Abstract: A process and system for mixing at least two independent streams in a mixing device having independent inputs for each of the at least two independent streams, comprising structure and steps for: (i) feeding a first stream, provided as a free-flowing solids stream, to a pressure chamber; (ii) compressing the first stream in the pressure chamber; (iii) feeding the first stream to a first input in the mixing device; (iv) feeding a second stream to a second input in the mixing device; (v) impinging the first stream and the second stream on one another in a mixing device to produce a mixture; and (vi) dispensing the mixture from the mixing device; wherein the compressing in Step (ii) is conducted to achieve a substantially linear relationship between a weight of the first stream and a period of impingement of the weight in Step (v). Preferably, the process and system are adapted to produce a filled foamed isocyanate-based polymer (e.g. filled polyurethane foam).