FIELD OF THE INVENTION

The present invention relates to methods and apparatus for additive

manufacturing and/or three-dimensional (3D) printing of 3D objects. BACKGROUND TO THE INVENTION

Known 3D printing technology include stereo-lithography (STL), where a laser is scanned selectively across a bath of photopolymer resin curing particular areas of the surface. The level of the uncured resin is increased slightly and the process repeated. While STL produces good detail rendition, multi-material printing is challenging and impractical as the entire resin bath needs to be changed, which can introduce cross- contamination issues.

Another known rapid prototyping technology is fused deposition modelling (FDM), which makes use of a heated thermoplastic extruded directly onto a print bed. Software control is used to divide an object into many fine threads that are extruded individually in layers to manufacture the part. As the thermoplastic cools it hardens into a functional part. However, thermoplastic printing processes typically involve temperatures in excess of 200°C, making them incompatible with many materials, and 'smart' printed objects.

Soft structures have been identified as the solution to problems faced in automation expansion as well as for wearable devices. Silicone has been recognised as the material of choice with the optimal blend of attributes. The soft robot manufacturing methods presently popular are neither convenient nor efficient. At present, additive manufacturing techniques suitable for rigid materials are highly advanced in comparison to the techniques available for silicone and soft materials.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a 3D printing method, apparatus, and/or system for printing silicone, or at least provide the public with a useful choice.

In a first aspect, the invention broadly consists in a method for printing a silicone structure with a 3D printer, the 3D printer comprising a liquid bath configured to receive a liquid, a print-bed, and silicone deposition member, the method comprising :

depositing silicone onto the print-bed via the silicone deposition member, the deposited silicone is the silicone structure, and maintaining a relative liquid level, where the relative liquid level is the level of liquid in the liquid bath relative to the silicone structure such that the silicone structure is at least partially submerged in the liquid .

In one embodiment, the silicone structure is substantially submerged in the liquid.

In one embodiment, the method further comprises depositing silicone onto the silicone structure.

In one embodiment, the relative liquid level is maintained constant. In another embodiment, the relative liquid level is maintained at a variable level dependent on the volume of silicone printed.

In one embodiment, the step of maintaining the relative liquid level comprises increasing amount of liquid in the liquid bath.

In another embodiment, the step of maintaining the relative liquid level comprises changing the height of the print-bed. Preferably, the print-bed is lowered into the liquid bath. Preferably, the volume of liquid in the liquid bath is constant.

In one embodiment the, liquid is pre-heated to a curing temperature. In another embodiment, the water bath comprises a heating member. Preferably, the heating member maintains the liquid at substantially a curing temperature.

In one embodiment, the liquid cures the silicone structure.

In one embodiment, the print-bed is a heat bed . Preferably the heat bed is heated to a heat bed curing temperature. Preferably, the heat bed curing temperature is 100°C.

In one embodiment, the curing temperature is between 50°C and 100°C.

Preferably, the curing temperature is between 85°C and 100°C. More preferably, the curing temperature is less than the boiling point of the liquid in the liquid bath. Even more preferably, the curing temperature is greater than 5°C less than the boiling point of the liquid in the liquid bath.

In one embodiment, the heated liquid reduces the silicone curing time.

In one embodiment, the liquid is water. Preferably, the liquid has a surfactant added to it. In another embodiment, the liquid comprises a surfactant. In another embodiment, the liquid comprises water and a surfactant.

In one embodiment, the 3D printer further comprises one or more additional silicone curing devices. In one embodiment, the additional silicone curing devices may be any one or more of the following :

• heated print-bed, • infra-red emitter, and/or

• soldering rework gun.

In one embodiment, a hardware system is configured to execute and/or carry out the steps of the method.

In a second aspect, the invention broadly consists in a 3D printing device for printing a structure, wherein the improvement comprises :

the structure being a silicone structure,

a liquid bath configured to receive a volume of liquid,

a relative liquid level, where the relative liquid level is the level of liquid in the liquid bath relative to the silicone structure,

wherein, when the 3D printer is printing, the relative liquid level is maintained such that the silicone structure is at least partially submerged in the liquid .

In one embodiment, the silicone structure is substantially submerged in the liquid.

In one embodiment, the relative liquid level is maintained constant. In another embodiment, the relative liquid level is maintained at a variable level dependent on the volume of silicone printed.

In one embodiment, the relative liquid level is maintained by increasing amount of liquid in the liquid bath.

In another embodiment, the relative liquid level is maintained by changing the height of the print-bed. Preferably, the print-bed is configure to be lowered into the liquid bath. Preferably, the volume of liquid in the liquid bath is constant.

In one embodiment the, liquid is pre-heated to a curing temperature. In another embodiment, the water bath comprises a heating member. Preferably, the heating member maintains the liquid at substantially a curing temperature.

In one embodiment, the liquid cures the silicone structure.

In one embodiment, the print-bed is a heat bed. Preferably the heat bed is heated to a heat bed curing temperature. Preferably, the heat bed curing temperature is 100°C.

In one embodiment, the curing temperature is between 50°C and 100°C.

Preferably, the curing temperature is between 85°C and 100°C. More preferably, the curing temperature is less than the boiling point of the liquid in the liquid bath. Even more preferably, the curing temperature is greater than 5°C less than the boiling point of the liquid in the liquid bath. In one embodiment, the heated liquid reduces the silicone curing time.

In one embodiment, the liquid is water. Preferably, the liquid has a surfactant added to it. In another embodiment, the liquid comprises a surfactant. In another embodiment, the liquid comprises water and a surfactant.

In one embodiment, the 3D printer further comprises one or more additional silicone curing devices. In one embodiment, the additional silicone curing devices may be any one or more of the following :

• heated print-bed,

• infra-red emitter, and/or

· soldering rework gun.

In a third aspect, the invention broadly consists in an add-on for a 3D printing device comprising a print-bed and a material depositing member, wherein the add-on comprises:

a liquid bath configured to receive a volume of liquid, the print-bed is located within the liquid bath,

a relative liquid level, where the relative liquid level is the level of liquid in the liquid bath relative to the height silicone structure,

a structure height tracking module, the structure height tracking module tracks the height of the structure being 3D printed and maintains the relative liquid level such that the silicone structure is at least partially submerged in the liquid .

