By

V. THULASIKANTHAssistant Professor

Mechanical Engineering Departmentvtkvsk@gmail.com

1

Rapid Prototyping and Tooling

Rapid Prototyping and ToolingIn the development of a new product, there is a need to produce a single prototype, of a designed part or system before allocating large amounts of capital to new production facilities or assembly lines.

Consequently, a working prototype is needed for design evaluation and troubleshooting before a complex product or system is ready to be produced and marketed.

The traditional method of fabricating a prototype part is machining, which can require significant lead times.

A virtual prototype, which is a computer model of the part design on a CAD system, may not be adequate for the designer to visualize the part.

It certainly is not sufficient to conduct real physical tests on the part, although it is possible to perform simulated tests by finite element analysis or other methods.

Rapid Prototyping (RP) is a family of fabrication methods to make engineering prototypes in minimum possible lead times based on a computer-aided design (CAD) model of the item.

RP technologies, a solid physical part can be created in a relatively short time

The designer can therefore visually examine and physically feel the part and begin to perform tests and experiments to assess its merits and shortcomings.

Rapid prototyping serves as an important tool for visualization and for concept verification.

With suitable materials, the prototype can be used in subsequent manufacturing operations to produce the final parts. Sometimes called direct prototyping, this approach can serve as an important manufacturing technology.

Rapid-prototyping operations can be used in some applications to produce actual tooling for manufacturing operations. Thus, one can obtain tooling in a matter of a few days.

Advantages of RPT

Rapid-prototyping processes can be classified into three major groups:

Subtractive: As the names imply, subtractive processes involve material removal from a workpiece that is larger than the final part.

Virtual processes: Use advanced computer-based visualization technologies.

Additive processes: build up a part by adding material incrementally to produce the part.

Classification of RPT

Subtractive ProcessesThe traditional method of fabricating a prototype part is machining, which can require significant lead times.

This approach requires skilled operators using material removal by machining and finishing operations one by one-until the prototype is completed.

To speed the process, subtractive processes increasingly use computer-based technologies such as the following:

Computer-based drafting packages, which can produce three-dimensional representations of parts.

Interpretation software, which can translate the CAD file into a format usableby manufacturing software.

Manufacturing software, which is capable of planning the operations required to produce the desired shape.

Computer-numerical-control (CNC) machinery with the capabilities necessary to produce the parts.

If a prototype is required only for the purpose of shape verification, a soft material (usually a polymer or a wax) is used as the workpiece in order to reduce or avoid any machining difficulties.

The material intended for use in the actual application also can be machined, but this operation may be more time consuming, depending on the machinability of the material.

Depending on part complexity and machining capabilities, prototypes can be produced in a few days to a few weeks.

Other starting materials can also be used, such as wood, plastics, or metals (e.g., a machinable grade of aluminum or brass).

The CNC machines used for rapid prototyping are often small, and the terms desktop milling or desktop machining are sometimes used for this technology.

Maximum starting block sizes in desktop machining are typically 180 mm (7 in) in the x-direction, 150 mm (6 in) in the y-direction, and 150 mm (6 in) in the z-direction

VIRTUAL PROCESSESVirtual prototyping is a software-based method that uses advanced graphics and virtual-reality environments to allow designers to view and examine a part in detail.

This technology, also known as simulation-based design, uses CAD packages to render a part such that, in a 3-D interactive virtual environment, designers can observe and evaluate the part as it is being drawn and developed.

Virtual prototyping has been gaining importance, especially because of the availability of low-cost computers and simulation and analysis tools.

Additive operations require elaborate software.

The first step is to obtain a CAD file description of the part.

The computer then constructs slices of the three-dimensional part.

Each slice is analyzed separately, and a set of instructions is compiled in order to provide the rapid-prototyping machine with detailed information regarding the manufacture of the part.

Starting Materials in Material Addition RP

1. Liquid monomers that are cured layer by layer into solid polymers

2. Powders that are aggregated and bonded layer by layer

3. Solid sheets that are laminated to create the solid part

Additive rapid-prototyping operations all build parts in layers, which are typically 0.1 to 0.5 mm thick and can be thicker for some systems.

