2. RECOATING CYCLE
• Recoating is a process of establishing a new layer
of fresh resin over the previously cured layer.
Although the recoating mechanism in machine
manufactured by different some what. Most SLA
machine use a blade and an elevator.
• A successful recoating step is one that is capable
of establishing a fresh layer of liquid resin of
thickness, with adversely affect the accuracy of
the product.
3. • However owing the effect related to the
viscosity and surface tension properties of the
liquid resin, it is not easy to meet the
required.
• As a result one need to adopt quite a complex
recoating cycle involving several stages.
4. First Stage
• At the start of the recoating cycle, the surface
or the previously completed part is level with
the resin surface level.
5. Second Stage
• The elevator fully immerses the previously
completed part under computer control to
allow the resin to flow over the part.
• Hence this step is sometime called ‘deep dip’.
However the resin of surface will not become
level immediately owing to the high viscosity
of the resin. The resin will take quite
sometime before it assume a level surface
over the entire part.
6. • Hence there will be a depression in the resin
surface above the part. The depression can be
quite significant over regions with trapped
resin volume.
• Such volume occur when they are pockets of
liquid resin within the part interior that do not
have connecting passage to the rest of the
liquid in the vat.
• In time, the resin fills the depression under
the influence of gravity and the resin surface
become level once again.
7. Third Stage
• It start once the depression closed reasonably.
The platform is elevated until the top of the part
is above the resin surface.
• As a result there will be layer of thickness
exceeding Lp, resting on part’s upper surface.
This excess resin needs to be removed by
executing a scrapping step immediately after the
platform is elevated.
• The blade is moved over the part at a certain
velocity and with gap equal to Lp between the
bottom of the blade and the top of the part.
8. • The gap is controlled by the height of the
elevator, if the layer thickness created at this
stage significantly deviated from Lp, the accuracy
and quality of the final part will be deteriorate.
• If the layer is too thick, adjacent layer may not
even attach to each other. If the part is too high.
• The blade might hit the part during the scraping
process since the part top surface has already
been elevated above the resin level.
• The rest of the resin is not disturbed during the
scrapping process, for many part geometrics, the
optimum scraping period is about 5second.
9. Fourth Stage
• The elevator is lowered against such that the part
is in the right position for scanning the next layer.
• The resin surface at the beginning of this stage is
still disturbed. In particular owing to finite
surface tension effect there is usually a visible
crease at a solid to liquid interface around the
perimeter of the part.
• Experiment have shown that this crease fades
away exponentially over time. Hence a waiting
period is introduced for resin surface to blend
and reach the surface.
10. Fifth Stage
• The Z wait period is usually determined through a
compromise between surface non-uniformity and
built time.
• The problem of non-uniformity can be
particularly problematic when thin layer are
used.
• Therefore one need to choose a longer Z wait
when large Lp is chosen. These observation might
give impression of unduly long Z wait.
• However fortunately with current technology it is
possible to complete computation and
adjustment of resin level layer with one second.
14. • FDM is second most widely used rapid
prototyping technique after SLA.
• The technology was first developed in 1988
and patented in 1992.
• A linear thermoplastic filament is unwounded
from a coil under the action of a pair of drive
wheels and supplied via resistively heated zone
(liquefier) to the extrusion nozzle.
• The nozzle squeezes out the fused filament in a
manner analogous to a toothpaste tube and
deposits the material at the desired location
on the partially constructed part.
15. • Polymer flow through the FDM tip is not by
simple shear but a combination of shear and
elongation, so it becomes necessary to
recognize the viscoelastic nature of the
polymer.
• The original filament materials were polyofin
and polyamide, more recent materials are ABS
and modified high impact ABS material.
• The material is heated to slightly above its
flow point, so it solidifies quickly, The
incoming cold filament acts as a piston and
pushes the liquefied material forward towards
the FDM tips.
16. • The tip is nozzle with a mechanism to allow
the flow the melted plastic to be turned ON or
OFF.
• The nozzle is mounted on a mechanical stages
that can be moved in both horizontal and
vertical direction the location of nozzle over
the work area is controlled in accordance with
the .stl file of the part supplied to the
machine.
• Since entire system is contained within a
chamber held at a temperature slightly above
its flow point but below to its melting point.
17. • Only a small amount of additional thermal
energy is need to be supplied by the nozzle to
cause the plastic melt.
• This provide much better control of process.
As a result it is possible to form short
overhanging features. However in general
supports need to be planned and included in
the .stl file.
• In particular, down facing surface at angle
smaller than 45˚ from the horizontal plane
need to be supported.
18. • When a dual or multiple head FDM machine is
available, the support can be built from a softer
material so they can be broken away easily.
• Alternatively, they can be built from a water-
soluble material so that they can be simply
washed away after part building had been
completed.
• FDM parts can be exhibit internal defect of a
several forms, voids, pores, delamination, cracks
etc., these can arise from several reasons, eg:
FDM system hardware and software limitation,
poor material characteristics and poor process
optimization.
19. • Material shrinkage associated with
temperature and phase changes can make the
accuracy of FDM parts.
• The main process variable that FDM process
planner needs to consider are:
– Build orientation
– Layer thickness
– Road width
– Air gap
– Build temperature
20. Build orientation
• This decision need to be made within the CAD
environment. The important consideration are
part Z-height, the angular orientation of each
facet with respect to xy plane.
• As in the case of SL and SLS different part
orientations are evaluated for their impact on
total build time including data preparation
time, material deposition time and processing
time.
21. Layer Thickness Lp
• It refers to the travel in the Z-direction of the
table. Usually, different layer thickness value
are associated with different FDM tips.
• As in any layer manufacturing process, the
stair-stepping effect increases with increasing
Lp.
• Owing to Stair- stepping the volume of the
actual part will be smaller than the volume of
the part as specified in the CAD model.
• Since larger volumetric error implies larger
dimensional errors.
22. Road Width
• It refers to the width of the position path.
Different tips and machine have different road
width.
• Usually Wb > Lp, hence the bead cross-
sections can be assumed to be elliptical.
23. Air Gap
• It is the space between two horizontally
adjacent denser structure.
• Positive air gap leads a loosely packed
structure.
• Larger gaps lead to longer build times.
24. Build Temperature
• It refers to the temperature of the heating
element that control the viscosity of the
extruded plastic.
• If the temperature is too low, the filament can
while emerging from the tip.
25. Materials
• Several material are available for FDM process
investment casting wax, nylon, polypropylene,
polycarbonate, poly-phenylsulfone,
Acrylonitrile butadiene styrene(ABS) and
polymer bound advance ceramics including
Bio-ceramics and polymer bound metals.
• Fused Deposition Ceramics (FDC) uses ceramic
materials which is blended with thermo plastic
polymer.
26. Strength
• Machine is less expensive.
• No post curing is required.
• Process can be carried out unattended.
• These is little wastage of material.
• Variety of material can be used and material
change over is very fast and simple.
27. Weakness
• Automotive
• Aerospace
• Consumer Products
• Electronics
• Biomedical
• Other industries
Application
• Process is slow on bulky parts.
• Strength is low in vertical direction
