If you have flats that are parallel to the PDMS surface, then the first unsupported layer of the flat will be only 0.03 mm thick. When the layers are this thin, they are not very strong and will need lots of supports. Try using the preview to see layer by layer the print as it is exposed. The preview will show the unsupported areas in red. If you see areas more than about one to two mm that are red, you will likely need more support. Tilting the model will reduce the unsupported layers.