Methods and thermal structures for additive manufacturing

The present disclosure generally relates to methods for additive manufacturing (AM) that utilize thermal dissipation support structures in the process of building objects, as well as novel thermal dissipation support structures to be used within these AM processes. The thermal dissipation support structures include at least one sacrificial structure that is separated from the object by a portion of unfused powder. The sacrificial structure increases a thermal dissipation rate of at least a portion of the object such that such that thermal gradients in the object remain below a specified threshold that prevents deformation of the object.

INTRODUCTION

The present disclosure generally relates to methods for additive manufacturing (AM) that utilize support structures in the process of building objects, as well as novel support structures to be used within these AM processes.

BACKGROUND

AM processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), AM encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. A particular type of AM process uses an energy beam, for example, an electron beam or electromagnetic radiation such as a laser beam, to sinter or melt a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. Different material systems, for example, engineering plastics, thermoplastic elastomers, metals, and ceramics are in use. Laser sintering or melting is a notable AM process for rapid fabrication of functional prototypes and tools. Applications include direct manufacturing of complex workpieces, patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of AM processes.

Selective laser sintering, direct laser sintering, selective laser melting, and direct laser melting are common industry terms used to refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. For example, U.S. Pat. Nos. 4,863,538 and 5,460,758 describe conventional laser sintering techniques. More accurately, sintering entails fusing (agglomerating) particles of a powder at a temperature below the melting point of the powder material, whereas melting entails fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material. Although the laser sintering and melting processes can be applied to a broad range of powder materials, the scientific and technical aspects of the production route, for example, sintering or melting rate and the effects of processing parameters on the microstructural evolution during the layer manufacturing process have not been well understood. This method of fabrication is accompanied by multiple modes of heat, mass and momentum transfer, and chemical reactions that make the process very complex.

FIG. 1is schematic diagram showing a cross-sectional view of an exemplary conventional system100for direct metal laser sintering (DMLS) or direct metal laser melting (DMLM). The apparatus100builds objects, for example, the part122, in a layer-by-layer manner by sintering or melting a powder material (not shown) using an energy beam136generated by a source such as a laser120. The powder to be melted by the energy beam is supplied by reservoir126and spread evenly over a build plate114using a recoater arm116travelling in direction134to maintain the powder at a level118and remove excess powder material extending above the powder level118to waste container128. The energy beam136sinters or melts a cross sectional layer of the object being built under control of the galvo scanner132. The build plate114is lowered and another layer of powder is spread over the build plate and object being built, followed by successive melting/sintering of the powder by the laser120. The process is repeated until the part122is completely built up from the melted/sintered powder material. The laser120may be controlled by a computer system including a processor and a memory. The computer system may determine a scan pattern for each layer and control laser120to irradiate the powder material according to the scan pattern. After fabrication of the part122is complete, various post-processing procedures may be applied to the part122. Post processing procedures include removal of excess powder by, for example, blowing or vacuuming. Other post processing procedures include a stress relief process. Additional thermal, mechanical, and chemical post processing procedures can be used to finish the part122.

The apparatus100is controlled by a computer executing a control program. For example, the apparatus100includes a processor (e.g., a microprocessor) executing firmware, an operating system, or other software that provides an interface between the apparatus100and an operator. The computer receives, as input, a three dimensional model of the object to be formed. For example, the three dimensional model is generated using a computer aided design (CAD) program. The computer analyzes the model and proposes a tool path for each object within the model. The operator may define or adjust various parameters of the scan pattern such as power, speed, and spacing, but generally does not program the tool path directly.

It is possible that during laser sintering/melting portions of a three-dimensional object may dissipate heat in an undesirable manner. For example, a melted portion that is surrounded by unmelted powder may be thermally insulated by the unmelted powder. Such a melted portion may not cool and solidify quickly enough. For example, if the melted portion does not solidify before the recoater116applies a new layer of powder, the melted portion may be disturbed by the recoater116. Additionally, such a melted portion may solidify but retain sufficient heat to facilitate large thermal gradients within the three-dimensional object leading to deformation of the part. Such deformation may lead to a finished object that does not meet geometrics specifications or to the part contacting the recoater during the build process, further deforming the part, damaging the recoater, and/or disrupting the build process.

