Patent Description:
The term "additive manufacturing" (AM) refers to various processes used to synthesise a three-dimensional object (hereafter simply referred to as an "object" or "part"). Certain AM techniques are sometimes referred to as "3D printing".

In AM, parts are typically manufactured by digitally slicing a three-dimensional computer-aided design (CAD) model into two-dimensional layers or images. These layers are then manufactured by curing, consolidating, melting or otherwise forming these layers from a raw material, typically in the form of a powder or fluid.

In the Inventors' experience, AM generally provides a number of advantages over traditional manufacturing methods. These advantages include the ability to manufacture highly complex parts which allows for weight reduction, integration of more functionality into parts and part count reduction. The process also obviates the need for tooling, leading to cost and/or material saving.

A number of AM processes employ an energy source in the form of a high energy beam (e.g. laser or electron beam) to melt or sinter powdered material in a material bed in layers to ultimately form a desired part. These processes are hereinafter jointly referred to as "powder bed fusion processes".

In powder bed fusion processes, the energy source is directed by a scanning unit, based on the geometry of a CAD model, to ensure that the desired layers are formed. The material bed is supported on a build platform which is incrementally lowered in one direction (hereinafter generally referred to as "the Z-direction") as each new layer of the object is formed. A fresh layer of material is then added to the material bed before the next layer is scanned.

A number of AM processes also require some form of preheating to be carried out. Preheating strategies are employed to elevate the temperature of raw material before consolidating the raw material into a final form. Preheating may be used in an attempt to ensure that material is more easily processed and/or processed at a higher rate, or to remove moisture from the material prior to consolidation. The presence of moisture may lead to porosity and/or other defects in the object.

Further, when producing AM parts using energy beams, residual stresses form in the material due to solidification shrinkage of the weld pool. In certain materials, these residual stresses are relatively high and, when producing larger parts, can cause the parts to distort and/or crack. Material can be preheated and the rate of cooling controlled to reduce or relieve such stresses.

In existing AM systems, a build platform is typically mounted to an actuating arrangement, e.g. one or more electrically driven linear actuators located below the build platform, which enables the build platform to be incrementally moved in the Z- direction, thereby permitting the incremental deposition of fresh layers of raw material onto the build platform.

Furthermore, the build platform is typically configured to move within a material retaining unit or housing. As the build platform moves incrementally in the Z- direction, a material deposition arrangement deposits new layers of material onto the build platform, thereby essentially filling the material retaining unit with material. Preheating is conducted either by heating the build platform itself, by heating the material retaining unit or by heating the material deposited on the build platform from the top.

To ensure high accuracy of parts, it is important to guide the build platform along its path in a sufficiently precise manner. Certain AM systems rely on the actuating arrangement to guide the build platform, while other AM systems employ dedicated guiding systems.

The Inventors have found that known AM systems are generally effective and sufficiently accurate when the build platform is relatively small, i.e. when the build platform defines a generally small area in a horizontal or a X-Y plane. For example, in a typically "small" implementation, the AM system may have major build platform dimensions of about <NUM> x <NUM> and operate at temperatures of up to <NUM>. However, the Inventors believe that known AM systems face a number of issues when relatively large build platforms are employed at higher temperatures due to the effects of thermal expansion.

When operating at relatively high temperatures, thermal expansion of AM apparatus components and in particular the build platform may cause distortion, mechanical jamming, failures of the actuating arrangement, and the like. Certain materials require relatively high preheating temperatures to reduce residual stresses in parts and to inhibit crack-forming. The required preheating temperature is material dependant and can range from a few hundred degrees Celsius to more than <NUM>. As an example, a <NUM> long titanium grade <NUM> baseplate may increase <NUM> or more in length when preheated at <NUM>.

In cases where a relatively small build platform is employed, a single, central actuator or guide pillar may be used to displace the build platform. Such configurations typically perform relatively well under high temperatures, as the effects of thermal expansion on the single central actuator or pillar are not substantial. However, these configurations do not provide enough structural support for larger build platforms.

