Meaning
Geometric geometric parameters for food contact testing define the relationship between the surface area of a plastic container and the volume of food it holds. The surface to volume contact ratio is a fundamental factor in determining the concentration of chemicals that will leach from the plastic into the food over time. In the European regulatory framework, the standard assumption is that one kilogram of food is packed in a container with a surface area of six square decimetres.
However, as the size of the container decreases, the relative surface area increases, leading to a much higher potential for migration. This means that a small yogurt cup will experience a higher concentration of migrants than a large bulk container made from the same resin.
Migration Concentration
The physics of diffusion dictates that the total amount of a substance that leaches out of the plastic is proportional to the area in contact with the liquid. For a fixed amount of migrant, a smaller volume of food will result in a higher concentration, which is the value that is measured and compared against safety limits. This is why the surface to volume contact ratio is so important for the safety assessment of small format packaging.
If a resin is barely compliant in a large bottle, it will likely fail the migration test when used for a small single serve portion. Moulders must account for this geometric effect when selecting resins for different sizes of the same product line. The concentration of additives must be carefully controlled to ensure safety across all packaging formats.
Testing Standardization
To ensure consistency between different laboratories, migration results are often converted back to the standard ratio of six square decimetres per kilogram. This allows for a direct comparison of the inherent migration potential of different materials regardless of the actual size of the test sample. When a lab performs a migration test, they measure the real surface to volume contact ratio of the test cell and then apply a mathematical correction factor.
This ensures that the final reported value reflects the worst case scenario for a standard package. This standardized approach prevents small containers from appearing more dangerous than they are and large containers from appearing safer. It provides a level playing field for the evaluation of all food contact plastics.
Regulatory Compliance
Manufacturers must state the intended use of their packaging, including the smallest size it will be used for, to ensure that the safety assessment is accurate. If a part has a very high surface to volume contact ratio, the producer may need to use a resin with a lower additive content or a more effective barrier layer. This is particularly relevant for caps, seals and gaskets where the contact area is small but the migration could be significant due to the nature of the material.
The declaration of compliance must include information about the ratios used during the testing to allow downstream users to verify that the part is safe for their specific application. Understanding this geometric relationship is a key part of the design process for any food or medical grade plastic component. It ensures that the final product remains within the legal limits for chemical transfer.
