WO2015082923A1

WO2015082923A1 - Additive manufacturing

Info

Publication number: WO2015082923A1
Application number: PCT/GB2014/053598
Authority: WO
Inventors: Ben Ian Ferrar; Jake Samuel UFTON; Jason Blair JONES
Original assignee: Renishaw Plc
Priority date: 2013-12-04
Filing date: 2014-12-04
Publication date: 2015-06-11
Also published as: GB201321390D0

Abstract

This invention concerns an additive manufacturing method for building an object layer-by-layer. The method comprises successively depositing layers of material from a transfer member (103b) on to a build platform (102) to build an object from a resulting stack of the layers (104). Between deposition of successive layers, the deposited layers are treated to form softened or melted material onto which a subsequent layer is deposited. Each subsequent layer of material is deposited from the transfer member (103b) with sufficient pressure such that the subsequent layer is embedded into the softened or melted material. The invention also concerns apparatus for carrying out this method.

Description

ADDITIVE MANUFACTURING

Summary of Invention This invention concerns additive manufacturing methods and apparatus for building an object layer-by-layer. The invention has particular, but not exclusive, application to apparatus, such as electrophotographic printing apparatus, wherein successive layers of material are deposited from a transfer member on which each layer is formed in a desired shape onto a build platform.

Background

As advances are made in 3D printing, or additive manufacturing (AM), from prototyping applications to the direct manufacture of end-use parts, the need for improved and consistent mechanical part properties is repeatedly highlighted.

Over the last twenty years innumerable advances have contributed to improved mechanical properties of AM parts, including optimisation of machine processing parameters (LIAO, H. T. & SHIE, J. R. 2007. Optimization on selective laser sintering of metallic powder via design of experiments method. Rapid Prototyping Journal, 13, 156-162.), energy exposure variations (AJOKU, U., SALEH, N., HOPKINSON, N., HAGUE, R. & ERASENTHIRAN, P. 2006b. Investigating mechanical anisotropy and end-of-vector effect in laser-sintered nylon parts. Proceedings of the Institution of Mechanical Engineers Part B-Journal of Engineering Manufacture, 220, 1077-1086.) including double energy exposure

(WEGNER, A. & WITT, G. 2012. Correlation of Process Parameters and Part Properties in Laser Sintering using Response Surface Modeling. Physics Procedia, 39, 480-490) and numerous material developments. Various post-processing procedures including: heat treatment, shot peening, vibratory grinding, infiltration, coating (SALEH, N., HOPKINSON, N., HAGUE, R. J. M. & WISE, S. 2004. Effects of electroplating on the mechanical properties of stereolithography and laser sintered parts. Rapid Prototyping Journal, 10, SOS- SIS.); aging (COOKE, W., TOMLINSON, R. A., BURGUETE, R., JOHNS, D. & VANARD, G. 201 1. Anisotropy, homogeneity and ageing in an SLS polymer. Rapid Prototyping Journal, 17, 269-279.), Hot Isostatic Pressing (HIPping) (AGARWALA, M. K. & BOURELL, D. L. Densification of Selective Laser Sintered Metal Parts by Hot Isostatic Pressing. Proceedings of the 5th Solid Freeform Fabrication Symposium., 1994 Austin, TX, USA. University of Texas Press, 65-73) and plating (SALEH, N., HOPKINSON, N., HAGUE, R. J. M. & WISE, S. 2004. Effects of electroplating on the mechanical properties of stereolithography and laser sintered parts. Rapid Prototyping Journal, 10, 305-315) have yielded some improvement; however they can be complicated or compromised by geometric complexity (which limits access or makes parts difficult to hold for downstream operations) negating some of the advantages of using AM, and making in-process enhancement of mechanical properties attractive.

Despite the improvements, parts made by AM typically have inferior mechanical properties when compared to conventional formative and moulding techniques. For example, AJOKU, U., HOPKINSON, N. & CAINE, M. 2006a, Experimental measurement and finite element modelling of the compressive properties of laser sintered Nylon-12. Materials Science and Engineering: A, 428, 21 1-216 demonstrated a 10% lower modulus from compression tested laser sintered Nylon- 12 compared to when it was injection moulded. Ajoku et al. (2006b) suggested that the lack of "molecular chain alignment", achieved by material flow during injection moulding, contributed to different properties in laser sintered Nylon-12. Furthermore, KRUTH, J. P., LEVY, G., KLOCKE, F. & CHILDS, T. H. C. 2007. Consolidation phenomena in laser and powder bed based layered manufacturing. CIRP Annals - Manufacturing Technology, 56, 730-759. found that compression moulded polyamide 12 outperformed the same material when laser sintered in tensile and impact strength tests.

The understanding that pressure during the build process can enhance part properties in AM is acknowledged in the literature, however it has proven difficult to implement and quantify any added benefit. Over a decade ago, KUMAR, A. V. & ZHANG, H. X. 1999. Electrophotographic powder deposition for freeform fabrication. SolidFreeform Fabrication Proceedings, 647-653 stated their intention of thermally fusing parts by "application of pressure and heat via a compacting device" in a process under development. Kumar reaffirmed his position four years later when he discussed a method for controlling the compaction of each layer (KUMAR, A. V. & DUTTA, A. 2003. Investigation of an electrophotography based rapid prototyping technology. Rapid Prototyping Journal, 9, 95-103). Researchers have adapted laser sintering (LS) equipment to incorporate powder compaction via the use of a roller. NIINO, T. & SATO, K. 2009. Effect of Powder Compaction in Plastic Laser Sintering Fabrication. Solid Freeform Fabrication Proceedings. Austin, TX document a 33% increase in impact strength and 4% increase of the elongation at break when compacting the powder bed with the spreading of each new layer.

With the exception of laminated object manufacture and ultrasonic consolidation (as described by RAM, G. D. J., ROBINSON, C, YANG, Y. & STUCKER, B. E. 2007, Use of ultrasonic consolidation for fabrication of multi-material structures. Rapid Prototyping Journal, 13, 226-235), the application of pressure in commercial and semi-commercial AM systems is limited to highly localized deposition areas (such as controlling the road/bead width in fused deposition modelling/extrusion) and the incidental forces of material deposition are so low that it is difficult to establish any significant contribution to part strength.

