WO2015174919A1

WO2015174919A1 - A method and an apparatus for geometrical verification during additive manufacturing of three-dimensional objects

Info

Publication number: WO2015174919A1
Application number: PCT/SE2015/050548
Authority: WO
Inventors: Katarina GUSTAFSSON
Original assignee: Gustafsson Katarina
Priority date: 2014-05-14
Filing date: 2015-05-13
Publication date: 2015-11-19
Also published as: SE537864C2; SE1450569A1

Abstract

The present invention relates to an apparatus and a method for geometrical verification of three-dimensional objects during manufacturing of the objects. The manufacturing process comprises applying successive layers based on a digital 3D design model of the object, and causing each of the layers to fuse after applying the layer. The verification method comprises repeatedly for each layer recording one or more images of the present layer, creating a digital 3D disk model of the present layer based on said one or more images, creating a digital 3D copy of the object being manufactured based on the 3D disk model of the present layer and 3D disk models of previous layers, and verifying the geometry of the object being manufactured based on the 3D copy of the object being manufactured and the 3D design model of the object.

Description

A method and an apparatus for geometrical verification during additive manufacturing of three-dimensional objects

Field of the invention The present invention relates to a method and an apparatus for geometrical verification of three dimensional objects during additive manufacturing of the object. Additive manufacturing is also known as 3D printing.

Prior art

Additive manufacturing of an object includes applying successive layers of a material based on a digital 3D design model, such as a CAD model, of the object, and solidifying each of the layers to create a solid object. The material in the layer is, for example, in powder form. These layers, which correspond to virtual cross sections from the design model, are fused to create the final shape.

Additive manufacturing or 3D printing is a process of making a three-dimensional solid object of virtually any shape from a digital model. 3D printing is achieved using an additive process, where successive layers of material are added on top of each other in different shapes.

Additive Manufacturing (AM) means great freedom in design compared to conventional production methods, which mostly rely on removal of material by methods such as cutting methods. The physical object is built from for example a powder bed, layer by layer, forming a complete product. Extremely complex external and internal geometries and surfaces can be created, and by that comes the need to be able to verify this geometrically. Verification of AM objects using excising methods is time consuming and costly, and sometimes not even achievable with conventional techniques.

Complex geometries and in particular internal geometry can, to some extent, be verified using Computer Tomography (CT). The analysis, however, is time consuming and therefore costly and limited by the resolution which decreases along with need for higher penetration power (Voltage). This means that objects in high density material and/or larger objects in medium to high density materials cannot be analyzed with sufficient accuracy. There are off course objects suitable for CT-inspection. The disadvantage then lies in that fact that it is still a separate activity following after a finished production, adding to the total lead time. Furthermore the number of components being verified at the same time is highly limited when using CT-inspection.

WO2013/098054 discloses a method for detecting defects in three-dimensional articles. The method described in WO2013098054 (Al) is configured to detect errors in the fused layer, by recording at least one image of a melted layer (n) and then recording at least one image of the next melted layer (n +1) and then compare these with a corresponding plane in the 3D design model.

US2013/0343947A1 discloses a method for monitoring a generative production process in real time. The last produced layer is optically detected using 2D optical detection and the installation space is thermally detected when applying a layer. 2D images of the last produced layer is recorded. The recorded image is superimposed on a reference image of the last produced layer for the purpose of evaluation. The method focuses on continuously detecting defects, such as processing defects and foreign particle, in the last produced layer.

Object and summary of the invention

It is an object of the present invention to provide an improved method for geometrical verification of three-dimensional objects during additive manufacturing of the object.

This object is achieved by the method as defined in claim 1.

A method for geometrical verification of three-dimensional objects during manufacturing of the objects, wherein the manufacturing process comprises applying successive layers based on a digital 3D design model of the object, and fusing each of the layers after the layer has been applied. The method is characterized in that it comprises repeatedly for each layer: recording one or more images of the present layer,

creating a digital 3D disk model of the present layer based on the one or more images, creating a digital 3D copy of the object being manufactured based on the 3D disk model of the present layer and 3D disk models of previous layers, and

verifying the geometry of the object being manufactured based on the 3D copy of the object being manufactured and the 3D design model of the object.

