NL2026789B1

NL2026789B1 - Flow control in an extruder head

Info

Publication number: NL2026789B1
Application number: NL2026789A
Authority: NL
Inventors: Arnoldus Hubertus Gerardus Joosen Marcus
Original assignee: Ultimaker Bv
Priority date: 2020-10-29
Filing date: 2020-10-29
Publication date: 2022-06-21
Also published as: WO2022093019A1

Abstract

An extruder head for an FFF printing system is described comprising an extruder channel and a heating element (7) for heating part ofthe extruder channel so as to melt a printing material. The extruder channel comprises a ﬁrst cylinder (2) and a second cylinder (4) connected to the first cylinder, optionally via an intermediate transition part (3), wherein the extruder head further comprises a Peltier device (10;30) arranged to locally cool a region (11) ofthe second cylinder so as to make the printing material in the region (11) less or non-flowable. The Peltier device comprises a first heat conductive element (12;32), a second heat conductive element (13;33), and a plurality ofthermo-electric units (14;34) arranged between the first and second heat conductive element. [Figure 1]

Description

Flow control in an extruder head Field of the invention The present invention relates to an extruder head for a fused filament fabrication printing system.

The invention also relates to a fused filament fabrication printing system comprising such an extruder head.

Finally, the invention relates to a method of FFF printing using such a fused filament fabrication printing system.

Background art Fused filament fabrication (FFF) is a 3D printing process that uses a continuous filament of a thermoplastic material.

The filament is fed from a filament supply through a heated extruder head and is deposited through a print nozzle onto an upper surface of a build surface.

The extruder head (also referred to as print head) may be moved relative to the build surface under computer control to define a printed shape.

In certain FFF printing devices, the extruder head moves in two dimensions to deposit one horizontal plane, or layer, at a time.

The build surface, or the extruder head, is then moved vertically by a small amount to begin a new layer.

In this way a 3D printed object can be produced made out of a thermoplastic material.

In an FFF printer comprising multiple extruder heads, or an FFF printer capable of selecting one of multiple extruder heads using a tool changer, a 3D object can be made using more than one 3D printable material.

The materials may be selected for their properties, which make them suitable for a particular purpose, including but not limited to colour, flexibility, tensile strength, wear resistance etc.

The combination of multiple materials can significantly increase the usefulness of the 3D printed object.

Typically, many layers of such a multi-material object consist of more than one material.

This requires switching between materials at least once per layer.

An extruder that is currently not in use but is kept at the flowing temperature for the material that it contains may extrude material undesirably.

This effect is called oozing and can be due to thermal expansion, material relaxation or gravity.

Any oozed material is lost to the printing process and may leave artefacts on the printed object if not removed.

Retracting the material before parking the unused extruder may not suffice to prevent oozing.

Additionally, after material has oozed from the extruder, the melt channel must be filled (primed) to resume printing with a known flow.

Current mitigations for the oozing problem are prime and wipe towers, and nozzle wiping arrangements.

Priming and/or wiping the nozzle prior to using it takes time and spends material, both of which impact the cost of obtaining a 3D printed part from a printer.

Also flow control is a challenge for FFF printers.

It takes time to heat the material to the correct printing temperature, so the melt chamber (i.e. hot-end) is usually large and contains a significant amount of material.

Combined with the expansion properties of most FFF materials, this makes flow control before the liquefier quite challenging: the response to the feeding and retraction of the filament by a feeder is neither linear nor fast at the shortest timescales involved.

Flow control of the liquid material, i.e. after the melt chamber, is not much easier.

While it would allow fast response, it requires valves or other mechanical moving parts in the 'melt' which make the extruder head more complex and heavier.

