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INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY Department of Mechanical Engineering

Course : ME-617 Rapid Product Development Instructor : K.P. Karunakaran Date : October 7, 2011 (Friday) 9:30PM

L-T-P-C : 3-0-0-6I Semester 2011-12

Duration : 90 min.Instructions: Answer all the questions; answer as briefly as

possible.

Roll No

Marks /60

TEST 5: Rapid Prototyping (Introduction, LOM, FDM, Liquid Based Processes)

Part A (1 mark each)

1. Name at least two other technologies than RP that use divide and conquer strategy. Finite Elements Analysis (FEA) Constructive Solid Geometry (CSG) 2. What are the two significant steps in automation? i. Automation of process ii. Automation of process planning 3. What are the three most significant considerations in the selection of a RP machine? i. Material ii. Minimum layer thickness iii. Support mechanism 4. What are the two reasons for poor surface finish in RP? i. Stair-step error due to layered manufacturing

ii. Uprooting or leftover material at the interface of model and support when the support is removed.

5. What are the methods to convey laser beam from a fixed source to the required spot in

LOM and SLA respectively? LOM: Appropriately oriented mirrors moving on X and Y slides

SLA: Galvanometer (a mirror having tilt about two orthogonal axes with matching variation of beam focus to ensure focus on the liquid surface)

6. What are the roles of laser in LOM and SLA respectively? LOM: Cutting (does not demand high precision)

SLA: Polymerization (requires high precision) 7. What is the significance of convex hull in OptiLOM?

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Convex hull is completely visible from any pair of opposite directions. Therefore, it has infinite parting directions/surfaces. This feature helps in choosing the optimal stock halves.

8. What tool is used to cut the paper laminates in Kiras RP machine? How are the laminates

joined in this machine?

Blade is used. The toner used in laser print is used as the binder.

9. What raw material is used in Solidimension RP machine? What is meant by inhibition in its context?

Transparent polythene sheet with adhesive on one side (similar to cello-tape). A liquid is selectively dispensed where the sheet should not stick. This liquid dissolves the glue and inhibits adhesion. Hence, it is known as inhibition.

10. Name at least two RP processes that are capable of making quasi-hollow objects. i. SLA ii. FDM 11. You have studied five RP processes that can make transparent objects. All their names

appear in this questions paper somewhere or the other. List all these five processes. You will receive zero marks even if you miss one of them.

i. SLA ii. SGC iii. Object iv. SolidDimesnion v. EnvisionTech 12. What is the support mechanism in FDM? State its recent enhancement. Similar to model material, a separate sacrificial material in wire form is deposited to

create the support in each layer. Both these materials are alternately dispensed by a twin-head. While the model material is deposited densely, the support is deposited coarsely, just sufficient enough to support. They are also of different colors so as to identify the support while removing.

Initially it was known as break-away support as the adhesion was weak enough for breaking by hand using simple tools. The latest support is soluble in water. So, after building the part, it is put in a tank with soap water that vibrates ultrasonically. The support slowly dissolves in it resulting in a clean part after a few hours. Stratasys refers to this support as Water Works (WW).

13. Which of the four liquid based processes we have studied has the most efficient use of the

photo-polymer? How?

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EnvisionTech. It builds the object from bottom. Therefore, at any time, liquid of only a layer thick is required unlike SLA where the entire tank has to be full.

14. Which surface is the most cumbersome to make in LOM? Horizontal planar surface. 15. Which make and model of RP machine is the most popular in Indian jewelry industry? 3D Systems SLA. Model name is Viper. 16. Which commercial RP machine can make color prototypes? ZCorps 3D Printer. 17. How is Z accuracy ensured in each of SGC and Model Maker II? By milling. SGC uses face mill and Model Maker II uses a long horizontal slab mill. 18. Mention at least two RP processes that use multiple tools. i. ZCorps 3D Printer ii. Object 19. Nesting is the method of building more than one prototype simultaneously within the

working volume. Which of [LOM, SGC, FDM, SLA] has the least advantage of nesting? Why?

FDM. Since many RP processes have substantial overhead for every layer, height of the object greatly influences the build time. So, nesting parts in 3D is beneficial in such processes. This overhead is the least in FDM and hence nesting does not influence significantly.

