INTRODUCTORY MECHATRONICS

Friday 12th November 2010Rapid Prototyping

Alberto Parmiggiani (alberto.parmiggiani@iit.it)

RAPID PROTOTYPING

Rapid prototyping is the automatic construction of physical objects using additive manufacturing technology.

The first techniques for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts.

“SEMANTIC” PROLEMS

The RP field is “relatively” new (1980s)

RP techniques are under constant development.

There has not been so far the time to establish widely accepted terminology for given processes

GOOGLE RESULTSName ResultsRapid Prototyping 9.170.000Solid Freeform Fabrication 92.400Rapid Manufacturing 111.000Layered Manufacturing 38.9003D Printing 325.000Layered Fabrication 9.440Additive Manufacturing 303.000

VERY COMPLEX GEOMETRIES

APPLICATIONS

RP is used for a wide range of applications:– Design studies– Creation of lost moulds (investment casting)– Manufacturing production-quality parts in relatively

small numbers (depends on the technique)– Assembly studies– Surgery planning– Fine arts / sculpture

APPLICATIONS EVOLUTION

1980s

2010(s)

PROCESS OVERVIEW

PRE-PROCESSING

FABRICATION

CAD

Computer Aided Design– Computer technology for the process of design– Mathematical description of 2D/3D objects and shapes

PART SLICING

ADDITIVE PROCESS

STEPEFFECT!

THE “STEP” EFFECT• Because of the layered process, the

model has a surface composed of stair steps

• Sanding can remove the stair steps for a cosmetic finish

• Model build orientation is important for stair stepping and build time

• In general, orienting the long axis of the model vertically takes longer but has minimal stair steps

• Orienting the long axis horizontally shortens build time but magnifies the stair steps

• For aesthetic purposes, the model can be primed and painted.

RESIDUAL STRESSES

• Inherent phenomenon because of the layered nature of the process

• Mainly caused by thermal expansion/contraction effects

• Considerably lowers the resistance of parts

• Can be relieved with post-curing treatments

OVERHANGING FEATURES

UNDERCUT FEATURES

SUPPORTS

SUPPORTS AGAIN...

RP MACHINES PARAMETERS

Resolution– The higher the better– Generally different in the x-y vs. z direction– Generally in the order of 50μm to 100μm

Scan speed– How fast can the parts be built– Generally in the order of 6e-9m3/s

Build volume– Sets a maximum size for part dimensions– Generally in the order of 0.02m3

RP MACHINES PARAMETERS

Material– Fundamental parameter– Determines the functionality of the final product– Often polymers (easier to process)

Cost– Of the machine (fixed)– Of the material (generally the most relevant cost in

running a RP machine)

TECHNOLOGIESElectron beam melting

– Fully fused void-free solid metal parts from powder stockElectron beam freeform fabrication

– Fully fused void-free solid metal parts from wire feedstockFused deposition modeling

– Fused deposition modeling extrudes hot plastic through a nozzle, building up a model.

Laminated object manufacturing– Sheets of paper or plastic film are attached to previous layers by either sprayed

glue, heating, or embedded adhesive, and then the desired outline of the layer is cut by laser or knife. Finished product typically looks and acts like wood.

Laser engineered net shaping– A laser is used to melt metal powder and deposit it on the part directly. This has

the advantage that the part is fully solid (unlike selective laser sintering) and the metal alloy composition can be dynamically changed over the volume of the part.

Polyjet matrix– PolyJet Matrix Technology (developed by Objet geometries) is the first technology

that enables simultaneous jetting of multiple types of model materials

TECHNOLOGIESSelective laser sintering

– Selective laser sintering uses a laser to fuse powdered nylon, elastomer, or metal. Additional processing is necessary to produce fully dense metal part.

Shape deposition manufacturing– Part and support material are deposited by a print head and then machined to

near-final shape.Solid ground curing

– Shines a UV light on an electrostatic mask to cure a layer of photopolymers, uses solid wax for support.

Stereolithography– Stereolithography uses a laser to cure liquid photopolymers.

Three-dimensional printing– This label encompasses many technologies of modern 3D Printers, all of which use

inkjet-like printheads to deposit material in layers. Commonly, this includes thermal phase change inkjets and photopolymer phase change inkjets.

Robocasting– Robocasting refers to depositing material from a robotically controlled syringe or

extrusion head.

