Indirect Metal Three-Dimensional Printing

Field

This invention relates to 3D printing of objects and molds. In particular, the invention relates to a method and apparatus for extruding viscous material feedstock in a 3D printing process, wherein the feedstock comprises a build material and a binder.

Background

Three-dimensional (3D) printing is an additive manufacturing process that is widely used in rapid prototyping. Unlike traditional subtractive processes (e.g., milling, turning, drilling, etc.), which involve cutting material away from a billet to create the final part, 3D printing is an additive process, which involves building a part by adding material layer-by-layer to create a final object. There are several 3D printing processes that are commonly used for creating prototypes from a polymer material. These processes include fused deposition modelling (FDM), stereolithography (SLA), and selective laser sintering (SLS).

Fused deposition modelling (FDM) involves extruding thermoplastic resin through a small nozzle and depositing it layer-by-layer to create a final object. Stereolithography (SLA) involves selectively curing a photopolymer resin with a UV laser layer-by-layer as the printed object is lifted out of the resin. Selective laser sintering (SLS) involves selectively melting a thermoplastic polymer powder with a laser layer-by-layer to create a final part.

The most significant drawback of such 3D printing technologies is that the strength of the 3D printed parts is limited by the strength and thermal properties of the polymer resin being printed. Consequently, polymer 3D printing technologies are not suitable for industrial testing or end-use applications (e.g., aerospace, automotive, etc.) that require parts with high strength and high temperature resistance.

Direct metal laser sintering (DMLS) is an additive manufacturing process that is used to create metal parts for high strength and high temperature applications. DMLS is a direct 3D printing process that uses a high-powered laser (typically a Yb-fiber optic laser) to locally melt metal powder to form a solid metal part layer-by-layer. A DMLS machine has a build platform coated with a thin layer of metal powder. Once the high-powered laser has selectively melted the metal powder on the build platform, the machine deposits a new layer of metal powder on top and the process repeats itself until a solid metal object is built layer-by-layer.

The DMLS process is able to create functional metal parts with high strength and high temperature resistance. However, it is prohibitively expensive due to the cost of the high- powered laser, the complex powder bed apparatus, and the highly refined metal powder material used in the process. Thus, the DMLS process is limited to high-end applications that can justify the costs involved.

To avoid the costs of the DMLS process, various indirect metal 3D printing processes can be used to create metal parts for high strength and high temperature applications. The two most notable indirect 3D printing processes are used in conjunction with either sand casting or bulk sintering processes to create a final metal part.

By combining sand casting with 3D printing, fully dense isotropic metal parts can be indirectly 3D printed. The 3D printing process can be used to create the sand casting mold layer- by-layer. Molten metal can then be poured into the 3D printed mold to form the final metal object. This indirect 3D printing process is based on the industrial sand casting process and uses the same sands used in the industry to print the molds. It does not require high-powered lasers or highly refined metal powders and is therefore much more affordable than the DMLS process.

By combining bulk sintering with 3D printing, nearly fully dense isotropic metal parts can be indirectly 3D printed. The 3D printing process can be used to print metal powder with a polymer binder layer-by-layer to create a "green" part with the shape of the final object. The green part can then be placed in a high temperature sintering furnace to remove the polymer binder and consolidate the metal powder to form the final object. This indirect 3D printing process is based on the industrial metal-injection-molding (MIM) process and can use the same readily available MIM metal powders. Since this process does not require highly refined metal powders or high-powered lasers, it is much more affordable than the DMLS process.

While these two indirect metal 3D printing processes offer alternatives to DMLS, they are still limited by the complexity of the currently available machines that are used to create metal parts using these processes.

Prior methods for using 3D printing to create casting molds involve powder bed systems similar to those used in SLS 3D printers. In these sand 3D printers, a thin layer of casting sand is spread over the build area and a print head then passes over the area and selectively applies a resin binder to that layer by jet-printing the binder onto the bed in a manner similar to that of a 2D inkjet printer. Another layer of casting sand is then spread over the previous layer and the process repeats itself until the metal casting mold is complete. The layers may be rapidly cured during the printing process after each individual layer is printed, and/or the printed mold may be cured after the printing process is complete to harden the binder. The printed mold can then be used in the casting process. The drawback of this method is that is requires complex apparatus to maintain the sand bed and curing system for the printing process, in addition to a specialized print head to facilitate the selective jet-printing of the polymer binder. This significant complexity increases the cost of these machines limiting the applicability of this process to a small range of high-end metal parts.