The Inventors have found that the effects of thermal expansion are amplified when the size of the build platform is increased. In cases where a relatively large build platform is employed, a plurality of actuators or guide pillars is typically employed. For instance, the build platform may be rectangular in bottom view, with an actuator being provided at or towards each of the four bottom corners of the build platform to support and displace the build platform. The Inventors have found that such configurations may be affected by thermal expansion to a significant extent, as thermal expansion of the build platform may more easily cause the guide pillars or actuators to distort, jam and/or fail.

Thermal stresses induced in the build platform when it is constrained by the actuating arrangement may cause the build platform itself to become distorted. A distorted build platform may make it impossible to produce a powder layer of uniform thickness at the start of the additive manufacturing process. This could result in failure of a build due to delamination from the build platform.

The present invention aims to address the issues identified above, at least to some extent.

<CIT> discloses a device for manufacturing three-dimensional models by means of a 3D printing process, whereby a build platform for application of build material is provided and a support frame is arranged around the build platform, to which said support frame at least one device for dosing the particulate material and one device for bonding the particulate material is attached via the guiding elements and the support frame is moveable in a Z direction, which essentially means perpendicular to the base surface of the build platform, said movement provided by at least two vertical positioning units on the support frame. In this respect, it is provided that the positioning units are respectively separate components and are arrangeable on the support frame independently from one another and the location and orientation of such can be adjusted independently from one another.

<CIT> discloses a three-dimensional shaping apparatus including a support frame, a material supply part supported by the support frame, a shaped article placing part which is supported by the support frame and on which a material supplied from the material supply part is placed, an input part that previously inputs thereto the moving amount of a table, a storage part that stores the moving amount input thereto from the input part, and a controller that controls the material supply part and the shaped article placing part. The shaped article placing part includes a table on the upper surface of which a shaped article is placed and a drive part that drives the table. The controller moves the table by the moving amount stored in the storage part.

<CIT> discloses an additive fabrication device configured to form layers of material on a build platform, each layer of material being formed so as to contact a container in addition to the build platform and/or a previously formed layer of material, and may comprise a build platform, a container, and a plurality of mechanical linkages each independently coupled to the container and configured to move the container relative to the build platform.

<CIT> discloses an additive fabrication device with two linear guiding elements abutting the side surface of the build platform.

According to one aspect of the invention there is provided an additive manufacturing apparatus as defined in claim <NUM> of the appended claims.

The mounting means is a mounting arrangement provided by a pillar and bush arrangement or pillar and bearing arrangement by which the build platform is mounted in the additive manufacturing apparatus in order to prevent the build platform from being displaced in the X-Y plane relative to the mounting arrangement.

In some embodiments, the pillar may be fixedly mounted in the additive manufacturing apparatus, with the bush being mounted to the build platform and being configured to be displaced along with the build platform. In other embodiments, the bush may be fixedly mounted in the additive manufacturing apparatus, with the pillar configured to be displaced along with the build platform.

The build platform is generally rectangular in the X-Y plane. The mounting element is positioned at a corner or corner region of the build platform.

The guiding means include two guiding elements, each located at or near a respective corner region of the build platform.

The mounting means is mounted at a first corner region, a first of the guiding elements is mounted at a second corner region on a side of the build platform adjacent to the first corner region, and a second of the guiding elements is mounted at a third corner region on another side of the build platform adjacent to the first corner region.

Each guiding element includes or be provided by a linear guide which extends along the Z-axis and which substantially abuts a side surface of the build platform to prevent angular displacement of the build platform, in use, the linear guide being fixedly mounted in the additive manufacturing apparatus.

Each guiding element may further include a surface plate provided on the side surface of the build platform. In some embodiments, the surface plate does not form part of the guiding element and instead forms part of the build platform. The surface plate may be configured to mate with a corresponding linear guide in a sliding fashion, in use.

