Method of smoothing a surface of an additively manufactured part

The present invention provides apparatus for post-processing an additively manufactured polymer part, comprising a reservoir (402) for containing a liquid solvent; a processing chamber (408) in controllable fluid communication with the reservoir, and a controller (418) configured to controllably post-process an additively manufactured polymer part located in the processing chamber by the solvent responsive to at least one parameter associated with the part. A method for post-processing an additively manufactured polymer part is also provided.

BACKGROUND

Related Field

The present invention relates to additive manufacturing processes. In particular, but not exclusively, the present invention relates to post-processing of polymer parts which have been made by a powder based additive manufacturing process, such as laser or infrared sintering, or an extrusion based process such as fused deposition modelling (FDM).

Related Art

A motion simulator is a mechanism that can create, for an occupant, the effects or feelings of being in a moving vehicle and includes a motion system i.e. at least one motion generator and an associated control system. Motion simulators are used, professionally, for training drivers and pilots in the form of driving simulators and flight simulators respectively. They also are used, industrially, in the creation, design, and testing of the vehicles themselves.

Additive manufacturing (AM) is a process of joining materials to make objects from 3D model data, typically layer-upon-layer, as opposed to subtractive manufacturing techniques, such as milling or cutting. AM processes are generally classed according to the machines and technologies used, e.g. laser or extrusion, and the state of the raw material used, i.e. liquid-based, powder-based, or solid-based. The different processing conditions encountered in AM produce different mechanical properties and surface finishes, particularly compared to traditional manufacturing techniques. For example, the surface finish of injection moulding is determined by the mould, whereas the outer surface of AM parts has a rough feel and is visually dull as a result of the layer-upon-layer process. Such a surface finish is often undesirable, particularly if the part is intended for an aesthetic application. Furthermore, surface roughness may adversely affect the mechanical performance of the AM part, such as tear strength, tensile and/or bending strength, Young's modulus of the material, and fracture strain. Additionally, a rough surface finish may be difficult to clean, may stain easily, or may cause undesirable damage to adjacent parts in use.

As such, AM parts are often post-processed after manufacture in an attempt to smooth the often undesirable rough outer surface and sometimes also to remove support structures used in the manufacturing process. The two key categories of post-processing are primary and secondary post-processing. Primary post-processing typically includes the mandatory steps that must be performed on all AM parts to make them suitable for use in any application. These steps vary by technology but generally include cleaning and support structure removal. Secondary post-processing includes optional part finishing to improve the aesthetics and/or function of the part. Most commonly, secondary post-processing includes sanding, filling, priming, and painting. Typically, polymer AM parts are mechanically tumbled, sanded or coated to improve the surface appearance and feel. However, these primary and secondary post-processing steps are manual which adds significant cost and time to the manufacturing process in view of the significant hands-on labour which is required, and which thus potentially erodes the benefits of AM in some applications. Such manual post-processing can also introduce unquantifiable dimension variation in the final part geometry and it is difficult, if not impossible, for a part finisher to obtain exactly the same amount and degree of finishing for each part, i.e. a repeatable and reproducible surface finish. For intricate parts with delicate features, a duplicate part is often manufactured just in case the original is damaged or broken during the post-processing stage, which undesirably further increases costs, energy and materials. Fine detailing on an AM part may be irrevocably lost as a result of manual post-processing techniques. Furthermore, the manual post-processing techniques are ‘line of sight’ processes and are not suitable for smoothing hidden/intricate features, such as inner surfaces of lattice structures, for example.

As such, the perception that AM techniques offer a digital workflow that is simple, fast and automated is only accurate up to the moment the AM parts are removed from the AM machine. As soon as the AM parts enter the post-processing phase, the automated, push-button process becomes a manual operation that undesirably impacts time, cost, materials, energy and quality.

The use of a solvent vapour, such as Acetone, to improve the surface finish of AM parts primarily made from ABS type materials is known. Such a treatment relies on a suitable solvent to dissolve some of the rough outer surface of the AM part to enhance the surface smoothness, gloss and potentially the mechanical properties of the part itself. However, these known solvent treatments are also manual and labour intensive and thus suffer from the same disadvantages as the mechanical techniques described above. Furthermore, known solvent treatments are not suitable for improving the surface finish of Nylon 12 which is the most common polymer used for laser sintering and which is part of the polyamide group of materials which are particularly resistant to chemicals such as Acetone. In addition, in some cases there has been found to be an increase in weight of the AM part after using a solvent post-processing treatment to improve surface finish. This undesirable weight gain can be up to 8% and has been found to be a result of water absorption (particularly in Nylon™ materials) in the surface layers of the AM part.

BRIEF SUMMARY

It is an aim of certain embodiments of the present invention to provide an automated method of post-processing an AM polymer part to improve the surface finish thereof without compromising the integrity, appearance or performance of the part.

It is an aim of certain embodiments of the present invention to provide an automated method of post-processing an AM polymer part to improve the surface finish thereof which is efficient and consistent in terms of quality, time, cost and materials.

It is an aim of certain embodiments of the present invention to provide an automated method of post-processing an AM polymer part to improve the surface finish thereof by exposing the AM part to a solvent in a controlled manner to achieve a desired surface finish without undesirable weight gain.

It is an aim of certain embodiments of the present invention to provide an automated method of post-processing an AM polymer part in an ‘intelligent’ manner whereby machine learning from multiple apparatuses is used to improve the processing parameters and efficiency of the part finishing process.

It is an aim of certain embodiments of the present invention to provide apparatus for carrying out an automated method of post-processing an AM polymer part to improve the surface finish thereof in a controlled and automated manner which is efficient and environmentally friendly.

According to a first aspect of the present invention there is provided apparatus for post-processing an additively manufactured polymer part, comprising: a reservoir for containing a liquid solvent; a processing chamber in controllable fluid communication with the reservoir; and a controller configured to controllably post-process an additively manufactured polymer part located in the processing chamber by the solvent responsive to at least one parameter associated with the part.

Optionally, the at least one parameter comprises a material of the part, a desired surface roughness of the part, and/or a geometric property of the part including surface area, volume, dimension, and/or part complexity.

Optionally, the apparatus further comprises a vacuum pump operably controllable by the controller and configured to apply a negative pressure to an interior of the processing chamber.

Optionally, the controller is configured to selectively operate the vacuum pump for a predetermined time to apply a negative pressure in the processing chamber.

Optionally, the negative pressure is around 10-400 mbar.

Optionally, the apparatus further comprises a solvent delivery valve operably controllable by the controller to allow a predetermined amount of solvent to be selectively drawn into the processing chamber by the negative pressure applied therein.

Optionally, the apparatus further comprises a solvent delivery system located upstream of the processing chamber and operably controllable by the controller for selectively receiving solvent from the reservoir and introducing a predetermined amount of solvent into the processing chamber.

Optionally, the solvent delivery system comprises a pump and a dosing valve for controlled delivery of the predetermined amount solvent into the processing chamber.

Optionally, the solvent delivery system further comprises a heating element configured to controllably heat the predetermined amount of solvent to a predetermined solvent temperature.

Optionally, the predetermined solvent temperature causes the solvent to vaporise.

Optionally, the solvent is introduced into the processing chamber via a solvent distribution system.

Optionally, the solvent distribution system comprises at least one nozzle for controlled distribution of solvent vapour into the processing chamber.

Optionally, the apparatus further comprises a heater for controlled heating of an interior of the processing chamber to a predetermined chamber temperature.