Prior methods for using 3D printing to create green parts for bulk metal sintering involve either the powder bed systems used in SLS 3D printers, or the extrusion systems used in FDM printers. These methods are outlined below.

In one method which utilizes a powder bed system, a thin layer of metal powder is spread over the build area and a print head then passes over the area and selectively applies a resin binder to that layer by jet-printing the binder onto the bed in a manner similar to that of a 2D inkjet printer. Another layer of metal powder is then spread over the previous layer and the process repeats itself until the green part is complete. The layers may be rapidly cured during the printing process after each individual layer is printed, and/or the part may be cured after the printing process is complete to harden the binder. The printed green metal powder part can then be placed in a furnace to complete the bulk sintering process. The drawback of this method is that is requires complex apparatus to maintain the powder bed and curing system for the printing process, in addition to a specialized print head to facilitate the selective jet-printing of the polymer binder. This significant complexity increases the cost of these machines limiting the applicability of this process to a small range of high-end metal parts.

In a second method which utilizes an extrusion system, metal powder is first impregnated with a polymer resin material in the form of rods which serves as the feedstock. This feedstock is then processed through a nozzle and extruded in a similar way as in FDM 3D printing in order to form the shape of the green part layer-by-layer. The green part may then be post-processed to remove the binder, before it is placed in a sintering furnace where the remaining binder is removed and the metal powder is consolidated into a solid metal object. The drawback of this method is that is requires complex apparatus to utilize the metal powder impregnated with polymer resin in the form of individual rod feedstock. The machine must be designed to autonomously feed the rod feedstock into the extrusion apparatus during the printing process after the previous rod is extruded. The process used to fabricate the rod feedstock, along with the feed mechanism and controls required for handling this rod feedstock, increases the complexity of these machines, which in turn increases the cost, limiting the applicability of this process to a small range of high-end metal parts.

Summary

According to one aspect of the invention, there is provided an extrusion apparatus for fabricating a three-dimensional (3D) object from a viscous material feedstock, comprising: a nozzle that deposits the viscous material to build the 3D object from the deposited viscous material; a viscous material feedstock delivery system that delivers the viscous material to the nozzle; and a curing apparatus associated with the nozzle that produces curing energy and focusses the curing energy at the nozzle tip to initiate curing of the viscous material as it exits the nozzle.

In one embodiment, the delivery system comprises at least one pump that pumps the viscous material feedstock and at least one actuator that pressurizes the extrusion apparatus.

In one embodiment, the delivery system comprises at least one syringe pump.

In various embodiments, the actuator may operate mechanically, pneumatically, or hydraulically, to facilitate flow of the viscous material.

In one embodiment, the actuator comprises a motor and lead screw.

In one embodiment, the delivery system comprises at least two pumps and a mixing component; wherein a first pump pumps a build material and a second pump pumps an additive for the build material; and wherein the mixing component receives and mixes the build material and the additive to provide the viscous material.

The object may be a metal powder green object, a casting mold, or a component of a casting mold.

In various embodiments, the additive is a binder comprising a photo-initiator and/or a thermo-initiator. In one embodiment, the additive is a binder comprising a two-part epoxy including a resin and a hardener.

In one embodiment, first and second parts of the epoxy are delivered independently by first and second pumps, and the parts of the epoxy are mixed with the build material to form the viscous material feedstock.

In one embodiment, the apparatus further comprises first and second actuators that pressurize the first and second pumps, respectively, to generate a selected mix ratio for the epoxy components.

In one embodiment, outputs of the at least two pumps are connected to inputs of a merging chamber of a nozzle, wherein at least two pressurized viscous materials are merged into a single viscous material flow.

In various embodiments, dimensions and/or relative angles of input directions of each of the inputs of the merging chamber are selected to reduce or minimize pressure build up in the viscous material flow, to ensure steady flow of material into the nozzle.

In one embodiment, the relative angles are less than or equal to 60 degrees.

In one embodiment, a length of the nozzle orifice is between lx and 2x the nozzle orifice diameter to prevent pressure buildup of the viscous material while minimizing die swell.

