Not Applicable
Not Applicable
The present invention relates generally to graphic prototyping, and more specifically, to a selective laser sintering process that is uniquely adapted to yield fully dense parts.
In today's world of rapid prototype selective laser sintering, production of quality parts faces certain challenges. First, parts and/or other details made of nylon typically exhibit poor strength properties due to their low density levels, which commonly fall around 60% density. The resulting strength characteristics of such parts are below the rigorous requirements for flight quality performance parts. In addition, despite careful design and consistent production, parts produced utilizing selective laser sintering may not possess uniform strength characteristics throughout the part. When analyzed, sintered parts may often exhibit varying densities across the part, which results in non-uniform and inconsistent strength characteristics. Therefore, even though a part may meet density requirements, the microstructure of the part may exhibit varying strength properties, which may result in unpredictable and unacceptable performance. Indeed, in order to produce parts that may be useful for applications such as aerospace, manufacturing, and the like, the part density and the uniformity/consistency of the parts created through selective laser sintering must improve.
Traditional selective laser sintering was developed in the early 1990's and soon became an alternative manufacturing process through which complex parts could be created. Selective laser sintering became a viable alternative for forming plastic parts because of its cost effectiveness, speed, and simplicity relative to other processes such as injection molding, blow molding, machining, and the like. Through this process, various plastics could be formed into complex part designs. Additionally, the parts designs may be communicated to the selective laser sintering apparatus directly from the corresponding computer model. Thus, through selective laser sintering, a three-dimensional complex part could be formed with substantial benefit to the user.
The selective laser sintering process involves several steps. First, a powder material is selected which exhibits desired material properties. Various considerations, such as the melting temperature, the softening temperature, the material density, and the diameter of the particles of the powder, are all important selection criteria. Various inventions and compositions directed to specifically formulated sintering powders have been developed, which yield respective advantageous properties.
An apparatus utilized to perform selective laser sintering typically includes a vertically movable piston, a rolling mechanism, and a laser control mechanism, all of which are housed within a temperature controlled environment of the apparatus. In order to produce the part, the roller mechanism deposits a layer of powder onto the piston. This area onto which the powder is deposited is also known as the part bed. The thickness of the layer of powder may be approximately 125 micrometers thick, but may vary. The layer of powder is then sintered utilizing the laser control apparatus. Sintering may be defined as heating the powder to a temperature whereat contiguous surfaces of the individual particles of the powder are melted together through viscous flow, with at least a portion of each of the individual particles remaining solid. Therefore, within the apparatus, the laser control mechanism directs laser energy toward the layer of powder until sintering occurs. The laser control mechanism may sinter the powder in precise geometrical configurations. Upon finishing the sintering scan of the layer, a first slice is formed, meaning a first cross-sectional region of the part.
Subsequent to formation of the first slice, the piston is lowered and the roller mechanism distributes a subsequent layer of powder onto the part bed. The laser control apparatus then sinters the subsequent layer according to a corresponding subsequent cross-section for the part. As these steps are repeated, and additional slices are formed, the slices become sintered and united into a single coherent part. Particles of powder adjacent the surfaces of the part must not be sintered or heated to their melting temperature so that when the part is removed from the part bed, excess powder may be easily dislodged from the parts surface and reused in subsequent sintering processes. If particles of the powder adjacent to the boundaries of each of the slices are heated to their melting temperature, these particles may adhere to the slice and other particles, which results in a loss of part definition.
However, one of the drawbacks of present selective laser sintering processes has not been addressed by recent developments. Specifically, the selective laser sintering process typically creates parts that are less than fully dense, which in turn limits the applications in which selective laser sintered parts may be used. The term “fully dense” may refer to a part that has no measurable porosity or has reached 99.5% of the theoretical density of that material. The term “near fully dense” refers to a porous part that has a density that is 80-95% of a fully dense part. During the sintering process, due to the interstitial spacing of the particles of the powder, fully dense parts are difficult to achieve. Many applications have been directed towards powders and methods that seek to fill these interstices so as to enhance the density and consistency or uniformity of the part. However, such powders result in additional expense, complexity, and do not guarantee the creation of a fully dense part.
Due to the interstitial gaps and resulting porosity of sintered parts, the strength properties of such parts are typically inferior to the corresponding strength properties of isotropic parts. For example, such critical properties as tensile strength, compression, hardness, stress, elasticity, and the like, limit the range of applications in which sintered parts may be used. Specifically, one such application includes flight quality parts. In this application, the strength property requirements for parts are very demanding and may preclude the use of non-fully dense parts.
