<I am a noob so please bear with me>
I am having a problem with my prints deforming. The attached photo is of a 60mm OD 50mm ID X 20mm tall sleeve. as you can see when i take it off the printer it is smaller in the middle than at either end. A nice effect, apart from the fact I needed parallel sides

I am using PLA @200°C with a bed temp of 70°C at 25mm/s. I have tried both Slic3r and Cura Engine, both have similar outcomes.
Any advice thankfully received....