Patent Description:
Fused filament fabrication (FFF) is a 3D printing process that uses a continuous filament of a thermoplastic material. Filament is fed from a coil through a moving, heated print head, and is deposited through a print nozzle on the growing work. The print head may be moved under computer control to define a printed shape. Usually, the print head moves in two dimensions to deposit one horizontal plane, or layer, at a time. The work or the print head is then moved vertically by a small amount to begin a new layer.

Mostly filament coils are arranged on a spool which may be mounted on a housing of the 3D printing device. The filament is fed to the print head by way of a feeder. In FFF filament fabrication there are two main types of printing devices, referred to as Direct feeder printing device and Bowden printing device. In a direct feeder printing device, the feeder is arranged in or on top of the print head. A filament is fed to the feeder from a spool of filament. In a Bowden printing device, the feeder is arranged external from the print head, and the filament is guided through a Bowden tube from the feeder to the print head. In both scenarios the feeder generates a feeding force for feeding the filament through the print head and a pulling force for unrolling the spool.

In systems where a filament spool is stored in a special storage compartment, the unrolling of the spool may require additional force for several reasons. Systems are known that comprise an additional feeder for feeding filament from the storage compartment to the feeder of the printing device. Such additional feeders are also referred to as Prefeeders.

When using a prefeeder, this prefeeder needs to be operated in line with the actions of the (printer) feeder so as not to cause any disturbance in the filament feeding process. Today's systems for alignment of feeders and prefeeders use complex measuring devices for sensing misalignment between the feeder and the prefeeder. Such measuring devices are often very error prone.

Publication <CIT>) discloses an FFF printer and a method with force feedback for printing non-uniform filaments. The patent application does not disclose an FFF printing system with a feeder and a prefeeder. In the described FFF printer, a filament is fed through a guide tube. In an embodiment, the guide tube is cut into two parts, and a stress/strain gauge is placed between the two parts for measuring the force applied during the extrusion. The FFF printer is configured to transport the filament through the transport channel. Due to the presence of the gauge, the transport channel comprises an upstream part and a downstream part which are associated to each other via a pressure sensor for sensing a force-related parameter for controlling deposition of the 3D printable material. Based on the sensor signal, the control system may control the force, for instance to maintain a constant diameter of the filament escaping from the printer nozzle. The pressure sensor is connected between a housing of the gauge and an end stop coupled to one of the tube parts. In this way the two tube parts are coupled via the pressure sensor, which is of course needed in order to let the sensor measure a pressure value. The measuring range may be sufficient for this application, but it will not suffice when needing accurate alignment of a feeder and a prefeeder.

One of the objects of the invention is to provide a measuring device which is less error prone as compared to the prior art solutions.

A first aspect of the invention provides an FFF printing system, the FFF printing system comprising:.

The filament path length measuring device comprises:.

A second end of the first flexible tube is connected to a connection point of the FFF printing system.

The FFF printing system further comprises a processing system arranged to receive the measurement data and to control the prefeeder depending on the measurement data. The filament path length measuring device is arranged to detect a misalignment between the feeder and the prefeeder. Measurement signals are sent to the processing system to correct any misalignment. By measuring the path length change of the filament in the flexible tube, the actions of a prefeeder can be aligned with the actions of the feeder. An advantage of the usage of a slider and a contactless sensor is that no, or very little force is required. So, no additional unwanted force is applied to the filament. In this way more accurate measurements are achieved, and the control of the feeders can be optimized.

Depending on the type of printing system, the flexible tube may be connected to different connection points. In an embodiment, the flexible tube is connected an upstream side of the feeder. In another embodiment, the flexible tube is connected a downstream side of the prefeeder. In yet another embodiment, the system comprises a merger module arranged to merge filaments coming from different spools, wherein the flexible tube is connected a downstream side of the filament merger module.

In an embodiment, the FFF printing system comprises a further processing system arranged to control the feeder and arranged to communicate with the processing system. This embodiment is advantageous in the situation wherein an FFF printer is combined with a separate filament feeding system, wherein both systems have their own processing system specialized in their own tasks, and able to communicate with each other. The processing system may be arranged to communicate status information on the filament feed status to the further processing system arranged in the FFF printer. Using such a modular system requires little redesign of the FFF printer. Only some software reprogramming is needed.

