Why does my model look smooth in SolidWorks but jagged when I render it?

Nine times out of ten this is the image quality setting on the model itself, not anything in the render software. SolidWorks only exports an approximation of your curved faces when it hands the file over, and if that approximation is coarse you get exactly what you're describing, flat little segments and visible lines along edges that should be smooth.

In SolidWorks, go to the document properties and look for Image Quality. There are sliders there for shaded and draft quality, one for the model on screen and one for tessellation on export. Crank the tessellation one way up. You'll see the on-screen display get smoother too, and if your part is small the file gets a bit heavier, but for a single part or a small assembly that's nothing to worry about. Do this before you export or import into the renderer, because the render software just works with whatever curve data it was given. Fix it at the source like you said and it carries through every time. One more thing to check. Some render tools let you set how finely the imported geometry gets subdivided when it comes in. If you allready cranked the SolidWorks side and it still looks chunky, look for an import or geometry quality option in the render software itself and push that up as well. Also make sure you're re-importing the part after changing the setting, not just refreshing an old file that's already loaded, because the old coarse version will stick around otherwise. I ran into the same thing with some welded frames I was rendering for quotes. Cylindrical runs on the tube looked like octagons til I found that slider. Bumped it up once, saved it into my template, and never thought about it again. If you set it in your part template it becomes the default for everything you make going forward, which sounds like what you want.

Yeah that slider is the fix, but heads up if you're doing big weldments. I render alot of frames and handrails, and cranking it up on a huge assembly made my file slow to open and the renders took forever. What I do now is bump the quality just on the parts that actually show curves, leave the plain flat stuff alone. Worked fine and keeps things snappy. Also curious which renderer you ended up using? Some of them have thier own smoothing option on import like the other reply said, but they don't all handle it the same way.

Ha, this takes me back to the PhotoWorks days when there was no easy fix, you either lived with the chunky edges or smoothed things by hand in some other program. Brutal. One thing I'd add is why the screen fools you, SolidWorks blends the shading between facets so everything looks round, but the renderer shows the real underlying shape. Easy way to confirm you've actually fixed it: make a plain cylinder in a throwaway file, push the quality up, run it through the same export you always use, and compare before and after. Two minutes of checking beats re-rendering a whole assembly twice.

This actually explains something I saw last week. I sent a buddy a picture of a bracket I made with big rounded corners and he said the corners looked like they were cut from a stop sign. On my screen they looked perfectly round, so I thought he was messing with me. Sounds like the same thing going on here, the display is just smoothing things over for my eyes. Quick one though, once I move that slider up, does it stay saved with the file itself? Like if I email the part to someone else, do they get the smoother version too, or is it just a setting on my machine?

Agree with the guy who said only bump the quality on parts with curves. I render a lot of tanks and rolled parts, and when I first set everything in a big assembly to max it took ages to rebuild after every little change. Now I leave the sheet flat panels alone and only touch the parts that actually show round faces in the shot. Also found that once a part is saved with the higher setting it keeps it, so I only had to do it once per part and future assemblies pulled it in automatically.

Update: found the image quality slider under document properties and bumped it up, and the faceted look is gone. Render looks decent now. But the second reply hit home because my big handrail assemblies did get sluggish with the setting cranked everywhere. So does the quality need to be bumped on each individual part before it goes into an assembly, or is there a way to do it in one shot?