Patent Publication Number: US-2017348911-A1

Title: Deposition print head

Description:
FIELD OF THE INVENTION 
     The invention relates to a deposition print head, a deposition print head assembly, a deposition printer and a method of controlling the deposition print head. 
     BACKGROUND OF THE INVENTION 
     Deposition print heads are known in printers for depositing fluid material on a solid surface. A well-known example is a deposition print head in a three dimensional printer which deposits a molten material to be deposited on a solidified body of the same or a similar material. While controlling the position of the deposition print head in space, the deposition print head can for example be used for creating three dimensional objects. 
     Deposition print heads known in the art can comprise a heating element, a feed connected to the heating element for the material to deposit, such as plastic material, and a nozzle connected to the heating element. The heating element can comprise a heat source and a heater body and a filament channel through the heater body from the feed to the nozzle for heating depositing material which is guided through the filament channel. 
     A problem with such a deposition print head is that the heating element body needs to be maintained at a suitable temperature so that the depositing material obtains the right consistency. The heating element body has a certain heat capacity and needs to be in a constant heated state while printing, even while intermittently printing. This causes undesired oozing of molten filament material and thereby fouling a deposition printer comprising the deposition print head. Furthermore continuous heating of filament material can cause material degeneration. 
     Moreover, intermittent operation of such a heater would cause a delay in operating the deposition print head, since if the heater is turned off during printing for saving energy, it needs to be reheated. Due to the heat capacity of the heating element body this would take some time. 
     Furthermore the heating element body needs a certain heat capacity in order to maintain its operating temperature, causing a minimal size for the heating element and thereby limiting a number of depositing print heads on a deposition print head support. Plural deposition print heads are desired for allowing an object to be printed in different colors and/or materials, without manual intervention for changing print heads. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to overcome the disadvantages in the deposition print heads in the art. 
     The object is achieved in a depositing print head comprising a non-susceptive or low susceptive sleeve, a susceptive element having a filament channel, the susceptive element arranged inside the sleeve, wherein the susceptive element is susceptive to a magnetic field. The filament channel is for feeding a thermoplastic filament in a feed direction. The depositing print head further comprises an exciter arranged around the susceptive element, wherein the exciter is arranged for generating a magnetic field compatible with the susceptive element. The depositing print head further comprises a nozzle attached to one end of the susceptive element. 
     By applying an alternating electric current or voltage to the exciter, the exciter is activated. It will generate an alternating magnetic field compatible with the susceptive element. 
     The magnetic field allows the susceptive element to be heated without direct wired contact to an electric power supply. 
     In an embodiment, the susceptive element comprises a ferromagnetic material having a field reluctance causing a hysteresis in response to the alternating magnetic field, causing the susceptive element to heat up. The heated susceptive element subsequently heats up the thermoplastic filament. By supplying sufficient energy to the exciter, the susceptive element reaches a temperature for melting the thermoplastic material of the filament. By feeding the filament through the filament channel, the molten filament material is extruded through the nozzle opening. The extruded molten filament material can be deposited on an object to be created, where it solidifies or cures. 
     The susceptive element has a low heat capacity, allowing fast response to excitation by the exciter. Thus deposition printing can be performed intermittently without continuously heating up the thermoplastic filament material. Exciting the susceptive element can be accurately timed while feeding the filament, thus preventing oozing or other filament material loss. 
     In an embodiment according to the invention, the susceptive element comprises a low conductive material. Heating is achieved by eddy currents in the susceptive element material. The use of a magnetic field allows advantageous use of skin effect on the susceptive element material such that the susceptive element heats up at its surface. 
     In a further embodiment, the low conductive material is applied at an inner wall of the susceptive element, thus allowing the generated heat to be transferred to the filament in the filament channel. 
     In an embodiment according to the invention, the exciter is arranged around the sleeve. This allows the exciter to be clear, i.e. insulated from the heated susceptive element. 
     In an embodiment according to the invention, the exciter comprises an induction coil wound around the sleeve. This allows a magnetic field to be generated inside the windings of the coil for exciting the magnetically susceptive element corresponding to the exciter. 
     In an embodiment according to the invention, the sleeve comprises an insulating, low susceptive material with high heat shock resistance. This allows intermittent operation of the deposition print head, intermittently exciting the susceptive element to a high temperature. 
     Preferably, the low susceptive material of the sleeve comprises quarts glass. 
