050 mm and surface finish are the best amongst all the processes
o Model building can take place unattended
o Capable of high detail and thin walls
o SLA is also used in direct Rapid Tooling through QuickCast
Limitations: The disadvantages of using this technology are
o Experience and expertise is required in deciding support structures. The model has to be modified for this purpose
o Material is toxic and hazardous
o Part strength is less and may undergo warpage in presence of excess moisture
o Post-curing of the part is required and may result in slight distortion
o The material has a finite shelf life and needs to be replaced (even if unused) after a period of about two years
o The part becomes brittle over a period of time
Solid ground curing (SGC)
SGC also makes objects out of photo-polymer. Instead of drawing out each cross-section by a laser light (as done in stereo-lithography), an entire cross-section in a single operation is exposed using photo-masks in SGC. Hence, the productivity of this process is many times that of SLA. This approach was originally developed and commercialised by Cubital of Israel. In SGC, each cross-section is imaged onto an erasable mask plate produced by charging the plate via an ionographic process and then developing the image with an electrostatic toner (like the photocopying process). The mask is then positioned over a uniform layer of liquid photo-polymer, and an intense pulse of UV light is passed through it to cure the material in the unmasked zones.
Uncured photo-polymer is removed from the layer with a vacuum system and replaced with a low melting point, water-soluble wax that serves as the sacrificial support. After the wax has cooled, the layer is milled to produce a flat surface. As can be seen from the Figure 3, this process involves two cycles, one for the preparation of the mask and the other for the preparation of the physical model and support parts of the layer.
Wiping off the toner erases the pattern on the exposed mask, and the entire process is repeated. After the part has been completed, the wax is removed by melting. The various processes used to implement SCG are performed at different stations. A unique feature of the photo-masking approach is its capability to build multiple parts in a single batch by packing them within the working volume.
Advantages: The benefits of this process can be listed as follows:
o This is one of the fastest processes since curing takes place simultaneously at all desired points
o Its high productivity owing to its ability to build a set of nested parts makes it suitable for service bureaus. Build time is independent of the number of parts being made at a time. Therefore, it can act as a production machine
o No external support structures are required
o No warping or curling of the part takes place, as there is no post-curing operation
o Large variety of photo-polymers can be used for building the parts
o Accuracy of the parts is good
Limitations: The limitations of this process can be listed as follows:
o There is lot of wastage of material and wax. The resin picked up by the aerodynamic wiper and vacuum during the milling process cannot be reused. Additionally, the material, which does not form the part of the model, but gets exposed to the UV light, needs to be replaced. Only fifty per cent of this affected material can be converted into usable form
o The cost of the machine is the highest amongst all RP machines since it involves several systems such as photo-masking system, vacuum system, milling system etc
o The process operation is complex and maintenance cost is high since it has several sub-systems
o It requires a huge compressor and its operation is noisy. Therefore, this machine is not suitable for office environment
o Monitoring of the building process is required
o Material has a finite shelf life and needs to be replaced after a certain period, even if not used
o Wax is sticky and difficult to remove
Selective laser sintering (SLS)
SLS process was originally developed at the University of Texas, Austin, USA and then commercialised by DTM Corporation, USA. It was subsequently developed and marketed as EoS, Germany. In SLS, a layer of powdered material is spread out and leveled over the top surface of the moving structure A laser then selectively scans the layer to fuse the areas defined by the geometry of the cross-section; the laser energy also fuses the layers together. The material, which doesnt fuse remains in place as the support structure. After each layer is deposited, the platform lowers the part by the thickness of the layer, and the next layer of powder is deposited. When the shape is completely built up, the part is separated from the loose supporting powder. Several types of materials are in use, including plastics, waxes, and metal alloys with low melting temperature. This process has been successfully used for making steel die inserts for short run production. For making steel dies on DTMs SLS machine, the raw material is steel powder with each steel particle coated with a polymer that acts as binder. The same machine is used for non-metals as well as steel prototypes. During the operation, only the binder coating is fused keeping the particles together. In the end, a green part is obtained. This green part is put in a special oven to complete the sintering. The binder evaporates leaving it a porous part. Subsequently, it is put inside another chamber for several hours to impregnate the pours with copper. Copper impregnation is required to get dense parts and good polish.
