Patent Application
20150273582
The present disclosure relates to methods for printing three-dimensional (3D) parts with additive manufacturing systems. In particular, the present disclosure relates to methods for printing 3D parts with the use of magnetic support media.
Additive manufacturing systems (e.g., 3D printers) are used to print or otherwise build 3D parts from digital representations of the 3D parts (e.g., AMF and STL format files) using one or more additive manufacturing techniques. Examples of commercially available additive manufacturing techniques include extrusion-based techniques, jetting, selective laser sintering, powder/binder jetting, electron-beam melting, and stereolithographic processes. For each of these techniques, the digital representation of the 3D part is initially sliced into multiple horizontal layers. For each sliced layer, a tool path is then generated, which provides instructions for the particular additive manufacturing system to print the given layer.
For example, in an extrusion-based additive manufacturing system, a 3D part may be printed from a digital representation of the 3D part in a layer-by-layer manner by extruding a flowable part material. The part material is extruded through an extrusion tip carried by a print head of the system, and is deposited as a sequence of roads on a substrate in an x-y plane. The extruded part material fuses to previously deposited part material, and solidifies upon a drop in temperature. The position of the print head relative to the substrate is then incremented along a z-axis (perpendicular to the x-y plane), and the process is then repeated to form a 3D part resembling the digital representation.
In fabricating 3D parts by depositing layers of a part material, supporting layers or structures are typically built underneath overhanging portions or in cavities of 3D parts under construction, which are not supported by the part material itself. A support structure may be built utilizing the same deposition techniques by which the part material is deposited. The host computer generates additional geometry acting as a support structure for the overhanging or free-space segments of the 3D part being formed. Support material is then deposited from a second nozzle pursuant to the generated geometry during the printing process. The support material adheres to the part material during fabrication, and is removable from the completed 3D part when the printing process is complete.
The present disclosure is directed to a system for printing a 3D part using a layer-based additive manufacturing technique with the use of magnetic support media. The system includes a build chamber, a media hopper, and a transfer zone interconnecting the build chamber and the media hopper. The system also includes one or more print heads (e.g., extrusion and jetting heads) and/or development engines (e.g., electrophotography-based engines) for printing layers of a 3D part from one or more part materials. In coordination with the part material printing, the system includes a transfer mechanism (e.g., a media rake) that applies layers of the magnetic support media to the build chamber.
Furthermore, the system includes a plurality of electromagnets configured to generate a magnetic field in the build chamber. This magnetically couples the applied layers of the magnetic support media to produce a self-supporting bed of the magnetically-coupled media. In some aspects the magnetic support media includes a core of one or more ferromagnetic materials, and a shell of one or more materials that promote adhesion to the part material, and more preferably that are soluble in a solution or water. In further aspects, the build chamber, the media hopper, and the transfer zone are each heated to reduce curling and warping of the 3D part.
The present disclosure is also directed to a system for printing a 3D part, which includes a build chamber, a media hopper configured to retain a supply of a magnetic support media, and one or more print heads that are configured to print layers of the three-dimensional part from one or more part materials in the build chamber. The system also includes a planarizing mechanism configured to apply layers of the magnetic support media from the media hopper to the build chamber, and a plurality of magnets configured to generate a magnetic field in the build chamber.
The present disclosure is also directed to a method for printing a 3D part. The method includes generating a magnetic field in a build chamber of an additive manufacturing system, and printing layers of the 3D part in the build chamber. The method also includes transferring layers of a magnetic support media to the build chamber in coordination with the printing of the layers of the 3D part, and magnetically coupling the transferred layers of the magnetic support media in the build chamber with the generated magnetic field to produce a self-supporting bed of the magnetically-coupled media.
In some aspects, the method for printing a 3D part includes generating a magnetic field in a build chamber of an additive manufacturing system, and printing one or more layers of the 3D part in the build chamber. The method also includes transferring a layer of a magnetic support media to the build chamber such that the transferred layer of the magnetic support media is substantially level with the one or more printed layers of the 3D part. The method further includes magnetically coupling the transferred first layer of the magnetic support media in the build chamber with the generated magnetic field to produce a self-supporting bed of the magnetically-coupled media.
Unless otherwise specified, the following terms as used herein have the meanings provided below:
The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the present disclosure.
