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The Research Direction of FDM in the field of Accuracy & surface finish are mainly categorized in the development of Accuracy improvement, Surface Roughness, Slicing, Part Orientation and Part Quality.
4.1 Accuracy Improvement:
In 2010 Masood S.H and his team evaluated and validated the shape accuracy of fabricated models using FDM. They have observed the High level of conventionality with regard to the surface feature of the anatomical part.
Chang D -y and Huang B -H are made some studies on Profile error and Extruding aperture for the RP parts using the FDM process, where their studies explored the effects of extruding parameters, including contour depth, part raster width, and raster angle, on quality characteristics by Taguchi's method.
For Accuracy improvement Sood AK and his team studied the effect of five parameters such as layer thickness, part build orientation, raster angle, air gap, and raster width along with their interactions. Their experimental results point out that the dimension of the measured part at each time is above the anticipated value beside the thickness direction however the length, width and diameter of the hole of test part are less than the anticipated value. So as to increase the complete dimensional accuracy they liked using artificial neural network (ANN).
This time Sood AK and his team are done some more work on the effect of five parameters on three responses such as tensile, flexural and impact strength of test specimen. Those tests were conducted on Central Composite Design (CCD) based and validity of models was tested by using Analysis of Variance (ANOVA). In conclusion, concept of desirability function is used for maximizing all responses simultaneously.
Anoop and his team observed that the shrinkage is dominant along length and width direction of built part. They also observed the positive deviation from the required value in the thickness direction. Finally they adopted grey Taguchi's method to gain optimum level of process parameters to minimize percentage change in length, width and thickness simultaneously.
Pennington and team examined the dimensional accuracy of the parts produced by FDM. They have taken several parts made up of acrylonitrile butadiene styrene (ABS) with six features commonly created by stratasys FDM2000. They have done analysis of 12 different dimensions on parts and they found that the part size, location in the work envelop and envelop temperature had a major effect on the dimensional accuracy.
Gregorian and his team improved the dimensional accuracy of the parts produced by the prototyping machine FDM1650 by showing the optimal shrinkage compensation factors (SCF). By analysing several results they concluded that the best SCF for the machine FDM1650 was 1.007 or 0.7%.
4.2 Surface Roughness:
Galantucci and his team worked on the improvement of the surface finish by performing chemical dipping based on immersion in a dimethyl ketone- water solution. In their process they have been examined tensile and bending mechanical properties by designing and performing four composite designs (CCDs) of experiments. Their results have been verified by testing an FDM marine turbine blade employed to generate energy.
This time Galantucci and his team studied the effects of FDM machining parameters on ABS prototypes surface finish. They concluded that the chemical post treatment does not require human intervention and it has led to a significant improvement in surface finish at the cost of a negligible change in the prototype size.
In the year 2009 Daekeon and team developed a new approach to model surface roughness in FDM. They have done a theoretical model to prompt surface roughness distribution according to changes in surface angle is presented by considering the main factors that significantly affect surface quality. And their proposed expression was verified by implementation and comparison with empirical data.
Wang and his team have done some research on how to improve and enhance the surface micro-precision of the parts fabricated by FDM. They have evaluated the equations of surface roughness in according to the FDM sample parts with special design for experimental measurement and to a physical model reflecting the outer shape characters of a FDM prototype. And they have compared the values of the measuring surface to the calculation values. Moreover they have analysed and studied the surface roughness deviation between the measuring values and calculation values. And they proposed some solid measures to reduce the surface roughness of the FDM parts based on the influencing principles of FDM process parameters and special post processing of FDM prototype parts.
In the Year 2002 Pulak and his team made an attempt to reduce the surface roughness in the FDM made parts. They have developed a simple material removal method namely Hot Cutter Machining (HCM). They have adopted a fractional factorial design of experiments, with two levels and four process parameters to understand the effect of various process variables. They were used ANOVA to find the significance index for process variables and confidence level for the statistical model developed for the surface roughness of hot cutter machining surface. Finally they have concluded that the proposed machining method was able to produce surface finish of the 0.3µm with 87% assurance level.
Perez has studied the roughness parameters gained through the use of the FDM manufacturing process. The roughness average (Ra) for the results of experimental analysis of the parts made from FDM and root mean square (RMS) average (Rq) were obtained through the use of the FDM manufacturing processes. He was also studied the dimensional parameters in order to determine the capability of the FDM process for manufacturing parts.
Kun tong and his team were using the stereolithograhy (STL) file-based compensation method to a new slice file format compensation approach for FDM. In this approach the confused effects of all errors in a FDM machine were mapped into a virtual parametric machine error model. In their research a 3D artifact was built on the FDM machine and differences between its actual and nominal dimensions were used to estimate the coef¬cients of the error functions. A slice ¬le compensation method was developed and tested on two types of parts as a means for further improving the error compensation for feature form error improvement. The two compensation methods were compared. Finally compensation method applied to slice ¬le format is developed for FDM machines and its limitations were explored. Based on the experimental study, dimensional accuracy of parts was considerably improved by the software error compensation approach.