Additive Processes

The common approach to prepare the control instructions (part program) in all of the current material addition RP techniques involves the following steps

1.Geometric modeling. This consists of modeling the component on a CAD system to define its enclosed volume.

Solid modeling is the preferred technique because it provides a complete and unambiguous mathematical representation of the geometry.

For rapid prototyping, the important issue is to distinguish the interior (mass) of the part from its exterior, and solid modeling provides for this distinction.

2. Tessellation of the geometric model. In this step, the CAD model is converted into aformat that approximates its surfaces by triangles or polygons, with their vertices arranged

to distinguish the object’s interior from its exterior.

The common tessellation format used in rapid prototyping is STL, which has become the de facto standard input format for nearly all RP systems.

3. Slicing of the model into layersIn this step, the model in STL file format is sliced into closely spaced parallel horizontal layers. Conversion of a solid model into layers is illustrated.

These layers are subsequently used by the RP system to construct the physical model.

By convention, the layers are formed in the x-y plane orientation, and the layering procedure occurs in the z-axis direction.

For each layer, a curing path is generated, called the STI file, which is the path that will be followed by the RP system to cure (or otherwise solidify) the layer.

There are many different ways to 3D print an object. But nearly all of them utilize computer aided design (CAD) files.

CAD files are digitalized representations of an object. They're used by engineers and manufacturers to turn ideas into computerized models that can be digitally tested, improved and most recently, 3D printed.

In 3D printing or additive manufacturing CAD files must be translated into a "language," or file type, that 3D printing machines can understand.

Standard Tessellation Language (STL) is one such file type and is the language most commonly used for stereolithography, as well as other additive manufacturing processes.Since additive manufacturing works by adding one layer of material on top of another, CAD models must be broken up into layers before being printed in three dimensions.

STL files "cut up" CAD models, giving the 3D printing machine the information it needs to print each layer of an object.

Production of STL file from 3d cad model

What is a STL File?A STL file is a format used by Stereolithography software to generate information needed to produce 3D models on Stereolithography machines. In fact, the extension "stl" is said to be derived from the word "Stereolithography."

A slightly more specific definition of a stl file is a triangular representation of a 3D object. The surface of an object is broken into a logical series of triangles (see illustration at right). Each triangle is uniquely defined by its normal and three points representing its vertices.

The stl file is a complete listing of the xyz coordinates of the vertices and normals for the triangles that describe the 3D object.

Often a stl file can be termed "bad" because of translation issues.

In many CAD systems, the number of triangles that represent the model can be defined by the user. If too many triangles are created, the stl file size can become unmanageable.

If too few triangles are created, curved areas are not properly defined and a cylinder begins to look like a hexagon (see example below).

When creating a stl file, the goal is to achieve a balance between unmanageable file size and a well-defined model with smooth curved geometries.

How to create a STL file?

Most CAD software packages offer stl conversion add-ins.

If we have access to conversion software, stl translation is relatively simple as long as you have a clean-surfaced 3D model and a high-end computer.

Traditionally when converting to a stl file, the user is given several options for resolution (sometimes called chord height, triangle tolerance, etc.). Depending upon the size of the model, the geometry of small details, and the overall curvature of the part, the tolerance can typically be set to .001 inch for average models.

Small parts or models with fine details may require a tighter tolerance.

Additive Process techniques include;Stereolitliograpliy,

MultiJet/ PolyJet modeling,

Fused deposition modeling,

Ballistic-particle manufacturing,

3D printing,

Selective laser sintering,

Electron-beam and

Laminated-object manufacturing.

Stereolithography This was the first material addition RP technology, dating from about 1988 and introduced by3DSystems, Inc. based on the work of inventor CharlesHull.

(STL) is a process for fabricating a solid plastic part out of a photosensitive liquid polymer using a directed laser beam to solidify the polymer

Part fabrication is accomplished as a series of layers, in which one layer is added onto the previous layer to gradually build the desired three dimensional geometry.

The stereolithography apparatus consists of

(1) a platform that can be moved vertically inside a vessel containing the photosensitive polymer

(2) a laser whose beam can be controlled in the x-y direction.

At the start of the process, the platform is positioned vertically near the surface of the liquid photopolymer, and a laser beam is directed through a curing path that comprises an area corresponding to the base (bottom layer) of the part.

