NL2035263B1

NL2035263B1 - FFF printing device with a liquefier assembly having multiple temperature sensors

Info

Publication number: NL2035263B1
Application number: NL2035263A
Authority: NL
Inventors: Fanzio Paolo; Hermanus Welling Kornelis; Andreas Versteegh Johan
Original assignee: Ultimaker Bv
Priority date: 2023-07-05
Filing date: 2023-07-05
Publication date: 2025-01-13

Abstract

A Fused Filament Fabrication system (1 ;20;1000) is described. The system comprises a liquefier tube (2) having an inlet (21) and an outlet (22). A heater (5) is arranged to heat at least a part of the liqueﬁer tube. A controlling system (30;40) is arranged to select one or more ofthe temperature sensors (6,7) of the liquefier depending on a first parameter indicative of a print material type and a second parameter indicative ofa flow rate through the liqueﬁertube, and to use measurement data from the selected sensor(s) to control the heater (5). [Figure 1 ]

Description

FFF printing device with a liquefier assembly having multiple temperature sensors

Field of the invention

The present invention relates to a Fused Filament Fabrication (FFF) system. The invention also relates to a method of controlling an FFF liquefier assembly, and to a computing device comprising one or more processing units arranged to perform such method, 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, or any other suitable 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 some 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, or any other suitable print material. The filament may be fed into the print head by means of a filament feeder. A filament feeder may comprise one or more gripper wheels, and optionally one or more idle wheels, to create an appropriate gripping force on the filament. The print head may comprise one or more liquefiers that are arranged to receive the filament and melt (i.e. liquefy) the print material. Heating of the liquefiers may be achieved by arranging one or more heaters on or around each of the liquefiers.

The liquefiers are arranged to deposit the melted material through the respective nozzles on the build plate or on a previous layer of the printed object. A pressure is needed to push the molten material through the liquefier and nozzle. Such pressure is influenced by nozzle size, nozzle geometry, melt rheology, set volumetric flow rate and liquefier temperature. During feeding of the filament into the liquefier, the feeder wheels apply a force via the filament to enable the material to be extruded at the set volumetric flow rate. This force is referred to as the extrusion force. It is noted that a too high extrusion force may cause an unwanted increase in the feeder wheel slip, causing a reduction in the actual extruded volumetric flow rate compared to the set values (commonly referred as under-extrusion). So, the maximum achievable volumetric flow rate is limited by a maximum allowable extrusion force. However, the extrusion force increases by increasing the set volumetric flow rate and/or by reducing the temperature of the melt. Controlling the melt temperature is then imperative to enable high extrusion rate (thus high printing speed).

The usual way to control the melt temperature (i.e. keep it constant) is to place a temperature sensor in contact with the heater and control the heater temperature. However, the melt temperature is intrinsically not-uniform since the solid filament enters at room temperature in the liquefier and the heat transfer is slow: it will need time to enable a full melting of such filament.

This leads to the creation of the temperature gradient inside the liquefier.

At high filament feed rate, the (relatively cold) filament is pushed towards the nozzle at a higher rate, leading to a decrease of the temperature (and an increase of viscosity) and to a consequent dramatic increased pressure in the liquefier, which will result in an unwanted level of feeder slip. A possible solution could be to increase the temperature setpoint in such a way that the material is fully melted, even at high filament feed rates. However, this may lead to burning and degradation of the print material in the liquefier which may cause a clog in the nozzle, and thus affecting the extrusion process in a negative way.

Summary of the invention

The aim of the present invention is to provide an FFF system wherein at least one of the problems mentioned above is solved.

According to a first aspect of the present invention, there is provided a Fused Filament

Fabrication system comprising a liquefier tube having an inlet and an outlet. The system also comprises a nozzle arranged at the outlet of the liquefier tube, and a heater arranged to heat at least a part of the liquefier tube. A first sensor is arranged to measure a first temperature at a first location of the liquefier tube, and a second sensor is arranged to measure a second temperature at a second location of the liquefier tube downstream the first location. The system further comprises a controlling system arranged to select at least one of the first sensor and the second sensor depending on a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefier tube. The controlling system is arranged to use measurement data from the selected sensor(s) to control the heater.

In one embodiment, the Fused Filament Fabrication system is a FFF printing device, also referred to a FFF 3D printer. In another embodiment, the Fused Filament Fabrication system is a liquefier as such, having its own controller arranged to perform the above-mentioned selection of sensors.

The first parameter may be a generic material type name, like ABS or PLA. The first parameter can also be a material brand name like ULTIMAKER TOUGH PLAT, or a material code, or any other parameter indicative of the print material type. The second parameter may be a set flow rate for a specific trace, or for a specific trace type (e.g. bottom layer). The second parameter may also be the feed rate for a specific trace or specific trace type, or any other parameter indicative of a flow rate through the liquefier tube.

By selecting a temperature sensor location as the control node for controlling the heater depending on the two parameters, the behaviour of the liquefier can be controlled, optimized for the filament type and filament flow rate used. It is noted that the position of the temperature sensor used, will actually enable the tuning of the melt temperature with the final goal of having a melt temperature high enough to keep a low melt viscosity, which results in a reduced pressure drop and a feeder slip, while on the other hand having a melt temperature low enough to prevent material degradation due to burning, which may lead to clogging and failed extrusion or printing process. The balance between these two aspects can be controlled by selecting the proper temperature sensor(s).