Therefore, there is a need in the art for a repeatable, reliable selective laser sintering process that produces parts that are fully dense. There is also a need in the art for a process that produces parts that have uniform and consistent density levels. There is a need in the art for an efficient, cost-effective, selective laser sintering process that produces a homogeneous part microstructure. Additionally, there is a need in the art for a selective laser sintering process which produces fully dense parts that meet specific dimensional requirements.
A manufacturing method is provided for enhancing density of a three-dimensional object defining at least one or more substantially flat surfaces. The method comprises the steps of: (a) applying a layer of powder at a target plane; (b) completely melting selected locations of the layer corresponding to a cross sectional slice of the object, the cross sectional slice defining a portion of the flat surface; and (c) repeating steps (a) and (b) to create successive layers to form the flat surface of the object in layerwise fashion, wherein at least one flat surface is oriented at an angle with respect to the target plane.
According to an aspect of the present invention, all of the flat surfaces of the object may be oriented at an angle with respect to the target plane. Preferably, a given flat surface may be oriented at an angle between 10 to 45 degrees with respect to the target plane. The object may be variously configured to include a plurality of flat surfaces, and such flat surfaces may be an exterior surface of the object or an interior surface of the object.
Step (b) of the method may include directing energy onto the layer to liquefy the selected locations thereof. In this regard, a laser may be used to direct between 45 to 60 Watts of energy onto the layer. Due to the fluence (power per unit area) of the laser, the complete melting of the powder may be facilitated. However, it is contemplated that other parameters may also be modified in order to achieve complete melting of the powder. For example, the layer may be between 0.003 to 0.006 in. in thickness. Also, the scan spacing may be within the range of 0.010 and 0.014 in. Finally, the scan count may be modified, but may preferably be 2.
In a further aspect of the present invention, the cross sectional slice of the object may define a cross sectional periphery, and step (b) may further include offsetting the selected locations from the cross sectional periphery corresponding to an offset value. This modification may aid in mitigating against “swelling” of the part as described below.
As it is contemplated that the configuration and mapping of the layer may be done via computer programs, other steps may be utilized in performing an implementation of the present method. For example, the method may further include the steps of: (a) identifying the flat surface of the object; (b) orienting a model of the object such that at least one flat surface thereof is oriented at an angle with respect to the target plane; and (c) dividing the model into a plurality of cross sectional slices, each of the slices being oriented parallel with respect to the target plane. Upon completion of these steps, the model may be reviewed in order to determine whether an optimal cross sectional slice for each layer is achieved. Such a determination may be made by each individual operator and take into consideration design requirements and other specifications related to a given project.
In accordance with another embodiment of the present invention, a manufacturing method is provided for enhancing density of a three-dimensional object defining a substantially flat dominant surface. The method comprises the steps of: (a) applying a layer of powder at a target plane; (b) completely melting selected locations of the layer corresponding to a cross sectional slice of the object, the cross sectional slice defining a portion of the dominant surface; and (c) repeating steps (a) and (b) to create successive layers to form the dominant surface of the object in layerwise fashion, wherein the dominant surface is not aligned parallel with respect to the target plane.
The dominant surface may be variously classified. For example, the object may define a plurality of substantially flat surfaces each defining a respective individual surface area and the dominant surface may define the largest individual surface area of the substantially flat surfaces. The dominant surface may be an exterior surface or an interior surface.
In yet another embodiment of the present invention, a manufacturing method is provided for enhancing density of a three-dimensional object defining at least one or more substantially flat surfaces, with each of the substantially flat surfaces being classifiable as a major or minor surface. The method comprises: (a) applying a layer of powder at a target plane; (b) completely melting selected locations of the layer corresponding to a cross sectional slice of the object, the cross sectional slice defining a portion of a given flat surface; and (c) repeating steps (a) and (b) to create successive layers to form the given flat surface of the object in layerwise fashion, wherein each of the major surfaces is not aligned parallel with respect to the target plane.
The major and minor surfaces may be variously classified. For example, each of the substantially flat surfaces may define an individual surface area and collectively define a cumulative flat surface area. In such a case, the surface area of a given minor surface may be less than 10% of the cumulative flat surface area or less than 5% of the cumulative flat surface area. The major and minor surfaces may be an interior surface or an exterior surface of the object.
