A method and apparatus for filling an expandable in-ear device having in-flow and out-flow canals including a plurality of chambers with elastic membranes in which the components of a settable polymer are held under high relative pressure. The component chambers are connected to a common mixing element which mixes the components before they are applied to the in-flow canal of an in-ear device. A sealed relief chamber is connected to the out-flow canal of the in-ear device and receives excessive mixed compound as it exits the in-ear device. The relief chamber is in contact with the elastic membranes of the component chambers such that when the elastic membranes contract the pressure in the relief chamber decreases, maintaining the pressure within a pre-determined pressure range.