In view of the above, it can be appreciated that there are problems, shortcomings or disadvantages associated with AM techniques, and that it would be desirable if improved methods and structures for controlling temperatures during AM processes were available.

SUMMARY

In one aspect, the disclosure provides a method for fabricating an object. The method includes: (a) irradiating a layer of powder in a powder bed with an energy beam in a series of scan lines to form a fused region; (b) providing a subsequent layer of powder over the powder bed by passing a recoater arm over the powder bed from a first side of the powder bed to a second side of the powder bed; and (c) repeating steps (a) and (b) until the object and at least one support structure is formed in the powder bed. The at least one support structure includes a sacrificial structure separated from the object by a portion of unfused powder. A rate of thermal dissipation from areas of each cross-section of the object are increased by a presence of the sacrificial structure to maintain a thermal dissipation rate of each layer to be within specification or control limits of thermal dissipation for the object.

These and other aspects of the invention will become more fully understood upon a review of the detailed description, which follows.

DETAILED DESCRIPTION

During various additive manufacturing processes such as DMLM and DMLS, heat from a previously scanned portion of an object may impact the scanning of a nearby portion of the object. For example, the heat may lead to unintentional melting or sintering of powder, which may result in unintentionally fused portions of the object or an otherwise deformed object. If an entire object (e.g.,202,204, and206) were to be heated uniformly, for example in an oven with the temperature ramped up slowly to some final value such that no significant thermal gradient forms within the object, the object would expand as it heats, but it would do so uniformly over the entire volume of the part with the volume change being described by the material coefficient of thermal expansion. In general, solids tend to expand when heated and contract when cooled. In a laser additive manufacturing process, thermal gradients are inevitably created within the object being manufactured. The material close to the most recently welded or sintered layer gets very hot and wants to expand while the material far from the weld remains at a much lower temperature and does not expand. This results in a virtual tug-of-war between the thermal stress of expansion in hot material and the material strength of the cooler material. If the thermal stress is sufficient to overcome the material strength, the part will deform, resulting in a failure to meet a geometric specification or even cracking of the part. As thermal gradients in this process are inevitable, the challenge is to control the thermal gradient such that the thermal stress does not exceed the strength of the material and result in a deformed or cracked finished part. The thermal gradient required to cause such deformation will depend on the thermal and mechanical properties of the material, as well as the geometry of the part and thermal properties of the surrounding powder bed.

The disclosure provides for thermal dissipation supports for regulating thermal gradients in the object during fabrication. For example, a thermal dissipation support may be added to a three-dimensional computer model to provide a structure that increases a rate of thermal dissipation from the object. The presence of the thermal dissipation support increases the thermal dissipation rate of the object such that the thermal dissipation rate for each cross-section is within specification or control limits of thermal dissipation for the object. For example, a control limit may specify that a thermal dissipation rate for each cross section is within 80 percent of an average thermal dissipation rate for the object. The thermal dissipation support may absorb heat away from the object. A thermal dissipation support may be a sacrificial support that is not part of the object and is separated from the object by a portion of unfused powder. For example, a sacrificial support may be built next to the object and may be discarded when performing post-processing on the object. In an aspect, the thermal dissipation support has a greater mass than the object and absorbs heat from the object. Moreover, the sacrificial structure has a higher thermal conductivity than unfused powder. Accordingly, the sacrificial structure helps dissipate heat. For example, when the sacrificial structure is in contact or close proximity to the build plate114, the sacrificial structure dissipates heat into the build plate114. The methods disclosed herein for fabricating an object using thermal dissipation supports may be performed by the apparatus100(FIG. 1), a person operating the apparatus100, or a computer processor controlling the apparatus100.

FIG. 2illustrates a vertical cross-sectional view of an example object200and thermal dissipation support210according to an aspect of the present disclosure. The object200has a generally hour-glass shape including a base portion202, a narrow middle portion204, and a wider top portion206. The base portion202is built directly on the build plate114. The base portion202has a horizontal cross-section with sufficient area to allow for cooling. For example, the base portion202is built on the base plate114and has sufficient thermal coupling for heat to dissipate from a recently melted layer. Accordingly, the thermal dissipation support210may be spaced apart from the object200in the layers forming the base portion202. The thermal dissipation support210, however, includes a leg portion214that provides physical support for the thermal dissipation support210and also thermally couples the thermal dissipation support210to the build plate114.