In EoSs SLS process, there is one machine for each material, EOSINT-P for polymer, EOSINT-C for ceramic and EOSINT-M for metallic prototypes. There is no binder coating on the metallic particle and the metallic powder is not strong steel but one with a lower melting point. The laser used for making metallic parts is sufficiently powerful to fuse the metallic particles. The metallic particles in EoS process apparently do not require post-sintering as well as copper impregnation. However, its laser is more powerful. For making ceramic moulds, the sand particles are coated with a binder as is done for steel tools in the case of DTMs SLS process.
Advantages
o Any material that can be converted into powders and can be bonded together by fusing its particles at a reasonably low temperature (about 350-500 °C) can be used for making the parts in SLS process. Materials commonly used for making parts in this process are nylon, ABS and investment casting wax (ICW)
o This is the only commercially available direct RP process to make prototypes out of metals. Hence, this is useful for toolmakers
o This can also produce ceramic moulds cavities directly and hence there is no need for patterns
o Parts obtained are tough
o No external support structures are required
o No post curing is required for non-metals. Only metal parts require sintering
o Functional metal and ceramic parts can be obtained
o There is no wastage of material
Weaknesses
o This is one of the costliest processes
o Surface finish of parts is grainy
o Parts are porous in nature
o The building operation needs to be monitored
o Long time is required to heat up the material chamber before building the parts and to cool it down after the building is over
o The parts are brittle
Fused deposition modeling (FDM)
FDM was first developed and commercialised by Stratasys Inc, USA. In this approach, a continuous filament of a thermoplastic material (polymer or wax) through a resistively heated nozzle is deposited to fill the contours of the desired slice. An explanatory sketch of the FDM process is shown in Figure 5. The raw material is in the form of a wire of about 3 mm diameter. Using a pinch wheel drive, it is fed into an extrusion chamber, which is kept at a temperature slightly above its flow point. The thermoplastic wire itself acts as the piston initially in the extrusion chamber, which subsequently gets melted and pushed out through the nozzle. The filament coming out of the nozzle solidifies relatively quickly after it exits the nozzle. It is possible to form short overhanging features without the need for explicit support in this process. In general, however, explicit supports are needed. These support structures are drawn out as thin coarse wall sections that can easily be removed upon completion. There is a separate extrusion head for depositing support material.
Advantages
o The process is very simple and the machine is less expensive
o A variety of materials can be used and the material changeover, which involves only changing the head, is very fast and simple
o No post-curing is required
o There is little wastage of material
o The part building can be carried out unattended
o The material has a large shelf life and remains unaffected if not removed from the packing provided.
Limitations
o Surface finish and delicate features are inferior to other processes
o The process is slow since the entire contour is to be filled
o The strength is low in the vertical direction
o Accuracy and surface finish is poorer as compared to the other RP processes.
Rapid tooling (RT)
Another important phase of product development is the manufacture of production tooling. Characteristically, all tooling require quite a long time for production and reasonable investment. So, many efforts have been put in to improve this phase to achieve savings in time and cost. Traditional sand casting has limitation of simplistic forms and dimensional accuracy. Investment casting offers better potential in terms of complex form geometry, better dimensional accuracies, and excellent surface finish. Machining processes for metal removal using conventional machine tools or latest NC machines, EDM, wire cutting machine and so on, give very high dimensional accuracies but these machining operations are time consuming and costly.
World over, manufactures and toolmakers are shifting from routine methods to rapid tooling for prototype and production purposes. This not only saves time and money but also gives a competitive edge at the crucial stage of product development. There are two main application areas of rapid tooling: Soft tooling, using silicon RTV moulds, epoxy tools or spray metal tooling for manufacturing 10 to 1000 plastic parts. Using conventional moulding processes, these moulds could also be used for metal parts to be made by investment casting or by centrifugal casting of low melting alloys of zinc and aluminium. Currently many companies are offering systems and technologies for making these soft tools.
