Mumbai university > mechanical engineering > sem 7 > cad/cam/cae

Marks: 6M, 10M

Difficulty: Medium

Selective Laser Sintering

In Selective Laser Sintering (SLS) process, fine polymeric powder like polystyrene, polycarbonate or polyamide etc. (20 to 100 micrometer diameter) is spread on the substrate using a roller.

Before starting CO2 laser scanning for sintering of a slice the temperature of the entire bed is raised just below its melting point by infrared heating in order to minimize thermal distortion (curling) and facilitate fusion to the previous layer.

The laser is modulated in such a way that only those grains, which are in direct contact with the beam, are affected. Once laser scanning cures a slice, bed is lowered and powder feed chamber is raised so that a covering of powder can be spread evenly over the build area by counter rotating roller.

In this process support structures are not required as the unsintered powder remains at the places of support structure. It is cleaned away and can be recycled once the model is complete.

Fused Deposition Modelling

In Fused Deposition Modelling (FDM) process a movable (x-y movement) nozzle on to a substrate deposits thread of molten polymeric material.

The build material is heated slightly above (approximately 0.5 C) its melting temperature so that it solidifies within a very short time (approximately 0.1 s) after extrusion and cold-welds to the previous layer as shown in figure.

Various important factors need to be considered and are steady nozzle and material extrusion rates, addition of support structures for overhanging features and speed of the nozzle head, which affects the slice thickness.

More recent FDM systems include two nozzles, one for part material and other for support material. The support material is relatively of poor quality and can be broken easily once the complete part is deposited and is removed from substrate.

In more recent FDM technology, water-soluble support structure material is used. Support structure can be deposited with lesser density as compared to part density by providing air gaps between two consecutive roads.

3D Printing.

The terms Rapid Prototyping and 3d printing are often used along each other. And they have similarities. For example, both rapid prototyping (RP) and 3D printing technologies build models layer by layer from STL data. But there are still some differences.

3D printers usually make smaller parts.

Most 3D printers are limited to making parts that will fit roughly in a cube 8 inches on a side. On the other hand, rapid prototyping machines provide a build chamber at least 10 inches on a side, and some have build chambers approaching 3 x 3 x 2 feet and much larger. Being smaller on the outside means there’s less room to build parts inside.

A smaller build envelope also means that it’s not possible to make as many parts at the same time. That’s lower throughput efficiency than an RP machine, but not usually a concern for the applications 3D printers are intended for.

3d Printing costs less

The cost difference per part between 3D printing and rapid prototyping systems can be significant. Including material, machine depreciation, system maintenance and labor, a part built using rapid prototyping technology can cost nearly twice as much compared to 3D printing.

3D printers are cheaper to maintain and feed.

You can expect to spend several hundreds to a few thousand dollars per year to maintain a 3D printer and keep it fed with materials, but it costs several tens of thousands of dollars each year to maintain a rapid prototyping system. Simply replacing a laser in a stereo lithography machine can cost $20,000, and filling a big vat with photopolymer can easily cost more than $50,000.

Less Material choices for 3d Printers.

Although the list of materials is growing each month, 3D printers don’t provide the same range of materials as RP machines. Some classes of materials such as ceramics and metals are not available at all.

But it’s possible to make adequately functional parts for many applications, and the materials available are certainly appropriate for concept modeling, a frequent use of 3D printers. At the hobbyist level some thermoplastic extrusion machines are actually capable of using a wide range of plastics and material costs are a lot lower.

3d Printers are less complex, and easier to use

Usually 3d printers require much less or even no training at all in contrast to rapid prototyping machines. It’s possible to be making parts pretty much right out of the box with some professional-level technologies. And the simplicity comes at the price of flexibility.

Unlike Rapid Prototyping machines, you may not be able to adjust or select many build parameters, or change them on the fly. Also, be forewarned that at the low-cost hobbyist level the equipment is not very “Plug & Play” yet.

Some of what you save by buying one of these very low cost machines is likely to be offset by the extra time required to get the equipment running reliably and learning to use it.