In an embodiment, the liquefier tube comprises a heat break separating the liquefier tube into a hot-end and a cold-end, wherein the first sensor is located at an upstream outer end of the hot-end. A sensor located just after the heat break enables a temperature sensing, and thus controlling of the temperature of the material as it starts to melt.

In an embodiment, the second sensor is located at a downstream outer end of the liquefier tube. Such a sensor measures the temperature of the material just before deposition, which enables a controlled layer deposition and adhesion onto the build surface or previous layers.

In an embodiment, the second parameter is the feed rate of filament fed through the liquefier tube. The feed rate is the rate of the feeding of the filament by the feeder. This parameter is needed for the feeder to operate, so it is a parameter which is available to the FFF printing device, and can be used to decide on which sensor to use for the control of the heater of the liquefier assembly.

In an embodiment, the system comprises a third sensor arranged to measure a third temperature at a third location of the liquefier tube between the first location and the second location. The third sensor is advantageous in case of a relatively long liquefier tube. The intermediate sensor can be used to substitute for the first sensor, which at certain conditions, might be located at a location where the filament in the tube is not molten yet (above the meniscus).

In an embodiment, the controlling system is arranged to select at least one of the first sensor and the second sensor further depending on a third parameter indicative of a flow resistance of the liquefier tube and the nozzle. The third parameter may be a nozzle orifice cross- section or a cross-section of the liquefier tube. By making the selection of the sensor(s) dependent on the degree of flow resistance, a more sophisticated control of the print process can be achieved.

In an embodiment, the controlling system is arranged to select at least one of the first sensor and the second sensor further depending on a fourth parameter indicative of a trace type of print traces to be deposited. This is advantageous for example in situations wherein a specific trace type needs a tight temperature control before deposition, such as an outer wall, to achieve proper dimensional accuracy. Other traces, such as infill traces, which could be optimized for speed, require a tight temperature control just after the heat-break, so as to optimize the melting process.

In an embodiment, the controlling system is arranged to access a look-up table, the look- up table comprising information on which of the first sensor and the second sensor to select depending on at least one of the first parameter and the second parameter. The look-up table may comprise a number of records, each of the records comprising a material type field, a feed rate range field, a first control field, and optionally a secondary control field.

In an embodiment, the heater is arranged along most part of the hot-end of the liquefier tube. Such an elongated heater is favourable in case the liquefier tube is relatively long.

According to a second aspect of the present invention, there is provided method of controlling an FFF liquefier assembly. The liquefier assembly to b controlled comprises a liquefier tube, a heater arranged to heat at least a part of the liquefier tube, a first sensor arranged to measure a first temperature at a first location of the liquefier tube, and a second sensor arranged to measure a second temperature at a second location of the liquefier tube downstream the first location. The method comprises obtaining a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefier tube, and selecting at least one of the first sensor and the second sensor depending on the first parameter and the second parameter, for control of the heater.

According to a third aspect of the present invention, there is provided a computing device comprising one or more processing units, the one or more processing units being arranged to perform the method as described above. The computing device may be a PC (personal computer). It may also be a 3D printing device as such.

According to a fourth aspect of the present invention, 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 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 part of a FFF device according to an embodiment of the invention;

Figure 2A shows an example of a look-up table to be used by the controlling system to find which of the temperature sensors should be used;

Figure 2B shows another example of a look-up table to be used by the controlling system;

Figure 3A-3D show graphs related to measurements performed using the first sensor as the sensor to control the heater;

Figure 4A-4D show graphs related to measurements performed using the third sensor as the sensor to control the heater;

Figure 5A-5D show graphs related to measurements performed using the second sensor as the sensor to control the heater;

Figure 6 schematically shows an example of an FFF device, according to an embodiment of the invention;

Figure 7 schematically shows a computing device according to an embodiment, and

Figure 8 shows a flow chart of a method 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. 5

Detailed description of embodiments

Figure 1 schematically shows part of a FFF device 1 according to an embodiment of the invention. The FFF device 1 comprises a liquefier 20, also referred to as liquefier assembly 20. It is noted that the liquefier 20 may be removably coupled to the FFF device 1, so that it can easily be removed and/or replaced.

The liquefier 20 comprises a liquefier tube 2 having an inlet 21 and an outlet 22 with a nozzle 23 arranged at the outlet 22 of the liquefier tube 2. The FFF device 1 device also comprises a feeder 3 arranged to feed filament 4 into the liquefier tube 2 at the inlet 21.

The liquefier 20 further comprises a heater 5 (also referred to as heater arrangement) which is arranged to heat at least part of the liquefier tube 2. In this example, the liquefier tube 2 comprises a heat break 24 thermally separating the liquefier tube into a hot-end, see 26, and a cold-end, see 25. The heat break 24 may be a relatively thin part of the liquefier tube 2, as shown in the example of Figure 1. The cold-end 25 may comprise cooling fins 27 arranged to cool the cold-end 25.

A first sensor 6 is arranged to measure a first temperature at a first location of the liquefier tube 2. In this example, the first sensor 6 is located at an upstream side of the hot-end 26 of the liquefier tube 2. A second sensor 7 is arranged to measure a second temperature at a second location of the liquefier tube 2 downstream the first location. In this example the second sensor 7 is located at the outlet 22 of the tube 2 near the nozzle 23.

