Field of the invention

The present invention relates to a method of printing a three-dimensional (3D) object with an additive manufacturing device, such as an FFF printer. The invention also relates to a method of generating instructions for printing a three-dimensional object with an additive manufacturing device. The invention also relates to a computing device arranged to perform the method of generating instructions and to a computer program product.

Background art

This invention relates to additive manufacturing and more specifically to fused deposition modelling (FDM). FDM is a material extrusion method of additive manufacturing where materials are extruded through a nozzle and joined together to create 3D objects. A specific example of FDM is fused filament fabrication (FFF). 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.

Before a print can be started on an FFF device, a calibration is needed to determine the distance of the build plate relative to the nozzle tip. Preferably, this distance is determined in several locations so that a height map of the surface of the build plate can be made. The distance between build plate surface and nozzle tip can be measured using different sensors. These sensors may be optical or mechanical sensors, such as a mechanical probe. This probe can be the nozzle tip itself. In all these mentioned methods, certain inaccuracies in the measurements are unavoidable.

Due to these inaccuracies in the bed level measurements, the first printed layer can either be deposited with too much space or too little space leading to under extrusion or over extrusion, respectively. Too much under extrusion can cause printed traces to stick to the nozzle rather than staying on their place on the build plate. Moreover, it could lead to adhesion issues. On the other hand, over extrusion can cause material to bulge out over the top of the first layer, which can cause subsequent layers to be too wide. Moreover, over extrusion leads to a buildup of pressure in the system, which can only be released in consecutive layers. This can cause the first couple of layers to be wider than the CAD model, which is commonly known as an elephant’s foot. Summary of the invention

The aim of the present invention is to provide a method of printing that overcomes the problems arising from the inaccurate build plate level measurements.

According to a first aspect of the present invention, there is provided a method of printing a three-dimensional (3D) object with an additive manufacturing device. The method comprises determining a nominal flow rate for a first layer of the 3D object using a trace width, a layer height, and a print speed. The method also comprises the printing one or more wall traces on a build surface of the device using a flow rate that is higher than the nominal flow rate so as to cause over extrusion, and printing a number of skin traces on the build surface of the device using a flow rate that is lower than the nominal flow rate so as to cause under extrusion. The wall traces are printed before the printing of the skin traces starts. It is noted that the wall traces in the first layer, together with the skin traces, will form the bottom of the resulting 3D object.

Due to the over extrusion during the deposition of the wall traces, adhesion of these traces is insured even in case of any error in bed level measurement. Over extrusion is preferred if a distance between the nozzle tip and the build surface is larger than expected. Although over extrusion will help the first layer to stick onto the build surface, a pressure in the print head will build up. This may cause for some problems as for example in accurate printing of the subsequent layers. Therefore, before starting to print a next layer, the pressure in the print head is released by way of later on printing the skin traces with under extrusion.

In an embodiment, the over extrusion and/or under extrusion is achieved by using a flow rate correction factor to correct the flow rate. The flow rate correction factor is also referred to as the extrusion correction factor. The flow rate correction factor is used to correct (i.e. adjust) the nominal flow rate. The nominal flow rate is defined as the nominal amount (volume) of printed material per unit time. This nominal amount per unit time is determined by the trace width, the layer height, and the print speed.

Optionally, the printing of the one or more wall traces is performed using a flow rate correction factor of at least 110%. Such a values for the correction factor will result in a secure adhering to the build surface. In an embodiment, the correction factor lies in a range of 110%- 140%.

Optionally, the printing of the number of skin traces is performed using a flow rate correction factor equal or lower than 90%. In an embodiment the correction factor lies in a range of 70%-90%. Such values will result in voids between the traces, and thus release of the over pressure while at the same time securing that the skin is still sticking to the build surface.

Optionally, the wall traces are printed starting at an outer wall trace and then printing inward. This enables an exact positioning of the outer wall of 3D object since no neighbouring traces will be present yet.

