Fluid products, particularly highly filled materials, are manufactured and cast in a continuous system utilizing feedforward composition quality control and plug flow. Liquid and solid raw are supplied to, and combined in, a continuous fluid flow system in which plug flow is established. Quantities of materials supplied are measured and averaged over time to determine composition proportions, which are compared to specification. Running averages are maintained for successive serial portions of the continuous plug flow stream. In-specification product is diverted to casting and off-specification to waste, after a time delay so that appropriate diversion is accomplished for each particular, composition-determined serial portion of the product flowstream.