Directional orientations such as “above”, “below”, “top”, “bottom”, and the like are made with reference to a layer-printing direction of a 3D part. In the embodiments shown below, the layer-printing direction is the upward direction along the vertical z-axis. In these embodiments, the terms “above”, “below”, “top”, “bottom”, and the like are based on the vertical z-axis.
The term “build plane” refers to a plane in which the layers of a 3D part are arranged, and is perpendicular to the layer-printing direction of the 3D part.
The term “providing”, such as for “providing a consumable material”, when recited in the claims, is not intended to require any particular delivery or receipt of the provided item. Rather, the term “providing” is merely used to recite items that will be referred to in subsequent elements of the claim(s), for purposes of clarity and ease of readability.
The terms “about” and “substantially” are used herein with respect to measurable values and ranges due to expected variations known to those skilled in the art (e.g., limitations and variabilities in measurements).
The present disclosure is directed to a system and method for printing a 3D part using a layer-based additive manufacturing technique, where layers of the 3D part are supported by magnetic support media during the printing process. As briefly mentioned above, supporting layers or structures are typically built underneath overhanging portions or in cavities of 3D parts under construction, which are not supported by the part material itself. The support material itself is typically either solution or water soluble, such as disclosed in Hopkins et al., U.S. Pat. No. 8,246,888; or may have a structure that facilitates it to be broken away from the 3D part, such as disclosed in Crump et al., U.S. Pat. No. 5,503,785.
While these support materials provide many advantages for 3D printing applications, the soluble materials require time, solution chemicals, heat, and/or agitation to dissolve them away from the 3D part. Similarly, the break-away materials require labor and time to physically break them away from the 3D part. In either case, the support materials are typically not reusable, which can increase material costs, particularly in situations where the support material is more expensive than the part material.
In comparison, as shown in
Any suitable deposition-based additive manufacturing system may be used to print the 3D part, such as extrusion-based systems, jetting systems, electrophotography-based systems, and the like. System 10 is an example extrusion-based system for printing or otherwise building 3D parts using a layer-based, additive manufacturing technique. System 10 includes enclosed unit 12, which is preferably an enclosed environment having build chamber 14, media hopper 16, and transfer zone 18. Enclosed unit 12 may be heated (e.g., with circulating heated air) to reduce the rate at which the part materials solidify after being deposited (e.g., to reduce distortions and curling). Examples of suitable heating temperatures for enclosed unit 12 include those disclosed in Batchelder et al., U.S. Pat. No. 5,866,058.
As discussed below, the magnetic support media may include metallic materials having high thermal conductivities, and may form a bed that encases the 3D part. As such, when the magnetic support media are heated in enclosed unit 12, they can more evenly distribute the thermal gradients of the 3D part, which accordingly can reduce or eliminate curl in the 3D part.
In alternative embodiments, enclosed unit 12 may be omitted and/or replaced with different types of build environments. For example, a 3D part may be printed in a build environment that is open to ambient conditions or may be enclosed with alternative structures (e.g., flexible curtains).
Build chamber 14 is the region of enclosed unit 12 in which 3D parts (e.g., 3D part 20) are printed in a layer-by-layer manner. Media hopper 16 is a reservoir offset from build chamber 14 for storing a supply of the magnetic support media (referred to as media 22). Transfer zone 18 is the region of enclosed unit 12 that interconnects build chamber 14 and media hopper 16, and allows media 22 to be transferred from media hopper 16 to build chamber 14 in an incremental manner, as explained below.
At build chamber 14, system 10 also includes chamber walls 24, magnets 26, build platen 28, and platen gantry 30. Chamber walls 24 are the lateral walls and floor of build chamber 14, and are preferably fabricated from one or more non-magnetic materials that are capable of withstanding the elevated temperature of enclosed unit 12 (e.g., non-magnetic metals, such as stainless steel, aluminum, and the like). In some embodiments, chamber walls 24 may be multi-layered walls that include an inner surface layer or film of the one or more non-magnetic materials.
Magnets 26 are a series or assembly of electromagnets or other controllable magnets (e.g., actuatable magnetic chucks) connected to the external surface of chamber walls 24, preferably around the entire lateral perimeter of build chamber 14 (e.g., on all four walls 24 of build chamber 14). In some embodiments, magnets 26 may also be secured to platen 28, such as shown. In the shown embodiment, magnets 26 are configured to receive electrical currents from system 10 to generate the magnetic fields within build chamber 14.
