WO2022033657A1

WO2022033657A1 - Method for optimized infill generation in slicing for additive manufacturing, computer program product and cad/cam data set

Info

Publication number: WO2022033657A1
Application number: PCT/EP2020/072411
Authority: WO
Inventors: Matthias Dürr
Original assignee: Siemens Aktiengesellschaft
Priority date: 2020-08-10
Filing date: 2020-08-10
Publication date: 2022-02-17

Abstract

Method for optimized infill generation in slicing for additive manufacturing, Computer program product and CAD/CAM data set A method is proposed for generation of an optimized infill pattern for an additive manufacturing process worked in slicing for a work piece. The solution lies in a three-step approach that implements a "constructed" pattern, in contrast to a static pattern, as already known. The underlying concept is based on the positioning of boundary points (BP1 - BPn) and optional supporting points (SP11 - SP34) that are connected by routing (preferably) one continuous thread across these points (111, 112) in such a manner, that a "meaningful" pattern results from it. "Meaningful" in the case of art could be an aesthetic effect.

Description

Method for optimized infill generation in slicing for additive manufacturing, computer program product and CAD/CAM data set

Additive manufacturing (AM) has and continues to experience considerable market and technological growth in market value over the next decades. One of the primary drivers for this growth is the increased freedom afforded to the design of both the external form and internal structure of fabricated parts. This freedom presents greater opportunities in optimizing parts mechanical properties, (such as strength, stiffness and mass) , which in turn leads to enhanced performance whilst potentially reducing material use and hence, environmental impact. Realizing this potential will further increase the viability of AM for a greater range of engineering contexts .

The following basically applies to the common varieties of e.g. Fused Filament Fabrication (FFF) , Fused Deposition Modeling (FDM) , Stereolithography (SL) , Selective Laser Melting (SLM) , Selective Laser Sintering (SLS) , however the explanations and examples refer to FDM/FFF.

Print preparation in Additive Manufacturing typically generates the machines' programs by slicing a 3D model of the desired part and creating manufacturing instructions for the respective slices. In this process several characteristic structures are employed: shell, infill, supports, etc.

However, while determining the geometry of the shell is straightforward, the design of the infill, meaning the interior of the body as defined by the shell leaves a plentitude of options, which are covered far from optimal by the present solutions . While the obvious approach would be to entirely fill the interior of the shape this gives away the benefit of obtaining lightweight , material-ef ficient parts by j ust filling in as much material as required . Furthermore , the successive introduction of thermal energy in a printed slice raises questions with regard to the in-process heat distribution and the related path planning in order to avoid hot spots during the generation process .

Hence several ways to generate the infill have been established, typically involving the proj ection of a predefined pattern with selected adaptations onto the area to be infilled, example of commonly used patterns are shown in figure 4 , linear 41 , hexagonal 42 , maroccan star 43 , cats fill 44 , sharkfill 45 , Line 46 , rectilinear 47 , concentric 48 , Hilbert curve 49 , Archimedean chords 40 . Another approach of filling is the use of random lines that crisscross the area in question .

One disadvantage of the known approach is , that symmetric parts of a printed workpiece are often not symmetric on the microscopic internal scale , presumably because the pattern was applied starting at one edge of the part , but not aligned to the symmetry axis .

Another problem with largely static patterns is the lack of consideration for the requirements of the process .

For FDM/ FFF a key quality influence is the continuous generation and deposition of the melt material . Ideally, the extruder would be turned on at the start point of a slice , moved along all the required paths and turned of f only once the slice is completed . I f an interruption is required, the filament will be retracted in order to prevent oozing and then needs to be pushed forward before the actual extrusion and deposition can restart . These processes are highly disruptive and have a high potential for defects in the resulting part ( like holes , cobweb or seams ) . One approach is to replace standard infill geometries with elements based on finite element analysis FEA. The finite element method ( FEM) is the most widely used method for solving problems of engineering and mathematical models . However, the concept of standard patterns as a basis for the overall in- fill-design is not disputed here : https : / / link . springer . com/ article/ 10 . 1007 / s40964- 017- 0034-y

It is the purpose of the invention to of fer a new method for an optimi zed infill generation in Additive Manufacturing, that is easy to perform and overcomes the abovementioned disadvantages .

This task is solved by the method according to patent claim 1 . The task is also solved by a computer program product according to patent claim 9 . Further the task is solved by a CAD/CAM data set according to claim 11 .

A method is proposed for generation of an optimi zed infill pattern for an additive manufacturing process worked in slicing for a work piece with the following steps by using a model representation of work piece slice :

- determination of outer shape of work piece slice intended for printing,

- determination of number of necessary boundary points on the outer shape , in particular an even number of boundary points ,

- placing of the determined boundary points on the outer shape of the work piece ,

- determination of sequence of boundary points to be passed during printing process ,

- using always shortest path of connection between successive boundary points ,

- j oining of all determined paths consecutively to form one single connected print path for the work piece slices .

