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DESIGNING FOR FEASIBLE

3D PRINTS

Index

Page

1. Introduction........................................................................................................2

2. Polyjet Printing About the Objet260 Connex3 .........................................................................3 Support Material .............................................................................................3 Finishing .........................................................................................................4 Minimum Wall Thickness...............................................................................4 Files to be submitted .......................................................................................5 Designing Assemblies .....................................................................................6

3. Stereolithography

About the Form 2 ............................................................................................8 Design guidelines ............................................................................................8 Files to be submitted .......................................................................................9

4. Fused Deposition Molding

About the Taz 6 .............................................................................................10 Support Material and Infill............................................................................10 Finishing .......................................................................................................11 Minimum Wall Thickness.............................................................................11 Files to be submitted .....................................................................................11 Designing Assemblies ...................................................................................12

5. Additional guidelines .......................................................................................14

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INTRODUCTION 3D printers can vary significantly, not only based on the underlying process type, but also by material and manufacturer. Designs that are feasible on one particular printer may be unsuccessful on another. This document details the capabilities of our printers and the geometric limits within which a design can print successfully. The Carnegie 3D Printing facility necessitates that newcomers as well as 3D printing experts go through guidelines for the 3D printer of choice before submitting orders. It is also strongly recommended to go through the “Additional Guidelines” pages which give suggestions on maximizing the accuracy and functionality of your design. Please reach out to us at Carnegie3DP@jhmi.edu, if you need help in making your design compliant with the posted guidelines.

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POLYJET PRINTING

About the Objet260 Connex3

Objet260 Connex3 from Stratasys is a polyjet printer- it builds parts by spraying drops of photopolymers onto a build plate which are then cured by UV light. There are a wide range of base materials available with material properties varying from rigid to rubber-like and transparent to opaque. These materials can be blended together to create new materials with intermediate material properties and different colors. In the Objet260 machine, up to 3 different base materials can be blended together in discrete ratios, resulting in 82 different blends. To know more about the materials that can be used on the printer, please visit stratasys.com/materials/polyjet .

1. Support Material The support material that we use for the models is called SUP706 and it is a transparent yellow gel-like substance that is soluble in water and alkali. All parts are manually cleaned and pressure-washed to remove this support material. The cost of cleaning the parts is added to final charge unless the customer offers to clean the model himself. However, if the model which is sent to us is very fragile or has interior features that are hard to be cleaned using our equipment, we will leave the model in the support-material casing and subtract any cleaning charge from the quote provided.

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SUP706 encasing the a fresh print Support material partially removed

Unless otherwise specified, all objects printed on the machine will be “solid” prints. In order to reduce the part cost, you can request the parts to be printed as “shells” (in the description section of the Carnegie_Work_Request_Form, or in your correspondence with us). This will hollow out the model. The hollow will be filled with the SUP706 material, which is generally cheaper than the model resin. However, this might interfere with desirable material properties such as transparency or strength.

2. Finishing

By default, all models will be printed with a matte finish. There is an option to print with a glossy finish which works well for designs with flat surfaces. Rounded surfaces such as this skull seen below, is glossy but has a matte finish in some places. These patches will need to be sanded down and coated with lacquer to obtain a uniformly glossy surface.

Print with glossy surface and matte patches Result after sanding and lacquering

3. Minimum Wall Thickness The minimal wall thickness that can be reliably printed is 0.6 mm. But a large surface that is only 0.6 mm thick is very likely to break during support material removal. If you plan on having large features (greater than 0.5 sq.mm surface area) which are less than 1.5 mm thick, consider using support structures such as ribs, gussets and fillets to reduce stress concentrations at the edges of the feature.

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Minimum Wall Thickness for Objet260 Connex3

4. Files to be submitted

All our printers require the design/CAD file to be in an STL format. Most CAD and mesh modeling software gives you the option of saving your model as an STL. There are also online conversion tools like 3dtransform.com, meshconvert.com and tinkercad.com, where you can upload your files and convert them for free. An STL saves the model as a mesh, which is a triangulated approximation of the surface of your actual geometry. It is a good practice to choose a tolerance/chord height of 0.01 mm so that the resultant mesh file has small enough triangles to fit the contours of your model. This is desirable when your model has round/spherical surfaces or lots of detail. For models with straight edges the resolution need not be high. There is also no necessity to increase the resolution beyond 0.01 mm because our printers will not be able to capture that amount of detail anyway.

Saving a part in STL format in Creo

You may also have the option of saving the STL as binary or ASCII format. It is better to go with the binary option as that results in smaller file sizes.

6 When submitting multiple parts for printing, it is recommended to save each part as a separate STL file. This way our printers can optimize the placement and orientation on the build-plate for shorter build-times.

