Predicting the microstructure of the as-cast HPDC (high-pressure die cast) product is valuable, because micro-scale features often determine its mechanical properties. To predict the microstructure, the effect of processing parameters such as pressure and cooling rates must be known. The object of this study is to create state-of-the-art models for predicting heat transfer in the HPDC process, and apply those models to predict the evolution of one feature of the microstructure: the size of polyhedral α-Fe intermetallic phase. In the study, we develop a new empirical correlation for the Nusselt number in water cooling channels. This can be used to validate heat transfer coefficients for water cooling channels in commercial software to assist in modelling heat transfer in the HPDC process. Additionally, we develop a model for impact pressure in HPDC, which augments the state-of-the-art Hamasaiid model for peak IHTC (interfacial heat transfer coefficient) in HPDC, and relaxes some of their empirical assumptions. We integrate the IHTC model as a custom boundary condition in FLUENT 18.1 using SCM and UDF-files. Finally, we predict the size of polyhedral Fe-rich intermetallics using commercial casting simulation NOVAFLOW&SOLID for cooling rates and classical solidification theory for intermetallic size, and validate the results using optical micrograph size measurements.