Optionally, the heater comprises a heating element configured to controllably heat at least one inner surface of the processing chamber.

Optionally, the apparatus further comprises a cooler for controlled cooling of an AM part located in the processing chamber.

Optionally, the controller is configured to selectively operate the vacuum pump for a further predetermined time to reapply a negative pressure in the processing chamber after a predetermined processing time has elapsed.

Optionally, the processing time is between around 5 seconds to around 120 minutes based on the at least one parameter.

Optionally, the vacuum pump is further configured to selectively vent the processing chamber to atmosphere when the negative pressure has been reapplied to extract solvent vapour from the processing chamber and from the part located therein.

Optionally, the apparatus further comprises a solvent recovery system to recover used solvent from the processing chamber.

Optionally, the solvent recovery system comprises a condenser to separate solvent from air.

Optionally, the solvent recovery system is fluidly connected with the reservoir and/or the solvent delivery system such that recovered used solvent can be returned thereto.

Optionally, the reservoir comprises a first compartment for virgin solvent and a further compartment for recovered solvent.

Optionally, the apparatus further comprises a user interface for a user to input the material of the part to be processed and the desired surface roughness, wherein the controller is further configured to execute a processing program based on at least the material and the desired surface roughness.

According to a second aspect of the present invention there is provided a method of post-processing an additively manufactured polymer part, comprising: locating an additively manufactured polymer part in a processing chamber; and responsive to at least one parameter associated with the part, controllably post-processing the part by a solvent controllably introduced into the processing chamber.

Optionally, the at least one parameter comprises a material of the part, a desired surface roughness of the part, and/or a geometric property of the part including surface area, volume, dimension, and/or complexity.

Optionally, the method further comprises selectively delivering a predetermined amount of solvent into the processing chamber.

Optionally, the method further comprises applying a negative pressure to an interior of the processing chamber for a predetermined time.

Optionally, the method further comprises using the negative pressure to selectively deliver the predetermined amount of solvent into the processing chamber.

Optionally, the method further comprises heating the predetermined amount of solvent to a predetermined solvent temperature to cause the solvent to vaporise prior to entering the processing chamber.

Optionally, the method further comprises selectively heating an interior of the processing chamber to a predetermined chamber temperature.

Optionally, the method further comprises heating at least one inner surface of the processing chamber.

Optionally, the method further comprises creating an energy potential between an AM part located in the processing chamber and the solvent.

Optionally, the method further comprises selectively cooling an AM part located in the processing chamber to create the energy potential and cause solvent vapour to condense on the AM part.

Optionally, the method further comprises reapplying a negative pressure in the processing chamber for a predetermined time after a predetermined processing time has elapsed.

Optionally, the method further comprises selectively venting the processing chamber to atmosphere when the negative pressure has been reapplied to extract solvent vapour from the processing chamber and from the part located therein.

Optionally, the method further comprises recovering used solvent from the processing chamber.

Optionally, the method further comprises returning recovered used solvent to the reservoir.

Optionally, the method further comprises inputting or selecting the at least one parameter associated with the part via a user interface.

Optionally, the method further comprises executing a processing program by a controller based on the at least one parameter.

According to a third aspect of the present invention there is provided a computer program that, when executed by a computer, performs the method according to the second aspect of the present invention.

According to a fourth aspect of the present invention there is provided a use of a solvent to controllably post-process an additively manufactured polymer part responsive to at least one parameter associated with the part.

Optionally, the use further comprises selectively creating an energy potential between an AM part and the solvent.

Optionally, the use further comprises selectively controlling a solvent temperature and an AM part temperature wherein the solvent temperature is greater than the AM part temperature.

Optionally, the use further comprises heating the solvent to a predetermined solvent temperature and cooling the AM part to create a desired temperature gradient therebetween to cause solvent vapour to condense on the AM part for a predetermined processing time.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

As shown inFIG.1A, apparatus100according to certain embodiments of the present invention includes a removable solvent cartridge102, a solvent dosing and heating system104for containing a solvent, a solvent distribution system106, a processing chamber108, a vacuum pump110, and a solvent recovery system112. The apparatus also includes a control chamber114connected to the solvent dosing and heating system104and an air heater pump116for venting heated air into the processing chamber108. The processing chamber108also contains a part support system109such as a rack or the like.

A first control valve Vi is located between the solvent cartridge102and the solvent dosing/heating system104. A second control valve V2is located between the solvent dosing/heating system104and the solvent distribution system106. A third control valve V3is located between the solvent dosing/heating system104and the control chamber114. A fourth control valve V4is located downstream of the chamber, aptly at the vacuum pump110, to allow the apparatus, particularly the processing chamber108, to be selectively vented to atmosphere. A fifth control valve V5is located between the solvent recovery system112and the solvent dosing and heating system104/solvent cartridge102. Valve V5is a diverter valve which selectively controls the flow of recovered solvent to the solvent dosing and heating system104or the cartridge102as described further below.

The apparatus100further includes a controller118having a user interface, e.g. a touchscreen display, to allow an operator to input predetermined parameters associated with the AM part to be post-processed, as will be described further below. An operator may interact with the controller either at the apparatus or remotely via a wired or wireless connection by using an application stored on a computing device such as a tablet or mobile phone. The controller118is electrically connected to the controllable components through a computer system such as a Raspberry Pi™ or other open or closed sourced system, to the systems and sensors of the apparatus100to allow the controller to automatically and selectively control the apparatus, whilst also receiving feedback signals during a post-processing operation, as described further below. The controller is also configured to record apparatus ambient operating conditions, namely ambient temperature AT1and humidity AH1while the apparatus is in use. The controller is aptly connected to an emergency stop button ES1to immediately stop the machine should it be required. A sensor AP1is also present to stop the apparatus100in the event of any maintenance access panels being removed during operation. The controller118is also in read/write communication with a data storage medium, such as a cloud-based database120, to allow operating parameters, programs and data to be accessed and/or stored. For illustrative purposes only, the flow of solvent in a liquid state during the postprocessing operation is shown by a dot-dashed line and referenced SPL. The flow of solvent in a vapour state is shown by a relatively thick solid line and referenced SPv. The flow of water is referenced WP and the flow of air is referenced A. The control links are shown in a relatively thin line and referenced C.

FIG.2illustrates a flow diagram showing the steps of a method S200of postprocessing an AM part using the apparatus100according to certain embodiments of the present invention. As illustrated at step S202, a ‘pre-packaged’ and sealed cartridge102containing a suitable solvent is selected and removably coupled to the apparatus100via a suitable connection102csuch as a quick release mechanism or the like. As shown inFIG.1B, the solvent cartridge102includes two compartments; one for virgin solvent and another for used solvent. A sensor SLi senses the virgin solvent level and a sensor S1_3senses the used solvent level. The virgin solvent compartment has at least an outlet and the used solvent compartment has at least in an inlet for returning used solvent to the cartridge as described further below. The cartridge102includes a closed source electronics chip102awhich communicates with the controller118to allow sensor signals to be operably sent thereto. The cartridge102also includes a quick release coupling handle mechanism102bfor handling the cartridge when connecting to and removing from the apparatus. The handle mechanism102bmay be coupled to the connection mechanism102csuch that the handle mechanism102bactuates a quick connect/release coupling mechanism102cto quickly and easily couple/remove the cartridge to/from the apparatus. A sensor QRi (not shown) may be provided to confirm to the controller118that the cartridge is properly located and connected to the apparatus.