The narrow middle portion204, however, has a smaller horizontal-cross sectional area. As the horizontal cross-sectional area of the object200decreases toward the narrow middle portion204, each subsequent layer of the object200takes less time to scan. Moreover, without the thermal dissipation support210, the narrow middle portion204is surrounded by unfused powder, which tends to thermally insulate the narrow middle portion204. For example, at a layer208, newly melted material may be unable to dissipate heat at a sufficient rate. The excess heat may lead to thermal gradients sufficient to cause warping or other deformations of the narrow middle portion204. A threshold temperature gradient for a material (e.g., the powder) defines a temperature gradient above which deformations are likely. In an aspect, excess heat or temperature gradients may occur when the horizontal cross-sectional area of the object200in a layer is less than a threshold. The threshold area may be determined based on a thermal dissipation rate of a first portion of the object that is below the current build layer. The thermal dissipation rate indicates a rate at which the first portion of the object cools. The thermal dissipation rate may be modeled based on, for example, the size of the first portion of the object and the structures or powder surrounding the first portion of the object. For example, a portion of the object surrounded by powder cools more slowly than a portion of the object connected to a lower portion of the object. The thermal dissipation rate is used to determine a threshold time until the first portion of the object solidifies or reaches a desired temperature within a threshold limit temperature. The threshold time can be converted into a cross-sectional threshold area based on the laser scan parameters. Additionally, a threshold dissipation rate indicates a minimum thermal dissipation rate necessary to prevent deformation of the object, for example, when the time for building a layer is fixed based on parameters of an AM apparatus.

Accordingly, the thermal dissipation support210is placed in close proximity to the object200in the layers forming the narrow middle portion204for absorbing heat from a portion that may be prone to excessive heat. For example, the thermal dissipation support210is separated from the narrow middle portion204by a thin portion of unfused powder212. The portion of unfused powder212has a minimum thickness sufficient to prevent the unfused powder212from fusing. The portion of unfused powder212, however, still conducts heat from the narrow middle portion204to the thermal dissipation support210. Additionally, the thermal dissipation support210has a greater mass than the object200, at least at the narrow middle portion204. The greater mass allows the thermal dissipation support210to absorb a large amount of heat energy and reduce the temperature of the narrow middle portion204. The thermal dissipation support210is a sacrificial support that does not form a portion of the completed object200. Instead, the thermal dissipation support210is removed from the object200and discarded or recycled. In an aspect, the thermal dissipation support210is designed to allow maintain a thermal dissipation rate of each layer of the object to be within 80 percent of an average thermal dissipation rate of the object. Accordingly, the thermal dissipation support210includes a larger fused region in layers where the object200has a smaller fused region.

The wider top portion206, once again, has a horizontal cross-section with sufficient area to allow for sufficient cooling. Accordingly, the thermal dissipation support210does not extend to the height of the wider top portion. In an aspect, although heat from the wider top portion206may dissipate toward the narrow middle portion204, the heat may also flow into the thermal dissipation support210via the narrow middle portion204.

As illustrated, the thermal dissipation support210at least partially surrounds the object200. In an aspect, the thermal dissipation support210includes one or more breaks or separation points such that the thermal dissipation support210may be easily removed from the object200. It should be appreciated that the shape of the thermal dissipation support210may vary. The surface of the thermal dissipation support210that faces the object200generally conforms to the shape of the object200. For example, the facing surfaces of the object200and the thermal dissipation support210may have similar curvature to keep a consistent separation between the object200and the thermal dissipation support210. The separation distance may be varied to adjust a thermal dissipation rate of the object. Other portions of the thermal dissipation support210, however, serve primarily to increase the mass of the thermal dissipation support210. Accordingly, the shape of the thermal dissipation support210may be adapted to accommodate a particular object200and/or other objects within a build. For example, the leg214may be located anywhere that the object200does not contact the build plate114. The size and shape of the leg214may be adjusted to control thermal dissipation into the build plate114.