In this embodiment, each magnet 26 may be a coil electrically connected to system 10 via one or more electrical lines (not shown). The coil may optionally be wrapped around a ferromagnetic material (e.g., steel) to increase the magnetic coupling strength. This embodiment is beneficial for generating a strong magnetic coupling during a printing process (by inducing an electrical current through the coils of magnets 26), while also allowing the magnetic field to be disabled or otherwise reduced after the printing process (by stopping the electrical current) to readily remove 3D part 20, as discussed below.
Platen 28 is a platform having platen surface 32 on which media 22 is transferred to, and on which 3D part 20 is printed, in a layer-by-layer manner. Platen 28 is supported by platen gantry 30, which is a gantry assembly configured to move platen 34 along (or substantially along) the vertical z-axis.
Platen 28 preferably has a sealed engagement with chamber walls 24 to prevent media 22 from spilling below platen 28, while also allowing platen gantry 30 to move platen 28 along the vertical z-axis. For example, platen 28 may include a flexible gasket perimeter that allows platen 28 to move, while also preventing passage of media 22.
In some embodiments, platen surface 32 may also include a removable substrate such as a flexible polymeric film or liner on which media 22 is transferred and/or 3D part 20 is printed, such as an adhesive tape, a painted-on layer of adhesive, a cardboard liner, or a build tray such as is disclosed in U.S. patent application Ser. No. 13/791,005. In one embodiment, platen 28 may be fabricated from, or platen surface 32 may include a layer or film of, one or more ferromagnetic materials. This allows media 22 to also magnetically couple to platen 28 when magnets 26 generate the magnetic field. This can assist in holding the magnetically-coupled media 22 to platen 28, preventing the magnetically-coupled media 22 from moving laterally in the build plane, and also to assist in pulling media 22 downward when platen 28 is lowered.
As further shown, system 10 also includes print head 34, media rake 36, y-axis gantry 38, x-axis gantry 40, and consumable assembly 42. In the shown example, print head 34 is an extrusion head configured to receive a consumable filament from consumable assembly 42 for printing 3D part 20. Examples of suitable devices for print head 34, and the connections between print head 34 and y-axis gantry 38 include those disclosed in Crump et al., U.S. Pat. No. 5,503,785; Swanson et al., U.S. Pat. No. 6,004,124; LaBossiere, et al., U.S. Pat. Nos. 7,384,255 and 7,604,470; Leavitt, U.S. Pat. No. 7,625,200; Batchelder et al., U.S. Pat. No. 7,896,209; and Comb et al., U.S. Pat. No. 8,153,182.
In additional embodiments, in which print head 34 is an interchangeable, single-nozzle print head, examples of suitable devices for each print head 34, and the connections between print head 34 and y-axis gantry 38 include those disclosed in Swanson et al., U.S. Pat. No. 8,419,996. In jetting-based systems, print head 34 may be an inkjet head such as described in Kritchman et al., U.S. Pat. No. 8,323,017. In alternative embodiments, print head 34 may be replaced with an electrophotography-based development, transfer, and/or transfusion system, such as disclosed in Comb et al., U.S. Publication No. 2013/0186549. In further embodiments, print head 34 may be replaced with a drop-on-demand print head.
While illustrated with a single print head 34, system 10 may alternatively include multiple print heads 34. For example, in some preferred embodiments, system 10 may also include a second print head 34 to deposit a soluble support material (e.g., as disclosed in Hopkins et al., U.S. Pat. No. 8,246,888), where the soluble support material may interface between the printed part material and magnetic media 22. This can be beneficial for preventing magnetic media 22 from becoming entrained in the printed part material at the surfaces of 3D part 20. Accordingly, system 10 may print the soluble support material to encase 3D part 20 at any location where magnetic media 22 may contact it.
