Extrusion head for the additive manufacturing of a shaped body

Abstract

The invention relates to an extrusion head for the additive manufacturing of a shaped body, comprising a tubular body (1) which leads into an extrusion nozzle (3) and which forms a ferromagnetic lateral wall (2), and comprising at least one induction coil (10, 11) which surrounds the ferromagnetic lateral wall (2) of the tubular body (1) in a melting zone (4). In order to create advantageous heating conditions, according to the invention the ferromagnetic lateral wall (2) has, at least in an axial portion (5, 7) of the melting zone (4), a lateral-wall thickness which repeatedly alternatingly changes over the circumference of the lateral wall.

Description

[0001] for the additive manufacturing of a

[0002] Technical area

[0003] The invention relates to an extrusion head for the additive manufacturing of a molded body with a molding body opening into an extrusion nozzle and forming a ferromagnetic shell, and with at least one induction coil enclosing the ferromagnetic shell of the molding body in a melting zone.

[0004] State of the art

[0005] In order to advantageously provide the heat energy required for melting a thermoplastic polymer strand fed into an extrusion head, it is known to provide an electric induction heater instead of the usual electric resistance heater. This electric induction heater comprises a tubular body provided in the extrusion head, which receives the polymer strand to be melted, and at least one induction coil enclosing the ferromagnetic shell of the tubular body, which induces eddy currents in the ferromagnetic shell of the tubular body. The ferromagnetic shell, heated by the eddy currents, transfers its heat to the thermoplastic polymer strand guided through the tubular body. The polymer strand is thus heated and melted during its conveyance through the tubular body, in order to be discharged as a melt strand from the extrusion nozzle closing the tubular body.However, the thermoplastic polymer strands used for the additive manufacturing of molded bodies are sensitive to overheating, so a temperature profile dependent on the type of polymer strand is desired. For this purpose, it is known (WO 2016 / 102 669 A1) to axially assemble the casing of the tubular body from several ferromagnetic sleeves with the interposition of electrically non-conductive connecting rings and to assign separately controllable induction coils to the individual ferromagnetic sleeves, so that the tubular body can be heated inductively to varying degrees in successive axial sections. However, the resulting specification of a specific heating profile along the melting zone is achieved at the cost of considerable design effort and an excessively long melting zone.

[0006] To avoid the control effort required for multiple induction coils, it is also known (EP 3 148 293 A1) for extrusion heads used in the additive manufacturing of molded bodies to divide the ferromagnetic shell of the tubular body containing the polymer strand into several axial shell sections with different Curie temperatures, so that these shell sections can only be inductively heated up to the Curie temperature. Apart from the fact that axial shell sections made of different ferromagnetic materials are required for the construction of the tubular body, there is the disadvantage that a power supply exceeding the power requirement must be provided for the inductive heating.

[0007] For improved heat input into the polymer strand to be melted, it is also known in extrusion heads with an electrical resistance heater (WO 2017 / 183992 A1) to provide a heat conducting device in the form of a heat conducting body extending from the jacket of the tube body towards the middle of the tube in the tube body receiving the polymer strand, in order to be able to heat the polymer strand not only from the outside via the heat-conducting jacket of the tube body, but also in a core area via the heat conducting body.A disadvantage of the heat transfer caused by the electrical resistance heating from a heating block to the casing of the pipe body and to the heat conducting body is that the heat input into the polymer strand depends on the one hand on the cross-section of the heat-conducting material available for the heat flow and on the other hand on the length of the heat flow between the heating block and the polymer strand, so that a voluminous heat conducting body which is advantageous for the heat input permanently reduces the flow cross-section of the pipe body for the polymer strand to be melted, which not only results in unfavorable flow conditions for the polymer strand to be melted, but is also accompanied by an increase in the flow velocity of the polymer strand which reduces the heat input.Apart from that, the temperature profile along the melting zone cannot be adapted to the respective heat requirement for melting the polymer strand by means of such heat conducting bodies provided along the melting zone.

[0008] Description of the invention

[0009] The invention is therefore based on the object of designing an extrusion head for the additive manufacturing of a shaped body in such a way that the temperature profile along the melting zone of the tubular body can be largely predetermined with a comparatively simple design effort in accordance with the polymer strand to be melted.

