RAPID PROTOTYPING A NEW APPROACHMAYUR THOMBRE1, SIDDHARTH KUMAR JAISWAL2, SANDEEP NAIR3

1 Assistant Professor, Mechanical, SSITM BHILAI, Chhattisgarh, India, mayur007me@gmail.com2 Student, Mechanical, SSITM BHILAI, Chhattisgarh, India, ssiddharth.jaiswal@gmail.com

3 Student, Mechanical, SSITM BHILAI, Chhattisgarh, India, sreenair92@gmail.com

AbstractRapid Prototyping is a promising powerful technology that has the potential to revolutionize certain spheres in the ever changing and challenging field of medical science. The process involves building of prototypes or working models in relatively short time to help create and test various design features, ideas, concepts, functionality and in certain instances outcome and performance. The technology is also known by several other names like digital fabrication, 3D printing, solid imaging, solid free form fabrication, layer based manufacturing, laser prototyping, free form fabrication, and additive manufacturing. The first patent of an apparatus for production of 3D objects by stereolithography was awarded to Charles Hull whom many believe to be father of Rapid prototyping industry.

1. Introduction

Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale model of a part or assembly using three-dimensional computer aided design (CAD) data. What is commonly considered to be the first RP technique, Stereolithography, was developed by 3D Systems of Valencia, CA, USA. The company was founded in 1986, and since then, a number of different RP techniques have become available.

Fig.1 The RP wheel depicting the four major aspects of RP.

Rapid Prototyping has also been referred to as solid free-form manufacturing, computer automated manufacturing, and layered manufacturing. RP has obvious use as a vehicle for visualization. In addition, RP models can be used for testing, such as when an airfoil shape is put into a wind tunnel. RP models can be used to create male models for tooling, such as silicone rubber molds and investment casts. In some cases, the RP part can be the final part, but typically the RP material is not strong or accurate enough. When the RP material is suitable, highly convoluted shapes (including parts nested within parts) can be produced because of the nature of RP.

2. Some Basic Processes in Rapid Prototyping:Regardless of the different techniques used in Rapid Prototyping systems developed, there are some basic approaches, which can be described as follows:

1. Creating CAD Model: A model or component is modeled on a computer-aided design and computer-aided manufacturing (CAD-CAM) system. The model which represents the physical part to be built must be represented as closed surfaces which unambiguously define an enclosed volume. This means, the data must specify the inside, the outside and the boundary of the model. This requirement will become redundant if the modeling technique used is based on Solid modeling. Fig.2 shows the basic CAD model.

Fig.2 An example of a CAD model.

2. Conversion to StereoLithography (STL) Format: The solid or surface model to be built is next converted into a format “.STL” (StereoLithography) file format which originates from 3D systems. The STL file format approximates the surface of the model by polygons. Highly curved surfaces must employ many polygons, which mean that STL files for curved parts can be very large. However, there are some RP systems which also accept data in initial graphics exchange specifications (IGES) format provided it is of the correct “flavor”.

Fig.3 An example of an STL triangulation model.3. Slicing the STL File: A computer program

analyses a .STL file that defines the model to be fabricated and “slices” the model into cross-sections. The cross-sections are systematically recreated through the solidification of either liquids or powders and then combined to form a 3D model. In other way, thin cross-sections, solid lamination and thin laminations are glued together with adhesives to form a 3D model. The RP machines build one layer at a time from polymers, paper, or powdered metal.

Fig.4 The cone is sliced into fine layers.4. Cleaning and Finishing: This involves

removing the prototype from the machine and detaching any supports. Some photosensitive materials need to be fully cured before use.

Prototypes may also require minor cleaning and surface treatment. Sanding, sealing, and/or painting the model will improve its appearance and durability.

Fig.5 An example of a part milled using subtract RP (left) and actual product (right).

3. Methodology

The Rapid Prototyping processes include the following steps (see fig. 6).

Fig.6 The data flow of Rapid Prototyping process.

