To produce highly transparent flat film from polypropylene, the melt must be cooled as quickly as possible to form the cast film. With polypropylene, heat transfer in the cooling unit is associated with crystallization processes. These make it difficult to calculate cooling times and subsequently the geometry of the cooling unit. A mathematical method for designing cooling units for polypropylene melts is described. The calculated results are compared with experimental findings and recommendations for optimized cast film cooling are given.