Obviously the temporary support structures and materials used in conventional AM are not intended to sustain pressure/force on the same order of magnitude as forging, injection moulding or HIPping. Description of the Drawings

Figure 1 is a schematic illustration of a selective laser printing (SLP) process according to an embodiment of the invention; Figure 2 is an illustration of the pressure application step of an embodiment of the invention;

Figures 3a and 3b show cross-sections of Somos 201 at 500x as processed by SLS (a) and SLP (b); Figures 4a and 4b show fracture surfaces of tensile tested Somos 201 produced by SLS (a) and SLP (b) (Note: these images have different magnifications);

Figure 5 shows a stress-strain curve comparison of Somos 201 samples processed by SLS and SLP;

Figure 6 is a schematic illustration of a selective laser printing (SLP) process according to a further embodiment of the invention; and

Figure 7 is a schematic illustration of a selective laser melting/sintering (SLM/SLS) apparatus according to a further embodiment of the invention. Description of Embodiments

Referring to Figure 1 , a selective laser printing apparatus 101 is shown comprising a build platform 102 and a transfer member, in this embodiment, transfer rollers 103a and 103b, for retaining material and successively depositing layers of the material on to the build platform 102 to build an object from a resulting stack 104 of the layers.

The height of the build platform 102 is adjustable by a mechanism (not shown) such that a position of the build platform 102 relative to the transfer roller 103b can be adjusted. As the stack 104 of layers increases in height, the build platform 102 is moved downwards to accommodate the higher stack. The build platform 102 is also movable laterally relative to the transfer roller 103.

An infrared heater 105 is provided for heating deposited layers to consolidate the material by softening or melting the material.

In use, the build platform 102 moves from a position in which material is deposited from the transfer roller 103b to a position below the heater 105 wherein the deposited layer is consolidated by sintering or melting of the material. Through repeated shutting between the two positions, successive layers can be deposited one on top of the other. The layers are sufficiently heated and the time between deposition of a subsequent layer and heating of the underlying layer is sufficiently short such that a subsequent layer is deposited onto softened or melted material of the stack 104.

As best illustrated in Figure 2, for the deposition of each layer 119, the transfer roller 103b and build platform 102 are relatively positioned such that a layer 1 19 of material is deposited from the transfer roller 103b with sufficient pressure to embed the unfused material of the layer into the softened or melted material of the stack 104. The pressure applied to the stack 104 is also sufficient to close subsurface voids 118 within the stack 104, as illustrated in Figure 2. In particular, the transfer roller 103b and build platform 102 are relatively positioned to achieve plastic deformation of uppermost layers of the stack 104, but not to exceed elastic recovery in layers underlying the uppermost layers.

The intensity of the infrared heat provided by heater 105 and the time the stack 104 is exposed to the infrared radiation may be controlled to provide a known thermal gradient across the stack 104. The pressure required to achieve plastic deformation of uppermost layers and elastic deformation of lower layers for the material with the known thermal gradient can be determined empirically and is stored in a look-up table in a controller (not shown). For the deposition of each layer, the controller queries the look-up table and adjusts the position of the build platform for each layer in order to achieve the pressure identified in the look-up table. The maximum pressure will typically be less than 1 MPa, with typical pressures across the nip of around 0.13MPa.

Pressure is applied to the stack 104 at points that are located within the nip of the transfer roller 103b and the build platform 102. The transfer roller 103b and build platform move synchronously such that a point on an outer surface of the roller

103b is maintained substantially above the same point on the build platform 102 as the stack 104 moves across the nip. The speed at which the transfer roller 103b and build platform 102 is selected such that any point on the transfer roller 103b, which applies pressure to the stack 104, applies pressure for a sufficiently short time to prevent melting of unfused material during deposition through heat transfer from the stack. In this embodiment, the nip width is 2.5mm and the platform speed is 5m/min.

The material comprises electrically charged particles and a layer of material is formed on the transfer roller 103a using electrostatic forces to attract charged particles. A surface of the transfer roller 103a is charged using a charging device 106. The charge is an opposite charge to that of the charged particles. LEDs of an array of LEDs 107 are selectively activated to discharge selected areas of the surface. The selectively charged surface of the transfer roller 103a then passes a source of the charged particles such that the particles are attracted to the areas of the surface that remain charged to form a pattern of particles on the surface. This pattern of particles is then transferred to transfer roller 130b through contact with transfer roller 103b. The layer of material on transfer roller 103b is then deposited to form a layer on the build platform 102, as described above.

Between depositions of successive layers, residual charge in the deposited layers may be reduced or eliminated, for example as described in WO2012/164015. Preferably, any residual charge in the deposited layers is reduced to provide a substantially neutrally charged surface onto which the subsequent layer is to be deposited.

In a modification to the above described embodiment, the build platform is rotatable such that a direction different layers of the stack are deposited can be varied. In this way, any directional defects or properties of a layer associated with a direction of deposition of a layer will vary between layers reducing weaknesses in the final build compared to building the layers all in the same direction. The direction in which layers are deposited may be varied every layer and the angle through which the build platform is rotated may be selected such that the same (or opposite) direction for deposition is not repeated for a set number of layers. For example, an angle other than 0 or 180 degrees may be selected, such that the direction for deposition is not repeated every layer. The angle may be an angle other than 0, 45, 90 or 180 degrees. The angle may a number in degrees that is not a divisor of 360. In one embodiment, the angle is 67 degrees. The controller for controlling the apparatus carries out steps based on instructions of a computer program stored on a suitable data carrier. For example, the data carrier may be a suitable medium for providing the controller with instructions such as non-transient data carrier, for example a floppy disk, a CD ROM, a DVD ROM / RAM (including - R/-RW and +R/ + RW), an HD DVD, a Blu Ray(TM) disc, a memory (such as a Memory Stick(TM), an SD card, a compact flash card, or the like), a disc drive (such as a hard disc drive), a tape, any magneto/optical storage, or a transient data carrier, such as a signal on a wire or fibre optic or a wireless signal, for example a signals sent over a wired or wireless network (such as an Internet download, an FTP transfer, or the like).

