Design Workflow for AM: From CAD to PartDesign Workflow for AM: From CAD to Part Sanjay Joshi Professor of Industrial and Manufacturing Engineering Penn State University Industry Practicum:

Industry Practicum: Additive Manufacturing with Metallic MaterialsJuly 17-20, 2018, Penn State University – Innovation Park, PA USA © CIMP-3D@PSU 2018

Offered by:

Center for Innovative Materials Processing through Direct Digital Deposition (CIMP-3D)

The Pennsylvania State University & Applied Research Laboratory

July 17-20, 2018 University Park, PA

Design Workflow for AM:From CAD to Part

Sanjay JoshiProfessor of Industrial and Manufacturing Engineering

Penn State University

Industry Practicum: Additive Manufacturing with Metallic Materials

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Digital Workflow: CAD to Part

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SIMULATION


Input to AM Systems

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3D Part Design as Solid Model -CAD

3-D Scan Data (Point Cloud, Polygonal Mesh)

2-D Slice Data (MRI, CAT Scan)

STL File

Convert(Export)


Other Types of Data

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Point Cloud Data

MRI - Magnetic Resonance Imaging. Uses magnetism, radio waves and a computer to produce detailed images of your body's organs, tissues and structures.

• Different Formats• DICOM, Nifti, Minc

CT Scan Data

http://www.libe57.org/data.html

https://blog.nikonmetrology.com/wp‐content/uploads/2016/07/Additive‐Manufactured‐metal‐parts‐for‐CT‐inspection‐1024x576.jpg


A faceted representation of the boundary, where each facet is a triangle

The facets are created using a process called “tessellation”, which generates triangles that approximate the object boundary

STL File

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STL File – ASCII Format Example

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……………

Model Name

One triangle

(1,0,0)

(1,1,0)

(0,0,0)

(0,0,‐1)


STL Example

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• Units (request a 2D drawing)• Binary vs ASCII• Resolution & File Size

• Deviation Tolerance• Angle Tolerance


Process of tessellation into triangles creates an approximation

Approximation Errors

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Trade off Size andNumber of facetsError


Generating STL files from SolidWorks

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2 variables control the generation of triangles

• Deviation– Controls whole-part

tessellation– Small numbers generate

files with greater whole-part accuracy

• Angle– The angle setting refers to

the angular deviation allowed between adjacent triangles

– Controls smaller detail tessellation (0~30 deg)

– Small numbers generate files with greater small-detail accuracy, but they take longer to generate


Impact of Chordal Tolerance

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Errors of approximation

Much larger than original CAD file for a given accuracy parameter

CAD STL conversion provided by vendor as part of CAD system• CAD model may not be mathematically correct• Tessellation algorithms are not robust• Challenges controlling numerical errors• Difficulty tessellating curved surfaces

Problems with STL files

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364kb 2.19 MB‐70 MB


Gaps & Missing Triangles

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Tessellation of surfaces with large curvature can result in errors at the intersections of such surfaces leaving gaps or holes along edges of the part model


Overlapping & Intersecting Triangles

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Facets may sometimes intersect at locations other than their edges resulting in overlapping facets

Overlapping facets may result from numerical round off errors

during tessellation, since floating point numbers are used


When the normal to the triangle facets points in the wrong direction (flipped normal)

Confusion caused if specified normal is opposite to that implied by vertex ordering

Mis-Oriented (Flipped) Triangles

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1

2 3


• A violation of the vertex to vertex rule

• Triangles whose vertices touch the sides of adjacent triangles

Unmatched Triangle Sides (Bad Edges)

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• Shell – connected grouping of triangles• A single part must have one single shell• Noise shells caused by extraneous triangles• Overlapping shells – may cause the overlapping material to be printed

twice

Multiple Shells

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Dealing with Defective STL Files

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• STL files must be checked for validity and repaired before the slicing process

• Automatic repair of STL files is often performed by software supplied along with AM machine (or special repair programs)

• Examples include:- MAGICS, NETFABB, POLYGONICA (3rd party software)- INSIGHT, 3-D Sprint (supplied by machine vendors)- MeshLab (open source)


Fixing with Magics

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Data Redundancies

Unit Ambiguities

Scale Poorly with complex geometry (highly curved surfaces)

Cannot accommodate Colors, Multiple Materials, Texture

Growing gap between what machines can make vs. what STL can represent

NEW Specifications: - ISO / ASTM 52915 AMF specification

- 3MF ( Consortium of companies, Autodesk, 3D

systems, Dassault, EOS, FIR, GE, HO, Microsoft, Materialise, PTC, Siemens, Stratsys, etc.)

Other Probles/issues with STL Files

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SIMULATION


Build orientation impacts:• Build time• Requirements for support structures• Thermal behavior• Internal stress buildup• Part properties

Process Planning – Build Orientation

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Build Orientation – Multiple Objectives

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SIMULATION


• Support overhangs during build

• Base supports facilitate removal from build platform

• Anchor part to prevent distortion

• Different designs of supports

Process Planning – Support Structures

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Process Planning – Support Structures

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Source: http://utwired.engr.utexas.edu/lff/symposium/proceedingsArchive/pubs/Manuscripts/2012/2012-53-Krol.pdf

Supportoptions

Supportfailures



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SIMULATION


Generate the layer information

Slicing

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Input TrianglesSort Trianglesin Z Direction

Intersectwith Z plane

Create BoundaryPolylines

SmoothBoundary

OutputBoundary Data

Sliced Files

Increment Z


• Material between slicing planes is called “Layer”

• Part is built by sequentially building and stacking layers, resulting in a “quantized” part along the build axis in steps equal to the layer thickness

• Layer thickness depends on machine, material, and process parameters

Layers

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Staircase effect

Surface roughness of surfaces not orthogonal to build direction

Errors related to Slicing

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Dimensions in plane of the layer may be created oversized or undersized

Features along building axis may be moved up or down one layer, and features smaller than the layer thickness may disappear

Errors related to Slicing

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SIMULATION


“Tool path” planning and process parameters determine how each layer will be built:

• Laser power/energy density• Scan speed• Hatch spacing• Hatch orientation• Boundaries/contours• Internal filling (sparse/dense)

• Nearly all of these are determined once a material is selected• Each material has unique

“recipe” for making parts

Process Planning – Tool Path

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Execution and Build

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Performed on the AM machine

Consolidation of multiple parts into single build platform

Machine Set up- Control Software- Hardware setup- Parameter settings on the machine

• Gas Flow rates• Chamber temperatures• Other process variables not set in

programs


Cleaning & Removal from machine

Stress relief and heat treatment

Part and support removal

Secondary operations: Hot isostatic pressing (HIP) Improving surface finish (e.g., shot peening) Machining (e.g., threading, tolerances) Assembly (e.g., mating surfaces/interfaces)

Inspection and verification

Post Processing Considerations

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Simulations

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SIMULATION

Documents

Design Workflow for AM: From CAD to PartDesign Workflow for AM: From CAD to Part Sanjay Joshi Professor of Industrial and Manufacturing Engineering Penn State University Industry Practicum: