A good elastomer stopper for a vial container system needs to have the right properties. These properties are largely determined by the elastomer formulation, which comprises the base elastomer, fillers, plasticizers, and curing agents. The interplay among these components is crucial, as they determine performance. For example, hardness is affected by filler type and level, as well as by curing agent and level. West understands these issues, and therefore is able to develop formulations to offer maximum performance. An example is the formulation 4040/40, which was designed according to quality-by-design (QbD) principles.