A possible alternative to mechanical valves might be the use of a so-called ‘freeze valve’ within the extruder head arranged to locally solidify the molten material. Patent publication US6578596 B1 describes an extruder head comprising a heat chamber arranged inside a heated body, wherein thermoplastic filament is molten inside the heated chamber and wherein the molten filament is then guided through a flow tube towards a discharge orifice. A so-called valving region of the flow tube is cooled in order to locally remove heat from the flow tube, thereby solidifying the molten material inside the flow tube, shutting off the flow of material. The described device is employed to control a thermoplastic flow through the discharge orifice by selectively removing heat from the valving region in the following manner. The heated body is maintained at a temperature at which the thermoplastic is flowable, thereby maintaining the flow channel at flowable temperature in the inlet and outlet regions of the flow tube. A flow of coolant having a temperature lower than the lowest flowable temperature of the thermoplastic is selectively and controllably provided to a coolant inlet tube to valve on and off the flow channel. When the coolant flow is low or stagnant, heat from the heated body travels up from the outlet region of the flow tube and travels down from the inlet region of flow tube. This heat transfer from the heated body brings the valving region of the flow tube and the thermoplastic in the entire flow channel, up to a temperature at which the thermoplastic is flowable again. The flow channel is then said to be "valved on" again.

The above described arrangement involves a separate cooling unit which is connected to the extruder head via a number of tubes needed to supply the fluid coolant. This additional cooling unit and the accompanying tubing make the arrangement rather complex to fabricate and makes it prone to failure. Furthermore, the switching ‘on’ of the valve (i.e. opening it) is performed in a passive way in the sense that the heating of the valve region is caused by not cooling the flow channel and by waiting for the valve region to get sufficiently heated as a result of a heat flow coming from the hotter parts of the nozzle. This solution may be too slow for certain applications, and is in fact not fully controllable.

Summary of the invention The aim of the present invention is to provide an extruder head wherein a flow of molten filament material to the orifice of the nozzle can be blocked and unblocked, wherein at least one of the problems of the state of the art extruder head is solved.

According to a first aspect of the present invention, there is provided an extruder head for a Fused Filament Fabrication printing system. The extruder head comprises an extruder channel and a heating element for heating part of the extruder channel so as to melt a printing material, wherein the extruder channel comprises a first cylinder and a second cylinder connected to the first cylinder, optionally via an intermediate transition part. The extruder head further comprises a Peltier device arranged to locally cool a region of the second cylinder, also referred to as flow tube 40 so as to make the printing material in the region less or non-flowable. The Peltier device comprises a first heat-conductive element, a second heat-conductive element, and a plurality of thermo-electric units arranged between the first and second heat-conductive element.

By using a Peltier device arranged to locally cool the molten filament in the flow tube, the flow tube can be blocked and unblocked by suitable control of the Peltier device. The Peltier device only needs two power cables which is less prone to failure as compared to the tubing of the prior art systems. Furthermore, the Peltier device can also be used to reverse the heat flow so as to heat up the flow tube, which makes the valving on considerably faster as compared to the known systems. As compared to the knows solutions, the present invention enables a faster control of the flow of material. The ability to quickly valve off the flow of material when an extruder will be unused for some time, and to quickly resume the flow, may improve the print result, both in surface finish and part strength, as well as improve productivity.

In an embodiment, the second cylinder has a smaller inner diameter as compared to the first cylinder. An intermediate transition part may be arranged between the first and second cylinder to provide for a transition from a first diameter of the first cylinder to a smaller second diameter of the second cylinder. An advantage of a relatively small inner diameter of the second cylinder is that the region where the material is solidified due to cooling, can be smaller than the volume of the melt chamber. This will result in a faster switching on/off of the valve.

In an embodiment, each of the first and second heat-conductive elements is a flat plate having a central hole, the first and second heat-conductive elements being arranged in parallel, wherein the second cylindrical part of the extruder channel extends through the holes of the first and second heat-conductive elements, and wherein the first heat-conductive element are in contact with the second cylindrical part but the second heat-conductive element is not in contact with the second cylindrical part.

In an embodiment, a number of cooling fins are arranged at an outer surface of the second heat-conductive element. These cooling fins will result in a faster cooling of the second heat-conductive element, and will thus speed up the blocking of the valve region.

In an embodiment, each of the first and second heat-conductive elements comprises a cylinder, the first and second heat-conductive elements being co-axially arranged around at least a part of the second cylindrical part of the extruder channel. A number of cooling fins may be arranged around the second heat-conductive element to improve the cooling process. In this embodiment, the Peltier device may be a disc-shaped device wherein heat is pumped in and out in a radial direction. This configuration may allow a more compact construction measured along the axis of the flow tube than the parallel-plate configuration mentioned above.

