Cracks develop in Zinc Castings

All, I use zinc casting ZA5 to cast a crash ring around Stabilizer bars, very often we find after these parts are painted that we have cracks at the parting lines. We have really no expierence in the casting business,this is a new process and we need some advice on what could be causing this problem. Let me explain the process a little, we use zinc ingots that are melted into a melt pot at 435 C there is a gooseneck in the melt pot with a fill gap, the plunger extends pushing the zinc through the gooseneck and out of the nozzle into the machined dies filling the cavity and casting around the parts. One more thing the nozzle is above the zinc so it is heated to 575 C. the die are water cooled and we have a chiller controlling the temp to 80 F. Any advice would be greatly appreciated.