With present layer is meant the last produced layer. The invention enables real-time geometrical verification in three dimensions in additive manufacturing processes. The invention provides real-time verification of dimensions and geometry of the objects being produced. The verification is iterative, and runs simultaneously with the object production. The invention makes it possible to detect any deviation from the 3D design model in real time. The method is suitable for any type of manufacturing method that is based on layer by layer production, for example, selective laser sintering, direct metal laser sintering, electron beam melting, inkjet 3D-printing, fused deposition modeling, fused filament fabrication. The invention is particularly useful for powder bed technologies and blown powder technologies. The basic idea is to create, for each applied layer, one or more images and generate a 3D disk model of the current layer. The disk models of the layers are then compiled into a 3D copy of the physical object. The created 3D copy is then compared to the original 3D design model. For instance, the created 3D copy is compared to the original 3D design model with regards to allowed tolerances. The digital 3D design model is, for example, a CAD file. The method can then, based on allowed tolerances added to the original design file, be used to control and assure quality with regards to the geometry of the physical object. As the comparison, of the two models is iterative, the geometry verification is in real-time during ongoing building process.

The method makes it possible to verify complex internal surfaces also in high density material objects that cannot be verified by other available technologies. The method would spare the normal additional lead-time for geometry verification - when the object is finished, so is the verification. The real-time verification is not limited by the complexity of the object or the number of objects in the building chamber, an entire batch or even different objects can be analyzed simultaneously. Production of faulty components is avoided or at least greatly limited.

According to an embodiment of the invention, the digital 3D disk model of the present layer is created based on knowledge of the thickness of the layer. The thickness of the layers is, for example, known beforehand, or is determined based on the one or more images. The thickness of the layers is, for example, equal to the distance a base of a manufacturing apparatus moves downwards after each applied layer.

According to an embodiment of the invention, the method comprises defining a surface of the present layer based on the one or more images using image analyses, and creating said digital 3D disk model of the present layer based on the defined surface of the present layer. In this embodiment, a representation of the surface of the present layer is created by image analyses of the one or more imagers. The surface is, for example, a top surface of the present layer, and the digital 3D disk model of the present layer is created based on the defined surface of the present layer and knowledge of the thickness of the layer. The image analyses makes it possible to detect irregularities and variations in the surface of the present layer.

According to an embodiment of the invention, the images recorded are 3D images, and the digital 3D disk model of the present layer is created based on the 3D images. Using 3d images of the layer makes it possible to create the 3D disk model without previous knowledge of the thickness of the layer.

According to an embodiment of the invention, said one or more images are recorded by means of a laser scanner. The laser scanner creates a 3D image, a point cloud, of the surface of the present layer. One benefit from using a laser scanner is that it will reflect possible irregularities of the surface. According to an embodiment of the invention, the method comprises detecting one or more edges of the layer based on the one or more images, and creating said digital 3D disk model of the present layer based on the detected edges of the object. This embodiment provides a precise method to detect the position of the layer edge using the inflection point in the temperature gradient.

According to an embodiment of the invention, the one or more images show the temperature of the layer, and the one and more edges of the present layer is determined based on the temperature gradient in the image. One benefit from defining the surface based on temperature is that unexpected temperatures indicate an error in the fused layer.

According to a n embodiment of the invention, said recorded images shows the temperature of the layer, and the one or more edges of the layer are detected based on the inflection point of the temperature gradient in the image. One benefit from using the inflection point to detect the edge is that the inflection point will remain at the same position regardless of the cooling process for a time period long enough to record images.

According to an embodiment of the invention, the one or more images are recorded after the present layer has been fused and before the next layer is applied to enable detection of the temperature gradient.

