Patent Publication Number: US-2023139210-A1

Title: Pellet extruder for 3d printing

Description:
FIELD OF THE INVENTION 
     The present invention relates to a an extruder based print head for 3D printing, comprising a material feed unit for providing material in granular and/or liquid feed form, a material extrusion unit arranged to receive the material in granular and/or liquid feed form and transform the material into liquefied material (i.e. molten by temperature and/or pressure) in print form, and a nozzle unit in fluid communication with the material extrusion unit, the nozzle unit comprising an output channel and a nozzle (or output aperture) arranged to output the liquefied material. 
     BACKGROUND ART 
     International patent publication WO2018/086792 discloses a print head for a 3D printer with a feed for feedstock with variable viscosity, a melting zone comprising a temperature control element and an outlet opening for the liquid phase of said feedstock for conveying the feedstock from the feed zone into the melting zone comprises a plunger that can be inserted into said feed zone. 
     Chinese patent publication CN103692653B discloses a melt differential three-dimensional printer comprising a material melting unit, a droplet ejection unit, a cylindrical coordinate system forming unit and a frame. In the material melting unit, a servo motor drives a screw to rotate through a feeding port. Poured plastic pellets are mixed and sheared, and a heater fixed in a barrel to ensure the complete plasticization of the pellets through a temperature control, and the molten material is conveyed by the screw to the droplet ejection unit. In the droplet ejection unit, the molten material is transported to the valve body along a hot runner in a runner plate, driven by a linear servo motor. The linear servo motor drives a valve needle to reciprocate in the valve body, and the molten material is extruded out of a nozzle to form a melt droplet. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved print head for 3D printing of objects, which is based on extrusion of granular form feed material, e.g. pellets of a basic material, or liquid form feed material e.g. melted plastic. 
     According to the present invention, an extruder based print head as defined above is provided, further comprising an internal volume enlargement unit in fluid communication with the output channel of the nozzle unit, wherein the internal volume enlargement unit has a first internal volume in a first operational status and a second internal volume in a second operational status, wherein the second internal volume is larger than the first internal volume, and where the internal volume enlargement unit comprises an expansion volume within the material extrusion unit, and an actuator connected to an extrusion screw, which actuator is arranged to linearly move the extrusion screw within the expansion volume to retract the liquefied material into the expansion volume. 
     By creating an enlargement of the internal volume present in the printing head, any pressure in the liquefied material is taken away instantly, allowing the print head to be moved to a new print position without any dripping or stringing. 
    
    
     
       SHORT DESCRIPTION OF DRAWINGS 
       The present invention will be discussed in more detail below, with reference to the attached drawings, in which 
         FIG.  1    shows a cross sectional view of an extruder based print head for 3D printing according to an embodiment of the present invention; 
         FIG.  2    shows a cross sectional view of an extruder based print head for 3D printing according to a further embodiment of the present invention; 
         FIG.  3    shows a cross sectional view of an extruder based print head for 3D printing according to a multi-nozzle embodiment of the present invention; 
         FIG.  4    shows a schematic diagram of a control arrangement for the extruder based print head for 3D printing according to an embodiment of the present invention; 
         FIG.  5    shows a cross sectional view of an extruder based print head for 3D printing, according to an ‘Auger valve’ embodiment of the present invention, and 
         FIGS.  6 A-B  show a perspective view of a part of the material extrusion unit, according to two exemplary embodiments of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In 3D printing of plastics, the most popular technique known in the art is wire extrusion, more commonly known as Fused Deposition Modeling (FDM). In FDM, threads of solid plastic filament directly feed into the 3D printer heating chamber, where the plastic filament melts into a liquefied form. The liquefied plastic filament is compressed through a nozzle unit, and is forcibly dispensed onto a solid platform, where it re-solidifies. 
     In FDM, the desired object is typically built in layers. To print an object of a certain shape, the nozzle unit may have to move to a different location, causing the printing to stop. Typically, during the moving process, this leaves small strings of plastic behind due to plastic oozing out of the extrusion nozzle. This is known as stringing, and it may leave undesired strings of plastic on the finished object. To solve this issue, before subsequently moving to a different location for further printing, the plastic filament retracts back into the nozzle unit. This technique is commonly known as retraction, and as such is known in the art. 