Example

This study indicates the effects on mechanical properties of applying pressure incrementally throughout a build using a developmental AM process called Selective Laser Printing (SLP). It also answers the question of whether the cumulative effects of applying pressure layer by layer can achieve part properties on the order of those imparted by injection moulding.

Influence of Pressure on Polymeric Parts

This investigation directly compares the properties of a commercial thermoplastic elastomeric material developed for laser sintering called Somos 201 (3D Systems, USA) as processed by Selective Laser Sintering (SLS) and Selective Laser Printing (SLP).

Selective Laser Sintering material & process

The SLS® process is a commercially established AM technology which uses the thermal energy from a laser to fuse successive cross-sectional layers of powdered materials into complex shapes. It does not incorporate a means of applying pressure during the process and has been thoroughly documented in the literature (Kruth et al., 2003, GIBSON, I. & SHI, D. 1997. Material properties and fabrication parameters in selective laser sintering process. Rapid Prototyping Journal, 3, 129- 136). Somos 201 is a proprietary thermoplastic elastomer powder, engineered for use on laser sintering machines; although its composition is not known, it exhibits some properties similar to Polybutylene Terephthalate (PBT) or PBT/rubber blends (WIMPENNY, D. I., BANERJEE, S. & JONES, J. B. Laser Printed Elastomeric Parts and their Properties. Solid Freeform Fabrication Proceedings, 2009 Austin, TX, USA. The University of Texas, 498-506).

Ten tensile specimens with a 40 mm gauge length were manufactured in a single batch with XY or YX orientation as per ASTM F2921 in an SLS machine (Sinterstation, 3D Systems, USA) using the manufacturers' recommended parameters for Somos 201 (3D Systems, USA) which has a melt temperature of 156°C (3D Systems, 201 1). The specimens were de-powdered with compressed air (to remove support material), with no further post-processing. Cross sections of the specimens were examined by SEM (Model ΣΙΘΜΑ, Carl Zeiss SMT AG, Oberkochen, Germany) and tensile tested (M250-2.5kN, Testometric, England) with a cross-head speed of 50mm/min at 19°C and 40% RH. Specimens which were not tensile tested were cut, mounted in epoxy (Epo-thin, Buelher UK, Coventry, UK), ground, polished (using 9, 3 and 1 μηι grinding media), and carbon coated by sputtering in preparation for SEM imaging. The fractured surfaces of tensile test specimens were gold coated by sputtering and imaged. SEM imaging was performed in secondary electron mode, WD = 8.0mm, 5.0 kV and 500x magnification.

Selective Laser Printing material & process In order to enable Somos 201 to be laser printed in the SLP process it was classified with a d50 32μηι diameter average particle size (measured by laser diffraction) and surface coated with 0.5 wt.% of fumed silica according to the method developed by BANERJEE, S. & WIMPENNY, D. I. 2006. Laser Printing of Polymeric Materials. Solid Freeform Fabrication Symposium. Austin, TX, USA. It was paired with a suitable carrier (for charging and conveying the powder around inside of the laser printers) in order to achieve a mean charge to particle diameter ratio q/d of -0.6 fC/10 μηι, measured using a q/d meter (Epping GmbH, Germany). Tensile specimens as specified above were produced in an XY orientation (ASTM F2921) as a single batch using the SLP process, which is currently under development in the UK. In the SLP process, two-component industrial laser printers (CTG-1 C17-600, CTG PrintTEC, Germany) were used to sequentially print dry unconsolidated powder layers onto a rigid build platform as described by JONES, J. B., GIBBONS, G. J. & WIMPENNY, D. I. Transfer Methods toward Additive Manufacturing by Electrophotography. IS&T's NIP27 and Digital Fabrication 201 1 , Minneapolis, Minnesota, USA. Springfield, VA: Society for Imaging Science and Technology, 180-84 & JONES, J. B., WIMPENNY, D. I., GIBBONS, G. J. & SUTCLIFFE, C. Additive Manufacturing by Electrophotography: Challenges and Successes. IS&T's NIP26 and Digital Fabrication 2010, Austin, Texas. Springfield, VA: Society for Imaging Science and Technology, p. 549-553. After each print, the upper surface of the newly deposited powder layer was heated to 1 15° C (Tg of Somos 201 , determined experimentally) using a medium wave (3-25 μηι) 12 kW infrared heater (Infra-red Systems, England) as shown in Figure 1.

Please note that in this process no thermal input from lasers was used to fuse, cure, or sinter the material. (The term "laser printer" is technically a misnomer in this case since a strip of light emitting diodes (LEDs), instead of a laser, is used to create a charge pattern to which the toner-like powder material is electrostatically attracted.) After each printed layer was fused, the platform was dropped by 15 μηι (the layer thickness) and the cycle continued for each additional layer. An average pressure of 126 ± 9 kPa, as measured by pressure sensitive film and pressure analysis software (Pressurex & Topaq, Sensor Products Inc., USA), was applied at the nip (nominal width 2.5mm) between the final roller (80mm dia.) of the printer and the layers on the build platform as illustrated in Figure 2.

The magnitude of pressure applied does not significantly alter the melting point of the polymer (SEEGER, A., FREITAG, D., FREIDEL, F. & LUFT, G. 2004. Melting point of polymers under high pressure: Part I: Influence of the polymer properties. Thermochimica Acta, 424, 175-181). "Back transfer," where some material in the heated layers adheres and collects on the transfer roller, was avoided by transmitting the pressure to the semi-molten layers by using the new unfused powder layer as a buffer (see Figure 2). Thereby the unfused powder was impregnated into the softened consolidated layers, simultaneously closing subsurface porosity and overlying the stickiness of the underlying build surface, thus ensuring a clean transfer from the roller to the build. A platform speed of at least 5m/min. makes this event dynamic enough that the new powder layer does not have time to become liquefied (by heat transferred from the underlying layers), which prevents a "hot offset" condition where powder is melted sufficiently to experience a droplet spit as it exits the nip resulting in a partial deposit on the substrate with the remainder staying on the transfer roller.

