Department of Mechanical and MechatronicEngineering 3D Printing Guide

Wayne Swart

Last update: October 19, 2015

Contact PersonWayne Swart

M616021 808 3613

wswart@sun.ac.za

1 Foreword

Rapid prototyping, and especially 3D printing, has become an increasinglyuseful tool that equips engineers in both, the academic field and industry,with a quick method of manufacturing prototypes. However, it is importantnot to consider this tool as the go to manufacturing process. The purpose ofthis document is to present students with comprehensive methods to designfor 3D printing and to effectively evaluate their designs in terms of feasi-bility. It is expected of each student that intends to make use of the 3Dprinting facilities at the Mechanical and Mechatronic Department, to thor-oughly read through this guide and ensure that they adhere to the house rulesregarding these facilities BEFORE the technician in charge of the facilitiesis approached.

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2 Suitability for Additive Manufacture

There are many aspects to consider when designing a component, and severalof these aspects directly influence the manufacturing method that will berequired. As it is not the purpose of this document to educate students onmanufacturing processes and design methods, it is recommended that anydoubts regarding these topics are put to rest by consulting previous coursework. Some points to consider however are:

• Can the component be bought off the shelf?

• Can the component easily be manufactured by means of milling orturning etc.?

• How many parts will be manufactured?

• This process is not a quick way out of a tricky design or a remedy fortime constraint issues.

• Dont design a detailed CAD model and then decide on a manufacturingprocess (i.e. design for manufacture, not the other way around!).

• Can the cost of manufacture be justified by the intended function of thecomponent? (some really educated guesses regarding costing is possibleby using the component volume and other specifications given in thisdocument)

• If weight constraints are of importance dont default to 3D printingregardless of the shape of the component. High Density Polyethylene(HDPE) and Vesconite are materials that can be machined and serveas an alternative to metals.

• Absolutely no components with aesthetic features will be accepted formanufacturing!

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3 Different Processes

The department of Mechanical & Mechatronic Engineering have two types of3D printers at our disposal, namely an Objet 30 photolithographic printerfrom Stratasys, and a BabyBot fusion deposition printer from OpenHard-ware.

Generally, fusion deposition printers are much cheaper to maintain and havematerial costs ten times lower that the resins used by the Objet. The ma-terials used by these printers are also much more durable and tolerant ofambient temperatures than is the case for the Objets material. The onlydrawback to fusion deposition printing is that the accuracy is much lowerthan the photolithographic alternative.

The objective of this section is to help the reader to decide on a suitableprocess for manufacturing of his/her component. Typically, the photolitho-graphic option will be more suitable for components that require high shapeaccuracy and good surface finish while material strength and durability isof less importance, such as with aerofoils or rotor blade profiles for the pur-pose of flow test. It might also be necessary for parts with intricate detailor interior detail. If the component does not have stringent shape-, surfacefinish- or detail requirements, the default choice for manufacturing processwill fusion deposition modelling.

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3.1 Photolithographic Printing

The Objet 30 makes use of a line of nozzles on the printer head that deposita thin layer of resin onto the build surface which is then cured with UV lighton the return pass of the head. This sequence makes for long print times(a 150 gram component can easily take between 12 and 16 hours to build).The component is also surrounded by a support material to prevent warpingduring prints and allowing overhangs. The support material also makes itpossible to manufactured assemblies that would otherwise have been impos-sible to achieve (example a part within a cage). Since the entire componentis encased and filled with support material, a good rule of thumb is thatthere will be a weight ratio of 1:1 between the support and resin. Whengenerating quotes, the CAD model of the component is used to determine amore accurate estimate of the material weights that will be used, but the 1:1estimate will help designers to get an idea of costs before submitting designsfor official quotes.

Build PropertiesNett build volume: 294 x 192 x 148.6 mmLayer thickness: 28 micron (28x10−3 mm)Accuracy: 0.1 mmBuild time: Highly dependent on component complexity (and

obviously size) as well as orientation on bed.

Material PropertiesAs previously noted, the material is highly sensitive to ambient conditions.All prints should be kept at room temperature and low humidity. Directexposure to sunlight and/or increased temperatures will cause componentdeformation. Refer to the table below for specific material properties.

