Extruder valve for extrusion-based, thermoplastic material discharge and method for extrusion-based, thermoplastic material discharge

Abstract

Described are an extruder valve for extrusion-based, thermoplastic material discharge and a method for extrusion-based, thermoplastic material discharge, comprising a hollow channel which has two hollow channel openings opposite one another in the hollow channel longitudinal axis, one hollow channel opening of which serves to introduce a solid, thermoplastic material into the hollow channel and to whose other hollow channel opening a discharge nozzle with a nozzle opening is directly or indirectly connected, a cooling unit arranged along the hollow channel and thermally coupled to the hollow channel and a heating unit arranged along the hollow channel between the cooling unit and the discharge nozzle and thermally coupled to the hollow channel, and a valve piston which is mounted bidirectionally movable along the hollow channel axis and radially encloses the hollow channel. The extruder valve is characterized by the provision of a valve piston mounted bidirectionally movable along the hollow channel axis, radially enclosing the hollow channel along a first axial valve piston section, which encompasses the hollow channel opening facing the discharge nozzle, is slidably mounted in a recess within the heating block, and defines a chamber at least with the heating block and the discharge nozzle. The valve piston has a valve piston gate at the hollow channel opening facing the discharge nozzle, which is designed and arranged such that the valve piston gate can seal the discharge nozzle against material discharge upon contact with a nozzle gate surrounding the nozzle opening of the discharge nozzle.In addition, the valve piston is pre-tensioned along the hollow channel longitudinal axis relative to the discharge nozzle by means of a clamping means such that in an otherwise force-free state, the valve piston gate comes into contact with the nozzle gate surrounding the nozzle opening and, in addition, has at least one wall section delimiting the chamber, to which a projection surface is assigned in axial projection to the hollow channel longitudinal axis, the surface normal of which is oriented parallel to the hollow channel longitudinal axis.

Description

Technical FieldThe invention relates to an extruder valve for an extrusion-based thermoplastic material discharge, having a hollow channel which has two hollow channel openings lying opposite one another in the longitudinal axis of the hollow channel, one hollow channel opening of which serves for introducing a solid thermoplastic material into the hollow channel and the other hollow channel opening of which is directly or indirectly adjoined by a discharge nozzle having a nozzle opening, a cooling unit arranged along the hollow channel and thermally coupled to the hollow channel, and a heating unit arranged along the hollow channel between the cooling unit and the discharge nozzle and thermally coupled to the hollow channel, and a valve piston mounted bidirectionally movably along the hollow channel axis and radially enclosing the hollow channel is provided. Furthermore, a method for extrusion-based thermoplastic material discharge is described.Extruder valves of the generic type are typically used in additive component production and serve for the layer-shaped material discharge of softened thermoplastic material. For this purpose, the extruder valve is moved relative to the construction plane within the scope of the production process on the basis of a 3-D data set reflecting the spatial shape of the component. For a desirably high component quality in extrusion-based, additive manufacturing, an exact application of the thermoplastic material in the component plane at the desired location is required.Typically, the thermoplastic material is present as a piece of material in solid filament form, in which the material is fed continuously to the extruder valve by means of a conveying unit and is converted into a molten state therein with the aid of temperature and shear forces. This melt is applied via a discharge nozzle arranged in the extruder valve into or onto the construction plane of the component. For controlled volume application, the ratio between the conveying speed of the filament feed and the travel speed at which the extruder valve is moved relative to the construction plane is of great importance. It is also of particular importance that the softened thermoplastic material exits the discharge nozzle only between the beginning and the end of the filament advancement. In the case of certain materials, in particular softened thermoplastics having a low viscosity or having a high deformation energy, as occur, for example, in the case of thermoplastic foams, undesired outflow of the material from the discharge nozzle can take place after the end of the filament or material advancement, frequently referred to as "dripping". This undesired and uncontrolled material discharge causes material residues to be carried along or accumulated in regions of