In various embodiments, the nozzle comprises a static mixing chamber with a selected internal geometry, and/or an active mixing chamber that is externally actuated, to facilitate mixing of the viscous materials.

In one embodiment, the viscous material flow exits a nozzle orifice in a controlled manner coordinated with movements of a 3D printer head in X, Y, and Z axes to build a sand casting mold or metal powder green object layer-by-layer via a 3D printing process.

In one embodiment, the apparatus further comprises a pump that delivers a non-reactive fluid material to an input of the nozzle.

In one embodiment, the apparatus further comprises a pump that delivers a support material to an input of the nozzle.

In various embodiments, the curing apparatus comprises at least one curing device that emits light and/or heat at the nozzle tip where the viscous material exits the nozzle output orifice. In one embodiment, the curing apparatus includes a cooling apparatus that prevents the at least one curing device from exceeding an operating temperature range during the printing process.

In one embodiment, the cooling apparatus includes a temperature sensor and an electronically controlled feedback loop, by which the temperature range of the at least one curing device is maintained.

In various embodiments, the nozzle has a geometry that allows light and/or heat produced by the curing apparatus to be applied directly to the nozzle tip.

Embodiments may be integrated into a 3D printer.

In one embodiment, the apparatus comprises a post-curing apparatus associated with a chassis of the 3D printer, wherein the post-curing apparatus directs light and/or heat at a build area of the 3D printer.

In one embodiment, the apparatus comprises a designated surface area of the 3D printer chassis for mounting and dissipating heat.

In one embodiment, a chassis of the 3D printer includes an enclosure with an array of mountings for securing heating devices and insulating an internal volume of the 3D printer.

According to another aspect of the invention there is provided method for fabricating a three-dimensional (3D) object, comprising: extruding a viscous material feedstock through a nozzle, wherein the nozzle deposits the viscous material to build the 3D object; and exposing the viscous material to curing energy at a nozzle tip to initiate curing of the viscous material as it exits the nozzle.

In one embodiment, the viscous material feedstock comprises at least two components, wherein a first component is a build material selected from casting sand and metal powder, and a second component is at least one binder.

In various embodiments, the object is a metal powder green object, a casting mold, or a component of a casting mold.

In one embodiment, the binder is a resin comprising a photo-initiator and/or a thermo- initiator.

In one embodiment, the binder is a two-part epoxy comprising a resin and a hardener. In one embodiment, the method comprises mixing the build material and the at least one binder to produce the viscous material feedstock. In one embodiment, each part of the epoxy is independently mixed with the build material to form two independent viscous materials, which are then mixed together to build the 3D object.

In one embodiment, mixing of the independent viscous materials is controlled by controlling their relative delivery rates to generate a selected extrusion ratio according to a desired mix ratio of the two epoxy parts.

One embodiment comprises delivering each component of the viscous material feedstock using a separate delivery apparatus; wherein outputs of each delivery apparatus are connected to inputs of a merging chamber of the nozzle; wherein an output of the merging chamber is mixed in the mixing chamber into a single viscous material flow that is deposited by the nozzle.

According to various embodiments, dimensions of nozzle features and/or relative angles of input directions of merging chamber inputs are selected to reduce or minimize pressure build up in the viscous material flow, to ensure steady flow of material into the nozzle.

In one embodiment, the relative angles are less than or equal to 60 degrees.

In one embodiment, wherein a length of the nozzle orifice is between lx and 2x a nozzle orifice diameter to prevent pressure buildup of the viscous material while minimizing die swell.

In various embodiments, the mixing chamber comprises helical internal geometry, and/or the mixing chamber is externally actuated, to facilitate mixing of the viscous materials.

Embodiments may be implemented using a 3D printer.

In various embodiments, the viscous material exits a nozzle orifice in a controlled manner coordinated with spatial movements of the 3D printer in the X, Y, and Z axes to build the sand casting mold or metal powder green object layer-by-layer.

In one embodiment, a component of the viscous material feedstock comprises a non- reactive fluid material and the method comprises delivering the non-reactive fluid material to the nozzle. The method may comprise purging residual viscous material in the nozzle during material transitions during 3D printing.

In one embodiment, a component of the viscous material feedstock comprises a support material, and the method comprises delivering the support material to the nozzle.

