File Preparation

As we know, RP is a technology for converting 3D solid modelling, virtual, CAD files into physical models. Since there are many different types of CAD file format, it is necessary for CAD files to be converted so that the RP machine can read them. Nearly all RP machines use the same file input method for model files. This method is called the STL format after the stereolithography process that first defined it. STL files are surface approximations of the solid models generated within the CAD system. The approximation is performed by forming the closed surfaces using planar triangles. If the triangles are very small, then the surface approximation can be very accurate, although for curved and freeform surfaces (like medical models) the file sizes can be very large.

The reason for the triangle meshing is that it makes the slice preparation quite straightforward for the RP operating system. Slices are basically generated by checking the intersection of

Figure 1.5 CAD part showing the triangular facets of the STL data

triangles with a plane that represents the Z height for a particular layer. Each intersecting triangle will form a vector that is part of an outline for the object layer. Vectors must be connected up to form a complete outline contour. Since it is possible there will be many contours forming a particular slice, it is also necessary to determine nested contours. Contours nested inside another represent a cavity or hole, for instance, while further nesting may represent other regions of the object to be fabricated. Conversion to STL format is a relatively simple process and there is generally no distinction between different objects. Separate objects could be combined into the same STL file. Sometimes this is helpful if the objects interlock in some way. Chain links, for example, could have each link physically separate from each other, but they can be interlocking to form a complete chain. The links do not require to touch each other in the STL file and so space can be formed around each element so that they can freely move within the chain. This could be helpful for complex medical modelling of anatomical joints or other interlocking bone regions.

The STL files, once read into the RP machine, generally require some software processing corresponding to the machine itself. Machines that can use different materials may also require different set-up parameters for each material, and these should be selected at this stage. Often there is a need to make an assessment of the complexity of the geometry to ensure the part is going to be built optimally. Some examples include:

  • Orientation of build. Since machines are heterogeneous in character, parts can vary in quality if built in different orientations. Finer detail is generally found on upward-facing surfaces, for example. Also, parts will vary in build time, depending on the height of build.
  • Thick and thin wall decisions. Thin-walled structures may require additional energy or material to ensure effective build. Thick-walled components may correspondingly require less energy or material to avoid part distortion or unnecessary wastage. In such cases, fill patterns may be chosen as an option.
  • Complex geometry. Some RP machines may require consideration relating to the complexity of part geometry in terms of trapped volumes, overlapping features, etc.
  • Support removal. Related to the above, complex geometry components may have supports that cannot be easily removed once the part is complete. Decisions on whether to modify or remove these supports must be made.

Many of these issues are particularly relevant to medical models since these are often complex, freeform models that are not designed using any specific engineering principles. Such models represent a challenge to RP model makers, and therefore considerable expertise is often required to build such models effectively.

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