The liquefier 20 may as well comprise an optional third sensor 8 arranged to measure a third temperature at a third location of the liquefier tube between the first location and the second location. The advantage of such a third intermediate sensor will be explained in more detail below.

The heater 5 may be a resistive heater arranged in or on the wall of the hot-end 26. Other types of heaters are conceivable, such as inductive heaters. The type of heater is not relevant for the invention. In operation, the heater 5 heats up the hot-end of the liquefier tube 2 and thus the print material present inside the liquefier tube 2, which will result in molten material 10. Due to a pressure created by the feeder 3, the molten material will be pushed out of the nozzle 23 and will be deposited on a build plate 12. Once deposited, the print material will cool off and will solidify, resulting in printed traces 14 which may be stacked in order to create a 3D object, as will be appreciated by the skilled person.

In the example of Figure 1, the system 1 also comprises a flow sensor 15 arranged to measure the feed rate of the filament (i.e. filament feed rate), and thus indirectly, the flow of the print material through the liquefier tube 2. As is appreciated by the skilled person, the volumetric flow rate can be calculated by multiplying the filament feed rate times a cross-section of the filament. So for example, if the filament diameter is 2.85 mm and the feed rate is 2 mm/sec than the flow rate is (2.85/2)2 x TT x 2 = 12,76 mm3/sec.

The FFF device 1 further comprises a controlling system. The controlling system in this embodiment comprises a main controller 30, and a heat controller 40. The heat controller 40 may be arranged to control the heater 5 using measurements from at least one of the first sensor 6 and the second sensor 7. This means that the heat controller 40 is arranged to use temperature measurements from the first sensor 6, or from the second sensor 7, or from both the first sensor 6 and the second sensor 7. Depending on the sensor(s) used, the liquefier 20 will behave differently with regard to how an increase of the flow rate will affect the extrusion force on the filament 4, as will be discussed in more detail below.

In this embodiment, the main controller 30 is arranged to also control the feeder 3, see arrow 31. The feeder 3 may be controlled in such a way that a specific filament feed rate is set for depositing the print material at the correct volumetric flow rate. It is noted that slip may occur between the feeder wheels and the filament 4. The slip will increase if the force on the filament 4 increases. In order to determine the actual feed rate of the feeder 3, a feed rate sensor 15 may be provided. The feed rate sensor 15 may comprise a wheel that is pushed against the filament for simply measuring the flow rate. Since the wheel of the feed rate sensor does not force the filament 4 to move, there is no risk of slip between the sensor wheel and the filament 4. The main controller 30 may be arranged to receive input from the feed rate sensor 15, see arrow 32. The input 32 may be used to control the feeder 3 in a feedback control loop.

As mentioned above, the heat controller 40 is arranged to receive measurement data from the sensors, 6, 7 and 8 and to control the heater 5, see also arrow 41 which indicates a power line from the heat controller 40 to the heater 5. The heat controller 40 may comprise e.g. a

PID controller arranged to calculate an electrical power level for the heater 5 depending on the inputs coming from the selected sensor(s). The heat controller 40 may receive control instructions, see arrow 42, from the main controller 30. These control instructions may comprise information on which of the sensors 6, 7, 8, is to be used for controlling the heater 5. It is noted that in the example of Figure 1, the heat controller 40 is separate from the main controller 30.

Alternatively, the heat controller 40 may be an integrated part of the main controller 30.

The main controller 30 may receive print instructions, see arrow 33, received via a communication network (not shown) of via a USB memory. The print instruction may comprise G- code including instructions on how to move the nozzle relative to the build plate, using which feed rate, using which temperature and also using which print material type. Below an example of a part of a G-code file is shown: ;EXTRUDER_TRAIN.O.MATERIAL.GUID:44a029e6-e31b-4c9e-a12f-9282e29a92ff ;EXTRUDER_TRAIN.O.NOZZLE .DIAMETER:0.4

TYPE:SKIRT

G1 F600 20.2

G1 F2700 EQ

G1 F1800 X139.943 Y99.008 E0.07015

G1 X144.011 Y94.94 E0.1423

G1 X148.558 Y91.751 E0.21194

The controlling system 30,40 may be arranged to obtain parameters indicative of a print material type, a filament flow rate, and a liquefier flow resistance (e.g. nozzle diameter).

In an embodiment, the controlling system is arranged to determine the parameters using information out of the print instructions. For example, the first parameter indicative of a print material type can be obtained by identifying/finding the value MATERIAL.GUID in the G-code.

The second parameter indicative of a flow rate through the liquefier tube can be obtained by combining the E-value of a trace (i.e. the extrusion length), the F-value of a trace (i.e. the tool speed), the width of a trace (extracted from the X,Y and E-values, and the layer height (extracted from the Z-value). If needed the filament feed rate can be calculated using the determined flow rate.

The optional third parameter indicative of the liquefier flow resistance, can be extracted from the parameter EXTRUDER_TRAIN.0.NOZZLE.DIAMETER, see example of the G-code above, to obtain the nozzle diameter.

