additive manufacturing module 10

Additive Manufacturing – Module 10

Spring 2015

Wenchao Zhou

[email protected]

(479) 575-7250

The Department of Mechanical EngineeringUniversity of Arkansas, Fayetteville 1

Stereolithography

Polymer

Overview

Photo polymerization

2

Photopolymerization Processes

Processes

Vector scan – Stereolithography

Mask projection – Digital Light Processing

Two-photon lithography

Credit: Gibson, Rosen, Stucker, Textbook

Stereolithography

Polymer

Overview


3


Complexities

Energy source: UV

lamp or LASER Energy absorption Polymers

Photopolymerization

Diffusion

Fluid mechanics –

refill flow, buoyancy

Stereolithography

Polymer

Overview


PolymersPolymer: High molecular weight molecule made up of a small repeat unit (monomer).

Monomer: Low molecular weight compound that can be connected together Oligomer: Short polymer chainCopolymer: polymer made up of 2 or more monomers

Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-BAlternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-BBlock copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B


A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A

ClassificationThermoset: cross-linked polymer that cannot be melted (tires, rubber bands)Thermoplastic: Meltable plasticElastomers: Polymers that stretch and then return to their original form: often thermoset polymersThermoplastic elastomers: Elastic polymers that can be melted (soles of tennis shoes) 4

Stereolithography

Polymer

Overview


PolymersPolymer families

Polyolefins: made from olefin (alkene) monomersPolyesters, Amides, Urethanes, etc.: monomers linked by ester, amide, urethane or other functional groups

Natural Polymers: Polysaccharides, DNA, proteins


5

Monomer Polymer

Ethylene

H3CCH3

nRepeat unit

Polyethylene

CH3

CH3

n

CH3 CH3 CH3 CH3 CH3 CH3CH3

Propylene

Polypropylene

Polyolefins (CnH2n)

Stereolithography

Polymer

Overview


Polymers


6

PolyesterMonomer Polymer

CO2HHO2CHO

OH

O O

HO OH2

CH2

C O

nTerephthalic

acid

Ethyleneglycol

Poly(ethylene terephthalate

H

Ester

Polyamides

HO OH

O O

4H2N NH24

Adipic Acid 1,6-Diaminohexane Nylon 6,6HO N

HNH

H

O O

4 4n

CO2HHO2C

Terephthalic acid

NH2H2N

1,4-Diamino benzene

Kevlar

O

HO

OHN

HN H

n

Amide

Polyurethanes

HOOH

Ethyleneglycol

H2

COCN NCO

4,4-diisocyantophenylmethaneSpandex

H2

CHN

HN

O

HO

O

OH2

CH2

C O H

n

Urethane linkage

Stereolithography

Polymer

Overview


Polymers


7

CharacteristicsReally big molecules (macromolecules)Chain entanglement: Long polymer chains get entangled with each other.

When the polymer is melted, the chains can flow past each other.Below the melting point, the chains can move, but only slowly. Thus the plastic is flexible, but cannot be easily stretched.Below the glass transition point, the chains become locked and the polymer is rigid

Stereolithography

Polymer

Overview


Polymers


Characteristics

8

Stereolithography

Polymer

Overview


Polymers


9

SynthesisAddition polymerization: The polymer grows by sequential addition of monomers to a reactive site

Chain growth is linearMaximum molecular weight is obtained early in the reaction

Step-Growth polymerization: Monomers react together to make small oligomers. Small oligomers make bigger ones, and big oligomers react to give polymers.

Chain growth is exponentialMaximum molecular weight is obtained late in the reaction

Stereolithography

Polymer

Overview


Polymers


10

Chain-growth polymerization (typically addition polymerization)

Propagation

nAIn A A A A

n

A*

A A A A Am

In A A A An

A

*A A A A Am

Combination

*A A A A Am

In A A A An

A

B A A A Am

Disproportionation

TerminationReactive site is consumed

A

In A A A An

A

A*

Chain TransferNew reactive siteis produced

MWkpropagation

kter mination

MW

% conversion0 100

In*A

InitiationIn A A A A*

Stereolithography

Polymer

Overview


Polymers


11

Chain-growth polymerization (typically addition polymerization)

Ph

Radical

PhCO2• PhCO2

Ph

PhCO2

Ph Phn

Ph

Cationic

Cl3Al OH2H

Ph

HOAlCl3

Phn

H

Ph Phn

HOAlCl3

Ph

Anionic

C3H7 LiC4H9

Ph

Li+ Phn

C4H9

Ph Ph

Li+

n

Stereolithography

Polymer

Overview


Polymers


12

Step growth polymerization (typically condensation polymerization)