Construct the CAD model. Convert the CAD model to STL format. Check the fix STL file. Generate support structures if needed. Slice the STL file to form layers. Produce physical model. Remove support structures. Post-process the physical model.

4. Principle of Rapid Prototyping

Rapid prototyping works on the basis of adding or removing layers of material to form the desired

shape. The majority of commercial rapid prototyping system build object by adding one layer after another. For simplicity, it can be visualized as stacking slices of bread until complete three-dimensional bread loaf is achieved. Rapid prototyping is a highly automated layer manufacturing process. The object is designed in any solid modeling software (CAD) and the data is converted into a standard format widely known as standard triangularisation language (STL) which is understandable by the rapid prototyping machine. Rapid prototyping software receives data in this format and creates a complete set of instructions for fabrication on rapid prototyping machine such as tool path, layer thickness, processing speed, etc. Rapid Prototyping machine then manufactures the object. Upon completion of a three-dimensional model, it is subjected to post processing treatment for removing support material that was used to support overhang features during fabrication.

Fig.7 Rapid Prototyping cycle for Rapid Product Design.

5. Various Rapid Prototyping ProcessesThe various Rapid Prototyping processes is illustrated in table 1.5.1 StereoLithographyStereo lithography is developed via 3D solid modeling systems. It is a popular RP process; approximately 60% of RP operations can use the SLA process. The SLA stands for Stereo Lithography apparatus.5.1.1 Stereo Lithography Process PreparationTo create a part using the SLA process, the following steps should be done:1. The STL file is generated by CAD software.2. The generated file is provided into the RP machine.3. The support file is generated.4. The STL file and support file are sliced with the slice program.5. The BFF file is generated.6. The BFF file is uploaded into the RP machine.The material container is an element of the machine and never removed unless to change resin. The platform is mounted onto the elevator of the machine and moves up and down within the resin container.

Fig.8 StereoLithography process.5.1.2 Stereolithography ProcessThe Stereolithography Process is illustrated in the fig.8. A directed laser beam reproduces the shape of slice files onto the build material. Supports are built first. The region laser traced onto the build material cures, solidifying the first layer of the laser trace. Support structures are reproduced concurrently as needed. The platform with the first layer of the part and/or support is let down the surface of the build material. The solidified build material is wiped to level it and avoid buildup of material at one end of the container. The platform is lowered one layer below the surface of the build material. The previous steps are repeated until the part and supports are built layer by layer. When the part is completed, the model is detached from the machine and the supports are manually removed from the part. The part is cleaned, postcured in an ultraviolet postcuring apparatus, and finished as required.

5.1.3 Advantages of Stereolithography:The following are the advantages of the SLA process:1. Productivity: Parts can be fabricated in a short period of time (hours/days), depending on the design complexity without the need for machining or tooling. Slight waste is created.2. Unattended operation: The SLA process can run from build start to build finish without the need of a worker.3. Accuracy: The accuracy of SLA parts is good. Several parts can be built in the same build envelope. SLA systems are very reliable.4. Surface finish and appearance: The surface finish and shape of produced parts are very good.5. Part strength: Part strength is excellent; therefore parts are used for the following tests: stress, airflow, fluids, and photoelastic analysis.

5.1.4 Disadvantages of Stereolithography:

Photocurable resins such as those used in SLA are brittle. Use of these resins causes the following problems:1. Cracking: Cracking of SLA-built parts is common when parts are drilled and have fasteners driven into them.2. Changing in dimension due to absorption of moisture: Because the material continues to cure and absorb ambient moisture from the air, parts generated from SLA resins face some problems such as changes in dimension and distorting.3. Support creation and subsequent removal: Numerous parts produced via SLA require the construction of supports. As a result, manual removal and disposal of the supports are essential.4. Cleaning and curing: Some SLA resin-based parts must be cleaned using a solvent such as alcohol.5. Long Build Time: Because SLA run time is proportional to the volume of the designed part, so building of large solid parts may require long time.