     In an embodiment according to the invention, the sleeve comprises at least one additional susceptive element, the exciter overlapping the at least one additional susceptive element. Multiple susceptive elements allow the filament material to be subjected to a temperature profile. This allows for example to gradually increase the filament material to a melting temperature. 
     In an embodiment according to the invention, the deposition print head further comprises a spacing element between each pair of susceptive elements. This allows buffering of the thermoplastic filament material. The spacing element acts as a buffer to keep a continuous steady flow of filament material towards the nozzle. 
     In a further embodiment according to the invention, the spacing element comprises a temperature sensor. The temperature sensor allows the use of a control circuit to control a temperature of a susceptive element preceding or succeeding the spacing element in the feeding direction of the thermoplastic filament. 
     In an embodiment according to the invention, the exciter is subdivided in a portion per each susceptive element. This allows control of energy transfer per each portion of the exciter, such that a pre-defined temperature profile is achieved. 
     In an embodiment according to the invention the exciter has a different energy transfer ratio for each portion of the exciter. The exciter can be supplied as a single unit, where the portions each transfer a pre-determined amount of energy to the respective corresponding susceptive element. 
     In an embodiment according to the invention, the exciter has a different number of windings for each portion of the exciter. This is advantageous for an inductive susceptive element and exciter combination where the exciter is manufactured as an induction coil. Less windings causes a lower energy transfer, a higher number of windings causes a higher energy transfer level. 
     In an embodiment according to the invention, each exciter portion is separated from another exciter portion. This allows separate energy transfer control for each portion. Thus different temperature profiles can be generated for the filament while being fed through the deposition print head, without changing a hardware configuration of the printer. 
     In an embodiment according to the invention, each exciter portion has a separately controllable power supply. Induction coils can easily be controlled by varying at least one of current, voltage and frequency of the supply. 
     In an embodiment according to the invention, the nozzle comprises a susceptive material which is the same as the susceptive material of the susceptive element. This allows the nozzle to be heated by an extension of the exciter, or by a separate exciter. This helps the molten filament material to remain in a molten state until it is extruded from the nozzle. This also allows the deposition print head to start up by melting filament material inside the nozzle after a prolonged state of inactivity, when the filament material has already solidified. 
     In an embodiment according to the invention, the nozzle is attached to one end of the susceptive element. This prevents the molten filament material to leak towards the inside of the sleeve and/or to the outside of the deposition print head. 
     In another aspect of the invention, the object is achieved in a deposition print head assembly comprising a support structure and at least one deposition print head as described above. 
     In another aspect of the invention, the object is achieved in a deposition printer, comprising holding means for holding an article to be deposition printed, positioning means, and control means for controlling a position of the positioning means, and a deposition print head assembly as described above. The deposition printer further has a temperature controller for controlling at least one filament channel element temperature. 
     The object is also achieved according to another object of the invention in a method of deposition printing, comprising intermittently activating an exciter while feeding thermoplastic filament in a feed direction in a deposition print head as described above. 
     In an embodiment according to the invention, the deposition print head comprises a plurality of susceptive elements. The method further comprises activating the respective exciters for each susceptive element at a different energy level. 
     In an embodiment according to the invention, the method further comprises activating the respective exciters for each susceptive element at a different energy level according to a temperature profile. 
     In an embodiment according to the invention, the temperature profile has an ascending temperature slope and subsequently a descending temperature slope in the feed direction. This allows gradual and homogeneous heating of the filament material. 
     In an embodiment according to the invention, the method further comprises first preheating the thermoplastic filament to a gel temperature, second preheating the thermoplastic filament to a temperature above a melt temperature, third heating the thermoplastic filament to the melt temperature. This last step maintains the thermoplastic filament at the melt temperature. 
     In an embodiment according to the invention, the method further comprises buffering the thermoplastic filament between second preheating and the third heating. This allows the thermoplastic material of the filament to heat up in a portion of the filament channel while passing through a susceptive element to obtain a homogeneous and suitable viscosity in the spacing element. This compensates for irregularities of a cross section of the thermoplastic filament, which may not be constant. 
     In a further embodiment according to the invention, the method further comprises controlling a filament channel temperature. This allows accurate implementation of temperature profiles along the filament channel. This makes the deposition print head according to the invention versatile, adaptable and applicable for different filament materials 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    shows a cross section of a deposition print head according to an embodiment according to the invention. 