In an aspect, the horizontal cross-sectional area of the thermal dissipation support210in any layer is inversely proportional to the horizontal cross-sectional area of the object200. The total horizontal cross-sectional area of the thermal dissipation support210and the object200may remain substantially constant such that the previously build layers are able to dissipate the heat from a newly added layer. For example, the total horizontal cross-sectional area of the object200and the thermal dissipation support210may vary by less than 10 percent while the horizontal-cross sectional area of the object210is less than a threshold area.

The thermal properties of the object400may be determined according to a thermal model. An example thermal model is described in, D. Rosenthal, “The theory of moving sources of heat and its application to metal treatments,” Transactions of the American Society of Mechanical Engineers, vol. 68, pp. 849-866, 1946, which is incorporated herein by reference. Variations of the Rosenthal model are described in N. Christenson et al., “The distribution of temperature in arc welding,” British Welding Journal, vol. 12, no. 2, pp. 54-75, 1965 and A. C. Nunes, “An extended Rosenthal Weld Model,” Welding Journal, vol. 62, no. 6, pp. 165s-170s, 1983, both of which are incorporated herein by reference. Other thermal models are described in E. F. Rybicki et al., “A Finite-Element Model for Residual Stresses and Deflections in Girth-Butt Welded Pipes,” Journal of Pressure Vessel Technology, vol. 100, no. 3, pp. 256-262, 1978 and J. Xiong et al., “Bead geometry prediction for robotic GMAW-based rapid manufacturing through a neural network and a second-order regression analysis,” Journal of Intelligent Manufacturing, vol. 25, pp. 157-163, 2014, both of which are incorporated herein by reference. A thermal model may be used to determine the need for the thermal dissipation support210and the dimensions thereof based on a three-dimensional computer model (e.g., a computer aided design (CAD) model) of the object200.

In an aspect, the analysis or modeling of an object200for any given layer is based on the immediately preceding layers and not any subsequent layers. The subsequent layers have not yet been fabricated and do not affect the thermal dissipation of the given layer. For example, the threshold for the horizontal cross-sectional area of the object200may be based on the layer212as well as a number of preceding layers. In another example, if an aspect ratio of the object200exceeds a threshold, a thermal dissipation support210may be added to the CAD model.

FIG. 3illustrates another example of a thermal dissipation support310for the object200. As illustrated inFIG. 3, two layers having the same horizontal cross-sectional area of the object400may have different sized layers of the thermal dissipation support310. For example, the widest portion of the thermal dissipation support310is located at layer320slightly above the narrowest portion of the object200. In the layer322, the object200has the same cross-sectional area as in the layer320, but the thermal dissipation support310does not need to be as large because the layers immediately preceding layer322have more mass than the layers immediately preceding layer320. As illustrated, the layers of the narrow middle portion204immediately above the base portion202may be thermally decoupled from the leg314. By adapting the thermal dissipation support310to the object200based on a thermal model of the object200including the layers immediately preceding a current build layer, the total mass of the thermal dissipation support310may be reduced in order to save resources such as build time, unfused powder, and energy.

In an aspect, the apparatus100further includes a thermal sensor such as a pyrometer or a thermal imaging camera. The thermal sensor provides information (e.g., a temperature) regarding the powder bed112or a portion of the object200. The thermal sensor is used to determine thermal properties of the object200such as the thermal dissipation rate. The thermal properties of the object200are then used to dynamically adjust the dimensions of the thermal dissipation support210or310during the build. In another aspect, the dimensions of the thermal dissipation support210or310are adjusted for a subsequent build. For example, when empirical evidence from a build procedure indicates that deformation of the object is due to excessive thermal gradients during a build procedure a new thermal dissipation support may be added or an existing thermal dissipation support may be modified for a subsequent build. The empirical evidence may include measurements of a fabricated object from a previous build.

FIG. 4illustrates a perspective view of another example thermal structure410for transferring heat from an object400. The thermal structure410and the object400may be manufactured in the same manner discussed above using the apparatus100ofFIG. 1.

