Field of the invention

The present invention relates to a fused filament fabrication method for manufacturing a 3D object. The invention also relates to a method of generating instructions for printing three- dimensional (3D) objects with an FFF printer. The invention also relates to a computing device comprising one or more processing units arranged to perform the method of generating instructions and to a computer program product.

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 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.

Currently there are two main methods of 3D printing with crystallizable materials: printing mostly amorphous and printing mostly crystalline. Both methods can be followed up with an annealing step to increase the degree of crystallinity. Typically, the state in which these materials are printed in is decided by the build volume temperature (i.e. the temperature within the build chamber of the FFF printer). If this temperature is below the glass transition temperature, the material stays amorphous. If it is above the crystallization temperature, the resulting part will end up crystalline. The degree of crystallinity can be estimated by different analytical methods, and it typically ranges between 10 and 80%, with crystallized polymers often called "semi-crystalline".

Most of the current print strategies within FFF are focused on uniformity i.e. keeping the printed part properties the same over a print. Semi-crystalline materials are printed with the idea that the part is either fully amorphous or ‘fully’ crystalline, to have uniform properties and to improve printability. Some strategies revolve around annealing amorphous parts to make them crystalline. But these annealing strategies have a big disadvantage. It is not possible to use the material properties of the crystalline and the amorphous phase at the same time. If different properties are required locally, typically this is solved by using a second material or by changing the toolpath (lower infill, higher surface roughness, etcetera).

Publication US20170182560 A1 describes a printer that fabricates an object from a computerized model using a fused filament fabrication process and a bulk metallic glass build material. By heating the bulk metallic glass at an elevated temperature in between an object and adjacent support structures, an interface layer can be interposed between the object and support where the bulk metallic glass becomes crystallized to create a more brittle interface that facilitates removal of the support structure from the object after fabrication. It is noted that when using metallic glass as a build material for both the object and the support, and creating a crystallized interface layer, the object can easily be removed with the application of mechanical force. However, this separation will result in a rough surface at the separated support and the object. This will require additional post-processing of the printed object to smoothen the surface of the printed object.

In an alternative embodiment mentioned in US20170182560 A1 , the support structure is fabricated from a high-temperature polymer or other material that will form a weak bond to the metal glass build material. Although this alternative may result in a clean surface of the printed object, it requires a dual nozzle system as well as two different materials which will slow down the whole manufacturing process considerably.

Summary of the invention

The aim of the present invention is to provide an FFF printing process for manufacturing a 3D object supported during printing by a support structure, using a single semi-crystalline polymer, wherein the 3D object can easily be removed from the support structure without the need for post-processing.

According to a first aspect of the present invention, there is provided an FFF method for manufacturing a 3D object, the method comprising:

- printing the 3D object using a build material comprising a semi-crystalline polymer, the 3D object comprising at least one overhang;

- printing at least one support structure underneath the at least one overhang wherein the support structure is printed using the build material, wherein the support structure comprises an interface top layer contacting the 3D object, wherein the interface top layer has a degree of crystallinity greater higher than the degree of crystallinity of at least those parts of the 3D object that get into direct contact with the top layer, wherein the higher degree of crystallinity of the top layer is achieved by keeping traces of deposited material for the top layer within a crystallization temperature range of the build material for a longer crystallization time as compared to traces forthose parts of the 3D object that get into direct contact with the top layer.

The top layer does adhere to the underlying layer of the support, because the top layer is still amorphous before it cools down and becomes more crystalline. By printing the top layer relatively hot, slow and/or with little cooling, this layer is given time to become more crystalline. It is noted that the appropriate crystallization time will depend on the type of build material. The top layer crystallizes after it is already bonded to the layer below, i.e. to the underlying layer of the support structure.

When depositing a build layer on top of this top layer, a different printing technique is used wherein the build layer is printed using the same material but with lower temperature and/or higher print speed, so that this layer becomes (more) amorphous. While printing this build layer, there will be not enough energy and/or time to let the (crystallized) top layer melt again, so the build layer on top does not stick to the top layer properly. This will facilitate the removal of the support structure from the 3D object after printing without the need for any post-processing.

In an embodiment, a part of the support structure not being the top layer has a degree of crystallinity that is less than the degree of the crystallinity of the top layer. The degree of crystallinity of the part of the support structure not being the top layer may be in a range of 0%- 15%. Such degree’s result is a relatively amorphous support structure which is relatively flexible but still strong enough to support the overhang(s).

