FIELD

The present disclosure relates to a method for processing additively manufactured components used as models for metal investment castings, and to a system for performing, and a part obtainable by, the same.

BACKGROUND

Investment casting is a well-known method for manufacturing metal components by pouring liquid metal into a cavity having a shape corresponding to that of a desired part.

Recently, investment casting processes have benefited from advancements in additive manufacturing technologies, since such processes can create intricate polymer casting models whilst eliminating the need for complex and expensive tooling.

Typically, casting models are printed and then placed in a furnace until they reach a desired strength. The casting models are then immersed in liquid wax to seal and smooth the surface of the casting model. The casting models are then infiltrated with epoxy resin for added strength before being mounted with a support and a feed section for receiving liquid metal during the casting process.

Prior to casting, ceramic coatings are applied to the casting models. The additively manufactured models are then burnt out from the casting, leaving only a duplicate, ceramic mould. Molten metal is then poured into the mould and cooled, after which the metal cast can be removed from the mould to obtain a desired part.

However, due to the layer-by-layer nature of current additive manufacturing methods, additively manufactured casting models tend to exhibit rough, powdery surfaces, as demonstrated by part 20 shown in Figure 1a. Therefore, parts obtained from such models often exhibit a high surface porosity, which in turn results in castings having a high water absorption, which can cause the castings to fail. One known solution to this problem involves immersing the additively manufactured casting models into wax after they are printed. As shown in Figure 1 b, by using this method, the additively manufactured part 20 can be provided with a smooth and sealed surface layer 12. However, the additional surface layer 12 present on the exterior of the additively manufactured part 20 is not always uniformly distributed and can substantially alter the thickness (T) of the part 20, which results in a loss of dimensional accuracy.

Embodiments of the present disclosure aim to provide a solution to at least one of these aforementioned problems.

SUMMARY

A first aspect of the disclosure provides a method for processing an additively manufactured investment casting model, the method comprising providing an additively manufactured investment casting model, placing the additively manufactured investment casting model into a processing chamber, introducing a chemical vapour into said processing chamber, carrying out an application step, wherein the chemical vapour is condensed onto at least a portion of the surface of the additively manufactured investment casting model so as to transform at least some of the surface material of the additively manufactured investment casting model from a first state, wherein the surface material is substantially rigid, to a second state, wherein the surface material is capable of movement, and wherein, during the application step, the temperature within the processing chamber is maintained between 10°c and 40°c, carrying out a re-distributing step, wherein the transformed surface material of the additively manufactured investment casting model is re-distributed across at least a portion of the surface of the additively manufactured investment casting model and carrying out a drying step, wherein the transformed surface material is re-solidified, and wherein during said drying step the temperature within the processing chamber is maintained between 30°c and 50°c and the pressure within the processing chamber is maintained at or below 1000 mbar.

Additively manufactured investment casting models tend to exhibit very low glass transition temperatures and so vapour processing of such models has not previously been possible since such methods risk deforming the models (due to the raised temperatures typically required to maintain a chemical in a vapour form). Advantageously, it has been found that by using the aforementioned method, it is possible to vapour process the surface material of additively manufactured investment casting models at reduced temperatures, thereby improving the smoothness and porosity of the surface of said models without causing them to deform. Subsequently, parts that are cast using said models can exhibit a reduced porosity, whilst still maintaining a high degree of definition and accuracy.

In exemplary embodiments, the application step comprises selecting a chemical such that, when the chemical is applied onto at least a portion of the surface of the additively manufactured investment casting model, the viscosity of the transformed surface material is below 10,000cps.

Advantageously, by selecting a chemical capable of transforming the surface material to a viscosity below 10,000cps, the surface material is able to be re-distributed under the influence of gravity, which helps to achieve a smooth, less porous surface finish without the need for any further processes or interactions with the part.

In exemplary embodiments, the chemical is one of a solvent, an acid, or an ionic liquid.

In exemplary embodiments, the chemical is provided in the form of a chemical vapour; a chemical mist; a chemical spray and/or a liquid immersion.