In an embodiment, the at least one sensor comprises a Hall sensor. An advantage of using a Hall sensor is that it allows for an analogue position detection of the slider. A continuous analogue signal is produced, which is easy to pre-process.

In an embodiment, the at least one sensor comprises two Hall sensors interacting with a single magnet mounted on the slider. Due to a combination of two Hall sensors interacting with an intermediate magnet, the slider can have a larger stroke as compared to that of a device having a single Hall sensor, given a certain sensitivity of a Hall sensor and a certain magnet strength. In an embodiment, the device comprises an analogue circuitry arranged to combine the output signals of the two Hall sensors, to obtain an analogue signal indicative of the change of position of the slider. A further advantage of using two Hall sensors is that such a configuration yields a high signal/noise ratio where it matters most: at both ends of the range of motion of the slider. The outputs of the two sensors may be combined so that only one IO pin is needed and there is no need for digital processing.

In an embodiment, the measuring device comprises a torsion spring having two spring arms, each of the spring arms being arranged to counteract movement of the slider at an outer end of a stroke of the slider. The spring arms allow for movement of the slider but counteract the movement of the slider at the outer end of its stroke. Preferable, the Hall sensors are arranged to detect the slider position in a mid-region and at the two outer regions where movement the slider counteracted by the spring arms.

In an embodiment, the slider comprises a number of fingers coaxial arranged around a central axis, wherein side walls of the fingers facing the central axis, together with structures arranged in the housing, define a channel for guiding the filament.

The fingers can surround the filament and will slide relative to the structures in the housing. They will guide the filament through a channel without creating a stepped path, and there will be no unwanted ridge for the filament to catch.

In an embodiment, the slider comprises three fingers. Such a configuration is relatively easy to fabricate using injection moulded techniques. Furthermore, if more than three fingers would be used, they would have reduced the wall thickness which is less favourable, since the device would then be flimsy and more difficult to produce.

In an embodiment, the housing of the measurement device is mounted to outer wall of the FFF printing system. Placing the filament path measuring device at the outside of the system, makes the device easily accessible for a user who may need to perform calibration methods. It should be noted that the measurement could alternatively be placed within the system so as to hide the device in order to protect it against outer forces and/or for aesthetic reasons.

In an embodiment, the FFF printing system is a modular system comprising a printing apparatus and a filament feeding system. In this embodiment the processing system may be arranged in the filament feeding system and configured to communicate with a processor of the printing apparatus. In this way the processing system can receive instructions for controlling the prefeeder in line with the printer feeder. An advantage of such a modular system is that in case of malfunctioning, only one of the modules needs to be replaced or repaired.

In an embodiment, the first flexible tube has an S-shape. This special shape can avoid axial forces on the tube and thus on the slider and thus on the filament. So, the slider will only experience lateral forces caused by a change of filament path length.

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.

<FIG> schematically shows an example of a filament path length measuring device <NUM> according to an embodiment of the invention. The filament path length measuring device <NUM>, also referred to as measuring device <NUM> comprises a housing <NUM> comprising a first opening <NUM> for passing through of a filament <NUM>. The measuring device <NUM> also comprises a slider <NUM> slidably arranged in the housing <NUM>. The slider <NUM> comprises a filament channel <NUM> for passing through of the filament <NUM>. At an outer end, the slider <NUM> is connected to a flexible tube <NUM> for guiding the filament <NUM> to for example an input of a printer feeder (not shown). The housing <NUM> comprises a second opening <NUM> through which the slider <NUM> can, at least to some extent, leave the housing <NUM>. In this embodiment, the slider <NUM> is an elongated part that forms a telescopic part of the device <NUM>.