Optionally, between printing one or more wall traces and printing a number of skin traces, the method comprises printing a number of brim traces on the build surface of the device. The brim will decrease the risk of the object getting loose from the build surface during printing. Optionally, the brim traces are printed on the build surface using over extrusion. Using over extrusion of the brim also increases the adhesion to the build surface in case of incorrect measurements of the build surface.

Optionally, the printing of the number of brim traces is performed using a flow rate correction factor of at least 110%. In an embodiment, the correction factor lies in a range of 110% -140%.

Optionally, the brim traces are printed starting at an inner brim trace and then printing outwards. By starting at the inner brim trace, the first trace can be deposited with optimal accuracy. So the trace can be laid down at a distance small enough to stick to the outer wall trace, by large enough to be able to easily tear off the brim after printing.

Optionally, before printing one or more wall traces, the method comprises the printing of one or more skirt traces on the build surface of the device. An advantage of printing a skirt is that the flow of the printing material is brought to a stable level before the parts of the object are printed.

Since every FDM printer has some inaccuracies in the positioning of the nozzle relative to the build surface, this method is useful for any FDM printer. The FDM printer may e.g. be an FFF printing device, or a pellet printer arranged to melt polymer pellets and extrude the melted material on a build plate surface.

According to a further aspect, there is provided a method of generating instructions for printing a three-dimensional (3D) object with an additive manufacturing device, the method comprising receiving a digital 3D model of the 3D object, and virtually dividing the 3D model into a number of layers to be printed. The method also comprises determining a nominal flow rate for a first layer of the 3D object using a trace width, a layer height, and a print speed.

The method also comprises generating instructions for the additive manufacturing device to print the first layer of the 3D object wherein the instructions comprise instructions for printing one or more wall traces on a build surface of the device using a flow rate that is higher than the nominal flow rate so as to cause over extrusion and for printing a number of skin traces on the build surface of the device using a flow rate that is lower than the nominal flow rate so as to cause under extrusion.

Optionally, the instructions comprise instructions for printing the one or more wall traces using a flow rate correction factor of at least 110%.

Optionally, the instructions comprise instructions for printing the number of skin traces using a flow rate correction factor equal or lower than 90%.

Optionally, the instructions comprise instructions for printing the wall traces starting at an outer wall trace and then printing inward.

Optionally, the instructions comprise instructions for printing a number of brim traces between printing the one or more wall traces and printing the number of skin traces.

Optionally, the instructions comprise instructions for printing the brim traces using over extrusion. Optionally, the instructions comprise instructions for printing the number of brim traces using a flow rate correction factor of at least 110%. Optionally, the instructions comprise instructions for printing the brim traces starting at an inner brim trace and then printing outwards.

Optionally, the instructions comprise instructions for printing one or more skirt traces before printing the one or more wall traces.

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 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 top view of an example of a first layer of a 3D object used to explain the invention;

Figure 2 schematically shows a top view of the wall traces of Figure 1 according to an embodiment;

Figure 3 shows a flow chart of a method of printing a three-dimensional 3D object with an additive manufacturing device, according to an embodiment of the invention;

Figure 4 schematically shows an example of an FDM device which can be used to perform the method of printing;

Figure 5 shows a flow chart of a method of generating instructions for printing a three- dimensional object, according to an embodiment of the invention, and

Figure 6 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 top view of an example of a print of a first layer 1 used to explain the invention. The first layer 1 comprises a skirt 11 , a brim 13, one or more wall traces 12 and a skin 14. The 3D object to be manufactured in this example has a round bottom surface which is actually formed by the round skin 14 and the surrounding wall traces 12. The brim 13 and the skirt 11 are sacrificial parts as will be clear to the skilled person. The arrows in Figure 1 refer to a preferred order of printing the traces within a certain subsection of the first layer 1 . In an embodiment of the method first the skirt 11 is printed to stabilize the flow. Adhesion of the skirt 11 is not too important, but it is nice if it sticks to the brim (printed later) so it easy to remove for the user. Since the width of the skirt 11 is normally only one or two lines width, no pressure in the print head is build up yet. Next, the wall traces 12 are printed from outside to inside, which is indicated by the small arrow direction inwards. The wall traces 12 are over extruded to ensure good adhesion to the build surface 20. Due to the over extrusion, a pressure build up occurs in the print head (i.e. in the liquefier). This build up depends on the number of wall traces printed, so for example if only two walls are printed, the build up will be less as compared to printing three or more wall traces.