An illustrative and presently preferred embodiment of the invention is shown in the accompanying drawings in which:
a is a side view of an object fabricated with a flat surface being aligned with the target plane;
b is a side view of an object fabricated with a flat surface being oriented at an angle, or at least not aligned parallel with the target plane, in accordance with another embodiment of the present invention;
Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only and not for purposes of limiting the same,
According to an aspect of the present invention, a manufacturing method is provided for enhancing density of the three dimensional object 42 defining at least one or more substantially flat surfaces 46. The method comprises of steps: (a) applying the layer 30 of powder 32 at the target plane 34; (b) completely melting the selected locations of the layer 30 corresponding to the cross sectional slice 40 of the object 42, the cross sectional slice 40 defining a portion of the flat surface 48; and (c) repeating steps (a) and (b) to create successive layers 30 to form the flat surface 46 of the object 42 in layerwise fashion, wherein at least one flat surface 46 is oriented at an angle with respect to the target plane 34. As shown in
A first inventive aspect of an embodiment of the present method includes the complete melting of the powder 32 in order to achieve a near-fully dense, if not fully dense object 42 (referred to herein as “part” or “object 42”). Traditional rapid prototyping processes, such as selective laser sintering, utilize a similar apparatus as described herein with the exception of utilizing lower laser fluence. Sintering, by definition, is a process of forming objects from a powder by heating the powder at a temperature below its melting point. The addition of laser energy is sufficient to cause the exterior of the particles to become viscous while the interior of the particles remains solid. The viscous exterior allows the particles to fuse to each other and thereby form a sintered part. As a result of the sintering process, such parts typically demonstrate interstitial gaps or porosity, and may achieve, albeit rarely, a maximum density of up to 95% of theoretical density. In addition, the microstructure of sintered parts is clearly layered, which may also compromise the structural integrity of the parts. Therefore, although these processes may create parts that are dimensionally accurate, such parts are not structurally sound and are therefore typically used only as models. Thus, these processes do not create a “flight quality” part—one that is not only dimensionally accurate, but also demonstrates the strength properties required for a given application. In contrast, implementations of the present invention are consistent and repeatable processes that create parts exhibiting strength properties of a near fully dense or fully dense part due in part to the complete melting of the powder. Complete melting of the powder also tends to eliminate other microstructural problems associated with layerwise fabrication of parts. In particular, complete melting of the part tends to create a homogenous (not layered) microstructure. As a result, the strength properties of completely melted parts are much greater than their selective laser sintered counterparts. Finally, completely melted parts are also able to achieve up to 99.5% of theoretical density.
Another inventive aspect of an embodiment of the present invention, as shown in
Secondly, the aforementioned orientation also tends to mitigate against warping of flat surfaces 46. Upon melting the selected portions 38 of the layer 30, differences in the weight and density of the selected portions 38 and the powder 32 below or around the selected portions 38, may cause warping. Indeed, if not properly supported by the surrounding powder 32, the selected portions 38 (as well as the part in general) may warp. Through small incremental advances in the horizontal direction coupled with advances in the vertical direction, the selected portions 38 of each successive layer 30 may be supported by the melted selected portions 38 of the layers 30 therebelow.
Therefore, as mentioned above, in order to mitigate against “burn through” and to enhance dimensional accuracy and proper fabrication of the object 42—the successive layers 30 of the object 42 are created with the flat surface 46 of the object 42 defined thereby being oriented at an angle with respect to the target plane 34.
Referring now to
According to an aspect of the present invention, the completely melting step may include directing energy onto the layer 30 to completely melt the selected locations thereof. Thus, as shown in
The powder 32 may be an amorphous powder 32, an organic polymer, or a metal. The powder 32 material may be selected according to the strength properties required by a given application. Implementations of the present invention have been determined to provide density levels consistently at 99.5% of theoretical density, in other words, full density. Therefore, a “flight quality” part may be produced utilizing implementations of the present invention. It is contemplated that implementations of the present invention may be utilized to provide a viable alternative to the costly manufacture of parts through injection molding or machining. The potential savings in manufacturing costs may be significant, especially when only a limited number of parts are required. Thus, through implementations of the present invention, “flight quality” parts may be produced without incurring the expense of a costly injection mold or machine expenses.
A further advantageous aspect of the present invention includes enhancing the microstructure of the object 42. Through the complete melting of the powder 32, the object 42 microstructure becomes homogenous. This is achieved through placing the powder 32 into a stable state through the melting process. Thus, by deliberately and completely melting the powder 32 corresponding to the each cross sectional slice 40 at each layer 30, the resulting part will have been manufactured by transitioning from a solid to a liquid and back to a solid, upon cooling.