Suitable solvents include protonic polar solvent and non-proton polar solvents, such as, but not limited to, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), dimethylformamide, sulphuric acid, m-cresol, formic acid, trifluoroacetic acid, and benzyl alcohol. Aptly a solvent that exhibits strong hydrogen bonding properties may be used. This property will allow the solvent to process substances that serve as hydrogen bond acceptors, such as Nylons™ (Polyamides).

At step S204, at least one AM polymer part is placed in the processing chamber108via an access door thereof, such as a scalable lid located on an upper surface of the apparatus for easy access, and on to an optional support structure109, such as racking, located in the chamber. The support structure109is configured to ensure the part is fully exposed to solvent introduced into the chamber. The access door/lid is closed and the chamber108is sealed. The chamber108is sealed once a process chamber door sensor Do/Dc indicates that the door is closed.

Aptly, via the user interface of the controller118, or automatically in response to the chamber being sealed and/or a part being located therein, a systems check is run by the controller118to ensure the apparatus100is in a ‘neutral’ state and ready to process AM parts. The neutral state check ensures that all control valves are in the correct positions, the solvent cartridge contains a sufficient amount of virgin solvent and has been correctly coupled to the apparatus, and the process chamber door/lid is securely closed.

At S206, a user inputs a digital CAD file into the controller118for the AM part to be processed either via the Internet/Ethernet/wireless connection from a program/application or from a database of pre-stored CAD data files. The CAD file details the parts to be processed in the orientation and position they are located on the support structure109in the processing chamber108. This ensures repeatability and reproducibility and also allows accurate calculation of processing parameters by understanding the surface area and ‘complexity’ of the parts. At S208, via the user interface of the controller118, the user selects the material of the AM part and the desired surface finish, e.g. roughness, which in turn determines the required processing program to be selected and run by the controller. The part material may be selected from a non-exhaustive list of materials including Nylon 12 (PA220 Duraform™ PA), Nylon 11 (Duraform™ EX Natural, Duraform™ EX Black), Thermoplastic Polyurethane (TPU), TPE-210 elastomer materials, ABS, or the like. The touchscreen display may list a number of selectable materials for the user to select responsive to the AM part to be post-processed. The surface roughness may be selected from a range of around 1.5 μιη to the ‘as printed’ surface roughness (i.e. no smoothing), in around 1 μιη increments. Alternatively, a surface finish may be selected based on finish descriptions, e.g. matt or gloss, and/or visual examples displayed on the display of the user interface or mobile computing device.

At step S210, the processing program is selected by the controller118in response to the selected part material and desired surface roughness and is then executed.

As shown inFIG.1C, the solvent dosing/heating system104includes a solvent dosing chamber104aand a solvent level sensor S1_2to sense a level of solvent therein, a solvent dosing pump104band a pressure sensor Spi to sense a pressure of the solvent being pumped thereby, a further control valve VIA, and a solvent heating element104c, such as a heating coil, and a temperature sensor Sti to sense a temperature of the solvent in the heating system104.

At step S212, the first control valve Vi is opened by the controller118and a predetermined amount of solvent to at least partially fill the chamber104aof the solvent dosing/heating system104, aptly around 1000 ml, is transferred from the cartridge102to the solvent dosing/heating system104using the solvent dosing pump104bat a controlled solvent pump dosing speed SDs. At step S214, the first control valve Vi is closed. A predetermined dose of liquid solvent is transferred from the dosing chamber104ato the solvent heater coil104cvia valve V1A by the solvent dosing pump104b. The ‘dose’ of liquid solvent is heated through the solvent heater coil104cto a predetermined temperature Sti by suitable heating means, e.g. an electrical heating element located in or around the solvent heater coil104c. The solvent quantity is calculated as a function of the type of solvent being used, the volume of the processing chamber108, the surface area of the part/s, the processing temperature (a function of solvent boiling point) and the processing pressure (vacuum). The dosing amount from the solvent dosing chamber through the heating coil is around 25-200 ml. The relationship determines the correct amount of solvent in order to achieve the required uniform layer thickness across all the parts to be processed. Alternatively, the cartridge102itself may be heated by a separate or integral heating means, such as an electrical heating element disposed in the cartridge, to elevate the temperature of the solvent therein to the predetermined temperature Sri. In such an embodiment, the solvent dosing/heating system104may not be required and the solvent may be introduced into the processing chamber108directly from the cartridge instead of via the dosing/heating system104. The solvent is heated to enable a range of solvents with different boiling points to be processed effectively. The solvent temperature Sti may be between room temperature and 100° C. depending on the type of solvent to be used for the AM part being processed. Some solvents may not require heating. Preheating the solvent to transform it into a vapour phase before it is introduced into the processing chamber108allows a lower pressure differential (vacuum) to be applied to the solvent, and therefore less energy, to transform the solvent from a liquid state to a vapour state and draw the solvent vapour into the processing chamber108. In addition, the ability to heat the solvent can speed up the thermodynamic process of solvent evaporation.

At step S216, a negative pressure is then applied inside the processing chamber108by the vacuum pump110for a vacuum time VTi to create a reduced chamber pressure VPi of about around 10-200 mbar, and aptly around 70 mbar. Aptly, an absolute vacuum may be created in the processing chamber108. The critical pressure as measured by sensor ACPI in the processing chamber108is defined as the point below which the solvent will evaporate at room temperature, i.e. at the critical point when the solvent is in the vapour phase before entering the gaseous phase if the pressure/temperature was increased.

At step S218, the second and third control valves V2and V3are opened and the pressure difference between the solvent dosing/heating system104and the processing chamber108and the control chamber114draws the solvent into both chambers108,114. In view of the pressure differential between the solvent vapour at atmospheric pressure and the selected vacuum pressure VPi applied to the chambers108,114, the ‘dose’ of solvent is instantly drawn into the processing chamber108and the control chamber114and instantly fills the chambers108,114. The AM part/s located in the processing chamber108and a test coupon/specimen located in the control chamber114are fully surrounded by solvent vapour. The dosing quantity is controlled by the solvent dosing/heating system104and is defined as the amount of solvent required to fully saturate the vapour phase. This is governed by the vapour-liquid equilibria of the specific solvent in relation to the temperature and pressure of the vapour and liquid phases. As shown inFIG.1D, the solvent distribution system106includes three distribution channels106aeach having a plurality of apertures equally spaced to ensure even and rapid introduction of the solvent vapour phase into the vacuum process chamber108. As an alternative, or in addition, to a negative pressure being applied to the processing chamber108, a positive pressure may be applied to the solvent to urge the same into the chambers108,114from the solvent reservoir104.

At step220, the second and third control valves V2and V3are closed.

At step222, the fourth control valve V4is opened and ambient air heated by a suitable heater H1(not shown) of the heater pump116to a predetermined temperature aT is drawn or pumped into the processing chamber108. With increased temperature and pressure in the processing chamber108, the solvent vapour instantly condenses onto the AM part/s to form a uniform residue and even film of solvent in the liquid state on the part's outer surfaces. Alternatively, the second control valve V2may be opened to expose the processing chamber108to the solvent dosing/heating system104and thereby to additional solvent to control the desired condensation rate.

The condensed liquid solvent film is maintained on the AM part/s for a predetermined part exposure time PET1which can range from around 5 seconds for Nylon 12 parts to around 10 minutes for TPU type parts. As illustrated inFIG.3, the predetermined part exposure time PETi determines the final surface roughness of the part/s and can be controlled to around 1.5 μιη.