         FIG. 1 b    shows a side view of the deposition print head according to the invention. 
         FIG. 2 a    shows a susceptive element according to an embodiment according to the invention. 
         FIG. 2 b    shows a susceptive element and nozzle according to an embodiment according to the invention. 
         FIG. 2 c    shows a spacing element according to an embodiment according to the invention. 
         FIG. 2 d    shows a sleeve according to an embodiment according to the invention. 
         FIG. 3 a    shows a deposition print head according to the embodiment according to the invention. 
         FIG. 3 b    shows a side view of the deposition print head of  FIG. 3 a    according to an embodiment according to the invention. 
         FIG. 4 a    shows a cross section of a further embodiment of the deposition print head according to the invention. 
         FIG. 4 b    shows a side view of the deposition print head of  FIG. 4 a    according to an embodiment according to the invention. 
         FIG. 5  shows a deposition print head assembly according to an embodiment according to the invention. 
         FIGS. 6 a  and 6 b    show temperature profiles according to embodiments of the invention. 
         FIG. 7 a    shows an arrangement of parts of a deposition print head according to an embodiment of the invention. 
         FIG. 7 b    shows a side view of the arrangement of  FIG. 7   a.    
         FIG. 8  shows a block diagram of temperature control of a deposition print head according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 1 a    shows a cross section of a deposition print head  100  having a sleeve  102 , a susceptive element  103 , an exciter  105  and a nozzle  106 . The susceptive element  103  is arranged inside the sleeve  102 . The susceptive element  103  has a tubular shape leaving a channel  104  for allowing feed through of thermoplastic filament. The exciter  105  is arranged around the susceptive element  103 . The nozzle  106  can preferably be attached to the susceptive element  103 , but can also be attached to the sleeve  102 . The exciter  105  is connectable to an energy source such as an electric power supply, and when supplied it generates a field compatible with the susceptive element  103 . 
     The exciter  105  can be an induction coil combined with a ferromagnetic tubular element as susceptive element  103 . Ferromagnetic materials for the susceptive element  103  include iron, iron alloys. Also materials with low conductivity can be used such as steel, carbon, tin, tungsten, which cause heating up by means of eddy currents induced by the magnetic field from the exciter  105 . 
     When activated using an alternating voltage or current supply, the exciter generates an alternating magnetic field. The susceptive element  103  captures the alternating magnetic field. Due to hysteresis of the ferromagnetic material of the susceptive element  103  and/or the eddy currents as described, the susceptive element  103  heats up. The thus heated susceptive element  103  heats the thermoplastic filament fed through the filament channel  104  to a melting temperature of the thermoplastic material of the filament. The feeding causes sufficient pressure to cause the molten filament to be pressed towards the nozzle  106 , where it is extruded. By continuous positioning the deposition print head  100  according to a predetermined pattern, the extrusion can be used for depositing the molten thermoplastic material, which when allowed to solidify, forms small portions i.e. layers on an object to be formed. By gradually forming these layers a complete object can be formed. 
     A space  109  between the susceptive element  103  and the sleeve  102  allows reduction of heat transfer from the susceptive element  103  to the sleeve  102 . The induction coil  105  is made from windings of for example copper wire. In alternative embodiment, the coil  105  can be integrated in the sleeve. 
       FIG. 1 b    shows side view of the deposition print head  100  from  FIG. 1 . 
       FIG. 2 a    shows an outline of susceptive element  103 . The susceptive element  103  comprises a body having a bobbin shape, comprising a tubular portion  201  and at the both ends of the tubular portion  201  comprising rims  202 . The filament channel  104  has an inner diameter d 1 . The tubular portion  201  has an outer diameter d 2  and the rims  202  have an outer diameter d 3 , such that d 1 &lt;d 2 &lt;d 3 . 
     The tubular portion  201  and rims  202  can be manufactured from an insulating temperature shock resistant material such as quartz glass or a ceramic material, having an inner layer of the ferromagnetic and/or low conductive material as described. Alternatively, the tubular portion  201  and rims  202  can be manufactured from the ferromagnetic and/or low conductive material as described 
       FIG. 2 b    shows the susceptive element  103  of  FIG. 2 a    having the nozzle  106  attached to a lower end of the susceptive element  103 . This prevents leakage of molten thermoplastic filament material into the sleeve  102 . 