The controlling system 30,40 may select at least one of the first sensor 6 and the second sensor 7 depending on the first and second parameter, and optionally the third parameter. In an embodiment, the controlling system 30,40 is arranged to use the first and second parameters to access the look-up table in order to find the appropriate temperature sensor(s) to use for sensing the temperature in the hot-end. The main controller 30 may comprise a memory 34 for storing the look-up table. Alternatively, the look-up table may be part of a database separate from the main controller 30, but still accessible by the main controller 30 via a link or a network (not shown).

In a further embodiment, the heat controller 40 comprises a memory 44 for storing a look- up table comprising information on which sensor to use depending on the first and second parameter. In this embodiment, the look-up table in the main controller 30 may be absent, or may be used in combination with the look-up table in the heat controller 40.

Figure 2A shows an example of a look-up table to be used by the controlling system 30,40 to find which of the temperature sensors 8, 7, 8 need to be used when depositing material during a printing process. If needed, before accessing the table, the controlling system 30,40 may convert the received print speeds into the feed rates using e.g. the nozzle diameter and the filament diameter. The look-up table of Figure 2A comprises five columns and twenty rows, also referred to as records. The five columns of Figure 2A relate to: the liquefier type, the material type, the feed rate, which temperature sensor to use as the first control, and which temperature sensor to use as a secondary control if any.

First let us assume that the object is printed using PLA, and the feed rate at a certain track or layer is 0.8 mm/s. Using the table in Figure 2A, the controlling system 30,40 will then use the first sensor 6 (S_top) as the temperature sensor. In this table the sensor 6 is referred to as

S_top being the most top sensor on the liquefier tube. Sensor 7 is referred to as S_bottom, and optional sensor 8 is referred to as S_mid.

Now let us assume that the object is printed using ABS, and the feed rate at a certain track or layer is 0.4 mm/s. Using the table in Figure 2A, the controlling system 30,40 will then use the S_top as the first control and will use S_bottom as the secondary control. Measurements from the first control and the secondary control may be combined in several ways when using their values for controlling the heater 5. For example, measurement from both sensors may have equal weight in the calculations used by the controlling system. Alternatively, measurement of one of the sensors may have a higher weight than the other sensor. It is also conceivable to use one sensor solely for the control loop and the other sensor as a fail safe for out of bound conditions.

Figure 2B shows another example of a look-up table to be used by the controlling system 30,40 to find which of the temperature sensors 8, 7, 8. This table contains five columns: the material type, the feed rate, use of S_top, use of S_mid and use of S_bottom. In the last three columns a percentage can be filled in upfront so that the controlling system knows which of the sensor need to be used and with which percentage.

Now let us assume that the object is printed using ABS, and the feed rate at a certain track or layer is 0.4 mm/s. Using the table in Figure 2B, the controlling system 30,40 will then use the S_top and will use S_bottom. Measurements of S_top will be weighted with a weight of 80% while measurement of S-bottom will be weighted with a weight of 20%.

It is noted that the tables of Figure 2A and 2B are merely examples and that the look-up table may have different formats, content and values.

To better understand the advantages of the invention, some results of lab tests are now discussed. The lab tests were performed using a liquefier as described with reference to Figure 1, using an additional force sensor arranged to measure an extrusion force. The build plate was left out, so the print material was not deposited on a build surface, but could simply flow into a waste bin. The tests were focussing on the relation between the feed rate and the extrusion forces. The force sensor was arranged between the feeder and the liquefier so as to measure the forces experienced by the filament when being fed into the entrance of the liquefier. The liquefier was installed with three sensors as were described above with reference to Figure 1.

The setpoint temperature has been chosen to be 240 °C for extrusion of PC (Polycarbonate). In order to reach such temperature inside the melt in steady state (no extrusion) a calibration has been performed using a temperature sensor located inside the melt. If the temperature control was performed using S_top, then T_top needed to be equal to 228 °C. If the temperature control was performed using S_top, then T_mid needed to be equal to 244*C. If the temperature control was performed using S_top, then T_bottom needed to be equal to 217*C.

Figure 3A-3D relate to measurements performed using sensor S_top (i.e. sensor 6) as the sensor to control the heater 5. Measurements were performed using a low feed rate, which was increased during measurements to a (relatively) high feed rate. In our case we used different filaments with a diameter of 2.85 mm. The low feed rate was set to 0.1 mm/s and the high feed rate was set to 1.4 mm/s. The diameter of the liquefier tube was 3.0 mm, and the length was about 45 mm, with the hot-end having a length of about 20 mm. The tests were performed using

PC, or different print materials such as PLA (Polylactic acid), ABS (Acrylonitrile butadiene styrene), PP (Polypropylene), Nylon, TPU (Thermoplastic polyurethan).

Figure 3A is a graph showing the temperature measured by the sensor S_top for a low and a high feed rate. As can be seen from the graph, the temperature is 228 °C for both feed rates. This is of course to be expected since the temperature is controlled using the top sensor

S_top.

Figure 3B is a graph showing the temperature measured by the sensor S_mid for a low and a high feed rate. As can be seen from the graph, the temperature is 247 °C at the low feed rate and increases to 275 °C at the high feed rate. So there is an increase of ATm = 28 °C.

Figure 3C is a graph showing the temperature measured by the sensor S_bottom for a low and a high feed rate. As can be seen from the graph, the temperature is 221 °C at the low feed rate and increases to 254 °C at the high feed rate. So there is an increase of ATb = 33 °C.