5.2 Laminated Object ManufacturingLaminated object manufacturing is a RP system that utilizes 3D solid model CAD data to build up prototype models from sheet material.5.2.1 Laminated Object Manufacturing Process PreparationTo create a part using the LOM process, the following steps should be done:1. 3D solid model CAD data is entered into the process controller.2. The software generates cross-sectional slices.3. The paper feeder is loaded with the desired material.

5.2.2 Laminated Object Manufacturing ProcessThe LOM process is shown in fig.9. A sheet of build material is placed over the LOM machine’s elevator platform on which a build plate rests. The elevator moves up until the plate makes contact with the build material. The initial layer of material is laminated to

a layer of tape on the elevator platform. A laser beam cuts the cross-sectional outline of the initial layer of material. The laser beam cuts a crosshatch design into the excess material to assist the removal process. The laser beam also cuts a wall that supports the model and crosshatched excess material. The elevator platform raises to detach the traced design from the roll of build material, and then lowers. A second layer of build material is advanced. The elevator platform rises to make contact with the build material. The heated laminating roller presses down the second layer to bond it to the initial layer. The previous steps continue until the complete model is completed. The laminated stack and surrounding wall and plate are removed from the elevator platform. The surrounding wall is detached. Excess external and internal material is removed to reveal the model.

Fig.9 Laminated Object Manufacturing.

5.2.3 Advantages of Laminated Object Manufacturing:The following are the advantages of the LOM process:1. Inexpensive: Both material investment and the LOM process are very inexpensive.

Table 1. Various Rapid Prototyping ProcessS.No.

Process Description Strength Finishing(Additive layers)

Layer Thickness

1 Stereolithography(SLA)

Laser cured photopolymer

2,500-10,000 (psi)

Good 0.051-0.152 mm

2 Selective Laser Sintering (SLS)

Laser sintered powder

5,300-11,300 (psi) Fair/Good 0.102 mm

3 Fused Deposition Modeling (FDM) Fused extrusions 5,200-9800

(psi) Fair 0.127-0.330 mm

4 Three Dimensional Printing (3DP)

Liquid binder inkjet printed onto powder Low Fair 0.089-0.203

mm

5 Poly-Jet (Pjet) UV-curved jetted polymer

7,200-8,750 (psi) Good 0.015-0.30

mm

6 Rapid Injection Moulding (RIM)

Injection moulded using Al tooling

3,100-20,000 (psi)

Excellent moulded smooth -

2. No toxic materials: Only nontoxic fumes from laser cutting are generated. The fumes can be vented.3. Accuracy: LOM parts were once prone to dimensional nonstability, particularly in the Z-axis. Improvements in materials and the process of the machine can reduce this problem.4. Semi-attended operation: The LOM process can run unattended.

5.2.4 Disadvantages of Laminated Object Manufacturing:There are many limitations of using the LOM process. They include the following:1. Dimensional stability: Until they are sealed, LOM papers are susceptible to swelling because of humidity. Even though improvements in the process and materials can reduce this problem, LOM parts have some Z-axis accuracy problems. Z height should be supervised during the building process.2. Internal cavities: Some parts require stopping the process to eliminate the waste. Moreover, the part should be split or delaminated to clean out interior cavities, after which the part can be bonded back together.3. Postproduction time: Depending on the complexity of the designed model, postproduction time to eliminate the whole waste can be labor intensive.4. Secondary processes: To generate accurately functional parts, secondary processes are required.

6. ConclusionRapid prototyping has a strong impact on productivity, which means getting a product from concept to prototype to reality in as fast and as an inexpensive method as possible. Conventionally, producing prototypes, testing them, and approving them have been a slow and costly process. Conventional prototyping can take a long period of time from weeks to months, depending on the complication of the design and the method used. The RP can assist radically in reducing the prototype life cycle and the related costs. Moreover, RP computerizes the fabrication of prototypes from 3D CAD designs. A physical prototype provides more information about a product compared with the information that can be obtained from the design drawing. The empirical method presented in this paper can increase both the prediction accuracy and application areas of Rapid prototyping.

References

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[6] www.efunda.com