3D Printing Method of Rapid Prototyping and Errors That Can Occur

At present rapid prototyping can be treated as a generic term for a collection of different manufacturing methods, which enable the fast realization of a solid, 3D-model starting from computer aided construction or design data (CAD) using generative fabrication processes.

The following technology-based definitions have been established:

  • Rapid Prototyping (RP): model generation using generative techniques
  • Rapid Tooling (RT): mold making applying generative and replication techniques
  • Rapid Manufacturing (RM): small scale fabrication with RP or RT methods

In general, RP is used in product development, RT in mold making for the realization of replication tools and RM for the small scale and in future for mass fabrication. All generative RP methods have in common that the 3D- structure is produced layer by layer by the deposition of suitable materials.

Phases of manufacturing

In conventional manufacturing there are the different phases of the making a new product.

Prototyping

One of the phases is the making a prototype of the product with which manufacture can observe all the possible defects and shortcomings in the process of product creation. Except that with a model of the product, a manufacturer can do different tests such as aerodynamic tunnels tests, hydrodynamic tests etc…

So because the “time on the market” time phase in a modern manufacturing has to be as short as possible, there is a need to make prototyping faster and more reliable.

Modern techniques such as 3D CAD drawings and additive manufacturing create many new possibilities.

Reverse engineering

Outside of conventional methods of making a product there are new techniques for making and improving products. One of these techniques is reverse engineering. In reverse engineering you create a replica of an existing product (for example in medical devices).

With reverse engineering there is few different ways you can scan an object to duplicate it’s design:

  • Contact Method
  • Non-Contact Method

There are also two ways of generating a CAD model:

  • Cross sectional method
  • Method based on the polygonal meshes

Generally, processing of the scanned data is done in the following order, where not all of these steps are necessary or sensible for all input data:

Format conversion > Pre-processing > Surface reconstruction > Post-processing > Viewing

Source: http://www.math.tamu.edu/~rdevore/MURI/MURI_documentation.html

One common format to save these scanned files to for editing is an STL format.
 All the points of cloud data in the pre-processing phase are connected into the surface form. In the surface reconstruction phase errors can occur between connecting the surfaces in the final form of the 3D CAD data object.

In that case there will need to be corrections made to the design in one of several CAD programs***. This process is referred to as a fix-up. Whereby errors in the file produced during scanning are identified and then corrected to ensure quality output files for prototyping.

STL (Stereolithography) is the oldest commercial RP technology on the market.

There are three phases of the Stereolithography method:

  1. Preprocessing
  2. Processing
  3. Postprocessing

Preprocessing

The first phase of preprocessing is a getting an STL file (stereolithography file). You can get the CAD files from the a designer, engineer, reverse engineering etc…

Next the STL file is processed with an slicing software for example 3D Lightyear. 3D Lightyear TM software inputs STL or SLC files, which it then prepares for part building on any of 3D Systems’ SLATM systems.

In the slicing phase of the process the 3D CAD STL files are “cut” or “sliced” into layers that will be layers of the 3D printing part. Software that cuts the 3D File into a series of layers outputs a G-code file containing instructions tailored to a specific type of 3D printer (FDM printers, Resin printers etc.). With G code the machine is told in which x,y and z coordinates the printer head or delivery system should place material such as filament or resin. Thus the term “additive manufacturing”. Depending on the design of the printer this could be a filament in the case of FDM (Fused deposition Modeling) or resin for those that use laser curing.

Print quality, failure rates and resolution depends of the manufacturer and the type of printing. Printer resolution describes layer thickness and X-Y resolution in dots per inch (dpi) or micrometers (μm). Typical layer thickness is around 100 μm (250 DPI), although some machines can print layers as thin as 16 μm (1,600 DPI). X-Y resolution is comparable to that of laser printers. The particles (3D dots) are around 50 to 100 μm (510 to 250 DPI) in diameter.