An interface between the linear guide and its corresponding surface plate may be aligned with a centre of the mounting means along the X-axis or Y-axis, as the case may be. The centre of the mounting means may be a centre point of the guide pillar in the X-Y plane.

The build platform guiding arrangement may further include a displacement arrangement configured to displace the build platform along the Z-axis of the additive manufacturing apparatus. The displacement arrangement may include at least two spaced apart linear actuators. The linear actuators may be angularly displaceable and/or include angularly displaceable components in order operatively to compensate for thermal expansion or contraction of the build platform to maintain the build platform in a generally parallel orientation relative to the X-Y plane.

One or both ends of each linear actuator may include or be mounted to angularly displaceable joints.

The build platform guiding arrangement may include three or more linear actuators of the type described above. In one embodiment, the build platform guiding arrangement may include four linear actuators arranged so as to define four corners of a rectangle when the guiding arrangement is viewed from the bottom.

The linear actuators may have opposed ends, a first end of each linear actuator may be mounted to a support structure (e.g. a lower region of the additive manufacturing apparatus) and second end of the linear actuator may be mounted to a bottom of the build platform.

The second end of the linear actuator may protrude above the build platform, with a side of the linear actuator being attached to the build platform by way of an angularly displaceable component, e.g. a joint.

The build platform may be a primary build platform which defines a working area of the additive manufacturing apparatus, in use.

Alternatively, the build platform may be a secondary build platform which is mounted or mountable to a primary build platform. In such cases, a primary build platform is also provided, in use, and the linear actuators are directly coupled to the secondary build platform and are configured to displace the primary build platform by displacing the secondary build platform.

The primary build platform may have dimensions different from the dimensions of the secondary build platform.

A material retaining unit may further be provided. The material retaining unit may include a plurality of walls which are configured to conform closely to outer side edges of the primary build platform so as to retain a material bed operatively deposited in the working area defined by the primary build platform.

The primary build platform may be spaced apart from the secondary build platform along the Z-axis of the additive manufacturing apparatus, i.e. the primary build platform may be located above the secondary build platform. The secondary build platform may in turn be located above the actuating arrangement. At least one heating and/or cooling unit may be provided in a space defined between the platforms. Thermal insulation and/or a sensor arrangement (e.g. temperature sensors) may also be provided in the space.

The primary and second build platforms may operatively be mounted such that they are substantially parallel to each other and to the X-Y plane.

Also disclosed, but not claimed, is an additive manufacturing apparatus, which includes a displacement arrangement which includes at least two linear actuators configured to displace a build platform of the additive manufacturing apparatus along a Z-axis of the additive manufacturing apparatus, wherein the linear actuators are angularly displaceable and/or include angularly displaceable components in order operatively to compensate for thermal expansion or contraction of the build platform to maintain the build platform in a generally parallel orientation relative to an X-Y plane defined by an X-axis and a Y- axis of the additive manufacturing apparatus.

The linear actuators may be angularly displaceable in an X-Z plane defined by the X- axis and the Z-axis of the additive manufacturing apparatus and/or in a Y-Z plane defined by a Y-axis and the Z-axis of the additive manufacturing apparatus in order operatively to maintain the build platform in a generally parallel orientation relative to the X-Y plane.

The additive manufacturing apparatus typically further includes:.

According to a second aspect of the invention, there is provided a method of supporting a build platform of an additive manufacturing apparatus of the first aspect as defined in claim <NUM> of the appended claims.

Displacement of the build platform may be effected by at least two linear actuators, each actuator having two connection regions, one of which is connected to a support structure and the other of which is connected to the build platform, the method including connecting the connection regions of the actuator to the support structure and the build platform such that they are angularly displaceable relative to the support structure and build platform to compensate for relative lateral movement between the ends of the actuator as a result of differential thermal expansion of the build platform and the support structure.

The invention will now be further described, by way of example, with reference to the accompanying conceptual drawings.