In exemplary embodiments, the chemical is one of: 1 ,1 ,1 ,3,3,3-hexafluoro-2-propanol (HFIP); acetone, dichlorobenzene, dichloromethane (DCM); dimethylformamide; sulphuric acid; m-cresol; formic acid; trifluoroacetic acid; benzyl alcohol; 1 ,2,4 trichlorobenzene; tetrahydrofuran; 2-methyltetrahydrofuran; Xylene and/or Dimethyl sulfoxide (DMSO).

In exemplary embodiments, the transformed surface material of the additively manufactured investment casting model is re-distributed across at least a portion of the surface of the additively manufactured investment casting model under the influence of gravity only.

In exemplary embodiments, the application step comprises dissolving at least a portion of the surface material of the additively manufactured investment casting model. In exemplary embodiments, the application step comprises melting at least a portion of the surface material of the additively manufactured investment casting model.

In exemplary embodiments, the application step comprises applying a chemical vapour onto at least a portion of the surface of the additively manufactured investment casting model.

Advantageously, applying the chemical as a chemical vapour can help to improve the efficiency of the application step, when compared to other known methods.

In exemplary embodiments, the application step comprises condensing the chemical vapour onto at least a portion of the surface of the additively manufactured investment casting model.

Advantageously, condensing the chemical vapour onto the additively manufactured investment casting model can help to more uniformly treat the surface of the part.

In exemplary embodiments, the chemical applied during the application step has a condensation temperature, and the method further comprises the step of, prior to the application step, cooling the additively manufactured investment casting model to a temperature below the condensation temperature of the chemical.

Advantageously, cooling the additively manufactured investment casting model to a temperature below a condensation temperature of the chemical vapour can help to improve the amount of chemical condensation onto the surface of the part, which in turn can help to improve the effectiveness of process.

In exemplary embodiments, the additively manufactured investment casting model is cooled to a temperature below 0°C.

Advantageously, cooling to temperatures below 0°C can help to further promote condensation of chemical vapour onto the part, particularly when processing at low temperatures.

In exemplary embodiments, the additively manufactured investment casting model is cooled to a temperature below -10°C. Advantageously, cooling to temperatures below -10°C can help to even further promote condensation of chemical vapour onto the part, particularly when processing at low temperatures.

In exemplary embodiments, the step of cooling the additively manufactured investment casting model comprises cooling the additively manufactured investment casting model for a pre-determined time period. In exemplary embodiments, the step of cooling the additively manufactured investment casting model comprises cooling the additively manufactured investment casting model for at least 30 minutes.

Advantageously, cooling for at least 30 minutes can help to achieve more uniform cooling across the additively manufactured investment casting model.

In exemplary embodiments, the application step further comprises placing the additively manufactured investment casting model into a processing chamber and introducing the chemical vapour into said processing chamber.

In exemplary embodiments, at least one wall of the processing chamber is maintained above the condensation temperature of the chemical during the application step.

Advantageously, maintaining at least one wall of the processing chamber above the condensation temperature of the chemical helps to prevent any chemicals condensing onto the surfaces of the processing chamber, thereby helping to reduce chemical wastage and improving process efficiency.

In exemplary embodiments, at least one wall of the processing chamber is maintained at a temperature in the range of 10°C to 40°C.

It has been found that maintaining the walls of the processing chamber at a temperature in the range of 10°C to 40°C is suitable for reducing condensation. In exemplary embodiments, the application step further comprises placing the additively manufactured investment casting model into a processing chamber, introducing the chemical vapour into said processing chamber, and increasing the pressure within the processing chamber so as to apply the chemical onto at least a portion of the surface of the additively manufactured investment casting model.

Advantageously, increasing the pressure of the processing chamber can help to better optimise the thermodynamic conditions required for allowing the chemical to condense onto the surface of the part, and therefore a greater amount of chemical condensation onto the surface of the part can be achieved during the application step.

This can help to further improve the effectiveness of the process.

In exemplary embodiments, after the chemical vapour has been introduced into the processing chamber, the pressure within the processing chamber is increased into the range of 200 mBar to 1000 mBar.

Advantageously, increasing the pressure of the processing chamber into the range of 200 mBar to 1000 mBar can help to further optimise the thermodynamic conditions required for allowing the chemical to condense onto the surface of the part.