       FIG. 2 c    shows a spacing element  108  for spacing apart two susceptive elements  103  in the sleeve  102 . The spacing element has an annular shape and can be manufactured from the same material as the sleeve  102 . An inner diameter of the spacing element  108  matches the inner diameter d 1  of the susceptive elements  103 , whereas an outer diameter of the spacing element  108  matches the inner diameter d 3  of the sleeve  102 . 
       FIG. 2 d    shows a sleeve  102  having a tubular shape, and an inner diameter d 3  for receiving at least one susceptive element  103 . When multiple susceptive elements  103  are inserted in the sleeve  102 , these susceptive elements can be separated and spaced apart by spacing elements  108  between the susceptive elements  103 . 
     The sleeve  102  has an outer diameter d 4 &gt;d 3  providing sufficient wall thickness for thermal insulation of the susceptive element  103  to protect the surroundings of sleeve  102 . 
     The sleeve  102  is made of a thermally insulating and heat shock resistant material. Preferably quartz glass is used. Possible alternatives include ceramic material. 
       FIG. 3 a    shows a deposition print head  100  having a multiple susceptive elements  103  separated by spacing elements  108 . Susceptive elements  103 , the spacing elements  108 , sleeve  102 , exciter  105  and nozzle  106  are centered around a common filament channel  104 . The exciter  105  in  FIG. 3 a    can be subdivided in portions, each portion corresponding with a susceptive element  103 . 
       FIG. 3 b    shows the deposition print head  100  of  FIG. 3 a    in a side view, and with driver circuits  302  connected to the respective portions  301  of the exciter  105  of the deposition print head  100 . The portions  301  of exciter  105 , when electrically separated, allow individual excitation of these portions. In the case of the exciter  105  being formed by an induction coil, each coil corresponding to a portion can be activated at a different energy level by means of a dedicated driver circuit  302 . Such a driver circuit  302  can be an amplifier for amplifying an RF-signal. The amplified RF-signal is supplied to the portions  301  of the exciter  105  via electrical connection leads. The energy levels of the RF-signals supplied to the respective portions  301  of the exciter  105  are controllable by a control device (not shown), in accordance with a temperature profile. 
     RF-signals for exciting the ferromagnetic susceptive elements  103  can have a frequency in a wide range of hundreds of kilohertz to several Megahertz or tens of Megahertz, which frequency range depends on the material and layer thickness, and/or resistivity of the susceptive element material. 
       FIG. 4 a    shows a cross section of an alternative configuration for the deposition print head  100  having three susceptive elements  103 .  FIG. 4 b    shows a side view of the deposition print head  100  of  FIG. 4 a   . The susceptive elements  103  are arranged in the common sleeve  102  and separated by spacing elements  108  as in the deposition print head of  FIGS. 3 a  and 3 b   . From  FIGS. 4 a    and also  FIG. 4 b    it should be apparent that exciter  105  can be single coil wound around the common sleeve  102 . The induction coil shows a varying density of windings around the sleeve  102  for each susceptive element  103 . This allows a different energy level for each respective susceptive element  103  with a single exciter  105 . 
     In the example of  FIG. 4 b   , the lower portion  401  of the exciter  105  has the most windings for the corresponding susceptive element  103 , and will therefore have the highest energy transfer level compared to the middle portion  402  and upper portion  403 . It should be clear to the skilled person that the energy transfer level per portion  401 ,  402 ,  403  can also be the same for each portion  401 ,  402 ,  403 . The energy transfer level is chosen to accommodate a temperature profile to be created in a longitudinal direction along the deposition print head  100 . 
       FIG. 5  shows a deposition print head assembly having multiple print heads  100  mounted in a vessel  501 . The vessel  501  allows coolant  504  to be introduced via inlet  502  and to be discharged via outlet  503 . The coolant  504  includes water, oil, any other liquid. The coolant may also be a gas such as air. The coolant  504  allows the deposition print heads  100  to be operated during prolonged time periods. The vessel  501  can be mounted onto a platform which is connected to a positioning system of a deposition printer. 