When the predetermined part exposure time PETi for the part/s being processed has been reached, the vacuum pump110(at step S224) applies a negative pressure VP2of about around 10-200 mbar to the processing chamber108for a predetermined time VT2. The reapplication of a negative pressure to the processing chamber108increases vapour pressure to return the condensed solvent from a liquid state to a vapour state.

At step S226, the fourth control valve V4is opened and the processing chamber108is vented to atmosphere through the vacuum pump110and the solvent vapour is fully removed from the surface of the processed AM part/s and from the processing chamber108itself. A carbon filter is present on the external atmosphere manifold and heater pump to ensure no solvent vapour is released into the atmosphere. No residual vapour remains on the surface of the part/s, no residual trace is left, and in turn the post-processing of the part/s is immediately stopped. This not only ensures the process is fully controlled, but also some applications, such as for medical or dental devices, require the parts to be fully clean and safe to use, and also to possess a particularly accurate surface roughness which would not be achievable if any further processing of the part occurred as a result of any remaining solvent residue on the surfaces of the part.

A solvent sensor SSi is located in the processing chamber108to sense the presence of solvent vapour therein and send a signal to the controller118indicating whether or not solvent exists in the chamber108. At step S228, the extraction/drying step is repeated until no solvent (Volatile Organic Compounds-VOCs) is detected within the chamber108by the solvent sensor SSi located therein. The solvent vapour extracted from the processing chamber108may be passed through a solvent recovery unit112, which consists of a Peltier module heat pump system or the like. The solvent vapour is condensed across the Peltier heat pump system in the solvent recovery system112at a predetermined temperature CT1, and the condensed liquid (solvent and water in view of the heated ambient air being introduced into the processing chamber at step S222) is collected in a liquid trap at the bottom of the solvent recovery system112. The liquid (containing both the solvent and water) is then distilled through a heater coil at a temperature HCT1above the solvent's boiling temperature, but below the water's boiling temperature, i.e. 100° C.). This vaporises the solvent, while keeping the water in the liquid phase. The re-vaporised solvent is then passed back through the Peltier heat pump to condense from a vapour to a liquid. The water is recovered in the waste water container122. Alternatively, a molecular sieve may be used to remove the water from the system. The recovered solvent is then returned to the solvent dosing/heating system104via the fifth control valve Vs. The apparatus100therefore forms a closed loop. The solvent is used for around a hundred operations or until a maximum surface area processed has been reached, whichever is sooner, before being recovered and sent to the used solvent compartment of the cartridge102via the diverter valve Vs for safe disposal. Before the cartridge102reaches it end of useful life and needs replacing, the electronic chip102awhich is configured to monitor how often solvent is recycled through the cartridge using sensor Nc will automatically communicate with the controller118such that a new cartridge is automatically or manually ordered for delivery. The used cartridge is removed and a new cartridge containing ‘fresh’ solvent is inserted for the next operation. The water product from the distillation is directed to a waste water container122for safe collection and disposal by the operator. The waste water container122includes a water level sensor WLI (not shown) to indicate when the container needs emptying or replacing. At step S230, in response to signal SSi indicating no solvent vapour remains in the processing chamber108, the controller118stops the vacuum pump110and closes the fourth valve V4. The processing program ends and the operator is notified by an audible and/or visual indication. At step S232, the access door of the apparatus100is opened to retrieve the post-processed AM polymer part/s from the processing chamber108. The process is repeated as desired. As shown inFIG.1E, a test coupon114b, e.g. a dog bone specimen, which is made by the same AM process and from the same material as the AM part/s being processed is located in the control chamber114which is not subject to a vacuum and is thus subject to potential water uptake from the surrounding atmosphere.

During post-processing, the test coupon is subjected to the solvent vapour for the same solvent exposure time of PET1as the AM part being processed and the weight of the test coupon is continually monitored by suitable means, such as a load cell114c. There is an increase in weight with improvement in surface roughness. This relationship has been quantified and the mass gain, which can be up to 8%, is known to be due to water absorption in the surface layers and not uptake of solvent. The original part weight calculated from the CAD data (under vacuum and without water uptake) is compared to a test coupon outside the vacuum chamber (not under vacuum and subject to water uptake). The test samples require the same surface area to volume ratio as the part being smoothed in order to provide accurate feedback.

The relationship between sample weight increase as a function of solvent exposure time PET1is used to calculate the actual achieved surface roughness. To ensure the relationship between the control chamber114and the processing chamber108conditions are in calibration, the correlation therebetween is generated through experimental feedback and results from iterative testing for a known sample quantity and surface finish per type of material. Alternatively, a non-contact optical method such as laser measurement and/or a white light interferometer or laser scanning confocal microscope could be used on the sample part in the control chamber114to determine surface roughness changes in real time. In this manner, it is possible to continuously assess if the AM part/s being smoothed in the processing chamber108and to ensure that the parts have been processed to the correct surface roughness. If the desired surface roughness has not been achieved, using feedback from the load cell data (or direct non-contact surface roughness measurement) in the control chamber114, the processing parameters may be adjusted in real time and the AM part/s in the processing chamber108will automatically be processed again until the desired surface roughness is achieved. Opening valve V3a(as shown inFIG.1E) at the end of the process allows the control chamber114to be exposed to the vacuum in the processing chamber108thus safely evacuating any solvent vapour and removing any solvent present on the surface of the sample located therein.

As such, automatic measurement of a test coupon's weight gain using a load cell whilst the AM part is being post processed accurately controls the amount (volume and time) of solvent exposure to the AM part being processed and in turn the degree of part smoothing required to achieve the desired surface finish. Furthermore, the relationship between weight gain in the test coupon and surface finish of the AM part will be created as part of the data sets generated, and the controller118is configured to use this feedback and learn over time what parameters are required to obtain a desired surface finish responsive to part material, geometry, surface area, and/or solvent type. This also enables real time verification of results and processing in the form of a closed feedback loop.

For each operation, the controller118monitors and logs all the operating variables as listed in the tables ofFIGS.4to6and uploads them to a read/write database stored on a web-based server120or the like.

A further example according to certain embodiments of the present invention will now be described. As shown inFIG.7, apparatus400for post-processing an AM part includes a removable reservoir402, a solvent dosing and heating system404, a solvent distribution system406, an AM part processing chamber408, a vacuum pump410, and a solvent recovery system412.

The solvent dosing and heating system404includes a peristatic pump405and a valve407to selectively deliver a predetermined amount/dose of solvent from the reservoir402to the processing chamber408via the solvent distribution system406. The solvent dosing and heating system404further includes a solvent flow meter409and a heating element411in the form of a heating plate or coil. A thermocouple413is provided at the heating element411to monitor a temperature STI thereof. The solvent distribution system406ensures solvent vapour is distributed evenly across the chamber408. It consists of at least one distribution channel with an aperture located, preferably centrally, within the process chamber408. Aptly, one aperture is provided per approximately 30 litres of effective process chamber volume. For a process chamber having a volume of greater than around 30 litres, more apertures are desirable to ensure the process conditions for polymer components situated at different locations within the chamber are the same. For example, a process chamber408having an effective volume of around 90 litres would require at least three apertures distributing solvent vapour across the chamber.

Within the solvent dosing and heating system404between the peristatic pump405and the valve407there may be provided another valve471to extract a liquid solvent sample out of the machine400for testing purposes.

