Feature article: PIM International, Vol.4 No. 2 June 2010, pages 53-56, 1834 words

Author: Richard W. Bono, Scott A. Sorensen, Gail R. Stultz , The Modal Shop, Inc. , USA

Contact: Gail Stultz, NDT Product Manager, The Modal Shop, Inc., 3149 E. Kemper Rd., Cincinnati, OH 45241, USA

Metal injection moulded parts are widely used in a number of safety-critical applications, from medical instruments through to automotive, firearms and aerospace products.

Richard W. Bono and colleagues from The Modal Shop Inc. explain how the ‘Resonant Acoustic Method’ of Non-Destructive Testing (RAM NDT) has been adapted to provide MIM producers with an effective in-line system to check 100% of parts for cracks and other flaws.

Introduction 

Resonant Inspection (RI) is commonly used for quality assurance testing of powder metal components, providing a volumetric whole body inspection, sensitive to both external and internal structural flaws or anomalies.

This technique measures a metal component’s mechanical resonances and compares an individual part’s signature to a template generated from a statistically significant sample space of ‘good’ parts.

Traditionally it has been limited to small to medium components with sizes ranging from about a ‘dime to a dinner plate’ given the requirements to fixture and excite the part effectively. However, using a new methodology that employs a drop testing fixture, very small components such as those commonly manufactured by powder metal MIM processes can be 100% inspected reliably, quickly and cost-effectively....

Further sections of this feature include:

- Science of resonant inspection

- Practical application of resonant acoustic method

- Resonant acoustic method applied to small MIM parts

- Case study with experimental results

- Conclusion 

Figures and Tables:

Fig. 1 NDT RAM process flow

Fig. 2  Typical acoustic signature, power spectrum to 50 kHz, for a metal part, shown as processed by NDT RAM with criteria ranges (indicated with green “boxes”) at seven natural resonant frequencies

Fig. 3  Data showing a typical frequency shift detected by NDT-RAM

Fig. 4  Small MIM parts used in medical instruments

Fig. 5  NDT DTF Drop Test Fixture with LanSharc data processor and laptop controller

Fig. 6  Part dropping on force sensor

Fig. 7  Data from multiple MIM parts overlaid at approximately 20kHz

Fig. 8  Standard Deviation of all parts at each of the five frequency ranges

Table 1  Typical structural defects commonly detectable by resonant inspection technique for powder metal, cast and forged processes