Early samples experienced deformation from applying too much pressure layer by layer or insufficient pressure for efficient transfer. To correct this, the nip pressure was adjusted empirically to achieve plastic deformation in the uppermost layers (to close porosity) and not exceed elastic recovery in the remaining underlying layers (to prevent part distortion). After the build was complete, the heaters were switched off and the specimens were allowed to cool to room temperature. Once cool, the specimens were peeled off of the platform, prepared for SEM and tensile tested according to the same method and parameters as described in 2.1 for the SLS specimens. Figures 3a and 3b show a comparison of SLS produced samples at the start (far left) and break (left middle) of the tensile test, and the SLP produced samples at the start (right middle) and just prior to break (far right).

Although additional types of testing were planned, experimental work using the Somos 201 material was discontinued early because it was not an ideal particle shape (See Wimpenny et al., 2009) or distribution, which made it prone to hot offset and pre-maturely aging/damaging the upstream printer components (which resulted in costly repair that was untenable to afford twice). Although experimental work is on-going with alternative materials (with more suitable particle shapes and distributions), there will not be the opportunity to compare results directly with commercially available SLS materials as in this case.

Results

A comparison of cross-sectional SEM images (Figures 3a and 3b) of Somos 201 illustrates fundamental differences between the SLS and SLP processes.

For SLS (Figure 3a) particles are highly coalesced making identification of individual particles difficult. Scattered voids on the same order of magnitude as the feedstock powder are present throughout the part. For SLP processed samples (Figure 3b) the boundaries between individual particles are still evident, but internal porosity above a few micrometres is absent.

Figures 4a and 4b show the fractured surfaces of the samples after tensile testing. The facture mode for SLS samples (Figure 4a) is a combination of plastic deformation and brittle fracture. The fracture surfaces for the SLP samples (Figure 4b) exhibited plastic deformation with a finer structure than the SLS parts (note the different magnifications in the images).

Figure 5 shows the stress versus strain behaviour of three samples (with the highest, medium, and lowest elongation at failure) from each sample set; and Table 1 compares, the tensile tested results of the SLS and SLP processed specimens.

Table 1 - Comparison ofSomos 201 properties as processed by SLS and SLP

Elongation UTS Elastic Modulus

(%) (MPa) (MPa)

SLS 136 ± 28% 4.9 ± 0.4 16.3 ± 0.9

SLP 513 ± 35% 10.4 ± 0.4 17.9 ± 0.1

The SLS specimens demonstrated properties slightly better than the (published) material specification (3D SYSTEMS 201 1. Data Sheet: SOMOS 201 Material for SLS® Systems): average percent elongation at break of 136 ± 28% (110%) with an ultimate tensile strength (UTS) of 4.9 ± 0.4 MPa (n/a) and 19.4 ± 0.3 MPa (15.5 MPa) modulus of elasticity. The SLP samples elongated an average of 513 ± 35% at break with UTS of 10.4 ± 0.4 MPa and elastic modulus of 17.9 ± 0.1 MPa.

2.4. Discussion

These results highlight the 2x increase in UTS and nearly 5x increase in the ductility which the SLP processing route imparted over SLS for Somos 201. The following discussion provides evidence to disambiguate the influence of material composition from that of pressure and surmise their respective contributions to the resulting mechanical properties. Effect of material composition on mechanical properties

Since the opportunity to compare feedstock powders with nearly identical compositions in polymeric AM is rare, the influence of fumed silica (0.5 wt. % added for SLP processing) on the mechanical properties of thermoplastic elastomers is considered. According to Wimpenny et al. (2009) fumed silica with size of 170-250 nm was demonstrated to be an aggressive sinter/fusing inhibitor for Somos 201. For example, when bulk processed in a mould without silica or applied pressure in an oven at 180° C, Somos 201 specimens had an average UTS of 6.3 ± 0.7 MPa; however with 0.5 wt. % fumed silica, the same processing conditions yielded an average UTS of 1.0 ± 0.4 MPa (Wimpenny et al., 2009). The mechanical properties of other silica filled thermoplastic elastomers and alternative inorganic fillers provides further context to assess any contribution to mechanical properties by the silica in the SLP samples. Aso et al. (2007) explored the effect of filling a Hytrel thermoplastic elastomer with both micro- and nano-silica which scarcely had any effect on the average elongation at break at 0.5 wt. %. The maximum effect of both the micro- and nano-silica filler was obtained with a 3 wt. % loading which increased the average elongation at break from 315 to -337% (Aso et al., 2007). For reference, VOSS, H. & KARGER-KOCSIS, J. 1988. Fatigue crack propagation in glass-fibre and glass-sphere filled PBT composites. International Journal of Fatigue, 10, 3-11 filled PBT with 11 vol. % of 4-70 μηι (17 μηι average particle size) glass spheres, which did not bond well to the matrix and limited the amount of plastic deformation the matrix was able to undergo reducing it toughness. Although the size, shape, and % fill of glass spheres are not representative of the research presented here, it provides context for a PBT composite with similar filler chemistry which exhibited the opposite effect of what was shown with the SLP samples.

Since the inclusion of silica in this research was intended only to achieve appropriate charging and flow characteristics for laser printing the powder, its inclusion did not follow state of the art practice for micro- or nano-scale composite fillers. For example, it was used untreated (JESIONOWSKI, T., BULA, K., JANISZEWSKI, J. & JURGA, J. 2003. The influence of filler modification on its aggregation and dispersion behaviour in silica/PBT composite. Composite Interfaces, 10, 225-242), its dispersion was not undertaken expressly avoiding agglomeration (NICHOLS, G., BYARD, S., BLOXHAM, M. J., BOTTERILL, J., DAWSON, N. J., DENNIS, A., DIART, V., NORTH, N. C.