The measuring device <NUM> also comprises at least one sensor <NUM> arranged to detect a change of position of the slider <NUM> relative to the housing <NUM> to obtain measurement data indicative of a path length change of the filament <NUM>. The measurement data can be communicated to a processing system (see arrow). The sensor <NUM> preferably comprises a contactless sensor <NUM> cooperating with a signal generator/reflector <NUM>. The contactless sensor <NUM> may be an optical sensor, such as a time-of-flight sensor. Preferably, the sensor <NUM> comprises a Hall sensor <NUM> wherein the signal generator <NUM> comprises a magnet <NUM>. The housing <NUM> of the measuring device <NUM> may be coupled to a second flexible tube <NUM> depending on the application of the measuring device <NUM>. <FIG> also shows a first limiter <NUM> for limiting the movement of the slider <NUM> to the left, and a second limiter <NUM> for limiting the movement of the slider <NUM> to the right. In this very schematic example, the first limiter <NUM> is coupled to an inner wall of the housing <NUM>, and the second limiter <NUM> is coupled to an outer wall of the slider <NUM>, but it will be clear to the skilled reader that other solutions are conceivable.

<FIG> schematically shows an example of a filament path length measuring device <NUM> according to a further embodiment. As compared to <FIG>, the measuring device <NUM> now comprises two Hall sensor <NUM>, <NUM> which interact with a single magnet <NUM>. Both Hall sensors <NUM>, <NUM> provide input for an electrical circuitry <NUM> which is arranged to combine the signals of the Hall sensors <NUM>, <NUM> to obtain a signal indicative of the displacement of the slider <NUM> relative to the housing <NUM>. The two Hall sensors <NUM>, <NUM> face in opposite direction and both sense the magnetic field of the magnet. Due to this combination of the Hall sensors <NUM>,<NUM> and the intermediate magnet <NUM>, the slider <NUM> can have a larger stroke as compared to that of the embodiment of <FIG>, given a certain sensitivity of one Hall sensor or magnet strength. A double arrow in <FIG> indicates the larger stroke. To enable this larger stroke of the slider <NUM>, a repositioning of the limiters <NUM> and <NUM> is needed, see <FIG>.

<FIG> shows an electrical schema incorporating the two Hall sensors <NUM>, <NUM> and the magnet <NUM>. The magnet <NUM> is mounted onto the slider <NUM>, see <FIG>, so the magnet <NUM> is movable relative to the two Hall sensors <NUM>, <NUM>. Both Hall sensors are fed using a voltage of Vinp. In this example, the output of each Hall sensor is connected to a terminal of a resistor, see resistors <NUM>, <NUM>. These two resistors <NUM>, <NUM> are connected at their other terminals to form an output node S. If two identical Hall sensors are used, and the resistors <NUM>, <NUM> have equal value (R1=R2) then an output can be generated as shown in <FIG> is a graph of the output S of the scheme of <FIG> as a function of the position of the magnet <NUM> being the slider position relative to the housing, also referred to as 'Decoupler position'. In <FIG> the value of S is indicated as a percentage of the input value Vinp of the Hall sensors. In this example the decoupler position varies between <NUM> and <NUM>. The graph of <FIG> can mainly be divided in three regions: a first region showing a non-linear decreasing curve between <NUM>-<NUM>, a second region with a nearly linear and slowly decreasing line, and a third region with a non-linear decreasing curve between <NUM>-<NUM>. It is noted that a gradient of the curve in <FIG> directly relates to the sensitivity of the measuring device. So, in the first and third region, the sensitivity is higher as compared to the second region. This is advantageous as the higher signal/noise ratio increases position sensor accuracy where it matters most: close to both limiters and at both springs.

In a simple embodiment, the housing <NUM> is an outer cylinder and the slider <NUM> is a thinner inner cylinder movable within the outer cylinder wherein the filament can pass through both cylinders, see <FIG>. In <FIG> the cylinders of <FIG> are shown again but with their width shown. In <FIG> the filament is inserted into an opening <NUM> at the left and pushed to the right. This situation may occur if a new filament is fed from the prefeeder to the feeder. As can be seen in the drawing, the filament <NUM> may get stuck at a threshold (or ridge) at the entrance of the inner cylinder <NUM>. In the figures described below this problem is solved using special designs of the slider and the housing.