20. Which RP process was related to the story of the thirsty crow and in what context?

SLA. If the entire height is not used, the bottom may be filled with some blocks so as to hold least photopolymer in the tank.

Part B (Weightage is mentioned against each question)

1. Expand the acronyms: LOM, FDM, SLA, SGC, SLS, 3DP. [3] LOM : Laminated Object Manufacturing FDM : Fused Deposition Modeling SLA : Stereo-Lithography Apparatus SGC : Solid Ground Curing SLS : Selective Laser Sintering 3DP : 3D Printing

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2. Ignoring the time taken in support building and removal, interlayer overheads and gains due to nesting multiple objects in the working envelope, the build time of an object in RP machines is proportional to one of the following of the object: [a] volume, [b] surface area and [c] height. Tick the appropriate cells in the following table in this regard. Briefly justify each answer. [3+3]

LOM FDM SLA SGC Objet Perfactory

Volume Surface area

Height

LOM FDM SLA SGC Objet Perfactory Volume

Surface area Height

3. Match the following in the most appropriate way: [3] [A] FDM [a] Hollow sphere without any perforation [B] Perfactory [b] Array of mirrors [C] 3DP [c] Easiest support [D] LOM [d] Grid cutting [E] ZCorp [e] Color prototyping [F] SLA [f] Photo-elasticity models

[A] FDM [a] Hollow sphere without any perforation [B] Perfactory [b] Array of mirrors [C] 3DP [c] Easiest support [D] LOM [d] Grid cutting [E] ZCorp [e] Color prototyping [F] SLA [f] Photo-elasticity models 4. State two significant differences between SLA and Perfactory. [2]

SLA uses laser while Perfactory uses array of mirrors. Building from bottom. So, laser is at the top while mirror display is at the bottom. Laser has to scan the entire layer whereas the mirror display gives a single flash to

cure the entire layer. SLAs layer thickness heavily depends on accurate control of laser focus in

conjunction with the galvanometer position and scanning speed. On the other hand, accurate mechanical movement of the plate ensures layer accuracy. So, Perfactory has better accuracy on layer thickness than SLA.

5. Which RP process has fine grid cutting? What is its purpose? Why did it not become

popular? [2] LOM.

It is required to separate at the horizontal planar surfaces. It takes more time. So, the users prefer to tilt the object so as to avoid the horizontal

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surfaces. 6. What are the significant advantages of LOM? Highlight its major limitations. [2] Advantages of LOM:

Only boundaries are to be addressed and not their interiors. Therefore, fast process since speed is proportional to the surface area of the object and not to its volume.

It employs CO2 laser which is cheaper. Since used only for cutting, control is simpler than for curing or sintering.

No protective environment is required. However, proper exhaust is required to take away smoke.

Paper is very cheap. It gives strong wood-like parts (paper is made from wood; here, wood is made from

paper). Ideal as patterns for casting. LOM concepts are used in Laminated Tooling & Manufacture.

Limitations of LOM:

Fixed layer thickness Grid cutting takes much more time than object cutting. Decubing also is time-consuming. Horizontal surfaces pose problems. Although it is solvable, it has not been done till

date. Prone for fire accidents. Poor product development and marketing strategies. Hence, it is almost dead.

7. An object has a planar surface slant at 45. When it is built in layers of 0.2mm, what is

will be the average roughness (Ra) on this plane? [4]

Ra = [0.2 cos(45)]/2 = 0.707. 8. What is the innovation adopted in LOM? What are the benefits it led to? [2]

Unlike the traditional cut and then paste approach, LOM employed paste and then cut approach. The following are the benefits:

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Support mechanism became inherent and simple as the remaining stock acted as support.

Number of pieces handled at any time is one. This is achieved either by a simple indexing of the roll of paper or replacement of the cut sheet.