In an embodiment, each of the number of thermo-electric units has four flat outer surfaces and two curved outer surfaces so as to fill up a space in between two co-axially arranged cylinders with different diameters. Using such rounded wedge-shaped units provides for an optimal heat transfer between the first and second heat conductive elements.

In an embodiment, the second cylinder has an inner diameter in a range between 0.2 mm - 1.5 mm. These dimensions showed good results during tests.

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According to a further aspect, there is provided a fused filament fabrication printing system comprising at least one extruder head as describe above.

The printing system may comprise a controlling system arranged to control the Peltier device.

This controlling system may also be arranged to control other functions of the printing system such as heating the build surface, moving the gantry and/or controlling the feeders.

The controlling system may be arranged within a housing of the printing system.

Parts of the controlling system may be arranged in or on the extruder head, and complementary parts may be arranged elsewhere such as in an inner space of the system near the bottom or walls.

In an embodiment, the controlling system is arranged to control the Peltier device so as to cool down the region of the second cylinder if material flow out of the extruder head needs to be stopped, and heat the region if the material flow out of the extruder head needs to restart.

The controller may be arranged to control the Peltier device using e.g. a lookup table that comprises information on the value of the electrical current needed, and the time needed to sufficiently stop the flow of material, and the same may account for the heating to unblock the extruder heads.

The lookup table may contain heuristic data gathered during experiments to fine tune the process for different printing materials.

The lookup table may be store in a memory of the controlling system.

In an embodiment, the controlling system is arranged to control the Peltier device so as to adjust the flow of material through the second cylinder by properly adjusting an electrical current through the Peltier device.

According to a yet further aspect, there is provided a method of FFF printing, the method comprising: - providing a fused filament fabrication printing system as described above; - controlling the Peltier device so as to cool down the region of the second cylinder if material flow out of the extruder head needs to be stopped, and heat the region if the material flow out of the extruder head needs to restart.

Brief description of the drawings These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings, Figure 1 schematically shows a cross section of a part of an extruder head according to an embodiment of the invention; Figure 2 schematically shows a perspective view of the Peltier device according to the embodiment of Figure 1; Figure 3 shows a further embodiment of the extruder head with a different Peltier device; Figure 4A shows a perspective view of the Peltier device according to an embodiment; Figure 4B schematically shows a cross section of the Peltier device of Figure 4A in a plane perpendicular to a main axis of the flow tube; Figure 5 shows a further embodiment of the extruder head wherein the Peltier device is enclosed by an extended heater that envelops the melt chamber as well as part of the flow tube;

Figure 6 shows yet a further embodiment of the extruder head wherein the Peltier device is enclosed by an extended heater that envelops the melt chamber as well as most part of the flow tube; Figure 7 schematically shows a fused filament fabrication (FFF) printing system according 5 to an embodiment of the invention, and Figure 8 shows a flow chart of a method of FFF printing according to an embodiment of the invention.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

Detailed description of embodiments Figure 1 schematically shows a cross section of a part of an extruder head 1 according to an embodiment of the invention. The extruder head 1 comprises an extrusion channel embodied by a first cylindrical part 2, an intermediate part 3, and a second cylindrical part 4. As is shown in Figure 1, the first cylindrical part 2 has an inner diameter d1 and the second cylindrical part 4 has an inner diameter d2, wherein d2 is smaller than d1. So, the second cylindrical part 4 is thinner than the first cylindrical part 2. A typical value for the inner diameter d2 lies in a range between

0.2 mm and 1.5 mm. The thinner part 4 is also referred to as the flow tube 4. At its outer end the flow tube 4 has an orifice for depositing molten plastic material which may have a bore diameter smaller than d2.

The extruder head 1 is supplied with a filament 6 by means of a filament feeder (not shown). The extruder head 2 also comprises a heating element 7 for heating part of the extruder channel. The heating element 7 may be controlled by a controlling system in order to make the filament 6 melt when it arrives in the melt chamber 8. The melt chamber 8 is also referred to as the hot-end of the extruder head 1. The extruder head 1 also comprises a cooling element 9 for keeping the extruder channel sufficiently cold at the so-called cold-end of the extruder head 1.