& SHERWOOD, J. D. 2002. A review of the terms agglomerate and aggregate with a recommendation for nomenclature used in powder and particle characterization. Journal of Pharmaceutical Sciences, 91 , 2103-2109), nor was any silane (or other) coupling agent employed (FUAD, M. Y. A., ISMAIL, Z., ISHAK, Z. A. M. & OMAR, A. K. M. 1995. Application of rice husk ash as fillers in polypropylene: Effect of titanate, zirconate and silane coupling agents. European Polymer Journal, 31 , 885- 893, Jesionowski et al., 2003). The omission of best practice almost certainly reduced the effectiveness of the silica as a composite filler in the SLP samples, when compared to its intended use as a composite filler in the references. This notion is reinforced by FU, S.-Y., FENG, X.-Q., LAUKE, B. & MAI, Y.-W. 2008. Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites. Composites Part B: Engineering, 39, 933-961 who demonstrated that toughness "decreases monotonically" with increased loading of unmodified nano-silica, attributed to poor interfacial adhesion between the silica and the polymer.

As shown by bulk processing of untreated silica in Somos 201 and the review of other thermoplastic elastomers with micro- and nano-silica filler, the composition change to the Somos 201 and the low wt. % loading used for this research, were not sufficient to account for the magnitude of improvement shown in these results. For that reason, it is deemed unlikely that the improved mechanical properties were derived from the change to material composition alone, even if the pressure enabled a beneficial interaction between the near nano-silica particles and the Somos 201 matrix. Furthermore, previous research has shown that the effect of average particle size on melt/sintering behaviour of Somos 201 was not the primary cause either. Investigation of tensile properties from bulk processed Somos 201 in a mould without any applied pressure made from two unfilled narrow particle size distributions (centred at 17 and 30 μηι average particle sizes), never exceeded a UTS of 2.5 MPa with 75% elongation and 1.2 MPa with 1 10% elongation respectively (BANERJEE, S. 201 1. Development of a Novel Toner for Electrophotography based Additive Manufacturing Process. PhD, De Montfort University).

Effect of pressure on mechanical properties

It can be argued that, the relative inferiority of mechanical properties in SLS processed specimens stems from the propensity of SLS to produce porous parts, particularly in amorphous materials as documented separately by Kruth el al. (1998) and Yan et al. (201 1). This tendency is greatly reduced in semi-crystalline materials where a majority proportion of material is fully molten at the target processing temperature promoting better particle coalescence (especially where there is no overlap between the temperature ranges of melting and crystallization), for example polyamide 11 and 12; however it is not entirely eliminated (Zarringhalam et al., 2006). Researchers have also shown that the SLS process does not fully melt all of the powder particles (regardless of their degree of crystallinity) consolidated into a solid body as documented separately by Shi et al. (2004) and Zarringhalam et al. (2009). These phenomena make the SLS process susceptible to defects inclusion, particularly from voids inside of the un-melted central region of particles or interstitial packing density voids which are shielded from laser energy by large particles. It is proposed that the voids in Figures 3a are an illustration of such defects which act as stress risers contributing to the initiation and propagation of cracks. Figure 4a corroborates the presence of these defects which resulted in brittle fracture after the more amorphous domains of the polymer matrix had plastically deformed. The lack of voids in the microstructure of the SLP specimens (Figure 3b) is evidence that the application of pressure closed the voids, thus reducing crack initiation and propagation features in the icrostructure. The presence of particle boundaries in the microstructure of the parts after consolidation can be explained because the consolidating temperature in SLP

(1 15° C) never exceeded the melt temperature (156° C, 3D Systems, 2011) of Somos 201. This reduced the thermal aging to the polymer and implies that SLP consolidation for these samples was analogous to a solid-state sintering process. This consolidation process relies on the layer by layer application of pressure and temperature to promote a high degree of particle cohesion which imparts strength with flexibility. Figure 4b illustrates the extensive polymer chain entanglement which accounts for the exclusive plastic failure mode of these specimens. Furthermore, the extensive network of particle interfaces created by this consolidation method mitigated against cracking mechanisms which propagate through homogenous microstructural domains and provided a much longer mean path through the interfaces.

For comparison, Somos 201 may be moulded with parameters similar to PBT, which is injection moulded with a pressure of 40-70 MPa on the projected area of the part according to GOODSHIP, V. (ed.) 2004. Arburg Practical guide to injection moulding, England: Smithers Rapra Press. Although the magnitude of pressure (-0.13 MPa) applied layer by layer during the SLP process was arguably insignificant when compared to typical injection moulding pressures, its cumulative effect over 65 layers has been demonstrated to contribute to mechanical properties on the same order of magnitude as moulded or dynamically vulcanized PBT/rubber blends as shown by OKAMOTO, M., SHIOMI, K. & INOUE, T. 1994. Structure and mechanical properties of poly(butyleneterephthalate)/rubber blends prepared by dynamic vulcanization. Polymer, 35, 4618-4622 and AOYAMA, T., CARLOS, A. J., SAITO, H., INOUE, T. & NIITSU, Y. 1999. Strain recovery mechanism of PBT/rubber thermoplastic elastomer. Polymer, 40, 3657-3663.

Furthermore, the repeated application of pressure allows the use of smaller, lighter and substantially weaker machine(s) to produce similar mechanical properties as conventionally processed materials. In addition to imparting improved properties on the SLP process, these results indicate applicability to a wider range of materials and processes. The use of pressure demonstrated improved consolidation of long chain length polymers, indicating the potential to process amorphous materials, or those with multiple melting temperatures, to near full density. Additionally, the application of pressure could improve the mechanical properties of parts made by various AM processes, especially equipment with high density/strength supports (such as a powder bed configurations, as previously attempted by Niino and Sato (2009) or extrusion approaches). In order to achieve a high degree of polymer chain entanglement, it is advantageous to apply pressure while the neighbouring feedstock material is in a softened or melted state (as demonstrated by this research), such as is realized with area-wide exposure to thermal, chemical or other consolidation means. Conclusions

The potential to impart increased mechanical properties through the use of layer by layer application of pressure in AM has been demonstrated with polymeric parts. The benefits of applying pressure are most evident when paired with an area-wide fusing method (rather than a point wise fusing method such as a laser) and have the potential to generate material properties comparable with or superior to conventionally produced parts. The successful incorporation of pressure may also increase the potential range of materials which can be processed via additive technologies.