In one embodiment, the silicone structure is substantially submerged in the liquid.

In one embodiment, the relative liquid level is maintained constant. In another embodiment, the relative liquid level is maintained at a variable level dependent on the volume of silicone printed.

In one embodiment, the relative liquid level is maintained by increasing amount of liquid in the liquid bath.

In another embodiment, the relative liquid level is maintained by changing the height of the print-bed. Preferably, the print-bed is configure to be lowered into the liquid bath. Preferably, the volume of liquid in the liquid bath is constant.

In one embodiment the, liquid is pre-heated to a curing temperature. In another embodiment, the water bath comprises a heating member. Preferably, the heating member maintains the liquid at substantially a curing temperature. In one embodiment, the liquid cures the silicone structure.

In one embodiment, the print-bed is a heat bed. Preferably the heat bed is heated to a heat bed curing temperature. Preferably, the heat bed curing temperature is 100°C.

In one embodiment, the curing temperature is between 50°C and 100°C.

Preferably, the curing temperature is between 85°C and 100°C. More preferably, the curing temperature is less than the boiling point of the liquid in the liquid bath. Even more preferably, the curing temperature is greater than 5°C less than the boiling point of the liquid in the liquid bath.

In one embodiment, the heated liquid reduces the silicone curing time.

In one embodiment, the liquid is water. Preferably, the liquid has a surfactant added to it. In another embodiment, the liquid comprises a surfactant. In another embodiment, the liquid comprises water and a surfactant.

In one embodiment, the 3D printer further comprises one or more additional silicone curing devices. In one embodiment, the additional silicone curing devices may be any one or more of the following :

• heated print-bed,

• infra-red emitter, and/or

• soldering rework gun.

In a fourth aspect, the invention broadly consists in a 3D printing device for printing a silicone structure, wherein the 3D printing device comprises:

a liquid bath configured to receive a volume of liquid,

a relative liquid level, where the relative liquid level is the level of liquid in the liquid bath relative to the height silicone structure,

a print-bed,

a silicone depositing member,

wherein the 3D printing device is configured to maintain the relative liquid level, such that the silicone structure is at least partially submerged in the liquid .

In one embodiment, the silicone structure is substantially submerged in the liquid.

In one embodiment, the relative liquid level is maintained constant. In another embodiment, the relative liquid level is maintained at a variable level dependent on the volume of silicone printed. In one embodiment, the relative liquid level is maintained by increasing amount of liquid in the liquid bath.

In another embodiment, the relative liquid level is maintained by changing the height of the print-bed. Preferably, the print-bed is configure to be lowered into the liquid bath. Preferably, the volume of liquid in the liquid bath is constant.

In one embodiment the, liquid is pre-heated to a curing temperature. In another embodiment, the water bath comprises a heating member. Preferably, the heating member maintains the liquid at substantially a curing temperature.

In one embodiment, the liquid cures the silicone structure.

In one embodiment, the print-bed is a heat bed. Preferably the heat bed is heated to a heat bed curing temperature. Preferably, the heat bed curing temperature is 100°C.

In one embodiment, the curing temperature is between 50°C and 100°C.

Preferably, the curing temperature is between 85°C and 100°C. More preferably, the curing temperature is less than the boiling point of the liquid in the liquid bath. Even more preferably, the curing temperature is greater than 5°C less than the boiling point of the liquid in the liquid bath.

In one embodiment, the heated liquid reduces the silicone curing time.

In one embodiment, the liquid is water. Preferably, the liquid has a surfactant added to it. In another embodiment, the liquid comprises a surfactant. In another embodiment, the liquid comprises water and a surfactant.

In one embodiment, the 3D printer further comprises one or more additional silicone curing devices. In one embodiment, the additional silicone curing devices may be any one or more of the following :

• heated print-bed,

· infra-red emitter, and/or

• soldering rework gun.

In a fifth aspect, the invention broadly consists of a product produced by the method according to the first aspect. In one embodiment, the product is a soft robot member.

In a sixth aspect, the invention broadly consists of a product produced by the devices according to any one of the second, third, or fourth aspects. In one embodiment, the product is a soft robot member.

In another aspect the invention broadly consists of method for printing a silicone structure, the method comprising : depositing silicone onto a print-bed via a silicone deposition member, and maintaining a liquid level of a liquid in a liquid bath such that the deposited silicone is substantially submerged in the liquid. In another aspect the invention broadly consists of method for printing a silicone structure, the method comprising :

depositing silicone onto a print-bed via a silicone deposition member,

maintaining a liquid level of a liquid in a liquid bath, and

maintaining a substantially constant temperature of the liquid.

In another aspect the invention broadly consists of method for printing a silicone structure, the method comprising :

depositing silicone onto a print-bed via a silicone deposition member, wherein the print-bed is located in a liquid bath,

maintaining a substantially constant temperature of a liquid in the liquid bath, wherein the temperature is substantially 90 degrees centigrade.

In another aspect the invention broadly consists in a 3D printing device for printing silicone, the device comprising a liquid bath configured to receive a volume of liquid, wherein, when the 3D printer is printing, a relative liquid level, being the level of liquid in the bath relative to the silicone being printed, is maintained in the liquid bath such that the silicone being printed is at least partially submerged in the liquid .

The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

The phrase "hardware system" as used in this specification and claims is intended to mean, unless the context suggests otherwise, any form of computing, processing or programmable electronic device, platform or system including, but not limited to, portable or non-portable consumer electronic devices such as smartphones, cellphones, tablets, e-Reader or e-book devices, laptops, and notebooks, gaming machines or consoles, server, smart televisions, general purpose computers such as desktop computers, specific purpose computers or the like, and is intended to include one or more linked or communicating hardware or processing devices or systems which work together.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

As used herein the term '(s)' following a noun means the plural and/or singular form of that noun.

As used herein the term 'and/or' means 'and' or 'or', or where the context allows both.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

The invention consists in the foregoing and also envisages constructions of which the following gives examples only. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described by way of example only and with reference to the drawings, in which :

Figure 1 shows a graph of cure times for various silicones.

Figure 2 shows a 3D printing experiment using a solder rework gun.

Figure 3 shows a graph of the change in viscosity of silicones over time.