STL (STereoLithography) is a file format native to the stereolithography CAD software created by 3D Systems. STL is also known as Standard Tessellation Language.

This file format is supported by many other software packages; it is widely used for rapid prototyping and computer-aided manufacturing.

This and subsequent curing paths are defined by the STL file (step 3 in preparing the control instructions described in the preceding).

The action of the laser is to harden (cure) the photosensitive polymer where the beam strikes the liquid, forming a solid layer of plastic that adheres to the platform.

When the initial layer is completed, the platform is lowered by a distance equal to the layer thickness, and a second layer is formed on top of the first by the laser, and so on.

Before each new layer is cured, a wiper blade is passed over the viscous liquid resin to ensure that its level is the same throughout the surface.

Each layer consists of its own area shape, so that the succession of layers, one on top of the previous, creates the solid part shape.

Each layer is 0.076 to 0.50mm thick. Thinner layers provide better resolution and allow more intricate part shapes; but processing time is greater.

After all of the layers have been formed, the photopolymer is about 95% cured.The piece is therefore ‘‘baked’’ in a fluorescent oven to completely solidify the polymer.

Excess polymer is removed with alcohol, and light sanding is sometimes used to improve smoothness and appearance.

Photopolymers are typically acrylic [13], although use of epoxy for STL has also been reported.

The starting materials are liquid monomers. Polymerization occurs upon exposure to ultraviolet light produced by helium-cadmium or argon ion lasers.

Scan speeds of STL lasers typically range between 500 and 2500 mm/s.

The time required to build the part by this layering process ranges from 1 hour for small parts of simple geometry up to several dozen hours for complex parts.

Other factors that affect cycle time are scan speed and layer thickness.

The part build time in stereolithography can be estimated by determining the time to complete each layer and then summing the times for all layers.

A common rapid-prototyping process-one that actually was developed prior to fused-deposition modeling-is stereolit/aography (STL). This process (Fig. 20.6) is based on the principle of curing (hardening) a liquid photopolymer into a specific shape. A vat containing a mechanism whereby a platform can be lowered and raised is filled with a photocurable liquid-acrylate polymer. The liquid is a mixture of acrylic monomers, oligomers (polymer intermediates), and a photoinitiator (a compound that undergoes a reaction upon absorbing light).

Multijet/Polyjet ModelingMultijet Modeling (MJM) or Polyjet process is similar to inkjet printing, where print heads deposit the photopolymer on the build tray.

Ultraviolet bulbs, alongside the jets, immediately cure and harden each layer, thus eliminating the need for any postmodeling curing that is needed in stereolithography.

The result is a smooth surface of thin layers as small as 16µm that can be handled immediately after the process is completed.

Two different materials are used: One material is used for the actual model, while a second, gel-like resin is used for support Each material is simultaneously jetted and cured, layer by layer.

When the model is completed, the support material is removed with an aqueous solution.

Build sizes are fairly large, with an envelope of up to 500 X400X200 mm.

These processes have capabilities similar to those of stereolithography and use similarresins .

The main advantages are the capabilities of avoiding part cleanup and lengthy postprocess curing operations, and the much thinner layers produced, thus allowing for better resolution.

Selective Laser SinteringSelective laser sintering (SLS) is a process based on the sintering of nonmetallic or (less commonly) metallic powders selectively into an individual object.

The bottom of the processing chamber is equipped with two cylinders:

I.A powder-feed cylinder, which is raised incrementally to supply powder to the part-build cylinder through a roller mechanism.

2. A part-build cylinder, which is lowered incrementally as the part is being formed.

First, a thin layer of powder is deposited in the part-build cylinder.

Then a laser beam guided by a process-control computer using instructions generated by the three-dimensional CAD program of the desired part is focused on that layer, tracing and sintering a particular cross section into a solid mass.

The powder in other areas remains loose, yet it supports the sintered portion. Another layer of powder is then deposited; this cycle is repeated again and again until the entire three-dimensional part has been produced.

The loose particles are shaken off, and the part is recovered.

The part does not require further curing-unless it is a ceramic, which has to be fired to develop strength.