Things to be aware of when producing a 3D CAD drawing for printing:

  1. Each object (part) must be in positive (x,y,z) CAD space.
  2. Distance between the CAD object and the beginning of the coordinate system beginning it should be as less as possible
  3. The height of an object should be as small as possible in the coordinate system. The result will be a design with the least number of layers possible. Also results in a faster built and quicker prototyping.
  4. It is necessary to made a part on that way that to make at less as possible zones that can cause holding of the photopolymer after the process of printing. To avoid this volume that holding a photopolymer can be make a new geometry of the object with CAD software or it can be made a proper holes on that place which should allow flowing of the polymer from that volume. That kind of holes should be close after that.
  5. With a proper orientation of the part make as less that is possible number of surfaces that are over angle because of the making a “step” surfaces after printing.>
  6. Curvilinear structures if needed should be kept in the horizontal plane because most printers have a much higher print resolution in the horizontal plane than the vertical plane resulting in much high quality prints.
  7. Because of the not hard enough structure of the printed part during the process of printing, sometimes there is a need for the support mounts of the structure of the printed part. Support mounts can be generated in the CAD or in a special software (etc. LightYear).

Processing

After that pre-processing is a process of processing. With one word it is a process of the printing the part. It depends of the printing machine.

Post-Processing

The third phase of printing a part is a postprocessing. In this phase the model is being fisished to remove debris or excess material applied during the print as well as performing surface finishing processes. Additionally for many materials hardening the surface is necessary to strengthen the part and add durability for handling and testing.

Typical Post processing steps:

  • Taking the object from the machine’s platform
  • Cleaning of the object
  • Solidification of the part with drying (post curing)
  • Taking a support mounts from the part.
  • Next the part is taken to the platform of the machine, it should be cleaned with a proper cleaner. It can be alcohol or acetone or other cleaner that it suitable for this purpose. After cleaning remaining fluid could be removed with a compressed air.
  • Process of a solidification of the part (post curing) are done in a special furnace. For most photopolymers temperature is about 100°C.
  • Remaining support mount are taking off from the part with special tools (knives).

 

Checking and corrections of the errors of STL data files

Existence of the errors in STL data files, which are get from a 3D designer, engineer or modelist, is possible even when the 3D CAD model is made without mistakes.

To perform STL file data correct you will need a special software such as:

  • Solidview/Pro
  • 3D Verify
  • MagicsRP

Common errors that can be found in contents of STL data files are:

  • Rifts
  • Wrongly oriented normal
  • Not corrected normal
  • Not corrected sections
  • False internal structures
  • Unsteadiness

Degenerations of the facets

In ideal cases, facets from the STL data file should formatted with closed shells which are made a volume of the model. But if any facet are missing, the shell will opened, and the rifts will be made. These rifts will result in large structural differences between the inner and outer facets of the model.

Wrongly orientated normal

Directions of the normals on some facets can be printed on other side from the other facets. Because those facets will be mismatching from the rest of the surface “parts”.

Not corrected normal

Not corrected normal can show in STL data file in the case when some normal do not coincide with normal on the node of the facet.

Not corrected sections

It shows when the lines of the sections of two facets do not coincide with their edges. In that case they will be made “folded” facets.

False internal structures

Errors in geometric algorithms, when closing the rifts in STL data files, can result in misrepresentation of the false internal “walls” and the structures. Causes of the false internal structures can also be and the solid modeling of the files which do not corresponds with the demands of the rapid prototyping (RP) technology.

Unsteadiness

Unsteadiness can show when is trying to merge two different STL files.

References, Literature and sources:

  • Rapid Prototyping and Rapid Tooling Techniques for the Manufacturing of Silicon, Polymer, Metal and Ceramic Microdevices
    • T. Hanemann, W. Bauer, R. Knitter, and P. Woias
    • Forschungszentrum Karlsruhe, Institut f. Materialforschung III, Postfach 3640, D-76021 Karlsruhe, Germany
Albert-Ludwigs-Universit ̈at Freiburg, Institut f. Mikrosystemtechnik, Georges-Koehler-Allee 102, D-79110 Freiburg, Germany
  • Rapid Prototyping and Rapid Tooling M. Plancek
    • University of Novi Sad 2009
    • Geometric Modeling Based on Polygonal Meshes
  • Scientific research:
    • Leif P. Kobbelt (Organizer), Stephan Bischoff, Mario Botsch Kolja Kähler, Christian Rössl, Robert Schneider, Jens Vorsatz

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