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiments described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

An embodiment of a build platform guiding arrangement <NUM> forming part of an additive manufacturing apparatus according to the invention is illustrated in <FIG>. The build platform guiding arrangement <NUM> includes a mounting means in the form of a mounting arrangement <NUM> and two guiding elements <NUM>, <NUM>. The mounting arrangement <NUM> is mounted to a build platform <NUM>.

The build platform <NUM> is generally planar and rectangular in an X-Y plane of an additive manufacturing apparatus (not shown) in which the guiding arrangement <NUM> is operatively installed. The build platform <NUM> is configured to be displaced along a vertical or Z-axis of the additive manufacturing apparatus in use. The axes X, Y and Z are indicated in <FIG>.

The mounting arrangement <NUM> is provided by a pillar and bush or bearing arrangement <NUM>, <NUM> mounted to a corner region of the build platform <NUM>. The pillar <NUM> is configured to be fixedly mounted in the additive manufacturing apparatus such that it extends along the Z-axis, while the bush <NUM> is mounted concentrically about the pillar <NUM> and is configured to be vertically displaced together with the build platform <NUM>, in use.

The mounting arrangement <NUM> serves to permit displacement of the build platform <NUM> along the Z-axis, while preventing substantial linear displacement of the build platform <NUM> along the X-axis or Y-axis relative to the pillar <NUM>.

The guiding elements <NUM>, <NUM> are provided by elongate linear guides which are rectangular in cross-section. The guiding elements <NUM>, <NUM> are configured to be fixedly mounted in the additive manufacturing apparatus such that their lengths extend along the Z-axis and such that they abut different side surfaces of the build platform <NUM>.

Each guiding element <NUM>, <NUM> is located near a respective corner region of the build platform <NUM>. As illustrated in <FIG>, the mounting arrangement <NUM> is mounted to a first corner region of the build platform <NUM>, a first of the guiding elements <NUM> is mounted such that one of its sides abuts a second corner region on a side of the build platform <NUM> adjacent to the first corner region, and a second of the guiding elements <NUM> is mounted such that one of its sides abuts a third corner region on another side of the build platform <NUM> adjacent to the first corner region.

In this embodiment, surface plates <NUM>, <NUM> are provided on the side surfaces of the build platform <NUM>. Each surface plate <NUM>, <NUM> has an exterior face which is configured to abut against a corresponding one of the guiding elements <NUM>, <NUM> to facilitate sliding motion of the build platform <NUM> along the lengths of the guiding elements <NUM>, <NUM>. A material with anti-frictional properties at high wear and high temperature conditions, such as aluminium bronze, may be used in the surface plates <NUM>, <NUM>.

An interface between each guiding element <NUM>, <NUM> and its corresponding surface plate <NUM>, <NUM> in the X-Y plane is aligned with a centre point "C" of the pillar <NUM>, as is clear from <FIG>.

In use, the fixedly mounted guiding elements <NUM>, <NUM> guide the build platform <NUM> accurately along the Z-axis. The guiding elements <NUM>, <NUM> serve to prevent angular displacement of the build platform <NUM> about the Z-axis (in the X-Y plane) while permitting unconstrained thermal expansion/contraction in the X and Y directions (indicated by the arrows "P" and "Q" in <FIG>).

The guiding elements <NUM>, <NUM> and surface plates <NUM>, <NUM> permit thermal expansion of the build platform <NUM> by providing a sliding surface between each guiding element <NUM>, <NUM> and its corresponding surface plate <NUM>, <NUM> in the X-Z or Y-Z plane, as the case may be.

In order further to illustrate the embodiment of <FIG>, <FIG> additionally illustrate a frame <NUM> of the additive manufacturing apparatus to which the guiding arrangement <NUM> is mounted. The frame <NUM> is configured to remain stationary, in use.

