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Determining dimensions and dimensional stability is key in the development of plastic applications. Some polymers (such as PA) demonstrate a complex behavior in dimensional stability. They have significant mold shrinkage, going through thermal cycles, they demonstrate significant post shrinkage and once assembled, they grow as they pick up moisture.
GE Advanced Materials’ Plastics Application Technology Center is well equipped to characterize these properties in an accurate way. With a coordinate measuring device, the dimensions X, Y, and Z can be measured accurately. Conventional technology measures with single point measurement devices such as mechanical probes, optical cameras, and single point lasers. A disadvantage of these single point measurements is that only a limited number of points are measured, leaving most of the complex surface unexamined, A recent innovation in the field of inspection, the non-contact laser line scanning, overcomes this challenge.
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GE Advanced Materials has installed a Metris LC50 line scanner on a Wenzel CNC coordinate machine in the Plastics Application Technology laboratory. This scanner is able to scan an object at almost 20000 points per second over a width of approximately 50 mm. The scanner also enables GE to rapidly capture an accurate 3D digital copy of the objects. In the development of car body panels GE Advanced Materials’ application technology has invested in measuring technology over the years and, this tool helps advance the process of determining the dimensional behavior of applications made in GE materials to higher quality levels.
The following example shows the advantages of this system in application development
The following example shows the advantages of this system in application development support. As GE promotes its Noryl* GTX (a PPE/PA Blend) for online painted fenders, the maximum possible dimension changes are achieved in this system, namely:
- Molding large parts with tool shrinkage of 1.5%.
- Going through E-coat bake ovens up to 200 °C results in a post shrinkage of 0.5%
- Part conditioning by absorbing more than 1% moisture over time, which leads to a moisture growth of about 0.2%.
Throughout different steps in the process, it is now possible to make a digital copy of the car fender by way of a CNC-based scanning program. The fender is completely scanned in 43 sub-scans at different angles in 47 minutes time. The different angles of the Renishaw PH10 are qualified with an automatic procedure before the measurement. Although the fender has a very dull, black surface, the laser light is sufficiently high to obtain enough reflection to obtain high quality measurements. The end result of scanning is a point cloud of more than 800.000 points. To speed up the data calculations, the point cloud is reduced using a curvature-based filter. Typically, the resolution is decreased to result in a point cloud of approximately 50,000 points.
Beside the scanning software, the CADcompare software based on AutoCAD Mechanical Desktop is used to compare point clouds to CAD models (IGES / VDA) or other previously measured point clouds – thereby enabling the visualization of dimensional changes at all stages in the process.
There are alternative ways to align the data for comparison. An n-point alignment is commonly used. In this technique specific points in the point cloud are roughly aligned with the corresponding areas in the CAD model.
To obtain a satisfactory result, it is important to select areas where the distortion is expected to be low. The calculation of the delta values with the CAD model is iterated (changing the position of the point cloud slightly) to finally result in the most optimal fit - also called the best fit optimization. Once the best fit is achieved, the deviations are visualized in a 3D color map. In addition to the full 3D color map, it is also possible to compare sections sliced through the pointcloud and CAD model.
Another method is to align fixed points in the point cloud with corresponding fixed points in the CAD model, a technique called feature based alignment. The fixed points can be part of the scanned application, but it is also possible to use references in the same coordinate system of the part. These points are also scanned and incorporated as part of the model. By overlaying these points with the CAD model, a direct compare is immediately possible.
The industrial process of online painted car fenders contains process stages that affect the dimensions of the fender. Most important stages occur during the injection molding-, the assembly- and the paint bake processes. To get an understanding of the dimensional behavior every critical step is analyzed by scanning the fender and comparing it to the closest theoretical model. After molding, the fender is positioned on a frame as freely as possible, excluding effects of assembly and even gravity on the dimensions. A scan can be compared with the tool CAD model (scaled for mold shrinkage). In this phase the advised part to the CAD alignment method is a best fit. In the next step the fender is mounted on a body frame (or actual body), using the appropriate assembly technique (fixing to the body); before the application goes through an e-coat paint bake, the fender is scanned. After cooling down from the e-coat bake, the part is re-scanned. Effects of assembly can be analyzed - as well as the dimension change resulting from post shrinkage of the material - by comparing both point clouds. Additionally, the final scan can be compared with the nominal CAD model of the application and its final performance can be judged. The preferred method for the alignment is the feature based alignment.
In the example used, three metal spheres are mounted on the body frame as reference points. By knowing the 3D coordinates of those spheres, they can also be added to the CAD model. By overlaying the spheres from the point cloud to the spheres in the CAD model, the absolute deviation (i.e. in assembly location and due to deformation from applied stresses) is calculated.
When the comparisons indicate unsatisfactory part defects, it is then possible to find the root cause of the possible defects in the process. This method is much faster and delivers many more details than ever before since the user is able to analyze the 3D scanning results of every step in the process. Another advantage is that with single point measurements most of the part is not measured, possibly hiding potential defects. As such the tooling, including different processing conditions, can be optimized more efficiently. Another advantage is that the assembly or fixing technology can be developed at a faster rate. The final evaluations on the fender are reported in easy to understand color plots where the part behavior can be interpreted at a single glance.
Another application based on point cloud technology is the reverse engineering. Reverse engineering allows the creation of a surface model or CAD file from a scanned part. This is a value-adding technique for the marketing units of GE Advanced Materials since the engineering and design units can use digitized products (metal, thermo-sets, etc.) for a redesign of the application towards the use of thermoplastic materials.
The Metris LC50 scanner has proved in several projects to deliver high value to both internal as well as external customers (application developers, molders, Auto OEM’s, etc.). The main advantage is that this technology brings more specificity and accuracy to 3D dimensional performance at a much faster rate.
J.W Heuseveldt
GE Advanced Materials
Global Application Technology
Part Performance
Download: GE Advanced Materials inspects car panels with 3D laser scanner (PDF file)
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