SLA

Highlights of Stereolithography– The first Rapid Prototyping technique and still the most

widely used– Inexpensive compared to other techniques– Uses a light-sensitive liquid polymer– Requires post-curing since laser is not of high enough power

to completely cure (but long-term curing can lead to warping)– Parts are quite brittle and have a tacky surface– Support structures are typically required– Process is simple: There are no milling or masking steps

required.– Uncured material can be toxic: ventilation is a must.

THE SLA PROCESS • A vat of photosensitive resin

contains a vertically-moving platform• The part under construction is

supported by the platform that moves downward by a layer thickness (typically about 0.1 mm) for each layer• The laser beam traces out the

shape of each layer and hardens the photosensitive resin.

THE SLA PROCESS

The laser beam traces out the shape of each layer and hardens the photosensitive resin.

THE “SLA” PROCESS

• Uncured resin is removed and the model is post-cured to fully cure the resin.

• During fabrication, if extremities of the part become too weak, it may be necessary to use supports to prop up the model

• The supports can be generated by the program that creates the slices, and the supports are only used for fabrication.

SLA

The first stereolitography rapid prototyping process, was developed by 3D Systems of Valencia, CA, USA, founded in 1986.

SLSSelective Laser Sintering– Patented in 1989– Considerably stronger than SLA (sometimes structurally

functional parts are possible)– Laser beam selectively fuses powder materials: nylon,

elastomer, and metal.– Advantage over SLA: Variety of materials and ability to

approximate common engineering plastic materials– Process is simple: There are no milling or masking steps

required– Living hinges are possible with the thermoplastic-like

materials.– Powdery, porous surface unless sealant is used. Sealant also

strengthens part– Uncured material is easily removed after a build by brushing

or blowing it off.

THE SLS PROCESS• An SLS machine consists of two

powder magazines on either side of the work area.• The levelling roller moves powder

over from one magazine, crossing over the work area to the other magazine.• The laser then traces out the

layer• The work platform moves down

by the thickness of one layer and the roller then moves in the opposite direction• The process repeats until the part

is complete.

Selective Laser Sintering (SLS) is a registered trademark by DTM of Austin, TX, USA

The process was patented in 1989 by Carl Deckard, a University of Texas graduate student.

OTHER SIMILAR TECHNOLOGIES

Direct Metal Laser Sintering (DMLS)– Developed by: EOS

3D Systems

Selective Laser Melting (SLM)– Developed by: MTT

ConceptLaser

OTHER SIMILAR TECHNOLOGIES

Laser Engineered Net Shaping (LENS)– Developed by Sandia National

Laboratories– Commercialized by Optomec

Electron Beam Melting (EBM)– Developed by ARCAM

SLA vs. SLSMaterial Properties

– The SLA (stereolithography) process is limited to photosensitive resins which are typically brittle.

– The SLS process can utilize polymer powders that, when sintered, approximate thermoplastics quite well.

Surface Finish– The surface of an SLS part is powdery, like the base material whose

particles are fused together without complete melting.– The smoother surface of an SLA part typically wins over SLS when

an appearance model is desired. – If the temperature of uncured SLS powder gets too high, excess

fused material can collect on the part surface. This can be difficult to control since there are many variables in the SLS process

– In general, SLA is a better process where fine, accurate detail is required

– However, a varnish-like coating can be applied to SLS® parts to seal and strengthen them.

SLA vs. SLSDimensional Accuracy

– SLA is more accurate immediately after completion of the model, but SLS is less prone to residual stresses that are caused by long-term curing and environmental stresses

– Both SLS and SLA suffer from inaccuracy in the z-direction (neither has a milling step), but SLS is less predictable because of the variety of materials and process parameters.

– The temperature dependence of the SLS process can sometimes result in excess material fusing to the surface of the model, and the thicker layers and variation of the process can result in more z inaccuracy

– SLA parts suffer from the "trapped volume" problem in which cups in the structure that hold fluid cause inaccuracies

– SLS parts do not have this problem. Machining Properties

– In general, SLA materials are brittle and difficult to machine– SLS thermoplastic-like materials are easily machined.

SLA vs. SLSSize

– SLS and SLA parts can be made the same size, but if sectioning of a part is required, SLS parts are easier to bond.

Investment Casting– The investment casting industry has been conservative about

moving to RP male models– SLS models made from traditional waxes, etc. are preferred since

SLA resins do not melt but burn to form ashSupport Structures

– SLA parts typically need support structures during the build– SLS parts, because of the supporting powder, sometimes do not

need any support, but this depends upon part configuration. Marks left after removal of support structures for parts cause dimensional inaccuracies and cosmetic blemishes.