The cooling element 9 may comprise a number of cooling fins arranged around the extruder channel, which fins may be cooled by forced air coming from one or more fans (not shown in Figure 1).

In this embodiment, the extruder head 1 further comprises a Peltier device 10 arranged to locally cool a valve region 11 of the second cylinder 4. The Peltier device 10 comprises a first heat conductive element 12, a second heat conductive element 13, and a plurality of thermo-electric units 14 arranged between the heat conductive elements 12, 13. In this example, the Peltier device 10 further comprises a number of cooling fins 15 which are arranged on the second heat conductive element 13.

The thermo-electric units 14 may comprise a number of P-type thermo-electric units and a 40 number of N-type thermo-electric units which are connected electrically in series and thermally in parallel. If the thermo-electric units are connected to a power supply, a current passes through the thermo-electric units 14, and the Peltier device 10 will start working as a heat pump, wherein the first heat conductive element 12 gets cold and the second heat conductive element 13 gets hot, or vice versa depending on the direction of the current.

When no current is applied to the Peltier device 10, the flow tube 14 will be heated by the heat coming from the heating element 7 via the heat conductive first cylindrical part and the intermediate part 3. Once the current is applied to the Peltier device 10, the first heat conductive element 12 will become cold. Heat coming from the flow tube 4 is transferred through the Peltier device 10 to the second heat conductive element 13. Due to the relatively low thermal mass and its small diameter, the flow tube 4 can be cooled off very fast. As a result, the specially arranged Peltier device 10 around the cooling section 11 of the flow tube 4 can quickly cool down a small amount of material in the valve region 11 below the glass transition temperature, thereby providing a fast flow control of this ‘freeze valve’. Of course, this may be combined with a suitable flow control of the filament feeder so as to avoid too much pressure in the melt chamber 8.

Preferably, the feed rate of the filament feeder is such as to allow for thermal expansion of the material that was recently brought into the hot-end. Retracting the solid filament will be required to allow for this expansion. A control system (see also Figure 7) may be arranged to schedule the filament retraction and Peltier current based on knowledge of the thermal properties of the flow tube and the material being used.

The thermoelectric units comprise both P- and N-type materials units which are connected electrically in series and thermally in parallel. The Peltier effect occurs when current flows between two dissimilar conductors or semiconductors. When this occurs, the charge carriers flowing through the material will transfer heat from one side to the other, allowing this effect to be used to create a heat pump having no moving mechanical parts, gases or fluids. This makes it very low maintenance and in principle completely vibration-free.

The first and second cylinder forming the extrusion channel are in fluidic communication and may share the same axis. However, the first and second cylinder do not need to be aligned. Alternatively, they may have different orientations, such as described in the prior art wherein the axis of the second cylinder is perpendicular to the axis of the first cylinder.

The Peltier device only needs to cool down the material from the printing temperature to a non-flowing temperature. For example, PLA may be printed using a printing temperature of around 210°C, and will be less or non-flowable in the range of 120°C and 170°C. So, when cooling the PLA material in the valve region to around 160°C, the valve will get blocked. This means cooling off of the flow tube with only 50°C. If PP is used as material the printing temperature may be about 215°C, but the non-flowing temperature needs to be lower than 150°C, so the valve tube needs to be cooled about 65°C which is a larger cooling step as compared to PLA, but still relatively small.

The Peltier device only has to create this relatively small temperature difference to obtain the flow control effect.

The efficiency of a Peltier device depends strongly on the temperature difference between the hot and cold terminals of the device: the efficiency decreases sharply with an increased temperature gradient. To limit the drop in efficiency, the outside of the Peltier device 10 can be maintained at an elevated temperature rather than the ambient temperature in the vicinity of the extruder head. As can be seen in Figure 1, the heater 7 and the Peltier device 10 are not in contact and thus thermally separated. In this case it is assumed that the airflow along the hot-end (which may be forced by a fan) is sufficient to keep the outside of the Peltier device 10 at a useful temperature.

The airflow may actually be the other way around if this part of the hot-end is inside an insulated compartment and convection or radiation from the build compartment dominates the air flow around heater 7. Even without forced airflow this arrangement may work.