The processing chamber408includes a removable lid415and a part support system417for supporting one or more AM parts to be post-processed, such as a rack, hooks, frame, or the like. The inner walls of the processing chamber408are aptly heatable by a suitable heating element/s and a thermocouple451is provided to monitor a temperature CT1thereof. The lid415is also aptly heatable by a suitable heating element/s and a thermocouple421is provided to monitor a temperature thereof. A pressure sensor423is provided to monitor a pressure inside the processing chamber408and a temperature sensor, e.g. a further thermocouple,419is provided to monitor a temperature therein. A switch/sensor425is also provided to sense a state of the lid relative to the processing chamber, i.e. open or closed.

Further thermocouples453,455, or the like, are provided inside and outside the apparatus to monitor internal and external ambient temperatures, and a humidity sensor457monitors ambient humidity. A pair of control valves427a,427bare provided between the processing chamber408and a stack of activated carbon filters416to allow the apparatus, particularly the processing chamber408, to be selectively vented to atmosphere. One of the valves is to allow a relatively small vent to atmosphere whilst the other valve is to allow a relatively large, e.g. complete, vent to atmosphere. A second valve429is located between the processing chamber408and the vacuum pump410for selectively recovering solvent from the processing chamber408.

Pump thermocouples459,461,463are also provided to monitor a temperature of the pump body and at the inlet and outlet thereof. A condenser valve431is provided between the vacuum pump410and the solvent recovery system412to isolate the vacuum pump for maintenance purposes. Condenser thermocouples465,467are provided at the inlet and outlet of the solvent recovery system412, and inside the condenser, and a pressure sensor469is also provided to monitor a pressure within the condenser.

A second peristatic pump433is provided between the solvent recovery system412and the solvent reservoir402for returning recovered solvent from the solvent recovery system412to the reservoir402. A further control valve435is provided between the second peristatic pump433and the solvent recovery system412and a used solvent flow meter437is provided downstream of the pump433. A further valve473is provided between the condenser412and the reservoir402to extract a sample of the recovered solvent for testing purposes.

A condenser air valve439is provided between the solvent recovery system412and the stack of filters416. Furthermore, a chilled air valve441is provided between the solvent recovery system416and the processing chamber408.

Optionally, a further control valve (not shown) may be located between solvent dosing and heating system404and the processing chamber408. In this case, the solvent dosing and heating system404would act as a high pressure vessel able to preheat solvent in liquid or vapour phase to a particular temperature before releasing it into the processing chamber.

The apparatus400further includes a controller418having a user interface, e.g. a touchscreen display, to allow an operator to input predetermined parameters associated with the AM part to be post-processed, as will be described further below. An operator may interact with the controller either at the apparatus or remotely via a wired or wireless connection by using an application stored on a computing device463such as a tablet or mobile phone. The controller418is electrically connected to the controllable components of the apparatus400through a computer system such as a Raspberry Pi™ or other open or closed sourced system, to the systems and sensors of the apparatus400to allow the controller to automatically and selectively control the apparatus, whilst also receiving feedback signals during a post-processing operation, as described further below. The controller is also configured to record apparatus ambient operating conditions, namely ambient temperature and humidity while the apparatus is in use. The controller is aptly connected to an emergency stop switch/button (signal EM1-see table ofFIG.10) to immediately stop the machine should it be required. A sensor (signal AP1) may also be present to stop the apparatus400in the event of any maintenance access panels being removed during operation. The controller418may also be in read/write communication with a data storage medium, such as a cloud-based database, to allow operating parameters, programs and data to be accessed and/or stored. For illustrative purposes only, the flow of solvent in a liquid state during the postprocessing operation is shown by a dot-dashed line inFIG.7and the flow of solvent (or air) in a vapour state is shown by a relatively thick solid line inFIG.7.

FIG.8illustrates a flow diagram showing the steps of a method of post-processing an AM part using the apparatus400as illustrated inFIG.7according to certain embodiments of the present invention.

As illustrated at step S402, a ‘pre-packaged’ and sealed reservoir402containing a suitable solvent is selected and removably coupled to the apparatus400via a suitable connection such as a quick release mechanism or the like. As shown inFIG.7, the solvent reservoir402includes two compartments420,422; one for virgin solvent and another for used/recovered solvent. A level sensor443senses the virgin solvent level in the virgin solvent compartment and a level sensor445senses the used solvent level in the used solvent compartment. The virgin solvent compartment420has at least an outlet and the used solvent compartment has at least an inlet for returning used solvent to the cartridge, as described further below. The cartridge402includes a closed source electronics chip which communicates with the controller418to allow sensor signals to be operably sent thereto. The cartridge402also may include a quick release coupling handle mechanism for handling the cartridge when connecting to and removing from the apparatus. The handle mechanism may be coupled to the connection mechanism such that the handle mechanism actuates a quick connect/release coupling mechanism to quickly and easily couple/remove the cartridge to/from the apparatus. A sensor may be provided to confirm to the controller418that the cartridge is properly located and connected to the apparatus.

Suitable solvents include protonic polar solvent and non-proton polar solvents, such as, but not limited to, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), dimethylformamide, sulphuric acid, m-cresol, formic acid, trifluoroacetic acid, and benzyl alcohol. Aptly a solvent that exhibits strong hydrogen bonding properties may be used. This property will allow the solvent to process substances that serve as hydrogen bond acceptors, such as Nylons™ (Polyamides). At step S404, at least one AM polymer part is placed in the processing chamber408via the access lid415and on to the part hooking structure417located in the chamber. The support structure417is configured to ensure the part is fully exposed to solvent introduced into the chamber. The access lid415is closed and the chamber408is sealed. A processing chamber door sensor425indicates that the lid is closed and the processing chamber is sealed. A corresponding signal L1(see table ofFIG.10) from the lid sensor425indicating the lid is closed is detected by the controller418.

Aptly, via the user interface of the controller418, or automatically in response to the chamber408being sealed and/or a part being located therein, a systems check is run by the controller418to ensure the apparatus400is in a ‘neutral’ state and ready to post-process AM parts. The neutral state check ensures that all control valves are in the correct positions, the solvent cartridge contains a sufficient amount of virgin solvent and has been correctly coupled to the apparatus, and the process chamber door/lid is securely closed.

At S406, a user inputs information relating to the AM part to be post-processed into the controller418. The information includes, but is not limited to, material, surface area, volume, geometry, complexity and/or orientation/position within the processing chamber. The part material may be selected from a non-exhaustive list of materials including Nylon 12 (PA220 Duraform™ PA), Nylon 11 (Duraform™ EX Natural, Duraform™ EX Black), Thermoplastic Polyurethane (TPU), TPE-210 elastomer materials, Acrylonitrile butadiene styrene (ABS), Acrylonitrile styrene acrylate (ASA), or the like. Other polymers which can be processed using an apparatus and method according to certain embodiments of the present invention include Polyamide 6, Polyamide 11, Polyamide 12, Polycarbonate (PC), Polypropylene (PP), Polyvinylidene fluoride (PVDF), Polyphenylene sulfide (PPS), Polyether ether ketone (PEEK), Ethylene propylene rubber (EDPM), Nitrile rubber (NBR), Thermoplastic elastometers (TPE), ULTEM™ 9085, ULTEM™ 1010, or the like. Selecting the part material from a pre-stored list of materials ensures repeatability and reproducibility of the process.

At S408, via the user interface of the controller418, the user selects the desired surface finish, e.g. smoothness/roughness, which, together with input parameters from step S406, determines the required processing program to be selected and run by the controller (see table ofFIG.11). The surface roughness may be selected from a range of around 1.Oμιη to the ‘as printed’ surface roughness (i.e. no smoothing). Alternatively, a surface finish may be selected based on finish descriptions, e.g. matt or gloss, and/or visual examples displayed on the display of the user interface or mobile computing device.