In an embodiment, the brim 13 is printed after the wall traces 12 are finished. The brim can also be printed with some over extrusion to ensure good adhesion. In that case a significant pressure in the print head may build up. However, this helper structure will be removed afterwards, and no second layer will be printed on top. So, over extrusion during printing of the brim 13 is no problem. Lastly, the skin 14 is printed using under extrusion to release the pressure build up during printing of the walls 12 and the brim 13. It is noted that since the skin 14 is connected to the walls 12 the under extrusion will not cause any adhesion issues. The skin 14 may comprise a number of concentric circles, or a number of parallel lines, or any other pattern of traces for filling the surface within the wall traces 12.

It is noted that since the last subsection of the first layer 1 (i.e. the skin 14) is printed using under extrusion, the pressure build up in the print head is released. So when starting a next layer, the is no unwanted pressure in the print head that might cause the problems known from the prior art, such the elephants foot.

In table 1 some possible values for the extrusion rate correction factor are listed for the different subsections of the first layer. The rows in table 1 indicate a preferred order of printing the subsections (also referred to as line types) skirt, walls, brim and skin. It noted that the skirt 11 is optional. Instead of printing the skirt 11 , a purge blob may be printed at a distant from the object to initiate the flow.

Table 1

In table 1 three combinations of deposition values are listed. These values are only examples, and many other values and combinations are conceivable. The brim 13 can be absent, see last column. If the brim is present, it can be printed using under extrusion, nominal extrusion or over extrusion. In case of printing the brim 13, over extrusion is preferred, so the correction factor is preferably above 100%, such as for example 110%. As mentioned above, the walls 12 are printed using over extrusion while the skin 14 is printed using under extrusion to release the pressure in the print head before starting a next layer. Different levels of over extrusion are conceivable, such as using a correction factor of 110%, 120% or even higher. Also different levels of under extrusion are conceivable, such as using a correction factor of 90%, 80% or even lower.

Depending on the shape and size of the different subsections the optimal flow rate correction factors might be different (e.g. depending on the relative sizes), but a general setting will already greatly reduce the described problems.

Figure 2 schematically shows a top view of the wall traces 12 of Figure 1 according to an embodiment. In this embodiment, the wall 12 comprises three traces 121 , 122 and 123. According to an embodiment, the outer wall trace 121 is printed first, followed by traces 122, 123 lying inward in respective to the outer wall trace 121 . The order of printing is indicated by the arrow. This also account for the arrows in Figure 1 . By first printing the outer trace of the wall, with the brim not (yet) printed, the outer wall trace 121 can be deposited very accurately without being influenced by any neighboring traces laid down before. This exact positioning of outer wall of the 3D object results in a dimensional accurate object, also in the first layer.

Figure 3 shows a flow chart of a method 30 of printing a three-dimensional 3D object with an additive manufacturing device, according to an embodiment of the invention. The method 30 comprises the determining 300 of a nominal flow rate for a first layer of the 3D object using a trace width, a layer height, and a print speed. The method comprises an optional step of printing 301 one or more skirt traces on the build surface of the device. The method also comprises the step of printing 302 one or more wall traces on a build surface of the device using a flow rate that is higher than the nominal flow rate so as to cause over extrusion. The method comprises an optional step of printing 303 a number of brim traces on the build surface of the device. And the method comprises the step of printing 304 a number of skin traces on the build surface of the device using a flow rate that is lower than the nominal flow rate so as to cause under extrusion.

Figure 4 schematically shows an example of an FDM device 1000, in this case a FFF printing device 1000, which can be used to perform the method of printing as described above.