     3D plastic printing is also possible using pellet extrusion. In this technique, instead of plastic filament thread, granules directly feed into the heating chamber. The advantage of pellet extrusion is that this does not limit granules to a plastic material, and metallic or ceramic compound materials may also be used. A further advantage is that plastic granules can be significantly cheaper than threads of plastic filament, saving printing costs. The disadvantage of pellet extrusion is that the liquefied material does not comprise a retraction mechanism as the pellets are gravity fed. This severely limits the scope of the object printing, since the nozzle unit cannot move to a different location without significant stringing. There is a need in the art to overcome this disadvantage. 
     The present invention embodiments provide solutions for obtaining retraction in a 3D printing system that employs the pellet extrusion technique. 
       FIG.  1    shows a cross sectional view of an extruder based print head  1  for 3D printing according to an exemplary embodiment of the present invention. A material feed unit  2  receives the material in granular and/or liquid feed form. The granular and/or liquid feed material passes through the material feed unit  2 , and enters a material extrusion unit  3 . Once liquefied, the material enters a nozzle unit  4  having an output channel  5  in fluid connection with a nozzle  6 . 
     In the embodiment shown in  FIG.  1   , the material extrusion unit  3  comprises a compression chamber  31 , an extrusion screw  32  and one or more temperature control elements  33 . The material extrusion unit  3  is arranged to compress the granular and/or liquid feed material within the compression chamber  31 . The temperature control element  33  is arranged to provide heat to the material extrusion unit  3 , and works in operative association with the compression chamber  31 . The temperature control element  33  may comprise, for example, heating and/or cooling elements at various positions in the material extrusion unit  3  (or even in further parts of the extruder based print head  1 , see below). The temperature control element is e.g. implemented as high-resistance wires near the output side of the material extrusion unit  3  as shown in the exemplary embodiment of  FIG.  1   , that heat to a high temperature when an electrical current is passed through them. Additionally or alternatively, the temperature control element  33  is implemented as channels with heating or cooling fluid connected to an external heating/cooling unit (not shown). 
     The temperature control element  33  may be positioned in different segments or zones, e.g. at the entrance of the granulate near material feed unit  2 , in the middle of the compression chamber  31  (typically for arranged for cooling) and/or at the bottom of the material extrusion unit  3 , below the compression chamber  31  and around flow channels  5  or nozzle  6  of the nozzle unit  4 . The material provided in granular feed form transforms into a liquefied print form in the material extrusion unit  3 , from heat provided by the temperature control element  33 , and/or the internal shear and compression provided by the compression chamber  31  and extrusion screw  32 . Herein after, the wording ‘liquefied material in print form’ may also be referred to as ‘liquefied material’. The liquefied material passes through the material extrusion unit  3 , and enters the nozzle unit  4 . Note that a further part of the temperature control element  33  may be arranged as a cooling unit blowing cooling air directed at the nozzle  6 . 
     It is noted for the material extrusion unit  3  receiving, from the material feed unit  2 , material provided in liquid feed form, heat from the temperature control element  33  and pressure from the compression chamber  31  and extrusion screw  32  may still be provided to further heat/melt the material in liquid feed form and e.g. change the viscosity and/or temperature thereof. In this respect, the material extrusion unit  3  may still modify the properties of the material provided in liquid feed form, and transform it to a liquefied (i.e. flowable) material in print form for 3D printing, and output the liquefied material in print form to the nozzle unit  4 . 
     As shown in the embodiment of  FIG.  1   , the nozzle unit  4  comprises an output channel  5  and nozzle  6 , arranged to further output the liquefied (i.e. flowable) material (in print form for 3D output printing). The extruder based print head  1  further comprises an internal volume enlargement unit  7  in fluid communication with the nozzle unit  4  and output channel  5 . The liquefied material enters the output channel  5  from the material extrusion unit  3 , and flows through the output channel  5  to the nozzle  6 . The liquefied material dispenses through the nozzle  6 . This allows printing of the desired object on a platform, with a continuous output flow of the liquefied material. 
