NL2026617B1 - Printing of continuous fibres in 3D printed objects - Google Patents

Abstract

A fused filament fabrication system (40) is described comprising a build surface (41), a first print head (43) arranged to receive a thermoplastic filament (45), to heat the thermoplastic filament, and to deposit molten filament onto the build surface. A second print head (44) is arranged to receive a continuous fiber (47) and deposit the continuous fibre. A controller (50) is arranged to control the first print head and the second print head, to feed the first print head with the thermoplastic filament while moving the first print head relative to the build surface, so as to print a first layer (62) wherein a predefined slot (52) is left open, and to feed the first print head with the thermoplastic filament while moving the first print head relative to the build surface, so as to print a second layer (63) wherein the second layer comprises overhangs (65) at both sides of the slot so as to partly cover the slot (52). The controller is further arranged to feed the second print head with the continuous fiber, while moving the second print head over the open slot so as deposit the continuous fiber into the slot, and to feed the first print head or a third print head with the, or a further, thermoplastic filament while moving the first print head, orthe third print head, over the open slot so as to fill the slot with the, orthe further, thermoplastic material.

Description

Printing of continuous fibres in 3D printed objects Field of the invention The present invention relates to a fused filament fabrication system and to a method of fused filament fabrication. The invention more particularly relates to manufacturing 3D objects with integrated continuous fibers using a fused filament fabrication system. Background art Fused filament fabrication (FFF) is a 3D printing process that uses a continuous filament of a thermoplastic material. Filament is fed from a filament supply through a moving, heated print head, and is deposited through a print nozzle onto an upper surface of a build plate. The print head, also referred to as extrusion head, may be moved relative to the build plate under computer control to define a printed shape. In certain FFF devices, 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. In this way a 3D printed object can be produced made out of a thermoplastic material.

Recently, FFF printers were developed that can print composite parts consisting of a continuous fiber and an extruded plastic filament. Publication US2019/0217525 A1 describes an FFF printer for printing with a composite filament. The printer comprises a controller that controls a print drive and filament drive to relatively move the print head and the print bed at a deposition velocity matching the feed velocity of the core reinforced filament, and to apply pressure with the nozzle to continuously compact the core reinforced filament as the core reinforced filament is translated through the nozzle and deposited upon the print bed. Due to the applied force the reinforced filament can be bent once it leaves the print head. The applied force is also used to deposit the filament into a previous layer of deposited material. In an embodiment, a special compression roller is used to impart a compressive force onto the filament during deposition. By combining a print head that deposits a thermoplastic material with a print head depositing a core reinforced filament, a printed object can be manufacture comprising a reinforcement structure. Furthermore, 3D objects can be produced that comprise specially shaped wiring. An example is the printing of an antenna within a plastic 3D object.

Although the prior art printers can manipulate the continuous fiber in the X-Y plane, making very sharp corners with the continuous fiber in the X-Y plane seems difficult. This is because the continuous fiber is deposited in a thermoplastic layer wherein the fiber is stiffer and stronger than its surrounding thermoplastic material. This may result in rounded curves at corners that supposed to be sharp curves. This disadvantage of the prior art reinforced filament printers considerably limits the number of applications.

Summary of the invention

The aim of the present invention is to provide an FFF system that is able to manufacture 3D printed objects with integrated fibers having a layout with sharp curves.

According to a first aspect of the present invention, there is provided a fused filament fabrication system comprising a build surface, a first print head arranged to receive a thermoplastic filament, to heat the thermoplastic filament, and to deposit molten filament onto the build surface or a previously deposited layer, and a second print head arranged to receive a continuous fiber and deposit the continuous fibre. Optionally the system comprises a third print head arranged to receive a further thermoplastic filament, to heat the further thermoplastic filament, and to deposit further molten filament.

The system also comprises a controller arranged to: - control movement of the first print head and of the second print head relative to the build surface; - feed the first print head with the thermoplastic filament while moving the first print head relative to the build surface, so as to print a first layer wherein a predefined slot is left open; - feed the first print head with the thermoplastic filament while moving the first print head relative to the build surface, so as to print a second layer wherein the second layer comprises overhangs at both sides of the slot so as to partly cover the slot; - feed the second print head with the continuous fibre, while moving the second print head over the open slot so as deposit the continuous fiber into the slot; - feed the first print head or the third print head with the, or the further, thermoplastic filament while moving the first print head, or the third print head, over the open slot so as to fill the slot with the, or the further, thermoplastic material.