the component over which the discharge nozzle is moved. For this reason, dimensional stability of the additively manufactured component can no longer be ensured.Prior ArtExtruder valves of the type mentioned at the beginning have the components mentioned at the outset, so that thermoplastic material present in solid form is initially fed to a hollow channel and conveyed through the latter under pressure, wherein the thermoplastic material is cooled within a first hollow channel section and heated and liquefied along a subsequent hollow channel section before it is discharged through a nozzle opening of a discharge nozzle arranged downstream of the hollow channel in the form of a plastic material strand. In filament-based additive manufacturing, it is sufficient in many thermoplastic materials to reverse the material feed to the end of the discharge and thereby to reduce the pressure in the melt within the extruder valve. This is a common and practicable solution, especially in the case of relatively high-viscosity material melts.A further possibility for preventing undesired dripping is achieved by spatially tilting or inclining the extruder valve together with the discharge nozzle, so that its nozzle opening is not positioned vertically with respect to the substrate and the force of gravity accordingly does not act parallel to the material outflow. In this way, too, uncontrolled dripping of material, especially of higher-viscosity melts, can be avoided. However, this requires constructional measures for inclining the extruder valve.A discharge nozzle closure by means of a disk at the nozzle outlet, which is capable of closing the nozzle opening, is described in the publication EP 3 020 550 A1. Several mechanics are used here.The publication DE 27 09 609 C2 discloses a valve-controlled inlet part for an injection molding unit, which has a plurality of injection nozzles, the nozzle openings of which can each be closed by means of a valve needle, which can be actuated mechanically, hydraulically or pneumatically. These nozzle closures are optimized for the injection process into a cavity and must withstand a high counterpressure. Therefore, active, usually hydraulic, valve closing forces are required.Self-opening and closing needle locks are also known. The publication KR 20-0366741 discloses a type of nonreturn valve which results in a positive fit and thus a closing of the valve when it flows back into the nozzle.KR 10-2013-0044553 discloses a spring force driven valve shutter mechanism in which a plurality of springs cooperate to cause opening and closing of the valve opening.The publication US 5402351 discloses a closure mechanism for an extruder valve arrangement based on the per se known needle closure principle.The documents CN1 07 244 074 A and KR 101 734862 B1 each disclose an extruder valve having a nozzle orifice which is always open. Only the volume throughput can be varied through the discharge nozzle by enlarging the outlet nozzle opening and by reducing the discharge nozzle opening. Complete closing of the outlet nozzle opening is not provided. Thus, both extruder valves each provide a capillary-shaped, central outlet channel through which constant minimum material throughput is discharged.The document DE 10 2020 109 847 A1 discloses a printing device for a 3D printer, having a printing nozzle head mounted so as to be deflectable by a piezoelectric or pneumatic drive, the spatial position of which defines a closing and opening position within a printing nozzle frame.SUMMARY OF THE INVENTIONThe object of the invention is to develop an extruder valve for an extrusion-based thermoplastic material discharge, having a hollow channel which has two hollow channel openings which are opposite one another in the longitudinal axis of the hollow channel, one hollow channel opening of which serves for introducing a solid thermoplastic material into the hollow channel and at the other hollow channel opening of which there is connected indirectly or directly a discharge nozzle having a nozzle opening, a cooling unit which is arranged along the hollow channel and is thermally coupled to the hollow channel, and a heating unit which is arranged along the hollow channel between the cooling unit and the discharge nozzle and is thermally coupled to the hollow channel, and a valve piston which is mounted such that it can be moved bidirectionally along the hollow channel axis and radially surrounds the hollow channel, in such a way that a dripping of flowable thermoplastic material is produced directly on and after the end of material discharge from the discharge nozzle, In particular, low viscosity can be excluded. Furthermore, it is also important to specify a method for extrusion-based thermoplastic material discharge which avoids dripping and thus ensures dimensional stability during the production of components by means of additive process technology.The solution of the object on which the invention is based is specified in claim 1. The subject matter of claim 12 is a method according to the invention for extrusion-based thermoplastic material