[0010] The invention achieves the stated object in that the ferromagnetic shell has a shell thickness that repeatedly changes alternately over the shell circumference, at least in one axial section of the melting zone.

[0011] Through the interaction of the sheath thickness, which repeatedly changes alternately around the sheath circumference, and a magnetic field excited at a corresponding frequency by the induction coil in the ferromagnetic sheath, a skin effect can be advantageously utilized. As a result, the induced eddy currents concentrate in a surface layer region of the sheath, so that the sheath is heated more strongly in the axial section of the sheath with the alternately changing sheath thickness in the surface area than in the other axial sections with a uniform thickness over the circumference. With the aid of a ferromagnetic sheath designed according to the invention with an alternating sheath thickness at least in one axial section, the heat input along the melting zone can thus be controlled according to a predetermined temperature profile.

[0012] Since the penetration depth of the magnetic excitation field of the induction coil for a given ferromagnetic material depends particularly on the excitation frequency, different effects for heat input into the polymer strand can be achieved by exciting the induction coil with a frequency that depends on the period length of the alternatingly changing sheath thickness and the thickness difference. This means, for example, that voluminous ribs, which result from thickness changes with a comparatively large period length and a comparatively large thickness difference, can provide larger amounts of heat with comparatively lower excitation frequencies due to the greater penetration depth of the magnetic excitation field.For shorter period lengths and smaller thickness differences and thus ribs with a smaller cross-section, a field excitation with a higher frequency is required for a lower penetration depth in order to ensure a corresponding skin effect and, depending on this, to be able to introduce a larger amount of heat into the polymer strand.

[0013] Since exploiting the skin effect requires repeated alternating thickness changes across the circumference of the ferromagnetic sheath, the sheath can form a circular-cylindrical flow channel for the polymer strand to be melted throughout the melting zone, so that the flow conditions of the tube body for the polymer strand cannot be adversely affected by the sheath thickness changing alternately around the circumference in an axial section of the melting zone. However, if the ferromagnetic sheath forms a circular cylinder on its outer surface in the area of ​​the melting zone, the axial ribs resulting from the alternating thickness changes protrude radially inward, resulting in an enlarged surface area and thus a larger heat transfer area for the polymer strand.

[0014] In order to be able to follow a predetermined course of heat input along the melting zone to an extent sufficient for the melting process, the ferromagnetic sheath in the region of the melting zone can have at least two sections spaced apart from one another in the axial direction with a sheath thickness that changes alternately over the sheath circumference, wherein the changes in the sheath thickness are selected differently in the two axial sections of the melting zone, so that different amounts of heat are introduced into the polymer strand due to the different configurations of the ferromagnetic sheath in these melting zone sections.This is particularly successful when an induction coil is assigned to each of the two axial sections with a jacket thickness that alternates around the jacket circumference, and when the two concentrically arranged induction coils can be excited at different frequencies that depend on the period length of the alternately changing jacket thickness and the thickness difference of the associated section. With alternating excitation of the two induction coils, a magnetic excitation field with different frequencies is alternately effective in the ferromagnetic jacket along the entire melt zone, so that a lower excitation frequency has an effect, particularly in the area of ​​a melt zone section with more voluminous ribs, and a higher excitation frequency has an effect in the area of ​​a melt zone section with ribs of a smaller volume.By appropriately adjusting the rib design and the excitation frequency, the extrusion head can be adapted to different requirements regarding gentle heat transfer into the polymer strand to be melted. Brief description of the invention.

[0015] The drawing shows an example of the subject matter of the invention.

[0016] Fig. 1 shows the tubular body of an extrusion head according to the invention, provided with an inductive heater, for the additive manufacturing of a shaped body in an axial section,

[0017] Fig. 2 shows a section through the casing of the pipe body along the line ll-ll of Fig. 1 on a larger scale,

[0018] Fig. 3 is a section along the line III-III of Fig. 1 on a larger scale, Fig. 4 is an embodiment of a tubular body in an axial section and Fig. 5 is a section along the line VV of Fig. 4 on a larger scale.