     The internal volume enlargement unit  7  comprises an plunger channel  71  in fluid communication with the nozzle  6  and a plunger  72  moveably arranged within the plunger channel  71  for retracting the liquefied material. In a further embodiment, the plunger  72  comprises a valve device  73 , arranged to close off the output channel  5 , subsequently stopping the output flow of the liquefied material through the nozzle  6 . In one embodiment, the valve device  73  is the end part of plunger  72  which is arranged to move up and down in a housing of the internal volume enlargement unit  7 . Alternatively, the valve device  73  can withdraw out of the plunger  72 , and similarly, the valve device  73  can insert back into the plunger  72 . 
     In the embodiment shown in  FIG.  1   , the internal volume enlargement unit  7  has a first internal volume in a first operational status, where the plunger  72  is in the lower position P 1 , and a second internal volume in a second operational status, where the plunger  72  is in the middle position P 2 . The second internal volume is larger than the first internal volume. 
     In more general wording, the present invention embodiments relate to an extruder based print head for 3D printing comprising a material feed unit  2  for providing material in granular and/or liquid feed form, a material extrusion unit  3  arranged to receive the material in granular and/or liquid feed form and transform the material into liquefied material (e.g. molten by temperature and pressure) in print form, and a nozzle unit  4  in fluid communication with the material extrusion unit  3 , the nozzle unit  3  comprising an output channel  5  and a nozzle (or output aperture)  6  arranged to output the liquefied material. The extruder based print head further comprises an internal volume enlargement unit  7  in fluid communication with the output channel  5  of the nozzle unit  4 , wherein the internal volume enlargement unit  7  has a first internal volume in a first operational status and a second internal volume in a second operational status, wherein the second internal volume is larger than the first internal volume. A sudden local volume creation near the nozzle  6  takes away any pressure present in the output channel  5 , and prevents flow of molten material through the nozzle  6 . 
     In a further exemplary embodiment, the plunger  72  is initially in the lower position P 1 . The valve device  73  is implemented as the lower end part of the plunger  72 , which is movable up and down in a housing of the internal volume enlargement unit  7 . If the end of the valve device  73  is in (lower) position P 1 , it closes off the output channel  5 , and stops the output flow of the liquefied material through to the nozzle  6 . If the plunger  72  then moves in an upwards direction within the plunger channel  71  from the lower position P 1  until the valve device is just under the output channel  5  (middle position P 2 ), effectively an internal volume is added to the output channel  5  part towards the nozzle  6 , thereby retracting a small local volume of liquefied material in an upwards direction back into the plunger channel  71 . This releases any pressure present in the liquefied material close to nozzle  6 , and effectively prevents stringing. If the plunger  72  is moved further up, with valve device  73  in upper position P 3 , the extruder based printing head  1  is in an operational status, as liquefied material can flow though the output channel  5  to the nozzle  6 . 
     If the plunger  72  is moved with valve device  73  in position P 1 , and then to position P 2 , the extruder based print head  1  can move to a different location without any liquefied material dripping out of the nozzle  6 . Once in position, the plunger  72  can then move in a upward direction within the plunger channel  71  from the middle position P 2  to upper position P 3 . This re-opens the output channel  5  for output flow of the liquefied material through to the nozzle  6 , allowing the printing to resume in a controlled manner. 
     In a further embodiment, the flow of liquefied material through output channel  5  is initially stopped by halting the extrusion screw  32  of the material extrusion unit  3 , when the valve device  73  of the plunger  72  is in upper position P 3  (open output channel  5 ). In order to then take away pressure in the output channel  5  and prevent liquefied material from dripping out of nozzle  6 , the plunger  72  is further retracted into plunger channel  71 , until the valve device  73  is in position P 4 . This creates the internal volume enlargement which is sufficient to prevent stringing from the nozzle  6 . 
     In the embodiment shown in  FIG.  1   , the material extrusion unit comprises a cylindrical housing  35 , arranged to protect the material extrusion unit  3 . The compression of the granular material in the compression unit  3  generates substantial pressures that could be destructive to the extrusion unit  3 , which is less in this embodiment because of the cylindrical structure. The cylindrical housing  35  is e.g. made of a material that is able to sustain the pressures generated. 
     In a further alternative embodiment, the valve device  73  may comprise a small opening, arranged to restrict the output flow of the liquefied material. The small opening may comprise, for example, a hole with a specific diameter. The valve device  73  may be arranged to allow rotation thereof, such that either the small opening faces the output channel  5 , restricting the output flow of the liquefied material, or the small opening does not face the output channel  5 , completely stopping the output flow of the liquefied material. 