According to an embodiment of the invention, said recorded images show the temperature of the layer, and a plurality of images are recorded at different points in time during a cooling process of the layer, and said digital 3D disk model of the present layer is created based on said plurality of images of the present layer. Accordingly, the images can be recorded at different temperatures. The benefit is that the inflection point will remain at the same position regardless of the cooling process in a limited time window.

According to an embodiment of the invention, said one or more images are recorded by means of an I R camera adapted to measure infrared light. An I R camera makes it possible to detect temperatures and is less sensitive to smoke and aerosols. Thus, the possible need for ventilation of the building chamber is reduced or eliminated.

According to an embodiment of the invention, said one or more images are recorded by means of a stereo camera, and preferably using an IR stereo camera. By using a stereo camera it is possible to obtain a very precise measurement of the object being built. The high precision of the stereo camera is utilized to create images of each layer and then using advanced image analysis and based on a large number of disks, a 3D model for real time comparison/geometry verification can be created. The verification is also made based on specified tolerances for the objects to be manufactured. The specified tolerances are determined with respect to the 3D design model . According to an embodiment of the invention, the method comprises:

- defining a first 3D tolerance model with minimum dimensions based on the 3D design model of the object and allowed minimum tolerances for the object,

- defining a second 3D tolerance model with maximum dimensions based on the 3D design model of the object and allowed maximum tolerances for the object, and

- verifying that the dimensions of the 3D copy of the object being manufactured is within the first and second tolerance model. The geometry of the object being manufactured is verified based on a 3D tolerance model with minimal dimensions, and a 3D tolerance model with maximum dimensions. It is verified that the surface of the 3D copy is between the surfaces of the two tolerance models.

According to an embodiment of the invention, the method comprises verifying that the dimensions of the 3D copy of the object being manufactured is within the first and second tolerance model, taking into account possible thermal expansion.

According to an embodiment of the invention, the manufacturing process of the object is allowed to continue as long as the dimensions of the object being manufactured are within specified tolerances. If the specified tolerances cannot be held, an action is initiated. The action can, for example, be any of the following actions: alerting an operator, and/or automatically stopping the manufacturing process, and/or the process itself will automatically take correctional actions.

It is easy to realize that the method according to the invention, as defined in the appended set of method claims, is suitable for execution by a computer program having instructions corresponding to the steps in the inventive method when run on a processor unit. Even though not explicitly expressed in the claims, the invention covers a computer program product in combination with the method according to the appended method claims.

This object is also achieved by the apparatus as defined in claim 12. The apparatus comprises:

one or more devices are arranged to record one or more images of each of the layers of the object during manufacturing of the object,

an image processing module configured, during manufacturing of the object, to create digital 3D disk models of the layers of the object based on the one or more images, and to create a digital 3D copy of the object being manufactured based on the 3D disk models created so far for the object, and

a verifying module configured, during manufacturing of the object, to verify the geometry of the object being manufactured based on the 3D copy of the object being manufactured and the 3D design model of the object.The image processing module 32 carries out image analyses or image processing of the recorded images and creates the 3D disc models based on the outcome from the image analysis. In the following term image processing and image analysis are used equivalently.

According to an embodiment of the invention, the device is an IR camera adapted to measure infrared light. Using an IR camera makes it possible to detect the position of the layer edge using the inflection point in the temperature gradient.

According to an embodiment of the invention, the device is a stereo camera, and preferably an IR stereo camera. This provides the ability to record three-dimensional images. According to an embodiment of the invention, the device is a laser scanner.

Brief description of the drawings

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 shows an example of a temperature gradient and inflection point in a layer after fusing of the layer.

Fig. 2 shows a flow diagram of an example of a process flow including a method according to the invention.

Fig. 3 shows a block diagram of an example of an apparatus according to the invention.

Detailed description of preferred embodiments of the invention An additive manufacturing process comprises applying successive layers of a material based on a digital 3D design model of the object, and fusing each of the layers by heating the layers after applying the next layer during a fusing process. The material is, for example, metal or polymer. The next layer is applied on top of the previous layer. The digital 3D design model is, for example, a CAD model for the object defining a desired shape of the object to be manufactured. In the following an embodiment of a method for geometrical verification of three-dimensional objects during manufacturing of the objects by an additive manufacturing process will be described. This embodiment uses the temperature gradient after the heating of the layer to determine the contour of the present layer. However, the invention is not limited to the use of temperature gradients.

