2009
09MMP607 - Rapid Prototyping, Tooling and Manufacture Coursework
David Coombes A911652
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
Contents 1.0
Executive Summary ..................................................................................................................... 3
2.0 Process Description and Suitability ................................................................................................... 3 2.1 Stereolithography ......................................................................................................................... 3 2.2 Selective Laser Sintering ............................................................................................................... 4 2.3 Fused Deposition Modelling ......................................................................................................... 4 3.0 Calculations ....................................................................................................................................... 5 3.1 Stereolithography ......................................................................................................................... 5 3.1.1 Production Calculations ......................................................................................................... 5 3.1.2 Machine Cost ......................................................................................................................... 5 3.1.3 Labour Cost ............................................................................................................................ 5 3.1.4 Material Cost .......................................................................................................................... 5 3.2 Selective Laser Sintering ............................................................................................................... 5 3.2.1 Production Calculations ......................................................................................................... 5 3.2.2 Machine Cost ......................................................................................................................... 6 3.2.3 Labour Cost ............................................................................................................................ 6 3.2.4 Material Cost .......................................................................................................................... 6 3.3 Fused Deposition Modelling ......................................................................................................... 6 3.3.1 Production Calculations ......................................................................................................... 6 3.3.2 Machine Cost ......................................................................................................................... 6 3.3.3 Labour Cost ............................................................................................................................ 7 3.3.4 Material Cost .......................................................................................................................... 7 4.0 Selection and Justification ................................................................................................................ 7 5.0 Issues ................................................................................................................................................. 8 6.0 Bibliography ...................................................................................................................................... 9
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David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
1.0 Executive Summary Additive manufacturing techniques, such as the ones considered in this study, have numerous advantages over more common manufacturing methods. The major advantage over conventional manufacturing is the geometric freedom. Many geometries produced by additive manufacturing methods could not be produced any other way, mostly due to the inherent layer by layer production techniques. All three processes compared in this study have their individual advantages and disadvantages. The accurate components produced using SL, is combined with poor material properties for the application and expensive parts. SLS produces parts with very good properties at a low price. These advantages are counteracted by the high levels of shrinkage reducing the dimensional accuracy. FDM process produces parts that have the requirements to meet the needs for this application. This, coincided with the predictable levels of shrinkage and the good accuracy, makes FDM the best suited process. Even though FDM has the lowest build volume and the parts produced are not the cheapest. The process produces parts that are best suited the application and these small issues can be overcome easier than the issues in the other processes.
2.0 Process Description and Suitability 2.1 Stereolithography Stereolithography is “a layer manufacturing technology in which the layers are formed by using a laser to cure the surface of a bath of photo-sensitive polymer resin in the desired shape.” (ProQuest, 2009). The process consists of the build platform being lowered into the vat of resin, the depth relates directly to thickness of each individual layer. Once the platform has been lowered and a sweeper passed across the surface to ensure the resin is level. Next the 2D profile is scanned onto the surface, and the process is repeated identically for each subsequence layer. “As the laser scans across the top surface of the resin, it imparts UV energy into the photopolymer, which causes the material to solidify.” (Grimm, 2004). Due to the repetitive nature of the process, subsequent layers are bonded to the pervious layers, forming a complete solid model. “Typical layer thicknesses range from 0.03mm to 0.25mm.” (Grimm, 2004). With the part bonded to the build platform, support structures are used at the beginning of each build. These support structures have two roles, firstly they support subsequent layers. Secondly, the supports attached to the build platform act as sacrificial material. When the part is removed from the platform, it requires the breaking of material. To preserve the surface finish of the part, supports are broken off, keeping the part fully intact. Stereolithography provides a high level of dimensional accuracy, partially due to the small amount shrinkage experienced. Also prior to any post processing “Stereolithography delivers the best surface finish”. (Grimm, 2004). The smoothest surface is found to be the top surface, stepping can affect the side walls and the removal of supports can have a detrimental effect of the bottom surface. One of the problems with Stereolithography is the poor material properties experienced. The materials used within this process experience additional shrinkage if exposed to; heat, moisture or chemical agents. New materials have reduced the sensitivity to moisture, swelling and changes may still occur if exposed to heat or excessive amount of water. Despite the very high accuracy and surface finished experienced, the poor material properties ensure this process is not well suited for this application.