Therefore, when using pressures in the range of 200 mBar to 1000 mBar, an even greater amount of chemical condensation onto the surface of the part can be achieved during the application step. This can help to even further improve the effectiveness of the process.

Advantageously, drying the additively manufactured investment casting model helps to remove any excess chemicals from the model after processing, and can therefore help to efficiently prevent over-exposure of the part.

In exemplary embodiments, the drying step comprises heating the additively manufactured investment casting model to a temperature above a boiling point of the chemical applied to at least a portion of the surface of the additively manufactured investment casting model during the application step.

Advantageously, heating the additively manufactured investment casting model to a temperature above a boiling point of the chemical can help to more efficiently dry the additively manufactured investment casting model. In exemplary embodiments, the additively manufactured investment casting model is formed from a material having a glass transition temperature, and the drying step comprises heating the additively manufactured investment casting model to a temperature below the glass transition temperature of the material of the additively manufactured investment casting model.

Advantageously, heating the additively manufactured investment casting model to a temperature below its glass transition temperature can help to reduce the likelihood of the additively manufactured investment casting model becoming damaged or deformed during the drying process.

In exemplary embodiments, the drying process is performed within a, or the, processing chamber, and the drying step comprises reducing a pressure within the processing chamber.

Advantageously, reducing the pressure of the processing chamber can help to better optimise the thermodynamic conditions required for allowing the chemical to re vaporise. Therefore, it is possible to vaporise the chemical at much lower temperatures than would otherwise be possible.

This can help to further reduce the likelihood of the part becoming damage or deformed during the process (due to excessive heat). In exemplary embodiments, during the drying process, the pressure within the processing chamber is reduced into the range of 50 mBar to 600 mBar.

Advantageously, reducing the pressure of the processing chamber into the range of 50 mBar to 600 mBar can help to further optimise the thermodynamic conditions required for allowing the chemical to re-vaporise.

Therefore, it is possible to vaporise the chemical at even lower temperatures than would otherwise be possible, and hence the likelihood of damage being caused to the part (due to excessive heat) can be even further reduced. In exemplary embodiments, the additively manufactured investment casting model is a polymeric part.

In exemplary embodiments, the additively manufactured investment casting model is a part comprising Polymethyl Methacrylate (PMMA).

In exemplary embodiments, during the application step, the pressure within the processing chamber is maintained at or below 1000 mbar.

In exemplary embodiments, during the application step, the pressure within the processing chamber is maintained between 200 and 1000 mbar.

In exemplary embodiments, during the drying step, the pressure within the processing chamber is maintained between 50 and 600 mbar.

Advantageously, reducing the pressure of the processing chamber into the range of 50 mBar to 600 mBar can help to further optimise the thermodynamic conditions required for allowing the chemical to re-vaporise.

In exemplary embodiments, the method further comprises the step of, prior to the application step, cooling the additively manufactured investment casting model to a temperature below 0°C.

Advantageously, cooling to temperatures below 0°C can help to further promote condensation of chemical vapour onto the part, particularly when processing at low temperatures.

In exemplary embodiments, the method further comprises the step of, prior to the application step, cooling the additively manufactured investment casting model to a temperature below -10°C.

Advantageously, cooling to temperatures below -10°C can help to even further promote condensation of chemical vapour onto the part, particularly when processing at low temperatures. In exemplary embodiments, the method further comprises cooling the additively manufactured investment casting model during the application step such that a surface temperature of the additively manufactured investment casting model remains below the condensation temperature of the chemical vapour.

In exemplary embodiments, the temperature within processing chamber is maintained between 30°c and 50°c during introduction of the chemical vapour into the processing chamber. Advantageously, maintaining the temperature between 30°c and 50°c can help to further optimise the thermodynamic conditions required for allowing the chemical to vaporise.

In exemplary embodiments, the pressure within the processing chamber is maintained at or below 1000 mbar during introduction of the chemical vapour into the processing chamber.

In exemplary embodiments, the pressure within the processing chamber is maintained between 50 and 600 mbar during introduction of the chemical vapour into the processing chamber.