A variety of materials can be used in this process, including polymers (such as ABS, PVC, nylon, polyester, polystyrene, and epoxy), wax, metals, and ceramics with appropriate binders.

It is most common to use polymers because of the smaller, less expensive, and less complicated lasers required for sintering.

With ceramics and metals, it is common to sinter only a polymer binder that has been blended with the ceramic or metal powders.

The resultant part can be carefully sintered in a furnace and infiltrated with another metal if desired.

Thickness 0.075 to 0.50mm

Fused deposition modeling

In the fused-deposition-modeling process a gantry robot controlled extruder head moves in two principal directions over a table, which can be raised and lowered as needed.

A thermoplastic filament is extruded through the small orifice of a heated die.

The initial layer is placed on a foam foundation by extruding the filament at a constant rate while the extruder head follows a predetermined path.

When the first layer is completed, the table is lowered so that subsequent layers can be superimposed.

Some parts are difficult to manufacture directly, because once the part has been constructed up to height a, the next slice would require the filament to beplaced in a location where no material exists beneath to support it.

The solution is to extrude a support material separately from the modeling material.

The use of such support structures allows all of the layers to be supported by the material directly beneath them.

The support material is produced with a less dense filament spacing on a layer, so it is weaker than the model material and can be broken off easily after the part is completed.

The layers in an FDM model are determined by the extrusion-die diameter, which typically ranges from 0.050 to 0.12 mm.

This thickness represents the best achievable tolerance in the vertical direction. In the x-y plane, dimensional accuracy can be as fine as 0.025 mm-as long as a filament can be extruded into the feature.

A variety of polymers are available for different applications. Flat wire metal deposition uses a metal Wire instead of a polymer filament, but also needs a laser to heat and bond the deposited Wire to build parts.

Close examination of an FDM-produced part will indicate that a stepped surface exists on oblique exterior planes.

If this surface roughness is objectionable, a heated tool can be used to smooth the surface, the surface can be hand sanded, or a coating can be applied (often in the form of a polishing Wax ).

The overall tolerances are then compromise d unless care is taken in these finishing operations.

Electron-beam MeltingA process similar to selective laser sintering and electron-beam welding electron-beam melting uses the energy source associated with an electron beam to melt titanium or cobalt-chrome powder to make metal prototypes.

The workpiece is produced in a vacuum; the part build size is limited to around 200 x200 x180 mm. Electron-beam melting (EBM) is up to 95% efficient from an energy standpoint (compared with 10-20% efficiency for selective laserSintering)

The titanium powder is actually melted and fully dense parts can be produced. A volume build rate of up to 60 cm3/hr can be obtained, with individual layer thicknesses of 0.05 0-0.200 mm.

Hot isostatic pressing also can be performed on parts to improve their fatigue strength.

Although applied mainly to titanium and cobalt-chrome to date, the process is being developed for stainless steels, aluminum, and copper alloys.

In the three-dimensional-printing (3DP) process, a print head deposits an inorganic binder material onto a layer of polymer, ceramic, or metallic powder.

This RP technology was developed at Massachusetts Institute of Technology.

Three-dimensional printing (3DP) builds the part in the usual layer-by-layer fashion using an ink-jet printer to eject an adhesive bonding material onto successive layers of powders.

The binder is deposited in areas corresponding to the cross sections of the solid part, as determined by slicing the CAD geometric model into layers.

The binder holds the powders together to form the solid part, while the unbonded powders remain loose to be removed later.

While the loose powders are in place during the build process, they provide support for overhanging and fragile features of the part.

When the build process is completed, the part is heat treated to strengthen the bonding, followed by removal of the loose powders.

To further strengthen the part, a sintering step can be applied to bond the individual powders.

The part is built on a platform whose level is controlled by a piston.

Process will be as follows

(1)A layer of powder is spread on the existing part-in-process.

(2)An ink-jet printing head moves across the surface, ejecting droplets of binder on those regions that are to become the solid part.

(3)When the printing of the current layer is completed, the piston lowers the platform for the next layer.

Starting materials in 3DP are powders of ceramic, metal, or cermet, and bindersthat are polymeric or colloidal silica or silicon carbide.

Typical layer thickness ranges from 0.10 to 0.18 mm. The ink-jet printing head moves across the layer at a speed of about 1.5 m/s, with ejection of liquid binder determined during the sweep by raster scanning.