Referring to Figure 6, an alternative embodiment is shown. Features of this embodiment that are the same or similar to features of the first embodiment described with reference to Figures 1 and 2 have been given the same reference numerals but in the series 200. In this embodiment, a separate compacting member in the form of platen 208 is provided for applying pressure to the stack 204. The platen 208 is provided such that the stack 204 can be compacted using the platen 208 before the deposited layers are consolidated using the heater 205. Accordingly, the platen 208 contacts a layer of unfused/unconsolidated material at the top of the stack 204. The platen 208 has a cooling device 229, such as cooling channels, therein for cooling the platen 208 below the temperature of the material in the stack 204. In this way, contact of platen 208 with the stack 204 may quench the material of the stack 204 as well as compressing the stack 204 to close subsurface voids. The platform 201 is lowered into a contained build volume 209. Support material, different to the material used to build the object, is deposited within the contained build volume to fill the spaces within the contained build volume 209 around the object being built so as to support the stack 204. The support material is delivered using a hopper and wiper mechanism (not shown) similar to those found in SLM apparatus, such as that disclosed with reference to Figure 7. The support material may have a different sintering/melting temperature to the material deposited using the transfer roller 203b, such as a lower sintering temperature, such that the support material is not integrated into the object when heated but forms a "part- cake" around the object that can be separated from the object after the object has been built.

To aid compaction of the consolidated material a voltage source 220 may be connected with the compacting member 208 to charge the compacting member to an opposite polarity to that of the charged particles in the stack 204. The charge on the compacting member 208 generates an electric field that repels these charged particles applying a further compressive force to the stack 204.

Referring now to Figure 7, a further embodiment of the invention is shown. Features of this embodiment that are the same or similar to features of the first and second embodiments, described with reference to Figures 1 and 2, and 6, respectively, have been given the same reference numerals but in the series 300. A selective laser melting/sintering (SLM/SLS) apparatus is shown comprising a build platform 301 lowerable into a build volume defined by chamber walls 309. Powder 312a can be spread across the build area above the build platform 301 using a wiper 314 to form a layer of the powder bed 312. In this embodiment, powder 312a is deposited onto a surface from hopper 315 and wiper 314 pushes the powder heap across the bed. However, it will be understood that the powder could be dispensed from a piston arrangement, wherein powder contained in a storage volume is pushed upwardly to provide a set amount of powder to be pushed across the powder bed by the wiper 314.

A laser beam 305a generated by a laser 305b is directed towards selected areas of the powder bed 312 by movable optics 305c to act as a consolidation device 305 for consolidating the powder in the selected areas by sintering/melting. A platen 308 is slidably movable from a position to the side of the powder bed 312 to a position above the powder bed 312. When above the powder bed 312, the build platform 301 can be raised such that the platen 308 engages the powder bed 312 to compress the powder bed 312 between the platen 308 and build platform 301. This process may be carried out after a fresh layer of powder 319 has been laid across the powder bed 312 but before consolidation of areas of this powder layer 319 such that an unconsolidated powder layer is provided between the platen 308 and any consolidated material 304. In this way, the chance of consolidated powder adhering to the platen 308 is reduced. The compression process may be carried out for every layer or may be carried out only after a predetermined number of layers have been consolidated.

In an alternative embodiment, shown in dotted lines in Figure 7, a brush 316 and powder dispensing line 317 form a coating device arranged for coating the surface of the platen 308 that contacts the powder bed with a layer of unconsolidated powder.

In a further embodiment, rather than or in addition to raising the build platform to compress the powder bed 312 against the platen 308, the platen 308 may be forced towards the powder bed by suitable mechanical mechanisms. Such an embodiment may be suitable when the material being consolidated is very hard and requires a large force to close subsurface voids formed in the consolidated material, for example in metal selective laser melting/sintering.

Alterations and modifications to the described embodiments can be made without departing from the invention as defined herein. For example, the transfer roller 103b or platen 308 may be provided with a cooling device for cooling the transfer roller/platen to quench consolidated material. In Figure 6, the compacting device may be provided to compact the material after being heated by the heater. In such an embodiment, a coating device may be provided for coating the platen 208 with powder to avoid adherence of the softened or melted material to the platen.

Claims

1. An additive manufacturing method for building an object layer- by- layer, comprising:

successively depositing layers of material from a transfer member on to a build platform to build an object from a resulting stack of the layers, wherein, between deposition of successive layers, the deposited layers are treated to form softened or melted material onto which a subsequent layer is deposited, and each subsequent layer of material is deposited from the transfer member with sufficient pressure such that the subsequent layer is embedded into the softened or melted material.

2. An additive manufacturing method according to claim 1 , comprising applying sufficient pressure to close subsurface voids within the stack.

3. An additive manufacturing method according to claim 1 or claim 2, comprising applying pressure to achieve plastic deformation of uppermost layers of the stack, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers at the top of the stack, but not to exceed elastic recovery in layers underlying the uppermost layers.

4. An additive manufacturing method according to any preceding claim, comprising controlling the relative position of the transfer member to the build platform to achieve a desired pressure.

5. An additive manufacturing method according to claim 4, comprising controlling the relative position of the transfer member to the build platform to vary the pressure for different layers during building of the stack.

6. An additive manufacturing method according to claim 5, comprising varying the pressure to achieve plastic deformation of uppermost layers of the stack but not to exceed elastic recovery in layers underlying the uppermost layers.

7. An additive manufacturing method according to any one of claims 4 to 6, comprising, between depositing of layers, heating the stack to soften or melt material onto which a subsequent layer is deposited, controlling the heating to provide a desired thermal gradient in the stack, and varying the pressure based upon the expected properties of the material when subjected to the thermal gradient.

8. An additive manufacturing method according to any one of the preceding claims, comprising applying pressure to the stack with the transfer member of less than 5MPa, optionally, less than 1 MPa and further optionally, around 0.13MPa.