The second area related to hard tooling for production purpose is being explored. Using quick cast SLA parts or investment casting frequency division multiplexing (FDM) patterns; one can cast accurate core and cavity tooling by shell investment casting process. The rapid prototyping parts could also be used for electroformed nickel tools or spray nickel shells. One more route exploited is for making electrodes for electrical discharge machine (EDM) of moulds for plastic parts. By using RP model as a pattern, a female abrasive filled epoxy die is made. This is used to grind away the graphite block for preparing graphite electrode for use in EDM of moulds.
The potential rewards of rapid tooling are enormous and newer avenues are being explored everyday.
Benefits of rapid prototyping
To the product designer: The product designers can increase the part complexity with little effect on lead-time and cost. They can optimise part design to meet customer requirements, with little restrictions like material wastage or large thin walls, which are otherwise imposed by machining operations. There is improved design creativity and valuable feedback on the design can be obtained from various sources.
To the manufacturer: The manufacture can avail of the benefits like early realisation of profit, reduction in cost due to reduction in wastage and scrap, reduced labour content, reduced inventory, assembly and inspection costs due to reduction in parts count. All testing and design changes are carried out before spending any money on tooling. Design problems can be solved before they become tooling glitches or manufacturing fiascos. The RP parts act as important communication tools between the designers, engineers, and vendors. Different people interpret 2-D drawings differently and clearing this confusion costs valuable time. When everyone can look at the part itself, inspect it, even touch it - design concepts are clarified and misunderstandings disappear. Design errors like thin walls, misaligned apertures, and inner walls crossing outer walls, etc cannot be identified in 2D drawings or 3D models. RP helps in locating and solving these problems early before costly scrapping or reworking of tools is required. The tooling can be done only when the concept is refined and the data is verified. In short, rapid prototyping and tooling are manufacturing tools that enable industries to optimise information at front end of design and manufacturing process and thereby work smarter and faster.
To the marketer: The market greatly benefits from these techniques because of the advantages like reduced time to market, reduced risk of product failure in the market, manufacturing of products that meet customer requirements and possibility of test-marketing of new products. Limited editions of a variety of conceptual prototypes of the same product can be launched followed by mass production of the most successful ones.
To the consumer: The consumer can buy products, which meet individual needs and wants. There is a much wider diversity of offerings to choose from.
Applications of RP&T
Prototypes made using rapid prototyping & tooling (RP&T) systems find applications as:
Concept models: Designers always prefer to present form ideas in mock-up models or prototype of product, for final presentations. Quick RP models become very handy for this purpose. Though little expensive, the time saved is enormous and one can make very intricate details even like snap-on to demonstrate the working of fitments.
Models for market research: With RP technology, several different variations in design models can be made simultaneously and the best one can be selected. Few accurate copies of the selected one could be produced in a short time to get a feel of the market.
Rapid tooling: Toy manufactures are exploiting rapid tooling methods to make injection moulds to introduce a number of designs in the market in peak season. Final hard tooling is done only for those toys, which perform best in the market.
Tender model: Many automotive companies have started realising the advantages of providing a physical three-dimensional part model to a sub-contractor for a quote. This helps the vendor to decipher the CAD file quickly and assists them in deciding the parting lines, which is a very important step in tool design.
Wind tunnel models: The models of planes, automobiles, trains, buildings, and structures are tested for performance in wind tunnels. Accurate RP models can help in obtaining reasonably good results.
Models for stress analysis: Many RP models are directly used for experimental stress analysis and other analytical methods.
Medical applications: RP parts are being increasingly used to manufacture one-off replicas of bones. The scanned (CT or MRI) data of affected bone areas is used to prototype the part, which in turn is used to create ceramic shells for investment casting of metallic replacement parts.
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