In one advantageous embodiment of the invention, the determined boundary points are placed equidistantly along the determined outer shape of the work piece . The proposed method shows additional support points , that are positioned in the interior of the work piece slice representation . This gives the work piece more stability especially when the interior to fill is more spaceous . The determination of the print path passed during printing process is a sequence of alternating boundary points and support points .

Another advantageous embodiment of the method calculates the distance between boundary points and / or the position of the support points based on preceding load simulations of the work piece during the design phase . Considering these it might be not advantageous to place the markers equidistantly but to support those areas , where stronger forces are predicted, e . g . according to the intended use of the work piece .

One further advantageous embodiment of the method ensures , that the positions of the Support Points SP are forming an evenly spaced array around the center of the interior of the slice . The radius of the spaced array is about 1 / 3 of distance between the center of the interior and the outer shape .

One further embodiment of the invention shown as a further step is , that after determination of outer shape of the work piece slice intended for printing, the work piece is decomposed in smaller parts and the method steps are applied on every one of the smaller parts independently . ( Divide and conquer ) .

At the step of determination of number of necessary boundary points on outer shape , no boundary points are used at a work piece with a radius smaller than 2 , 5 mm because it is presumed that the printed structure then is stable enough in itsel f and no further infill printing is needed . Given that an optimal distance between two boundary points is about 5 , 0 mm, this might be obvious . The problem is also solved by a computer program product for modelling a work piece for generation with an additive manufacturing process represented as a stack of slices , of fering an optimi zed infill pattern for the work piece with the following steps by using a model representation of work piece slice :

- determination of number of necessary boundary points on outer shape , in particular an even number of boundary points ,

- using always shortest path of connection between successive boundary points ,

- j oining of all determined paths consecutively to form one single connected print path for the work piece slice .

Further embodiments of the invention are contained in the dependent patent claims .

Examples for carrying out the invention are also shown in the figures , as follows :

Figure 1 with an example shape and Boundary Points BP, Figure 2 the same example shape and additional Support

Points SP

Figure 3 further example shapes ,

Figure 4 shows state of the art patterns , and

Figure 5 a flow chart , showing the steps of the method .

The problem is solved by a three-step approach that implements a "constructed" pattern, in contrast to a static pattern, as already known, thus providing ample opportunity for optimi zation, e . g . with respect to weight of the work piece , processed material , process speed or mechanical properties of the work piece . The underlying concept is based on the positioning of boundary points and optional supporting points that are connected by routing (preferably) one continuous thread across these points in such a manner, that a "meaningful" pattern results from it . "Meaningful" in the case of art could be an aesthetic ef fect , in the case of AM this would address the optimi zation obj ectives mentioned above .

It is worth hinting that there is a strong analogy between the use of a single thread for the convenience of the artist and the uninterrupted generation of one string of melt as used in the Additive Manufacturing techniques described above .

With the proposed technique , also very complex free forms can be approached :

One advantageous embodiment of the invention shows a decomposition of the shape into smaller segments with regard to accessibility of boundary points by a thread and construction of regional infill with and without support points . Very extensive geometrical designs based on boundary and support points are imaginable , presumably generated by the repetitive application of design rules .

The following Figure 3 gives an impression of the possibilities provided by the technique i f applied to geometric primitives , like a triangle 31 , a square 32 or a circle 33 , even without the use of support points . Obviously, the distribution of the infill and the creation of an empty core ( desirable for some optimi zation obj ectives ) depends on the distribution of the boundary points and the routing rules applied to the thread .

One potential problem can be seen in the "circle"-design 33 : with an unfortunate routing strategy and/or without support points , an excessive aggregation of thread/ filament in one spot can result , thus posing a signi ficant risk for the FDM/ FFF process stability . The approach based on the aforesaid is defined as follows, and can also be seen in Figure 5: a) Positioning of boundary points BP

After determination of the (outer) shape of workpiece or object, 51, the boundary points BP provide the interface between the skin of the object and the internal reinforcement infill. Obviously, they provide support of the skin OS against external mechanical stress.

A number of boundary points BP1, ... BPn are positioned along the circumference of the slice on the inside of the skin OS.

In a first approximation, the boundary points BP1, ... BPn are distributed in an equidistant manner along the circumference of the slice 54, in order to achieve a closed loop their number must be even. The distance between the respective BP depends on the loads to be handled, a typical advantageous value for plastic parts with a mostly static function (housings, bottles, ..) is 5mm.

In more advanced scenarios, the distance between Boundary Points, BP1, ...BPn could be varied depending on rules, load simulations etc. A simple rule would be that parts of the skin involving a small radius (e. g. below 2.5mm) do not require any additional infill (the skin section in question could hence be skipped) , as the local geometry provides sufficient stiffness by itself. However, further optimizations are possible. b) Positioning of support points SP1, SP2,...

Support points are needed to provide a defined exchange of stresses between filament segments, to control the distribution of infill across the area and to avoid entanglement / overcrowding between filament segments. For basic geometries and load cases (as shown in Figure 3) they may not be needed, however the benefit becomes clear in Figure 2 where the dis- tribution of infill varies depending upon the number and location of support points .