Y-axis gantry 38 and x-axis gantry 40 are a pair of gantry assemblies configured to move the retained print head 34 and media rake 36 in the top region of enclosed unit 12. As shown, print head 34 is retained by y-axis gantry 38, which is supported by x-axis gantry 40 and is configured to move the retained print head 34 along the y-axis, as illustrated in
X-axis gantry 40 is also configured to move media rake 36 along the y-axis to transfer media 22 from media hopper 16, across transfer zone 18, and to build chamber 14. Accordingly, media rake 36 preferably extends across the entire width of enclosed unit 12 along the y-axis.
In the shown embodiment, print head 34/y-axis gantry 38 and media rake 36 are supported by the same x-axis gantry 40. In alternative embodiments, print head 34/y-axis gantry 38 and media rake 36 may be supported by separate x-axis gantries 40. In this case, a first x-axis gantry 40 may be used to move print head 34/y-axis gantry 38 in (or substantially in) the horizontal x-y build plane at the top end of build chamber 14, and a second x-axis gantry 40 may be used to move media rake 36 along the x-axis, across media hopper 16, transfer zone 18, and build chamber 14.
In either embodiment, print head 34 is preferably capable of being moved out of the way from media rake 36 when x-axis gantry 40 moves media rake 36 across build chamber 14. For instance, print head 34 may be configured to servo upward, allowing media rake 36 to pass below it. Alternatively, y-axis gantry 38 may be configured to move print head 34 along the y-axis to a parking location to avoid interference with media rake 36.
Consumable assembly 42 may contain a supply of a part material (e.g., a filament) for printing 3D part 20. Suitable part materials for 3D part 20 include those disclosed and listed in Crump, U.S. Pat. Nos. 5,121,329 and 5,340,433; Comb et al., U.S. Pat. No. 7,122,246; and Batchelder, U.S. Pat. Nos. 8,215,371; 8,221,669; and 8,236,227.
Media rake 36 is a planarizing transfer mechanism configured to transfer media 22 from media hopper 16, across transfer zone 18, and to build chamber 14. This applies a thin layer of media 22 onto the previously transferred and magnetically coupled media 22 in build chamber 14. Accordingly, media rake 36 may be any suitable transfer mechanism capable of transferring new media 22 across the top surface of the media 22 already retained in build chamber 14. In some embodiments, media rake 38 (or other suitable transfer mechanism) may also planarize the top surface of 3D part 20 and/or media 22 to assist in maintaining desired layer heights.
At media hopper 16, system 10 also includes hopper walls 44, elevator platen 46, and platen gantry 48. Hopper walls 44 are the lateral walls and floor of media hopper 16, and may also be fabricated from one or more non-magnetic materials that are capable of withstanding the elevated temperature of enclosed unit 12 (e.g., non-magnetic metals, such as stainless steel, aluminum, and the like). In some embodiments, hopper walls 44 may be multi-layered walls that include an inner surface layer or film of the one or more non-magnetic materials.
Elevator platen 46 is a second platform that is configured to move media 22 upward in an incremental manner. Elevator platen 46 is supported by platen gantry 48, which is a gantry assembly configured to move elevator platen 46 along (or substantially along) the vertical z-axis, generally parallel the movement of platen 28. This arrangement allows elevator platen 46 to provide a supply of media 22, which may be transferred across transfer zone 18 to build chamber 14 with media rake 36 (or other suitable transfer mechanism).
Elevator platen 46 preferably has a sealed engagement with hopper walls 44 to prevent media 22 from spilling below elevator platen 46, while also allowing platen gantry 48 to move elevator platen 46 along the vertical z-axis. For example, elevator platen 46 may also include a flexible gasket perimeter that functions in the same manner as discussed above for platen 28.
Media hopper 16 may be filled with a new supply of media 22 in any suitable manner. For instance, media hopper 16 may include a top port for pouring media 22 into media hopper 16. Alternatively, media 22 may be actively fed into media hopper 16 from a feed mechanism, or media hopper 16 (or a portion thereof) may be removable from system 10 and replaced with a filled media hopper 16. In yet a further alternative embodiment, media hopper 16 may be located on a top side of enclosed unit 12, allowing dosed amounts of media 22 to pour into transfer zone 18 under gravity.
The magnetic support media 22 may be derived from one or more ferromagnetic materials, such as iron, ferrite, nickel, and alloys thereof. Preferably, media 22 is provided as a powder or other small particulates that allow media 22 to be readily transferred from media hopper 16 to build chamber 14, and to achieve a thin layer with a smooth surface and a thickness corresponding to a sliced layer of 3D part 20. Furthermore, media 22 may have multiple particle sizes to provide a dispersion of smaller particles that fill in the interstitial spaces between larger particles.