<FIG> is a perspective view of a slider <NUM> according to an embodiment. The slider <NUM> can be fabricated as a single moulded part, but alternatively could be assembled using subparts being glued or welded or screwed together. Alternatively, the slider <NUM> could be manufactured using an additive manufacturing technique such as FFF. The slider <NUM> comprises a first cylindric body <NUM>, a second cylindrical part <NUM> having a slightly smaller diameter, and two spring holders <NUM>, <NUM> coupled at two opposing sides to the second cylindrical body <NUM>. The spring holders <NUM>, <NUM> are arranged to each hold a torsion spring (not shown). It is noted that instead of two springs, only one torsion spring could be used. Two torsion springs arranged at both sides of the slider are preferred since the avoid any unwanted momentum on the slider. The slider <NUM> further comprises a magnet holder <NUM> for storing the magnet <NUM>, see also <FIG>. The slider <NUM> also comprises three fingers <NUM> which extend from the first cylindrical body <NUM> in an axial direction parallel to a main axis of the slider <NUM>. Each of the fingers <NUM> has a side wall <NUM> facing the main axis of the slider <NUM>. As such the side walls <NUM> of the fingers <NUM> define a channel <NUM>. This channel <NUM> is shown in <FIG> which shows a front view of the slider <NUM>. Since in this embodiment both the first and second cylindrical part <NUM> and <NUM> are hollow, the filament <NUM> can pass through the slider <NUM> from one side to the other.

In an embodiment the fingers of the slider <NUM> cooperate with structures arranged in the housing. <FIG> shows an example of a cross section of the fingers <NUM> together with an additional set of fingers <NUM> (dotted fingers) arranged in the housing, which fingers extent in the axial direction in the same way as the fingers <NUM>, but in the opposite direction. The fingers <NUM> make an angle β with β = <NUM> degrees. In this example the fingers <NUM> are of equal dimensions as the fingers <NUM> and also make an angle equal to β. So, the fingers <NUM> by themselves enclose, at least partly, the same channel <NUM>.

If the slider <NUM> comprising the fingers <NUM> moves relative to the housing comprising the fingers <NUM>, the fingers <NUM> move relative to the fingers <NUM> but due to their configuration, they intertwine and form a channel at a region where they overlap. They also form the channel <NUM> at regions where they do not overlap. Now if a filament <NUM> is fed through the measurement device <NUM>, the filament will not meet a threshold as was the case at <FIG>. So, the risk of getting a filament stuck in the measuring device is minimized.

It is noted that this solution of sets of fingers mating may also work using sets of four (or even more) fingers. Preferably the side walls of the fingers facing the main axis are curved to form parts of the outer circumference of the circular channel <NUM>.

The housing <NUM> may comprise a bottom part and an associated top part for closing the bottom part, once the slider is installed in the bottom part. <FIG> is a perspective view of a bottom part <NUM> of the housing <NUM>. As can be seen from <FIG>, the housing in this embodiment is a beam shaped box. Before placing the slider <NUM> in the bottom part <NUM>, two torsion springs are arranged at both side of the slider onto the spring holders <NUM>, <NUM>. Each of the torsion springs comprises two arms which are indicated in <FIG> with reference numbers <NUM>, <NUM>, <NUM> and <NUM>. The arms <NUM> and <NUM> (referred to as the `lower arms' <NUM>, <NUM>) will meet abutments <NUM>, <NUM> respectively once the slider has moved sufficiently into the housing. Similarly, the arms <NUM> and <NUM> (referred to as the 'upper arms' <NUM>, <NUM>) will meet associated abutment when the slider <NUM> moves sufficiently out of the housing. As such, the arms of the torsion spring function as limiters mentioned in <FIG>, see <NUM> and <NUM>, but now in a biased manner. Note that the abutments can be positioned at different distances from the torsion spring mounting boss, so that each abutment results in a different force, as it flexes the torsion spring closer to/further from the mounting boss.

Two of the three fingers <NUM> of the slider <NUM> make contact with a support structure <NUM> arranged in the bottom part <NUM>. The support structure <NUM> comprises two surfaces making an angle β equal to <NUM> degrees. The angle may depend on the angle between the fingers <NUM> of the slider <NUM> as will be explained below when discussing <FIG>.

<FIG> shows an example of a top part <NUM> to be coupled to the bottom part <NUM> shown in <FIG>. In this particular embodiment, the top part <NUM> comprises two walls <NUM> with angled outer ends. In a closed state of the housing <NUM>, these two walls <NUM> will be arranged at both sides of one of the fingers <NUM> of the slider. This is shown in <FIG> which shows a cross section of the slider <NUM> at the location of the fingers and the support structure <NUM>. As can be seen from <FIG>, a channel is now formed and enclosed by the three fingers <NUM>, the two walls <NUM> and the support structure <NUM>. Although different in design, the creation of an extendable and threshold-free channel is similar to that described with reference to <FIG>.