In an embodiment, the degree of crystallinity of the top layer lies above 25%. Such a relative high degree of crystallinity of the top layer significantly reduces the adhesion between the top layer on the 3D object above, enabling easy removal of the support structures.

The degree of crystallinity of the top layer may be achieved by using different techniques which may be combined as well. These techniques may comprise the controlling a fan of an FFF system during deposition, the controlling a print speed of a printhead of the FFF system, the controlling a nozzle temperature, the controlling a geometry of a trace, and suitable planning of the traces. All these techniques influence the amount of energy dissipation from the deposited traces and thus will influence the crystallization process. The specific settings for these techniques will need to be tuned by way of testing the different techniques separately or in combination.

In an embodiment, the degree of crystallinity of the top layer is increased by means of ironing the top layer using an ironing means such as a nozzle. This technique can be used after a complete layer has been deposited or part of the layer. If a nozzle comprises a flat under surface, this surface can be used to iron the top layer. Alternatively a separate ironing tool could be used.

In an embodiment, the build material comprises PEEK, PEKK, PPS, PA, PLA and PET. These materials proved to give good results.

In an embodiment, the complete 3D object after printing has a degree of crystallinity below 10%. Such a low degree of crystallinity results in optimal bonding of the layers of the 3D object.

In an embodiment, the support structure comprises one or more upstanding meandering thin walls, and a platform on top of the meandering walls, the platform comprising at least one support layer arranged under the top layer, the at least one support layer having a degree of crystallinity less than that of the top layer.

According to a further aspect, there is provided a method of generating instructions for printing three-dimensional (3D) objects with an FFF printer, the method comprising:

- receiving a 3D model of a 3D object;

- identifying overhangs in the 3D model;

- generating models of support structures that support the overhangs during a print process;

- generating instructions for the FFF printer to print the 3D object and the support structures using build material comprising a semi-crystalline polymer, wherein a top layer of the support structures touching the 3D object is deposited using print settings that cause a higher degree of crystallinity of the top layer as compared to those parts of the 3D object that get into direct contact with the top layer, wherein the higher degree of crystallinity of the top layer is achieved by keeping traces of deposited material for the top layer within a crystallization temperature range of the build material for a longer crystallization time as compared to traces for those parts of the 3D object that get into direct contact with the top layer.

According to a further aspect, there is provided a computing device comprising one or more processing units, the one or more processing units being arranged to perform the method of generating instructions as described above.

According to yet a further aspect, there is provided a computer program product comprising code embodied on computer-readable storage and configured so as when run on one or more processing units to perform the method of generating instructions 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 cross-section of a 3D object generated by using an FFF method according to an embodiment;

Figure 2 schematically shows a cross-section of the support structure according to an embodiment;

Figure 3 schematically shows a cross-section of the support walls in a plane parallel to the build plate;

Figure 4 schematically shows part of a cross-section of a spherical 3D object which is partly supported by a support structure;

Figure 5 is a graph of the crystallization half-time for PEEK with different molecular weights;

Figure 6 schematically shows part of a print head that can be used to deposit material onto a build plate;

Figure 7 is a flow chart of an FFF method according to an embodiment;

Figure 8 shows a flow chart of a method of generating instructions for printing three- dimensional (3D) objects with an FFF printer, according to an embodiment of the invention, and

Figure 9 schematically shows a computing device according to an embodiment.

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 3D object generated by using an FFF method according to an embodiment. The 3D object in this example comprises a plateau 10 and two legs 11 , 12. Both the plateau 10 and the legs 11 , 12 extend in a direction perpendicular to the drawing plane. The 3D object is printed on a print plate 6 of a FFF printing system using a layer- by-layer deposition method. The part of the plateau 10 between the two legs 11 ,12 constitutes an overhang which needs support during printing to avoid printing errors and/or unwanted surface roughness at the bottom of the plateau as will be appreciated by the skilled person. Today’s slicing programs are able to calculate the place and the size of the support needed for supporting such overhangs. In the example of Figure 1 , it is assumed that only the middle part of the overhang needs support. A support structure 14 is created underneath the overhang.

In this embodiment, the degree of crystallinity of a top layer 15 of the support structure is increased during, or after, printing that layer. The rest of the support structure 14 as well as the 3D object is printed with a lower degree of crystallinity, and may even be amorphous. The top layer 15 of the support structure is also referred to as the interface layer 15 since it forms an interface between the 3D object and the more amorphous part of the support structure 14 below.

The advantage of creating a more crystallized top layer 15 is that it adheres very well to the more amorphous support structure 14, but it adheres poorly to the more amorphous object printed on top. This makes it easy to break off the entire support from the 3D object and leave a nice smooth surface of the print.

