QMT Features: May 2013
CT metrology for plastics
Component qualification and tool error correction in light metal and plastic injection moulding: The benefits of highly automated 3D computed tomography


In addition to today's widespread use of high-resolution computed tomography (CT) in quality control,  continuing development of this technology has taken a giant leap forward and is now being used in measuring procedures. 3D metrology using CT allows plastic and light metal components to be measured non-destructively, which, until recently, were not able to be inspected and measured using current methods due to their complex internal structures.

The actual physical measurement is taken by scanning a series of 2D X-ray projection images. To perform this, the specimen is positioned on a precision manipulation system and, during the measurement, is completely rotated through 360° on a precision rotation platform. The quality of the raw data – and naturally the accuracy of all subsequent evaluations of the digitally reconstructed 3D volume – is strongly influenced by the sharpness of the X-ray images, which is heavily dependent on the quality of the X-ray source and detector, including the precision and stability of the manipulation system. You could therefore say: the more effectively the CT measurement system performs this first step, the more precisely the measurement task can be performed.

In addition to a stable system structure that is adapted to the specific application at hand, data processing is the key to successful measurement using computer tomography. The complete process chain can be fully automated using the GE Inspection Technologies phoenix datos|x CT software's click & measure|CT function. This function not only reduces operation time, but also the user's influence on the CT results, which therefore results in even greater repeatability. The process just needs to be programmed once for a work piece, and the complete scanning and reconstruction process is fully automated, including volume optimisation and surface extraction. Thanks to the automatic activation of 3D metrology software such as PolyWorks by InnovMetric, an automatically generated test report can be available in less than an hour.

Effective 3D measuring of a virtual component

A significant advantage of CT is the ability to carry out fast visual checks using nominal/actual value comparisons. (See Fig. 1) For example, as part of first sample report, the finished part can be quickly and easily analysed to check whether it complies with crucial functional dimensions and shape specifications. This provides considerable potential for time and cost savings in many application fields where there is comparable precision compared with the mainstream coordinate measurement technology. PolyWorks offers excellent analysis functions for the polygon models generated by CT data. The user has a number of alignment options available to him, from best-fit, the plane/vector point, through to reference points.
PolyWorks' simple yet exactly parameterisable functions for generating features from polygon models make it a user-friendly and effective tool. In addition to comparisons of nominal and actual value geometries, a range of geometric dimensioning and tolerancing calculations are possible.

The user can choose from various comparative methods and colour scales to show a polygon model's two-dimensional deviation from the CAD reference. Surface comparisons, for example, can be shown directly on the polygon model and/or CAD as a false colour image so that deviations can be seen at a glance. For a detailed analysis, comparison points give the user an exact and repeatable display of deviations from defined nominal value points on the CAD. In addition to its analysis functions, PolyWorks offers versatile measuring tools such as sections, 2D and 3D callipers, profile guides and gap and flush measurements.

PolyWorks can also be used in combination with CT systems as a production aid. Automated processes are greatly simplified thanks to the series parametric measurement function with statistical analysis and macros.
The new automatic project update feature also means that every change to a parameter is immediately applied to alignments, comparisons, measuring tools and reports. Each measurement is therefore reproducible, parameterisable and repeatable. This allows its users to realise considerable time and cost savings.
CT in injection moulding

The German company F. & G. Hachtel GmbH & Co. KG has been benefiting from its use of an industrial phoenix|x-ray computed tomograph by GE since 2008. The company has decades of professional experience in engineering, tool and mould construction and injection moulding. Hachtel uses computed tomography for plastic component qualification and tool adjustments, and also offers this as a service. The following two examples show how using CT directly leads to noticeably increased levels of productivity at Hachtel GmbH.

Fast component qualification:

Complex plastic components are often assessed using geometric dimensioning and tolerancing in combination with RPS alignment. With plastic components alone – whose shape often deviates from ideal CAD specification due to warpage effects – this procedure often results in misinterpretation and incorrect conclusions for the tool adjustment process. This is clearly demonstrated by the plastic components shown in Fig. 2. During the analysis, minimal deviations in the alignment surfaces that are defined in the drawing (left) lead to geometric dimensions and tolerances that are clearly exceeded (Fig. 2 middle component). This leads to the conclusion that substantial geometrical deviations could jeopardise the function of the component. However, the good usability of the sample in question and the positive result of the functional tests contradicted this. A simulated change in the alignment (Fig. 2, right component) also showed that the dimensional compliance of the component was very good overall. Here, in contrast to the CT method, the classic coordinate measurement method only provides measurements in a table format. This means that it is rarely possible to correctly interpret a component's dimensional compliance. In contrast to laser scanning technology, the use of CT means a complete description of the geometry can always be generated, and therefore various alignment philosophies can be tested. In the example above, a time-consuming, expensive and unnecessary tool adjustment process costing approximately €6,000 was avoided. Instead, just 4 hours and €750 were needed to test and qualify the component. 

Tool adjustments:

The nominal/actual value comparison of the threaded ring component (Fig. 3 left)  illustrates good compliance of the component in the threaded area. However, considerable deviations to the CAD geometry are apparent on the outer edge of the component. This deformation causes problems during assembly, and tool adjustment was therefore necessary in this case. Instead of the difficult-to-interpret individual measurements produced by classic coordinate measurement technology, the CT analysis gives the manufacturer immediate access to the information and dimensional allowances required to correct the moulding insert (see Fig. 3 right). The corrected tool design was available after just 4 hours. A renewed CT scan after correcting the mould confirmed the success of the corrective action after just one recursive loop (see Fig. 4).

These examples demonstrate the potential of computed tomography for qualifying plastic components. The phoenix v|tome|x L walk-in computed tomography system from GE Inspection Technologies located at Hachtel's headquarters in Aalen (see Fig. 5) allows large components measuring up to 400 x 1200 mm to be analysed, and light-metal, injection-moulded parts to be tested (see Fig. 1). As a service provider, the company offers the analysis and qualification of components and assemblies in combination with 3D metrology and non-destructive component testing.l

www.ge-mcs.com/phoenix
  
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Rob Tremain Photographer
www.4exposure.co.uk
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