The FFF printing device 1000 comprises a print head 1002 also referred to a deposition head 1002. At its outer end the print head 1002 comprises a nozzle 1004 where molten filament can leave the deposition head 1002. A filament 1005 is fed into the print head 1002 by means of a feeder 1003. Part of the filament 1005 is stored in a filament storage which could be a spool 1008 rotatably arranged onto a housing (not shown) of the FFF printing device, or rotatably arranged within a container (not shown) containing one or more spools. The FFF printing device 1000 comprises a controller 1007 arranged to control the feeder 1003 and the movement of the print head 1002, and thus of the nozzle 1004, relative to the build surface. The controller 1007 may comprise one or more processing units 1070. By executing suitable instructions on the processing units 1070, the FFF printing device 1000 may be arranged to perform the method as described in Figure 3. The instructions may comprise G-code produced by the computing device 210 shown in Figure 6. In this embodiment, the FFF printing device further comprises a Bowden tube 1009 arranged to guide the filament 1005 from the feeder 1003 to the print head 1002. The FFF printing device 1000 also comprises a gantry arranged to move the print head 1002 at least in one direction, indicated as the X-direction. In this embodiment, the print head 1002 is also movable in a Y-direction perpendicular to the X-direction. The gantry comprises at least one mechanical driver 1014 and one or more axles 1015 and a print head docking unit 1016. The print head docking unit 1016 holds the print head 1002 and forthat reason is also called the print head mount 1016. It is noted that the print head docking unit 1016 may be arranged to hold more than one print head, such as for example two print heads each receiving its own filament. The feeder 1003 is arranged to feed and retract the filament 1005 to and from the print head 1002. The feeder 1003 may be arranged to feed and retract filament at different speeds to be determined by the controller 1007. A build plate 1018 may be arranged in or under the FFF printing device 1000 depending on the type of printing device. The build plate 1018 may comprise a glass plate, or metal plate or any other object suitable as a substrate. In the example of Figure 4, the build plate 1018 is movably arranged relative to the print head 1002 in a Z-direction, see Figure 4. It is noted that instead of a build plate, other build surfaces may be used such as surfaces of movable belts.

Figure 5 shows a flow chart of a method 50 of generating instructions for printing a three- dimensional object with an additive manufacturing device, according to an embodiment of the invention. The method 50 comprises receiving 500 a 3D model of a 3D object. The method also comprises virtually dividing 501 the 3D model into a number of layers to be printed. The method also comprises determining 502 a nominal flow rate for a first layer of the 3D object using a trace width, a layer height, and a print speed. The method also comprises generating 503 instructions for the additive manufacturing device to print the first layer of the 3D object wherein the instructions are generated for printing one or more wall traces 504 on a build surface of the device using a flow rate that is higher than the nominal flow rate so as to cause over extrusion, and generating instructions for printing a number of skin traces 505 on the build surface of the device using a flow rate that is lower than the nominal flow rate so as to cause under extrusion. It is noted that the steps 504 and 505 are together also referred to as step 503.

By controlling the flow of each subsection in the initial layer as well as controlling the order of the line types (i.e. segment), the pressure build up can be prevented and sufficient adhesion guaranteed, since every line type has its own requirements in terms of adhesion and required pressure.

Figure 6 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 6 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 the method of generating instructions for printing a three-dimensional object with an additive manufacturing device, as described above.

The instructions may comprise G-code comprising instructions for the additive manufacturing device on how to move the print head relative to the build surface, with which speed, flow rates, etc. The G-code may also comprise values for a flow rate correction factor. This correction factor can be explicitly provided in the G-code as a correction factor, which is taken into account by the printer firmware. Alternatively, the correction factor can be included in the normal printing commands, i.e. the flow rate values already are corrected with the factor before the G- code is generated. In yet another embodiment, the different subsections of the first layer mentioned in Fig 1 are ‘tagged’ in the G-code. So for example a tag ‘begin of brim’ and ‘end of brim’ is inserted in the G-code. The additive manufacturing device may then be arranged to recognize these tags and apply a certain correction factor to each subsection of the first layer. In this embodiment, the flow rate correction factors may be stored in a memory of the additive manufacturing device which will be read by the one or more processing units 1070 (see also Figure 4).

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