<FIG> schematically shows an example of an FFF printing system <NUM> comprising a filament feeding system <NUM> and an FFF printing apparatus <NUM> on top of the filament feeding system <NUM>. <FIG> shows a back view of the FFF printing apparatus <NUM> and the filament feeding system <NUM>. In this embodiment, the filament feeding system <NUM> comprises the above mentioned decoupler <NUM>, a prefeeder <NUM> and a processing system <NUM>. The FFF printing apparatus <NUM> comprises a printer feeder <NUM> and a further processing system <NUM>. The processing system <NUM> is connected to the processing system <NUM> of the filament feeding system <NUM>. This connection may be embodied by a communication cable <NUM>. Alternatively, the connection may be a wireless connection. It is noted that the example of <FIG> shows a modular FFF printing system. It is noted that the two modules <NUM> and <NUM> could be integrated into one printing FFF printing system where there is no need for two separate processing systems, and a single processing system will suffice.

As can be seen from <FIG>, the decoupler <NUM> is coupled to the prefeeder <NUM> via a tube <NUM>, which may be a flexible tube. The slider <NUM> of the decoupler <NUM> is coupled to the printer feeder <NUM> via a flexible tube <NUM>. As can be seen from the figure, the flexible tube <NUM> is formed as an S-shaped tube. Furthermore, the printer feeder <NUM> is coupled to a print head (not visible in <FIG>) via a flexible tube <NUM>, also referred to as Bowden tube <NUM>.

<FIG> shows a perspective view of the FFF printing system <NUM> of <FIG> in which the front of the system is visible. In this example the FFF printing apparatus <NUM> comprises a print head <NUM>. The print head <NUM> comprises a nozzle (not visible in <FIG>) where molten filament can leave the print head <NUM>. The filament <NUM> is fed into the print head <NUM> by means of a printer feeder <NUM>. The FFF printing apparatus <NUM> also comprises a gantry arranged to move the print head <NUM> at least in an X-direction. In this embodiment, the print head <NUM> is also movable in a Y-direction perpendicular to the X-direction. The gantry comprises at least one mechanical driver (not shown) and one or more axles <NUM> and a print head docking unit <NUM>. The print head docking unit <NUM> holds the print head <NUM> and for that reason is also called the print head mount <NUM>. It is noted that the print head docking unit <NUM> may be arranged to hold more than one print head, such as for example two print heads each receiving its own filament. A build plate <NUM> may be arranged in or under the 3D printer <NUM> depending on the type of 3D printer. The build plate <NUM> may comprise a glass plate or any other object suitable as a substrate. In the example of <FIG>, the build plate <NUM> is movably arranged relative to the print head <NUM> in a Z-direction, see arrows in <FIG>. The FFF printing apparatus <NUM> also comprises a user interface <NUM> for showing information and for receiving instructions from the user.

The printer feeder <NUM> is arranged to feed and retract the filament <NUM> to and from the print head <NUM>. The printer feeder <NUM> is arranged to feed and retract filament at different speeds to be determined by the processing system <NUM>. A retraction may be needed in case a different type of filament is needed, another print head takes over the printing, the printing process is paused, or in case the filament spool is nearly empty (end-of-filament). As mentioned above, the feeding and retraction of filament by the printer feeder <NUM> needs to be aligned with the operation of the prefeeder <NUM>.

<FIG> schematically shows elements of a single nozzle FFF printing system comprising a filament feeding system <NUM>. <FIG> shows a schematic cross section wherein the filament feeding system <NUM> comprises a box shaped housing having a front door <NUM>. In this embodiment, the filament feeding system <NUM> further comprises a container <NUM> for storing filament spools. In this embodiment the container <NUM> is air conditioned. The container <NUM> is also referred to as the conditioned cabinet <NUM>. A dehumidifier <NUM> is arranged to extract air out of a non-conditioned cabinet <NUM>, dehumidify the air, and then send it into the conditioned cabinet <NUM>. The filament feeding system <NUM> also comprises a power supply <NUM>, a prefeeder module <NUM> and a processing unit <NUM>. The filament feeding system <NUM> also comprises a filament merger module <NUM> which is arranged to merge filament channels coming from multiple filament entries into a single merger exit path M07.