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David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
2.2 Selective Laser Sintering Selective laser sintering is “A layer manufacturing technology in which the layers are formed by using a laser to bond the surface of a bed of powder material in the desired shape.” (ProQuest, 2009). Similar to Stereolithography, with a laser tracing individual layers causing a chemical change and bonding them to previous layers. Differences occur when considering the material; laser sintering used a powdered material. This material is added to a heated chamber layer at a time, this material is then scanned using a CO2 laser. The laser “imparts thermal energy into the powder bed, which causes the material to sinter (fuse)” (Grimm, 2004). Again the depth of the powder placed, is related directly to the depth of each layer, and thus accuracy. “Typical layer thicknesses range from 0.1mm to 0.15mm.” (Grimm, 2004). Selective laser sintering does not require support structures, with the part being built in a volume of powder, the surround powder acts as support for overhanging edges. Recent advances in materials used within SLS means that this process provides the widest latitude in material properties, resulting in parts applied to functional testing and tooling applications. SLS produces generally accurate parts; issues can arise with shrinkage though. This larger rate of shrinkage (3-4%) “increases the tendency for the prototype to warp, bow or curl.” (Grimm, 2004). The plastic prototypes produced by SLS process are dimensionally stable as soon as they are removed from the machine, only excessive heat will distort the part, as with many plastics. Metallic parts need to be infiltrated fully before they are dimensionally stable, usually in a furnace and infiltrated with bronze. Due to the nature of the sintering process, the surfaces on a SLS part display rough and porous characteristics. This is a function of particle size within the powder and can be improved by reducing the powder size. Even though the materials available within this process are of a wider variety than most processes, dimensional accuracy can hinder the process. Tolerances of 0.5mm may make this process unsuitable for the part in question.
2.3 Fused Deposition Modelling Fused deposition modelling is a completely different process to both mentioned above. Fused deposition modelling does not use a laser, but a nozzle extrudes heated filament. By heating the thin filament, the material is built up layer by layer, and the 2D cross section is extruded by the nozzle. The nozzle moves in the X and Y plane only, with the part lowering to enabling building in the Z plane. The filament, in its semi-molten state is passed through the tip of the nozzle and a “road” is deposited. “The thickness and depth of the road are defined by the diameter of the extrusion tip and velocity of the extrusion head” (Grimm, 2004). The part is built onto a platform again, which is lowered after each slice is completed, this dictates the layer thickness. Typical layer thicknesses are between 0.13 and 0.30mm. As the part is built in free space, supports throughout are vital. Material properties are one of FDMs greatest strengths. Strengths very similar to that of injection moulded ABS can be experienced. The accuracy experienced within fused deposition modelling is “equal to those of Stereolithography and better than those of selective laser sintering.” (Grimm, 2004). One is issue can be that shrinkage can be extremely high, but current technology can accurate predict, and compensate for, this variable. Just like injection moulded parts, the parts do not change with time or environment, and remain stable in almost all situations. Due to the extruded nature of the material, surface finish can be a problem. This can be partially overcome if careful consideration goes into the part orientation. The top surface usually requires a larger amount of work to gain a high level of surface finish, when compared to vertical walls. With the parts having good properties, and relatively good surface finish, this process seems to be relatively well suited to the application.