Creating layer images

For each powder bed layer an analysis cycle is performed. One thin layer of a material, for example metal or polymeric powder, is applied on top of a previous layer. Heating, for example with laser or electron beam, starts and depending on the complexity of the present layer one or more layer image analysis sequences will be performed. The analysis sequences will be synchronized with the heating sequence. The edge of the current layer is characterized by a distinct temperature gradient and can thus be detected, i.e. the position of the edge is equal to the inflection point of the temperature gradient. See figure 1. The inflection point is defined as the point where the second derivative of the temperature gradient changes sign. At least one device, such as a camera (30) is arranged to record one or more images of each of the layers of the object during manufacturing of the object. The camera(s) is, but not necessary, located inside a cabinet together with a laser/electron beam equipment to obtain a high quality image of each layer after the fusing process. Hence the lenses will be exposed to metal vapor from the fusing process. IR light has the ability to penetrate a fume filled environment and thus IR stereo cameras are a suitable choice for this type of manufacturing process.

To avoid sublimation on to the camera lens it can be purged with an inert gas. To protect the camera's electronics from intense heat radiation during the fusing sequences, it can be equipped with a mechanical shutter.

Depending on the complexity of the object, choice of production method, powder material and other limitations that may occur during the development of the idea, a number of options in high-speed image-based methods could become significant, including IR camera(s), IR stereo camera(s), conventional (visible light) camera(s), conventional (visible light) Stereo camera(s) and Laser scanner(s).

To obtain a high quality image using IR cameras the temperature gradient should be distinct and heat loss to surrounding powder bed must not be too big. Yet it is preferably if the whole layer is completely melted (i.e. the production cycle of the specific layer is finished). Several image analysis runs may become necessary and must then be synchronized to add short pauses to the laser or electron beam fusing process during the production of one layer (i.e. the fusing must stop while the image is created). Aiming for process optimization, acquire control of the melt pool is essential. Often lack of control of the melt pool is the reason for reduced quality. During the image analyses where the layer edge is detected, information about surface defects and melt pool quality is also obtained. By verifying the surface quality in real-time, adjustments can be made to correct the fusing sequence and avoid too hot or too cold areas in the next layer.

Create a disk model of the present layer

A digital 3D disk model of the present layer is created based on the recorded images.

There are optional ways to add thickness to the layer image. An accurate 3D layer model, a disk model, can be created using image analysis to obtain knowledge about a top surface and an edge of the previous layer as well as the new top layer surface and edge, and combining this with knowledge of the distance between the two layers (the production table moves downwards at a fixed rate, for instance 20μιη, after each completed layer).

One area of interest would be to test if stereo cameras for IR vision, not only can detect the top surface but also the vertical surface of the actual layer below the powder bed top and thus create an even more accurate copy of the layer. In any case advanced image analysis is necessary to achieve a high quality digital copy for precise and fast geometry verification.

In order to determine the propagation of the last applied layer based on the inflection point, and to put this in relation to the design model, the object's temperature gradients should be determined. At least the horizontal temperature gradient should be determined. Preferably, also the vertical temperature gradient should be determined. The temperature gradient is determined based on the recorded IR images. Create 3D copy of the object

A digital 3D copy of the object being manufactured is created based on the 3D disk model of the present layer and 3D disk models of the previous layers.

By using high precision IR stereo photography to create copies of each layer in the powder bed after laser or electron beam fusing (depending on choice of AM technique), a large number of disk models can be created and compiled into a volume, a 3D digital model (a 3D copy of the physical object). The method may also be called reverse engineering, but the advantage of using an 3D image of each layer is to obtain a digital based model which reflects not only external but also internal geometries/surfaces as well as built in imperfections, pores etc. Parallel to the object being physically built a 3D digital copy is created in real time.