It is noted that certain parts of the 3D object may have a higher degree of crystallinity as well. For example the plateau 10 may comprise an elongated semi-crystalline part to make the plateau 10 more strong. Also in such an embodiment, the rest of the 3D object is more or less amorphous, at least at those parts that get into contact with the top layer 15.

Figure 2 schematically shows a cross section of the support structure 14 according to an embodiment. The support structure comprises a support roof 14A and support walls 14B. The top layer 15 of the support structure has a high degree of crystallinity while the rest of the structure is more amorphous (i.e. having a lower degree of crystallinity). In an embodiment, the degree of crystallinity of the part of the support structure not being the interface layer lies in a range of 0%- 15%.

Figure 3 schematically shows a cross section of the support walls 14B in a plane parallel to the build plate 6 (see also Figure 1). The walls 14B follow a meandering pattern. Such a pattern creates sufficient support while using relatively little print material. The support roof 14A preferably supports the whole top layer 15. In an embodiment, the support roof 14A comprises a number of layers of printing material, extending over the same area as the crystalline top layer 15.

It is noted that the invention is not limited to the support of horizontal overhangs. The invention can be used in all situations where support is needed. Figure 4 schematically shows part of a cross section of a spherical 3D object 20 which is partly supported by a support structure 30. A typical layer of the spherical 3D object 20 comprises a number of wall traces 22, 23 and infill traces 24. The wall traces 22, 23 may be circular traces forming the outer wall of the 3D object 20. The infill may be optional. The infill may have different structures as will be appreciated by the skilled person. The infill may be a 100% infill, or it may be a partly open infill, such as a 20% infill. The support structure 30 may comprise a number of amorphous traces 31 wherein traces contacting the 3D object 20 are crystallized forming the interface layer, see dashed traces 32 in Figure 4. The crystallized traces 32 may be circular traces similar to the wall traces 22, 23 of the 3D object. It is noted that the track of the traces 32 will depend on the local needs for support and the slicing strategies used.

The crystalline top layer may be created by depositing build material and keeping a trace of deposited material near the optimal crystallization temperature of the build material for an appropriate crystallization time. The appropriate time may depend on the materials used, the nozzle temperature, the build volume temperature, the air flow rate and the print speed.

Figure 5 is a graph of the crystallization half-time for PEEK with different molecular weights. The graph was published in “Isothermal crystallization of poly (ether ether ketone) with different molecular weights over a wide temperature range”, Seo, J, Gohn, AM, Dubin, O, et al. in Polymer Crystallization 2.1 (2019):e10055. Figure 5 shows a curve 52 for PEEK 650G, a curve 53 for PEEK 450G and a curve 54 for PEEK 150G.

From the graph of Figure 5 it can be seen that crystallization rates are the highest at an optimal crystallization temperature range. Optimal crystallization rates for these types of PEEK lie in a temperature range between 200 - 250 °C. It is noted that crystallization is also possible outside this range, but that more time is needed at those temperatures. It is clear from Figure 5 that temperatures below 175 °C and above 325 °C will not be appropriate.

The temperature range in which crystallization of a material takes place, is also referred to as the crystallization temperature window. According to an embodiment of the invention, a print process is provided in which the time the deposited material spends in the crystallization temperature window is controlled by means of a nozzle temperature, a print speed and/or local reheating, in order to control the crystallization degree of individual traces of the interface layer.

The proposed method can be performed using a known print head, but using different printing settings and/or instructions as compared to the state of the art.

Figure 6 schematically shows part of a print head 60 that can be used to deposit material onto a build plate 6. The print head 60 comprises a liquefier having an inlet 61 for letting in a filament 5, a cold end 62 and a hot-end 63. Between the hot-end 63 and the cold-end 62, a heat break 64 is present. The hot-end 63 comprises a heating element 65 arranged to heat the hot-end 63 so as to melt the filament 5, see molten filament 6 in the hot-end 63. The print head 60 further comprises a nozzle 66 and two fans 67 arranged to blow air onto the deposited material, i.e. onto the deposited traces in order to cool the traces. In this figure a simplified print is shown comprising a first layer 41 , a second layer 42 and a third layer 43. The nozzle 66 of the print head 2 comprises an orifice and an ironing surface around the orifice. The ironing surface can be used to iron the traces during deposition or iron the traces later on in order to reheat the traces so that they are crystallized. The deposited traces are kept in a crystallization temperature range (i.e. window) for a sufficient long time in order to enable crystallization of those traces. Cooling down the traces quickly (and keeping them cool) will result in material having a lower degree of crystallinity.

