rp_background_fall10.20100907.4c867881b19c71.39891687 by fanzhongqing


									ME350 - Rapid Prototyping Background Information

1 Overview
   1.1 Background
        This laboratory will introduce you to a technology called Rapid Prototyping. Rapid
Prototyping can be defined as a group of techniques used to quickly fabricate a scale model of a
part or assembly using computer aided design (CAD) data. There are several advantages (as well
as limitations) with these techniques as compared to more traditional subtractive processes, such
as milling or turning.

        In rapid prototyping, the machine reads in data from a CAD drawing, and lays down
successive layers of liquid or powdered material, and in this way builds up the model from a
series of cross sections. These layers, which correspond to the virtual cross section from the
CAD model, are glued together or fused (often using a laser) automatically to create the final
shape. The primary advantage to this type of “additive” construction is its ability to create almost
any geometry (excluding trapped negative volumes).

        The word "rapid" is relative: construction of a model with contemporary machines
typically takes 3 to 72 hours, depending on machine type and model size. Used in micro
technologies "rapid" is correct, the products made are ready very fast and the machines can build
the parts in parallel.

        Some major techniques of rapid prototyping are Selective Laser Sintering (SLS),
Stereolithography (SL) using Stereolithography Apparatus (SLA), and photopolymer jetting
using Objet Eden 350.

(Reference: http://en.wikipedia.org/wiki/Rapid_prototyping)

   1.2 Definition of Rapid Prototyping
       In the manufacturing arena, productivity is achieved by guiding a product from concept
to market quickly and inexpensively. Rapid Prototyping technology aids this process. It
automates the fabrication of a prototype part from a three-dimensional (3D) CAD drawing. This
physical model conveys more complete information about the product earlier in the development
cycle. Rapid Prototyping can be a quicker, more cost-effective means of building prototypes as
opposed to conventional methods.

ME350 - Rapid Prototyping Background Information

        The new Rapid Prototyping technologies are additive processes. Liquid RP methods can
be categorized by material: photo-polymer, thermoplastic, and adhesives. Photo-polymer systems
start with a liquid resin, which is then solidified by exposure to a specific wavelength of light.
Thermoplastic systems begin with a solid material, which is then melted and fuses upon cooling.
The adhesive systems use a binder to connect the primary construction material. Rapid
Prototyping systems are capable of creating parts with small internal cavities and complex
geometries. Also, the integration of Rapid Prototyping and compressive processes has resulted in
the quicker generation of patterns from which molds are made.

   1.3 STL File
          The CAD data are converted to the .stl file format through a translator. The .stl file is an
industry standard interface between 3D CAD solid models and Rapid Prototyping systems. The
.stl file consists of x, y, z coordinates, which represent triangles.

        A major concern with the .stl format is in its representation of curved surfaces. Triangular
facets can precisely describe planar surfaces, but can create some complexity when representing
curved surfaces. Specifying excessive precision will create an enormous .stl file and result in
long processing times, whereas less precision will create a less than accurate model. An
“important” characteristic of a .stl translator is that it allows workers to control the chord-height
deviation between the actual CAD model surface and the triangular facets approximating it. This
value is the maximum distance that a point on a triangle can deviate from the true surface of the

   1.4 Supports
       Most parts built with a rapid prototyping system require some form of support structure.
These supports sustain the part in place as it is being generated. They are thin walls usually
arranged in a web or grid pattern. When materials change from liquid to solid, they shrink. This
loss in volume results in curl, which necessitates support structures. Also, any overhanging part
geometries must be supported. Some rapid prototyping technologies do not require
supplementary software to generate support structures. Rather, they may use the raw material or
secondary material for support.

ME350 - Rapid Prototyping Background Information

   1.5 Staircase Effect
        In any rapid prototyping (RP) process, the layer by layer building process introduces an
error on the amount of material used compared to the volume specified by the computer aided
design model. This error causes the staircase effect on the surface and adversely affects the
dimensional accuracy as well as surface finish for different part build orientations. Inclined and
curved surfaces show staircase effects more predominantly than other surfaces. The orientation,
at which the part is built, has a significant effect on the quality of various surfaces of the part.
The orientation of part also affects other factors such as the build time, the complexity of support
structure, shrinkage, curling, trapped volume, and material flow in many rapid prototyping
processes. There is no way the staircase effect can be completely eliminated. However, the effect
can be reduced by decreasing the layer thickness and by orienting the part such that the effect of
overall staircase error is greatly reduced.