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David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
3.0 Calculations 3.1 Stereolithography 3.1.1 Production Calculations Parts per build (N) = 120 Platform build time (hours, T) = 20 Production rate (per hour, R = N/T) = 120/20 = 6 Operation time (hours, HY) = 8544 x 80% = 6835.2 Production volume (per year, V = R x (HY)) = 6 x 6835.2 = 41011.2 3.1.2 Machine Cost Initial Machine & Ancillary Cost (E) = £500,000 Depreciation (per year, D = E/8) = £500,000/8 = £62,500 Maintenance (per year, M) = £70,000 Machine cost (per year, MC = D + M) = £62,500 + £70,000 = £132,500 Machine cost (per part, MCP = MC/V) = £132,500 / 41011.2 = £3.23 3.1.3 Labour Cost Operator Cost (per hour, OP) = £15 Labour Time (hours, LT) = 4 Labour Cost (per build, L = OP x LT) = £15 x 4 = £60 Labour Cost (per part, LCP = L/N) = £60 / 120 = £0.50 3.1.4 Material Cost Material (weight per part, SLMass) = 0.01kg Material Cost (per Kg, SLCost) = £150 Material Cost (per part, SLMCP = SLMass x SLCost) = 0.01kg x £150 = £1.50 Total Cost (per part, TC = MCP + LCP + SLMCP) = £3.23 + £0.50 + £1.50 = £5.23
3.2 Selective Laser Sintering 3.2.1 Production Calculations Parts per build (N) = 300 Platform build time (hours, T) = 40
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David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
Production rate (per hour, R = N/T) = 300/40 = 7.5 Operation time (hours, HY) = 8544 x 70% = 5980.8 Production volume (per year, V = R x (HY)) = 7.5 x 5980.8 = 44856 3.2.2 Machine Cost Initial Machine & Ancillary Cost (E) = £400,000 Depreciation (per year, D = E/8) = £400,000/8 = £50,000 Maintenance (per year, M) = £20,000 Machine cost (per year, MC = D + M) = £50,000 + £20,000 = £70,000 Machine cost (per part, MCP = MC/V) = £70,000 / 44856 = £1.56 3.2.3 Labour Cost Operator Cost (per hour, OP) = £15 Labour Time (hours, LT) = 8 Labour Cost (per build, L = OP x LT) = £15 x 8 = £120 Labour Cost (per part, LCP = L/N) = £120 / 300 = £0.40 3.2.4 Material Cost Material (weight per part, LSMass) = 0.05kg Material Cost (per Kg, LSCost) = £30 Material Cost (per part, LSMCP = LSMass x LSCost) = 0.05kg x £30 = £1.50 Total Cost (per part, TC = MCP + LCP + LSMCP) = £1.56 + £0.40 + £1.50 = £3.46
3.3 Fused Deposition Modelling 3.3.1 Production Calculations Parts per build (N) = 49 Platform build time (hours, T) = 25 Production rate (per hour, R = N/T) = 49/25 = 1.96 Operation time (hours, HY) = 8544 x 90% = 7689.6 Production volume (per year, V = R x (HY)) = 1.96 x 7689.6 = 15071.62 3.3.2 Machine Cost Initial Machine & Ancillary Cost (E) = £80,000
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David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
Depreciation (per year, D = E/8) = £80,000/8 = £10,000 Maintenance (per year, M) = £10,000 Machine cost (per year, MC = D + M) = £10,000 + £10,000 = £20,000 Machine cost (per part, MCP = MC/V) = £20,000 / 15071.62 = £1.33 3.3.3 Labour Cost Operator Cost (per hour, OP) = £15 Labour Time (hours, LT) = 2 Labour Cost (per build, L = OP x LT) = £15 x 2 = £30 Labour Cost (per part, LCP = L/N) = £30 / 43 = £0.61 3.3.4 Material Cost Material (weight per part, FDMMass) = 0.01kg Material Cost (per Kg, FDMCost) = £250 Material Cost (per part, FDMMCP = FDMMass x FDMCost) = 0.01kg x £250 = £2.50 Total Cost (per part, TC = MCP + LCP + FDMMCP) = £1.33 + £0.61 + £2.50 = £4.44
4.0 Selection and Justification The product, which is a light bulb housing, could potentially be working at high temperatures. Incandescent light bulbs are notoriously inefficient and a large percentage of the energy input is transferred into heat rather than light. This temperature can reach values of “477 deg F (247 deg C)” (DeHaan, 1997). This is the worst case scenario and values experienced by the housing would be significantly lower, rarely exceeding 100 °c. The housing is to be used within a military application and needs to be robust and long lasting to fulfil the product specification. A tolerance of +/- 0.5mm is required, but surface finish is not a critical issue, providing the surface is excessively rough. Stereolithography Advantages
