When kinking, flattening or rippling of larger size conduit is resulting, it’s best to replace the rollers and shafts before replacing the clamping jaw. Usually the rollers have worn out and are just due to be replaced. This will cure about 95% of all these issues. Also, make sure that there is an ample amount of conduit sticking out past the clamping jaw. If the end of the conduit is too close to the clamping jaw, it will start to flatten the conduit as it is bent due to the force being put on the conduit. The bending shoe and conduit may also need to be sprayed with a bit of WD-40. If the conduit is sticking to the bending shoe, it will start to pull and stretch the conduit which can cause deformation in the conduit.
If you are having this problem with PVC-Coated Rigid Conduit, try cleaning the shoe and conduit with rubbing alcohol. This is recommended by one of the leading manufactures in PVC-Coated Conduit.
Gardner Bender FAQ 