A temperature sensor 15 may be arranged on the outer surface of the flow tube 4 so as the measure the temperature near the valve region 11 which may be used to control the current of the Peltier device 10. A further temperature sensor may also be arranged at or in the heating element 7 to measure the temperature of the melt chamber (not shown in Figure 1).

Figure 2 schematically shows a perspective view of the Peltier device 10 according to the embodiment of Figure 1. Figure 2 shows the first heat-conductive element 12 and the second heat-conductive element 13 where both elements 12,13 are flat plates having a central hole, one of which is visible, see hole 23 in the second heat conductive element 13. Both heat-conductive elements 12, 13 are arranged in parallel, having the thermo-electric units sandwiched in between except for a central region where no thermo-electric units are located. As can be seen from Figure 1, the second cylindrical part 4 of the extruder channel extends through the holes of the first and second heat-conductive elements. It is noted that the first heat-conductive element 12 is in contact with the second cylindrical part but the second heat-conductive element 13 is not in contact with the second cylindrical part 4. This is a result of the fact that the hole 23 in the second heat-conductive element 13 is wider than the outer diameter of the flow tube 4.

The heat-conductive plates 12, 13 can be made from a metal such as copper or bronze, but it may alternatively be made out of a ceramic material. Other materials are conceivable as will be appreciated by the skilled person.

Figure 3 shows a further embodiment of the extruder head 1 with a different Peltier device

30. The Peltier device 30 comprises two cylindrical shaped conductive elements 32, 33. In between the cylindrical shaped conductive elements 32, 33, a number of thermo-electric units 34 is arranged. A first heat-conductive element 32 is arranged around the flow tube 4, whereas a second heat-conductive element 33 is arranged around the first heat-conductive element 32 and in contact with cooling fins 35. The thermo-electric units 34 are arranged between the cylindrical shaped conductive elements 32, 33.

Figure 4A shows a perspective view of the Peltier device 30 according to an embodiment. The Peltier device 30 is substantially disc-shaped and has a hole 36 in the centre. Figure 4B schematically shows a cross section of the Peltier device 30 in a plane perpendicular to a main 40 axis of the flow tube 4. Figure 4B shows a number of P-type thermo-electric units 34 and a number of N-type thermo-electric units 34’ which are connected electrically in series and thermally in such a way that heat is pumped in a centrifugal direction or in a centripetal direction. The arrows in Figure 4B indicate a situation wherein heat flows in the centrifugal direction.

If the thermo-electric units 34, 34’ are connected to an electrical power supply, an electric current passes through the thermo-electric units 34, 34’ and the Peltier device 30 will start working as a heat pump, wherein the first heat-conductive element 32 gets cold and the second heat- conductive element 33 gets hot, or vice versa depending on the direction of the current. As can be seen from Figure 4B, the flow tube 4 is inserted into the hole 36 of the Peltier device 30.

In the embodiment of Figures 4A and 4B, the first conductive element 32 is much thicker than the second conductive element 33. As a result, the thermal mass may be considerable which may be disadvantageous in case a fast switching of the freeze valve is requested. In order to decrease the size of the first heat-conductive element 32, this disc-shaped element may comprise one or more cut-outs so as to decrease the total mass, and thus the thermal mass. A possible arrangement of cut-outs may result in the first conductive element 32 having a cross section that is similar to a wheel having spokes. The spokes will then transfer the heat in a centrifugal direction from the central part of the conductive element 32 towards the outer surface and towards the thermo-electrical units 34, 34’.

Figure 5 shows a further embodiment of the extruder head 1 wherein the Peltier device 30 is enclosed by an extended heater that envelops the melt chamber 8 as well as part of the flow tube 4. Thermal separation is provided by not having the flow tube 4 in direct contact with the heater 7. This arrangement allows for a stable, high reference temperature on the outside of the Peltier element, thus limiting the temperature gradient across the Peltier device and thereby improving the efficiency of the thermo-electric process.

Peltier devices can only transfer heat (thermal energy), they don't just set a temperature.