In various embodiments, the curing energy comprises light and/or heat.

In one embodiment, the binder is a resin comprising a photo -initiator and the curing energy is light. In one embodiment, the binder is a resin comprising a thermal initiator and the curing energy is heat.

In one embodiment, the method comprises disposing at least one curing device with the nozzle, wherein the curing device focusses curing energy at a nozzle tip where the viscous material exits a nozzle output orifice.

In one embodiment, the method further comprises preventing a temperature of the at least one curing device from exceeding an operating temperature range during printing.

In one embodiment, the method further comprises directing curing energy at a build area of the 3D printer.

Brief Description of the Drawings

For a greater understanding of the invention, and to show more clearly how it may be carried into effect, embodiments will be described with reference to the accompanying drawings, which are intended for illustration purposes only, and do not limit the scope of the invention. In the drawings:

FIG. 1 shows a schematic diagram of an extrusion apparatus for viscous material feedstock in accordance with an embodiment of the invention.

FIG. 2 shows a schematic diagram of a nozzle apparatus secured to a 3D printer apparatus in accordance with an embodiment of the invention.

FIG. 3 shows a schematic diagram of an alternative extrusion apparatus in accordance with another embodiment of the invention.

FIG. 4 shows a schematic diagram of a merging chamber in accordance with an embodiment of the invention.

FIG. 5 shows a schematic diagram of a static helical mixing chamber in accordance with an embodiment of the invention.

FIG. 6 shows a schematic diagram of an externally actuated active mixing chamber in accordance with an embodiment of the invention.

FIG. 7 shows a schematic diagram of a nozzle tip, a rapid-curing apparatus, and a cooling apparatus in accordance with an embodiment of the invention. Detailed Description of Embodiments

The apparatus and methods described herein overcome the drawbacks and limitations of the prior art by providing improvements that reduce the complexity and increase the utility of indirect 3D printing for fabricating casting molds, components of casting molds, or green objects or parts for bulk metal sintering.

According to one aspect of the invention, there is provided a feedstock extrusion apparatus, wherein the feedstock comprises a build material (e.g., casting sand, metal powder, etc.) together with a binder and any other additives, in the form of a fluid viscous material. The feedstock extrusion apparatus includes components for storing, optionally mixing, and delivering the fluid viscous material to one or more nozzle. Such components include a pump that pumps the viscous material and an actuator that pressurizes delivery of the feedstock. The pump may be a syringe pump (e.g., comprising a piston and cylinder). The actuator may operate mechanically (e.g., using a motor and lead screw), pneumatically, or hydraulically, to facilitate flow of the viscous material. The output of the syringe pump may be connected to the nozzle either directly or remotely (e.g., via tubing) to allow the pressurized viscous material to flow through the nozzle. The extrusion apparatus deposits the viscous material through the nozzle layer-by-layer to build up an object using a 3D printing process.

The binder used in the viscous material may be a resin containing a photo-initiator and/or a thermal initiator. The binder is activated by light and/or heat, as appropriate, immediately as it exits the nozzle to enable rapid curing of the viscous material as it is extruded. This gives the object sufficient green strength during 3D printing, after which the green object may be post- cured by light and/or heat to further solidify the object. Post-curing may not be necessary in certain use cases if the printed object can be sufficiently cured during printing. At this point, cured sand molds will be ready for casting and cured metal powder green parts will be ready for debinding and sintering.

Alternatively, the binder used in the viscous material may be a two-part epoxy consisting of the resin and hardener. Each of the epoxy components may be independently mixed with the build material to form two independent viscous materials, each with a different component of the binder. According to this embodiment, components of the extrusion apparatus may provide for mixing by, for example, containing the independent viscous materials within independent syringe pumps and either two independent (i.e., respective) actuators, or a single actuator. For embodiments configured with two independent actuators, the relative extrusion of each syringe pump may be controlled independently for each actuator through software to generate an extrusion ratio proportional to the required mix ratio of the epoxy components. For embodiments configured with a single actuator, the relative extrusion of each syringe pump may be controlled using a single actuator with a mechanical, pneumatic, or hydraulic coupling, utilizing a pressure ratio between the two syringe pumps to generate an extrusion ratio proportional to the required mix ratio of the epoxy components. For both epoxy extrusion apparatus configurations, both syringe pumps may be simultaneously actuated to extrude the viscous materials containing each epoxy component.