     In a further embodiment, the valve device  73  may also comprise a small hollow pathway within the body of the plunger  72 , arranged to restrict or stop the output flow of the liquefied material. 
     The opening on either end of the small hollow pathway is arranged between a side surface of plunger  72 , and an end surface of the plunger  72 . The small hollow pathway may comprise, for example, a I′ shaped hollow cylinder with a specific diameter, and is in operative association with the output channel  5 . Furthermore, two or even more pathways may be present with different internal diameters, with their respective openings exiting on the side surface of plunger  72 . The plunger  72  may also rotate such that either (one of) the opening(s) of the small hollow pathway on the side surface of the plunger  72  either faces the output channel  5 , restricting the output flow of the liquefied material, or the small opening does not face the output channel  5 , stopping the output flow of the liquefied material. The opening of the small hollow pathway on the end of the plunger would be in-between position P 2  and P 3  as shown in  FIG.  1   , and the end face of plunger  72  in position P 1 . Material extraction would be possible by rotating the plunger device  73  to stop output flow of the liquefied material, and moving the plunger  72  upwards to position P 2 , for extraction of the remaining liquefied material. Printing may resume by moving the plunger downwards towards position P 1 , and re-rotating the plunger for output flow of the liquefied material. 
       FIG.  2    shows a cross sectional view of an extruder based print head for 3D printing according to a further embodiment of the present invention. In this embodiment, the liquefied material dispenses through the nozzle  6 , but retracts into a different local volume than in the embodiments described above with reference to  FIG.  1   . 
     In the embodiment shown in  FIG.  2   , the internal volume enlargement unit  7  comprises an expansion volume S 1 -S 2  within the material extrusion unit  3  itself, and an actuator  34  connected to the extrusion screw  32  for rotation thereof to compress material towards output channel  5 . The internal volume enlargement unit  7  and material extrusion unit  3  are also arranged for retracting the liquefied material to prevent dripping. The actuator  34  is arranged to linearly move extrusion screw  32  within the expansion volume S 1 -S 2 . The internal volume of the compression chamber  31  with an end of the extrusion screw  32  at position S 2  is larger than the internal volume at position S 1 . 
     The liquefied material dispenses through the nozzle  6 . By using an actuator  34  to linearly move the extrusion screw  32  in a direction to expand the internal volume by moving the end face of extrusion screw  32  from position S 1  to position S 2 , the liquefied material present below the end of the extrusion screw  32  retracts into the expansion volume S 1 -S 2 . This effectively prevents stringing. 
     In the embodiment shown in  FIG.  2   , the extruder based print head  1  may then move to a different print location. By using an actuator  34  to linearly move the extrusion screw  32  in a direction to reduce the internal volume back again from position S 2  to position S 1 , and actuate the extrusion screw  32  for rotation thereof, the liquefied material compresses, allowing the printing of the desired object to resume in a controlled manner. 
     In the embodiment shown in  FIG.  2   , the actuator  34  may also be arranged to drive the extrusion screw  32  in a reversible rotation direction. The actuator  34  may be driven, for example, by a belt drive, or by using a secondary screw thread. A rotation of the extrusion screw  32  in a reversible rotation direction retracts the liquefied material back into the internal volume enlargement unit  7  then formed by the internal volume of compression chamber  31  (left of the material feed unit  2  in the embodiment shown in  FIG.  2   ). This prevents stringing, and the extruder based print head  1  may move to a different location for further printing. 
       FIG.  3    shows a cross sectional view of an extruder based print head for 3D printing according to a multi-nozzle embodiment of the present invention. In this embodiment, the liquefied material dispenses through multiple nozzles, allowing the retraction of the liquefied material in multiple nozzle units  4 ,  4 ′. Thus, in a further embodiment, one or more secondary nozzle units  4 ′ in fluid communication with the material extrusion unit  3  are provided. The multiple nozzles units  4 ,  4 ′ may comprise different nozzles  6 ,  6 ′, e.g. having different size apertures. A larger aperture improves printing speed for fast assembly of printed objects. A smaller aperture improves precision for providing fine printed objects. 