9. An additive manufacturing method according to any preceding claim, comprising moving the transfer member relative to the build platform such that any point on the transfer member, which applies pressure to the stack, applies pressure for a sufficiently short time to limit heat transfer from an underlying layer to material on the transfer member to below that which would cause adherence of the material to the transfer member.

10. An additive manufacturing method according to any preceding claim, comprising moving the transfer member relative to the build platform such that any point on the transfer member, which applies pressure to the stack, applies pressure for a sufficiently short time to limit heat transfer from an underlying layer to material on the transfer member such that the material on the transfer member is not fully melted during deposition through heat transfer from the underlying layer.

1 1. An additive manufacturing method according to claim 9 or claim 10, wherein the transfer member is a transfer roller forming a nip width with the build platform across which pressure is applied by the transfer roller to the stack supported by the build platform, the transfer roller moved relative to the build platform such that points on the build platform pass through the nip width within the sufficiently short time.

12. An additive manufacturing method according to any one of claims 9 to 1 1 , comprising any point on the transfer member, which applies pressure to the stack, applies pressure to the stack for less than 0.05s and preferably, less than 0.03s.

13. An additive manufacturing method according to any one of the preceding claims, wherein the material comprises electrically charged particles, the method further comprising, between depositing of successive layers, reducing residual charge in a deposited layer of the stack onto which a subsequent layer is to be deposited.

14. An additive manufacturing method according to claim 13, comprising reducing a residual charge in the deposited layer to provide a substantially neutrally charged surface onto which the subsequent layer is to be deposited.

15. An additive manufacturing method according to any preceding claim, wherein the transfer member is a transfer roller and the build platform is rotated for the deposition of different layers of the stack to vary the direction across the stack that the different layers are formed.

16. An additive manufacturing method for building an object layer-by-layer, comprising:

successively consolidating layers of material on to a build platform to build an object from a resulting stack of the layers, wherein consolidation of the layers comprises heating the layers such that material of the stack is softened or melted, and

between depositing successive layers, applying pressure to the stack using a compacting member to close subsurface voids within the stack.

17. An additive manufacturing method according to claim 16, comprising providing a layer of unsintered or partially sintered material between a surface of the compacting member and the stack, for example the surface of the compacting member may be coated with unsintered or partially sintered material before engaging the stack to apply pressure to the stack or unsintered or partially sintered material may be provided as a top layer of the stack when engaged by the compacting member.

18. An additive manufacturing method according to claim 16 or claim 17, comprising applying pressure to achieve plastic deformation of uppermost layers of the stack, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers at the top of the stack, but not to exceed elastic recovery in layers underlying the uppermost layers.

19. An additive manufacturing method according to any one of claims 16 to 18, comprising controlling the relative position of the compacting member to the build platform to achieve a desired pressure.

20. An additive manufacturing method according to claim 19, comprising controlling the relative position of the compacting member to the build platform to vary the pressure for different layers during building of the stack.

21. An additive manufacturing method according to claim 20, comprising varying the pressure to achieve plastic deformation of uppermost layers of the stack but not to exceed elastic recovery in layers underlying the uppermost layers.

22. An additive manufacturing method according to claim 20 or claim 21 , wherein consolidation of the layers comprises heating the stack to soften or melt material onto which a subsequent layer is consolidated, controlling the heating to provide a desired thermal gradient in the stack, and varying the pressure based upon the expected properties of the material when subjected to the thermal gradient.

23. An additive manufacturing method according to any one of claims 16 to 22, comprising applying pressure to the stack using the compacting member of less than 5MPa, optionally, less than 1 MPa and further optionally, around 0.13MPa.

24. A method according to any one of claims 16 to 23, comprising cooling the compacting member below a temperature of layers of material of the stack.

25. A method according to any one of claims 16 to 24, wherein the compacting member is a roller or platen.

26. A method according to any one of the preceding claims, comprising forming the stack in a contained volume.

27. A method according to any one of the preceding claims, comprising building the object by successively consolidating layers of a first material and second material on to a build platform to build an object from a resulting stack of layers of the first material, the second material consolidated around the first material to support the first material during the build and separable from the first material in a post-processing step.

28. A method according to claim 27, wherein the consolidated first material plastically deforms under a lower pressure than the consolidated second material.

29. A method according to claim 27 or claim 28, wherein the first material has a lower melting point than the second material.

30. A method according to any one of claims 27 to 29, comprising heating the stack of the first and second materials to melt the first material but only partially sinter the second material.

31 . A method according to any one of claims 16 to 30, comprising, successively, laying layers of powder material to form a powder bed and consolidating powder material of each layer with a focussed energy beam, such as a laser or electron beam, to form the stack, wherein the compacting member is arranged such that pressure can be applied to the powder bed using the compacting member to close voids in the consolidated material, the method optionally comprising providing a layer of unsintered or partially sintered powder between the compacting member and the consolidated material.

32. A method according to any one of claims 16 to 30, wherein consolidating the material comprises successively transferring layers of charged particles from a transfer member onto the build platform, the method further comprising applying a voltage to the compacting member to generate an electric field that produces a repulsive force between the compacting member and charged particles in the stack.

33. A method according to claim 32, comprising, after applying pressure using the compacting member, reducing a residual charge in the deposited layer to provide a substantially neutrally charged surface onto which a subsequent layer is to be deposited.

34. A method according to claim 25, wherein consolidating the material comprises successively transferring layers of material from a transfer member onto the build platform, the compacting member applying pressure to the stack separately from the transfer member.

35. An additive manufacturing apparatus for building an object layer-by-layer, comprising:

a build platform,

a transfer member for retaining material and successively depositing layers of the material on to the build platform to build an object from a resulting stack of the layers,

a heater or chemical treatment unit for treating the deposited layers, between depositions of successive layers, to form softened or melted material onto which a subsequent layer is deposited,

wherein the transfer member and build platform are relatively positioned such that each subsequent layer of material is deposited from the transfer member with sufficient pressure such that the subsequent layer is embedded into the softened or melted material.

36. An additive manufacturing apparatus according to claim 35, wherein the transfer member and build platform are relatively positioned to apply a pressure to close subsurface voids within the stack.