The output of each epoxy syringe pump may be connected, either directly or remotely

(e.g., via tubing), to a nozzle which may contain a merging chamber with multiple inputs to allow the multiple pressurized viscous material flows to converge into a single viscous material flow. This nozzle may contain a static mixing chamber with helical internal geometry, and/or an active mixing chamber that is externally actuated, to facilitate mixing of the two-part epoxy components in the viscous materials. After passing through the mixing chamber, the viscous material flow then exits the single nozzle orifice in a controlled manner to build the sand casting mold or metal powder green part layer-by-layer. The final 3D printed object may then be post- cured to further solidify the object. Post-curing may not be necessary in certain cases if the printed object can be sufficiently cured during printing. At this point, cured sand molds will be ready for casting and cured metal powder objects will be ready for debinding and sintering.

In embodiments where a two-part epoxy resin is used as the binder, care should be taken to ensure that the chemical reaction between each part of the epoxy is not occurring in the mixing chamber while the extrusion system is inactive for a prolonged period of time (i.e., when the printer is not is use). In order to prevent unwanted chemical reactions in the mixing chamber which would lead to solidification of the viscous build material and clog the nozzle, an additional non-reactive fluid material (e.g., water) may be used to dilute and/or inhibit the chemical reaction when the extrusion system is inactive for a prolonged period of time. The inhibitor fluid may be implemented using the same type of extrusion apparatus described above for the fluid viscous materials, or it may be implemented using an auxiliary extrusion system. The reaction inhibiting fluid may feed into the same multi -input nozzle used for the two-part build material. Once a print is finished, the extrusion system containing the reaction inhibiting fluid is actuated and the fluid purges the remaining two-part build material from the mixing chamber to prevent the chemical reaction from clogging the nozzle during the prolonged period of inactivity.

Embodiments of the viscous material extrusion apparatus and methods described herein provide a solution for the use of support material in indirect metal 3D printing processes. The use of support material enables greater design freedom for the 3D printed objects, allowing a 3D printer to produce sand casting molds and metal powder green objects with steep overhangs and unsupported geometry. Embodiments of the extrusion apparatus may also be applied to the support material required for each process. The support material (e.g., ceramic powder, sodium silicate powder, PVA powder, etc.) can be mixed together with a binder and any other additives necessary to take the form of a fluid viscous material. The viscous material serves as the support material feedstock. The binder may be, for example, a photopolymer/thermosetting resin, or a two-part epoxy resin. In one embodiment the support material extrusion apparatus may be substantially identical to the extrusion apparatus described above for the binder system that is used.

Each type of indirect 3D printing process utilizes a different support material. For a bulk sintering process, two different approaches may be used for the support material. One approach employs a high temperature refractory ceramic powder (e.g., alumina powder, zirconia powder, etc.) or a high temperature refractory sand powder (e.g., sodium silicate glass powder, etc.) mixed with a binder for use as a viscous support material. The support material may be printed/cured in the same manner as the build material. Once the green metal powder object is printed, the entire object together with the support structure still attached may be placed in the sintering furnace at the sintering temperature required for the metal powder build material. The metal sintering temperature is lower than the sintering temperature required for the refractory ceramic/sand powder support structure. Consequently, as the binder is removed and the metal powder consolidates into a solid object, the support structure will not reach a high enough temperature for the refractory powder to consolidate and will simply be reduced to powder as the binder is removed.

Another approach to generating support material for a bulk sintering process employs PVA powder (or any other water soluble polymer powder) mixed with a resin binder for use as a viscous support material. This support material may be printed/cured in the same manner as the build material. Once the green metal powder object is printed, the entire object together with the support structure still attached may be placed in a water bath to dissolve the PVA powder (or equivalent water soluble polymer powder) support structure, prior to the final sintering process at high temperature.

The use of water soluble polymer powder as the base material incorporated into the viscous support material may also be applied to a sand casting indirect 3D printing process because it does not require the debinding step of the bulk sintering process which is often done at high temperature in a sintering furnace. The steps involved are substantially identical to those described above. Once the water soluble polymer supports are dissolved in the water bath and the 3D printed sand casting mold is completely dried, it is ready for the casting process.