The sweep time, together with the spreading of the powders, permits a cycle time per layer of about 2 seconds

Laminated-Object ManufacturingLaminated-object manufacturing produces a solid physical model by stacking layers of sheet stock that are each cut to an outline corresponding to the cross-sectional shape of aCAD model that has been sliced into layers.

The layers are bonded one on top of the previous one before cutting. After cutting, the excess material in the layer remains in place to support the part during building.

Starting material in LOM can be virtually any material in sheet stock form, such as paper, plastic, cellulose, metals, or fiber-reinforced materials.

Stock thickness is 0.05 to 0.50 mm .In LOM, the sheet material is usually supplied withadhesive backing as rolls that are spooled between two reels.

Otherwise, the LOM process must include an adhesive coating step for each layer.

The data preparation phase in LOM consists of slicing the geometric model using the STL file for the given part.

The slicing function is accomplished by LOMSliceTM, the special software used in laminated-object manufacturing. Slicing the STL model in LOM is performed after each layer has been physically completed and the vertical height of the part has been measured.

(1) LOMSliceTM computes the cross-sectional perimeter of the STL model based on the measured height of the physical part at the current layer of completion.

(2)A laser beam is used to cut along the perimeter, as well as to crosshatch the exterior portions of the sheet for subsequent removal.

The laser is typically a 25 or 50 W CO2 laser. The cutting trajectory is controlled by means of an x-y positioning system. The cutting depth is controlled so that only the top layer is cut.

(3) The platform holding the stack is lowered, and the sheet stock is advanced between supply roll and take-up spool for the next layer.

The platform is then raised to a height consistent with the stock thickness and a heated roller moves across the new layer to bond it to the previous layer.

The height of the physical stack is measured in preparation for the next slicing computation by LOMSliceTM.

When all of the layers are completed, the new part is separated from the excess external material using a hammer, putty knife, and wood carving tools.

LOM part sizes can be relatively large among RP processes, with work volumes up to 800 mm 500 mm by 550 mm (32 in 20 in 22 in). More common work volumes are 380 mm 250 mm 350 mm

Solid-ground Curing or solid based curingThis process is unique in that entire slices of a part are manufactured at one time. As a result, a large throughput is achieved, compared with that from other rapidprototyping processes. However, solid-ground curing (SGC) is among the most expensive processes; hence, its adoption has been much less common than that of other types of rapid prototyping, and new machines are not available. Basic all , the method consists of the following steps:I. Once a slice is created by the computer software, a mask of the slice is printed on a glass sheet by an electrostatic printing process similar to that used in laser printers. A mask is required because the area of the slice where the solid material is desired remains transparent.2. While the mask is being prepared, a thin layer of photoreactive polymer is deposited on the work surface and is spread evenly.3. The photo mask is placed over the work surface, and an ultraviolet floodlight is projected through the mask. Wherever the mask is clear, the light shines through to cure the polymer and causes the desired slice to be hardened.4. The unaffected resin (still liquid) is vacuumed off the surface.5. Water-soluble liquid wax is spread across the work area, filling the cavities previously occupied by the unexposed liquid polymer. Since the workpiece is on a chilling plate and the workspace remains cool, the wax hardens quickly. 6. The layer is then milled to achieve the correct thickness and flatness.7. This process is repeated-layer by layer-until the part is completed. Solid-ground curing has the advantage of a high production rate, because entire slices are produced at once and two glass screens are used concurrently. That is, while one mask is being used to expose the polymer, the next mask already is being prepared, and it is ready as soon as the milling operation is completed.

Rapid ToolingRapid-prototyping techniques have made possible much faster product development times, and they are having a major effect on other manufacturing processes.

When appropriate materials are used, rapid-prototyping machinery can produceblanks for investment casting or similar processes, so that metallic parts can nowbe obtained quickly and inexpensively, even for lot sizes as small as one part.

Such technologies also can be applied to producing molds for operations (such asinjection molding, sand and shell mold casting, and even forging), thereby significantly reducing the lead time between design and manufacture.

Several methods have been devised for the rapid production of tooling (RT) by means of rapid-prototyping processes. The advantages to rapid tooling include the following:

1.The high cost of labor and short supply of skilled patternmakers can be overcome.