discharge. Features which develop the concept of the invention in an advantageous manner can be taken from the subject matter of the dependent claims and from the further description, in particular with reference to the exemplary embodiments.The extruder valve according to the invention for an extrusion-based thermoplastic material discharge, which has the features contained in the preamble of claim 1, is characterized in that the valve piston is mounted in a recess inside the heating block so as to slide along a first axial valve piston section, which comprises the hollow channel opening facing the discharge nozzle, and delimits a chamber at least with this and the discharge nozzle. In addition, the valve piston has, on its hollow channel opening facing the discharge nozzle, a valve piston gate which is designed and arranged in such a way that, when it comes into contact with a nozzle gate surrounding the nozzle opening of the discharge nozzle and preferably facing the chamber, it is able to seal the discharge nozzle against material discharge. In this case, the valve piston is mounted in a prestressed manner along the longitudinal axis of the hollow channel relative to the discharge nozzle by means of a clamping means, preferably in the form of a helical spring, such that, in an otherwise force-free state, the valve piston gate comes into contact with the nozzle gate surrounding the nozzle opening.The formulation "otherwise force-free state" means that no further force acts on the valve piston apart from the clamping force or spring force caused by the clamping means. In this state, the valve piston closes the extruder valve completely against any material discharge.For the purpose of controlled opening of the extruder valve, the valve piston has, in the otherwise force-free state, at least one wall section which delimits the chamber and to which, in axial projection with respect to the longitudinal axis of the hollow channel, a projection surface is assigned, the surface normal of which is oriented parallel to the longitudinal axis of the hollow channel.Precisely this at least one wall section of the valve piston offers an engagement surface for the liquefied thermoplastic material collecting within the chamber due to the closed valve position, in order to move the valve piston from the closed valve position counter to the spring force acting on the valve piston and thus to open the extruder valve.The opening mechanism of the extruder valve according to the invention is thus based on the displacement effect of the liquefied thermoplastic material conveyed into the chamber under pressure. Since the chamber is delimited quasi-fluid-tightly by the heating block and the valve piston for the liquefied thermoplastic material, the bidirectionally movably mounted valve piston enables the only possibility for reducing the internal pressure of the chamber by the valve piston being raised from the nozzle opening by way of a force-applied displacement.The thermoplastic material is preferably conveyed as a cylindrical filament by means of a conveying unit through the hollow channel extending along the valve piston into the extruder valve. Along the conveying section extending through the hollow channel, the thermoplastic material is first subjected to cooling and then to heating, so that the thermoplastic material softens and opens into the chamber in a flowable or liquid state through a hollow channel opening along the valve piston. The cooling unit provides a thermal barrier along the hollow channel so that the heating of material along the filament is limited to the area of the heating block and the solid material area of the filament itself serves as a piston by the targeted separation from solid to flowable material state.As long as the valve piston gate rests in a sealing manner on the nozzle opening and the delivery of softened thermoplastic material into the chamber takes place, the chamber internal pressure acting from the sides of the softened thermoplastic material on the walls delimiting the chamber rises in the chamber. As soon as the pressure effect directed onto the valve piston by means of the softened thermoplastic material, which is oriented counter to the prestress or spring force, which prestresses the valve piston in the direction of the discharge nozzle opening under the action of force, exceeds, the valve piston is lifted from the discharge nozzle opening, as a result of which the extruder valve passes over into the open position.If, on the other hand, the chamber internal pressure decreases, due to a reduction or complete abrupt termination or kinematic reversal of the material supply of thermoplastic material into the extruder valve by means of the conveying unit, the chamber internal pressure decreases abruptly and the clamping means which acts on the valve piston moves the latter without interruption in the direction of the discharge nozzle for sealingly closing the discharge nozzle opening.In a preferred embodiment, the valve piston is designed as an elongate monolithic structural unit, preferably made of a metallic, but in