Using Magnetic Levitation for Non-Destructive Quality Control of Plastic Parts

therefore, during the development and use of a manufacturing process visual inspection may be the only form of quality control. In this paper, we demonstrate how magnetic levitation (MagLev) enables: i) the identifi cation of parts with embedded defects, ii) the separation of a defective part from a single batch, iii) the characterization of certain kinds of defects, and iv) the detection of counterfeit, high-value plastic parts. MagLev can suspend, localize, and orient [ 12 ] an object without contact with a solid surface, such as a gripper or solid wall, by balancing gravitational and magnetic forces. [ 13 ] Here, we show that the orientation of a part that is magnetically levitated depends strongly on both its shape and heterogeneity in density. This characteristic enables rapid identifi cation of a defective part by visual inspection of the levitation height or angle of orientation in comparison to the rest of the parts in the batch. The method is inexpensive, non-destructive, and straightforward to implement. The portability of the MagLev device has the potential to allow quality control of parts at the point-of-manufacturing, the point-of-sale, and the point-of-use. We used a similar MagLev device setup to those previously described. [ 13 ] Briefl y, the object is suspended in a container fi lled with a paramagnetic solution (e.g., aqueous manganese chloride, MnCl 2 , or paramagnetic ionic liquids) [ 14 ] and the container is placed between two NdFeB magnets oriented with like poles facing each other ( Figure 1 ). In this confi guration of the magnets, the magnetic gradient along the vertical centerline is approximately linear, and the vertical equilibrium position of the diamagnetic object within the device—its levitation height, h , measured as the distance between the face of the bottom magnet to the centroid of the object—correlates linearly with its volumeaveraged density s ρ (kg m −3 ), as shown in Equation 1 : [ 13 ]