The shape of piece handled is always same. 9. What is OptiLOM and what does it do? Explain its principle and benefits. [3]

OptiLOM is a preprocessor of STL file. It accepts the STL file of an object to be made and progressively fills, first the wholes and then the concave regions to get a convex hull. Finally it calculates a pair of optimal stock halves. So, OptiLOM accept the object as input and outputs a larger STL file consisting of (a) the original object, (b) various plugs that convert it into a convex hull and (c) a pair of optimal stock halves. When this collection, which is nothing but a box, is submitted to LOM machine, it will not find any situation requiring grid cutting and hence decubing. In other words, OptiLOM aggregates all grids that grow into tiny sticks into lumps so that laser is better utilized. While traditional LOM is analogous to Sand Casting, OptiLOM route is similar to injection molding.

10. Only the features that can be accessed can be manufactured. Therefore, a hollow closed

sphere with some wall thickness cannot be made. Agree or disagree with reasoning. If you disagree, then name the RP process(es) which can be used for building this object and explain how it is done. Are there any other methods than RP to achieve this? [3]

The statement is only partially true. It may be modified as Only the features that can be accessed at the time of manufacture can be manufactured. Therefore, a hollow closed sphere with some wall thickness cannot be made. Note that at the time of manufacture has been added. In an additive manufacturing, even the inside of the closed hollow sphere will be visible during manufacture. Therefore, the hollow sphere can be manufactured using an additive manufacturing process that does not leave support material inside. This is possible with FDM. Since the curvature is smoothly varying along vertical direction, FDM can build it without using support. EnvisionTech also can produce it in principle as the building is upside down permitting the draining of the trapped liquid.

11. How many axes of motion are typically required in RP? Which RP machine has single

axis? How are the two axes eliminated in this process? [2]

A typical RP machine will require 2.5 axes, i.e., the movement is either in XY plane or vertically. Solid Ground Curing (SGC) and EnvisionTech have single axis only. In SGC, the mask eliminates the need for X and Y motions while the same is achieved through the mirror display in the latter.

12. State the support mechanism used in each of [LOM, FDM, SLA, SLS, SGC, 3DP]. [3] LOM : Remaining stock of the paper. This is an inherent support mechanism.

FDM : Another sacrificial material with its own dispensing system. It is deposited coarsely. This may be break-away type or water-soluble.

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SLA : Since it is not possible to use a sacrificial material as the building takes place inside the liquid, the CAD definition itself is modified to create support structures of bristles, blades etc.

SLS : Remaining powder. This is an inherent support mechanism. SGC : Wax. This is not inherent; it is deposited separately. SLS : Remaining powder as in SLS. This is an inherent support mechanism.

13. Discuss the issues related to viscosity, surface tension and setting in of waves and ripples

in SLA. How are they overcome? [3]

When the vat (i.e., table or platform) dips by exactly the required layer thickness from the top of the liquid surface, liquid does not spread on top of the vat to create exactly the required layer thickness. This is due to the viscosity and surface tension. Therefore, it has to be dipped more and then brought up to the required level. This is known as deep dipping. After a deep dipping and coming back, the layer standing on the vat is more than required as the excess liquid does not drain down, again due to viscosity and surface tension. Therefore, a blade wipes off the excess liquid. This blade is known as Doctor Blade. The minimum layer thickness achievable is about 0.0127 in Viper. However, during these above movements of the vat or Doctor Blade, if any wave is set, its amplitude will be more than the layer thickness and one has to wait for a long time for the waves to settle down. Therefore, to avoid this, the entire vat is perforated and all the movements are very slow. This leads to a substantial time overhead per layer which is unavoidable. It is also recommended to locate these machines over vibration isolators and in ground floor!

14. Which was the first RP machine in India and who bought it? Why is this process not

popular? [2] Solid Ground Curing (SGC), also known as Photo-Masking. It was bought by Datar Switch Gears, Nashik in 1995.

It employs two sub-systems, one for mask creation and the other for layer creation, each with their sub-systems such as static charging of glass, toner application, toner removal, optical system, dispensers for photo-polymer and wax, suction system for photo-polymer, milling etc. The overall reliability, which is the multiplication of the reliabilities of these sub-systems, was very low. Essentially, the system was too complex with several sub-systems to be reliable. Such a machine is meant for continuous operations. No company will have continuous load for such a machine. Even bureau services could not engage this machine all the time. When the machine is not used for some time and to be restarted, clogging of the pipelines of wax and photo-polymer was a common issue.