Figure 4 shows an example 3D printer.

Figure 5 shows a 3D printing experiment using hydrogel.

Figure 6 shows an example 3D printer modified to print silicone.

Figure 7 shows an example 3D printer modified to print silicone.

Figure 8 shows example 2.5D printed silicone.

Figure 9 shows example 3D printed silicone.

Figure 10 shows example 3D printed silicone. Figure 11 shows example 3D printed silicone.

Figure 12 shows an embodiment of a liquid bath.

Figures 13A to 13C show embodiments for maintaining relative liquid level.

DETAILED DESCRIPTION

Various aspects of a 3D printing method, apparatus and/or system for 3D printing silicone will now be described with reference to the accompanying drawings.

Although 3D printing has had rapid growth in the last decade particularly within the prototyping of industrial use as well as general use within homes and the public, the devices printed has been limited to thermoplastics which means the parts are often solid, brittle and non-flexible. This has limited the use and prototyping of stretchable devices to more tradition methods which often are more time consuming for rapid prototyping. Designing a 3D printer that can print with a silicone material would solve this problem causing rapid prototyping and customization of flexible and stretchable parts.

Silicones are widely used in products around the world due to be flexible but are time consuming to make. This is due to needing a mould for the silicones which must be manufactured for each new part which slows down and increases the cost of prototyping of silicone devices. Developing an additive material process for silicone parts would increase the speed of prototyping and increase the number of people that had easy access to it. It would also allow for more complex flexible devices to be manufactured.

Many different types of silicone are known in the art. While the description herein may relate to only a select few silicones, a person skilled in the art will appreciate that the present invention may be modified, adjusted, and/or customised to work with other silicones. Example silicones include Smooth-On's EcoFlex 0030 (referenced as "EcoFlex 0030"), Dow Coming's 732 RTV silicone, and Sylgard's 184 (referenced as "Sylgard 184").

The present invention provides accelerated silicone curing times for use with standard silicone types. Accelerating silicone curing times gives for faster 3D printing times and may allow for silicone to be used as a viable rapid prototyping material.

Curing Silicone Experiments

A series of experiments were undertaken to establish the silicones Sylgard 184's and EcoFlex 0030's response to physical phenomena. This was required as 3D printing of silicone requires fast curing rates and the materials in use do not exhibit such rates under normal conditions. The silicone curing experiments described below may provide different advantages and disadvantages. These potential advantages and disadvantages are discussed under each section. Unaccelerated Curing

Table 1 gives the pot life and curing time of each material. These curing times are not suitable for working with a 3D printing, rapid prototyping environment or system.

Sylgard 184 EcoFlex 0030

Pot Life (hours) 1.5 0.75

Cure Time at 25°C (hours) 48 4

Table 1 - Curing properties of Sylgard 184 and EcoFlex 0030 Heat Bed

To determine the curing time for Ecoflex 0030 on the heat bed, a hotplate was used along with a custom made aluminium plate to be extruded onto, rested on top of the hotplate. This experiment was to find the ideal heat bed temperature for rapid curing while maintaining substance integrity. The aluminium plate was 5 mm thick to make sure there was even distribution of heat throughout the top of the plate. The aluminium plate temperature was measured using a thermometer. Curing time was the time taken for the silicone to solidify enough that a knife would peel the silicone instead of passing through the liquid. Based on these experiments, the curing time decreased as the temperature of the hotplate increased. This lead to the fast curing temperature of 200 degrees C which was almost instantaneous.

The cured silicones at 200 degrees C were observed to cure fast but with formed gas bubbles instead of the structure resulting in a more brittle and less stretchable material. This was thought to be caused by air bubbles in the silicone mixture due to improper mixing of the two parts. This was ruled out when using a vacuum-packed mixture and repeating the same experiment which resulted in the same bubbles forming in the material. This was most likely due to the intense temperature causing rapid uneven curing in the material or the uncured silicone having a lower temperature resistance then cured silicone which resulted in breaking down at this temperature. This lead to the ideal curing temperature of 150 degrees C which had a fast curing time (2-3 seconds) and no bubble formation in the material.

Figure 1 shows the time taken to cure the Sylgard 184 and EcoFlex 0030 silicones at various temperatures. The more steeply slanted line represents the Ecoflex 0030, and the shallower line represents the Sylgard 184.

Water Bath

Since direct contact heat from the heat bed proved potentially a viable method for curing the silicone, it was proposed that curing in heated water would have a similar result. An experiment was set up with 90 degree C water with hand extrusion of silicone into the water. Silicone was pre-stuck to the bottom of the container and water heated to 90 degrees C was then poured into the container. The syringe tip was then dragged across this first layer under the water and depending on the distance of the tip to the already set material, the silicone would stick to the previous material or it would float. Although it would cure in both situations reasonably quickly.

The problem with the silicone not sticking to the previous layer may be due to it wanting to float. Only when the syringe tip was dragged though the previous layer did the silicone stick. This proved hard to do manually so would require the experiment to be done using a 3D printer printing silicone to obtain a clearer result. Another problem encountered was that the silicone would cure in the nozzle of the syringe when the tip was dragged though the water. This resulted in an increase in force required for extrusion.

A successful deposition was found to have a cure time that was approximately 5-7 seconds.

Infra-red

A halogen bulb was acquired to emit infrared (IR) to test whether that the silicone

(EcoFlex 0030) would be cured by these wavelengths of IR. Silicone was extruded from a lOmL syringe fitted with a 23-gauge tip whilst the 75W halogen bulb was held within 10mm of the string. The silicone was deemed "cured" through the same metric as previously described . After 2 minutes, there was no observed curing of the silicone, so the experiment was halted. This lead to a repeat, but, with a 10: 1 silicone-Carbon mixture. It was hoped the Vulcan XC72R powdered carbon would absorb light energy and thus improve the cure time. However, after 1 minute the silicone-carbon black mixture also showed no significant signs of curing, so this too was aborted. Alternative silicone types, such as NuSil 4213 series, are more absorbent of IR and thus be a viable option.

These experiments effectively ruled out using heat from a halogen bulb as they exceeded realistic curing times by far. This may be due to the halogen having a non- focused energy point so all the heat energy is dissipated around the place so curing time is long.