The number of support points controls the maximum density of infill across the area . It is advantageous to use at least two support points , as a rule of thumb it can be said that the higher the number of ( clearly separated) support points , the lower the maximum density of the infill . The number of three support points can be seen as a robust first guess for an application .

In a first approximation, the location of the support points SP1 , SP2 , ... is determined as an evenly spaced array around the center of the area, the radius of the array points being 1 / 3 of the respective distance between center and circumference / skin .

Further optimi zation based upon detailed load cases and simulation, consideration of relations to neighboring slices etc . is possible . c ) Construction of pattern

Once boundary points BP1 , ..., BPn and support points SP1 , SP2 , ... have been defined, the routing of the filament thread needs to be determined .

In a first approximation, the starting point can be chosen randomly . The routing then follows basic rules :

- BP and SP are visited alternately,

- after a SP has been visited, the next visit from a BP to a SP must address another SP,

- the distance between two visited points (BP, SP ) must be as short as possible ,

- the loop must be closed, once all BP have been visited once , meaning, that the last point to visit is the starting point . Sketches 2 to 4 in Figure 2 show (probably imperfect ) examples of the execution of these rules , the results resemble a "round robin" model , whereby the visits to the SP toggle between the nearest SP, hence a distribution and limited crossing of the generated infill is achieved .

The method might further be improved by solutions to the "travelling salesman" problem : "Given a list of cities and the distances between each pair of cities , what is the shortest possible route that visits each city and returns to the origin city?" which is an NP-hard problem in combinatorial optimi zation . https : / /en . wikipedia . org/ wiki/ Trave lling_s al esman_problem

Advantages of the proposed solutions are an improved printing quality, improved resource ef ficiency, and improved product properties regarding to weight , stability, and the like .

The proposed functionality can be integrated in a computer program and of fers an improved and unique function in those CAx products to design work pieces , and improved results in own manufacturing .

The parametric construction is process aware of infill construction compared to the mapping of static infill patterns .

Claims

Patent claims

1. Method for generation of an optimized infill pattern for an additive manufacturing process worked represented as a stack of slices for a work piece with the following steps by using a model representation of work piece slice:

- determination of outer shape (OS) of work piece slice intended for printing (51) ,

- determination of number of necessary boundary points (BP1, ... BPn) on outer shape (53) , in particular an even number of boundary points (BP) ,

- placing of the determined boundary points on the outer shape (OS) of the work piece (54) ,

- determination of sequence of boundary points (BP) to be passed during printing process (57) ,

- using always shortest path of connection between successive boundary points (BP) , (58)

- joining of all determined paths consecutively to form one single connected print path for the work piece slice (59) .

2. Method according to patent claim 1, characterized in that the determined boundary points (BP1, ... BPn) are placed equidistantly along determined outer shape of the work piece.

3. Method according to one of the previous patent claims, characterized in that additional support points (SP11, SP12,...) are positioned in the interior of the work piece slice representation (55) and determination of print path passed during printing process is a sequence of alternating boundary points (BP1, ... BPn) and support points (SP11, SP12,...) .

4. Method according to one of the previous patent claims, characterized in that the distance between boundary points (BP1, ... BPn) and / or the position of the support points (SP1, SP12, ...) is based on load simulations of the work piece.

5. Method according to one of the previous patent claims, characterized in that the position of the Support Points (SP11, SP12, ...) are forming an evenly spaced array around the center of the interior of the slice and the radius of the spaced array is about 1/3 of distance between center of interior and outer shape.

6. Method according to one of the previous patent claims, characterized in that after determination of outer shape (OS) of work piece slice intended for printing, the work piece is decomposed in smaller parts and method steps are applied on every one of the smaller parts independently ( 52 ) .

7. Method according to one of the previous patent claims, characterized in that at the step of determination of number of necessary boundary points (BP) on outer shape, no boundary points are used at a work piece with a radius smaller than 2,5 mm.

8. Method according to one of the previous patent claims, characterized in that the distance between boundary points (BP1, ... BPn) is preferably 5 mm.

9. Computer program product for modelling a work piece for generation with an additive manufacturing process worked in slicing offering an optimized infill pattern for the work piece with the following steps by using a model representation of work piece slice:

- determination of number of necessary boundary points (BP) on outer shape, in particular an even number of boundary points (BP1, ..., BPn) , (53)

- placing of the determined boundary points on the outer shape (OS) of the work piece, (54) - determination of sequence of boundary points (BP1, ... BPn) to be passed during printing process, (57)

- using always shortest path of connection between successive boundary points (BP1, ... BPn) - joining of all determined paths consecutively to form one single connected print path for the work piece slice (58, 59) .

10. Computer program product according to claim 9, Working accordingly to one of the previous claims 2 to 8.

11. CAD/CAM data set containing information to produce a workpiece with an additive manufacturing process, the workpiece comprising an optimized infill pattern generated ac- cording to the method of one of the claims 1 to 8.