For ripping on the corners, you first need to figure out if it's a cold tear or a hot tear. If the material feels jagged then it's a cold one and you need to increase your preheat, if it's a hot tear (which I doubt) then you'd need to lower the temperature.
The next thing you can look at is where the tear is happening. Is it on interior doors, inside the press, or only on the outside. If it's on the interior, then you may be able to move the parts just slightly away from each other to give it more vinyl to stretch but if it's on an exterior side and there is room for a dummy jig to place on the bed that will help keep the material from sliding/pulling too much towards the edge of the press.
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