These temperature changes are due to the fact that increasing the feed rate, the solid filament will enter in the liquefier tube 2 at a higher speed lowering the temperature at the location of S_top. The heat controller 40 will then increase the heater power to keep T_top constant.

However, the heater spans over the entire length of the liquefier and this increased power leads to an increase of the temperature in the location S_mid and S_bottom.

Figure 3D is a graph showing the extrusion force measured by the force sensor arranged between the liquefier and the feeder, for a low and a high feed rate. As can be seen from the graph, the extrusion force stays almost constant, increasing only from 9 N to 13 N, so just 4 N.

Looking at the graphs of Figure 3A-3D, it can be noted that when using temperature sensor S_top as the control sensor, the temperature at the top of the hot-end will stay constant as well as the extrusion force (i.e. almost constant) when an increase in feed rate occurs. But both the temperatures of sensors S_mid and S_bottom increase when the feed rate increases. The increase of the S_bottom sensor showed to be quite high at certain tests. Temperature increases up to 254 °C were measured for PP. Such high temperatures could cause the PP material to degrade, especially during extrusion stops and pauses, which may result in a clogging of the nozzle. At the same time, an advantage of using S_top as the control sensor is that the extrusion force stays constant and relatively low so that the feeder slip is absent or constant. This is advantage for reliably controlling the filament feed rate.

Figure 4A-4D relate to measurements performed using sensor S_mid (i.e. intermediate sensor 8) as the sensor to control the heater 5. Measurements were performed using a low feed rate, which was increased during measurements to a (relatively) high feed rate. The low feed rate was set to 0.1 mm/s and the high feed rate was set to 1.4 mm/s.

Figure 4A is a graph showing the temperature measured by the sensor S_top for a low and a high feed rate. As can be seen from the graph, the temperature decreases from 227 °C to 203 °C when the feed rate is increased. So there is a decrease of ATt = 24 °C.

Figure 4B is a graph showing the temperature measured by the sensor S_mid for a low and a high feed rate. As can be seen from the graph, the temperature is 244 °C at both the low feed rate and the high feed rate. This is of course to be expected since the temperature is controlled using the middle sensor S_mid.

Figure 4C is a graph showing the temperature measured by the sensor S_bottom for a low and a high feed rate. As can be seen from the graph, the temperature is 217 °C at the low feed rate and increases to 228 °C at the high feed rate. So there is an increase of ATb = 11 °C.

Figure 4D is a graph showing the extrusion force measured by the force sensor arranged between the liquefier and the feeder, for a low and a high feed rate. As can be seen from the graph, the extrusion force increases from 9 N to a level of 21 N. So there is an increase of AF = 12 N.

Looking at the graphs of Figure 4A-4D, it can be noted that when using temperature sensor S_mid as the control sensor, the temperature at the mid-section in the hot-end will stay constant when an increase in feed rate occurs, see Figure 4B. But both the temperatures of sensors S_top and S_bottom will change when the feed rate is increased. The increase of the

S_bottom sensor ATb = 11 °C showed to be smaller as compared to the situation shown in Figure 3C for the same material. Temperature increases up to 228 °C were measured for PP. So the risk of material degrading is lower as compared to the situation with the S_top being the sensor to be used. At the same time, an advantage of using S_mid as the control sensor is that for longer hot end lengths (+50mm) it gives a better control point for intermediate speeds.

The temperature changes are due to the fact that increasing the feed rate, the solid filament will enter in the liquefier at higher speed lowering the temperature at the location of

S_top, lowering the temperature T_Top. The controller 40 will then increase the heater power to keep T_mid constant. This increased power leads to an increase of the temperature in the location S_bottom.

Figure 5A-5D relate to measurements performed using sensor S_bottom (i.e. sensor 7) as the sensor to control the heater 5. Measurements were performed using a low feed rate, which was increased during measurements to a (relatively) high feed rate. The low feed rate was set to 0.1 mm/s and the high feed rate was set to 1.4 mm/s.

Figure 5A is a graph showing the temperature measured by the sensor S_top for a low and a high feed rate. As can be seen from the graph, the temperature decreases from 224 °C to 196 °C when the feed rate is increased. So there is a decrease of ATt = 28 °C.

Figure 5B is a graph showing the temperature measured by the sensor S_mid for a low and a high feed rate. As can be seen from the graph, the temperature decreases from 242 °C to 29 °C when the feed rate is increased. So there is a decrease of ATm = 13 °C.

Figure 5C is a graph showing the temperature measured by the sensor S_bottom for a low and a high feed rate. As can be seen from the graph, the temperature stays at 217 °C for both the low feed rate and the high feed rate. This is of course to be expected since the temperature is controlled using the bottom sensor S_bottom.

Figure 5D is a graph showing the extrusion force measured by the force sensor arranged between the liquefier and the feeder, for a low and a high feed rate. As can be seen from the graph, the extrusion force increases from 9 N to a level of 32 N. So there is an increase of AF = 23 N.

Looking at the graphs of Figure 5A-5D, it can be noted that when using temperature sensor S_bottom as the control sensor, the temperature at the nozzle will stay constant when an increase in feed rate occurs. But both the temperatures at the top and the mid section will decrease when the feed rate is increased, see Figures 5A and 5B. The temperature changes are due to the fact that increasing the feed rate, the solid filament will enter in the liquefier at higher speed lowering the temperature at the location of S_top and S_mid. The controller 40 will then increase the heater power to keep T_bottom constant. However, such power increase is not high enough to also increase the temperature in the location S_mid and S_top. This is due to the fact that the temperature variation in function of the feed rate in the location S_bottom is usually smaller compared to the variation at the positions upstream, closer to the solid filament inlet.