(Reference: S.H. Masood, W. Rattanawong and P. Iovenitti, “A Generic Algorithm for a Best
Part Orientation System for Complex Parts in Rapid Prototyping,” Journal of Materials
Processing Technology, Volume 139, Issues 1-3, p110-116, 20 August 2003)

2 Basic RP Features
       (1) Build time is directly related to total volume of part material and support material,
           and also directly related to the number of layers in the vertical direction.

       (2) Support structures are required for any overhanging sections. Additionally, every RP
           machine deposits an initial layer of support structures that act as a reference plane.
           During preprocessing, this initial layer of support structures must be taken into
           consideration, meaning that the part should be placed at a certain level above from
           the base of the RP machine. For example, when doing the preprocessing for SLA,
           the part should be placed at least 10 mm above the base.

       (3) Support structures cause surface imperfections where they touch the part, so at the
           preprocessing step, the part should be oriented in such a way as to minimize
           imperfections or have them at places that matter the least. For example, if the
           prototype will be utilized as part of an assembly and the surface roughness of certain
           surfaces of the part are essential for the assembly, then the orientation must be
           considered carefully.

ME350 - Rapid Prototyping Background Information

3 Selective Laser Sintering (SLS)
 3.1 Summary of Selective Laser Sintering
    (1) Produces parts within a few hours directly from CAD data.

    (2) Current materials used in SLS include polyvinylchloride, polycarbonate, polyester,
        polyurethane, ABS, nylon, and investment casting wax.

    (3) Laser beam is used to sinter heat-fusible powders.

    (4) The powders are preheated to just below their melting point to facilitate bonding and
        reduce distortion.

    (5) Layer by layer, the powders are gradually bonded into a solid mass that forms the
        three-dimensional part.

    (6) Ideally suited for the economic production of small series and individualized
        products with complex geometries.

    (7) Compared to other rapid prototyping methods it has low investment costs.

    (8) After each layer is completed, a new layer of loose powder is spread across the
        surface using a counter-rotating roller.

    (9) Does not require support structures due to the fact that the part being constructed is
        surrounded by unsintered powder at all times.

    (10) In areas not sintered by the laser beam, the powders remain loose so they can be
         poured out of the completed part.

    (11) Since there are no support structures to remove detailed small parts can easily be
         made and not damaged in the post-processing.

    (12) Creates layers at a rate of up to 0.94 in/hr.

    (13) Minimum layer thickness is slightly larger than the other two RP techniques
        presented here (approximately 0.004’’)

ME350 - Rapid Prototyping Background Information

        SLS was developed and patented by Dr. Carl Deckard at the University of Texas at
Austin in the mid-1980s, under sponsorship of DARPA. The process uses a laser to fuse small
particles of plastic, metal, ceramic, or glass powders into a desirable 3-D shape. There are no
support structures because loose powder is just spread across the part after each layer is
complete. A Selective Laser Sintering machine is shown below:

                                   Illustration 1: SLS machine

   3.2 Machine: FORMIGA P 100
      The Ford Lab has the FORMIGA P 100 Selective Laser Sintering machine. The
machine's performance specifications are tabulated below.

            Specifications            Value

         Build Size                7.9’’ × 9.8’’ × 13’’

         Build Resolution          0.016”

         Modeling Material         Powder based plastics, metals, and ceramics

         Layer Thickness           0.004’’ (material dependent)

ME350 - Rapid Prototyping Background Information

4 Stereolithography (SLA)
  4.1 Summary of Stereolithography
       (1) SLA is the first and still the most widely used Rapid Prototyping technique.

       (2) Least expensive compared to FDM and Objet.

       (3) Uses a light-sensitive liquid polymer (photo-sensitive resin).

       (4) Requires post-curing under UV light since laser does not fully cure part.

       (5) Over curing (curing for longer than recommended) can lead to warping.

       (6) Parts tend to have tacky surfaces on sections that are in contact with the support
           material scaffolds.

       (7) Removal of the part from the machine can be quite messy.

       (8) Post-processing is a tedious process involving manual removal of the support
           structures using scalpels.