In embodiments where a viscous support material is used in conjunction with the viscous build material, a multi-input single-output nozzle, or two independent nozzles, must be used to accommodate this.

In embodiments where a multi-input single-output nozzle is used, the nozzle geometry is configured in such a way as to merge the viscous material flows from the multiple inputs into a single flow that exits through the nozzle orifice. The term "merging chamber" is used herein to refer to a portion of the nozzle that receives the multiple inputs and has an internal geometry that facilitates converging of the multiple viscous flows into a single output. For example, the merging chamber may have an internal geometry that improves the converging of the multiple flows by reducing resistance to flow. In some embodiments, a static mixing chamber or an actuated active mixing chamber is used in conjunction with the merging chamber. The static mixing chamber has a selected internal geometry that facilitates mixing of the merged viscous flows, such as ridges, bumps, splines, etc., or combinations thereof, in a selected or random arrangement. For example, in one embodiment, the static mixing chamber has ridges arranged in a helical pattern. In such embodiments, the viscous material that flows from the multiple inputs converges in the merging chamber and then flows into the mixing chamber. The viscous material is mixed in the mixing chamber, and then flows out of the nozzle orifice.

As the object is built layer-by layer through the 3D printing process, the machine switches between the build material and the support material. During each switch, it should be noted that there will be a volume of the previously printed material remaining in the merging chamber and mixing chamber in the nozzle. This residual material will need to be purged during the material switch. This purging may be incorporated into the 3D printing process as a purge tower printed adjacent to the object, or it can be extruded at a designated purging zone adjacent to the build area where a receptacle may be used to contain the purged material from each transition.

In the design of the multi-input single-output nozzle, the internal volume of the merging chamber may be minimized in order to minimize the amount of material that must be purged on each transition. This results in a tradeoff in the mixing chamber design. A longer mixing chamber will enable more thorough mixing of the viscous materials, especially if a static helical mixing chamber is used. However, a longer mixing chamber will also result in a larger mixing volume that needs to be purged on each transition from the build material to the support material, and vice versa. This larger mixing volume will increase the print time and material waste for each printed object. The length of the mixing chamber may be optimized based on the minimum length required to provide sufficient mixing of the viscous materials passing through the nozzle. This ensures consistent material flow and effective curing of the binder, and minimizes the printing time and material waste.

Alternatively, if two independent nozzles are used, their respective offsets in the X-axis,

Y-axis, and Z-axis must be compensated for in the control software. It should be noted that such a dual nozzle configuration offers an alternative solution if a water soluble support material is required. Rather than employing the viscous material extrusion technology as described above, the water soluble support material may be printed using an FDM print head that uses water soluble thermoplastic filament as the support material feedstock.

With regard to the mechanical design of the extrusion apparatus for viscous material extrusion technology, the materials used for the syringe pump, tubing, and nozzle apparatus should be appropriately selected for the application. These components may be opaque so as to prevent transmission of light which could result in premature curing of any photopolymer resin binder component of the viscous material contained within. In addition, these components may be thermally insulated from any nearby heat sources so as to prevent premature curing of any thermosetting resin binder component of the viscous material contained within.

Due to the substantial pressure required to extrude the viscous materials used in this process, the materials and seals used in the extrusion apparatus should be capable of

withstanding high pressures. Due to the abrasive nature of the build/support material component of the viscous material (e.g., metal powder, ceramic powder, sodium silicate powder, etc.) the syringe pump, tubing, merging chamber, mixing chamber, and nozzle components are preferably made of a wear resistant material. This is especially important for the nozzle tip material, as the nozzle output orifice will experience the most abrasion. Consequently, the nozzle tip may be of a wear resistant material (e.g., hardened steel), and/or it may have a wear resistant coating. The nozzle tip may also be easily interchangeable to allow for replacements in case of nozzle wear, and to facilitate the use of different nozzle orifice sizes for different uses and as may be required for specific objects being built.