The pillar <NUM> is mounted to a pillar support formation <NUM> in a corner region of the frame <NUM> and the guiding elements <NUM>, <NUM> are mounted to guiding support formations <NUM>, <NUM> in adjacent corner regions of the frame <NUM> by way of set screws <NUM>, <NUM>. The set screws <NUM>, <NUM> permit accurate alignment of the build platform <NUM>, in use.

<FIG> illustrate a displacement arrangement <NUM> which may provide a build platform guiding arrangement according to the invention or which may form part of a build platform guiding arrangement according to the invention, e.g. the guiding arrangement <NUM>.

The displacement arrangement <NUM> is configured to displace a build platform <NUM> of an additive manufacturing apparatus along the Z-axis of the additive manufacturing apparatus. In this embodiment, the displacement arrangement <NUM> includes four linear actuators <NUM> which operatively actuate and support the build platform <NUM>. The linear actuators <NUM> are arranged at the bottom of the build platform <NUM> so as to form four corners of a rectangle, when the displacement arrangement <NUM> is viewed from the bottom, thereby supporting the entire length and width of the build platform.

Each linear actuator <NUM> has opposed ends. The linear actuator <NUM> includes a first end <NUM> which is configured to be mounted to a support structure in the form of a lower region of the additive manufacturing apparatus, e.g. an apparatus base (not shown), and a second end <NUM> which is mounted to a bottom surface <NUM> of the build platform <NUM>.

Each end <NUM>, <NUM> of the linear actuator <NUM> is provided with an angularly displaceable joint <NUM>, <NUM>. In this embodiment, the joints <NUM>, <NUM> are ball joints.

As mentioned above, the linear actuators <NUM> are configured to displace the build platform <NUM> along the Z-axis of the additive manufacturing apparatus. Additionally, the linear actuators <NUM> are angularly displaceable by means of the joints <NUM>, <NUM> in order operatively to compensate for thermal expansion or contraction of the build platform <NUM> to maintain the build platform <NUM> in a generally parallel orientation relative to the X-Y plane.

In a first position shown in <FIG>, the linear actuators <NUM> extend along the Z- axis and there is thus a <NUM> degree angle between the length of each linear actuator <NUM> and the build platform <NUM>, as indicated by the angle α in <FIG>.

In a second position shown in <FIG>, thermal expansion of the build platform <NUM> has taken place, as indicated by the directional arrow "T" in <FIG>. To compensate for relative lateral movement between the ends <NUM>, <NUM> of the actuators <NUM> as a result of differential thermal expansion of the build platform <NUM> and the support structure to which the ends <NUM>, <NUM> are mounted, the actuators <NUM> are angularly displaced by way of the joints <NUM>, <NUM>, thereby maintaining the build platform <NUM> in an orientation generally parallel relative to the X-Y plane (i.e. in a horizontal orientation).

In the second position, therefore, there is a non-perpendicular angle between the base <NUM> and the length of each actuator <NUM> when viewed in the Y-Z plane, as indicated by the angle β in <FIG>. It will be appreciated that the extent of the angular displacement of the actuators <NUM> will vary, with the actuator <NUM> which is furthest from the pillar <NUM> experiencing the largest angular displacement. Although this may cause the base <NUM> to be at a slanted angle instead of being in a horizontal position, the Inventors have found that this deviation is typically within acceptable tolerance values.

It will be understood by those of ordinary skill in the art that the thermal expansion shown in <FIG>, and thus the angle β, is exaggerated in order to illustrate the principle of operation of the actuators <NUM>, and that the angle β may thus typically be closer to <NUM> degrees than the angle illustrated in the drawings.

The Inventors have found that, in some embodiments, the actuators <NUM> may move up to <NUM> degrees as a result of thermal expansion. Each actuator <NUM> may be mounted at a relatively small negative angle when at room temperature (e.g. at a negative angle of <NUM>°) to compensate for the angular displacement to which that specific actuator will be subjected in use. This may ensure that the actuators are vertically oriented when at operating conditions (e.g. <NUM>) and that the build platform is thus horizontal.