In the example aspect illustrated inFIG. 4, the thermal structure410includes a first thermal structure402and a second thermal structure404. The thermal structures402,404extend from first and second ends406,408of the object400. As shown inFIG. 4, each of the thermal structures402,404continues the contour of the ends406,408of the object400. For example, the object400curves and become narrower/thinner towards the ends406,408. The first and second thermal structures402,404respectively have similar thicknesses to the ends406,408. As illustrated inFIG. 4, the first thermal structure402has about the same thickness and height as the end406of the object400and the second thermal structure404has about the same thickness and height as the end408of the object400. The width of each of the first and second thermal structures402,404is about 0.5 to 1.5 the width of the object400or about 0.75 to about 1.25 the width of the object. The thermal structures402,404are relatively thin compared to the maximum thickness of the object400. For example, the ratio of the maximum point of thickness of the object400to the thickness of the thermal structures402,404may be about 5:1 to about 2:1, or about 4:1 to 3:1. Furthermore, the ratio of the height of the thermal structures402,404to the thickness of thermal structures402,404may be about 20:1 to about 10:1, about 18:1 to about 12:1, or about 16:1 to about 14:1. A length of the thermal structures402,404is based on a thermal mass of the object400. For example, the length of the thermal structures402,404is selected such that the total mass of the thermal structures402,404is greater than a mass of the object400. By having these relative dimensions to the object, the thermal structures improve the transfer of heat away from the object400.

FIG. 5illustrates a perspective view of an object500and a thermal structure512. The thermal structure512and the object500may be manufactured in the same manner discussed above using the apparatus100ofFIG. 1.

In the example aspect illustrated inFIG. 5, the thermal structure512includes a first thermal structure514and a second thermal structure516. The thermal structures514,516extend over opposing faces of the object500, e.g., such that the object500is sandwiched between the first and second thermal structures514,516. As shown inFIG. 5, each of the thermal structures514,516matches the contours of the object500. For example, the surfaces of the object500curve, where the rear surface has a greater curve than the front surface. The first (rear) thermal structure514similarly matches the curvature of the rear face of the object500while the second (front) thermal structure516matches the curvature of the front surface of the object500. As illustrated inFIG. 5, each of the first and second thermal structures514,516has about (or exactly) the same height and width as the object500. That is, the first thermal structure514about (or exactly) equally covers the surface area of rear surface while the second thermal structure516about (or exactly) equally covers the front surface. The ratio of the thickness of a thickest portion of the object500to each thickness of each of the thermal structures514,516may be about 5:1 to about 2:1, or about 4:1 to about 3:1. The thermal structures514,516may have a generally constant thickness such that in some areas, the thermal structures514,516are thicker than adjacent portions of the object500. A total mass of the thermal structures514,516is greater than a mass of the object500. By having these relative dimensions to the object, the thermal structures improve the transfer of heat away from the object through the front and rear of the object.

FIG. 6illustrates a perspective view of an object600with another example thermal structure618, where the thermal structure618includes thermal structures similar to bothFIGS. 4 and 5. The thermal structure618and the object600may be manufactured in the same manner discussed above using the apparatus100ofFIG. 1.

In the example aspect illustrated inFIG. 6, the thermal structure618includes a first thermal structure620, a second thermal structure622, a third thermal structure624, and a fourth thermal structure626. The first and second thermal structures614,616have the same characteristics and relative dimensions as thermal structures402,404discussed above with respect toFIG. 4. The third and fourth thermal structures618,620have the same characteristics and relative dimension as thermal structures514,516as discussed above with respect toFIG. 5. The ratio of the thickness of the third and fourth thermal structures624,626to the thickness of the first and second thermal structures620,622may be about 4:1 to about 1.5:1 or about 3:1 to about 2:1. By including all four thermal structures around the object600, the thermal structure620ofFIG. 6draws heat away from the object600on both ends and also both the front and rear surfaces. Moreover, the mass of the thermal structure618is at least twice the mass of the object600.

For each of thermal structures410,512,618, there may be a gap between the thermal structures and the respective object400,500,600so that the thermal structures do not contact the object. The gap may be filled with a thin portion of unmelted powder. The presence of the unmelted powder still provides a mechanism for thermal coupling of the object and the thermal dissipation support without requiring a direct connection between the support structure and the object. Each of thermal structures410,512,618may further include one more connecting ribs integrally connected with the respective object400,500,600. The connecting ribs would extend from any surface of the thermal structure that faces and/or abuts the object. The connecting ribs may be formed incrementally along the height of the thermal structures.

