Chow Y.S*, Lim Chin Chin, Michael Tay Ming Kiong
International Association of Forensic Sciences 17th Triennial Meeting,
21-26 August 2005, Hong Kong. (oral presentation)
Background: This case study highlights the usefulness of physical comparison to differentiate between mass-produced plastic bottles made using different moulds. A plastic bottle supplier suspected that the factory he contracted to manufacture plastic medicine bottles for him was clandestinely using his proprietary moulds to produce bottles for rival suppliers without his consent. The supplier submitted four sets of bottles for examination: twenty 60-ml control bottles, eighteen 60-ml questioned bottles, thirty 120-ml control bottles and ten 120-ml questioned bottles. Our laboratory was tasked to determine whether the 60-ml questioned and control bottles were produced by the same mould, and likewise, whether the control and questioned 120-ml bottles were produced by the same mould.
Methods and results: The physical dimensions of the four sets of bottles were measured. Each bottle was examined visually and found to be white and translucent with a treaded neck and two types of volumetric scales (“ML” and “OZ”) on its body. The macroscopic class characteristics on the body of the control and questioned bottles were found to be similar. The commercial labels at the bases of the bottles in all four sets were different between sets but similar within sets. From macroscopic examination of the treading on the neck of each bottle, the 60-ml control bottles could be divided into two groups: bottles in group A had treads ending above the “ML” scale markings and bottles in group B had treads ending above the “OZ” scale markings. The 60-ml questioned bottles could similarly be divided into groups A and B. For the 120-ml bottles, all questioned and control bottles had treads ending above the “OZ” scale markings.
The following features were compared using comparison microscopy: font size of commercial markings, dimensions of the volumetric scales, and unique random microscopic defects present in the treaded necks, commercial markings and volumetric scales. Microscopic characteristics and defects within groups A and B of the 60-ml control bottles and within groups A and B of the 60-ml questioned bottles were similar; but these four groups had different microscopic features from one another. The 120-ml control bottles were divided into two different groups based on microscopic characteristics and defects. Similarly, the 120-ml questioned bottles were also divided into two groups according to their microscopic features. The two groups of 120-ml control bottles were compared with those of the 120-ml questioned bottles and all four groups possessed significantly different microscopic features.
The unique microscopic characteristics on the bodies of the plastic bottles indicated that two different moulds produced the 60-ml control bottles. Likewise, two other different moulds produced the 60-ml questioned bottles. The two moulds used to manufacture the 60-ml control bottles were distinctly different from the two moulds used to manufacture the 60 ml questioned bottles. We found that two different sets of moulds produced the 120-ml control bottles and two other different moulds produced the 120-ml questioned bottles. The two moulds used to manufacture the 120-ml control bottles were distinctly different from the two moulds used to manufacture the 120-ml questioned bottles.
Conclusion: The factory therefore did not use the same moulds to produce the questioned and control bottles of the same size. The comparison of macroscopic and microscopic features on moulded bottles is useful for differentiating between different production lines using different moulds.