An example of a build platform guiding arrangement <NUM> not within the scope of the claims is shown in <FIG>.

In this example, a pillar <NUM> and bush <NUM> arrangement is provided centrally along one end of the build platform <NUM> and a single guiding element <NUM> is provided centrally along an opposing end of the build platform <NUM>. Furthermore, the guiding arrangement <NUM> includes only two linear actuators <NUM>.

The pillar <NUM> is mounted to one end of a frame <NUM> of an additive manufacturing apparatus (not shown), while the guiding element <NUM> is mounted to an opposing end of the frame <NUM>.

In this example, the guiding element <NUM> is elongate and T-shaped in cross- section. A wider side, or base, of the guiding element <NUM> is mounted to the frame <NUM> and a narrower side, or protrusion, of the guiding element <NUM> mates with a complementally shaped recess <NUM> in the corresponding end of the build platform <NUM>. The length of the narrow side or protrusion of the guiding element <NUM> and the depth of the recess <NUM> will be selected such that the build platform <NUM> may expand and contract in the X-direction without the free end of the narrow side or protrusion coming into contact with a root or base of the recess <NUM> to ensure that they remain in sliding engagement. With this arrangement, the build platform <NUM> can expand and contract in the Y-direction and the X-direction in an unconstrained manner.

Each end <NUM>, <NUM> of the linear actuator <NUM> is provided with an angularly displaceable joint <NUM>, <NUM>. In this example, the joints <NUM>, <NUM> are single axis pivot joints. The joints <NUM>, <NUM> are pivotable in the Y-Z plane.

The pillar <NUM>, the linear actuators <NUM>, the recess <NUM> and the guiding element <NUM> are aligned along a centre line "G" of the guiding arrangement <NUM>, as shown in <FIG>. The depth of the recess <NUM> will be sufficient to compensate for the thermal expansion of the build platform <NUM>.

It will again be understood that the thermal expansion shown in <FIG> (indicated by the directional arrow "H") is exaggerated in order to illustrate the principle of operation of the actuators <NUM>.

Another embodiment of a build platform guiding arrangement <NUM> according to the invention is shown in <FIG>. This embodiment may provide similar functionality or achieve similar results to the embodiments described with reference to <FIG>.

The guiding arrangement <NUM> includes a mounting arrangement <NUM> and two guiding elements <NUM>, <NUM> similar to those described with reference to <FIG>.

The guiding arrangement <NUM> includes four linear actuators <NUM>. Two linear actuators <NUM> are provided on each side of the build platform <NUM>, as is best shown in <FIG>. In this embodiment, each linear actuators <NUM> is configured to displace the build platform along the Z-axis by way of a lead screw and nut arrangement (not shown) forming part of the linear actuator <NUM>. It is envisaged that rack and pinion arrangements may be used instead of lead screw and nut arrangements to provide a similar configuration to the one illustrated in <FIG>.

Each linear actuator <NUM> includes a first end <NUM> which is configured to be mounted to a support structure in the form of a lower region of the additive manufacturing apparatus, e.g. an apparatus base (not shown), and a second end <NUM> which protrudes above the build platform <NUM>.

The first end <NUM> of each linear actuator <NUM> is provided with a ball joint <NUM>, while a side of the actuator <NUM> is connected to the build platform <NUM> by way of a further ball joint <NUM>. The ball joint <NUM> and the ball joint <NUM> are oriented generally transversely relative to each other, as is best shown in <FIG>. The ball joint <NUM> is configured to be displaced along the Z-axis together with the build platform <NUM>, in use.

The Inventors have found that the configuration of <FIG> may reduce the overall height of a build platform guiding arrangement and/or additive manufacturing apparatus.

<FIG> illustrates an example of a build platform configuration <NUM> in which a build platform guiding arrangement of the present invention can be employed.

The build platform configuration <NUM> includes a secondary build platform, or base <NUM>, which is attached to a conventional actuating arrangement <NUM>.