particular thermally highly conductive material, and encloses the hollow channel radially over the entire circumference. The hollow channel passing through the valve piston at least in regions opens open on one side for unhindered introduction of the solid thermoplastic material, preferably in the form of a solid cylindrical filament present as a piece of material, which filament can be conveyed into the hollow channel by means of a conveying unit known per se.In a preferred embodiment, the otherwise, preferably straight-cylinder-shaped hollow channel opens out via at least two openings oriented obliquely, preferably transversely to the longitudinal axis of the hollow channel, for example as a result of a bore which passes transversely through the valve piston and which is adjoined directly adjacent by the valve piston gate, which has a surface region which faces the nozzle gate and has a shape formed in a manner counter-contoured with respect to the latter, in order to be able in this way to form a positive connection with the discharge nozzle which seals off the softened thermoplastic material. Preferably, the surface region of the valve piston gate has gate cross sections which taper gradually or continuously in the direction of the nozzle gate, for example this gate cross section is designed to be conical, stepped or spherical.In a preferred embodiment, the valve piston has a straight hollow cylindrical basic shape which can be substantially divided into three axial valve piston sections, of which a first and second valve piston section each have an identical or similarly dimensioned outer diameter. The first and second axial valve piston sections are monolithically connected to one another via a third axial valve piston section, which however has a significantly smaller outer diameter. All three axial valve piston sections are traversed by the straight-cylindrical hollow channel, so that the wall thickness of the valve piston in the region of the third axial valve piston section is significantly smaller than the wall thickness within the first and second axial valve piston sections, respectively.The second valve piston section adjoining the freely accessible hollow channel opening in a central or direct manner is preferably joined axially fixedly to the cooling unit or monolithically connected to the latter.The first axial valve piston section opens slidingly in a recess within the heating block, relative to which recess the valve piston together with the cooling unit axially fixedly joined or monolithically connected thereto is mounted so as to be bidirectionally deflectable.For the bidirectional mounting of the valve piston relative to the heating block, a mounting plate is arranged spaced apart from the heating block along the hollow channel axis and connected to the latter via a rigid connecting structure.The clamping means is supported on one side on the mounting plate and is operatively connected with its opposite clamping means end indirectly or directly to the valve piston, forming a spring force oriented along the longitudinal axis of the hollow channel and in the direction of the discharge nozzle. Preferably, the clamping means end facing away from the mounting plate is supported on the cooling unit arranged along the second axial valve piston section.In this configuration, the clamping means is capable of pressing the valve piston gate in the axial direction against the discharge nozzle integrated within the heating block, which is either monolithically worked within the heating block or can be interchangeably inserted into the heating block in the form of a separate joining part, for example by means of a screw thread.In the state of the sealing joint between the valve piston gate and the nozzle gate of the discharge nozzle, at least a partial region of the surface region of the valve piston gate is arranged at a distance from the nozzle gate surrounding the nozzle opening and encloses with the latter a gap which is freely accessible from the chamber. Softened thermoplastic material is thus able to penetrate into this gap and, as a function of the chamber internal pressure increasing when the die position is closed and the thermoplastic material is guided in a controlled manner into the extruder valve, lift the valve piston counter to the spring force acting on it and release the die opening.In addition to the controlled supply of thermoplastic material into the chamber, the internal pressure of the chamber can also be influenced by means of the temperature which can be preset by means of the heating block. For this purpose, the heating block provides temperature control, i.e. a heat source, for example in the form of an electric heater or a heating medium circuit, which is guided through the heating block, and a temperature sensor are arranged inside the heating block.The valve arrangement designed according to the invention is moreover based on a novel operating principle which is based on a spring-loaded bidirectionally movably mounted valve piston arrangement. Thus, on the one hand, the spring-loaded, bidirectionally movably