When it becomes necessary to remove the thermal structure210,310,410,512,618from the respective object200,300,400,500,600, the operator may apply force to break the support structure free when connecting ribs are present. The thermal structure may be removed by mechanical procedures such as twisting, breaking, cutting, grinding, filing, or polishing. Additionally, thermal and chemical post processing procedures may be used to finish the object. When no connecting members are present and instead powder has been placed between the object and the thermal structure during manufacturing, the powder can simply be removed by blowing, for example.

In an aspect, multiple supports may be used in combination to support fabrication of an object, prevent movement of the object, and/or control thermal properties of the object. That is, fabricating an object using additive manufacturing may include use of one or more of: scaffolding, tie-down supports, break-away supports, lateral supports, conformal supports, connecting supports, surrounding supports, keyway supports, breakable supports, leading edge supports, or powder removal ports. The following patent applications include disclosure of these supports and methods of their use:

U.S. patent application Ser. No. 15/041,991, titled “METHODS AND LEADING EDGE SUPPORTS FOR ADDITIVE MANUFACTURING”, and filed Feb. 11, 2016; and

U.S. patent application Ser. No. 15/041,980, titled “METHOD AND SUPPORTS WITH POWDER REMOVAL PORTS FOR ADDITIVE MANUFACTURING”, and filed Feb. 11, 2016.

The disclosure of each of these applications are incorporated herein in their entirety to the extent they disclose additional support structures that can be used in conjunction with the support structures disclosed herein to make other objects.

Additionally, scaffolding includes supports that are built underneath an object to provide vertical support to the object. Scaffolding may be formed of interconnected supports, for example, in a honeycomb pattern. In an aspect, scaffolding may be solid or include solid portions. The scaffolding contacts the object at various locations providing load bearing support for the object to be constructed above the scaffolding. The contact between the support structure and the object also prevents lateral movement of the object.

Tie-down supports prevent a relatively thin flat object, or at least a first portion (e.g. first layer) of the object from moving during the build process. Relatively thin objects are prone to warping or peeling. For example, heat dissipation may cause a thin object to warp as it cools. As another example, the recoater may cause lateral forces to be applied to the object, which in some cases lifts an edge of the object. In an aspect, the tie-down supports are built beneath the object to tie the object down to an anchor surface. For example, tie-down supports may extend vertically from an anchor surface such as the platform to the object. The tie-down supports are built by melting the powder at a specific location in each layer beneath the object. The tie-down supports connect to both the platform and the object (e.g., at an edge of the object), preventing the object from warping or peeling. The tie-down supports may be removed from the object in a post-processing procedure.

A break-away support structure reduces the contact area between a support structure and the object. For example, a break-away support structure may include separate portions, each separated by a space. The spaces may reduce the total size of the break-away support structure and the amount of powder consumed in fabricating the break-away support structure. Further, one or more of the portions may have a reduced contact surface with the object. For example, a portion of the support structure may have a pointed contact surface that is easier to remove from the object during post-processing. For example, the portion with the pointed contact surface will break away from the object at the pointed contact surface. The pointed contact surface stills provides the functions of providing load bearing support and tying the object down to prevent warping or peeling.

Lateral support structures are used to support a vertical object. The object may have a relatively high height to width aspect ratio (e.g., greater than 1). That is, the height of the object is many times larger than its width. The lateral support structure is located to a side of the object. For example, the object and the lateral support structure are built in the same layers with the scan pattern in each layer including a portion of the object and a portion of the lateral support structure. The lateral support structure is separated from the object (e.g., by a portion of unmelted powder in each layer) or connected by a break-away support structure. Accordingly, the lateral support structure may be easily removed from the object during post-processing. In an aspect, the lateral support structure provides support against forces applied by the recoater when applying additional powder. Generally, the forces applied by the recoater are in the direction of movement of the recoater as it levels an additional layer of powder. Accordingly, the lateral support structure is built in the direction of movement of the recoater from the object. Moreover, the lateral support structure may be wider at the bottom than at the top. The wider bottom provides stability for the lateral support structure to resist any forces generated by the recoater.