However, it is noted that the temperature inside the valve region 11, see also Figure 3, only has to be controlled within a small range between a maximum value and a minimum value. The maximum value may be the same as the temperature in the liquefier (i.e. melt chamber), whereas the minimum value needs to be only a little less so as to increase the viscosity enough to stop the flow. For example, in case a PLA filament is used, the maximum value in the valve region can be e.g. 210°C and the minimum temperature can be e.g. 160°C.

Figure 6 shows yet a further embodiment of the extruder head 1 wherein the Peltier device 30 is enclosed by an extended heater 7 that envelops the melt chamber 8 as well as most part of the flow tube 4. In this case the heater 7 has a narrowing close to the orifice 5 so as to enable heating of the outer end of the flow tube 4. This arrangement significantly reduces the amount of material that can flow out of the nozzle uncontrollably, while actively and accurately maintaining the temperature of the extruded material.

Note that with or without the heater extending down to the end of the flow tube 4, the Peltier device 30 may be used to finely control the temperature of the material before it leaves the flow tube 4, heating or cooling it as required. This is an additional advantage of the Peltier 40 arrangement that is independent of its flow-controlling properties.

Figure 7 schematically shows a fused filament fabrication (FFF) printing system 70, also referred to as a 3D printer, according to an embodiment of the invention. The 3D printer 70 comprises an extruder head 1 as described above. In this example, a filament 75 is fed into the extruder head 1 (i.e. print head) by means of a feeder 73. Part of the filament 75 is stored around a spool 78, which could be rotatably arranged onto a housing (not shown) of the 3D printer, or rotatably arranged within a container (not shown) containing one or more spools. The 3D printer 70 comprises a controlling system 77 arranged to control the feeder 73 and the movement of the extruder head 1. The controlling system 77 may comprise a memory 76 for storing instructions for printing and for controlling the Peltier device(s). The latter may be done using a lookup table containing information on how much electrical current is needed to cool or warm the Peltier devices(s) and for how long (i.e. time period) the current needs to be applied. In this embodiment, the 3D printer further comprises a Bowden tube 79 arranged to guide the filament 75 from the feeder 73 to the extruder head 1. It is noted that alternatively the 3D printer may use a direct-drive print head, wherein the Bowden tube(s) may be absent.

The 3D printer 70 also comprises a gantry arranged to move the extruder head 1 at least in one direction, indicated as the X-direction. In this embodiment, the extruder head 1 is also movable in a Y-direction perpendicular to the X-direction. The gantry comprises at least one mechanical driver 84 and one or more linear guides 85 and a print head docking unit 86. The print head docking unit 86 holds the extruder head 1 and for that reason is also called the print head mount 86. It is noted that the print head docking unit 86 may be arranged to hold more than one extruder head, such as for example two extruder heads each receiving its own filament. Two fans 87 are arranged in the print head docking unit 86 to cool the extruder head 1 if needed. A build plate 88 may be arranged in or under the 3D printer 70 depending on the type of 3D printer. The build plate 88 may comprise a glass plate or any other object suitable as a substrate. In the example of Figure 7, the build plate 88 is movably arranged relative to the extruder head 1 in a Z- direction, see Figure 7. In an alternative embodiment, material is deposited on a conveyer belt which is arranged to move perpendicular to the X-direction, and wherein the surface of the belt makes an angle with the Y-direction that is smaller than 90°, such as 45°.

The feeder 73 is arranged to feed and retract the filament 75 to and from the extruder head 1. The feeder 73 may be arranged to feed and retract filament at different speeds to be determined by the controlling system 77.

The extruder head 1 is Figure 7 comprises a Peltier device 10 in order to activate the freeze valve as was described above. The Peltier device 10 is connected to the controlling system 77 so that the controlling system 77 can control the settings of the Peltier device 10 for blocking and unblocking of the flow tube of the extruder head 1.

Figure 8 shows a flow chart of a method of FFF printing according to an embodiment of the invention. The method 800 comprises providing 801 a fused filament fabrication printing system as described above, and controlling 801 the Peltier device 10 so as to cool down the region 11 of the second cylinder if material flow out of the extruder head needs to be stopped, and heat the region 11 if the material flow out of the extruder head needs to restart. Now an example is discussed on how the printing system can be used. In this example, the system comprises two extruder heads called A and B.