The merging chamber is designed to minimize the relative angle between the direction of each of the multiple input flows and the direction of the single output flow in order to minimize pressure build up in the viscous material and ensure steady flow of material into the nozzle, even at high feed rates. Additionally, the internal geometry of the nozzle output is optimized to minimize the extrusion pressure and ensure uniform viscous material flow out of the nozzle. The internal angle from the nozzle orifice may be minimized to ensure a gradual transition to reduce extrusion pressure and promote a steady flow of viscous material out the nozzle orifice. The length of the nozzle orifice may be selected as a multiple of the nozzle orifice diameter. This dimension may be optimized to minimize extrusion pressure without significantly increasing die swell.

With regard to the use of light and/or heat to rapidly cure the viscous material exiting the nozzle orifice, the point of application of the heat and/or light may be focused at the nozzle tip where the viscous material exits the nozzle output orifice. This concentrated curing energy facilitates rapid curing of the viscous material as soon as it exits the nozzle output orifice, so as to solidify the material and form the shape of the 3D printed object. The external nozzle geometry may be designed accordingly to allow for the light and/or heat to be applied directly to the nozzle tip. The angle of application of the light and/or heat may be selected to maximize the power density per unit area at the focal point coinciding with the nozzle tip.

This focused application of light and/or heat at the nozzle tip is achieved by a rapid- curing apparatus which is secured to or associated with the nozzle apparatus. In various embodiments, the rapid-curing apparatus includes one or more curing devices such as, but not limited to, LED, laser, IR heater, resistance heater, etc., which provide light and/or heat at the nozzle tip. The rapid-curing apparatus may include a cooling system (e.g., heat sinks, air cooling fans, water cooling jackets, etc.) to ensure that a device does not exceed its safe operating temperature range during the printing process.

In addition to the focused application of light and/or heat at the nozzle tip to facilitate rapid curing of the viscous material, a post-curing apparatus, which may include one or more additional curing devices such as, for example, LED, IR heater, resistance heater, etc., may be integrated into the apparatus of a 3D printer. The post-curing apparatus directs light and/or heat at the build area of a 3D printer to assist in curing the printed object throughout the entire printing process and to enable a prolonged post-curing period after the object is complete to ensure thorough curing of the resin binder. Integrating a post-curing apparatus into a 3D printer may allow post-curing to take place during printing, which reduces the time required for post- curing after the print is complete.

In embodiments where a post-curing apparatus is integrated with a 3D printer, the structure of the 3D printer chassis may be designed in accordance with the requirements of the post-curing apparatus. For example, a 3D printer chassis may be designed to allow the post- curing apparatus to be mounted to the chassis in such a way as to direct the light and/or heat at the build area. This may require the 3D printer chassis to be designed with designated smooth surface areas for adhesive backed LEDs, and/or an array of mounting holes for heating devices. For example, the chassis may be able to efficiently dissipate heat from the LEDs and/or insulate the internal volume of the 3D printer to allow the heating devices to increase the ambient temperature of the internal volume and enable thermosetting of a resin binder.

Embodiments of the invention will be further described in the below non-limiting example. Example

An apparatus for extruding and curing viscous material feedstocks to create a three- dimensional object based on indirect metal 3D printing is described. The object may be a metal powder green part, a casting mold, or a component of a casting mold. The apparatus may include components for storing, optionally mixing, and delivering the viscous material to one or more nozzle. FIG. 1 illustrates an embodiment of the feedstock extrusion apparatus described herein. The feedstock extrusion apparatus includes a syringe pump 110 including a cylindrical tube 101 and a piston 102. A motor 103 serves as an actuator in the system. A lead screw 104 connected to the motor 103 translates the rotational motion of the motor to linear motion of an extruder carriage 107. The lead screw 104 is coupled to the piston 102 by the extruder carriage 107. This enables extrusion of the viscous material through the tubing 106 in the direction of the arrows shown in FIG. 1. The viscous material feedstock is shown in the magnified view as build material powder particles 108 mixed with a resin binder 109.

Referring to FIG. 2, the tubing 106 from the extrusion apparatus is connected to a nozzle apparatus 202 and the arrows show the direction of viscous material flow. The nozzle apparatus 202 is secured to the 3D printer gantry 203 which is a part of the 3D printer apparatus 204. The 3D printer chassis 205 features a designated area for a post-curing apparatus 201 , depicted with dashed lines. The designated area 201 is present on all four sides of the 3D printer chassis 205 to mount curing devices and focus curing energy (e.g., heat, light) at the printed object from all sides.