The temperature decrease at the top of the hot-end will cause an increased flow resistance and thus an increase of the extrusion force. A too high extrusion force will increase slip in the feeder, which may result in unreliable deposition rates. However it is noted that when using the bottom sensor as the control sensor, the risk of degrading the print material is lower as compared to the situation with the S_top being the sensor to be used.

By arranging at least two sensors at different locations on/in the walls of the liquefier tube 2, see also Figure 1, the inventors have been able to use the advantages while avoiding the disadvantages of the specific sensors. As mentioned above, the middle sensor is an optional sensor, but may also improve the performance of the whole FFF device 1 for longer hot ends where intermediate sensors/control points are needed.

When printing with print materials with relatively large melting ranges, such as PLA or

ABS, switching the control to the top sensor (i.e. sensor 8) will keep the extrusion pressure low which enables an increased extrusion speed and consequently an increased printing speed, resulting in a decreased printing time for a specific 3D object. When printing with print materials with relative high risk of degradation at too high temperatures, such as PP or TPU, switching the control to the bottom sensor will provide for a liquefier with little risk of material degradation in the nozzle. It is noted that the selection of which sensor to use as the control node, will also be dependent on the filament feed rate; so for a specific material, the controlling system 30,40 will select a specific sensor depending on feed rate and the type of print material.

The feed rate data received by the controlling system may comprise the feed rate set for a specific trace, or an average feed rate over a certain track or area. When printing a wall trace, the print speed (and thus the feed rate) will normally be lower then when printing the infill traces. So, at the start of the infill printing, the controlling system may select a different temperature sensor that is more appropriate for that feed rate, for the specific print material used. At the end of the infill printing at a specific layer, the controlling system may switch back to the previously selected temperature sensor. To elaborate further on the invention, below some examples are given.

In case a layer, or traces need to be printed using high speed printing, then it is favourable to switch off the bottom sensor 7 and only use the top sensor 6.

In case a layer, or traces need to be printed using stable, low speed printing, with high quality inter-layer bonding using materials that can degrade in high temperatures, then it is favourable to use only the bottom sensor 7.

In case a layer, or traces need to be printed using print speeds lower than the maximum print speed for a liquefier, and in a relatively long liquefier (e.g. 100mm) with a large range of flow rates (e.g. 1 mm/s to 40 mm/s), then it is favourable to use the intermediate sensor 8 as the main control point (mimicking the top sensor of a shorter liquefier).

In case a layer, or traces need to be printed fast but with high details (switch fast printing in the infill and slow printing in the outer walls) then use combination of the two sensors

It is noted that the selection of the temperature sensor could be pre-set in the G-code or in the firmware of an additive manufacturing system. So the controlling system 30,40 can switch the sensors during printing but it is all set beforehand. Alternatively, the controlling system is arranged to access a look-up table to find the temperature sensor(s) to be used. The look-up table may be stored in a memory of the controller 30, a memory of the heat controller 40, or a memory of a remote device in communication with the controlling system 30,40.

Figure 6 schematically shows an example of an FFF device 1000, according to an embodiment of the invention. The FFF device is also referred to as the 3D printer 1000. The liquefier 20 comprises a liquefier tube 2. At its outer end, the liquefier tube 2 comprises the nozzle 23 where molten filament can leave the liquefier tube 2. A filament 4 is fed into the liquefier tube 2 by means of a feeder 3. Part of the filament 4 is stored in a filament storage which could be a spool 1008 rotatably arranged onto a housing (not shown) of the 3D printer, or rotatably arranged within a container (not shown) containing one or more spools. The 3D printer 1000 comprises a controller 30 arranged to control the feeder 3 and the movement of the liquefier tube 2, and thus of the nozzle 23. The controller 30 may also be arranged to communicate with the heat controller 40 as described with reference to Figure 1. The controller 30 may comprise one or more processing units 37 arranged to execute the print instructions. The print instructions may comprise the print speed data and material type data, the temperatures, as well as instructions for the gantry how to manipulate the print head 1002.

In this embodiment, the 3D printer further comprises a Bowden tube 1009 arranged to guide the filament 1005 from the feeder 3 to the liquefier tube 2. The 3D printer 1000 also comprises a gantry arranged tc move the liquefier tube 2 at least in one direction, indicated as the

X-direction. In this embodiment, the liquefier tube 2 is also movable in a Y-direction perpendicular to the X-direction and the Z-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 for that reason is also called the print head mount 10186. It is noted that the print head docking unit 1016 may be arranged to hold more than one liquefier, such as for example two liquefiers each receiving its own filament. The feeder 3 is arranged to feed and retract the filament 1005 to and from the liquefier tube 2. The feeder 3 may be arranged to feed and retract filament at different speeds to be determined by the controller 30.

A build plate 12 may be arranged in or under the 3D printer 1000 depending on the type of 3D printer. The build plate 12 may comprise a glass plate or any other object suitable as a substrate, such as a (coated) metal plate. In the example of Figure 6, the build plate 12 is movably arranged relative to the nozzle 23 in a Z-direction. It is noted that instead of a build plate, other build surfaces may be used such as surfaces of movable belts.