By first creating a slot into the object, a fiber can be deposited exactly at the correct locations in a reliable way. The created overhangs prevent the fiber from escaping out of the slot, which may be a risk particularly near or in the curves. The invention enables more complex toolpaths for the continuous fiber in the X-Y plane as compared to known systems.

In an embodiment, the predefined slot in the first layer is defined by printing two parallel traces with a space in between, and wherein the overhangs in the second layer are formed by printing two parallel traces on top and parallel to the underlying parallel traces of the first layer but with less space in between. By using these steps, a wall is created for the slot which is only two layers high. In the rest of these two layers, the traces can have arbitrary directions depending on the requirements for the infill for those layers. This way a printing layers is quite similar to printing objects with internal voids, so common slicing programs can be used to prepare the slicing of the object without much modification needed.

In an embodiment, the slot and the continuous fiber follow a path with one of or more rectangular corners. Such rectangular corners in the path of the fiber are advantageous when 3D printing e.g. electrical circuits within a non-conductive material, such as the polymers used in FFF printing. Most electrical circuits are designed so as to save space. This resulted in a history of designing circuit layouts with straight lines and rectangular curves. The invention provides for 40 printing such circuitry without the risk of wires (i.e. fibers) bending back in the sharp corners.

In an embodiment, the continuous fiber is an electrically conductive strand. This makes the fabrication of electrical connections and electrical circuitry possible.

The fiber may be a metal wire coated with a phenolic resin. This kind of fiber is easier to feed as compared to metal wires because of the higher friction of the resin coating with the feeder wheels. Furthermore, the coating will stick to the printed material which makes it easier to deposit the fiber into the slot.

In an embodiment the system comprises a feeder for feeding the continuous fiber through the second print head, wherein the feeder comprises at least one feeder wheel having a rubber contact surface. This rubber surface is advantageous when depositing metal since it increases the friction between the feeder wheel and the fiber to be fed.

According to a further aspect, there is provided a method of fused filament fabrication, the method comprising: - print a first layer of thermoplastic material on a build surface or previously printed layer, wherein a predefined slot is left open in the first layer; - print a second layer of thermoplastic material, wherein the second layer comprises overhangs at both sides of the slot so as to partly cover the slot; - deposit a continuous fiber into the slot, and then - fill the slot with a thermoplastic material.

The predefined slot in the first layer may be defined by printing two parallel traces with a space in between, and wherein the overhangs in the second layer are formed by printing two parallel traces on top and parallel to the by parallel traces of the first layer by with less space in between.

In an embodiment, the predefined slot and the fiber follow a path with one or more rectangular corners. As mentioned above this makes several layouts possible for designing circuitry and electrical connection within electrical devices.

According to a further aspect, there is provided a computer program product comprising code embodied on computer-readable storage and configured so as when run on a controller of an FFF system, giving the FFF system the ability to perform a method as described above.

According to a further aspect, there is provided a 3D object manufactured by the method as described above.

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 part of an FFF system of the prior art; Figure 2 schematically shows a cross section of an example of a print in the X-Z plane printed by the prior art system of Figure 1; Figure 3 schematically shows a top view a 3D printed object with a continuous fibre 40 deposited into the object using the system of the prior art shown in Figure 1;

Figure 4 schematically shows a part of an FFF system according to an embodiment of the invention; Figure 5 schematically shows a cross section of an example of a print in the X-Z plane which may be printed by the FFF system of Figure 4; Figure 6 schematically shows a cross section in the X-Z plane of a slot in a curve which may be printed by the FFF system of Figure 4, showing that the overhangs will block the fiber in the Z-direction so that the fiber will not escape from the slot; Figure 7 shows a cross section of part of the printed object with the slot filled; Figure 8 schematically shows a top view a 3D printed object printed on the built plate with the continuous fibre deposited into the 3D object using the FFF system according to an embodiment, and Figure 9 shows a flow chart of the method 90 of fused filament fabrication 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 part of an FFF system 1 of the prior art. The FFF system 1 comprises a build plate 2 which can be moved up and down in a Z-direction. The system 1 also comprises a first print head 3 and a second print head 4 which are movably arranged in a X-Y plane. The system 1 may comprise a gantry {not shown) which is arranged to manipulate the print heads and optionally the build plate 2. The first print head 3 is arranged to receive a thermoplastic filament 5 which is fed by a feeder 6 into the print head 3. The first print head 3 is arranged to heat the filament 5 and deposit molten filament onto the build plate 2. The second print head 4 is arranged to receive a further thermoplastic filament 7 which is fed by a feeder 8 into the print head