FDM

Fused Deposition Modelling – Standard engineering thermoplastics, such as ABS, can

be used to produce structurally functional models– Two build materials can be used, and latticework

interiors are an option– Parts up to 600 × 600 × 500 mm can be produced– Filament of heated thermoplastic polymer is squeezed

out like toothpaste from a tube– Thermoplastic is cooled rapidly since the platform is

maintained at a lower temperature– Milling step not included and layer deposition is

sometimes non-uniform so "plane" can become skewed– Not as prevalent as SLA and SLS, but gaining ground

because of the desirable material properties.

THE FDM PROCESS

• The FDM process was developed by Scott Crump in 1988.

• The fundamental process involves heating a filament of thermoplastic polymer and squeezing it out like toothpaste from a tube to form the RP layers.

• The machines range from fast concept modellers to slower, high-precision machines.

• The materials include polyester, ABS, elastomers, and investment casting wax.

FDM MACHINE SUPPLIERS

The Fused Deposition Modelling technology was developed and commercialized by Stratasys of Eden Prairie, MN, USA.

Hewlett-Packard recently entered the FDM market with a cheaper but lower quality machine.

POLYJET

– Simple technology, with easy support removal, and easy replacement of jetting heads

– The PolyJet Technology allows for high-speed raster build with no post-curing

– The selling company offers a wide variety materials with different mechanical properties, and colors

– The support material is the same for all models types (makes switching materials easy and fast)

– Elastomeric components can be printed– The PolyJet technology was introduced and patented by

Objet in early 2000

THE POLYJET PROCESS

• “Standard” layered process

• Photopolymer is cured instantly with a UV light housed on the printing head

MULTI-MATERIAL

• 3D printing system that jets multiple model materials simultaneously

• Unique ability to print parts and assemblies made of multiple model materials, with different mechanical or physical properties, all in a single build

PROBLEMS

Parts printed with the polyjet technology:• Degrade rapidly under the effect of UV light• Exhibit significant creep over time• Generally have poor mechanical properties

LOM

Laminated Object Manufacturing– Layers of glue-backed paper form the model– Low cost: (Raw material is readily available)– Large parts: Because there is no chemical reaction

involved, parts can be made quite large– Accuracy in z is less than that for SLA and SLS– Outside of model, cross-hatching removes material– Models should be sealed in order to prohibit moisture– Before sealing, models have a wood-like texture– Not as prevalent as SLA and SLS

THE LOM PROCESS•Material is usually a paper sheet

laminated with adhesive on one side, but plastic and metal laminates are appearing. •Layer fabrication starts with

sheet being adhered to substrate with the heated roller. •The laser then traces out the

outline of the layer. •Non-part areas are cross-

hatched to facilitate removal of waste material.

THE LOM PROCESS•Once the laser cutting is complete,

the platform moves down and out of the way so that fresh sheet material can be rolled into position. •Once new material is in position, the

platform moves back up to one layer below its previous position. • The process can now be repeated.• The excess material supports

overhangs and other weak areas of the part during fabrication. • The cross-hatching facilitates

removal of the excess material.

THE LOM PROCESS•Once completed, the part has a wood-

like texture composed of the paper layers. •Moisture can be absorbed by the

paper, which tends to expand and compromise the dimensional stability. • Therefore, most models are sealed

with a paint or lacquer to block moisture ingress. • The LOM developer continues to

improve the process with sheets of stronger materials such as plastic and metal.•Now available are sheets of powder

metal (bound with adhesive) that can produce a "green" part. The part is then heat treated to sinter the material to its final state.

LOM is a registered trademark by Helisys of Torrance, CA, USA.

3D PRINTING

• Allows to obtain realistic-looking models• Allows to obtain multiple-coloured parts• Resulting parts have poor structural properties • First company on the market: Z-Corp, Boston,

MA, USA• Layers of powder glued together.

3D PRINTING

MATERIALS/TECHNOLOGIES COMPARISON

Material Tensile strength [MPa]

Modulus of Elasticity [MPa]

Elongation at break [-%]

HDT [°C]

SLA photopolymer

65 3100 7 130

PolyJet photopolymer

60 2870 20 40-50

FDM polycarbonate

68 2280 4.8 130

FDM ABS 36 2413 4 90