Embodiments of the present invention tend to ensure microstructure homogeneity, i.e., that no interstitial gaps of air exist in the part and that the part does not have the microstructure weaknesses of one constructed in layerwise fashion. As is known in the art, interstitial gaps or porosity may decrease the strength properties of the object 42. In addition, a layerwise microstructure may allow the part to exhibit poor strength properties in shear, bending, and other stresses. However, according to aspects of the present invention, even though the part is fabricated in a layerwise fashion, the microstructure of the part does not exhibit properties of layerwise fabrication. Therefore, the part exhibits strength properties comparable to a part of the same configuration, for example, that is machined from a single block of homogenous solid material or injection molded.
As mentioned above, traditional rapid prototyping processes such as selective laser 16 sintering, create parts that exhibit a clearly layered microstructure and have correspondingly weak strength properties. Further, such parts are not near fully dense or fully dense. In contrast, the complete melting of the selected locations corresponding to a cross section of the object 42 ensures that successive cross sectional slices 40 of the object 42 completely melt and facilitate the creation of a homogenous microstructure. Additionally, the complete melting of each cross sectional slice 40 tends to ensure that the object 42 has uniform properties throughout, which may be advantageous over not only selective laser 16 sintering, but also over injection molding.
Referring again to
According to a preferred embodiment of the present invention, the flat surface 46 may be oriented at an angle between 10 and 45 degrees with respect to the target plane 34. Additionally, it is contemplated that the angle may be any angle up to 90 degrees with respect to the target plane 34. The determination of the angle may be dictated by the geometry or configuration of the part to be manufactured.
As illustrated in
According to another aspect of the present invention, the layer 30 may be between 0.003 to 0.006 in. in thickness. According to a preferred embodiment of the present invention, the layer 30 is 0.004 in. in thickness. It is contemplated that the layer 30 thickness may be modified depending on the configuration of the object 42, such as the cross sectional slice 40, and the properties of the powder 32, such as melting point, powder density, and the density of the material(s) from which the powder 32 is made.
The completely melting step (b) may further include modifying parameters such as scan count, scan spacing, and scan speed of the laser 16. According to a preferred embodiment of the present invention, the scan spacing may be between 0.010 to 0.014 in., and the scan count may be 2. Other parameters such as temperature within the part chamber 20 and roller speed may be manipulated to enhance the performance of implementations of the present method. For example, as may be appreciated by one of skill in the art, the selective laser sintering process often seeks to heat the powder within the part chamber to a temperature near the “softening” end temperature of the powder such that the powder may be easily sintered by a low powered laser, but without causing the powder to “cake” (or stick to itself or the part bed) upon being spread across the part bed by the powder distribution system. However, according to an implementation of the present method, it may be unnecessary to precisely maintain the temperature of the part chamber 20 near the “softening” temperature of the powder 32. Instead, the part chamber 20 may be maintained at a temperature to ensure even spreading of the powder 32 at the target plane 34.
As illustrated in
In another implementation of the present invention, a manufacturing method is provided for enhancing density of a three dimensional object 42 defining a substantially flat dominant surface 64. The method may comprise the steps of: (a) applying a layer 30 of powder 32 at a target plane 34; (b) completely melting selected locations of the layer 30 corresponding to a cross sectional slice 40 of the object 42, the cross sectional slice 40 defining a portion of the dominant surface 64; and (c) repeating steps (a) and (b) to create successive layers 30 to form the dominant surface 64 of the object 42 in layerwise fashion, wherein the dominant surface 64 is not aligned parallel with respect to the target plane 34.
Referring to
According to another implementation of the present invention, a manufacturing method is provided for enhancing density of a three dimensional object 42 defining at least one or more substantially flat surfaces 46, each of the substantially flat surfaces 46 being classifiable as a major surface 68 or minor surface 70. The method may comprise the steps of: (a) applying a layer 30 of powder 32 at a target plane 34; (b) completely melting selected locations of the layer 30 corresponding to a cross sectional slice 40 of the object 42, a cross sectional slice 40 defining a portion of a given flat surface 46; and (c) repeating steps (a) and (b) to create successive layers 30 to form the given flat surface 46 of the object 42 in layerwise fashion, wherein each of the major surfaces 68 is not aligned parallel with respect to the target plane 34.
Referring to
This description of the various embodiments of the present invention is presented to illustrate the preferred embodiments of the present invention, and other inventive concepts may be otherwise variously combined and implied. The appended claims are intended to be construed to include such variations except insofar as limited by the prior art.