The actuating arrangement <NUM> is provided by two electrically driven linear actuators <NUM>, <NUM> located below the base <NUM>. Supporting ends <NUM>, <NUM> of the actuators <NUM>, <NUM> are directly connected to a bottom surface <NUM> of the base <NUM>.

In use, linear motion of the actuators <NUM>, <NUM> causes vertical displacement of the base <NUM>, i.e. displacement along a Z-axis of an additive manufacturing apparatus in which the build platform configuration <NUM> is installed.

A primary build platform <NUM> is mounted to the base <NUM> by a mounting structure <NUM>. In this embodiment, the mounting structure <NUM> is in the form of a steel frame, which is bolted onto the base <NUM>. A heating assembly <NUM> is provided on top of the mounting structure <NUM>. The primary build platform <NUM> is thus attached to the mounting structure <NUM> via the heating assembly <NUM>.

It is envisaged that, in other embodiments, the mounting structure may be bolted or clamped directly to the primary build platform. A connection mechanism between the mounting structure and the primary build platform may permit adjustment by means of set screws, to ensure that the primary build platform can be aligned appropriately relative to other components of the additive manufacturing apparatus.

In use, vertical displacement of the base <NUM> causes displacement of the primary build platform <NUM>.

The primary build platform <NUM> can be displaced downwardly incrementally to permit fresh layers of raw material to be deposited in a working area <NUM> it defines, thereby to form a material bed <NUM>.

At least one material deposition arrangement (not shown) is typically provided for depositing the layers of material, with a suitable feeding mechanism being included for feeding material into the material deposition arrangement. Raw material may be consolidated using a high energy beam, directed by a scanning unit (not shown), as will be well understood by those of ordinary skill in the art.

A material retaining unit <NUM> is provided. The material retaining unit <NUM> includes a plurality of walls which are configured to conform closely to outer side edges of the primary build platform <NUM> so as to retain the material bed <NUM> operatively deposited in the working area <NUM>.

The primary and secondary build platforms <NUM>, <NUM> are mounted such that they are parallel to each other and to the X-Y plane.

In order to compensate for the effects of thermal expansion, the mounting arrangement <NUM> and guiding elements <NUM>, <NUM> of the build platform guiding arrangement <NUM> described with reference to <FIG> can be incorporated into the configuration <NUM> by mounting the m relative to the base <NUM>. Additionally, the actuating arrangement <NUM> can be replaced with the displacement arrangement <NUM> described with reference to <FIG>. Such an exemplary embodiment is illustrated in <FIG> and is generally indicated by reference numeral <NUM>.

As shown in <FIG>, the build platform guiding arrangement <NUM> would typically be incorporated into the base <NUM> and not the primary build platform <NUM> to prevent the guiding arrangement <NUM> from interfering with the material retaining unit <NUM>. However, in certain alternative configurations a guiding arrangement may be mounted directly to a primary build platform, i.e. a build platform defining an additive manufacturing apparatus' working area.

<FIG> illustrate another example of a build platform guiding arrangement <NUM> for an additive manufacturing apparatus not within the scope of the claims.

This example is conceptually similar to the embodiments described with reference to <FIG>. However, in this example, the pillar <NUM> and guiding elements <NUM>, <NUM> are mounted to the build platform <NUM> in a movable manner (along the Z-axis), while the bush or bearing <NUM> and the surface plates <NUM> and <NUM> are mounted to a frame <NUM> of the additive manufacturing apparatus in a fixed manner.

The guiding elements <NUM>, <NUM> are mounted to a bottom surface <NUM> of the build platform <NUM> by way of generally triangular flanges or gussets <NUM>, <NUM>. In use, the guiding elements <NUM>, <NUM> slidingly engage the plates <NUM>, <NUM> to guide the build platform <NUM> accurately along the Z-axis. The guiding elements <NUM>, <NUM> serve to prevent angular displacement of the build platform <NUM> about the Z-axis (in the X-Y plane) while permitting unconstrained thermal expansion/contraction in the X and Y directions.

