I have had this problem myself, usually when I was printing on a flash plate, or printing on ink... when you push the ink through the screen onto the fabric, the ink can embed itself into the shirt. when you push the ink through the screen on top of ink, the ink has nowhere to go except out to the side...(not to mention that after this occurs you have ink on the back of your screen and have to clean the screen before continuing).
I started using higher mesh screens when I print on a flash plate, stopped flooding the screen and don't apply so much pressure to the print. It's more of a personal thing... you need to just practice with it and build your own technique..
Hi Steve. Two things you need to check on the press itself. First you need to ensure that the bed is level. Unlike other processes, sublimation ink turns into a very fluid gaseous state, so you don't want the ink roaming anywhere it shouldn't. Second thing to check is that you don't have any ink on either your top or bottom platens. Even specks of ink will transfer every time you apply heat and pressure else.
Your most likely culprit however, is that you have the pressure set too high. Most tees press fine with just medium pressure.
If none of the above solve your problems, then you are probably looking at the garment itself. Especially if it is a poly cotton mix.
I've had the same problem myself before now. I resolved the problem by ensuring the printed design was within 1/4 inch (1 cm) of the edges of the transfer paper. If the ink is too close to the edges, then some could seep out of the sides. It is also true that you may be using too much pressure and/or specks of ink on the heat press surface can transfer. Try all of the above and I am sure you will resolve this. Good luck.
Too little pressure will cause this. If there is not enough pressure, when you press the print and the ink gasses, it can blow out to the sides. You want the ink to gas straight down into the fabric. If there is not enough pressure, it will follow the path of least resistance, which will be off to the side parallel to the fabric weave.
What causes blowout on shirts.... 