     In the embodiment shown in  FIG.  3   , the secondary nozzle units  4 ′ comprises secondary output channel  5 ′ and secondary nozzle  6 ′, arranged for output flow of the liquefied material. Each of the one or more secondary nozzle units  6 ′ further comprises of one or more secondary internal volume enlargement units  7 ′, in fluid communication with one or more secondary nozzle units  4 ′, one or more secondary output channels  5 ′, and the material extrusion unit  3 . 
     In the embodiment shown in  FIG.  3   , the one or more secondary internal volume enlargement units  7 ′ each comprises a secondary plunger channel  71 ′ and a secondary plunger  72 ′, arranged to retract the liquefied material. The secondary plungers  72 ′ comprises a secondary valve devices  73 ′, similar to the valve device  73  of the embodiments discussed above with reference to  FIG.  1   . 
     The one or more secondary internal enlargement units  7 ′ have a first operational status, where the one or more secondary plungers are in the one or more secondary lower positions P 2 ′, and a second operation status, where the one or more secondary plungers  72 ′ are in the one or more secondary upper positions P 3 ′. The second internal volume is larger than the first internal volume. 
     In the embodiment shown in  FIG.  3   , the retraction of the liquefied material in one or more of secondary nozzle units  4 ′ is identical to the retraction of the liquefied material in the nozzle unit  4  described in the first embodiment. Similarly, the operation of stopping the output flow in one or more of secondary nozzle units  4 ′ is identical to the operation of stopping the output flow in the nozzle unit  4  described in the first group of embodiments above having a single nozzle unit  4 . 
       FIG.  4    shows a schematic diagram of a control arrangement for the extruder based print head  1  for 3D printing according to an embodiment of the present invention. The control arrangement for the extruder based print head  1  comprises a control unit  8 , which is arranged to control the output flow of liquefied material. The control unit  8  connects to the material extrusion unit  3 , the internal volume enlargement unit  7 , and nozzle  6 . The control unit controls the nozzle  6  with one or more operational parameters associated with the material extrusion unit  3  and the internal volume enlargement unit  7 . 
     In an embodiment, the one or more operational parameters comprise a speed and/or direction control parameters, and/or heating/cooling control parameters of the material extrusion unit  3  and/or the nozzle unit  4 . The control unit  8  may control the speed and direction of the material extrusion unit  3  by, for example, changing the compression rate of the granular material in the compression chamber  31  and even retract by pumping the granular material back. The control unit  8  may control the heating function of the material extrusion unit  3 , by, for example, changing an amplitude of the electrical current through the high-resistance wires of the temperature control element  33 . 
     In a further embodiment, the one or more operational parameters further comprise an actuation of the internal volume enlargement unit  7  between the first and second operational status. The control unit  8  is e.g. arranged to control the valve device  73  and plunger  72 . This allows the control unit  8  to have the internal volume enlargement unit  7  to be in the first operational status, stopping the output flow of the liquefied material by withdrawing the valve device  73  out of the plunger  72 , and moving the plunger  72  upwards to extract the liquefied material. The control unit  8  can also equally have the internal volume enlargement unit  7  to be in the second operational status, restarting the output flow of the liquefied material by inserting the valve device  73  into the plunger  72 , and moving the plunger  72  downwards to recompress the liquefied material. 
     In the embodiment shown in  FIG.  4   , the control unit  8  is implemented as a multiple nozzle control unit which also connects to the material extrusion unit  3 , one or more secondary internal volume enlargement units  7 ′ and one or more secondary nozzles  6 ′. This allows the control unit  8  to control the output flow of the liquefied material from one or more secondary nozzles  6 ′, as discussed above with reference to  FIG.  3   . Furthermore, the control unit  8  may stop the output flow of the liquefied material from the nozzle unit  6 , and switch the output flow of the liquefied material to one of the one or more secondary nozzle units  6 ′. 
     In a further exemplary embodiment, a multiple nozzle control unit  8  is connected to the material extrusion unit  3  and the internal volume enlargement unit  7 . The multiple nozzle control unit  8  is arranged to switch between the nozzle unit  4  and one of the one or more secondary nozzle units  4 ′ by actuating the internal volume enlargement unit  7  to stop flow when switching nozzles. [claim  13 ] The internal volume enlargement unit  7  can be in fluid communication with the one or more secondary nozzle units  4 ′, or alternatively, each of the one or more secondary nozzle units  4 ′ is provided with a secondary internal volume enlargement unit  7 ′. In that embodiment, all secondary internal volume enlargement units  7 ′ are then also connected to the multiple nozzle control unit  8 , as shown in  FIG.  4   . 