Soldering Rework Gun

An experiment using a soldering rework gun to try curing of the silicone to test whether it was a viable curing method. Silicone was extruded by hand onto an aluminium plate with the soldering rework gun being focused on a part of the silicone. Initial testing of the curing method showed that the temperature needed to be maximum for the device which was 500 degrees C. This proved to be inefficient for curing with a low air speed. Increasing the flow of hot air decreased curing time, was found to but result in an increase in distortion of the silicone. A curing time of 6-7 seconds was obtained with a fast air flow rate but the amount of distortion of the silicone rendered it an non-preferable curing method.

Temperature Flowrate Result

350°C 0.25 Partial cure after 7s, still not fully cured after 20s

500°C 0.3 Partial cure after 6-7s, but significant distortion of string

500°C 0.25 Even with gun very close and very thin string/sausage insufficient curing after 5s

500°C 0.2 Low deformation, but insufficient curing after 5s

Table 2 - Curing time for silicone in soldering rework gun experiment In each test, a section of silicone string slightly larger than then diameter of the hot-air rework gun nozzle (8mm diameter) was subjected to hot air as shown in Figure 2.

Extruding into Hydrogel

Figure 5 shows red EcoFlex printed inside a clear bubbly hydrogel solution . A thick gel was obtained and watered down to form a hydrogel solution. EcoFlex was extruded manually with a long 18-gauge syringe needle into a helix shape. The container of hydrogel was placed in a warm water bath to accelerate curing and hereby reduce the likelihood of distortion prior to full cure.

Rising Water Level

When printing underwater, as discussed above under the heading "Water Bath" above, the silicone was found to cure in the nozzle and floated to the surface if it didn't bond to material already submerged . A solution was devised to use a rising water level system to cure the material. This would transfer the heat up to the most recent layers while not causing curing in the tip. It was also theorized that this would double as support material allowing voids to be printed.

3D Printer and Operating the 3D Printer

In this embodiment, the experiment from the "Rising Water Level" section above is used as the method for accelerated curing of the silicone. A person skilled in the art will appreciate that the other methods may be used in conjunction, or as an alternative to the printer and methods described in this embodiment.

In another embodiment, the "Rising Water Level" system is used in conjunction with any one or more of the following techniques described in the sections:

· Head Bed,

• Soldering Rework Gun,

• Intra-red, and/or

• Extruding into hydrogel.

These additional curing techniques, when used in conjunction with the rising water level technique, may increase the speed of curing and quality of the final cured silicone.

In this example, a Kellerman K8200 3D printer was obtained to be modified for the use of this project. A person skilled in the art will appreciate that other 3D printers may also be modified or used to a similar effect. The printer was originally a kit set printer that had modifications in the extrusion head as well as in the heat bed of the printer. The heat bed was previously modified to allow more current to decrease rise time for heat bed temperature. Some cables had to be replaced in the printer due to damage. Repetier host is a software that controls the printer through manual control as well as G-code that can be hand written or acquired through the use of the inbuilt Slic3r which converts an STL file into G-code ready for printing. A thin 3mm aluminium plate was manufactured to cover the heat bed of the printer to avoid damaging it through extrusion directly onto the heat bed, partially when scrapping off the print.

Figure 4 shows the original, unmodified 3D printer used in this example.

In this example, the 3D printer has been retrofitted to include the extrusion rig design described in the "extrusion rig design" section below. The retrofitting was conducted by way of convenience so that a new 3D printer didn't need to be constructed.

Alternatively, a new 3D printer may be constructed using the techniques described in the

"extrusion rig design" section above in addition to standard 3D printer elements.

Alternatively, an add-on, plugin, or enhancement may be designed using the techniques described in the "extrusion rig design" section such that the add-on, plugin, or enhancement can be incorporated into other 3D printers without substantial

modifications of the existing 3D printer. Extrusion Rig Design

Figure 6 shows an example extrusion rig 60 according to an embodiment of the invention. Also shown in Figure 6 is an example test print of a heart 65. The extrusion rig 60 is an example of a silicone depositing member or a silicone dispensing member.

Architecture

In this example, a powered syringe system 62 was designed that worked using a stepper motor 64 that was in line with the axis of the syringe 66 to avoid any unnecessary torques. This stepper motor 64 preferably pushes a plate down causing the plunger of the syringe 66 to plunge into the barrel of the syringe 66.

In the example printer of Figure 4, all the parts were 3D printed or laser cut except for the shafts and rods. This was due to needing this test extrusion rig quickly as to not have a long waiting period where no testing was conducted. The back plate needed to be withstand the most forces and was simple in design, which was laser cut. All 3D printed parts were printed on maximum quality and solid to make sure they wouldn't break under the stress of the system.

Extrusion Rig Motor Testing

Once the test printing extrusion rig 60 was assembled, it was tested using by plugging the stepper motor into the extrusion plug of the 3D printer. Manual control was then used to valid the design by movement in both directions of the motor. Water was then added into the syringe 66 to prove that the system would extrude a liquid before mounting it to the 3D printer.

Extrusion Rig Test Printing

The extrusion rig was tested by printing a love heart shape 65 part that was one layer thick and only one loop. This test was to validate that this extrusion rig system worked as an extrusion system for silicone. This test printing required changing parameter settings inside of the slicer program. These included making sure that only one layer and one loop was being printed as well as manually adjusting the flow rate to increase the amount of silicone coming out. It was found that the existing firmware requires the extruder to have reached a minimum temperature for extrusion to be permitted. Initially we allowed the ABS print-head to be heated and thus were able to test the extrusion system. Later it was found the permission requirement could be overrode using G-code command "M302". This prevented a slow drip of melted ABS. The ABS extruder had a step-up ratio of 5: 1. The software was calibrated to this, so 50mm extrusion resulted in a ~10mm vertical displacement of the syringe plunger.

Continuous Silicone Supply System

Several systems were fabricated to try and achieve a system capable of continuously supplying silicone, preferably mixing the two components during printing. A further requirement for the system is a narrow nozzle to enable printing the largest possible area inside the liquid bath.