Advantageously, reducing the pressure of the processing chamber into the range of 50 mBar to 600 mBar can help to further optimise the thermodynamic conditions required for allowing the chemical to vaporise.

A second aspect of the disclosure provides a system for processing an additively manufactured investment casting model, the system comprising: a processing chamber for receiving an additively manufactured investment casting model; a temperature control mechanism for controlling a temperature within the processing chamber; a pressure control mechanism for controlling a pressure within the processing chamber; an applicator for introducing a chemical vapour into the processing chamber; and a controller configured to control the system according to the method of the first aspect of the disclosure.

Advantageously, this system can help to facilitate improvement to the smoothness and porosity of the surface of additively manufactured investment casting models. Subsequently, parts that are cast using models processed using this system can exhibit a reduced porosity, whilst still maintaining a high degree of definition and accuracy.

Furthermore, this system can be easily automated and scaled which can improve the speed and economy at which parts can be processed.

In exemplary embodiments, the applicator comprises a chemical inlet and a heating plate, the heating plate being arranged for receiving the chemical from the chemical inlet and being further configured to vaporize the chemical, received from the chemical inlet, for introduction into the processing chamber.

Advantageously, providing a heating plate and a chemical inlet configurable for vaporising the chemical can help to facilitate more efficient application of the chemical onto the surface of the additively manufactured investment casting model.

In exemplary embodiments, the applicator comprises a perforated sheet arranged such that the chemical is introduced into the processing chamber via the perforated sheet.

Advantageous, the perforated sheet helps to achieve a more uniform distribution of the chemical vapour throughout the processing chamber. In exemplary embodiments, the system further comprises a cooler, configurable for cooling the additively manufactured investment casting model to a temperature below a condensation temperature of the chemical.

Advantageously, providing a cooler can help to achieve a greater amount of condensation onto the surface of the part, which, in turn, can help to improve processing effectiveness.

In exemplary embodiments, the cooler is a blast chiller.

In exemplary embodiments, the cooler is arranged so as to cooling the additively manufactured investment casting model whilst the additively manufactured investment casting model is located within the processing chamber.

Advantageously, the temperature control mechanism can help to control the temperature of the processing chamber so as to optimise the thermodynamic conditions of the processing chamber for each step of the process.

In exemplary embodiments, the temperature control mechanism comprises a heating element which comprises, or is located at, a wall of the processing chamber.

Advantageously, locating the heating element at the wall of the processing chamber helps to prevent any chemicals from condensing on the surfaces of the processing chamber, which in turn helps to reduce chemical wastage.

Advantageously, providing a pumping mechanism can help to control the pressure of the processing chamber so as to further optimise the thermodynamic conditions of the processing chamber for each step of the process.

In exemplary embodiments, the applicator is a nebuliser configurable for applying a chemical mist onto at least a portion of the surface of the additively manufactured investment casting model.

Advantageously, providing the applicator as a nebuliser helps the chemical to be applied onto the part without requiring the use of heat and, as such, the likelihood of damage being caused to the part (due to excessive heat) can be reduced. In exemplary embodiments, the controller is configurable for controlling the heating element such that at least one wall of the processing chamber is maintained above the condensation temperature of the chemical.

In exemplary embodiments, the controller is configurable for controlling the pumping mechanism so as to increase the pressure within the processing chamber in order to to apply the chemical onto at least a portion of the surface of the additively manufactured investment casting model.

In exemplary embodiments, the controller is configurable for controlling the pumping mechanism so as to increase the pressure within the processing chamber into the range of 200 mBar to 1000 mBar. In exemplary embodiments, the controller is configurable for controlling the heating element such that the additively manufactured investment casting model is heated to a temperature above a boiling point of the chemical applied to at least a portion of the surface of the additively manufactured investment casting model during the application step.

In exemplary embodiments, the controller is configurable for controlling the heating element so as not to exceed the glass transition temperature of the material of the additively manufactured investment casting model. In exemplary embodiments, the controller is configurable for controlling the pumping mechanism so as to reduce a pressure within the processing chamber.

In exemplary embodiments, the controller is configurable for controlling the pumping mechanism so as to reduce the pressure within the processing chamber into the range of 50 mBar to 600 mBar.

