STLs nee to represent solid parts, so you can't have a ton of open faces or surfaces. The part you posted looks like it was just hacked out of a larger model, so it has a ton of open faces. You can try automated repair like netfabb (
https://netfabb.azurewebsites.net/), it might be able to figure out what you're getting at, but the best approach is to close the surfaces properly.