Dual-Cartridge Passive Mixing

In this example, dual-cartridge epoxy mixing syringes were used. These would provide 50ml_ of print material, deemed to be sufficient in quantity for sample prints desired in this project and hence acceptable despite not offering continuous supply. Great effort went into the fabrication of this system with the milling of aluminium parts where the above described preliminary syringe extrusion system's 3D printed parts were not strong enough. This system was successfully manufactured but ultimately failed because a combination of misalignment, friction and lack of stepper motor strength resulted in an inability to extrude silicone and even test if mixing would work.

A person skilled in the art will appreciate that while in this given example, a non- ideal solution was found, using different materials and using routine approaches, a more suitable system could be designed to overcome the issues outlined above.

Dual Peristaltic Active Mixing

In this example, peristaltic pumps were used to pump the two parts of Ecoflex through and into an epoxy mixing nozzle. This example made use of SLA printing to manufacture a Y section part to combine the two Ecoflex parts from two inlets to one outlet going into the mixing nozzle. This example functioned fine with it pumping liquid through the nozzle. Problems were identified with the printing speed which required the peristaltic pump to rotate very slowly. This caused the Ecoflex to not be mixed properly resulting in no curing even after hours on the heat bed.

A person skilled in the art will appreciate that while in this given example, a non- ideal solution was found, further development could provide a suitable system that overcomes the issues outlined above.

Passive Mixing with Peristaltic Pumps

In this example, a peristaltic pump driven passive mixing system that employed the epoxy mixing syringe nozzles but replaced the cartridges and accompanying plungers with a peristaltic pump. This resulted in the design and fabrication of mixer nozzle connection pieces and pump plumbing using SLA printing technology as some of these parts had complex internal structures from which the removal of support material generated by a FDM printer would be difficult. The system was successfully

manufactured however, in this example, printing is not possible since the low flowrates required result in a lack of adequate mixing. Only at higher flowrates, such as when extruded by hand from the dual-cartridge syringe, the mixer nozzles performed. As this system did not accomplish the task either, it was decided to pursue a continuous supply pre-mix system. A person skilled in the art will appreciate that while in this given example, a non- ideal solution was found, further development could give a suitable system that overcomes the issues outlined above.

Pre-mix Peristaltic Pump System

In this example, shown in Figure 7, a hybrid approach from the previous systems outlined above was used. A peristatic pump was used to pump pre-mixed solution 100 of

Ecoflex from a container. This was pass through a nozzle tip at the bottom. A turned part was used as the connector between the pipe and the nozzle tip. This system used thin pipes to reduce chance of curing in the system. Due to using pre-mixed Ecoflex, we had to continuously extrude material to stop it curing in the system. This example gave larger prints than achieved by the initial example system and functioned properly.

As seen in Figure 7, the solution 100 is introduced to the water bath 101.

The first version of this example used 3mm ID hoses, however given the length of tube in use this resulted in a significant quantity of EcoFlex sitting in the hose. It was established that material in the hoses was partially curing before being printed, especially if there was delay between prints. This resulted in a switch to 1.59mm and 2mm ID tubes, resulting in a volume of 0.7ml_, a volume decrease of 180%.

Limitations of this example may also include the ability to seal hose connections to other hardware. Under greater pressure, which occurs when there is partial curing or the extrusion rate is too high, the system may begin to slowly leak, which may affect print quality as the calculated amount of material was not deposited. Heat through convection and radiation from the print bath initially encouraged partial curing in the pipes. This may be largely overcome through heat shielding and a fan blowing cooler ambient air towards the tubing. Cleaning of the system may prove to be difficult:

isopropyl alcohol may be flushed through but this may not dissolve the silicone and air of 4 bar pressure also may struggle to clear the system.

A combination of air and allowing the material to cure and then clearing with copper wire worked best.

Further, this example allowed a constant Z-height of the nozzle to be achieved and continual addition of EcoFlex.

Liquid Bath and Stage

In this example, the liquid being used is water and as such, the liquid bath is described as a water bath. A person skilled in the art will appreciate that other liquids may be used in place of water. In this example, the stage being used is a heated stage. The heated stage is described as a "heat bed". A person skilled in the art will appreciate that it is not essential that the stage be heated and is merely an option.

Figure 7 best shows the example water bath (labelled "Water Bath"). In this embodiment, the water needed to be heated in a controlled environment for printing. In this example, a water bath that had a kettle element in the bottom was placed on the heat bed. The kettle element was connected to a Bang-Bang temperature controller which had a PT100 probe place in the bath. This allowed for control of water

temperature. A person skilled in the art will appreciate that other temperature control algorithms are known and can be used in addition to, or instead of the PT100 bang-bang system.

Inside of the water bath was a 110 by 110 mm aluminium plate which acted as our new heat bed. This allowed for 75 x 80mm print space. This was attached using screws to allow levelling of the heat bed.

Gaps between the print-bed and container walls resulted in additional volume of water, which is undesired as it increases the volume of water that sloshes as the print- bed moves in the XY-directions. Scouring pads were inserted into these gaps to reduce the volume of water whilst still being water-permeable. Water-permeability was desired as it allowed convection of heated water from beneath the platform as well as slowing waves caused by print-bed motion.

Z-height is commonly an important parameter for 3D printing. For Z-height to be correct over the entire print area it is necessary to have a level print-bed, for this reason the existing print-bed was left underneath the water bath as it had levelling screws.

A hardware system may be used to control the water volume in the water bath. In this example, an Arduino controlled peristaltic pump system was developed for water level regulation. At the end of layer deposition, custom layer-change G-code paused the extrusion to wait for the water level increase to take place. The volume of water, added upon button press, was based off the average cross-sectional area of the part.

The new hardware system may be an add-on to a pre-existing hardware system associated with the pre-existing 3D printer. Communication between the hardware systems may be done via any standard hardware system interfaces such as Bluetooth, SPI, or I2C. Alternatively, the pre-existing hardware system of the 3D printer may updated with additional functionality to control the water level in the water bath.

Alternatively, the water bath may comprise a constant volume of water and the pre-existing hardware system of the 3D printer controls the height of the stage (or heat bed) such that the structure being printed is submerged or substantially submerged. A person skilled in the art will appreciate that the water level can be determined and adjusted using any one or more of the following variables or parameters:

• the total volume and/or surface area and/or cross-sectional areas of the silicone structure being built,

· the current volume and/or surface area and/or cross-sectional area of the silicone structure being built,

• the total volume of the water bath,

• the total volume of water already added to the water bath,

• the starting volume of water in the water bath,

· the height of the structure being printed, and/or

• the height of the heat bed.