A third aspect of the disclosure provides an additive manufacturing apparatus comprising the system according to the second aspect of the disclosure. A fourth aspect of the disclosure provides a part obtainable according to the method of the first aspect of the disclosure. We consider the term “vapour” to be defined as a compound which is in a gaseous state. We consider the term “mist” to be defined as a finely dispersed liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described with reference to the accompanying drawings, in which:

Figure 1a illustrates a schematic view of an additively manufactured part before any post-processing treatments have been applied;

Figure 1b illustrates a schematic view of an additively manufactured part after being applied with a known wax immersion treatment; Figure 2 illustrates a system for processing an additively manufactured part according to an embodiment of the present disclosure;

Figure 3 illustrates a schematic flow chart for a method according to an embodiment of the present disclosure;

Figure 4 illustrates a graph depicting optimal temperature and pressure conditions for processing and drying an additively manufactured part according to the method of the present disclosure;

Figure 5 illustrates a schematic view of an additively manufactured part processed according to the method of the present disclosure; and

Figure 6 illustrates an alternative system for processing an additively manufactured part according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 2 shows a system 100 for processing an additively manufactured part 20, in this case an additively manufactured investment casting model 20.

The system 100 includes a vacuum-tight sealable processing chamber 102, optionally made out of stainless steel, configured to receive the additively manufactured investment casting model 20. In the illustrated embodiment, the system 100 also includes a support in the form of a part stand 104 configured for supporting the additively manufactured investment casting model 20 within the processing chamber 102.

The system 100 also features an applicator 110 configured to apply a chemical onto at least a portion of a surface of the additively manufactured investment casting model 20, received within the processing chamber 102.

In the embodiment illustrated in Figure 2, the applicator 110 is made up of a chemical inlet 112, a heating plate 114 and a perforated sheet 116.

In use, a chemical, suitable for transforming a surface material of the additively manufactured investment casting model 20 from a first state, wherein the surface material is substantially rigid, to a second state, wherein the surface material is capable of movement, is supplied to the heating plate 114 via the chemical inlet 112. The heating plate 114 then heats the chemical applied thereto to a temperature above the chemical’s respective boiling point which causes the chemical to vaporise, ready for introduction into the processing chamber 102 via the perforated sheet 116.

The provision of the perforated sheet 116 helps to ensure a uniform distribution of the vaporised chemical within the processing chamber 102 during the process. However, it shall be appreciated that in other embodiments, the perforated sheet 116 may be omitted.

The system also includes a temperature control mechanism configured to control a temperature of the processing chamber. In the illustrated embodiment, the temperature control mechanism comprises a heating element 106. However, it shall be appreciated that in some embodiments, the temperature control mechanism may also include elements having a cooling function.

In the illustrated embodiment, a pair of heating elements 106a, 106b are provided which make up part of a wall of the processing chamber 102. The heating elements 106a, 106b are used to raise and lower the temperature of the processing chamber accordingly at each step of the processing method, as shall be described in later parts of this application, so as to obtain optimal thermodynamic processing conditions for each of the respective processing steps. Furthermore, by providing heating elements which make up part of the wall of the processing chamber 102, the system 100 is able to better avoid the unwanted condensation of chemicals on the surfaces of the processing chamber, which helps to reduce chemical wastage and also helps to improve the efficiency of the processing operation, as shall be described greater detail in later parts of this application.

The system also includes a pumping mechanism configured to control the pressure of the processing chamber during the process.

In the illustrated embodiment, the pumping mechanism is made up of a vacuum pump 108 configured to control a pressure within the processing chamber 102 and an outlet 107 configured to permit egress of excess chemical vapour from within the processing chamber 102 to a location external to the processing chamber, such as the atmosphere, via an activated carbon filter 109.

During operation, the pumping mechanism is used to raise and lower the pressure of the processing chamber to enable the system 100 to obtain optimal thermodynamic processing conditions for each of the respective processing steps, as shall be described in later parts of this application.

The system also includes a cooler 120 configured to cool the additively manufactured investment casting model 20 prior to the part 20 being placed into the processing chamber 102 for processing.