Verify geometry

The geometry of the object being manufactured is verified based on the created 3D copy of the object and the original 3D design model of the object.

To some extent the physical object will off course always deviate from the theoretical; what is interesting is how large the deviation is in relation to given constraints/tolerances. Thus the design model/original CAD file must be prepared with constraints/tolerances also taking into account thermal expansion. With regards to the allowed tolerances, a 3D tolerance model with minimum dimensions, as well as a 3D model with maximum dimensions is created. The shape of the real-time created 3D copy of the object must then fit in-between those two surfaces. By continuously comparing the real-time built 3D copy with the maximum and minimum dimension tolerance models, a continuous quality assurance of the building process can be performed.

Process control The quality assurance software can control the building process and allow production to continue as long as the dimensions are within the specified tolerance . On the contrary if the specified tolerances cannot be held the production process can be automatically stopped and /or the operator alerted and/or the process itself will start taking corrective actions. Hence production of faulty objects can be avoided and the scrap rate greatly reduced.

Further development could be adding intelligence to the apparatus, an adaptive behavior that based on knowledge from previous production runs (i.e. previous layer applications or finished objects) can, at an earlier state, correct a process that is about to exceed given tolerances and thereby avoid production to stop.

Process flow

Figure 2 shows a flow diagram of an example of a process flow including an example of a method according to the invention. The process flow may include, but is not limited to, the following steps:

1. First filling of powder in a building chamber

2. Powder preparation. Powder added.

3. Layer preparation. Powder leveling.

4. Camera shutter(s) close.

5. Gas purging of lens(es) start.

6. Layer fusing start.

7. Layer fusing stop.

8. Camera shutter(s) open.

9. IR image(s) of fused layer is recorded

10. Image analysis is performed: The edges of the layer is detected by tracking the inflection point of the temperature gradient. A complete image of the solidified surface including possible cavities is created.

11. A disk model for the present layer is created based on the layer image(s) and stored. 12. The disk models from each layer are compiled into a 3D copy of the physical object.

13. The 3D copy is compared to the original design file with regards to allowed tolerances and it is determined whether or not the geometry is within allowed tolerances.

14. If the geometry is not within allowed tolerances, an action is taken. The action is, for example, to alert an operator, and/or to stop the manufacturing process, or the process itself will automatically take correctional actions. Corrective feedback can be provided from the image analysis.

15. The base of the powder box is lowered a distance equal to the thickness of one layer.

16. Process steps 2 to 15 are repeated until the object is completed.

17. Camera shutter(s) close

18. Venting of fusing chamber start.

19. Gas purging of lens(es) stop. 20. The finished object(s) is/are removed

21. The process is repeated from step 1.

The implementation of the steps 10-13 are preferable made by a computing unit comprising software code portions, such as a computer program, comprising instructions for carrying out the steps of the method, and hardware, such as a processor, memory and input/output devices, for carrying out the instructions of the computer program.

Figure 3 shows an example of an apparatus for geometrical verification of three-dimensional objects. The apparatus comprises a device 30 arranged to record one or more images of each layer of the object during manufacturing of the object. The device 30 is, for example, a stereo IR camera or a laser scanner. A stereo camera is a type of camera with two or more lenses with a separate image sensor or film frame for each lens. This allows the camera to simulate human binocular vision, and therefore gives it the ability to record three-dimensional images, a process known as stereo photography. An IR camera, also called an infrared camera or thermal imaging camera or a thermo graphic camera, is a device that forms an image using infrared radiation, similar to a common camera that forms an image using visible light. Instead of the 450-750 nanometer range of the visible light camera, infrared cameras operate in wavelengths as long as 14,000 nm (14 μιη).