FIG. 3 depicts an embodiment of a single actuator used to control two independent syringe pumps in an implementation where the binder used in the viscous material feedstock is a two-part epoxy consisting of a resin and a hardener. This embodiment includes two cylindrical pumps 310a, 310b. The cylindrical tubes 301 a, 301b and pistons 302a, 302b are depicted for each syringe pump. A single motor 303 is used as the actuator in this embodiment. A lead screw 304 is connected to the motor 303 which translates the rotational motion of the motor to linear motion of the extruder carriage 307. The lead screw 304 is coupled to each piston 302a, 302b by the extruder carriage 307 according to a selected ratio which determines the amount of travel of each piston. For example, they may be connected with a 1 : 1 ratio, or other ratio such as 1 : 1.25, 1 :1.5, 1 :2, etc., in various embodiments. This enables simultaneous extrusion of each component of the viscous material through the tubes 306a, 306b in the direction of the arrows shown in FIG. 3 according to a selected mix ratio. In another embodiment, a separate motor may be used for each lead screw, and the speed of each motor may be controlled independently (e.g., using control software) to allow the system to be programmed for any desired mix ratio. In another embodiment, the lead screws may have different thread pitches or thread counts which are selected to achieve a desired mix ratio. FIG. 4 illustrates an embodiment of a merging chamber as described herein. The viscous material feedstock flows through tubing from two syringe pumps into the merging chamber inputs 401a, 401b, as shown in this embodiment. The viscous materials flow in the direction of the arrows, converging at a merging zone 402 and continuing in the direction of the arrows through the merging chamber exit 403. The relative angle between the directions of each of the input flows is shown as 60 degrees in this embodiment. This angle may be optimized according to a parameter such as, for example, a given viscosity of the material feedstock. Larger angles may be used, although larger angles may increase resistance to flow and therefore may not be desirable for certain material feedstock parameters. Generally, lower angles, within a range of about 10 degrees to about 60 degrees, may be used.

FIG. 5 illustrates an embodiment of a static helical mixing chamber as described herein. The viscous material feedstock flows from a merging chamber output, according to one embodiment, or directly from a single tube carrying the viscous material from the extrusion apparatus in another embodiment, into the static helical mixing chamber input 501. The viscous material is mixed as it passes through the static helical mixer 502 and continues in the direction of the arrows through the chamber exit 503.

FIG. 6 illustrates an embodiment of an externally actuated active mixing chamber as described herein. The viscous material feedstock flows from a merging chamber output, according to one embodiment, or directly from a single tube connected to the extrusion apparatus in another embodiment, into the externally actuated active mixing chamber input 601. The viscous material is mixed as it passes through the active mixer 602 and continues in the direction of the arrows through the chamber exit 603. In an embodiment, the active mixer 602 is actuated by rotation of the shaft 604 which may be connected to a motor or a similar rotating device.

FIG. 7 illustrates an embodiment of a nozzle tip, a rapid-curing apparatus, and a cooling apparatus as described herein. The viscous material feedstock flows from a merging chamber output, according to one embodiment, from a static or active mixing chamber output in another embodiment, or directly from a single tube connected to the extrusion apparatus in yet another embodiment, into the nozzle input 701. The viscous material flow converges as it passes through the nozzle tip 704 and flows in the direction of the arrows out the nozzle orifice 702. As the viscous material exits the nozzle orifice 702 it is rapidly cured by the curing apparatus 705 and forms an extruded line 703 on the build plate 707 or on a previous layer of the printed object 707. The rapid-curing apparatus 705 emits light and/or heat focused at the nozzle output as indicated by the dashed lines. The rapid-curing apparatus may be cooled by a cooling apparatus 706, which may be a cooling fan in one embodiment. The nozzle tip 704 may be secured via a luer lock connection, a threaded connection, or other suitable connection, to facilitate quick replacement in case of wear or when different orifice sizes are required. The internal angle from the nozzle orifice is shown as 60 degrees in this embodiment. Other angles may of course be selected.

All cited publications are incorporated herein by reference in their entirety. Equivalents

While the invention has been described with respect to illustrative embodiments thereof, it will be understood that various changes may be made to the embodiments without departing from the scope of the invention. Accordingly, the described embodiments are to be considered merely exemplary and the invention is not to be limited thereby.