In the illustrated embodiment, the cooler 120 is provided in the form of a blast chiller and is provided separately from the processing chamber 102. The blast chiller receives the part prior to processing, to allow the part 20 to be cooled. This helps to achieve optimal condensation of the chemical vapour onto the part during the processing operation, as shall be described in greater detail with reference to Figure 3.

Whilst the cooler is provided as a separate component in the illustrated embodiment, it shall be appreciated that in other embodiments, the cooler may be provided as an integral part of the processing chamber 102. For example, the cooler may be incorporated into the part stand 104 such that cooling can be imparted onto the part during operation (for example during application of the chemical onto the part). In some embodiments, the cooler may also be controllable via a controller so that the surface temperature of the part is maintained below the condensation temperature of the chemical vapour in order to promote condensation of the chemical onto the part during the application step. In other embodiments, the cooler may be provided in the form of one or more jets arranged so as to direct a cooling stream of gas onto the part.

A method for processing the additively manufactured part 20 shall now be described with reference to Figure 3.

Whilst the embodiment illustrated in Figures 2 and 3 describes a method of processing a part made from Polymethyl methacrylate (PMMA), it shall be appreciated that in other embodiments, parts made from other materials may also be processed using the method described below. Furthermore, it shall be appreciated that for other materials, different processing parameters may be used.

At step 201 of the illustrated method, an additively manufactured investment casting model 20 is provided and cooled to a temperature below a condensation temperature of the chemical to be applied to the additively manufactured investment casting model 20.

It shall also be appreciated that any suitable additive manufacturing method, such as FFDM or SLM, may be used for producing the investment casting model and therefore, for conciseness, the methods and systems used for additively manufacturing the investment casting model 20 shall not be described in detail within this application.

In the embodiment illustrated in Figure 2, following the manufacture of the investment casting model (via additive manufacturing methods), the model 20 is placed in a blast chiller 120 for a pre-determined amount of time to enable the model 20 to become sufficiently cooled.

In some embodiments, the model 20 may be placed in a cooler for at least 30min at a temperature of -18°C or less to achieve the desired amount of cooling. However, it shall be appreciated that the temperature and duration of the cooling process may vary.

The step 201 of cooling the model 20 helps to increase the amount of chemical vapour that condenses onto the surface of the part 20 during processing, since chemicals typically condense more readily onto lower temperature surfaces. Consequently, since greater amounts of chemical vapour can be condensed onto the model 20, a greater amount of surface material is subsequently transformed and re-distributed during the processing operation, as shall be described in greater detail below. This enables a smoother, less porous end product to be achieved.

Following step 201 , the PMMA model 20 is then loaded into the processing chamber 102 at step 202, ready for processing.

At step 203, a chemical solvent such as an acid, an ionic liquid or another component suitable for transforming the PMMA polymer is introduced into the processing chamber.

In the illustrated embodiments, a chemical is selected such that the PMMA polymer material at the surface of the additively manufactured investment casting model is dissolved in order to allow said material to be re-distributed. However, it shall be appreciated that in other embodiments, a chemical may be selected that is suitable for melting the PMMA polymer material at the surface of the additively manufactured investment casting, or any other suitable transformation mechanism, in order to allow said material to be re-distributed.

Examples of suitable chemicals include, but are not limited to 1 ,1 ,1 ,3,3,3-hexafluoro- 2-propanol (HFIP), dichloromethane (DCM), dimethylformamide, sulphuric acid, m- cresol, formic acid, trifluoroacetic acid, benzyl alcohol, 1 ,2,4 trichlorobenzene, tetrahydrofuran, 2-methyltetrahydrofuran, Xylene and Dimethyl sulfoxide (DMSO), or the like. The amount of solvent depends on the surface area and the number of parts to be processed.

In the embodiment illustrated in Figure 2, the chemical is introduced into the processing chamber 102 in the form of the vapour.

As specified previously, in the illustrated embodiment, the chemical, suitable for dissolving a surface of the additively manufactured part 20, is supplied to the heating plate 114, via the chemical inlet 112. The heating plate 114 then heats the chemical to a temperature above the chemical’s respective boiling point which causes the chemical to vaporise, after which the chemical vapour is introduced into the processing chamber 102, via the perforated sheet 116. Figure 4 shows a graph illustrating the optimal thermodynamic conditions for processing at various stages of the method of the illustrated embodiment. As can be seen in Figure 4, optimal processing conditions are obtained between 10°C and 40°C and between 200-1000 mBar of pressure.