4. The thermoplastic filament 7 comprises a continuous fibre. The continuous fibre can be a strand made out of carbon, metal, etcetera. The second print head 4 is arranged to heat the filament 7 and deposit molten filament together with the continuous fibre. In this example, the second print head 4 comprises a cutter 9 arranged to cut the thermoplastic filament 7 and its continuous fibre.

Figure 2 schematically shows a cross section of an example of a print in the X-Z plane printed by the prior art system 1 of Figure 1. Figure 2 shows eight traces 10 of a 3D printed object printed in two layers of deposited thermoplastic material. The eight trances 10 are deposited using the first print head 3. After having finished the second layer, the system 1 will use the second print head 4, see figure 1, to deposit a trace 12 of the thermoplastic filament with the continuous fibre. In this example, the second print head 2 will use sufficient force to iron the filament 7 into or onto 40 the previous layers. In this way, a fiber 14 is integrated in the 3D printed object. In the example of

Figure 2, the fibre 14, together with its surrounding thermoplastic 12, is deposited in the Y direction, but it may also be deposited in other directions depending on the design.

Figure 3 schematically shows a top view a 3D printed object 30 with a continuous fibre 14 deposited into the object 30 using the system 1 of the prior art shown in Figure 1. In this example 5 the 3D printed object 30 has a rectangular shape in the X-Y plane. A dashed line 13 indicates the proposed design of the continuous fibre 14. Figure 3 shows that in the corners the fibre 14 does not follow the dashed line 13. Since the fibre 14 has a certain stiffness and strength it is not possible with the prior art system 1 to lay down a fibre in the thermoplastic material having a layout with sharp curved lines. The second print head 4 is able to bend the filament with the continuous fibre 14 in the X-Z direction needed for deposition, but it is not equipped to bend it sufficiently in the X-Y plane. As a consequence, only rounded corners can be created in the fibre in the X-Y plane. This may be workable when using the fibres for reinforcing the 3D object, but is can become a problem when trying to integrate e.g. electric circuitry in a 3D printed object. Such circuitry requires accurately printed wiring so as to avoid failure and malfunctioning.

Figure 4 schematically shows a part of an FFF system 40 according to an embodiment of the invention. The FFF system 40 comprises a build plate 42 which can be moved up and down in a Z-direction. The system 40 also comprises a first print head 43 and a second print head 44 which are movably arranged in a X-Y plane. The system 40 may comprise a gantry (not shown) which is arranged to manipulate the print heads and optionally the build plate 42. The first print head 43 is arranged to receive a thermoplastic filament 45 which is fed by a feeder 46 into the print head 43. The first print head 43 is arranged to heat the filament 45 and deposit molten filament via a first nozzle 43’ onto a build surface 41 the build plate 42. The second print head 44 is arranged to receive a continuous fibre 47 which is fed by a feeder 48 into the print head 44. The continuous fibre 47 can be a strand made out of carbon, metal, etcetera. In an embodiment, the second print head 44 may comprise a heating element arranged to heat the fibre 47 and/or a coating material surrounding the continuous fibre 47. The second print head 44 may comprises a cutter 49 arranged to cut the continuous fibre 47. The fibre 47 leaves the second print head 44 at a nozzle 44’.

The fiber 47 may be a metal wire coated with a phenolic resin. This kind of fiber is easier to feed because of the higher friction with the feeder wheels. Furthermore, the coating will stick to the printed material which makes it easier to deposit the fiber into the slot.

In this example, Figure 4 also shows an optional third print head 83 arranged to receive a further thermoplastic filament 85, to heat the further thermoplastic filament 85, and to deposit further molten filament. An optional feeder 86 is arranged to feed the further filament 85 into the print head 83.