The apparatus further comprises an image processing module 32 configured, during manufacturing of the object, to create digital 3D disk models of the layers of the object based on the recorded images, and to create a digital 3D copy of the object being manufactured based on the 3D disk models created so far for the object, and a verifying module 34 configured to verify, during manufacturing of the object, the geometry of the object being manufactured based on the 3D copy of the object being manufactured and the 3D design model of the object. The image processing module 32 may carry out image analyses of the recorded images and creates the 3D disc models based on the outcome from the image analysis. The image processing module 32 and the verifying module 34 are, for example, software modules running on a computer 36. However, the image processing module 32 and the verifying module 34 may also be implemented by other processing means such as programming logic, such as FPGA, an ASIC, or a simple microprocessor. The output from the verifying module 34 can be a control signal or command to the manufacturing process to allow the manufacturing process to continue as long as the dimensions of the object being manufactured are within specified tolerances, and to take action, for example to stop the manufacturing process, if the specified tolerances cannot be held.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. It is possible to create the digital 3D copy of the object using image processing of images recorded by a conventional digital camera, and not using an IR camera and the inflection point. For example, one or more visible light cameras can be used to obtain surface images of the fused layer structure as well as the powder structure, and then the image analysis detects the difference in surface structure and thereby the edge.

Claims

1. A method for geometrical verification of three-dimensional objects during manufacturing of the objects, wherein the manufacturing process comprises applying successive layers based on a digital 3D wherein, the method comprises repeatedly for each layer:

- recording one or more images of the present layer (8 - 9),

creating a digital 3D disk model of the present layer based on the one or more images (10 -11),

creating a digital 3D copy of the object being manufactured based on the 3D disk model of the present layer and 3D disk models of previous layers (12), and

- verifying the geometry of the object being manufactured based on the 3D copy of the object being manufactured and the 3D design model of the object (13).

2. The method according to claim 1, wherein the method comprises defining at least one surface of the present layer based on said one or more images using image analyses, and creating said digital 3D disk model of the present layer based on the defined surface of the present layer.

3. The method according to claim 1, wherein the method comprises detecting one or more edges of the layer based on said one or more images, and creating said digital 3D disk model of the present layer based on the detected edges of the object.

4. The method according to claim 3, wherein said recorded images show the temperature of the layer, and the one or more edges of the layer are detected based on an inflection point of a temperature gradient in the image.

5 The method according to any of the previous claims, wherein said recorded images show the temperatures of the layer, and a plurality of images can be recorded at different points in time during a cooling process of the layer, and said digital 3D disk model of the present layer is created based on said plurality of images of the present layer.

6. The method according to any of the previous claims, wherein said digital 3D disk model of the present layer is created based on said one or more images and knowledge of the layer thickness.

7. The method according to any of the previous claims, wherein said one or more images are recorded by means of an IR camera adapted to measure infrared light.

8. The method according to any of the previous claims, wherein said images are recorded by means of a stereo camera.

9. The method according to any of the previous claims, wherein said images are recorded by means of laser scanner.

10. The method according to any of the previous claims, wherein the method comprises:

- verifying that the dimensions of the 3D copy of the object being manufactured is within the first and second tolerance model.

11. The method according to any of the previous claims, wherein manufacturing process of the object is allowed to continue as long as the dimensions of the object being manufactured are within specified tolerances, and performing an action if the specified tolerances cannot be held.

12. An apparatus for geometrical verification of three-dimensional objects during manufacturing of the objects, wherein the manufacturing process comprises applying successive powder layers based on a digital 3D design model of the object, and causing each of the layers to fuse after applying layer, wherein the apparatus comprises one or more devices (30) arranged to record one or more images of each of the layers of the object during manufacturing of the object, characterized in that the apparatus further comprises:

an image processing module (32) configured to create, during manufacturing of the object, digital 3D disk models of the layers of the object based on said one or more images, and to create a digital 3D copy of the object being manufactured based on the 3D disk models created so far for the object, and

- a verifying module (34) configured, during manufacturing of the object, to verify the geometry of the object being manufactured based on the 3D copy of the object being manufactured and the 3D design model of the object.

13. The apparatus according to claim 12, wherein said device is an IR camera adapted to measure infrared light.

14. The apparatus according to claim 12 or 13, wherein said device is a stereo camera.

15. The apparatus according to claim 12, wherein said device is laser scanner.