Manufacture and synthesis of a dark micro magnetic flake powder for forensic application

Abstract

In the study a novel method for synthesis of a dark magnetic flake powder for detecting latent fingerprint has been developed. Even though, flake powders ,of aluminium, brass and highly reflective magnetic iron flake are already in use in crime scenes , it is highly desirable to develop a suitable darker variety of magnetic flake powder for print development on light background surfaces.
In order to achieve rapid production of dark metal flakes, a new high energy prototype vibratory mill has been designed, manufactured and develo ed. The design concepts were developed following a comprehensive review of the various commercially available milling devices and undertaking some initial experimentation. The mechanical milling process which
results in changing the particle morphology of starting atomised iron powder to flaky shaped powder was investigated in terms of different milling phases like micro forging, fracture and agglomeration. The effect of milling process parameters on the flake quality was investigated. It was shown that the amount of stearic acid content, ball packing fraction and weight loading were important parameters in determining the final flake qualities. The quality of latent fingerprint development with the dark flake powder was investigated. Some of the flake powders produced excellent ridge quality details with good adherence quality on a range of background surfaces.
The present study has been able to establish the relationship between the visual characteristics ( dark) and adherence property of the flake fingerprint powder with respect to the particle dimensions and surface characteristics of the flakes. It was also found that finer flakes are darker in colour and the low weight percentage of stearic acid has produced the best
adherence quality for the dark flake powder. Some unique relationship between the flake dimensions with the nature of the print deposit was also established as it was found that slightly coarser flakes are suitable for heavy print deposit whereas finer flakes are more sensitive to aged prints. Further, the role of process variables in influencing the milling process were
established such that optimum conditions by vibration milling can be determined to obtain dark flakes of desired quality. While the weight loading significantly influenced the milling behaviour of the powder, the role of stearic acid as an additive was found to influence the surface quality of flakes, besides restricting particle welding. Increasing the pre determined
theoretical ball size does not result in faster milling, but increasing the ball packing fraction from 50% to 70% has proved to be more effective. Some relation with the oxidising behaviour of the powder with vibration milling has also been established. Moreover, the study has demonstrated that high energy milling by a vibration mill can be utilised for consistent production of a novel dark magnetic flake powder.