The FFF system 40 also comprises a controller 50. The controller 50 may be arranged within a housing of the FFF system 40 or it may be, at least partly, arranged remote from the other components of the FFF system 40. The controller 50 is arranged to control the movement of the first print head 43 and the movement of the second print head 44 relative to the build surface 41.

40 The controller 50 is also arranged to control the feeders 48, 48, which controlling is indicated by the dashed lines in Figure 4. The FFF system 40 may comprise a gantry (not shown in Figure 4) which is arranged to move the first print head 43 and second print head 44 relative to the build surface 41 of the build plate 42 in an X, Y and Z direction. The gantry may be a guide rail system that is desirably configured to move the print heads 43, 44 in a horizontal X-Y plane within the build chamber based on signals provided by the controller 50. The horizontal X-Y plane is a plane defined by an X-axis and a Y-axis, where the X-axis, the Y-axis, and the Z-axis are orthogonal to each other. In an alternative embodiment, build plate 2 may be configured to move along two axes (e.g., X-Z plane or the Y-Z plane), and the loaded print head 43, 44 may be configured to move along a single horizontal axis (e.g., the X-axis or the Y-axis). Other similar arrangements may also be used such that one or both of the build plate 42 and the loaded print head 43,44 are moveable relative to each other. Also, the build plate 42 may be replaced by a conveyer belt which comprises the build surface 41.

Figure 5 schematically shows a cross section of an example of a print in the X-Z plane which may be printed by the FFF system 40 of Figure 4. Figure 5 shows thirteen traces 60 of a 3D printed object 51 printed in three layers of deposited thermoplastic material, see layers 61, 62 and

63. The traces 60 are deposited using the first print head 43. As can be seen from Figure 5, the 3D printed object comprises a slot 52 in this example extending in the Y direction perpendicular to the plane of view. In this example the slot 52 is created by leaving a void in the middle layer 62 and a void in the top layer 63. The traces in the top layer 63 at the edges of the slot 52 are closer to each other than the traces in the middle layer 62 lying at the edges of the slot 52. As a result, overhangs 65 are created at both sides of the slot 52 and partly cover/close the slot 52.

After having finished the top layer 63, the FFF system 40 will activate the second print head 44, see Figure 4, and deposit a continuous fibre 47 in the created slot 52. Due to gravitational force, the deposited fiber 47 will normally stay at the bottom of the slot 52, as shown in Figure 5. In case the slot 52 comprises a sharp curve, the fiber 47 laid down in the slot 52 may, as a result of some tension, try to escape from the slot 52. But in the curve, the fiber 47 will get pushed to the outer walls of the slot 52 and will partly be positioned underneath the overhangs 65. The overhangs 65 will then block the fiber 47 in the Z-direction, as shown in Figure 6 so that the fiber 47 cannot escape from the slot 52.

If applicable, the fiber 47 is cut by way of operating the cutter 49. Operation of the cutter 49 may be done manually, mechanically or electrically, e.g. by the controller 50. Once the fiber 47 is laid down in the slot 52 at the requested locations, the slot 52 is filled with a thermoplastic material 70 using the first print head 43 or a further print head, see print head 83, arranged to deposit a material into the slot 52, see Figure 7 which shows a cross section of part of the printed object 51. In an embodiment, the slot 52 is filled using a curable material that is deposited using a special print head providing a fluid resin or other type of curable material.

In the example of Figures 5-7 the fiber 47 has a cross section which is less than the height of the middle layer 62. The cross section of the fiber 47 may dependent on the application. The fiber needs to be smaller than the opening created by the overhangs 65. Typical values for the diameter of the fiber 47 lie between 0.2 — 2 mm, other values are possible, such as diameters larger than 2 mm.

Figure 8 schematically shows a top view a 3D printed object 51 printed on the built plate surface 41 with the continuous fibre 47 deposited into the 3D object 51 using the FFF system 40 as described above. In this example the 3D printed object 51 has a rectangular shape in the X-Y plane. In this example, the fibre 47 also has a rectangular shape following the circumference of the printed object 51. As can be seen, the fiber 47 is laid down as a rectangular with sharp curves, which would not have been possible when using the prior art methods and systems.