Errors in STL files Sometimes there can be errors during file conversion or in the model geometry itself which results in STL files that are not water-tight. A water-tight STL has a mesh surface that has no open faces, inverted normals and other mesh errors. Please verify the water-tightness of your model before submitting it. Meshlab, Meshmixer and MakePrintable.com are free options for checking and repairing models. 5. Designing assemblies

a) Designing parts that will mate It is advisable to leave a minimum of 0.3 mm clearance between mating parts (a 5 mm pin will need at least a 5.3 mm diameter hole). However, depending on the accuracy of the printer, the required clearance maybe even 0.5 mm for a tight fit. It is difficult to estimate the exact clearance, as the dimensional accuracy is highly dependent on the shape, size, orientation of the build, speed of print, ambient temperature conditions etc. So, we suggest that you design the parts of your assembly such that they can be post-processed for mating. It is always safer to print the models in the maximum-material-condition (holes undersized, shafts oversized). This way the interior features such as holes, grooves etc., can be drilled open to the right size. Solid features such as shafts and pins can be sanded or filed down to the correct size. b) Printing assemblies directly The Objet260 can be used to print assemblies directly. For example, a fully-functional a planetary gearbox can be printed in place. Below is a toy car printed as an assembly from VeroClear and VeroWhitePlus. It would not have been possible to assemble these as separate parts without splitting the axle and wheel. You will need to provide a clearance of at least 0.3 mm between the parts of your moving assembly, otherwise, the different parts will fuse together even if they are assigned different materials.

Isometric view Bottom view

7 c) Fused assembly part The Objet260 also print fused assemblies (single object made up of different materials in discrete regions such as the head shown below). In order to print such objects, the different parts must be saved as separate STL files but in the same coordinate system that they were assembled in. lease look at the ‘Save As’ options in your CAD/ sculpting software that will allow you achieve this. The material specifications for each part can be specified in the file name of each STL that you upload. For example, in the heart model below, the CAD files could be named as Skull-VeroYellow.stl, Tumor-VeroMagenta.stl and Skin-VeroClear.stl.

Fused assembly with VeroClear, VeroYellow and VeroMagenta

Navigate to Additional Guidelines

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STEREOLITHOGRAPHY

About the Form 2

The Form 2 is a desktop Stereolithography apparatus which shoots UV-Laser light into a tank of photopolymer resin and cures the resin selectively, layer by layer, thus building the 3D object. FormLabs’ SLA printers build the objects ‘bottom-up’, i.e., the build platform is mounted upside down and pulls out cured resin layers from the tank. The lattice of support structures which hold the object to the build platform is autogenerated by the PreForm software which comes along with the printer. However, certain challenging designs may require custom supports to be built.

When the model comes off the printer, the residual resin is removed by dipping it in a vat of Isopropyl alcohol. The supports are then snipped off and the model is placed in a UV-curing station that enhances the strength of the photopolymer. The Form 2 allows only for a single material to be used at a time. It is to be noted that the Form 2 does not allow for completely enclosed cavities. Every cavity must have a drain hole of at least 3.5 mm to allow for the resin to drip out during the building. If not, the print will explode inside the printer.

FormLabs has an excellent set of design guidelines on their website: https://formlabs.com/3d-printers/design-specs/

Typically, we stock at least one of the rigid materials and one of the Class 1 biocompatible materials. Material specifications can be found at the following locations: • https://formlabs.com/materials/engineering/ • https://formlabs.com/3d-printers/tech-specs/ (scroll to Material properties for mechanical properties and SDS) • https://formlabs.com/materials/

All our printers require the design/CAD file to be in an STL format. Most CAD and mesh modeling software gives you the option of saving your model as an STL. There are also online conversion tools like 3dtransform.com, meshconvert.com and tinkercad.com, where you can upload your files and convert them for free. An STL saves the model as a mesh, which is a triangulated approximation of the surface of your actual geometry. It is a good practice to choose a tolerance/chord height of 0.01 mm so that the resultant mesh file has small enough triangles to

9 fit the contours of your model. This is desirable when your model has round/spherical surfaces or lots of detail. For models with straight edges the resolution need not be high. There is also no necessity to increase the resolution beyond 0.01 mm because our printers will not be able to capture that amount of detail anyway.

Saving a part in STL format in Creo

You may also have the option of saving the STL as binary or ASCII format. It is better to go with the binary option as that results in smaller file sizes.

When submitting multiple parts for printing, it is recommended to save each part as a separate STL file. This way our printers can optimize the placement and orientation on the build-plate for shorter build-times.