Now the course of the filament is described. S01 indicates the filament-spool separation where the filament leaves a spool <NUM>. S02 indicates a free filament arc. P01 indicates a prefeeder entry funnel. P02 indicates a first filament detector. P03 indicates a prefeeder drivetrain. P04 indicates a prefeeder path. P05 indicates a second filament detector. M01 indicates a first filament detector. M02 indicates a merger entry. M03a-M03d indicate merger pathways. M04-M06 indicate merger junctions. M07 indicates a merger exit path. M08 indicates a merger exit. D01 indicates a merger-decoupler Bowden tube. D02 indicates a decoupler entry. D03 indicates a decoupler slider. D04 indicates a decoupler gap. D05 indicates a decoupler exit. D06 indicates a decoupler-feeder connection. F01 indicates a printer feeder entry. F02 indicates a flow sensor. F03 indicates a printer feeder drivetrain. F04 indicates a printer feeder exit. H01 indicates a printhead Bowden tube. H02 indicates a print core entry. H03 indicates a cold end, and finally H04 indicates a hot end.

As can be seen from <FIG>, the slider D02 of the decoupler <NUM> is connected to the Bowden tube D01 which is beginning at merger exit M08. Because the tube D01 is a flexible tube, is can bend if needed. A bending of the tube will cause the movement of the slider in case too much filament builds up in this part of the tubing.

The decoupler <NUM> (i.e. the filament path length measurement device) provides information for the operation of the prefeeder <NUM> during filament loading, printing, and unloading of the filament. For example, during filament feeding, the prefeeder <NUM> will feed filament down the tubes, until the filament reaches the printer feeder entry F01. The filament may get blocked by a not yet activated printer feeder <NUM>. As a result, the filament tension in the Bowden tube D01 will increase and due to that, the slider D03 will slide out of the decoupler <NUM>. Movement of the slider D03 will be detected by the sensor (see also <FIG>) in the decoupler <NUM>, and the obtained measurement data may be used to activate, or adjust activation of, the print feeder <NUM>.

In case of feeding a dual nozzle printing apparatus, the filament feeding system <NUM> may comprise two filament merger modules <NUM>, two merger exit paths and two decouplers. A first decoupler will be coupled to a tubing leading to a first printer feeder, and a second decoupler will coupled to another tubing leading to a second printer feeder. In case, the printing apparatus comprises more than two nozzles, such as three, four or even more, a corresponding number of decouplers, filament merger modules and merger exit paths could be provided in the filament feeding system.

<FIG> shows a perspective view of the filament feeding system <NUM> according to a further embodiment. In this example, the filament feeding system <NUM> comprises a box <NUM> having a door <NUM>. The opening of the door <NUM> gives access to a number of bays arranged to store multiple filament spools <NUM>. In this example, the box <NUM> has six bays for storing six filament spools. This embodiment is suited for supplying two filaments to two print heads of a dual nozzle printing system. At each bay two filament entries <NUM>, <NUM> are arranged. These entries are connected via channels to a prefeeder arranged for each bay separately. So, in this embodiment, the filament feeding system <NUM> comprises six prefeeders. At each bay, the two filament entries <NUM>, <NUM> may be labelled, for example using numbers '<NUM>' and '<NUM>' or 'I' and 'II' so that the user can put the correct filament into the filament entry. Let's assume that a spool in bay <NUM> holds PLA, and that the user wants to print PLA via a first nozzle and another second material via a second nozzle. Then the user is prompted by the user interface to insert the PLA filament in the first filament entry labelled '<NUM>'.