       FIG. 6 a    shows a temperature profile of a deposition print head in feed direction  107 , x in  FIG. 6 a   , in accordance with one of the  FIGS. 3 a , 3 b , 4 a  and 4 b   .  FIG. 6 a    shows that while thermoplastic filament material passes through a first susceptive element  103 , a filament temperature increases in stage I to a first level indicated. This can be a pre-heating stage. Continuing to a second susceptive element  103  in stage II, the filament temperature is allowed to increase further. Passing by the last susceptive element  103  in stage III, the filament temperature reaches a maximum level. This is preferably a temperature above a melt temperature T m  of the thermoplastic filament material, allowing the filament material to be deposited by the nozzle  106  of the deposition print head  100 . 
     By varying the excitation, i.e. energy transfer, from the exciter  105  to the different susceptive elements  103 , the temperature level may vary indicated by the dotted line in  FIG. 6   a.    
       FIG. 6b  shows a more advanced temperature profile of a deposition print head in feed direction  107 , x in  FIG. 6 b   . Three pre-heating stages are shown indicated by I, II and III, allowing the thermoplastic filament feeding through the filament channel  104  starting from temperature T 0  to reach gel temperate Tg. Than in stage IV, the thermoplastic filament gel is heated to a temperature above the melt temperature Tm. This ensures that all of the thermoplastic filament material is molten when it reaches the last stage V where it is allowed to drop in temperature to the melt temperature Tm. Thus after stage V the thermoplastic filament material is homogenously molten before it exits the nozzle  106 . The various temperatures can be achieved by controlling the energy transfer levels between the exciter portions and respective susceptive elements  103  in the filament path towards the nozzle output in the feed direction  107 . A temperature drop can be realized by keeping the respective portion of the deposition print head at a predetermined temperature. Also a temperature drop can be realized by passing the filament through a spacing element which allows heat transfer from the filament channel  104  towards the sleeve  102 . 
       FIG. 7 a    shows an arrangement of susceptive elements  103  having two spacing elements  108  interposed between the elements  103  and sensing elements  701 . The sensing element comprises a temperature sensor  702 . All elements are normally arranged in a sleeve  102 , which is not shown in  FIG. 6   a.    
     The sensing elements  701  comprise an annular body  704  from for example the same material as the spacing elements  108  and have a filament channel  104  in communication with the filament channel  104  of the susceptive elements  103  and the spacing elements  108 . The temperature sensor  702  is positioned in a cavity  703  such that the sensor is in close proximity to the filament channel  104 . The temperature sensor  702  can be a resistive temperature device (RTD) such as for example a PT100 element. 
     The sensing elements  701  are suitable for measuring the filament temperatures T 1  and T 2  respectively. 
       FIG. 7 b    shows a cross section a sensing element  701  having a temperature sensor  702 . 
       FIG. 8  shows a block diagram of a process for controlling a temperature, e.g. T 1  of filament passing through the filament channel  104  of  FIG. 7 a    in a deposition print head as described above. 
     A set temperature  801  is compared with a measured temperature  812  by temperature sensor  811  corresponding to sensor  702  in  FIG. 7 a   . The difference temperature  803  is sent to a control unit  804  which converts the temperature difference in a control signal  805  which determines an energy level of an exciter  806 . An energy transfer  807  from exciter  806  to susceptive element  808  corresponding to a susceptive element  103  of  FIG. 7 a    causes the susceptive element  808  to heat up thermoplastic filament being fed through the filament channel of the susceptive element  808 . Temperature sensor  811  corresponding to temperature sensor  702  of  FIG. 7 a   , measure the temperature  810  (T 1 ) of the filament which passed through the susceptive element. 
     The embodiments described above are intended as examples only, not limiting the scope of protection of the claims as set out below. 
     REFERENCE NUMERALS 
     
         
           100  deposition print head 
           102  sleeve 
           103  susceptive element 
           104  filament channel 
           105  exciter 
           106  nozzle 
           107  feed direction 
           108  spacing element 
           109  space 
           201  body 
           202  rim 
           203  receiving space 
           301  additional heating element 
           302  driver 
           501  vessel 
           502  input 
           503  output 
           504  coolant 
           601  temperature profile 
           602  temperature profile 
           701  sensing element 
           702  temperature sensor 
           703  cavity 
           704  annular body 
           800  temperature control process 
           801  set temperature value 
           802  subtraction unit 
           803  temperature difference 
           804  control unit 
           805  excitation energy 
           806  exciter 
           807  heat transfer 
           808  susceptive element 
           809  hot filament feed 
           810  filament temperature 
           811  temperature sensor 
           812  measured temperature value