The Inventors have found that the configuration described with reference to <FIG> may obviate the need for a secondary build platform, at least in some applications. This aspect is illustrated in <FIG>, in which a set of angularly displaceable linear actuators <NUM> are shown as being mounted directly to the bottom <NUM> of the build platform <NUM> and to a base <NUM> of the frame <NUM>. <FIG> also illustrate a conventional material retaining unit <NUM> and a material bed <NUM> retained by the material retaining unit <NUM> (only in <FIG>).

The Inventors believe that the guiding arrangement of the present invention provides numerous advantages.

An arrangement is provided by which a build platform can be guided and actuated in such a manner that it is substantially unaffected by thermal expansion. The Inventors believe that the arrangement is suitable for use with relatively large build platforms and in relatively high temperature applications.

The Inventors have found that the present invention provides an effective technique for supporting a build platform of an additive manufacturing apparatus in a manner which compensates for thermal expansion of the build platform. Specifically, a vertical guide is provided at a periphery of the build platform along which the build platform is displaceable, and angular displacement of the build platform about the vertical guide is substantially inhibited whilst at the same time permitting unconstrained expansion and contraction of the build platform. This permits an additive manufacturing apparatus to handle high levels of preheating as it allows for substantial thermal expansion without imposing significant forces on the build platform and guide and/or actuation systems.

It is believed that the mounting element (e.g. the pillar <NUM>) provides a useful fixed reference point which is not affected by thermal expansion.

The present invention provides a displacement arrangement capable of compensating for thermal expansion or contraction of the build platform to maintain the build platform in a generally parallel orientation relative to the X-Y plane. Specifically, the linear actuators described herein, or components thereof, are angularly displaceable relative to a support structure of the additive manufacturing apparatus and the build platform to compensate for relative lateral movement between ends of the actuators as a result of differential thermal expansion or contraction of the build platform and the support structure.

The Inventors have found that the use of a secondary build platform may be advantageous in that it obviates the risk of the actuators interfering with the material retaining unit and permits the use of so-called "hanging" actuators, i.e. actuators that function under tension instead of compression. Embodiments of the invention, however, employ only a single, primary build platform.

The linear actuators may also serve to prevent significant angular displacement of the build platform.

Claim 1:
An additive manufacturing apparatus which includes a build platform (<NUM>) which is configured to be displaced along a vertical or Z-axis of the additive manufacturing apparatus and a build platform guiding arrangement (<NUM>) including:
mounting means (<NUM>) positioned at or near an extremity of the build platform in a horizontal or X-Y plane defined by an X-axis and a Y-axis of the additive manufacturing apparatus, wherein the build platform (<NUM>) is generally rectangular in the X-Y plane and wherein the mounting means (<NUM>) is a mounting arrangement provided by a pillar (<NUM>) and bush (<NUM>) arrangement or pillar and bearing arrangement by which the build platform (<NUM>) is mounted in the additive manufacturing apparatus, the mounting means being mounted at a first corner region and being configured operatively to permit displacement of the build platform (<NUM>) along the Z-axis and prevent substantial linear displacement of the build platform (<NUM>) along the X-axis and/or the Y-axis relative to the mounting means; and
guiding means comprising at least two guiding elements (<NUM>, <NUM>), each guiding element (<NUM>, <NUM>) including or being provided by a linear guide which extends along the Z-axis and which substantially abuts a side surface of the build platform (<NUM>), the linear guides being fixedly mounted in the additive manufacturing apparatus, the guiding means including a first linear guide mounted at a second corner region on a side of the build platform adjacent to the first corner region, and a second linear guide mounted at a third corner region on another side of the build platform adjacent to the first corner region thereby to permit unconstrained thermal expansion and contraction of the build platform (<NUM>) in the X-Y plane and prevent, in use, substantial angular displacement of the build platform (<NUM>) about the Z-axis in the X-Y plane.