Alternatively, a sensor may be used to determine the height of the structure and therefore the level of water required.

A person skilled in the art will appreciate that current water level and current water volume are directly derivable from one another by knowing the total volume and shape of the water bath.

In an alternative embodiment, the heat bed may have an adjustable height. The height may be adjusted such that the heat bed remains completely submerged in the water. The height of the heat bed way be such that the structure being printed on the heat bed is partially submerged in the water. Preferably, the height of the heat bed is set or adjusted such that highest point of the structure is not submerged in the water in the water bath. This alternative embodiment may be used in conjunction with water level control, or as an alternative to water level control.

This example 3D printer had movement in the X-Y directions on the base and Z direction on the printer head. This caused problems with the method of printing. In particular because of the large bath of water used. Movements in the base caused large ripples that sloshed the water around. This proved problematic as deposition was occurring on top of the water level. Changing to a printer that had the X-Y-Z movement directions in the printer head while the base/heat bed/water bath remained stationary would be the best case. Although due to the rate of change in the Z direction being so small, the base could theoretically have movement in the Z direction without comprising the print.

Other methods of reducing the water sloshing included reducing the bath size by adding walls to outside of the print bed. In this example, shown in Figure 12, the water bath had each side reduced by 40mm, making the print water volume less. This was achieved through adding sponges so that water could pass through but they still gave enough structure to reduce water bath area on the print bed. This reduced the sloshing of water but not enough. This was because the print size was still 150 by 150mm which is a large body of water.

Further improvements included reducing the speed at which the printer was moving. This decreased the magnitude that the water was sloshing around.

Unfortunately, this had the side effect of also decreasing the speed of the print. In this example, the speed was reduced by a factor of 4 down from 60 mm/s. This increased the printing time by a factor of 4. This improvement did result in a higher quality print.

Further investigations included adding gel to the water to stiffen the liquid to reduce the movement of the water. This caused problems with surface tension being too high so getting it to be the same level as the print was deemed too hard. It also didn't stop unwanted Z-direction movement of the water but instead oscillate a larger body of fluid. This method left us with unreliable results and was discarded.

In one embodiment, the hotter the water bath, the better for curing silicone. Increasing the water temperature to boiling point however would interfere with the silicone depositing/dispensing and result in a jagged, poorly printed 3D structure.

Preferably, the temperature of the liquid in the liquid bath is close to boiling point. More preferably, the temperature of the liquid in the liquid bath is slightly less than the boiling point of water.

Printing System Parameters

Since the printer employed in this example was designed and used for printing ABS all the parameters were tuned for this in the printer software Repetier Host and its slicing plug-in Slic3r. A summary of potential parameters adjusted for print optimisation is given below:

• Z-height and Z-resolution (layer thickness). Correct Z-height is vital for a

continuous extrusion stream; if this is too high globules are formed rather than a continuous stream. Layer thickness of less than 0.5mm resulted in very rapid curing and can result in dragging of the extrusion stream. Lowering the printing temperature may allow improved Z-resolutions to be achieved.

• Extrusion Multiplier - this is for fine-tuning the flow rate. It was necessary in various embodiments to scale this down by a factor of 5 for the preliminary syringe extrusion system as the filament extruder supplied with the system has a step-up gearing ratio of 5 : 1 between the stepper motor and the filament feed- wheel. For the pump system, this the value became 2. • Extrusion Diameter - this controls the extrusion stream width. It was set to the inner diameter of the hose, which was 2mm.

• Feed rate - this is calculated by Slic3r based off values entered for Χ,Υ,Ζ, bridges and gaps speed. To reduce sloshing of water we set the print speed to 15mm/s, which is a reduction by a factor of three from the default setting. Consequently, this resulted in a factor of three increase in print time. Very slow speeds caused extremely long printing time and resulted in frequent print failures due to material curing in the system.

• Infill - rectilinear infill needed to be used in various embodiments to ensure

100% infill. To reduce sloshing the infill direction in various embodiments was set at 90.

A person skilled in the art will appreciate that similar adjustment may be required to be made for other retrofitted 3D printer systems.

Printing Method

Below an example printing method is described. Where specific values are mentioned, a person skilled in the art will appreciate that these are approximate and can be adjusted to find the ideal values. For example, where 85°C has been suggested, a person skilled in the art will appreciate that other temperatures will also cure silicone.

For all prints according to one embodiment, EcoFlex 0030 is pre-mixed with a 50 : 50 ratio of the two components. If desired, dye may be added . Degassing was not required as the pumping procedure sufficiently negated bubbles that arose during mixing. Additionally, for all prints the print bath must preferably be filled to at least be in contact with the underside of the print-bed with the water-surfactant solution prior to powering on for water bath pre-heating. Z-height has been set as part of Printing System Parameterisation (as described in the "Printing System Parameters") and remains a constant even with nozzle replacement. Using the manual control settings, silicone is preferably pumped into the system until it has reached the nozzle. Finally, the part is prepared for printing by Slic3r and all axes are zeroed.

The initial layer is printed onto the dry, contaminant-free, print-bed. The print- bed is heated to 85°C through water contact on its underside. This may ensure good adhesion of the silicone, vital for a successful print. The initial layer preferably begins with three perimeter loops to ensure a smooth stream of silicone is being extruded prior to part printing.

Subsequent layers are printed either with rising water level, water or hydrogel submersion. Thin objects, up to 3 layers thick, can however be printed using simply a dry print-bed and allowing for heat to propagate through previous layers. As the print progresses new silicone must preferably be pre-mixed and added to the system. This may preferably done in small quantities to ensure it is consumed prior to partial curing. When using the rising water level procedure, upon layer change the button must preferably be pressed for addition of water via pump. If water-submersion of hydrogel are used, upon reaching the third layer the required quantity must be added to ensure submersion throughout the remaining print. Upon completion of the print, the part should preferably be left in the bath for a few additional seconds to ensure the last layer is fully-cured before removal.