mounted valve piston is able to seal the nozzle opening against any material discharge by means of a contact joint between valve piston, in particular its valve piston gate and discharge nozzle. On the other hand, the opening of the discharge nozzle is effected by displacement forces acting on the valve piston in the opposite direction to the spring force and exceeding the spring force, which forces act on the valve piston from the side of the liquefied thermoplastic material. In this case, the liquefied thermoplastic material flows into a chamber arranged downstream of the hollow channel, which is delimited in regions by the valve piston mounted such that it can move bidirectionally. When an increasing chamber internal pressure caused by the inflow of the liquefied thermoplastic material into the chamber is exceeded, the valve piston is displaced counter to the spring force, whereby the nozzle opening of the discharge nozzle covered by the valve piston opens.In a particularly advantageous manner, the hollow channel runs through the bidirectionally mounted valve piston, while the spring force generated by the clamping means acts on the valve piston through the hollow channel in the conveying direction of the thermoplastic material. The displacement forces forming within the chamber act on at least one surface region of the valve piston delimiting the chamber, so that the valve piston is raised by the displacement force acting in the opposite direction to the conveying direction of the thermoplastic material when the spring force acting in the conveying direction of the thermoplastic material is exceeded, and releases the nozzle opening.BRIEF DESCRIPTION OF THE INVENTIONThe invention is described below by way of example without limiting the general concept of the invention on the basis of exemplary embodiments with reference to the drawings. The following are shown: FIG. 1 is an external perspective view of an extruder valve designed according to the invention, FIG. 2 is a longitudinal sectional view of the extruder valve, FIGS. 3a, b are illustrations of alternatively constructed valve pistons, and FIGS. 4a, b are longitudinal sectional views of a valve piston designed according to the invention in the closed and open valve position with inserted thermoplastic material.WAYS OF CARRYING OUT THE INVENTION, INDUSTRIAL APPLICABILITYFIG. 1 shows a perspective external view of an extruder valve designed according to the invention, which is mounted on a mounting plate 1 which is connected to a manipulator unit, not shown, which can be deflected at least in the X and Y directions, for example in the form of a 3D printer.A heating block 4 is fixed at a distance from the mounting plate 1 via a connecting structure 2 in the form of two connecting struts. A heating element 6 and a temperature sensor 5 are fastened inside or on the heating block 4 in order to temperature control the heating block 4 in a controlled manner. The heating element 6 is preferably designed in the form of an electrically operable heating element. Alternatively or in combination, the heating block can be tempered via a heating channel through which heat medium is integrated therein.The heating block 4 also has a central bore which serves as a nozzle opening 10. Alternatively, a corresponding threaded bore can be provided within the heating block 4 for use with a separate discharge nozzle.The heating block 4 also has a further central bore 4' with a larger diameter than the nozzle opening 10, through which bore the valve piston 9 is inserted in a slidingly mounted manner, so that the latter together with the heating block 4 delimits a chamber 15. The valve piston 9 has at its lower end a valve piston gate 21 which, in the "closed" state of the extruder valve, rests on a nozzle gate 16 at the upper end of the nozzle opening 10. The valve piston 9 is designed in the manner of a straight hollow cylinder and encloses a straight-cylindrical inner hollow channel 12, the upper hollow channel opening 12' of which opens freely and the lower hollow channel opening 12'' of which opens into the chamber 15 via one or more bores 13 arranged transversely or obliquely to the longitudinal axis of the hollow channel, directly above the valve piston gate 21.FIGS. 3 a, b illustrate a respective valve piston 9. The valve piston 9 has the basic shape of a straight hollow cylinder and has three axial valve piston sections. The first valve piston section I is located directly adjacent above the lower hollow channel opening 12" and has an outer diameter which is formed slightly smaller than the central bore 4' within the heating block 4, so that the first axial valve piston section I is thermally coupled on the one hand to the heating block 4 but is mounted in a bidirectionally sliding manner in the latter on the other hand.The second valve piston section II has an outer diameter of similar size or an outer diameter of greater size than that along the first valve piston section I. Along the second axial valve piston section II, a cooling unit 14 is also axially fixedly connected. Alternatively, it can also be