In the example of Figure 5-7, the slot 52 is created with straight sections in the X directions and straight sections in the Y direction, but the sections may also be created in other directions depending on the design. Actually, the slot 52 can have arbitrary orientations and shapes as long as it is in the X-Y plane. It is noted that the invention is not limited to layouts having right angles. As will be appreciated by the skilled person, the described method and system can be used to make angles that are sharper than 90 degrees, such as angles between 30-60 degrees and maybe even sharper. Also angles between 90 and 180 degrees are possible.

Figure 9 shows a flow chart of the method 90 of fused filament fabrication according to an embodiment of the invention. The method comprises a step 91 of printing a first layer of thermoplastic material on a build surface or previously printed layer, wherein a predefined slot is left open in the first layer. In a next step 92, a second layer of thermoplastic material is printed, wherein the second layer comprises overhangs at both sides of the slot so as to partly cover the slot. Next, see step 93, a continuous fiber is deposited into the slot, and then the slot is filled with thermoplastic material, see step 94.

The invention provides for the ability to selectively print electrically conductive fibers within a structure. This enables the construction of desired electrical components in the structure. For example, electrically conductive and optically conductive continuous cores may be used to construct strain gauges, optical sensors, traces, antennas, wiring, and other appropriate components. Fluid conducting cores (e.g. thin tubes) might also be used for forming components such as fluid channels and heat exchangers. The ability to form functional components on, orin, a structure offers multiple benefits. For example, the described three-dimensional printing process and FFF system 40 may be used to manufacture printed circuit boards integrally formed in a structure; integrally formed wiring and sensors in a car chassis or plane fuselage; as well as motor cores with integrally formed windings. Examples of the materials deposited by the first print head 43 could be any of the non-limiting list: Polylactic acid (PLA), Tough PLA (TPLA), Thermoplastic Elastomer (TPE), Polyethylene terephthalate glycol (PETG), Polyamide (PA), Poly Carbonate (PC), Acrylonitrile butadiene styrene (ABS), PC-ABS, Poly propylene (PP), Polyvinylidene fluoride (PVDF), Polyphenylene sulphide (PPS), Polyetherketoneketone (PEKK), Polyethene (PE), Polyoxymethylene (POM).

Examples of the electrically conductive materials used for the fiber 47 are copper, gold and iron. It is noted that alternatively, the fiber 47 placed in the slots, is not electrically conductive, 40 such as carbon fibers, used to e.g. reinforce the 3D printed object.

In view of the above, the present invention can now be summarized by the following embodiments: Embodiment 1. A fused filament fabrication system (40) comprising: a build surface (41); a first print head (43) arranged to receive a thermoplastic filament (45), to heat the thermoplastic filament, and to deposit molten filament onto the build surface or a previously deposited layer; a second print head (44) arranged to receive a continuous fiber (47) and deposit the continuous fibre; optionally a third print head arranged to receive a further thermoplastic filament, to heat the further thermoplastic filament, and to deposit further molten filament; a controller (50) arranged to: - control movement of the first print head and of the second print head relative to the build surface; - feed the first print head with the thermoplastic filament while moving the first print head relative to the build surface, so as to print a first layer (62) wherein a predefined slot (52) is left open; - feed the first print head with the thermoplastic filament while moving the first print head relative to the build surface, so as to print a second layer (63) wherein the second layer comprises overhangs (65) at both sides of the slot so as to partly cover the slot (52); - feed the second print head with the continuous fiber, while moving the second print head over the open slot so as deposit the continuous fiber into the slot; - feed the first print head or the third print head with the, or the further, thermoplastic filament while moving the first print head, or the third print head, over the open slot so as to fill the slot with the, or the further, thermoplastic material.

Embodiment 2. Fused filament fabrication system according to embodiment 1, wherein the predefined slot in the first layer is defined by printing two parallel traces with a space in between, and wherein the overhangs in the second layer are formed by printing two parallel traces on top and parallel to the underlying parallel traces of the first layer but with less space in between.

Embodiment 3. Fused filament fabrication system according to any one of the preceding embodiments, wherein the slot and the continuous fiber follow a path with one of or more rectangular corners.

Embodiment 4. Fused filament fabrication system according to any one of the preceding 40 embodiments, wherein the continuous fiber is an electrically conductive strand.

Embodiment 5. Fused filament fabrication system according to any one of the preceding embodiments, wherein the fiber is a metal wire coated with a phenolic resin.

