RECOATING ISSUES

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.

• 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.

First Stage

• At the start of the recoating cycle, the surface or the previously completed part is level with the resin surface level.

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.

• 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.

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.

• 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.

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.

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.

FUSED DEPOSITION MODELLING

• 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.

• 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.

• 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.

• 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.

• 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.

• 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

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.

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.

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.

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.

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.

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.

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.

Weakness

• Automotive• Aerospace• Consumer Products• Electronics• Biomedical• Other industries

Application

• Process is slow on bulky parts.• Strength is low in vertical direction