[0019] Ways to implement the invention

[0020] The drawing schematically shows only the area of ​​an extrusion head used for the additive manufacturing of a molded body that is used to melt the supplied thermoplastic polymer strand. This thermoplastic polymer strand is fed by a conveyor device into a tubular body 1 held in a carrier, the ferromagnetic jacket 2 of which opens into an extrusion nozzle 3. The ferromagnetic jacket 2, which is inductively heated along a melting zone 4, has, in at least one axial section 5, a jacket thickness that repeatedly changes alternately around the circumference of the jacket. Due to this repeatedly changing jacket thickness, axial ribs 6 are formed in section 5 of the melting zone 4, distributed over the circumference of the jacket 2. According to Figs. 1 to 3, these ribs run on the inside of the jacket 2 of the tubular body 1, but according to Figs. 4 and 5, they can also be arranged on the outside of the jacket.

[0021] Along the melting zone 4, at least one section 5 with a repeatedly alternating thickness over the circumference of the shell is provided. However, the shell 2 can also form two or more such sections. In Fig.

[0022] 1 shows, in addition to section 5, a further section 7, in the area of ​​which the repeatedly alternating changing sheath thickness has a different period length 8 and a different thickness difference 9 compared to section 5, which results in a different cross-section of the ribs 6.

[0023] To advantageously utilize the skin effect in a section 5 with an alternating sheath thickness, provided that the period length 8 of the thickness change and the thickness difference are comparatively small, as can be seen from Figs. 2 and 5, a correspondingly higher excitation frequency for the magnetic excitation field must be provided to limit the penetration depth. For a design according to Fig. 3, i.e., in the case of comparatively large cross-sections for the ribs 6 resulting from the thickness differences, lower excitation frequencies for the magnetic excitation field are required to ensure a greater penetration depth.

[0024] As can be seen from Fig. 1, when providing two sections 5 and 7 with a jacket thickness that repeatedly changes alternately over the jacket circumference, two induction coils 10, 11 can be provided, which are matched, in particular with regard to their excitation, to the two sections 5, 7, which are differently designed with regard to the thickness differences, but extend concentrically over the entire melting zone 4. With alternating excitation of the two induction coils 9, 10, magnetic excitation fields with different frequencies act alternately in the jacket 2 along the entire melting zone 4.This means that, depending on the excitation frequency, different effects regarding the skin effect occur in sections 5 and 7 due to different penetration depths, with the result that different amounts of heat can be made available along sections 5 and 7 in order to ensure adaptation to a predetermined temperature profile along the melting zone 4.

[0025] If the jacket 2 has only one section 5 along the melting zone 4 with a jacket thickness that changes repeatedly and alternately over the jacket circumference, an induction coil extending over the length of the melting zone 4 is sufficient, regardless of the area of ​​the melting zone 4 over which the section of the jacket 2 with a jacket thickness that changes repeatedly and alternately over the jacket circumference extends.

Claims

Patent claims 1. Extrusion head for the additive manufacturing of a shaped body with a tubular body (1) opening into an extrusion nozzle (3) and forming a ferromagnetic jacket (2), and with at least one induction coil (10, 11) enclosing the ferromagnetic jacket (2) of the tubular body (1) in a melting zone (4), characterized in that the ferromagnetic jacket (2) has a jacket thickness that changes repeatedly and alternately over the jacket circumference, at least in one axial section (5, 7) of the melting zone (4).

2. Extrusion head according to claim 1, characterized in that the induction coil (10, 11) can be excited with a frequency dependent on the period length (8) of the alternately changing jacket thickness and the thickness difference (9).

3. Extrusion head according to claim 1 or 2, characterized in that the ferromagnetic jacket (2) has a circular-cylindrical outer surface in the region of the melting zone (4).

4. Extrusion head according to one of claims 1 to 3, characterized in that the ferromagnetic jacket (2) in the region of the melting zone (4) has at least two sections (5, 7) spaced apart from one another in the axial direction with a jacket thickness that changes alternately over the jacket circumference and that the changes in the jacket thickness in the two axial sections (5, 7) of the melting zone (4) are different.

5. Extrusion head according to claim 4, characterized in that an induction coil (10, 11) is assigned to each of the two axial sections (5, 7) with a jacket thickness that changes alternately over the jacket circumference, and in that the two concentrically arranged induction coils (10, 11) can be excited with different frequencies that depend on the period length (8) of the alternately changing jacket thickness and the thickness difference (9) of the associated section (5, 7).

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