The wanted crystallization of the deposited traces can for example be achieved by controlling the speed of the fans 67. By introducing increased airflow to a deposited trace, the trace cools faster and will spend less time in crystallization temperature window, resulting in a relatively low degree of crystallinity. Turning the fan off will have the opposite effect, resulting in a relatively high degree of crystallinity.

Alternatively or additionally, the print head 64 can be moved relative to the build plate 6 with adapted speed in order to create crystalline traces. By moving the printhead 60 faster, less heat is conducted from the nozzle into the traces. So a deposited trace cools down faster and surrounded traces are heated up less. This reduces time spend near the optimal crystallization temperature so that the material is less crystalline or even amorphous. Decreasing the print head speed has the opposite effect, creating a more crystalline material.

Alternatively or additionally, the nozzle temperature can be tuned so that the interface traces become crystalline. Increasing the nozzle temperature is the easiest way to pump more heat into the current trace and its surrounding traces. This will keep the trace near the optimal crystallization temperature for a longer time, while reheating the surrounding traces back towards the optimal crystallization temperature. This way, higher nozzle temperatures increase the local crystallization, lower nozzle temperatures have the opposite effect.

Alternatively or additionally, the geometry of a traces can be controlled. For example, thicker traces will cool down slower and thus will have a higher degree of crystallinity under the same environmental circumstances. Another option is to appropriately plan tracks (i.e. paths) of the traces. New traces may heat up neighbouring traces which will result in increased crystallinity of those neighbouring traces.

In an embodiment, the degree of crystallinity of the top layer is increased by means of ironing the top layer using an ironing means such as the nozzle 66. Ironing specific parts of the print, reheats the traces. By inputting the right amount of thermal energy (by balancing nozzle/tool temperature and speed), the degree of crystallization can be increased locally. It is noted that the nozzle geometry effects the thermal energy input into the printed layers: a larger contact surface and more radiation will both increase the thermal input. This has effect on how print speed, nozzle temperature and ironing introduce crystallinity.

The invention provides for a wider application range with a single material. It provides for a higher print quality, a higher production rate, lower costs, especially since separate support material is no longer required.

In an embodiment, the 3D object and the support are printed using polymers like PEEK (Polyetheretherketone), PEKK (Polyetherketoneketone), PPS (Polyphenylene sulfide), PA (Polyamide), PLA (Polylactic acid) and PET (Polyethylene terephthalate), taking full advantage of both the amorphous and crystalline properties of these polymers. Figure 7 is a flow chart of an FFF method 70 according to an embodiment. The FFF method for manufacturing a 3D object comprises the printing the 3D object using a build material, the 3D object comprising at least one overhang, see step 71 . The method also comprises the printing of at least one support structure underneath the at least one overhang wherein the support structure is printed using the build material, see step 72, wherein the support structure is amorphous except for a crystalline top layer contacting the 3D object, the 3D object being amorphous at least at a parts/layers that get into contact with the crystalline top layer. It is noted that the support and the 3D object may be created simultaneously in a layer-by-layer building process. So sometimes after steps 71 , step 72 may follow, and visa versa.

Figure 8 shows a flow chart of a method 130 of generating instructions for printing three- dimensional (3D) objects with an FFF printer, according to an embodiment of the invention. The method comprises receiving 131 a 3D model of a 3D object, and identifying 132 overhangs in the 3D model. The method 130 further comprises generating 133 models of support structures that support the overhangs during a print process. The method 130 also comprises generating 134 instructions for the FFF printer to print the 3D object and the support structures in build material, wherein a top layer of the support structures touching the 3D object is deposited using print settings that cause the interface layer to crystallize.

Figure 9 schematically shows a computing device 100 according to an embodiment. The device 100 comprises a processing unit 111 , an I/O interface 112 and a memory 113. The processing unit 111 is arranged to read and write data and computer instructions from the memory 113. The processing unit 111 may also be arranged to communicate with sensors and other equipment via the I/O interface 112. The computing device 100 may also comprise an interface 114 arranged to communicate with other devices via a LAN or WAN (not shown). Figure 12 also shows a display 115 which may be connected to the interface 112 so as to show information regarding a slicing process of a 3D object. The memory 113 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 memory 113 may comprise a computer program product comprising code configured to make the processing unit 111 perform one or more of the embodiments of the method as described above.

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. For example, instead of using FFF printers, other types of FDM printers may be used such as pellet printers. 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.