In a forensic comparison of materials it is usually the remit of the scientist to establish what, if any, association there is between different items.
In drugs cases involving packaging, the examination of plastic bags and cling films can often yield vital evidence to link (or not) a person with either another person or a location.
A number of physical methods have been developed to examine plastic commonly used for packaging drugs. These physical methods generally depend on identifying characteristics that are permanently left on the bag or film during the manufacturing process. Specialised lighting techniques are used called polarisation and shadowgraph.
Examples of such identifying characteristics are striations, pigment bangs and ‘tiger stripes’. It is commonly observed that striations are left on the outer and inner surface of plastic film, from the die and mandrel, and rollers used to extrude the molten plastic.
Typical striations lines seen in a plastic bag.
Plastic bag evidence can be extremely valuable when interpreted appropriately. For example, in a case where a large drug seizure has been made, and on searching a suspect’s house or vehicle some plastic bags or film are found, it is then possible to compare the plastic packaging found containing the drugs to plastic found in the suspect’s possession.
The visualisation of machining marks can either show that the plastic packaging has completely different marks and is unrelated, or that the packaging has similar marks and has been manufactured on the same machinery at around the same time, or that the packaging was joined together during manufacturing, before being cut into bags or smaller lengths of film and is therefore most probably from the same packet after manufacture.
It is important to be careful when making conclusions in this manner. All of the bags in one packet may not be in sequence. Also, during quality control procedures at the manufacturing plant, some bags may be removed from the packet for testing and replaced with bags from another packet or batch.
Therefore, if a plastic bag discovered in a suspect’s possession is found to match a bag containing illicit drugs, it can be said that the bags were manufactured on the same machinery at around the same time, or, in some cases, that the bags were manufacture, and were therefore from the same packet of bags.
However, if the plastic bag discovered in a suspect’s possession does not match the plastic bag containing illicit drugs, it cannot be said that the bags definitely did not come from the same packet.