In some embodiments, the one or more ferromagnetic materials may be provided as a core that is encased in one or more outer shells that promote adhesion to the part material. This adhesion is beneficial for reducing curl and/or warping of 3D part 20 by anchoring the layers of 3D part 20 during the printing process. Furthermore, in some embodiments, the shell(s) of media 22 are solution and/or water soluble to assist in removal of the portion of media 22 that adheres to the part material during the printing process. In these embodiments, the soluble materials (e.g., acid-functional polymers) may encase the ferromagnetic core material(s), and/or are modified for coupling to the ferromagnetic core material(s). These embodiments are particularly suitable for use in combination with a soluble support material that interfaces between 3D part 20 and media 22, allowing good adhesion to be achieved.
Transfer zone 18 is a bridging region between build chamber 14 and media hopper 16, which preferably has dimensions that prevent the generated magnetic field in build chamber 14 from affecting the media 22 in media hopper 16. Accordingly, transfer zone 18 includes transfer floor 50 between build chamber 14 and media hopper 16, which is a surface along which media 22 is transferred from media hopper 16 to build chamber 18.
Enclosed unit 12 may also optionally include region 12a extending beyond media hopper 16. Region 12a provides a suitable location for moving media rake 36 to initiate a transfer of coating of media 22 to build chamber 14. Region 12a may also provide a convenient location to park media rake 36 when not in use.
System 10 also includes controller assembly 52, which may include one or more control circuits (e.g., controller 54) and/or one or more host computers (e.g., computer 56) configured to monitor and operate the components of system 10. For example, one or more of the control functions performed by controller assembly 52, such as performing move compiler functions, can be implemented in hardware, software, firmware, and the like, or a combination thereof; and may include computer-based hardware, such as data storage devices, processors, memory modules, and the like, which may be external and/or internal to system 10.
Controller assembly 52 may communicate over communication line 58 with enclosed unit 12 (e.g., with a heating unit and/or air blower for chamber 12), magnets 26, platen gantry 30, print head 34, carriage 38, carriage gantry 40, platen gantry 48, and/or various sensors, calibration devices, display devices, and/or user input devices. While illustrated as a single signal line, communication line 58 may include one or more electrical, optical, and/or wireless signal lines, which may be external and/or internal to system 10, allowing controller assembly 52 to communicate with various components of system 10.
Prior to a printing process, controller assembly 52 may generate printing instructions for 3D part 20 based on a digital model of 3D part 20. For example, controller assembly 52 may slice the digital model into layers, generate tool paths for the sliced layers, and transmit the printing instructions to system 10.
Furthermore, as shown in
Controller assembly 52 may then direct x-axis gantry 40 to move media rake 36 along the x-axis in the direction of arrow 60 across media hopper 16, transfer zone 18, and build chamber 14. As shown in
Upon being applied to build chamber 14, the generated magnetic field preferably magnetically couples media 22 together to produce a self-support bed. As mentioned above, in some embodiments, media 22 may also magnetically couple to platen surface 32.
Controller assembly 52 then preferably directs x-axis gantry 40 to move media rake 36 back along the x-axis in the direction of arrow 62 to region 12a. After media rake 36 exits build chamber 14, controller assembly 52 may direct y-axis gantry 38 and x-axis gantry 40 to move the retained print head 34 around in the horizontal x-y plane in the top region of build chamber 14, above platen 28.
Controller assembly 52 may also direct print head 34 to draw successive segments of the consumable filaments from consumable assembly 42. Print head 34 then thermally melts the received successive segments such that the consumable filament becomes a molten material. The molten material is then selectively extruded from print head 34 and deposited onto the magnetically-coupled layer of media 22 to print one or more layers for 3D part 20, as shown in
After the layer is completed, controller assembly 52 may direct platen gantry 30 to lower platen 28 by one or more layer increments, as illustrated by arrow 64. Due to the non-magnetic material(s) of chamber walls 24, the magnetically coupled media 22 in build chamber 14 does not readily adhere to chamber walls 24, and lowers with platen 28. However, in some embodiments, magnets 26 at chamber walls 24 may be disabled when platen 28 is lowered to assist in having media 22 lower with platen 28.