First, extruder A is printing material, extruder B is unused but already warm. The Peltier device of extruder B is cooling, so no material comes out of extruder B (no oozing). If extruder A is almost done with its material on a specific layer, extruder A is permanently cooled and the filament is retracted in time, so that the flow stops. At the same time, extruder B is heated, so that the flow can start immediately when B is in the correct position. Then extruder A is moved away from the object, and extruder B is moved towards it. Now extruder B can print, because the process is so sophisticated that the flow starts exactly when extruder B is in the right place because at that moment (i) the temperature of the entire melting channel is right and (ii) the material is pressurized by the feeder. Now A is inactive.

In view of the above, the present invention can now be summarized by the following embodiments: Embodiment 1. An extruder head (1) for a Fused Filament Fabrication printing system, the extruder head comprising an extruder channel and a heating element (7) for heating part of the extruder channel so as to melt a printing material, wherein the extruder channel comprises a first cylinder (2) and a second cylinder (4) connected to the first cylinder, optionally via an intermediate transition part (3), wherein the extruder head further comprises a Peltier device (10,30) arranged to locally cool a region (11) of the second cylinder so as to make the printing material in the region (11) less or non-flowable, wherein the Peltier device comprises a first heat- conductive element (12;32), a second heat-conductive element (13;33), and a plurality of thermo- electric units (14;34) arranged between the first and second heat-conductive elements.

Embodiment 2. The extruder head according to embodiment 1, wherein the second cylinder has a smaller inner diameter as compared to the first cylinder.

Embodiment 3. The extruder head according to embodiment 1 or 2, wherein each of the first and second heat-conductive elements is a flat plate having a central hole, the first and second heat-conductive elements being arranged in parallel, wherein the second cylindrical part of the extruder channel extends through the holes of the first and second heat conductive elements, and wherein the first heat conductive element is in contact with the second cylindrical part, but the second heat-conductive element is not in contact with the second cylindrical part.

Embodiment 4. The extruder head according to any one of the preceding embodiments, wherein a number of cooling fins are arranged at an outer surface of the second heat-conductive element.

Embodiment 5. The extruder head according to embodiment 1, wherein each of the first and second heat-conductive elements comprises a cylinder, the first and second heat conductive elements being co-axially arranged around at least a part of the second cylindrical part of the extruder channel. Embodiment 6. The extruder head according to embodiment 5, wherein a number of cooling fins are arranged around the second heat-conductive element.

Embodiment 7. The extruder head according to embodiment 5 or 6, wherein each of the number of thermo-electric units (14;34) has four flat outer surfaces and two curved outer surfaces so as to fill up a space in between to co-axially arranged cylinders with different diameters.

Embodiment 8. The extruder head according to any one of the preceding embodiments, wherein the second cylinder has an inner diameter in a range between 0.2 mm - 1.5 mm.

Embodiment 9. A fused filament fabrication printing system (70), the system comprising at least one extruder head (1) according to any one of the preceding embodiments.

Embodiment 10. The fused filament fabrication printing system according to embodiment 9, wherein the printing system comprises a controlling system (77) arranged to control the Peltier device.

Embodiment 11. The fused filament fabrication printing system according to embodiment 10, wherein the controlling system (77) is arranged to control the Peltier device {10;30) so as to cool down the region (11) of the second cylinder if material flow out of the extruder head needs to be stopped, and heat the region (11) if the material flow out of the extruder head needs to restart.

Embodiment 12. The fused filament fabrication printing system according to embodiments 10-11, wherein the controlling system (77) is arranged to control the Peltier device (10;30) so as to adjust the flow of material through the second cylinder by properly adjusting an electrical current through the Peltier device.