       FIG.  5    shows a cross sectional view of an extruder based print head  1  for 3D printing, according to an ‘Auger valve’ embodiment of the present invention. Elements with the same function as in the embodiments shown in  FIGS.  1  and  2    are indicated by the same reference numerals. 
     In the Auger valve embodiment shown in  FIG.  5   , the material feed unit  2  may be connected, for example via a supply hose, to a syringe adapter assembly comprising a syringe barrel holding material in e.g. a liquid feed form. The syringe barrel may be e.g. permanently pressurized to provide material to the material feed unit  2 . The material may then be received by the material extrusion unit  3  and nozzle unit  4 , and dispensed through the nozzle  6 , as described herein. In general, the Auger valve embodiment may also allow for the correct dosing of material provided in e.g. liquid feed form, and high accuracy dispensing of the liquefied material in print form for 3D printing. 
     Similar to the  FIG.  2    embodiment described herein, in the Auger valve embodiment (shown in  FIG.  5   ), an expansion volume S 1 -S 2  is provided within the material expansion unit  3 , whereby the actuator  34  is connected to the extrusion screw  32  for rotation thereof to compress material towards output channel  5 , and linearly move the extrusion screw  32  within the expansion volume S 1 -S 2  to retract the liquefied material, thereby preventing stringing. Moreover, the actuator  34  may also be arranged to drive the extrusion screw  32  with a reversible rotation direction, as described herein. 
     It is noted that during operation of the actuator  34 , the extrusion screw  32  may first be driven in a reversible rotation direction and then, and at the same time, linearly retract within the expansion volume S 1 -S 2 , or vice-versa, to prevent stringing. 
     In addition, in an advantageous embodiment shown in  FIG.  5   , and also shown in a perspective view in  FIG.  6 A , the material extrusion unit  3  may comprise a sleeve  37 , wherein the sleeve  37  may be arranged to receive the material from the material feed unit  2 . 
     As shown in  FIGS.  5  and  6 A , the compression chamber  31  and extrusion screw  32  may be provided within the sleeve  37 , where, as shown in  FIG.  6 A , the sleeve  37  may comprise a funnel-shape body. From this standpoint, for material provided in liquid feed form to the material feed unit  2 , the funnel-shaped body of the sleeve  37  may allow for better output of the liquefied material to the nozzle unit  6 , and also allow better retraction of the liquefied material in the expansion volume S 1 -S 2 . 
     In the further advantageous embodiment shown in  FIG.  5   , and also shown in a perspective view in  FIG.  6 B , the material extrusion unit  3  may comprise a sealing element  38  connected to a part of the extrusion screw  32 . The sealing element  38  is connected to a top part of the extrusion screw  32 , e.g. above the threads of the extrusion screw  32  (as shown in  FIG.  6 B ). 
     The sealing element  38  is arranged to seal off a (top) part of the material extrusion unit  3 . Furthermore, during the linear movement of the extrusion screw  32  within the expansion volume S 1 -S 2 , the sealing element  38  is further arranged to press/compress against the liquefied material in the material extrusion unit  3 , thereby allowing for improved output of the liquefied material through to the output channel  5  in the nozzle unit  4 , and without any of the liquefied material leaking e.g. around the sides of the sealing element  38 . 
     From this perspective, the sealing element  38  may comprise a shape similar to the cross-section plane of the material extrusion unit  3 . To elaborate with a non-limiting example, if the material extrusion unit  3  comprises a cylindrical body, the sealing element  38  may comprise a disc-shaped element similar to cross-section of the cylindrical body. 
     According to an aspect of the present invention, a extruder based print head for 3D printing, as defined above, is provided, comprising a material feed unit for providing material in granular form, a material extrusion unit arranged to receive the material in granular form and transforming the material into liquefied material and a nozzle unit in fluid communication with the material extrusion unit, the nozzle unit comprising an output channel and a nozzle arranged to output the liquefied material. Furthermore, an internal volume enlargement unit is provided in fluid communication with the output channel of the nozzle unit, wherein the internal volume enlargement unit has a first internal volume in a first operational status and a second internal volume in a second operational status, wherein the second internal volume is larger than the first internal volume. 
     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.