Figure 7 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 7 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 controlling an

FFF liquefier assembly, as described herein. This method may comprise generating print instructions for a FFF printing device. The print 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.

In an embodiment, the memory 113 is also arranged to store a look-up table comprising information on which sensor to use depending on the first and second parameter. In this embodiment, there is no need for storing a look-up table in the FFF printing device since all information on the selection of temperature sensors can already be included in the print instructions. In an embodiment, the G-code may also comprise values for which of temperature sensors to control the heater. In the example below, the M109, M209 codes are added to select a specific temperature sensor (top_sensor and bottom_sensor respectively) for a specific part of the print. In the example, during printing of the infill traces, the top sensor 6 is used to control the heater, and during printing of the outer wall traces, the bottom sensor 7 is used to control the heater:

TYPE:INFILL select top sensor at 215 °C

M109 S215

G1 F600 20.2

G1 F3200 X139.943 Y99.008 E0.07015

G1 X144.011 Y94.94 E0.1423

G1 X148.558 Y91.751 E0.21194

G1 X153.816 Y89.288 E0.28476

G1 X159.313 Y87.817 E0.35612

G1 X165.017 Y87.318 E0.42792

G1 X170.704 Y87.817 E0.49951 ee

TYPE:OUTER WALL select bottom sensor at 200 °C

M209 S200

G1 F1800 X141.479 Y100.302 E12.61059

G1 X145.302 Y96.479 E12.67839

G1 X149.573 Y93.485 E12.7438

G1 X154.516 Y91.168 E12.81226

G1 X159.658 Y89.792 E12.87901

G1 X165.017 Y89.323 E12.94647

G1 X170.349 Y89.792 E13.0136

G1 X175.514 Y91.178 E13.08066

G1 X180.401 Y93.467 E13.14833

G1 X184.697 Y96.479 E13.21413

G1 X188.52 Y100.302 E13.28193

G1 X191.514 Y104.573 E13.34734

G1 X1983.83 Y109.514 E13.41577

G1 X195.207 Y114.658 E13.48255

G1 X195.676 Y120.015 E13.54998

G1 X195.207 Y125.349 E13.61713

G1 X183.821 Y130.513 E13.68418

Figure 8 shows a flow chart of a method of controlling an FFF liquefier assembly according to an embodiment. The liquefier assembly to be controlled comprises a liquefier tube, a heater arranged to heat at least a part of the liquefier tube, a first sensor arranged to measure a first temperature at a first location of the liquefier tube, and a second sensor arranged to measure a second temperature at a second location of the liquefier tube downstream the first location.

The method 800 comprises obtaining 801 a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefier tube. The method 800 also comprises selecting 802 at least one of the first sensor and the second sensor depending on the first parameter and the second parameter, for control of the heater 5. The method may be performed by a processor arranged on a mainboard of the FFF printing device, optionally is cooperation with a processor arranged on the liquefier assembly installed in the FFF printing device. Alternatively, the method 800 method may be performed by a processor arranged on the liquefier assembly. The method 800 may also be performed by a software program (e.g. a slicing program) running on a processor arranged on a PC.

In view of the above, the present invention can now be summarized by the following embodiments:

Embodiment 1. A Fused Filament Fabrication system (1;20;1000), the system comprising: - a liquefier tube (2) having an inlet (21) and an outlet (22); - a nozzle (23) arranged at the outlet of the liquefier tube; - a heater (5) arranged to heat at least a part of the liquefier tube; - a first sensor (6) arranged to measure a first temperature at a first location of the liquefier tube; - a second sensor (7) arranged to measure a second temperature at a second location of the liquefier tube downstream the first location, and a controlling system (30;40) arranged to: - select at least one of the first sensor (6) and the second sensor (7) depending on a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefier tube; - use measurement data from the selected sensor(s) to control the heater (5).

Embodiment 2. The system according to embodiment 1, wherein the liquefier tube (2) comprises a heat break (24) separating the liquefier tube (2) into a hot-end and a cold-end, wherein the first sensor is located at an upstream outer end of the hot-end (26).

Embodiment 3. The system according to embodiment 1 or 2, wherein the second sensor is located at a downstream outer end of the liquefier tube (2).

Embodiment 4. The system according to any one of the preceding embodiments, wherein the system comprises a third sensor (8) arranged to measure a third temperature at a third location of the liquefier tube (2) between the first location and the second location.

Embodiment 5. The system according to any one of the preceding embodiments, wherein the controlling system (30;40) is arranged to select at least one of the first sensor (6) and the second sensor (7) further depending on a third parameter indicative of a flow resistance of the liquefier tube and the nozzle (23).

Embodiment 6. The system according to embodiment 5, wherein the third parameter is a nozzle orifice cross-section.

Embodiment 7. The system according to embodiment 5, wherein the third parameter is a cross-section of the liquefier tube (2).

Embodiment 8. The system according to any one of the preceding embodiments, wherein the controlling system (30;40) is arranged to select at least one of the first sensor (6) and the second sensor (7) further depending on a fourth parameter indicative of a trace type of print traces to be deposited.

Embodiment 9. The system according to any one of the preceding embodiments, wherein the second parameter is the feed rate of filament fed through the liquefier tube.