In an embodiment, a decoupler <NUM> is arranged in a direct feeder printing system. <FIG> shows a direct drive print head assembly <NUM> having a direct feeder incorporated. The decoupler <NUM> is now arranged onto the direct drive print head assembly <NUM>, instead of remote from the print head like in the previous embodiments. A filament <NUM>, stored on a spool <NUM>, is led to the direct drive print head assembly <NUM> using a Bowden tube <NUM> and a prefeeder <NUM>. The spool <NUM> may be arranged in a spool holder <NUM>. It should be noted that instead of one spool holder there could be multiple spool holders next to each other, similar to the plurality of bays in the box <NUM> shown in <FIG>. The direct drive print head assembly <NUM> could comprise a single print head or multiple print heads. In case of a multiple print heads, multiple prefeeders and multiple Bowden tubes could be arranged.

<FIG> is a detailed view of the direct drive print head assembly <NUM> of <FIG>. As can be seen, the direct drive print head assembly <NUM> comprises a feeder <NUM> and a print head <NUM>. <FIG> also shows a decoupler <NUM> comprising a slider <NUM> slidably arranged in a housing <NUM> of the decoupler <NUM>. The decoupler <NUM> comprises a channel <NUM> for letting the filament <NUM> pass the decoupler <NUM>. The housing <NUM> of the decoupler <NUM> is mounted on the direct drive print head assembly <NUM>.

<FIG> is a detailed view of the prefeeder <NUM> of <FIG>. As can be seen, the prefeeder feeds the filament <NUM> into the Bowden tube <NUM>.

<FIG> schematically shows the processing system <NUM> of the filament feeding system according to an embodiment. The processing system <NUM> comprises a processing unit <NUM>, an I/O interface <NUM> and a memory <NUM>. The processing unit <NUM> is arranged to read and write data and computer instructions from the memory <NUM>. The processing unit <NUM> is also arranged to communicate with sensors, such as the measuring device <NUM>, and other equipment via the I/O interface <NUM>. The memory <NUM> may comprise a volatile memory such as RAM, or a non-volatile memory such as a ROM memory, or any other type of computer-readable storage. The processing system <NUM> may comprise several processing units. It is noted that the processing system <NUM> of the FFF printing apparatus may comprise the same elements as those described in <FIG>.

In an embodiment of the invention, the processing system <NUM> is arranged to control the prefeeder <NUM> using a state machine. In case the filament feeding system <NUM> comprises multiple prefeeders, the processing system <NUM> may be arranged to simulate state machine for each of the prefeeders. In an embodiment, the decoupler <NUM>,<NUM>,<NUM> generates several different signals depending on the stroke positions of the slider <NUM>, <NUM>, <NUM>.

It is noted that when printing, preferably the position of the slider <NUM> in the decoupler <NUM> is kept in the region where the two torsion springs (see also <FIG>) are not biased, so that the filament is offered tension-free to the FFF printing apparatus <NUM>. The FFF printing apparatus <NUM> should not 'feel' the filament feeding system <NUM> while printing, because pushing against or pulling on the filament directly results in over- or under-extrusion: the more or less passing through of filament than intended (after all, it is an open loop instruction, without feedback or correction).

Claim 1:
A fused filament fabrication, FFF, printing system (<NUM>), the FFF printing system comprising:
- a print head (<NUM>);
- a feeder (<NUM>;<NUM>) arranged to feed a filament (<NUM>) into the print head (<NUM>);
- a container (<NUM>) for storing the filament on one or more filament spools (<NUM>);
- a prefeeder (<NUM>) arranged to feed the filament from the spools to the feeder (<NUM>;<NUM>);
- a first flexible tube (D01;<NUM>;<NUM>) for guiding the filament (<NUM>);
- a filament path length measuring device (<NUM>) comprising:
- a housing (<NUM>) comprising a first opening (<NUM>) for passing through of the filament (<NUM>), and a second opening (<NUM>) opposite the first opening (<NUM>);
- a slider (<NUM>;<NUM>;<NUM>) slidably arranged in the housing, the slider comprising a filament channel (<NUM>) for passing through of the filament, wherein an outer end of the slider is connected to a first end of the flexible tube;
- at least one contactless sensor arranged to detect a change of position of the slider relative to the housing to obtain measurement data indicative of a path length change of the filament in the first flexible tube,
wherein a second end of the first flexible tube is connected to a connection point of the FFF printing system, and
wherein the FFF printing system further comprises a processing system (<NUM>) arranged to receive the measurement data and to control the prefeeder depending on the measurement data.