Are you using 1 or 2 clamps? You should be using 2 - plastic should never come out of the mold. You need to heat your plastic to 350 for it to cook, then back off on the heat and shoot them a little cooler. Make sure you don't have any air in your injector by holding it over your cup at a 45 degree angle and purging a little plastic. You will normally see plastic, air, then plastic. You are now ready to go.
Give that a try and see how they come out.
Jason