Embodiment 13. Method of FFF printing, the method comprising: - providing a fused filament fabrication printing system according to any of embodiments 10-12; - controlling the Peltier device (10;30) so as to cool down the region (11) of the second cylinder if material flow out of the extruder head needs to be stopped, and heat the region (11) if the material flow out of the extruder head needs to restart.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible and are included in the scope of protection as defined in the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

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Claims

CONCLUSIES

1. Een extruderkop (1) voor een filament samensmelt fabricage printsysteem, waarbij de extruderkop een extruderkanaal en een verwarmingselement (7) omvat voor het verwarmen van een deel van het extruderkanaal om een printmateriaal te smelten, waarbij het extruderkanaal een eerste cilinder (2) omvat en een tweede cilinder (4) verbonden met de eerste cilinder, optioneel via een tussenliggende overgangsdeel (3), waarbij de extruderkop verder een Peltier-inrichting (10;30) omvat die is ingericht om lokaal een gebied ( 11) van de tweede cilinder te koelen om het printmateriaal in het gebied (11) minder of niet-vloeibaar te maken, waarbij de Peltier-inrichting een eerste warmtegeleidend element (12; 32) omvat, een tweede warmtegeleidend element (13,33), en een aantal thermo-elektrische eenheden (14; 34) ingericht zijn tussen de eerste en tweede warmtegeleidende elementen.

2. Extruderkop volgens conclusie 1, waarbij de tweede cilinder een kleinere binnendiameter heeft in vergelijking met de eerste cilinder.

3. Extruderkop volgens conclusie 1 of 2, waarbij elk van de eerste en tweede warmtegeleidende elementen een vlakke plaat is met een centraal gat, waarbij de eerste en tweede warmtegeleidende elementen parallel zijn opgesteld, waarbij het tweede cilindrische deel van het extruderkanaal zich uitstrekt door de gaten van de eerste en tweede warmtegeleidende elementen, en waarbij het eerste warmtegeleidende element in contact is met het tweede cilindrische deel, maar het tweede warmtegeleidende element niet in contact is met het tweede cilindrische deel.

4. Extruderkop volgens één van de voorgaande conclusies, waarbij aan een buitenoppervlak van het tweede warmtegeleidende element een aantal koelribben is aangebracht.

5. Extruderkop volgens conclusie 1, waarbij elk van de eerste en tweede warmtegeleidende elementen een cilinder omvat, waarbij de eerste en tweede warmtegeleidende elementen coaxiaal zijn ingericht rond ten minste een deel van het tweede cilindrische deel van het extruder. kanaal.

6. Extruderkop volgens conclusie 5, waarbij om het tweede warmtegeleidende element een aantal koelribben is aangebracht.

7. Extruderkop volgens conclusie 5 of 6, waarbij elk van het aantal thermo-elektrische eenheden (14; 34) vier platte buitenoppervlakken en twee gebogen buitenoppervlakken heeft om een tussenruimte op te vullen tot coaxiaal geplaatste cilinders met verschillende diameters. 40

8. Extruderkop volgens één van de voorgaande conclusies, waarbij de tweede cilinder een binnendiameter heeft in een bereik tussen 0,2 mm - 1,5 mm.

9. Een filament samensmelt fabricage printsysteem (70), waarbij het systeem tenminste één extruderkop (1) omvat volgens één van de voorgaande conclusies.

10. Filament samensmelt fabricage printsysteem volgens conclusie 9, waarbij het printsysteem een regelsysteem (77) omvat dat is ingericht om de Peltier-inrichting te besturen.

11. Filament samensmelt fabricage printsysteem volgens conclusie 10, waarbij het besturingssysteem (77) is ingericht om het Peltier-inrichting (10;30) te besturen om het gebied (11) van de tweede cilinder af te koelen als het materiaal dat uit het extruderkop stroomt moet worden gestopt, en het gebied (11) te verwarmen als het materiaal weer uit de extruderkop moet gaan stromen.

12. Filament samensmelt fabricage printsysteem volgens één van de conclusies 10-11, waarbij het regelsysteem (77) is ingericht om de Peltier-inrichting {10; 30) te besturen om de materiaalstroom door de tweede cilinder aan te passen door een juiste afstelling van een elektrische stroom door het Peltier- inrichting.

13. Werkwijze voor FFF printen, waarbij de methode omvat: - het verschaffen van een Filament samensmelt fabricage printsysteem volgens één van de conclusies 10-12; - het besturen van het Peltier- inrichting (18; 30) om het gebied (11) van de tweede cilinder af te koelen als het materiaal uit de extruderkop moet worden gestopt, en het gebied (11) te verwarmen als het materiaal weer uit de extruderkop moet gaan stromen.