Embodiment 8. Fused filament fabrication system according to any one of the preceding embodiments, wherein the system comprises a feeder for feeding the continuous fiber through the second print head, wherein the feeder comprises at least one feeder wheel having a rubber contact surface.

Embodiment 7. A method of fused filament fabrication, the method comprising: - print a first layer of thermoplastic material on a build surface or previously printed layer, wherein a predefined slot is left open in the first layer; - print a second layer of thermoplastic material, wherein the second layer comprises overhangs at both sides of the slot so as to partly cover the slot; - deposit a continuous fiber into the slot, and then - fill the slot with a thermoplastic material. Embodiment 8. The method of fused filament fabrication according to embodiment 7, wherein the predefined slot in the first layer is defined by printing two parallel traces with a space in between, and wherein the overhangs in the second layer are formed by printing two parallel traces on top and parallel to the by parallel traces of the first layer by with less space in between. Embodiment 9. The method of fused filament fabrication according to embodiment 7 or 8, wherein the predefined slot and the fiber follow a path with one or more rectangular corners.

Embodiment 10. A computer program product comprising code embodied on computer-readable storage and configured so as when run on a controller of an FFF system, giving the FFF system the ability to perform a method according to any one the embodiments 7-9.

Embodiment 11. A 3D object manufactured by the method according to any one of the embodiments 7-9.

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 40 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.

Claims (11)

CONCLUSIONS A filament fuse fabrication system (40), comprising: - a build surface (41); - a first print head (43) adapted to receive a thermoplastic filament (45), to heat the thermoplastic filament and to deposit molten filament on the build surface or a previously deposited layer; - a second print head (44) adapted to receive a continuous fiber (47) and deposit the continuous fiber; - optionally a third print head adapted to receive a further thermoplastic filament, to heat the further thermoplastic filament and to deposit further molten filament; - a controller (50) arranged for: - controlling the movement of the first printhead and of the second printhead relative to the build surface; - feeding the first printhead with the thermoplastic filament while moving the first printhead relative to the build surface to print a first layer (62) in which a predefined slot (52) is left open; - feeding the first printhead with the thermoplastic filament as the first printhead is moved relative to the build surface to print a second layer (63) wherein the second layer includes overhangs (85) on both sides of the slot to partially covering the slot (52); - feeding the second printhead with the continuous fiber while moving the second printhead over the open slot to deposit the continuous fiber in the slot; - feeding the first printhead or the third printhead with the or the further thermoplastic filament while the first printhead or the third printhead is moved over the open slot to fill the slot with the or the further thermoplastic material. The filament fusion fabrication system of claim 1, wherein the predefined slot in the first layer is defined by printing two parallel tracks spaced apart, and wherein the overhangs in the second layer are formed by two parallel tracks on and parallel to , the underlying parallel tracks of the first layer, but with a smaller gap. A filament fuse fabrication system according to any one of the preceding claims, wherein the slit and the continuous fiber follow a path having one or more right angles. A filament fuse fabrication system according to any preceding claim, wherein the continuous fiber is an electrically conductive wire. 40 A filament fuse fabrication system according to any one of the preceding claims, wherein the fiber is a metal wire covered with a phenolic resin. A filament fusion fabrication system according to any preceding claim, wherein the system comprises a feeder for feeding the continuous fiber through the second printhead, the feeder comprising at least one feeder wheel having a rubber contact surface. A method of filament fusion fabrication, the method comprising: - printing a first layer of thermoplastic material on a build surface or a previously printed layer, leaving a predetermined slit open in the first layer; - printing a second layer of thermoplastic material, the second layer comprising overhangs on either side of the trench to partially cover the trench; - deposit a continuous fiber in the trench, and then - fill the trench with a thermoplastic material. The method of filament fusion fabrication according to claim 7, wherein the predefined slit in the first layer is defined by printing two parallel tracks with a spacing, and wherein the overhangs in the second layer are formed by two parallel tracks on, and parallel to, the parallel tracks of the first layer but with a smaller gap. The method of filament fusion fabrication according to claim 7 or 8, wherein the predetermined slit and the fiber follow a path having one or more right angles. A computer program product comprising code embodied in computer readable storage and configured to run on a controller of an FFF system, thereby enabling the FFF system to perform a method according to any one of claims 7-9. A 3D object manufactured according to the method according to any one of claims 7-9.