Stage 1

Consumptionof monomer

n n

Stage 2

Combinationof small fragments

Stage 3

Reaction of oligomers to give high molecular weight polymer

Condensation polymerization

PolyestersPolyamidesPolyurethanes (addition)

Stereolithography

Polymer

Overview


Polymers


13

Step VS Chain growth

Molecular weight

Stereolithography

Polymer

Overview


Polymers


14

Step VS Chain growth

Step-growth polymerization Chain-growth polymerization

Growth throughout matrixGrowth by addition of monomer only at one end or both ends of chain

Rapid loss of monomer early in the reaction

Some monomer remains even at long reaction times

Similar steps repeated throughout reaction process

Different steps operate at different stages of mechanism (i.e. Initiation, propagation, termination, and chain transfer)

Average molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length

Molar mass of backbone chain increases rapidly at early stage and remains approximately the same throughout the polymerization

Ends remain active (no termination) Chains not active after termination

No initiator necessary Initiator required

Stereolithography

Polymer

Overview


Polymerization


15

Producing radicals

Thermal decomposition: bond dissociation energy ~ 100 – 165

kJ/mol; temperature range 0 – 100C; slowPhotochemical: use energy from photo to break bonds, typically

UV light for high energy photonsRedox reagents: electron transfer reactions involving peroxides,

persulfates, and metallic ionsIonizing radiation: high energy radiation: γ rays, x-rays, α

particles, high speed electrons, etc.; Ionizing radiation: E = 10 keV

to 100 MeV, VS. 1 – 6 eV for visible to UV light; Least practical

A A∙+E

Stereolithography

Polymer

Overview




16

ReactantsMonomers: M; Initiators: IFree radicals: R·; Polymer chains: P·Oxygen: O2; Solvent

Reaction StagesInitiation; PropagationTermination; Inhibition

PMP

PMR

RI

p

p

k

k

hv

2

Initiation

(6)

(7)

(8)

dead

k

dead

k

dead

k

PRP

PPP

RRR

t

t

t

2

Propagation

(9)

(10)

PPP

PRP

p

p

k

k

Termination

(11)

(12)

(13)

dead

k

dead

k

ROP

ROR

tO

tO

2

2

2

2

Inhibition

(14)

(15)

12% conversion is a typically threshold value to be considered as solid

Stereolithography

Polymer

Overview




17

Credit: Dr. David Rosen @ Georgia Tech

Commercial materials: combinations of acrylates and epoxiesAcrylate: acrylic monomer + free radical photoinitiatorEpoxy: epoxy monomer + cationic photoinitiatorAcrylate and epoxy monomers do not react with one another. Result is interpenetrating polymer network.Variety of additives to stabilize resin, adjust viscosity, etc.

H = Hydrogen atomC = Carbon atomR = Radical group

Enables cross-linkingAcrylate Epoxy

AcrylatesFirst photopolymer for SLFree radical polymerizationHigh photospeedIssues of curled, warped parts (5 to 20% of shrinkage)

EpoxyCationic polymerizationMinimal volume change (ring-opening) (1 to 2% of shrinkage)Slower polymerization

Stereolithography

Polymer

Overview




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Commercial SL ResinsPhotosensitizer (increase photo-efficiency)Cationic reactive modifier (improves mechanical properties)Antifoaming agentsLeveling agentsThickening agentsAntioxidant Stabilizers (prevent unwanted reactions)

Acrylate and epoxy hybrids achieve synergyInterpenetrating Polymer Network (IPN)

Acrylate and epoxy monomers undergo independent polymerization.

Acrylate enhances the photospeed and reduces the energy requirement

of the epoxy reaction.

Acrylate polymerizes more extensively in the presence of epoxy;

plasticizing effect.

Hybrid system requires a shorter exposure to be cured than either of the

two monomers taken separately.

Acrylate may decrease the inhibitory effect of humidity on the epoxy

polymerization.

Stereolithography

Polymer

Overview




19


Reaction ratescontrolled by concentrations of photoinitiators [I] and monomers [M].

Average molecular weight of polymers is the ratio of the rate of propagation and the rate of initiation

vo = Rp/Ri [M]/[I]1/2

Ri is the rate of initiationRp is the rate of propagationIncreasing photoinitiator concentration increases polymerization rate, but decreases molecular weight.

