It starts out with the temperatures of the plastic. The temperature of the plastic coming out of the molding machine should be right down the middle of the manufacturer's spec. If you can run a mold here, you have some latitude down the road when things aren't so optimum. Setting up the molding machine to achieve this is important. It also is important that if you are using a hot runner mold, you set all of the temperatures throughout the manifold at the same temperature on the initial tryout. What temperature? The temperature of the front zone of the molding machine.
  You should think of the front zone of the molding machine and the hot runner as a thermos bottle, which keeps the melt at proper temperature. By the time the material gets to this location it does not have to be heated further, but should simply be maintained at the proper melt temperature until transferred into each cavity. Only by doing initial tryouts with all temperatures set this way can the capability of a hot runner system be tested. If the mold can't be run this way, someone should dig in and find out why. In the long run this will get a mold to the customer that doesn't cause splay, black specks and other issues that can easily be covered up by your wizard tryout person who can jack temperatures and squeeze a good part out of a half-baked mold.               

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