As such, once the chemical vapour has been introduced into the processing chamber, the temperature control mechanism and vacuum pump 108 are controlled to obtain a processing chamber temperature between 10°C and 40°C and a processing chamber pressure between 200-1000 mBar.

Chemicals tend to condense more easily at higher pressures and at lower temperatures and therefore, by altering the thermodynamic conditions within the processing chamber 102 at step 203 to lower the temperature and increase the pressure within the processing chamber 102, it is possible to cause the chemical vapour to condense onto the surface of the model 20, which subsequently leads to the surface material of the model 20 becoming dissolved by the chemical applied thereto. Furthermore, during step 203, the walls of the processing chamber are maintained above the chemical condensation temperature to help avoid chemical vapours from condensing on the chamber walls rather than on the model 20, to help reduce wastage and to ensure that any condensation of the chemical vapour is focussed at the model, rather than at other parts of the processing chamber.

As the chemical is condensed onto the surface of the part 20, the surface of the part undergoes a change of state from a first, undissolved state, in which the movement of the surface material of the part is substantially prevented, to a second, dissolved state, in which the movement of the surface material of the part is permitted.

Once dissolved, the dissolved surface material of the PMMA part 20 is then allowed to reflow under the influence of gravity. This material re-distribution closes the pores on the surface of the part, resulting in a smooth and water tight surface. For example, in the illustrated embodiment, following processing, the additively manufactured investment casting model 20 exhibits a fully sealed surface (i.e. a surface which is impermeable to fluids). In order to enable the surface material of the model 20 to reflow under the influence of gravity, the chemical selected for the surface treatment performed at step 203 is chosen so as to obtain a surface material having a viscosity below 10,000cps, when the surface material is in the second, dissolved state.

However, it shall be appreciated that whilst it is preferable for the surface material to flow under the influence of gravity only, since this avoids the need for any secondary smoothing processes, in other embodiments the step of re-distributing the surface material of the additively manufactured investment casting model may be performed using a tool or other suitable apparatus, such as an air blower.

Furthermore, in such alternative embodiments, it shall be appreciated that chemicals may be selected which obtain a surface material having a viscosity above 10,000cps, when the surface material is in the second, dissolved state.

Once the surface of the model is smoothed and sealed, any excess chemicals used for processing the model are extracted by decreasing the pressure and increasing the temperature within the processing chamber 102 at step 205.

As can be seen in Figure 4, optimal drying conditions are obtained between 30°C and 50°C and between 50 mBar to 600 mBar of pressure, since chemicals typically vaporise more readily at high temperatures and low pressures. Therefore, at step 205, the temperature control mechanism and vacuum pump 108 are again controlled to increase the heat within the processing chamber 102 to a temperature between 30°C and 50°C and to reduce the pressure of the processing chamber 102 to a pressure between 50 mBar to 600 mBar. In order to obtain re-vaporise the condensed chemical, the temperature of the processing chamber must be held above the respective boiling temperature of the chemical used during the process. Furthermore, by increasing the temperature of the processing chamber, a greater amount of chemical can be vaporised, which leads to faster drying times. However, as temperatures are increased, there is an increased risk of the model 20 becoming melted or damaged due to excessive heat. It has been found that a temperature in the range of 30°C and 50°C is able to achieve optimal drying, without risking damage to the parts being processed.

It has also been found that by reducing the pressure of the processing chamber 102, it is possible to vaporise any excess chemicals at lower temperatures, which helps to improve the efficiency of the drying process and also further reduces the risk of the model 20 becoming damaged during processing.

The temperatures used during the drying processing runs are also kept below the glass transition temperature of the model 20. This is to help further reduce the likelihood of the part 20 becoming damaged or deformed by the high temperatures.

Whilst it is preferable to dry the investment casting model 20 using the method described above, it shall be appreciated that in other embodiments, other suitable drying processes may also be used. Furthermore, in other alternatives, it shall be appreciated that the drying step may be omitted.