Errors in STL files Sometimes there can be errors during file conversion or in the model geometry itself which results in STL files that are not water-tight. A water-tight STL has a mesh surface that has no open faces, inverted normals and other mesh errors. Please verify the water-tightness of your model before submitting it. Meshlab, Meshmixer and MakePrintable.com are free options for checking and repairing models.

Navigate to Additional Guidelines

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FUSED DEPOSITION MOLDING

About the Taz 6

The Taz 6 is a Fused Deposition Molding 3D desktop printer made by LulzBot. It builds 3D objects by heating and extruding plastic filament in successive layers. The molten layer solidifies in air just in time before the next layer is added. The Taz 6 accepts a wide variety of rigid and flexible filaments, with new materials being added almost every few months.

1. Support material and infill

The Taz 6 uses a lattice-like support structure from the same material for overhanging geometry. However, using the Flexidually extruder tool, a combination of rigid and flexible support materials can be used. This allows dissolvable filaments such as PLA to be used as support material. The Flexidually tool can be also used to print objects with rubber-like materials such as ninjaFlex and PCTPE. Certain material combinations are not compatible with each other, so please check with us for more details. If your model has unsupported geometry that is hard to access, then it may not be possible to remove the support structures.

11 Unless otherwise specified, all objects printed on the machine will be “shell” prints (semi-hollow), with an infill of about 20%. You can request the infill settings to be changed in the description section of the Carnegie_Work_Request_Form, or in your correspondence with us.

2. Finishing

FDM printers have a shiny, yet rougher surface finish to them. Sanding and painting the objects can improve the appearance and the smoothness of the object. We perform this post-processing step upon request.

3. Minimum Wall thickness

The minimum recommended wall thickness is 2 mm. For larger walls, it is best to increase the thickness even more or add supporting gussets, ribs and fillets to reduce the stress concentrations.

4. Files to be submitted

All our printers require the model/CAD file to be in an STL format. Most CAD and mesh modeling software gives you the option of saving your model as an STL. There are also online conversion tools like 3dtransform.com, meshconvert.com and tinkercad.com, where you can upload your files and convert them for free. An STL saves the model as a mesh, which is a triangulated approximation of the surface of your actual geometry. It is a good practice to choose a tolerance/chord height of 0.01 mm so that the resultant mesh file has small enough triangles to fit the contours of your model. This is desirable when your model has round/spherical surfaces or lots of detail. For models with straight edges the resolution need not be high. There is also no necessity to increase the resolution beyond 0.01 mm because our printers will not be able to capture that amount of detail anyway.

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Saving a part in STL format in Creo

You may also have the option of saving the STL as binary or ASCII format. It is better to go with the binary option as that results in smaller file sizes.

When submitting multiple parts for printing, it is best to save each part as a separate STL file. This way our printers can optimize the placement and orientation on the build-plate for shorter build-times.

Errors in STL files Sometimes there can be errors during file conversion or in the model geometry itself which results in STL files that are not water-tight. A water-tight STL has a mesh surface that has no open faces or inverted normals. Please verify the water-tightness of your model before submitting it. Meshlab, Meshmixer and MakePrintable.com are free options for checking and repairing models. 5. Designing assemblies

5. a) Designing parts that will mate It is advisable to leave a minimum of 0.3 mm clearance between mating parts (a 5 mm pin will need at least a 5.3 mm diameter hole). However, depending on the accuracy of the printer the required clearance maybe even 0.5 mm for a tight fit. It is difficult to estimate the exact clearance, as the dimensional accuracy is highly dependent on the shape, size, orientation of the build, speed of print, ambient temperature conditions etc. So, it is recommended that you design the parts of your assembly such that they can be post-processed for mating. It is always safer to print the models in the maximum-material-condition (holes undersized, shafts oversized). This way the interior features such as holes, grooves etc., can be drilled or machined open to the right size later. Solid features such as shafts and pins can be sanded or filed down to the correct size. 5. b) Fused assembly part In order to print objects using the dual material extruder, the different material regions must be

13 saved as separate STL files but in the same coordinate system that they were assembled in. lease look at the ‘Save As’ options in your CAD/ sculpting software that will allow you achieve this. The material specifications for each part can be specified in the file name of each STL that you upload. For example, in the key-chain model shown below, the STL file for the letters region can be saved as letters-ABS.stl and the padding can be saved as padding-NinjaFlex.stl.

Fused assembly with parts in different Merged Coordinate systems coordinate systems

Navigate to Additional Guidelines

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ADDITIONAL GUIDELINES

The ‘Feasibility’ of a design does not always translate into accuracy or practicality of the design. The below guidelines address some finer details of 3D printing which can help ensure that the resultant parts have the expected fit and function.

