Using X-Ray Computed Topography Scans in the Casting Industry

CT scans offer high-quality resolution, both externally and internally, in a nondestructive manner and enable measurements to be taken on surfaces both outside and inside an object.

The penetration of X-rays prevents the CT scans from being affected by specific object characteristics such as transparent, reflective or dark surfaces, and/or shaded zones present on the object that can create problems with other 3D scanning techniques.

It is also possible to directly compare 3D CT reconstruction models to CAD models to display similarities or differences.

Figure 1 shows an Exhaust gas recirculation (EGR) housing for truck or car diesel engines that was cast by TCDC. This component is made up of two integrally cast stainless steel inserts and an aluminum casting. The three key objectives of the project include measuring the part, inspecting for internal integrity, and dimensionally comparing the exact manufactured casting with the formerly designed CAD model.

Exhaust gas recirculation housing

Figure 1. Exhaust gas recirculation housing

Figure 2 displays the 3D CT reconstruction of TCDC’s aluminum casting. It is possible to control the model in real time 3D and it can also be sliced through in any direction for internal inspection. The reconstruction process has complex algorithms that convert the stack of 2D X-ray images to a 3D volume model.

Angle, diameter, length, and other 2D measurements can be applied to a slice created from the cutting plane of a 3D volume. It is now possible to measure any defect, part, or feature within an assembly or structure without causing any damage to it.

(a) Reconstructed 3D CT volume (b) Reconstructed CT Volume showing virtual slice, and (c) Surface capabilities reconstructed from 3D volume.

Figure 2. (a) Reconstructed 3D CT volume (b) Reconstructed CT Volume showing virtual slice, and (c) Surface capabilities reconstructed from 3D volume.

3D CT Reconstruction

The 3D CT reconstruction has the potential to be transformed to a surface model. The operator chooses a threshold value of radio density that is set with the help of edge detection image processing algorithms. From this, a 3D model can be built and displayed on screen (Figure 3). Generally, models include polygons ranging from thousands to up to 50 million.

Image (c) in Figure 2 exhibits a surface reconstruction with a reflective surface. The user is provided with a variety of applications by the generated surface model.

The format of the output (points cloud, STL, WRL …) suits most rapid prototyping machines for modeling, CAD software for Reverse Engineering applications, Finite Element Analysis software for simulations, etc. In several situations, the polygon mesh produced by the CT system can be used in the applications discussed above without any changes and the resolution is generally higher than what is expected.

It is necessary to process the CT model to make it editable in order to alter or take measurements of the CT surface model. Modeling software, such as Polyworks, Rapidform and Geomagic, recommend semi-automatic tools to convert the polygon mesh to parametric CAD models and Nurbs Surfaces.

3D CT reconstruction

3D CT reconstruction

Figure 3. 3D CT reconstruction

Comparing 3D CT Image to CAD Model

Dimensional analysis is a vital application for model comparison. Extremely accurate dimensions on surfaces are provided by CT, and because of this the technology is often used for metrology studies. Measurements can be carried out either directly on the surface with Metrology software or any CAD, or it can automatically draw a comparison between the CT model and the CAD model.

A comparison can also be drawn between the CT model with another CT model. Processing was not required on the polygon mesh for TCDC’s casting application.

The CT surface file was compared directly with the dimensions of a different surface model, solid or verbatim. Comparison of a 3D CT model was reconstructed by NSI using a CAD model developed by TCDC. The examples discussed above highlight a dimensional comparison between the CT surface reconstruction produced by NSI and the Solidworks CAD model of the part offered by Twin Cities Die Casting.

Following the alignment of the two models, a simple 3D comparison option automatically produces a colored view displaying all the dimensional variations between the two models. In the above example, all of the dimensional differences between the actual CT surface (polygon mesh) and the Solidworks model are indicated by colors.

The green color shows the tolerances between -0.300 mm and +0.300 mm, while yellow highlights areas where the CT scan measurements are greater than the original CAD model; smaller measurements are indicated in blue.

Catering to particular preferences or projects is possible by changing the color code and the tolerance values. Numerical values are also an available option.

3D CT is now increasingly used as a viable tool in the foundry industry. This technology has experienced a rapid growth due to its decreasing prices, increased scan speeds, and user- friendly interfaces. Completely unique to CT is the ability to compare to a reference model as well as the ability to have highly accurate internal dimensions, without damaging the item.

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Conclusion

The advanced CT technology does not require shaded zones and works with a variety of surfaces and shapes. Resolution is also excellent and no post-processing work is required. The ability to obtain the object’s 3D internal structure in a nondestructive manner is considered to be greatest benefit, and this performance can be best achieved by the CT technology.

This information has been sourced, reviewed and adapted from materials provided by North Star Imaging, Inc.

For more information on this source, please visit North Star Imaging, Inc.

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