Embodiment 10. The system according to any one of the preceding embodiments, wherein the controlling system (30,40) is arranged to access a look-up table, the look-up table comprising information on which of the first sensor (6) and the second sensor (7) to select depending on at least one of the first parameter and the second parameter.

Embodiment 11. The system according to any one of the preceding embodiments, wherein the look-up table comprises a number of records, each of the records comprising a material type field, a feed rate range field, a first control field, and optionally a secondary control field.

Embodiment 12. The system according to any one of the embodiments 2-11, wherein the heater (5) is arranged along most part of the hot-end of the liquefier tube (2).

Embodiment 13. The system according to any one of the embodiments 1-12, wherein the

FFF system is an FFF printing device (1;100).

Embodiment 14. The system according to any one of the embodiments 1-12, wherein the

FFF system is an FFF liquefier assembly (20).

Embodiment 15. A method of controlling an FFF liquefier assembly, the liquefier assembly comprising a liquefier tube (2), a heater (5) arranged to heat at least a part of the liquefier tube, a first sensor (6) arranged to measure a first temperature at a first location of the liquefier tube, and a second sensor (7) arranged to measure a second temperature at a second location of the liquefier tube downstream the first location, the method comprising: - obtaining a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefier tube; - selecting at least one of the first sensor (6) and the second sensor (7) depending on the first parameter and the second parameter, for control of the heater (5).

Embodiment 16. A computing device (1;100;1000) comprising one or more processing units, the one or more processing units being arranged to perform the method according to embodiment 15.

Embodiment 17. A computer program product comprising code embodied on computer- readable storage and configured so as when run on one or more processing units (37;111) to perform the method according to embodiment 15.

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 as long as they are included in the scope of protection as defined in the appended claims. For example, instead of printing with thermoplastic material, other materials could be used such a filament of metal or glass. 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 within the scope of protection as defined in the appended claims. 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.

Claims

CONCLUSIONS

1. A Fused Filament Fabrication system (1;20;1000), the system comprising: - a liquefaction tube (2) having an inlet (21) and an outlet (22); - a nozzle (23) disposed at the outlet of the liquefaction tube; - a heater (5) configured to heat at least a portion of the liquefaction tube; - a first sensor (6) configured to sense a first temperature at a first location of the liquefaction tube; - a second sensor (7) configured to sense a second temperature at a second location of the liquefaction tube downstream of the first location, and - a control system (30;40) configured to: - select at least one of the first sensor (6) and the second sensor (7) in dependence on a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefaction tube; - using measurement data from the selected sensor(s) to control the heater (5).

The system of claim 1, wherein the liquefaction tube (2) comprises a heat break (24) dividing the liquefaction tube (2) into a hot end and a cold end, the first sensor being located at an upstream end of the hot end (26).

3. A system as claimed in claim 1 or 2, wherein the second sensor is located at a downstream end of the liquefaction tube (2).

4. A system according to any preceding claim, wherein the system comprises a third sensor (8) configured to measure a third temperature at a third location of the liquefaction tube (2) between the first location and the second location.

5. A system according to any preceding claim, wherein the control system (30; 40) is arranged to select at least one of the first sensor (6) and the second sensor (7) further depending on a third parameter indicative of a flow resistance of the liquefaction tube and the nozzle (23).

6. The system of claim 5, wherein the third parameter is a nozzle opening cross-section.

7. A system according to claim 5, wherein the third parameter is a cross-section of the liquefaction tube (2).

8. System according to any of the preceding claims, wherein the control system (30; 40) is arranged to select at least one of the first sensor (6) and the second sensor (7) further depending on a fourth parameter indicative of a trace type of print traces to be deposited.

9. A system according to any preceding claim, wherein the second parameter is the feed rate of the filament fed through the liquefaction tube.

A system according to any preceding claim, wherein the control system (30, 40) is adapted to access a lookup table, the lookup table comprising information on which of the first sensor (6) and the second sensor (7) to select depending on at least one of the first parameter and the second parameter.

11. A system as claimed in any preceding claim, wherein the lookup table comprises a plurality of records, each of the records comprising a material type field, an input speed range field, a first control field and an optional secondary control field.

A system according to any one of claims 2 to 11, wherein the heater (5) is disposed along the majority of the hot end of the liquefaction tube (2).

13. System according to any one of claims 1 to 12, wherein the FFF system is an FFF printing device {1;100).

The system of any one of claims 1 to 12, wherein the FFF system is an FFF liquefaction assembly (20).

15. A method of controlling an FFF liquefaction apparatus, the liquefaction apparatus comprising a liquefaction tube (2), and a heater (5) arranged to heat at least a portion of the liquefaction tube, and a first sensor (6) arranged to sense a first temperature at a first location of the liquefaction tube, and a second sensor (7) arranged to sense a second temperature at a second location of the liquefaction tube downstream of the first location, the method comprising: - obtaining a first parameter indicative of a print material type and a second parameter indicative of a flow rate through the liquefaction tube; - selecting at least one of the first sensor (6) and the second sensor (7) in dependence on the first parameter and the second parameter, for controlling the heater (5).

16. Calculating device (1;100;1000) comprising one or more processing units, the one or more processing units being adapted to perform the method according to claim 15.

17. A computer program product comprising code embodied on computer readable storage and configured to, when executed on one or more processing units (37; 111), perform the method of claim 15.