Rapid Prototyping 1

RAPID PROTOTYPING

STEREOLITHOGRAPHY

Presented By

SHIYAS.M.M

First Semester

M- Tech ( CIM )

MCET

RECOATING ISSUES

Recoating cycle Resin level control Gap control

RECOATING CYCLE

Recoating is the process of establishing a new layer of fresh resin over the previously cured layer.

A successful recoating step is one that is capable of establishing a fresh layer of liquid resin of thickness exactly equal to the desired thickness , Lp , within a reasonable short time.

Figure schematically illustrates a recoating cycle involves six stages ( a ) to ( e )

A RECOATING CYCLE

RECOATING CYCLE

At the start of the recoating cycle ( figure – a ) , the surface of the previously completed part is level with the resin surface level .

In the next stage , the elevator fully immerse the previously completed part under computer control to allow the resin to flow over the part ( figure- b ) .

Tcl ∞ Rcr µ / hd²

RECOATING CYCLE

The third stage starts once the depression has closed reasonably. The platform is elevated until the top of the part is above the resin surface. Figure – c shows the configuration at the end of this scraping process.

Next the elevator is lowered again such that the part is in the right position for scanning the next layer figure – e.

RECOATING CYCLE

A waiting period ( called ‘ Z ‘ wait ) is introduced for the resin surface to blend and reach the configuration shown figure – f.

ISSUES IN SCRAPING

ISSUES IN SCRAPING

Figure – a schematically illustrates resin flow while scraping over a solid substrate.

The blade width is Wb and Vb is the blade velocity.

L=g/2 Figure – b shows the experimentally observed

resin flow pattern when a blade scrapes over a trapped volume.

Resins with lower viscosity result in thicker layers.

RESIN LEVEL CONTROL

The liquid resin is levelled at the beginning of the part build sequence to ensure optimum laser focus.

A typical linear cell used in SL machines is about 5mm long and 2mm wide.

The typical laser spot size is only about 0.2mm. After a layer has been built , the level sensor

checks the resin level.

GAP CONTROL

Initially the blade is set at a baseline gap. Calculate the required up or down offset from

the baseline. The spot size is linearly related to the distance of

the resin level from the location of the laser beam’s waist.

It must be ensured that the level of resin is within the focal plane limits of the laser imaging system.

CURING AND ITS IMPLICATIONS

Degree of curing and green strength Effects during post curing

DEGREE OF CURING AND GREEN STRENGTH

Degree of cure ( % ) = ( 1- Hp / Hliq ) * 100

Where Hp and Hliq are the quantities of thermal energy released from the laser cured specimen and liquid resin respectively.

Y = Kp ( E-Ec ) | ( E / Ec -1 ) = small number

Where Y , photo modulus, Eex , excess exposure

Eex = E - Ec

Green flexure modulus ( GFM ) GFM = kwv ( Cd – Cdo ) { 2Wo / hs }²

Where kwv is the resin green strength constant for the WEAVE build style, and Cdo is the intercept on the x- axis of the extrapolated curve.

Generally , the GFM values of epoxy based resins have been found to be much larger than those of acrylate- based resins.

EFFECTS DURING POST-CURING

Post curing apparatus produced in the period 1988-1989

Mainly used UV emitting mercury arc lamps working at high pressure and irradiance values.

Actinic fluorescent lamps. Maximum temperature at post-cure is reduced to

about 45ºC. Thermal stresses and post-cure distortion are

reduced Improving part accuracy.

Maximum post-cure time ( tpc ) max is given by

( tpc ) max =1 / Vpc ( 3Mp / 4πρ ) 1/3

Where Mp is the mass of the part , and ρ is the density of the resin.

PART QUALITY AND PROCESS PLANNING

Shrinkage, Swelling , Curl and Distortion Surface Deviation and Accuracy Build Styles and Decisions Build-time and Build-cost Functional Prototyping using SL

SHRINKAGE, SWELLING, CURL AND DISTORTION

During polymerization , the resin can undergo between 5%v and 7% volumetric shrinkage.

Shrinkage leads to increase in part density. The degree of shrinkage strongly influenced by

the resin itself, part building style, and operational parameters.

A method of investigating post-cure shrinkage is the ‘Nine – Box ‘ method developed by 3D systems.

The nature of chemical reactions occurring during polymerisation can significantly affect shrinkage.

Swelling is usually specified as the percentage change in a dimension resulting from a 24 hour immersion of the part in liquid resin.

A major consequence of thermal and shrinkage effects in part distortion immediately after laser irradiation and after post-curring.

Curling is most severe over unsupported regions. Ϩ= Rc { 1-cos (l / Rc ) }=sl²/(2a) The curled layers can be modelled as an arc of

radius Rc = a/s where a is the layer thickness and s is the linear shrinkage.

Ϩ, distortion distance over an unsupported part region of horizontal length l.

SURFACE DEVIATION AND ACCURACY

Figure illustrates the stairstepping problem associated with layered manufacturing process in general.

The deviation of the actual surface from the desired surface increases with increasing value of Lp and profile angle σ.

Other methods of post-cure finishing include barrel tumbling, centrifugal tumbling, vibratory finishing, abrasive blasting, or abrasive flow finishing.

BUILD STYLES AND DECISIONS

While building a part , the borders are drawn first , and then the interior is drawn using one of the several available hatching patterns.

WEAVE TM STAR WEAVE TM ACES ACES is a more recent building style that aims

to produce Accurate Clear Epoxy Solid parts with highly improved dimensional stability.

BUILD-TIME AND BUILD-COST

Build-time estimation and control is of paramount importance.

Build time= recoat time + support draw time + border draw time + hatch draw time

Total part cost = labour cost + software cost + maintenance cost + laser cost +depreciation + resin cost + overheads

FUNCTIONAL PROTOTYPING USING SL

Reinforcement of plastics for improving the mechanical properties of parts has long been in practice.

SL has been used to produce ceramic parts whose mechanical , thermal , and electrical properties are close to those obtainable by conventional ceramic techniques.

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