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PROPERTY VALUE UNITS

Tensile strength 50-60 MPa

Polymerised density 1.18-1.19g

cm3

Elongation at break 15-25 %

Modulus of elasticity 2000-3000 MPa

Flexural Strength 60-70 MPa

Flexural Modulus 1900-2500 MPa

HDT, ◦C @ 0.45 MPa 45-50 ◦C

HDT, ◦C @ 1.82 MPa 45-50 ◦C

Izod Notched Impact 20-30J

m

Tg 48-50 ◦C

Shore Hardness (D) 83-86 Scale D

Rockwell Hardness 73-76 Scale M

Also note that the material used by this printer is not durable or wearresistant at all. The slightest bit of friction will cause the component to startdegrading by shedding debris dust similar to what happens to an eraser whenit is rubbed against paper.

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3.2 Fused Deposition Modelling

Fused deposition modelling (FDP), also known as plastic jet printing (PJP)is the process of melting plastic filament fed from a spool and depositing themolten plastic to the build surface through a small nozzle. As the plasticcools, it hardens and attaches to the previous build layer. There are threetypes of materials that can be used with this process as listed below:

Material Options

Acrylonitrile butadiene styrene (ABS): UNAVAILABLE

Polylactic acid (PLA): AVAILABLE

Polyvinyl alcohol (PVA): UNAVAILABLE

NB: Please be sure to check the latest version of this document for infor-mation about availability.

Build Properties

Nett build volume: 100 x 100 x 100 mm

Layer thickness: 0.3-0.4 mm

Accuracy: 0.3-0.5 mm

Build time: Highly dependent on component size and infill den-sity. Infill refers to the degree to which the com-ponents inner volume is reduced by creating hon-eycomb support structures instead of producing asolid part.

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Material Properties

PROPERTY ABS PLA UNITS

Tensile Strength 34,0 46,7 MPa

Flexural Strength 36,8 61,8 MPa

Compressive Strength 7,6 17,9 MPa

Un-notched IZOD Impact Strength 304.7 96.2J

m

Density 1.05 1.25g

cm3

Melting Temperature 105 175 ◦C

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4 Design for Printing

When designing a component that will be 3D printed, there are few thingsthat should be kept in mind. This section lists some of the common consid-erations.

4.1 General

This subsection discusses a variety of considerations that are relevant to boththe available 3D printing processes.

Support Material Slicer software, that convert the mesh files of a CADdesign into machine code that is used to perform the print, provide little tono control over the shape and position of support structures. Basically itcomes down to including or excluding the support structure altogether (forFDM). The orientation of a component will often determine the need forsupport material, which is almost always undesirable to have, especially forFDM prints since support material greatly affects the surface finish of thecomponent. Always make sure that the part is orientated in such a way thatthe need for support material is minimised. See the example below.

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Orientation does not require the gener-ation of support material, since thereare no overhangs to support.

Same component in such a way thatthere is an overhang which needs to besupported by extra material.

AccessibilityWhen designing a component that will require the generation of supportstructures, the designer needs to plan for the removal of the support. Forexample, a hollow cube with only one small hole in one of the sides will benearly impossible to clean without extensive chemical post processing. Evenwhen making use of chemicals for post processing some features cannot becleared of the support material completely, such as in the case of a narrowbent tube.

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Splitting ComponentsSometimes no orientation of a component will the optimal way to manu-facture the component. It is often more prudent to split a component intomultiple parts and assembling them afterwards than to simply print the com-ponent as-is.

Component design does not allow forany rotation that exclude the need forsupport generation.

Same component broken up into partsthat will simplify printing and is easyto assemble.

4.2 Objet Specific

OrientationAfter all other design specifications have been satisfied, orientation of the

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design can be brought into consideration. The Objet has a faster printingspeed along the x-axis. It is therefore desirable to orientate the componentin such a way that the longest dimension is along the x-axis. This is usuallyhandled by the technician in charge, but it is a good thing to keep in mindwhilst designing since print time directly affects cost.

Recommended Wall Thickness and ClearanceA minimum wall thickness of 0.6 mm is recommended and a clearance of atleast 0.2 mm is required between two parts that are to move relative to oneanother.

4.3 ABS/PLA Specific

Stresses and Direction of BuildWhen a load bearing component is being designed, be sure that the directionof the load will be normal to the build plan. That is, the part can be loadedvertically, but not horizontally. The reason for this is that the component isweakest between the layers since the attachment between layers do not allowfor full material strength. The same principle holds for torsional loads.

