Aluminium can be essential in reducing climate impacts as weight reduction is critical. Rheocasting is getting more and more attention from the electronics and automotive industries. The solid fraction in Rheocasting determines the processing outcome. The RheoMetal process is one of the leading processes with the most significant deviation from equilibrium, making presetting the slurry-making parameters difficult. A deeper analysis of the physics of the solid fraction deviation from equilibrium is made based on literature data using a simplistic mathematical model. The developed model confirms that the process is far from equilibrium and that the growth conditions of the freeze-on layer on the cooling agent used in the process determine the slurry temperature and cause the formation of excess solid fraction.