monolithically connected to the valve piston 9 along its second axial valve piston section II. Between the first and second valve piston sections I and II, a third valve piston section III is provided, the outer diameter of which is selected to be significantly smaller than the outer diameters in the region of the first and second valve piston sections I, II. In this way, the first and second axial valve piston sections are largely decoupled from one another thermally.The valve piston 9 illustrated in FIG. 3 a has a bore 13 oriented transversely to the longitudinal axis A of the hollow channel, which bore communicates with the lower hollow channel opening of the valve piston 9.In contrast to this, the valve piston 9 illustrated in FIG. 3 bhas two bores 13 which are each oriented orthogonally to one another and oriented transversely to the longitudinal axis A of the hollow channel.In the longitudinal sectional illustration according to FIG. 2, it can be seen that the thin-walled third axial valve piston section III lies above the heating block 4 in the "closed" state of the extruder valve. Above the thin-walled third axial valve piston section III, the cooling unit 14 is axially fixedly fastened to the outer side of the valve piston or integrated within the valve piston 9. The cooling unit 14 has bores 3, 8, see FIG. 1, through which a cooling medium, such as water or air, can be conducted.Between the mounting plate 1 and the cooling unit 14 there is mounted a clamping means, preferably in the form of a compression spring 7, which is supported on the one hand on the mounting plate 1 and on the other hand on the cooling unit 14 which is mounted axially fixedly with the valve piston 9. The compression spring 7 is mounted in a prestressed state between the mounting plate 1 and the cooling unit 14, so that it presses the valve piston 9 onto the nozzle slot 16 in the axial direction along the longitudinal axis A of the hollow channel. The spring travel is limited by a distance which can be set as much as possible between the upper end of the valve piston 9 and the mounting plate 1 and by the block length of the compression spring 7. Thus, the axial movement height of the valve piston 9 can be limited to a certain amount.In FIG. 2, the lower illustration shows a detailed view of the valve piston gate 21 together with the nozzle gate 16. The valve piston gate 21 shown in FIG. 2 is designed conically and has a cone angle α. Furthermore, the nozzle gate 16 surrounding the nozzle opening 10 is likewise of conical design, but has a cone angle β which is selected to be significantly greater than the cone angle α of the gate 21. The valve piston gate 21 has in this way a surface region 19 which is spaced apart from the nozzle gate 16. Precisely this surface region 19 has a projection surface 20 in axial projection with respect to the longitudinal axis A of the hollow channel, the surface normal of which is oriented parallel to the longitudinal axis A of the hollow channel. If softened thermoplastic material enters the gap 18, the surface region 19 is subjected to a displacement force F v which has a force component F* oriented orthogonally to the projection surface 20, by means of which force component the valve piston 9 is raised counter to the spring force acting along the longitudinal axis of the hollow channel by the compression spring 7.With reference to FIGS. 4 a, b, the mode of operation of the extruder valve designed according to the invention is explained below. Solid thermoplastic material 22, preferably in the form of a cylindrically shaped filament present as a state goods, is fed to the extruder valve by means of a conveying unit, not shown. There, the thermoplastic material passes through a bore 11 in the mounting plate 1 into the hollow channel 12 passing through the valve piston 9, and at the height of the heating block 4, the thermoplastic material 22 is converted into a flowable or liquid state by introduction of heat. At the transition point 17 from the solid to the flowable state, the thermoplastic material present in solid form, similar to a piston, causes a feed of the melt through the bores 13 introduced within the valve piston 9 into the chamber 15. By further supply of the thermoplastic material 17 and the temperatures within the heating block 4, a chamber internal pressure is formed within the chamber 15, which also acts on the valve piston gate 21 and here in particular on the freely accessible surface area 19 of the valve piston gate 21.Since the outer diameter of the valve piston 9 in the region of the first axial valve piston section I is selected to be only slightly smaller than the bore diameter within the heating block 4, it is ensured that, on the one hand, the valve piston 9 is mounted movably in a bidirectionally sliding manner within the heating block 4, but, on the other hand, it can be ruled out that softened thermoplastic material can escape from the rear between the heating block 4 and the valve piston 9. Thus, with an increasing chamber internal pressure, a pressure reduction can only take place in that the forces