The example printing process can also be described by the following the steps:

1. Mix equal parts A and B of Ecoflex and add to the extrusion system container

2. Cleaning the heat bed - Making sure there is no previous material in the way of the print

3. Homing all 3 X-Y-Z axes

4. Upload CAD and slice using the inbuilt Slic3r

5. Start print

6. Wait for first few layers to be printed -If doing less than 3 layers than next steps can be ignored.

7. After the third layer has been completed, Press button on water control circuit to add water into the system

8. If the storage of Ecoflex is running low throughout the print, mix equal parts A and B of Ecoflex and add to the extrusion system container.

9. Press button on water control circuit after each layer has completed to add more water into the system.

10. Once the print has finished, wait a few seconds before draining the water from the bath.

11. Gently remove printed part using a scraper if needed.

If doing an underwater print/in gel print, replace step 7 with the addition of enough water/gel to cover the entire completed print. Also skip step 9.

If doing a manual water level rising print, replace pushing the button with using a syringe for the addition of water.

In one embodiment, the method of printing comprises the steps:

1. depositing silicone onto the print-bed via the silicone deposition member, the deposited silicone forms the start of the silicone structure to be printed, 2. modifying the relative level of liquid in the liquid bath to the height of the silicone structure, such that the silicone structure is at least partially submerged in the liquid. For example, the relative level of liquid in the liquid bath to the height of the silicone structure may be modified by increasing the volume of liquid in the liquid bath. Alternatively, the height of the print-bed may be lowered so that the silicone structure currently printed on the print-bed is at least partially

submerged into the liquid,

3. printing the next layer of silicone on the current silicone structure,

4. repeat steps 3 and 4 until the entire structure is complete, and then

5. remove the silicone structure from the 3D printer.

Surfactants

In an embodiment, surfactants or other additives may be added to the liquid bath. The addition of surfactants reduces the surface tension between the liquid contents of the water bath and the 3D structure being printed .

The surfactants may be already present in the liquid contained in the water bath, or may be added during the 3D printing process as required.

With these additives a cleaner 3D printed structure can be produced with fewer flaws. By controlling how the liquid wets the printed silicone, more consistent water contact can improve accuracy of the product shape and edges.

Example surfactants include dish washing liquid.

Post Processing

Post processing is a method that is used in most manufacturing processes. The example prints had uneven edges and needed to be cut if they wanted to look completely square. Since this is common practice in most manufacturing processes, it was deemed acceptable to have a small amount of post-processing to tidy up the print. Most of the deformation in the edges can be blamed on the movement in the X-Y direction of the heat bed. Changing this to a printer that had X-Y direction in the printer head may significantly reduce the need for post-processing.

Layer Height

Selecting the right layer height was found in some configurations to be vital to the success to the print quality. If the layer height was too high then the material's surface tension may cause the silicone to come out in small droplets rather than a continuous flow. If the layer height was too low then the silicone may cure too fast from leaving the nozzle. This may result in the silicone sticking to the nozzle rather than the print bed. Adjusting temperature, potentially lowering it, might result in being able to print with lower layer height. Relative Liquid Level

Figures 13A through 13C describe various embodiments according to the present invention relating to the relative liquid height in a water bath 1304. The relative liquid height is the height of the liquid 1320 relative to the height of the structure being printed 1306, 1334. Figure 13A shows an example the 3D printing system 1302. The 3D printing system 1302 may be in the form of an add-on for an existing 3D printer, a new 3D printer, or a retrofitter 3D printer. The system 1302 comprises a water bath 1304, a print-bed member 1312, the print-bed member comprising a print-bed 1308 and a stand or standoff 1310. The house-like 3D printed structure 1306 has an associated height 1362. The water bath 1304 comprises a water 1305. The water 1305 has an associated water height 1364. The relative liquid level 1366 is such that the 3D printed structure 1306 is not submerged in the water 1305.

Figure 13B discloses a 3D printing system 1342, a variation of the system of Figure 13A wherein the print-bed member 1316 is adjustable. In this example, the height of the print-bed member 1316 is adjustable. The water level 1320 is higher than the height of the 3D printed structure 1322. In this example, the relative liquid level 1324 is such that the 3D printed structure is submerged in the water 1320.

The example shown in Figure 13B is configured to adjust the height of the print- bed 1308, or print-bed member 1316, such that the 3D structure being printed 1306 at the time is at least partially submerged in the water 1305. As explained above, maintaining the 3D structure 1306 in the water 1305 assists in accelerating the silicone cure time. Alternatively described, the silicone cure time is reduced when the silicone is in the water. In this example, the water is also heated to further reduce curing times. In one embodiment, the height of the print-bed 1308, or print-bed member 1316 is adjusted using a print-bed adjustment member 1314. In one embodiment, the height of the print-bed 1308 is adjust such that the 3D printed structure 1306 is at least partially submerged in the water 1305 during the printing of the 3D structure. Alternatively, the 3D printed structure 1306 may be completely submerged in the water 1305 while the 3D printed structure 1306 is being printed.

Figure 13C shows an alternative embodiment 1352 wherein the relative liquid level is maintained by increasing the volume of water 1305 in the water bath 1304. In this embodiment, the 3D structure being printed 1334 is completely submerged in the water 1305. As the height 1322 of the 3D printed structure 1334 increases due to it being 3D printed, the volume of water is increased such that the relative liquid level is maintained.

A person skilled in the art that the techniques described in relation to Figures 13B and 13C may be used in combination with one another. A system of this form requires a controller or hardware system capable of determining the liquid level and the structure height. Determining the liquid level and/or structure height can be conducted using routine measuring methods, or based on the volumes of liquid and/or silicone added.

A person skilled in the art will appreciate that various control systems may be used to adjust the liquid level and/or print-bed height. In the examples presented in Figures 13B and 13C, during printing, the control system adjusts either the print-bed height, or the total liquid volume, to maintain a constant liquid relative liquid height.

In the embodiments described in Figures 13B and 13C, the relative liquid level is maintained substantially constant. A person skilled in the art will appreciate that the relative liquid level may also be maintained in other ways. For example, the relative liquid level may be maintained to change linearly with amount of silicone printed such that there is enough thermal mass in the water to cure the silicone.

