A method for forming a polyurethane foam, said method comprising the steps of: supplying an isocyanate component from a source of isocyanate to a proportioner; supplying a resin component from a source of resin to a metering unit by a resin pump; supplying an additive having an unstable state from a source of additive to the metering unit by an additive pump, the metering unit including an electronic control system comprising a plurality of relays communicating with the resin pump and additive pump; and activating and deactivating the resin pump and the additive pump with the electronic control system, forming on demand a reactive mixture comprising the resin component and the additive by the metering unit by adjusting a ratio of the additive mixed with the resin component, wherein the metering unit is configured to adjust the ratio on the fly to accommodate for changes desired in characteristics of the polyurethane foam and changes in atmospheric and environmental conditions by adjusting a pump volume control on the additive pump; delivering the reactive mixture by the metering unit to an accumulator disposed between the metering unit and the proportioner and storing the reactive mixture in the accumulator for use on demand from the proportioner; demanding a supply of the reactive mixture with the proportioner; supplying the reactive mixture by the accumulator to the proportioner in response to the demand by the proportioner and mixing the reactive mixture with the isocyanate component by the proportioner to form the polyurethane foam; and tracking the amount of the reactive mixture in the accumulator by a low level sensor and a high level sensor in the accumulator, the level sensors (60. 62) communicating with the electronic control system to activate/deactivate the resin pump and the additive pump, and forming and delivering the reactive mixture to the accumulator when the amount of the reactive mixture in the accumulator is below a predetermined amount sensed by a low level sensor in the accumulator. supplying an isocyanate component from a source of isocyanate to a proportioner; supplying a resin component from a source of resin to a metering unit by a resin pump; supplying an additive having an unstable state from a source of additive to the metering unit by an additive pump, the metering unit including an electronic control system comprising a plurality of relays communicating with the resin pump and additive pump; and activating and deactivating the resin pump and the additive pump with the electronic control system, forming on demand a reactive mixture comprising the resin component and the additive by the metering unit by adjusting a ratio of the additive mixed with the resin component, wherein the metering unit is configured to adjust the ratio on the fly to accommodate for changes desired in characteristics of the polyurethane foam and changes in atmospheric and environmental conditions by adjusting a pump volume control on the additive pump; delivering the reactive mixture by the metering unit to an accumulator disposed between the metering unit and the proportioner and storing the reactive mixture in the accumulator for use on demand from the proportioner; demanding a supply of the reactive mixture with the proportioner; supplying the reactive mixture by the accumulator to the proportioner in response to the demand by the proportioner and mixing the reactive mixture with the isocyanate component by the proportioner to form the polyurethane foam; and tracking the amount of the reactive mixture in the accumulator by a low level sensor and a high level sensor in the accumulator, the level sensors (60. 62) communicating with the electronic control system to activate/deactivate the resin pump and the additive pump, and forming and delivering the reactive mixture to the accumulator when the amount of the reactive mixture in the accumulator is below a predetermined amount sensed by a low level sensor in the accumulator.