acting on the freely accessible surface area 19 of the valve piston gate 21 lead to an axial compression of the compression spring, as a result of which ultimately the valve piston 9 is lifted axially counter to the spring force. In this case, the flowable thermoplastic material flows through the nozzle opening 10 out of the extruder valve into the environment 23, see FIG. 4 a.By means of a corresponding controlled movement of the extruder valve by means of a suitable manipulator unit, for example a 3D printer, softened thermoplastic material reaches the construction level of an additive manufacturing process in this way. To close the nozzle opening 10, the advance of the solid thermoplastic material is stopped with the aid of the conveying unit or vice versa, see FIG. 4 b. In this case, the chamber internal pressure is suddenly reduced, especially since the solid region of the thermoplastic material 22 acting as a pressure piston exerts no pressure on the flowable thermoplastic material located within the chamber 15. The spring force exceeds at this moment the displacement force acting on the valve piston gate 21 due to the chamber internal pressure, whereby the valve piston 9 deflects downward and closes the nozzle opening 10, so that no further flowable thermoplastic material can escape from the nozzle opening.The interaction of the internal chamber pressure formed within the chamber 15 and the spring force of the compression spring axially acting on the valve piston realizes a self-opening and at the same time self-closing action of the extruder valve. A particular advantage of the extruder valve according to the invention is that the discharge nozzle is closed autonomously exactly at the moment at which the feeding of the solid thermoplastic material into the extruder valve is interrupted. Thus, no additional actuator synchronized with the motion kinematics, for example a 3D printer, is required for closing the discharge nozzle. Since the reaction time for valve closure takes place instantaneously, i.e. without a time delay after the termination of the material supply of the thermoplastic material into the extruder valve, the disadvantageous effect of the "dripping" explained at the beginning can be ruled out, so that a dimensional stability and thus a good quality of the component produced by using the extruder valve according to the solution is ensured.List of reference characters1 Mounting plate 2 Connecting struts 3 Bore 4 Heating block 5 Temperature sensor 6 Heating element 7 Compression spring 8 Bore 9 Valve piston 10 Nozzle opening 11 Bore 12 Hollow channel 13 Bore 14 Cooling unit 15 Chamber 16 Nozzle gate 17 Transition point 18 Gap 19 Free surface area 20 Projection surface 21 Valve piston gate 22 Strand-shaped solid thermoplastic material 23 Environment F V Displacement force F* Axial force component α Cone angle of the valve piston gate β Cone angle of the nozzle gate

Claims

Extruder valve for an extrusion-based thermoplastic material discharge, having a hollow channel (12) which has two hollow channel openings (12', 12") lying opposite one another in the longitudinal axis of the hollow channel, one hollow channel opening (12') of which serves for introducing a solid thermoplastic material into the hollow channel (12) and at the other hollow channel opening (12") of which there is connected indirectly or directly a discharge nozzle having a nozzle opening (10), a cooling unit (14) arranged along the hollow channel (12) and thermally coupled to the hollow channel (12), and a heating unit (4) arranged along the hollow channel (12) between the cooling unit (14) and the discharge nozzle and thermally coupled to the hollow channel (12), and a valve piston (9) mounted so as to be bidirectionally movable along the hollow channel axis and radially enclosing the hollow channel (12), characterized in that, the valve piston (9) is mounted in a sliding manner in a recess within the heating block (4) along a first axial valve piston section, which comprises the hollow channel opening (12") facing the discharge nozzle, and delimits a chamber (15) at least with the latter and the discharge nozzle, the valve piston (9) having a valve piston gate (21) at the hollow channel opening (12") facing the discharge nozzle, which gate is designed and arranged such that the valve piston gate (21), when it comes into contact with a nozzle gate (16) surrounding the nozzle opening (10) of the discharge nozzle, is able to seal the discharge nozzle against material discharge such that the valve piston (9) is mounted in a prestressed manner along the hollow channel longitudinal axis relative to the discharge nozzle by means of a clamping means (7), such that, in an otherwise force-free state, the valve piston gate (21) comes into contact with the nozzle gate (16) surrounding the nozzle opening (10), and that the valve piston (9), in the otherwise force-free state, has at least one wall section delimiting the chamber (15), to which wall section, in axial projection with respect to the longitudinal axis of the hollow channel, a projection surface is assigned, the