Stereolithography

Polymer

Overview




20


Material characteristics:Cd = cure depth = depth of resin cure as a result of laser irradiation [cm] [mils]Ec = critical exposure = exposure at which resin solidification starts to occur [mJ/cm2]Dp = depth of penetration of laser into a resin until a reduction in irradiance of 1/e is reached = key resin characteristic [cm] [mils]H(x,y,z) = irradiance (radiant power per unit area) at an arbitrary point in the resin = time derivative of E(x,y,z).[W/cm2]PL = output power of laser [W]Vs = scan speed of laser [m/s]Wo = radius of laser beam focused on the resin surface [cm]

Irradiance and Exposure

, , , ,0 pz DH x y z H x y e

Beer-Lambert law: irradiance at a depth z

For a Gaussian laser

2 2

02

0, ,0 ,0r W

H x y H r H e

0( ,0)

r

Lr

P H r dA

0 2

0

2 LPH

W

Stereolithography

Polymer

Overview




21


Irradiance and ExposureExposure is the energy per unit area and the time integral of irradiance

( ,0) ( ),0t

tE y H r t dt

2 22

2

2,0 or WL

o

PH r

We

t = W0/Vs

2 202

0

2( ,0)

y WL

s

PE y

W Ve

2 202

0

2( , , ) pz Dy WL

s

PE x y z

W Ve e

Exposure at a given coordinate:

Stereolithography

Polymer

Overview




22


Zone of influenceHow far away from the scan is beyond the scan’s influence?R = region around spot center that receives 99.99% of its exposure

2 221 0.9999oR W

e

R = 2.146W0

The zone of influence is proportional to the beam radius.

Characteristic exposure time

te 2 R/ Vs 4.3 Wo / VsFor laser scan velocity of 100 to 10000 mm/sec and W0 of 100 um.43 ms te 4.3 ms

Stereolithography

Polymer

Overview




23


Critical exposure (a material property)E = Ec

2 202

0

2( , , ) pz Dy WL

s

PE x y z

W Ve e

= Ec

Take logs of both sides to get cured line shape

*2 *

2

22 ln L

o p o s c

Py z

W D W V E

Stereolithography

Polymer

Overview




24


Line widthSet z* = 0 (for cured line shape equation) Lw = Wo (2 Cd / Dp)1/2

Line width is proportional to beam spot size.Line width increases as cure depth increases.

Cure depthExposure along the center of a scan vector is given as

max

20,0 L

o s

PE E

W V

Then, the relationship among Emax, Cd, Ec, and Dp can be derived as: Cd = Dp ln(Emax / Ec)

Stereolithography

Polymer

Overview




25


PhotospeedInformal term for sensitivity of resinResin characteristic, independent of SL machine, laser, optics system, etc.Indicated by Ec and Dp

The faster the resin can be scanned, the higher the photospeed

2d pC DL

s

o c

PV

W Ee

Mechanical properties of cured part Elastic modulus, tensile strength, and other properties improve as resin cures.Acrylates cure first (provide shape); epoxies cure slower to improve properties.Properties also improve with increased exposure, to a point.Then, resins age and properties change over time (weeks, months).

Stereolithography

Polymer

Overview




26


Length scales

dm << l << S << Dp < Wo < R << L

dm = length of polymer molecule 0.001 to 0.007mm,

l = laser wavelength = 0.325 to 0.351 mm,

S = shrinkage length = 1 to 7 mm

Dp = penetration depth = 0.1 to 0.2 mm = 100 to 200 mm,

Wo = laser spot size = 250 mm,

R = zone of influence of laser spot = 2.146 Wo = 540 mm,

L = typical part dimension = 0.1 m = 10000 mm.

Time scales

tt << tk << te << ts,o < ts,c << td

tt = time for a photon to traverse a layer = 10-12 seconds,

tk = photopolymer kinetic reaction rate = ~1-10 us,

te = exposure time for zone of influence = 50 to 2000 us,

ts,o = onset of measurable shrinkage = 0.4 – 1 s,

ts,c = completion of measurable shrinkage = 4 – 10 s

td = scan time for layer = 10 – 300 s.

Stereolithography

Polymer

Overview




27


Scan patternsWEAVE – late 1990STAR-WEAVE – (Staggered hatch, Alternating sequence, Retracted hatch), late 1991ACES – Accurate, Clear, Epoxy Solids, 1993. Necessary for emerging epoxy resins.Prior to WEAVE, scan patterns were ad hoc and did not work well

Stereolithography

Polymer

Overview




28Credit: Dr. David Rosen @ Georgia Tech

WEAVE ScanFirst pass, cure depth Cd(1) is achieved, based on exposure, Emax(1). Second pass, same amount of exposure is provided -- cure depth increases to Cd(2). Cd(2) = Dp ln[2 Emax (1) / Ec] = Dp ln[2] + Dp ln[Emax(1) / Ec]Cd(2) = Cd (1) + Dp ln[2] = Cd (1) + Dp(0.693)Second pass provides enough exposure to cure layer to previous layer.

ACES ScanProvides SLA machine operator with many scan pattern options.Objective: cure more resin in a layer before proceeding to next layer (98%).Accomplished by overlapping hatch vectors.X and Y scans, similar to STAR-WEAVE.

Stereolithography

Polymer

Overview


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additive manufacturing module 10

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