Pandey and his team were proposed adaptive slicing procedure to increase the part surface quality. In that procedure they found that the RP parts are implicitly assumed as rectangular but in reality the edge profiles of a RP parts are parabolic in case of FDM. So, they approached to an adaptive slicing based on the realistic build edge profile which was implemented using two approaches namely direct slicing and tessellated model (STL). In comparison to the earlier approaches on cusp height and area deviation using the rectangular build edge profiles, it was seen that the present technology can reduce the number of slices and hence build time. The major advantage of the present methodology was expressed in terms of standard Ra value.
Justin and his team developed a new effective approach to adaptive slicing. This method fabricates all parts and part- features independently of one other. This enables signi¬cant reduction in fabrication times relative to conventional adaptive build layer thickness techniques. The new approach was implemented on a FDM 1600 rapid prototyping system, together with a revised system calibration to ensure smooth surface transitions between dissimilar build layer thicknesses. They concluded that to implement local adaptive slicing on FDM 1600 rapid prototyping system, it was necessary to increase the extrusion temperature to prevent delamination when fabricating with thin build layers, and revise the calibration tables to enable accurate and smooth part-surfaces.
4.4 Part Orientation:
Masood and his team developed a methodology for computing the volumetric error for any orientation of the parts built by the FDM system. They have developed a mathematical technique to determine the optimum part orientation in RP processes based on minimum volumetric error values using the primitive volume approach. The methodology has been shown to work for various primitive volumes and for simple parts made from the primitives such as cylinders, cubes, spheres and pyramids. This procedure has also been veri¬ed experimentally for parts built on the FDM rapid prototyping system.
Thrimurthulu and his team approached to determine the optimum part deposition orientation in FDM process. The two challenging objectives namely build time and average part surface roughnesses were minimized by minimizing their weighted sum. They have used a real coded generic algorithm to achieve the optimum solution. They were also used adaptive slicing method simultaneously to get the optimum solution. They have concluded that the proposed methodology can be used to determine the optimum part deposition orientation for any complex part that might be completely freefoam.
4.5 Part Quality:
Q. sun and his team investigated the mechanisms controlling the bond formation among extruded polymer filaments in the FDM process. The bond quality was evaluated through measuring and analysing changes in the mesostructure and the degree of healing achieved at the interfaces between the adjacent polymer filaments. Experimental measurements of the temperature pro¬les were carried out for specimens produced under different processing conditions, and the effects on mesostructures and mechanical properties were observed. Parallel to the experimental work, predictions of the degree of bonding achieved during the ¬lament deposition process were made based on the thermal analysis of extruded polymer ¬laments.
In their research experimental results showed that the fabrication strategy, the envelope temperature and variations in the convection coef¬cient had strong effects on the cooling temperature pro¬le, as well as on the mesostructure and overall quality of the bond strength between ¬laments. The sintering phenomenon was found to have a signi¬cant effect on bond formation, but only for the very short duration when the ¬lament's temperature was above the critical sintering temperature. Otherwise, creep deformation was found to dominate changes in the mesostructure.
Anitha and her team have done the research on the critical parameters influencing the quality of the prototypes in FDM. They have used the Taguchi technique to assess the influence of the parameters on the quality characteristics of the prototypes. Their main object is to reduce the surface roughness of the parts produced by FDM. Their study was involved in a sample component and 18 models. And the roughness of these models was measured by substronic surface roughness measuring tester. After all these tests and analyses they have concluded that without pooling, only the layer thickness is effective to 49.37% at 95% level of significance but with pooling the layer thickness is effective to 51.57% at 99% level of significance. And the other factors road width and speed, contribute to 15.57% and 15.83% respectively at 99% level of significance.
Mukesh and his team study determine the development of Fused Deposition of Ceramics (FDC) and Fused Deposition of Metals (FDMet) in ceramics and metals respectively. They describes process improvements made in solid free foam (SFF) techniques, called as FDC and FDMet for fabrication of structural and functional ceramic and metal parts. They were based on existing SFF technique, FDM and use of commercial FDM systems. But the state of SFF technology and commercial FDM systems results in parts with several surface and internal defects which if not eliminated results severely limit the structural properties of ceramic and metal parts as a result produced. In detail the nature of these defects and their origins confers several novel strategies for elimination of most of those defects. So they have shown how some of those strategies will successfully implement to result in ceramic parts with structural properties comparable to those obtained in conventionally produced ceramics.
Chapter 5: Process Improvement in FDM
The Research development of FDM in Process improvement is categorized in to three groups they are support generation, Process parameter optimization and Finite Element modelling. Now we can discuss about the researches done on the each category.
5.1 Support Generation:
Huang and his team presented a robust algorithm to generate support for FDM. This algorithm uses slice data as input and to calculate the support slice layer by layer they have used the top down approach. In their report they have described in detail about the slice grouping, oriental bounding box calculation, offsetting and Boolean operation in the generation algorithm. Their algorithm provides necessary support not only for hanging surface but also for hanging vertexes and edges with o (n) time complexity, where n is the number of layers. Their algorithm uses the parts self-support ability and reduces support volume to the maximum extent..
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