2. There is a major reduction in lead time.

3. Hollow designs can be adopted easily so that lightweight castings can be produced more easily.

4. The integral use of CAD technologies allows the use of modular dies with base-mold tooling (match plates) and specially fabricated inserts. This modular technique can further reduce tooling costs.

5. Chill- and cooling-channel placement in molds can be optimized more easily,leading to reduced cycle times.

6. Shrinkage due to solidification or thermal contraction can be compensated forautomatically through software to produce tooling of the proper size and, in turn, to produce the desired parts.

The main shortcoming of rapid tooling is the potentially reduced tool or pattern life (compared to those obtained from machined tool and die materials, such as tool steels or tungsten carbides).

The simplest method of applying rapid-prototyping operations to other manufacturing processes is in the direct production of patterns or molds.

Example : Investment castingThe individual patterns are made in a rapid-prototyping operation (in this case, stereolithography) and then used as patterns in assembling a tree for investment casting.

As drawn in CAD programs, the parts are usually software modified to account for shrinkage, and it is then that the modified part is produced in the rapidprototyping machinery.

Example: 3DP3DP can easily produce a ceramic-mold casting shell or a sand mold in which an aluminum-oxide or aluminum-silica powder is fused with a silica binder. The molds have to be post processed in two steps: curing at around 150°C and then firing at 1000°-1500°C.

Example: Injection MoldingInjection molding in which the mold or, more typically, a mold insert is manufactured by rapid prototyping.

The advantage of rapid tooling is the capability to produce a mold or a mold insert that can be used to manufacture components without the time lag (typically several months) traditionally required for the procurement of tooling.

Furthermore, the design is simplified, because the designer need only analyze a CAD file of the desired part; software then produces the tool geometry and automatically compensates for shrinkage.

Room-temperature vulcanizing (RTV) molding/urethane casting can be performed by preparing a pattern of a part by any rapid-prototyping operation. The pattern is coated with a parting agent and may or may not be modified to define mold parting lines.

Liquid RTV rubber is poured over the pattern, and cures (usually within a few hours) to produce mold halves. The mold is then used with liquid urethanes in injection molding or reaction-injection molding operations

Epoxy or aluminum-filled epoxy molds also can be produced, but mold design then requires special care.

With RTV rubber, the mold flexibility allows it to be peeled off the cured part. With epoxy molds, the high stiffness precludes this method of part removal, and mold design is more complicated.

Thus, drafts are needed, and undercuts and other design features that can be produced by RTV molding must be avoided.

Other rapid-tooling approaches

Acetal clear epoxy solid (ACES) injection molding, also known as direct AIM, refers to the use of rapid prototyping (usually stereolithography) to directly produce molds suitable for injection molding.

The molds are shells with an open end to allow filling with a material such as epoxy, aluminum-filled epoxy, or a low-melting-point metal.

Depending on the polymer used in injection molding, mold life may be as few as 10 parts, although a few hundred parts per mold are possible.

Sprayed-metal tooling. In this process a pattern is created through rapid prototyping.

A metal spray operation then coats the pattern surface with a zinc-aluminum alloy. The metal coating is placed in a flask and potted with an epoxy or an aluminum-filled epoxy material.

In some applications, cooling lines can be incorporated into the mold before the epoxy is applied. The pattern is removed; two such mold halves are then suitable for use in injection-molding operations.

Keltool process. In the Keltool process, an RTV mold is produced based on a rapid-prototyped pattern, as described earlier.

The mold is then filled with a mixture of powdered A6 tool steel tungsten carbide, and polymer binder, and is allowed to cure.

The so-called green tool (green, as in ceramics and powdermetallurgy) is fired to burn off the polymer and fuse the steel and the tungsten-carbide powders.

The tool is then infiltrated with copper in a furnace to produce the final mold.

The mold can subsequently be machined or polished to attain a superior surface finish and good dimensional tolerances.

Keltool molds are limited in size to around 150 >< 150 >< 150 mm, so, typically, a mold insert suitable for high-volume molding operations is produced.

Depending on the material and processing conditions, mold life can range from 100,000 to 10 million parts.