When the 3D printed structured has completed its print and the silicone has cured, the relative liquid level will also change so that a user can remove the 3D printed structure without touching the hot water.

Printing Results

Various objects were printed to display the example 3D printer and methods from the "3D Printer and Operating the 3D Printer" section described above ability to fabricate geometric features.

2.5D Shapes

In the process of establishing printing parameters the system was used to print simple 2.5D shapes. The squares 82, 86 and circle 84 depicted in Figure 8 are 3 layers (1.5mm) thick where each layer is 0.5mm and prove the example system's ability to print highly accurate objects without water contact. Here, curing is taking place through the heating of the print-bed, which is warmed to 85°C through water contact on its underside.

Table 3 summarises the accuracy achieved when printing 2.5D objects:

Square Circle

(length x width x height) (diameter x height)

Desired (mm) 25 x 25 x 1.5 25 x 1.5

Measure (mm) 25.1 x 24.9 x 1.5 25.1 x 1.5

Table 3 - Accuracy of 2.5D objects Simple 3D Structures

Figure 9 shows the quality achievable currently with using the example 3D printer configuration with the rising water level and water-submerged techniques. Figure 9 shows particularly jagged edges for the sample produced under rising water level on the left. Figure 9 also shows the structure on the right created using the water-submerged technique. It is believed this may be radically reduced with a change of printer configuration to one where the print bath does not have X- or Y-direction motion which causes unfavourable sloshing of water. Water-submerged printing was affected less by sloshing water but saw increased rates of curing in the extrusion nozzle. A person skilled in the art will appreciate that these and other further configuration and/or customisation options may be used to improve the results presented . The results presented are indicative of a correctly functioning system and should not be used as examples of desired output.

Table 4 summarises the accuracy achieved for simple 3D prints:

Rising Water Level Water-submerged

(length x width x height) (length x width x height)

Desired Cuboid (mm) 25 x 25 x 5 25 x 25 x 5

Measure Cuboid (mm) 25 x 25 x 5 25 x 25 x 5

Table 4 - Accuracy of 3D structures

Hollow Structures

To demonstrate the feasibility of printing voids a bridge was printing using the rising water level technique as well as in the hydrogel technique.

Rising Water Level Technique

Figure 10 shows a bridge successfully printed in a water-surfactant solution clearly showing the potential of the rising water level procedure even with the evident flaws in the sample produced. The inconsistent material deposition was the result of partial curing of the material in the system, as it was noticeable that this increased as the print progressed .

Similar to the "Simple 3D Structures" examples above, the walls of the printed bridge were jagged. This defect was attributed to poor water level control in addition to sloshing of water.

Hydrogel Technique

Figure 11 below shows a successfully bridge successfully printed in hydrogel. EcoFlex was initially extruded onto the dry print-bed in the heated bath. After this initial layer had adhered and extrusion of consequent layers had begun, the hydrogel solution was poured onto the print-bed, submerging the so far deposited material. The remainder of the bridge print occurred with the nozzle dragging through the hydrogel solution.

While the hydrogel structure reduced sloshing and therefore jagged edges, it also increased the surface tension and therefore caused the mushrooming. See for example the mushrooming 102 in Figure 11. Adding a surfactant to the hydrogel dissolved this, hereby negating the stiffening effect. Some limitations of dragging the nozzle through gel are:

• a gel based system requires a very long needle for large prints,

• needle forces occur when printing in large bulks of gel,

· dragging of a needle through support material can distort prints, and

• if the gel solution is heated, curing in the needle may occur the same way as printing in submerged heated water.

Viscosity Measurements

Measurements of viscosity of some example silicones were made. In particular, the viscosity increase characteristics of Ecoflex 0030 and Sylgard 184 as time proceeded were measures. As can be seen from the graph plotted in Figure 3, viscosity linearly increases with time for EcoFlex 0030 and Sylgard 184. The more steeply slanted line represents the Ecoflex 0030, and the shallower line represents the Sylgard 184. The viscosity of these two materials were also taken every 15 seconds for 15 minutes to show the rate of change in viscosity as the two silicones cure. It must also be noted is that impure EcoFlex (e.g. due to air trapped in mixture) may significantly alter viscosity. These high numbers show that there would need to be a high force to deposit/dispense the silicone in any rig design.

In an embodiment, the 3D printer of the present invention uses a higher force than a standard 3D printer. In an embodiment, the 3D printer of the present invention uses a sufficient force to move the nozzle while printing silicone.

Alternative Embodiments and Enhancements

The following features may improve the quality of the 3D printer technology and techniques described.

· An active mixing system may allow high duration prints to run without constant refilling with freshly mixed silicone. This may help automate the system. When designing this system consideration should be given to the ability to clean the parts that come in contact with the two-component solution.

• Improvement of water level control through a pump and sensors as well as

evaporation rate adjustment is likely to improve the fidelity of prints in the Z- direction. Improving the water temperature control may also be beneficial to enable higher bath temperatures to be maintained with smaller temperature fluctuations.

• Switching to a 3D printer using a delta configuration or a XYZ-motion enabled printhead. It may be adequate to have XY-motion of the print head and Z- direction motion of the print bath. Using a different printer configuration may prevent water sloshing about in the bath, consequently improving print quality.

• The implementation of an improved retraction procedure to compensate for

compressibility of EcoFlex may prevent dripping as the printer is traversing gaps. · To prevent curing of EcoFlex in components closest to the heated bath thermal insulation or active cooling should be considered. Whilst designing this the ability to clean or replace parts should not be overlooked.

Advantages

The new Rising Water Level technique provides a number of potential advantages. Over the hydrogel technique, some advantages include:

• faster curing - i.e. it does not require post-curing,

• easier support material removal - water runs off,

• the overall scales better, a gel based system requires a very long needle for large prints,

· avoids needle forces that occur when printing in large bulks of gel, and

• avoids dragging a needle through support material which can cause distort prints.

In comparison to other 3D printing techniques, such as SLA, the Rising Water Level technique:

• does not require a vat full of print material,

· allows slow-cure materials to be used (in a vat these would cure over printing time),

• does not require light/UV-curable material, hereby generating a wider selection of materials,

• avoids needle forces that occur when printing in large bulks of gel, and

· avoids dragging a needle through support material which can cause distort prints.