Additionally, while print head 34 is printing the layer for 3D part 20, controller assembly 52 may also direct platen gantry 48 to raise elevator platen 46 to a suitable height, as illustrated by arrow 66. This positions the next amount of media 22 for transfer by media rake 36, as shown in
Controller assembly 52 may then again direct x-axis gantry 40 to move media rake 36 along the x-axis in the direction of arrow 60 across media hopper 16, transfer zone 18, and build chamber 14. As shown in
Upon being applied to build chamber 14, the generated magnetic field preferably magnetically couples the newly applied media 22 together with the previously-applied media 22 to grow the self-supporting bed in build chamber 14. Controller assembly 52 then again preferably directs x-axis gantry 40 to move media rake 36 back along the x-axis in the direction of arrow 62 to region 12a. After media rake 36 exits build chamber 14, controller assembly 52 may direct y-axis gantry 38, x-axis gantry 40, and print head 34 to print the next one or more layers of 3D part 20, as shown in
As can be appreciated from the above, system 10 encases 3D part 20 in a self-supporting bed of media 22. Metallic powder beds, such as found with media 22, can have much higher thermal conductivities than polymeric support materials. This can reduce curl of 3D part 20 since curl is driven by thermal gradients along 3D part 20. In fact, because the entire 3D part 20 can be held at the same temperature as it is built at (i.e., thermal gradients can be more evenly distributed), curl can be substantially reduced or even eliminated.
After the printing process is completed, controller assembly 52 may stop applying power to magnets 26, thereby disabling the magnetic field in build chamber 14. This allows the bed of media 22 to decouple back to the powder or particulate state. 3D part 20 may then be removed from the bed of powder/particulate media 22 and undergo one or more post-printing processes. For instance, after removal from system 10, the surface of 3D part 20 may be coated with a thin film of media 22 that is adhered to the printed layers of 3D part 20. In this case, the film may be removed using any suitable mechanism.
In one embodiment in which media 22 includes a shell of a solution or water soluble material, the printed 3D part 20 may be immersed in a bath, sprayed, or otherwise exposed to the solution or water. This dissolves the shell materials, causing the ferromagnetic cores of media 22 to fall away from the printed 3D part 20. The ferromagnetic cores of media 22 may be recollected and refurbished with new shell materials for use in subsequent printing processes with system 10, if desired.
The residual bed of media 22 in build chamber 14 that did not adhere to 3D part 20 may then be removed from build chamber 14 and directly reused in subsequent printing processes with system 10. This can provide significant cost savings to an operator of system 10, and reduces environmental waste, providing an eco-friendly support material. In one embodiment, the residual bed of reusable media 22 in build chamber 14 may be removed by opening a conduit at the base of build chamber 14, allowing the reusable media 22 to dump out of build chamber 14 into a collection reservoir (e.g., the same location as the overflow reservoir) (not shown). Alternatively, build chamber 14 may be tilted to pour any free media 22 back into media hopper 16.
Media 22 provides a convenient and efficient support material for printing 3D parts, such as providing support for overhanging portions and lateral wall support. Furthermore, each layer of media 22 can be applied to build chamber 14 at a relatively fast rate compared to additive manufacturing systems that print the support structures in a layer-by-layer manner. As such, system 10 can also substantially increase the printing throughput by eliminating the support material printing.
This is in addition to the relative fast removal of 3D part 20 from media 22. The ability to generate and disable the magnetic field in build chamber 14 allows the magnetic coupling of media 22 to be engaged during the printing process, and then disengaged after completion for removal of 3D part 20 with the substantial majority of media 22 being already detached. Any thin film of adhered media 22 may then be readily dissolved or otherwise removed from 3D part 20.
In a further aspect of the present disclosure, system 10 may be configured with one or more pick-and-place mechanisms, such as insert mechanism 68, as shown in
These inserted blocks 70 may also function in the same manner as media 22 for forming the self-supporting bed. When the magnetic field is disabled, blocks 70 may release from each other (and from media 22) and be reused in subsequent printing processes with system 10. While shown as cubic blocks in
Although the present disclosure has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.