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Cooling RateThe rate of cooling for the deposited plastic can influence the quality of theprint greatly. When certain parts of the component cool quicker than others,the temperature gradient induces thermal stresses which warp the printed

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part. This is frequently encountered in large- and high density infill com-ponents. Some ways to minimise the risk of this happening are to avoidunnecessarily large volumes and avoiding stress concentration features suchas sharp corners.

In cases where the cooling rate is too low, smearing tends to occur as theextruder head jumps between printing areas. This situation is most commonwhen the print area is very small such as in the case of spire-like structures.Methods of reducing the likelihood of this occurring include creating extrathin wall structures which can be broken off the part after the print and pro-viding a continuous printing path to ensure that the extruder will not jumpbetween print areas but rather transition between them smoothly. See thefigures below for a graphic example.

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Minimal build areas, such as in the caseof printing spire-like structures, doesnot allow sufficient cooling time whichmay cause smearing of material. Theextruder will also jump between thespires at accelerated speed which willpull them over.

The same structure can be achievedby inserting features that will ensurea continuous extruder path as well asincreased cooling time.

Recommended Wall Thickness and ClearanceA minimum wall thickness of 1 mm is suggested for FDM prints. Featureswith wall thicknesses this thin, should only be used for small parts or assupporting structures in larger prints. When designing components that willbe assembled into other parts, tolerances should be built into the dimensionsof the component. Inside diameters should be 0.5-0.8 mm larger and outside

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diameters should be 0.5-0.8 mm smaller. When the component to be printedhas holes, it is recommended that the holes diameters should be made atleast 1 mm less than the desired dimension and then drilled to the correctsize after manufacture.

5 Post Processing

For all the plastics listed above, any support material needs to be brokenaway from the part by hand.

ABSABS can be treated with acetone to give the component a smoother andglossy finish. Given the material properties it can also be machined to someextent.

PLAPost processing of PLA is limited to sanding and machining.

PVAPVA is water soluble and can therefore be treated accordingly.

Objet PrintsBecause of the high density of support structures typically generated forthese print, the support is removed with a high pressure water jet. This stepis also done by the responsible technician. Any support material that mightremain in finer features of the part by delivery time can easily be scraped off.These parts can also be chemically treated by soaking them in a 2% sodiumhydroxide solution (caustic soda, NaoH). For larger parts a soak time ofapproximately 2 hours while smaller parts and parts with wall thickness lessthan 1 mm should not be soaked for longer than 40 min. This will improvethe surface finish of the print as well as remove any support material residue.WARNING Use protective goggles and latex gloves. Sodium hydroxidesolution should not come into contact with skin or eyes. In case of accidentalcontact, immediately clean the area with running tap water.

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6 Procedure

Step 1:Read through this document completely and thoroughly.

Step 2:Create a technical drawings pack as illustrated in Section ?? Fill in theprovided drawings pack cover page and have the bundle reviewed and signedby your supervisor or the person in charge of the relevant cost point. Do notproceed until no more revisions are required.

Step 3:For each component, check the orientation of the CAD part and ensure it iscorrect. Remember that the X-Y plane is the ground plane of the printer.Thus, if you are looking at the X-Y plane you are seeing the top/bottom ofyour component.

Step 4:Convert your CAD files to .stl mesh files. Make sure that the format andunits are correct. Also avoid high resolution meshes. If you are convertingin Inventor, follow these steps:

• Click on the file button (the square button in the top left corner withthe IPRO logo).

• Now move the cursor to the export menu (NOT the save as menu) andselect theCAD format option.

• At the bottom of the browser window you should now have in front ofyou click on the dropdown menu just below the file name textbox andselect the STL files (*.stl) option.

• Now click on the Options button to the left of the Save button andchange the units of the file to millimetre.

• The rest of the options should be correct, but double-check that theformat is binary and the resolution is medium.

• Press OK to exit the options menu and then press Save.

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Your files are now ready to be sent for manufacture.

Step 5:Present the technician in charge with your signed drawings pack and CADfiles for revision. If there are queries from the technician, a quote will begenerated and sent to the person in charge of the presented cost point. Oncethe order has been approved, manufacture will commence. Production ofcomponents that do not adhere to the guidelines provided in this documentmay be refused.

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