At step S410, the processing program is selected by the controller418in response to the selected part material and desired surface roughness and is then executed. The processing programs are defined according to the required levels of energy to be equalised to process particular materials (see table ofFIG.11, and graphs ofFIGS.13and14), as described further below. The energy to be equalised depends on the solvent, the material, the desired smoothness, and in turn the required temperature differences between the material and the solvent. The higher the temperature difference, the higher the energy to be equalised. For example, for Polyamide 12 the minimum amount of energy to be equalised should be: Eq=k×(TH−TM)/TH=k×(21−15)/21=0.3k, whereas for Thermoplastic polyurethane (TPU) at least: Eq=k×(TH−TM)/TH=k×[39−(−20)]/39=1.5k, where k is coefficient representing heat transfer of different polymers, TH is the temperature of HFIP vapour and TM is the temperature of the polymer to be processed. The higher values will result in a different degree of surface processing, which when combined with processing time can produce varying levels of surface finish (seeFIG.12. The Eq levels have been described relatively in the table ofFIG.12as ‘low’ and ‘high’ for each material, i.e. Eq which is ‘low’ for TPU, would be ‘high’ for Polyamide 12, whereas Eq which is ‘low’ for Polyamide 12 would not be suitable for processing TPU). TM values were derived by the applicant and are provided inFIG.11. TH values depend on the process conditions derived by the applicant as tabulated inFIG.11and are derived accordingly from the Pressure-Temperature graph of the specific solvent (e.g.FIG.15for the HFIP solvent).

At step S412, the vacuum pump410is used to reduce the pressure within the processing chamber408, and the chamber wall heaters403preheat inner walls of the chamber to a predefined temperature. The pressure and temperature of the processing chamber408are based on the AM polymer part to be processed (see table ofFIG.11).FIGS.13and14show the solvent-specific thermodynamic relationships for Polyamide 12 and Thermoplastic Polyurethane respectively.

At step S414, the solvent dosing valve407of the solvent dosing and heating system404is opened by the controller418and a predetermined amount/dose of solvent (see table ofFIG.11) is transferred from the reservoir402to the heating element411within the solvent dosing/heating system404using the solvent dosing pump405at a controlled solvent pump dosing speed SDs (see table ofFIG.10). The solvent quantity is calculated as a function of the type of solvent being used, the volume of the processing chamber108, the surface area of the part/s, and the material of the parts (see table ofFIG.11). For example, if the processing chamber408is filled with Polyamide 12 components with overall surface area of 1000 cm2, then the amount of HFIP solvent required will be 0.08 ml/cm2×1000 cm2=80 ml (see table ofFIG.11for the quantity multiplier for the HFIP solvent, which will vary depending on the type of the solvent used).

At step416, the solvent within the solvent dosing/heating system404is then heated (provided with energy) to a predetermined temperature St1(see table ofFIG.10). The amount of energy (heat) provided to the solvent depends on the thermodynamic conditions of the process, which in turn is dictated by the type of polymer to be processed (see tables ofFIGS.10and11). As explained inFIGS.13,14,15and17, the solvent has to follow a particular thermodynamic path and condense on the polymer under specific conditions for the surface of the polymer to process. The solvent temperature St1in the solvent dosing/heating system404may be between room temperature and 100° C. depending on the type of solvent to be used for the AM part being processed (seeFIG.9showing the thermodynamic graph for the HFIP solvent). Some solvents may not require heating. As a result, the solvent is turned into vapour state and is delivered into the processing chamber408via the solvent distribution system406.

The solvent distribution system406includes at least one distribution channel having a at least one aperture located preferably centrally in the chamber to ensure even and rapid introduction of the solvent vapour therein. As described above, the number of apertures located across the chamber depends on the size of the chamber, with larger chambers requiring more apertures to ensure solvent vapour covers all polymer parts located therein uniformly. It was determined by the applicant that one centrally located aperture is sufficient to uniformly distribute solvent vapour across a processing chamber having a volume of around 30 litres.

Alternatively, the cartridge402itself may be heated by a separate or integral heating means, such as an electrical heating element disposed in the cartridge, to elevate the temperature of the solvent therein to the predetermined temperature St1. In such an embodiment, the solvent dosing/heating system404may not be required and the solvent may be introduced into the processing chamber408directly from the cartridge via the solvent distribution system406.

Another alternative may be a pressure expansion type of device where the solvent dosing/heating system404is isolated by the valves from both the reservoir402and the processing chamber408. In this case, the solvent dosing/heating system404becomes a high-pressure vessel where the solvent is heated until it reaches vapor state, then the vapor is superheated and is further provided with energy that in turn increases the pressure within the solvent dosing/heating system404. A valve separating the solvent dosing/heating system404and the processing chamber408is then opened and the pressure difference between the solvent dosing/heating system404with superheated vapor and the processing chamber408drives the vapor into the processing chamber408. As a result, the apparatus allows for the combination of the pre-heated solvent vapour and the higher or lower pressure in the processing chamber108to be achieved. The temperature of the AM polymer material may also be selectively controlled by pre-cooling using chilled air, or the like, delivered into the processing chamber408from, for example, the condenser/solvent recovery system412and via the control valve441located therebetween. This in turn allows various energy level differences (i.e. temperature gradients) to be achieved between the solvent vapour and the AM polymer part for different AM polymer parts, as shown inFIGS.16A-B16a&16b. These figures illustrate the process to create various levels of surface smoothness by controlling the available energy to be equilibrated Eq, when a) Eqis set to be relatively high by increasing the temperature of the solvent vapour TH and reducing the temperature of the AM polymer part to be processed TM; and b) when Eqis set to be relatively low. In the case when Eqis relatively high, more solvent will rapidly condense on the surface of the part which will result in more dissolution on the material surface and therefore a smoother surface. In comparison, when Eqis relatively low, less solvent will condense on the material surface which will lead to less dissolution and a less smooth surface.

After the solvent vapor is introduced into the processing chamber408, the AM part/s located therein are fully surrounded by solvent vapour. The dosing quantity is controlled by the solvent dosing/heating system404and is determined according to the processing requirements of the particular AM polymer part and/or as the amount of solvent required to fully saturate the vapour phase. In the latter case, this is governed by the vapour-liquid equilibria (e.g. seeFIG.15for the HFIP vapour-liquid graph) of the specific solvent in relation to the temperature and pressure of the vapour and liquid phases.

At step418, the solvent starts to condense on the AM polymer part located in the processing chamber408. The condensation is triggered due to the pressure and/or temperature conditions and the resulting vapour oversaturation governed by the vapor-liquid equilibrium line for the particular solvent (seeFIG.15). This is manipulated by various methods: lowering the temperature within the processing chamber408using the chamber heater403; increasing the pressure by introducing more solvent vapor into the processing chamber408via the solvent dosing/heating system404or opening the vent-to-air valves427a,b(seeFIGS.13,14and17).FIG.17illustrates the processing effects of controlling the pressure and temperature of the solvent in the processing chamber408and on the AM part/s.

For example, HFIP solvent in vapor state condenses on the polymer part quickly enough to form a boundary layer on top of the surface.FIG.18Aillustrates the absorption of the condensed solvent droplets into the porous polymer matrix caused by the capillary pressures of the pores, which undesirably occurs under standard conditions by conventional methods; andFIG.18Billustrates condensed solvent droplets rapidly accumulating on the surface of the porous polymer faster than absorption can take place, which is desirably achieved in accordance with certain embodiments of the present invention.

