / or at an end of the cavity opposite an overflow cavity (40) or an overflow channel.

4. Injection molding tool according to one of the preceding claims, wherein the injector (30) and the injection nozzle (20) are arranged on opposite sides of the cavity.

5. Injection molding tool according to one of the preceding claims, comprising at least two injectors (30) for introducing fluid into the cavity filled with plastic in order to displace liquid plastic melt from the interior of the solidifying workpiece (10) and thereby create at least one inner cavity (14), preferably at least two inner cavities (14) in the workpiece (10), wherein the injectors (30) are arranged in particular on the same side of the cavity and / or in the region of the first molded part.

6. Injection molding tool according to one of the preceding claims, wherein the first mold part is designed such that a first component (1) for forming the first axle holder (11) can be inserted into the cavity and can be back-injected and / or over-injected with plastic melt, wherein the injection molding tool is preferably further developed by the features of claim 2 and the second mold part is designed such that a second component (2) for forming the second axle holder (12) can be inserted into the cavity and can be back-injected and / or over-injected with plastic melt.

7. Injection molding tool according to one of the preceding claims, comprising mold inserts connected to the tool parts, which in the closed state to form the cavity, wherein at least one mold insert is detachably fastened and designed as a replacement part.

8. Set comprising an injection molding tool according to the preceding claim and at least one further mold insert, wherein at least one tool part is designed such that, in order to change the shape of the cavity, a mold insert detachably fastened to the tool part can be removed and a further mold insert can be attached in its place or a further mold part can be attached additionally.

9. A method for producing a pedal crank for a bicycle using a thermoplastic material by means of an injection molding tool according to one of claims 1 to 7, comprising the steps: Introducing plasticized plastic melt into the cavity via the injection nozzle (20) and at least one injection opening formed in the first mold part, Introducing a fluid into the cavity via the injector (30) after partial solidification of the workpiece (10), whereby liquid plastic melt is displaced from the interior of the solidifying workpiece (10) and an internal cavity (14) is created in the workpiece (10).

10. The method according to claim 9, wherein by introducing fluid via the injector (30), liquid plastic melt is pressed into an overflow channel and preferably further into a screw antechamber of an injection molding machine or into an overflow cavity (40) via at least one opening formed in a second mold part, which is provided for forming a second axle receptacle (12) surrounded by plastic in the workpiece (810).

11. Method according to claim 9 or 10, wherein gas, in particular nitrogen, and / or water is used as the fluid, wherein preferably at least two different fluids are introduced into the cavity via one or more injectors (30) and their fluid volume flows and / or pressure-time profiles are controlled separately.

12. Method according to one of claims 9 to 11, wherein at least one component (1, 2), in particular a threaded sleeve or a sleeve with internal teeth, is inserted into the cavity and is back- and / or over-molded with plasticized plastic melt, wherein preferably a component (1) is placed on the first molded part and / or a component (2) is placed on a second molded part to form a second axle holder (12) surrounded by plastic in the workpiece (10).

13. Method according to one of claims 9 to 12, wherein the injection of the fluid is controlled based on a pressure-time profile and / or based on a volume flow, wherein preferably a set fluid pressure is maintained over a defined period of time in the interior of the workpiece (10) and then fluid is drained, blown out, squeezed out and / or sucked out of the workpiece (10).

14. Method according to one of claims 9 to 13, wherein the cavity is only partially filled with plastic melt and, after partial solidification, liquid plastic melt is displaced by the introduction of fluid, wherein the liquid plastic melt is inflated in such a way that it adheres to an inner wall of the cavity and a hollow space (14) is formed in the interior of the workpiece (10), wherein the cavity is in particular completely covered with plastic melt.

15. The method according to any one of claims 9 to 14, wherein the plastic is a fiber-reinforced plastic, which in particular contains glass fibers, carbon fibers, basalt fibers and / or natural fibers, wherein the plastic preferably contains PP, PE, PA, PET, POM, PC, PPA, PPS and / or PEEK.

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