37. An additive manufacturing apparatus according to claim 35 or claim 36, wherein the transfer member and build platform are relatively positioned to apply a pressure selected to achieve plastic deformation of uppermost layers of the stack, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers at the top of the stack, but not to exceed elastic recovery in layers underlying the uppermost layers.

38. An additive manufacturing apparatus according to any one of claims 35 to 37, wherein the transfer member and build platform are arranged such that the relative position of the transfer member to the build platform can be varied to control the pressure applied to the stack by the transfer member.

39. An additive manufacturing apparatus according to claim 38, comprising a heater for heating the stack between depositing successive layers, the heater arranged to provide a desired thermal gradient in the stack, and the transfer member and build platform are arranged such that the relative position of the transfer member to the build platform can be varied based upon the thermal gradient.

40. An additive manufacturing apparatus according to any one of claims 35 to 39, wherein the transfer member is arranged for movement relative to the build platform such that any point on the transfer member, which applies pressure to the stack, applies pressure for a sufficiently short time to limit heat transfer from an underlying layer to material on the transfer member to below that which would cause adherence of the material to the transfer member.

41. An additive manufacturing method according to any one of claims 35 to 40, wherein the transfer member is arranged for movement relative to the build platform such that any point on the transfer member, which applies pressure to the stack, applies pressure for a sufficiently short time to limit heat transfer from an underlying layer to material on the transfer member such that the material on the transfer member is not fully melted during deposition through heat transfer from the underlying layer.

42. An additive manufacturing apparatus according to claim 40 or claim 41 , wherein the transfer member is a transfer roller forming a nip width with the build platform across which pressure is applied by the transfer roller to the stack supported by the build platform, the transfer roller and build platform arranged to move relative to each other such that any point on the transfer roller, which applies pressure to the stack, applies pressure for the sufficiently short time.

43. An additive manufacturing apparatus according to any one of claims 35 to 42, wherein the apparatus is arranged to transfer layers of electrically charged particles to build the stack, the apparatus further comprising a charge neutralisation device for reducing residual charge in a deposited layer of the stack onto which a subsequent layer is to be deposited.

44. An additive manufacturing apparatus according to any one of claims 35 to 43, wherein the transfer member is a transfer roller and the build platform is rotatable for the deposition of different layers of the stack to vary the direction across the stack that the different layers are formed.

45. An additive manufacturing apparatus for building an object layer-by-layer, comprising:

a build platform, a material deposition device for laying layers of material on to the build platform

a consolidation device for consolidating material of the layers by heating the layers such that material is softened or melted, and

a compacting member for applying pressure to a stack of the consolidated layers to close subsurface voids within the stack.

46. An additive manufacturing apparatus according to claim 45, comprising a coating device for coating a surface of the compacting member that contacts the consolidated layers with a layer of unsintered or partially sintered material.

47. An additive manufacturing apparatus according to claim 45, wherein the material deposition device, consolidation device and compacting member are arranged such that the consolidation device consolidates at least parts of an unconsolidated layer on top of the stack after the compacting member has applied pressure to the stack via the unconsolidated layer.

48. An additive manufacturing apparatus according to any one of claims 45 to 47, wherein the compacting member is positioned or positionable relative to the build platform to apply pressure to achieve plastic deformation of uppermost layers of the stack, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers at the top of the stack, but not to exceed elastic recovery in layers underlying the uppermost layers.

49. An additive manufacturing apparatus according to any one of claims 45 to 48, comprising a controller for controlling the relative position of the compacting member to the build platform to vary the pressure for different layers during building of the stack.

50. An additive manufacturing apparatus according to claim 49, wherein the consolidation device comprises a heater for heating the stack to soften or melt material onto which a subsequent layer is consolidated, the heater arranged to provide a desired thermal gradient in the stack, and the controller is arranged to control the relative position of the compacting member to the build platform to vary the pressure based upon the expected properties of the material when subjected to the thermal gradient.

51. An additive manufacturing apparatus according to any one of claims 45 to

50, wherein the compacting member comprises a cooling device for cooling the compacting member.

52. An additive manufacturing apparatus according to any one of claims 45 to

51 , wherein the compacting member is a roller or platen.

53. An additive manufacturing apparatus according to any one of claims 35 to 52, comprising a contained volume in which the stack is formed.

54. An additive manufacturing apparatus according to any one of claims 45 to 53, wherein the material deposition device is arranged for laying layers of powder material to form a powder bed and consolidating device is a focussed energy beam, such as a laser or electron beam, wherein the compacting member is arranged such that pressure can be applied to the powder bed using the compacting member, and optionally, a coating device is arranged to provide a layer of unsintered or partially sintered powder on a surface of the compacting member.

55. An additive manufacturing apparatus according to any one of claims 45 to 53, wherein the material deposition device comprises a transfer member for transferring layers of charged particles from the transfer member onto the build platform, a voltage source connected to the compacting member to generate an electric field that produces a repulsive force between the compacting member and charged particles in the stack.

56. A data carrier having instructions stored thereon, the instructions, when executed by a processor of an additive manufacturing apparatus, cause the processor to control the additive manufacturing apparatus to carry out a method according to any one of claims 1 to 34.

57. A method of generating instructions for controlling an additive manufacturing apparatus in which a transfer roller is arranged for depositing material onto a build platform, the build platform rotatable about an axis, the axis movable relative to the transfer roller, the apparatus forming an object by successively forming layers of material, each in a desired pattern, on the transfer roller and transferring each layer to the build platform with the build platform rotated to different orientations for different layers, the method comprising:- receiving geometric data defining an object to be built, determining sections of the object to be built as layers by the additive manufacturing apparatus and determining an orientation for each section to be formed on the transfer roller based upon an intended rotational orientation of the build platform when that section is to be deposited in the build platform.

58. A method of generating instructions for controlling an additive manufacturing apparatus according to any one of claims 34 to 53, that method comprising receiving geometric data defining an object to be built and determining sections of the object to be built as layers by the additive manufacturing apparatus, wherein a thickness of the sections is less than a thickness of a deposited layer of material before pressure is applied to the deposited layer to take into account compaction of the layer upon application of pressure.