1. Optimize material usage and strength Below is an ordinary box-design with relatively thin walls. Every wall (including the base) is supported by at least three other walls that it shares edges with. Though the model is feasible to be printed (provided that the wall thickness is at least 0.6 mm), the object is still susceptible to breakage with rough handling or short drops. Sharp edges, holes and sudden change in profile or material are some of the common design features that reduce the strength of the model. In this case, the boxes sharp corners can be smoothed out using fillets as shown. Note that both internal and external edges have been filleted.

Simple Box design Filleted design

Now the edges and corners are stronger, but the center of the walls which is away from the supporting edges is still weak. Ribs can be used to improve the rigidity of the walls without a significant increase in material usage as shown below. Ribs must be designed carefully for optimum strength. Coring of intersections and maintaining uniform wall thickness is not as important in 3D printing as compared to injection molding.

Ribbed and filleted

15 Gussets are similar to ribs and can provide strength by connecting adjacent walls together as seen in figure below. Gussets are also subject to some rules regarding placement, draft angle, thickness, orientation, height etc.

Plain bosses Gusseted bosses

2. Roundness The roundness of a feature is best preserved when its axes is perpendicular to the base plate (x-y plane). This is because the x-y resolution is usually smaller than the layer thickness. The difference in roundness between the perpendicular and parallel configurations is quite significant in FDM printing (Taz 6). If there is a feature in your model for which roundness is important, please let us know in the description section of the Carnegie_Work_Request_Form or in your email correspondence with us, so that we can choose a suitable configuration while printing.

The cylindrical feature on the top surface will be more round than the cylinder on the side surface

3. Font and Small Features

In general SLA and Polyjet (Form 2 and Objet260) are capable of reproducing small features and embossed or engraved text more accurately. Sans Serif fonts are recommended for smaller font sizes. Please refer to the table below for minimum text parameters for printability. It is advisable to increase the font size above the recommended minimum if the text is placed on complex geometry.

16 POLYJET SLA FDM Minimum Height 0.2 mm 0.2

mm 1.5 mm (top surface), 2 mm (side surface)

Minimum Stroke Width (boldness)

0.2 mm 0.3 mm

1 mm

Minimum Letter Spacing 0.2 mm 0.2 mm

0.6 mm

Minimum Font Size recommended

8 pt. 10 pt. 20 pt.

For pegs (or peg like protrusions) we recommend at least 1 mm diameter with any of the printers. Though 1 mm pegs can be printed (in case of the Form 2 and Objet260, the diameter can be as low as 0.3 mm), they may be too fragile. Similarly, holes less than 0.5 mm diameter may simply get filled up by expanding filament/resin. With the Taz 6 printer, at least 1.5 mm diameter is required for a hole to resolve (the resultant hole is still much smaller than 1.5 mm).

4. Consideration of supports 3D printers build parts one layer at time, like a wedding cake. Molten plastic/ liquid resins/ fine powders are cured by convection or by light in successive layers. It is not possible for our printers to build objects in thin air- there needs to be a base onto which the fluid materials can solidify. When there is overhang in the part, the printer will add some sort of support lattice made of the same or different material. Once the part has finished printing the support lattice can be removed by physical or chemical means depending on the type of support material being used. Many support materials can be dissolved in alkaline solutions, organic solvents or even water.

When using the Objet260 printer, the entrance hole to a support filled cavity must be at least 3 mm in order to remove the support material. If there is a long neck as shown in the figure below, the hole must be at least 5 mm.

The Form 2 printer needs drain holes of at least 3.5 mm diameter in hollowed parts to release the resin during the printing process. Buildup of resin will cause the part to explode.

5. Threads Acme threads are better reproduced on 3D printers in comparison to square threads. The Objet260 and Form 2 printers support as much as 20 threads per inch, while the Taz 6 can reproduce 5 threads per inch at diameters greater than 5 mm. For smaller diameters and thread sizes, it is best to 3D print a regular hole (with counter-bores and counter-sinks if required) and then use a tap and drill set to machine out the exact threads.

Another option is to use metal inserts for internal threads. 3D printed threads are likely to be stripped down by metal screws. Please consult the material specifications to check if the hardness and toughness is sufficient for using metal screws as fasteners.

17 6. Shrinkage and warping The Taz 6 printer is likely to produce parts which shrink by as much as 1% after cooling. This is separate from the dimensional accuracy of the printer. With any of the 3D printers there need some be some allowance in the design for inaccuracy. This can be achieved by printing at the maximum material condition (bigger walls, smaller holes) so that the resultant object and be sanded or drilled down to the correct size. This document is in no way an exhaustive resource on the subjects of 3D Printing or Product Design and the reader may well-benefit from reading other online literature in order to be able to create challenging geometries. But we hope that the guidelines above will eliminate a majority of the errors that could have been unforeseen by the reader.

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End of Document