Finally, at step 206, once the drying process is complete, the process is stopped and the processed model 20 is removed from the processing chamber 102.

An additively manufactured investment casting model 20 processed according to the method of the illustrated embodiment is shown in Figure 5.

As can be seen in Figure 5, the surface 22 of the additively manufactured investment casting model 20 is able to retain a much finer feature definition when compared to the parts coated with wax (see Figure 1 b).

Another major advantage of this illustrated method is that the vapour condensation process is more controllable compared to the wax dipping. The process includes a number of process parameters affecting the surface smoothing and sealing, including amount of solvent, process time, initial polymer material temperature, chamber pressure and chamber temperature. In contrast, the usual process of wax coating merely includes coating parts in liquid wax. The process of the present embodiment is also readily automatable and scalable meaning the parts can be processed faster and cheaper using appropriate process parameters for specific parts. In comparison, the wax dipping process is mostly manual and is difficult to customize for specific parts.

Whilst the method and system described in Figures 2 and 3 involve the application of a chemical vapour to an additively manufactured investment casting model, in other embodiments, the chemical may be applied onto the surface of the part in the form of a mist, spray or liquid immersion.

A system 300 for processing an additively manufactured part according to an alternative embodiment of the present disclosure, in which a chemical is applied in liquid form, shall now be described with reference to Figure 6. Similarly to the system 100 of the embodiment illustrated in Figure 2, the system 300 of Figure 6 includes a processing chamber 302, in this case an airtight processing chamber, for receiving an additively manufactured part 20 and a support provided in the form of part stand 304 for supporting the additively manufactured part 20 within the processing chamber 302.

Likewise, the system 300 also includes an outlet 307 configured to permit egress of excess chemical vapour from within the processing chamber 302 to a location external to the processing chamber, such as the atmosphere, via activated carbon filter 309. Flowever, unlike the system 100 of Figure 2, the applicator 310 of the system 300 of Figure 6 is provided in the form of a nebulizer comprising a chemical nebulizing container 312 and a carrier gas inlet 314.

The chemical nebulizing container 312 is configured to contain a chemical suitable for transforming a surface of the additively manufactured part 20. The carrier gas inlet 314 supplies a carrier gas to the chemical nebulizing container 312, which then mixes with the chemical contained therein, and carries said chemical, in the form of a chemical mist, into the processing chamber 302 for processing the additively manufactured part 20. The method of processing the additively manufactured part 20 using the system of Figure 6 is substantially the same as the method described in Figure 3 and so, for conciseness, shall not be described in any further detail.

Flowever, the key difference between the two systems and methods is that in the embodiment described in Figure 6, the application of the chemical onto the surface of the part 20 is performed via applying the chemical onto the surface of the part 20 in the form of a liquid mist (via a nebuliser), rather than as a vapour.

As such, when performing the method using the system of Figure 6, the steps 201 and 203 of cooling the part 20 and vaporising and condensing the chemical vapour onto the surface of the part 20 can be omitted.

EXPERIMENTAL EXAMPLE

As an example, the afore-described vapour-condensation methodology for processing two PMMA impeller cast models is described below.

The impellers were chilled for 30 min in a blast chiller at -18°C, before being processed.

The processing conditions of the processing chamber are as follows:

• Processing Chamber temperature: 30 °C

• Pleating Plate temperature: 80 °C

• Processing Chamber pressure: 350 mbar

• Volume of chemical solvent: 210 ml

• Processing time: 160 sec

The PMMA impeller casts were also subjected to the afore-described drying process. The drying process conditions were as follows:

• Processing Chamber temperature: 40°C

• Processing Chamber pressure: 80 mbar

The processing chamber temperature was set at a temperature which is higher than the boiling point of the chemical solvent, which in this case was 25°C for Plexafluoroisopropanol at the drying pressure of 80 mbar, but much lower than the temperature at which structural deformation of PMMA parts may occur (i.e. 50°C). As a result of the processing, 100% surface sealing, with required dimensional shrinkage of less than 1 mm was achieved for both impellers.

Although the disclosure has been described above with reference to one or more embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the disclosure as defined in the appended claims.