Hand working on the parting line also be one of the reason for wall thickness is not even. Of course, most of the reason is from the plastic chair 3D design and poor polishing on the cavities and cores. So when we start a plastic chair design, we have to consider the wall thickness. Sometimes, we need to make some area a little thicker to be sure that the chair is strong enough. But we have to consider seriously when it is need. As because the uneven thick will caused the injection not balanced and also will caused the cooling time not even. This will effect the injection speed slow down and also will caused the shrinkage marks. If the CNC milling is too rough, you can never get a chair with thickness even.