       (9) Price per kg is approx. $200-240/kg

       (10) System requires periodical fine tuning in order to maintain optimal operating

       (11) Uncured material can be toxic. Ventilation is important.

       (12) The material is not deposited layer by layer in SLA. Instead, each layer is cured by
            the laser. Hence, shifting problems that may occur in FDM do not occur in SLA. So
            the process is very stable.

       (13) Due to high dimensional accuracy, SLA parts can be used in assemblies requiring
            precision (e.g. Snap fit assemblies, microfluidic devices)

       (14) It takes long time to build a part that is tall in vertical direction. This is because after
            each layer is exposed the tray must lower to allow fresh liquid resin to cover the
           surface, then raise back up to set the thickness of the new layer. Waiting for the
           resin to flow takes time. This step generally takes between 20-60 seconds.

        Stereolithography (SLA), the first Rapid Prototyping process, was developed by 3D
Systems of Valencia, California, USA, founded in 1986. A vat of photosensitive resin contains a
vertically-moving platform. The part under construction is supported by the platform that moves
downward by a layer thickness (typically about 0.1 mm / 0.004 inches) for each layer. A laser

ME350 - Rapid Prototyping Background Information

beam traces out the shape of each layer and hardens the photosensitive resin. The
Stereolithography (SLA) System overall arrangement is shown below:

Illustration 4: Schematic of the SLA Process                   Illustration 5: SLA Viper Si2

  4.2 Machine: ViperTM SLA(R) System by 3D Systems
        The Ford Lab recently replaced the old SLA 250 by a new Viper Si2 system. The salient
feature of this system is as follows:

            Specifications           Value

         Build Size               10” ×10” ×10” (Larger machines can build parts up to
                                  20” ×20” ×24”.)

         Build Resolution         0.003”

         Modeling Material        Accura SI 40, photo-sensitive resin

         Layer Thickness           Minimum (0.001”), typical (0.004")

ME350 - Rapid Prototyping Background Information

5 Objet Eden 350
 5.1 Summary of Objet Eden 350
    (1) Objet can be thought of a hybrid RP that combines aspects of FDM and SLA in one
        machine: print head deposits droplets of material and immediately are cured by UV

    (2) Support structures and building materials are made of different materials in Objet.

    (3) Support structures are water soluble, but they do not dissolve easily in ultrasonic
        bath, they have to be cleaned by high-pressure water jets.

    (4) Building materials are generally elastomers or rigid plastics.

    (5) Layer thickness is smallest for Objet. Minimum layer thickness can go down to

    (6) For a generic part, Objet generally has the fastest build time among the 3 machines.

    (7) Price per kg is approximately $300-350/kg.

    (8) There is no intermediate waiting time between the deposition of layers, so
        construction can very rapid laterally, but vertically can be very slow since the layer
        thickness is small.

    (9) Objet generally deposits the largest amount of support material in the construction of
        the parts.

    (10) The System requires occasional cleaning of the print heads, adjustment of the UV
         lights, hence maintaining cost may be high compared to the other machines.

ME350 - Rapid Prototyping Background Information

                                Illustration 6: Objet Eden 350

  5.2 Machine: Objet Eden 350
        Ford Lab currently has an Eden 350 3-D Printer. Basic features of Objet machine are
listed below:

            Specifications           Value

         Build Size               13.4” ×13.4” ×7.9”

         Build Resolution         0.0016”

         Modeling Material        VeroBlue, VeroWhite, etc.

                                  Flexible, rubber-like TangoGray, TangoBlack,etc.

                                  FullCure 720 Transparent

         Layer Thickness          As small as 0.0006"

ME350 - Rapid Prototyping Background Information

6 Summary
         Below is a table that briefly compares the three RP techniques discussed:

        Property                         SLS                              SLA                      Eden350

     Raw Material            Plastic, metal, and ceramic        Only UV sensitive photo-    UV photo-polymers, but
                                      powders                          polymers             more variety than SLA

    Layer Thickness                    0.004”                       0.001" to 0.004"                0.0006"

   Support Structure                   None                     Same material (manually Different Material (removed
                                                                  removed by scalpel)      using high-pressure water)

     Surface Finish                   Powdery                           Smooth                      Smooth

    Build Resolution                   0.016”                           0.003”                      0.0016”
(smallest feature that can
       be created)


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