Q.4 Explains subtractive rapid prototyping process and compare with additive process.

                                                  OR

What are additive and subtractive processes? Give explains of both. Describe rapid prototyping operation. State its applications.

Ans. Additive process : The raw material for the manufacture of PCB by additive or semiadditive technique is basically an unclad laminate and copper which is pattern plated selectively only where it is the fully additive process, while load on etching is brought to a minimum in the semiadditive pattern plating technique.

         Selective plating on conducting areas is carried out by several methods such as :

(a) Selective desensitization of areas of the catalyst coated surface by exposure to light of suitable wavelength.

(b) Mechanical transfer of the sensitizer in the conducting path by image transfer by printing (stamping), screen printing or offset printing method.

      Subtractive process : This is the most conventional method of getting the circuit pattern. The basic material for fabrication is a laminate with copper foil bonded to the substrate. Through holes are drilled/.punched in these clad laminates. The image of the circuit pattern is produced either by printing and screen printing with ink resist or by photofabrication techniques.

       Rapid prototyping operation : In rapid prototyping techniques, real objects are produced directly from part models in the computer by using additive fabrication techniques like stereolithography for producing polymer based parts, selective laser sintering. For producing functional metallic parts, spray metal deposition for directly producing the tooling and moulds required to make pats, layered manufacturing technologies, 3D printing for producing ceramic moulds, metal dies and end use parts made of metal matrix compositions or ceramic directly form CAD models without intervening step or conversion. These techniques lead to much shorter product development cycle due to

(i) Faster and easier production of 3D physical prototype for checking the form, function and fit of the product and parts before finalizing design.

(ii) Feasibility of producing the end use functional parts directly from the CAD model.

          Rapid prototyping is the process of rapid creation of a physical solid model from the CAD drawing without the use of tools or traditional manufacturing processes, thus reducing the cycle time in manufacturing considerably. Stereolithography technique is used for this purpose. Stereolithography apparatus accept surface or solid model CAD data through a five depresented in a specific formal. The apparatus is programmed slice the CAD file into 2D cross sections of the object using computer generated sectioning process. A bath of photo sensitive resin is used so that when ultraviolet light strikes it, that position of resin is solidified. The apparatus

Directs a laser beam of UV radiation of appropriate power and wavelength and focuses it onto the surface of bath so that light beam draws the shape of the slice onto the liquid resin which gets solidified. The process is repeated, layer by layer until the three dimensional object is completely built. Thus through multiple layer solidification, the object is built over a span of time period depending on the resin used, the power of the laser, the exposure time and the configuration of the object. After last layer is completed, the created object is raised above the liquid resin, the excess or trapped resin is drained into the container. Cleaning solvent is used to wipe away any excess resin. The object is then fully polymerized using ultraviolet radiation in a special post curing device. Other techniques of producing prototype rapidly are selective laser sintering techniques, solid ground curing, fused decomposition modeling and laminated object manufacturing.