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Flow of the plastic in the mold is quite important to ensure adequate fill; to avoid short shots as well as voids. The Melt Flow Index (MFI) is one of the indices used to characterize the behavior of molten plastic. Quite often the MFI is published by the resin manufacturer. However, the data is quite often gathered under differing test conditions, so apples-to-apples comparisons become difficult. Instead, viscosity vs. shear rate diagrams are often available from the manufacturer.

Typically, viscosity decreases with increasing shear (an exception is thixotropic materials), so that design of the mold geometry can be used to optimize the flow. A relatively cool mold is important to decrease cycle time and to prevent degradation of the plastic due to prolonged heat exposure. However, the flowing plastic hardens on contact with a cold mold. Instead, shear in the flow can be used to maintain flow of the plastic. Shear can be increased by reducing runner cross section so that the flow is more turbulent.

A number of finite element flow modeling packages are available for injection mold design. Computer modeling is time and money intensive, but is necessary to save debug and rework time in a large, complex, multi-cavity mold tool. Some resin manufacturers will perform the computer modeling for their own resin in a customer's mold design.