At this stage the grains in the upper layer of the polymer part are dissolved and redistributed to form a smooth surface (seeFIGS.16A-B). The amount of condensed solvent per unit of time depends on the level of energy Eqto be equilibrated between the polymer and the solvent vapour. More solvent condensed per unit of time will result in bigger dissolution of the outer layer of the polymer part, hence a smoother surface; and, in contrast, a lower amount of solvent condensation on the surface will dissolve less of the surface which in turn means a less smooth surface finish. Thermodynamic conditions in the processing chamber408for the optimum energy to be equilibrated Eqwere derived by the applicant and vary for different polymers (see tables ofFIGS.11and12and the graphs ofFIGS.13and14). Condensation may also be triggered by the addition of inert gasses using additional storing chambers for those gasses to increase the pressure or addition of cooled air to reduce the temperature. To avoid condensation on the inner walls of the processing chamber408, the temperature of these walls is controlled to be just above the condensation line of the solvent (seeFIGS.15and17). As a result, the condensation of solvent vapor desirably occurs only on the AM polymer part/s.

The condensed liquid solvent film is maintained on the AM part/s for a predetermined part exposure time PETi, which can range from around 5 seconds for Nylon 12 parts to around 10 minutes for TPU type parts (see table ofFIG.12). The predetermined part exposure time PETi and the level of the energy to be equilibrated Eq, determines the final surface roughness of the part/s and can be controlled to around I.Oμιτι (see table ofFIG.12andFIGS.16A-B).

The rest of the processing chamber408is kept above the condensation point of the solvent to ensure the solvent condenses only on the part. For example,FIGS.13and14represent the process for Polyamide 12 (Nylon 12) and TPU materials respectively in terms of pressure-temperature conditions within the processing chamber.

At step420once the predetermined part exposure time PETi for the part/s being processed has been reached, the part smoothing process is complete. The time required to achieve various levels of surface finish in accordance with certain embodiments of the present invention can be seen in the table ofFIG.12.FIG.3illustrates the relationship between surface roughness and exposure time according to certain embodiments of the present invention.

Vacuum pump410then applies a negative pressure VP1of around 10-200 mbar to the processing chamber408for a predetermined time VT1. The reapplication of a negative pressure to the processing chamber108increases vapour pressure to return the condensed solvent from a liquid state to a vapour state. This is done to dry the processed part and recover any excess solvent (seeFIGS.19A-B). At step S422, the valve429between vacuum pump410and the solvent recovery system412is opened and the solvent vapor is fully removed from the processing chamber408through the solvent recovery system412where the solvent is collected in a liquid form and transferred to the recovered solvent compartment422of the reservoir402by the peristatic pump433. Solvent recovery system412may comprise a condenser with at least one column, or the like.

The apparatus400therefore forms a closed loop. The solvent is used until a maximum surface area processed has been reached, before being recovered and sent to the used/recovered solvent compartment422of the reservoir402for safe disposal. Before the reservoir402reaches its end of useful life and needs replacing, an electronic chip, which is configured to monitor how often solvent is recycled through the reservoir, will automatically communicate with the controller418such that a new reservoir is automatically or manually ordered for delivery. The used reservoir is removed and a new reservoir containing ‘fresh’ solvent is inserted for the next operation.

The carbon filter/s416is present on the external atmosphere manifold to ensure no solvent vapour is released into the atmosphere. No residual vapour remains on the surface of the part/s, no residual trace is left, and in turn the post-processing of the part/s is immediately stopped. This not only ensures the process is fully controlled, but also some applications, such as for medical or dental devices, require the parts to be fully clean and safe to use, and also to possess a particularly accurate surface roughness which would not be achievable if any further processing of the part occurred as a result of any remaining solvent residue on the surfaces of the part.

A solvent sensor (not shown) may be located in the processing chamber408to sense the presence of solvent vapour therein and send a signal to the controller418indicating whether or not solvent exists in the chamber408. At step S424, the extraction/drying step is repeated until no solvent (Volatile Organic Compounds-VOCs) is detected within the processing chamber408by the solvent sensor SSi located therein. At step S426, in response to signal SSi indicating no solvent vapour remains in the processing chamber408, the controller418stops the vacuum pump410and closes the valve429between the processing chamber408and the vacuum pump410. The processing program ends and the operator is notified by an audible and/or visual indication. At step S428, the access door/lid415of the apparatus400may be opened to retrieve the post-processed AM polymer part/s from the processing chamber408. The process is repeated as desired.

For each operation, the controller418monitors and logs all the operating variables as indicated in the tables ofFIGS.9and10and uploads them to a read/write database stored on a web-based server or the like. In use, a set of predetermined algorithms defining the various relationships between the operating parameters are selected and used to control the post-processing of an AM part based on its material, volume, surface area, geometry, and the desired finish, as outlined above.

An example according to certain embodiments of the present invention is now described. Information on the AM part/s to be post-processed is loaded into the controller118,418where the following parameters are acquired:Volume of each part—VpSurface Area of each part—APMax height, width and length of each part—to calculate the ‘bounding box’ volume Vb—L, W, H

From these parameters the following values are calculated:VpPart volume ratio (CPR)=1-Volume of part=VpVolume of bounding box=VbArea Ratio (CAR)=1-ApSurface of imaginery sphere=As=(4π)⅓(3Vp)⅔Thickness Ratio=CTn=1-mm′maxTmin˜Minimum Thickness of part Tmax−Maximum Thickness of partThe complexity factor CFis thus calculated as follows:CF=CTR+CAR+0.1CPRThe required surface roughness is input into the controller along with the part material type. The controller calculates the required part exposure time PET1in accordance with the relationship between surface roughness and time shown inFIG.3. 5B-0.4371Λ For example, for Nylon 12 this relationship follows the relationship T=ev−»«5λ Therefore, to achieve a specific surface roughness of 5 microns, the process PET1is calculated as follows: 5−0.4371Λ
T=e−1965)=3.7 seconds

This is then multiplied by the complexity factor CFto give the PET1. For the processing of moderately complex parts this would generate a CFof around 2.1 which would give a total processing time of 7.8 seconds. Overall the process can be described by the corresponding energy level Eq, values for which were derived by the applicant and provided in theFIG.12.

Depending on the AM polymer material type, the maximum energy Eqto be equilibrated between the HFIP TH and the polymer material™ can be derived from equation Eq=k×(TH−TM)/TH, where k is coefficient representing heat transfer of different polymers, TH is the temperature of HFIP vapour and TM is the temperature of the polymer to be processed. Relatively high Eqresults in rapid condensation of solvent vapour on the AM polymer part and glossy surface finish, whereas relatively low Eqresults in slower condensation speed and a matt surface finish (seeFIGS.16A-B). The predetermined algorithms are used by the controller to automatically control the key variables of the automated process to achieve the user's desired surface finish. It has been shown that apparatus according to certain embodiments of the present invention having a processing chamber408of around 400 mm high, around 400 mm deep and around 600 mm wide can enable batch processing of an AM polymer part having dimensions of about around 31×31×53 mm in approximately 20 seconds and thus processing of around 50 parts in around 12 minutes which is over 100 times quicker than the traditional, manual methods for post-processing an AM polymer part.