Good Surface Finish High Level of Accuracy
Disadvantages
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No Robust and Long Lasting Material Available Low Resistance to Environmental Factors High Cost
David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
Selective Laser Sintering Advantages
Good Material Properties Good Resistance to Environmental Factors Very Low Cost
Disadvantages
High Levels of Shrinkage Poor Level of Accuracy
Fused Deposition Modelling Advantages
Good Material Properties Resistant to Environment Accurately Compensate for Shrinkage
Disadvantages
Middling Cost
Within FDM the better surface finish is found on the vertical walls, with the top surface and the bottom surface requiring more work to get a good surface finish after production. If the part was orientated vertically, the outer surface would have a better surface finish and it would increase the number of parts within the build. It would also increase the time of the build, but the improved product quality would be the reason for this prolonged build time.
5.0 Issues By using highly anisotropic materials, properties can be directly effect by the orientation. Another issue can come in the rate of shrinkage; even though this can accurately compensated for, if this is done incorrectly, the part produced will be out of tolerance. The FDM process builds parts in free space; this requires a need for support structures. Programmes will automatically generate the supports, but extra time will be required to remove these supports and ensure the surface is free from any small pieces left from the supports. FDM has a relatively low production volume and can achieve a maximum of 15,000 parts per year. If higher levels of production are needed, further machines may need to be purchased.
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David Coombes – A911562
09MMP607 – Rapid Prototyping, tooling and Manufacture
Coursework
6.0 Bibliography Chua, C. K., & Leong, K. K. (1997). Rapid prototyping: principles & applications in manufacturing. New Jersey: John Wiley & Sons. Chus, C. K., Leong, K. F., & Lim, C. S. (2003). Rapid prototyping: principles and applications . London: World Scientific Publishing Co. Cooper, K. G. (2001). Rapid prototyping technology: selection and application. New York: CRC Press. DeHaan, J. D. (1997). Kirk's Fire Investigation. Prentice Hall. Grimm, T. (2004). Users guide to rapid protoyping. Society of Manufacturing Engineers , 161-218. Hague, R. J., & Reeves, P. E. (2000). Rapid prototyping, tooling and manufacturing. Shrewsbury: Rapra Technology Ltd. Hopkinson, N., Hague, R. J., & Dickens, P. M. (2006). Rapid manufacturing: an industrial revolution for the digital age. Wiley. Jacobs, P. F. (1996). Stereolithography and other RP&M technologies: from rapid prototyping to rapid tooling. Society of Manufacturing Engineers. Jacobs, P. F., & Reid, D. T. (1992). Rapid prototyping & manufacturing: fundamentals of stereolithography. Society of Manufacturing Engineers. Kamrani, A. K., & Nasr, E. A. (2006). Rapid prototyping: theory and practice. New York: Springer. Liou, F. W. (2007). Rapid prototyping and engineering applications: a toolbox for prototype development. CRC Press. Noorani, R. (2006). Rapid prototyping: principles and applications. New Jersey: John Wiley & Sons. ProQuest. (2009). Glossary. Retrieved December 28, 2009, from ProQuest: http://www.csa.com/discoveryguides/rapidman/gloss.php
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David Coombes – A911562