surface normal of which is oriented parallel to the longitudinal axis of the hollow channel.Extruder valve according to claim 1, characterized in that the valve piston (9) has a second axial valve piston section which thermally couples to the cooling unit (14), and that the first and second valve piston sections are monolithically connected by a third valve piston section which has a smaller valve piston cross section than the valve piston cross sections along the first and second valve piston sections.Extruder valve according to claim 1 or 2, characterised in that the discharge nozzle is monolithically connected to the heating block (4) or is joined as an insert part into the heating block (4).Extruder valve according to one of claims 1 to 3, characterised in that a mounting plate (1) is arranged spaced apart from the heating block (4) along the hollow channel axis and is rigidly connected thereto, that the clamping means (7) is joined to the mounting plate (1) so as to be supported on one side and is operatively connected to the valve piston (9) indirectly or directly with a clamping means end facing away from the mounting plate (1), forming a spring force oriented along the hollow channel longitudinal axis and in the direction of the discharge nozzle.Extruder valve according to claim 4, characterised in that the cooling unit (14) is joined axially firmly to the second axial valve piston section or is monolithically connected thereto, and that the clamping means end facing away from the mounting plate (1) is supported on the cooling unit (14).Extruder valve according to one of claims 1 to 5, characterised in that the valve piston gate (21) has a surface region which faces the die gate (16), with gate cross-sections which taper gradually or continuously in the direction of the die gate (16).Extruder valve according to claim 6, characterised in that the surface region (19) of the valve piston gate is formed conically, in a step-like or spherical manner.Extruder valve according to claim 6 or 7, characterised in that at least a partial region of the surface region (19) of the valve piston gate (21) in the otherwise force-free state is spaced apart from the nozzle gate (16) surrounding the nozzle opening (10) and encloses a gap (18) with the latter.Extruder valve according to one of claims 1 to 8, characterised in that the hollow channel opening (12") of the valve piston (9) facing the discharge nozzle opens into the chamber (15) via at least one bore (13) or opening inside the valve piston (9) oriented transversely or obliquely to the longitudinal axis of the hollow channel.Extruder valve according to claim 9, characterised in that the at least one bore (13) or opening is arranged directly adjacent to the valve piston gate (21).Extruder valve according to any of claims 1 to 10, characterized in that the mounting plate (1) provides a connection structure for mounting a conveying unit, which serves for the introduction of the thermoplastic solid material through the hollow channel opening (12') along the hollow channel (12).Method for extrusion-based thermoplastic material discharge, in which the thermoplastic material is supplied in solid form to a hollow duct (12) and conveyed by the latter under pressure, wherein the thermoplastic material is cooled within a first hollow duct section and heated and liquefied along a subsequent hollow duct section before it is discharged through a nozzle opening (10) of a discharge nozzle arranged downstream of the hollow duct (12) in the form of a plastic material strand, characterized in that the nozzle opening (10) is completely sealed against any material discharge by means of a contact joint between the valve piston (9) and the discharge nozzle, and in that the discharge nozzle is opened by displacement forces acting counter to the spring force on the valve piston (9) and exceeding the spring force, said displacement forces acting on the valve piston (9) from the side of the liquefied thermoplastic material, wherein the liquefied thermoplastic material flows into a chamber (15) arranged downstream of the hollow channel (12), which is delimited in regions by the valve piston (9) mounted such that it can move bidirectionally, and when an increasing internal chamber pressure caused by the inflow of the liquefied thermoplastic material into the chamber (15) is exceeded, the valve piston (9) is displaced counter to the spring force, whereby the nozzle opening (19) of the discharge nozzle covered by the valve piston (9) opens.Method according to claim 12, characterised in that the hollow channel (12) runs through the valve piston (9) and the spring force in the conveying direction of the thermoplastic material acts through the hollow channel (12) on the valve piston (9), and in that the displacement forces act on at least one surface region of the valve piston (9) delimiting the chamber (15), by means of which surface region the valve piston (9) is moved counter to the conveying direction of the thermoplastic material and releases the nozzle opening (10).

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