As a further example, the post-processing method according to certain embodiments of the present invention comprises selectively releasing a predetermined amount of 1, 1, 1, 3,3,3-Hexafluoro-2-propanol (HFIP) solvent into the processing chamber using positive or negative pressure. The amount of HFIP solvent required depends on the type and number of AM polymer parts to be processed. The method comprises providing solvent predetermined amount of energy and heating it to a specific temperature, which causes the solvent to vaporise prior to entering the processing chamber. The apparatus and method according to certain embodiments of the present invention are configured to create an energy (heat) potential between the AM polymer part and the HFIP solvent: the solvent is provided with a particular amount of energy and is vaporised, whereas the AM polymer part may be pre-cooled to a particular temperature so that there is an energy difference (entropy) between the solvent vapour and the AM polymer part. Liquid HFIP is turned into a vapor and then is rapidly condensed on the surface of the polymer material. Due to the energy difference between the solvent vapour and the AM polymer part, the polymer material is not fast enough to absorb the solvent condensate droplets within its matrix, instead solvent condensate droplets accumulate on the upper layer of the polymer. At this stage the polymer grains in the upper layer are dissolved and redistributed to form a smooth boundary layer. As a result, the material particles coalesce and the upper layer is sealed, preventing solvent intrusion/absorption deeper into the polymer matrix. Thermodynamic energy conditions for enough condensate to be created on the part surface vary for different AM polymers. This particle surface sealing effect may in turn have further applications in water-sealing applications. The condensation speed is aptly increased relative to condensation under standard conditions to ensure the polymer surface is processed. The reason for this is the porous structure of the polymer surface, which results in the gradual absorption of the condensate droplets, i.e. sponge-water absorbing behaviour. This is especially the case for the elastomers such as Thermoplastic Polyurethane (TPU). Due to this adsorption phenomena rather than accumulating on the surface of the polymer, solvent would normally be absorbed within the polymer matrix. However, by increasing condensation speed the porous polymer is not able to absorb condensation droplets fast enough, as a result the condensation droplets accumulate on the surface of the polymer rather than be absorbed, which in turn dissolves and smoothens the surface of the AM polymer part. In the closed system (processing chamber) the availability of energy to be equilibrized during the process has to be increased, i.e. the entropy of the system reduced. This in turn increases the condensation speed. The maximum energy Eqto be equilibrated between the HFIP TH and the polymer material™ can be derived from equation Eq=k×(TH−TM)/TH, where k is coefficient representing heat transfer of different polymers, TH is the temperature of HFIP vapour and TM is the temperature of the polymer to be processed. To achieve the required equilibration energy level in the system, the temperature of the HFIP is increased using in-built heating system, whereas the polymer materials may be cooled using either in-built or external blast-freezing system. Pressure within the chamber is also manipulated to help achieve the required difference in the energy to be equilibrated. There is a threshold level for the energy equilibration for different polymers to be processed. For example for the Polyamide 12 the minimum amount of Energy to be equalised should be at least: EQ=k×(TH−TM)/TH=k×([(−1)×21]−25)/21=0.2k, whereas for the Thermoplastic polyurethane at least: EQ=k×(TH−TM)/TH=k×[39−(−20)]/39=1.5k. TM values were derived by the applicant and are provided inFIG.11. TH values depend on the process conditions derived by the applicant as tabulated inFIG.11and are derived accordingly from the Pressure-Temperature graph of the specific solvent (e.g.FIG.15for the HFIP solvent). The processing chamber walls are aptly kept above the HFIP condensation conditions to ensure the condensation happens preferentially on the polymer parts instead of the chamber itself. Once processing is finished the HFIP is converted back into the vapor phase to remove it from the polymer. A vacuum is applied to help residual solvent be removed from the processed polymer. The amount of HFIP getting into the matrix of the polymer material is minimized by fast condensation process. As a result, the polymer grains in the upper surface coalesce and act as a sealing boundary layer. During the last stage of processing the venting is turned on and the chamber temperature and pressure are adjusted to ensure any residual solvent inside the polymer matrix is recovered (seeFIGS.22and23).FIG.22illustrates thermogravimetric results of the processed polymer material, indicating the presence of residual HFIP and the temperature at which it can be recovered.FIG.23illustrates a chromatograph showing the efficiency of recovery of the HFIP from the processed polymer. The solid grey line represents the amount of leftover HFIP in the processed polymer that has not undergone solvent removal procedure, whereas the solid black line indicates the amount of leftover HFIP in the processed polymer sample that has undergone solvent removal procedure. The dashed black line is provided for reference and shows amount of leftover HFIP in non-processed polymer sample. This ensures there is no unwanted weight gain or residual chemicals in the AM polymer part.

Thermogravimetric analysis and chromatography experiments were carried out to find the most effective processing chamber temperature-vacuum parameter combination that recovers residual solvent (see table ofFIG.24).

There is therefore provided the use of 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) condensate to process the surface of additively manufactured (AM) polymers by the manipulation of thermodynamic system and the energy levels of the HFIP and the polymer. Apparatus in accordance with certain embodiments of the present invention is configured to manipulate the thermodynamic system and create favourable energy conditions for the AM polymers to be smoothed. The apparatus and processing method are configured to treat the surface of several polymer parts manufactured by additive manufacturing methods (e.g. 3D printing). The surface of treated polymers becomes smoother compared to non-treated surfaces and, in particular, the smoothness level can be controlled with a set of predefined parameters as described herein. HFIP condensate treats the surface of the polymer material in the quick process which is achieved by maximizing the energy to be equilibrated (entropy) between HFIP and the polymer, i.e. heating up the HFIP while cooling polymer material. In accordance with certain embodiments of the present invention, a new use of HFIP is provided by treating the surface of the polymers, particularly TPU, by HFIP condensation on the polymer through control of available equilibration energy between HFIP and the polymer material. The apparatus according to certain embodiments of the present invention is configured to process any polymer part made from a 3D printing/additive manufacturing process. For example, the apparatus is able to process parts created from multiple polymer groups such as Nylon 12 (PA220 Duraform™ PA), Nylon 11 (Duraform™ EX Natural, Duraform™ EX Black), Thermoplastic Polyurethane (TPU), and TPE-210 elastomer materials, or the like. In addition, the apparatus is able to process other polymer materials such as ABS. An AM part made by fused deposition modelling (FDM), laser sintering (LS), and high speed sintering (HSS) and multi-jet fusion can be finished using the apparatus and method according to certain embodiments of the present invention.

The processing method according to the present invention may desirably seal the surface of porous materials, such as TPU and Polyamide 12, which might be advantageous where the AM part is to be used in water-sealing applications or where water tightness of the material is particularly important (seeFIGS.20A-B). This particle surface sealing effect may in turn have further applications in water-sealing applications. The processing method according to the present invention may also desirably improve the tensile strength of the AM polymer part/s. The control of the drying step of the processing method according to certain embodiments of the present invention desirably avoids continued processing of an AM part which would otherwise occur due to the presence of unevaporated solvent on the part's surface. Use of the pre-packaged and interchangeable solvent reservoirs/cartridges eliminates the need to handle solvents in a fume cupboard for example. A closed loop system as described above allows recovery of solvents used to enable total solvent consumption from the cartridge to be minimised thus reducing the operation costs and environmental impact in terms of solvent creation and disposal etc.

Whilst the apparatus and method according to certain embodiments of the present invention have been described herein with reference to a ‘static’ processing chamber, the invention could be implemented as part of a ‘dynamic’ conveyor-like system which move AM parts through a 3D printing device and automated inspection/monitoring system.

The applications for the apparatus and method according to certain embodiments of the present invention are wide ranging, and may be suitable for processing any AM part which requires a desired finish for aesthetic and/or functional purposes, such as footwear, automotive interior/trim components, and dental/medical devices, or the like.