There are some pretty impressive online fabrication houses to look into that can help you work through manufacturing process options: Fathom, Stratasys-direct, Protolabs, ICO Mold, Xometry
If you want a DIY approach to producing parts you could think about casting polyurethane parts in Silicone RTV molds from a 3D printed buck. This is something you could do at home without much extra support hardware, maybe just a vacuum chamber. Check out Smooth-On (smooth-on.com).
You mentioned rotomolding which sounds like you are looking for large parts. Because of the high cost of rotomolding tooling I would recommend looking into fiberglass construction for dimensionally large short run parts. If you can find a local CNC shop you can have foam bucks machined and then take them to a fiberglasser to create a run of parts or one-offs.
DIY fiberglassing is also very possible on a reasonable budget.
For most molding processes you will need to get familiar with draft angles, part lines, minimum wall thickness, sprue placement, etc. These are things that will have an impact on your parts during the design phase.
Very generally speaking on manufacturing, it is good to get into the habit of defining your budget, how big the parts you want to make are, how many you need and what material you want them in. As you answer those questions the better fabrication methods for a part will become more obvious.
Final (and probably unpopular) piece of advice; I would stay away from buying a CNC machine or other large equipment until you know exactly what you need. If your goal is to make finished prototypes and products then it's usually better to spend your time understanding the various manufacturing methods vs learning to be a machine operator for a specific process.
If you want a DIY approach to producing parts you could think about casting polyurethane parts in Silicone RTV molds from a 3D printed buck. This is something you could do at home without much extra support hardware, maybe just a vacuum chamber. Check out Smooth-On (smooth-on.com).
You mentioned rotomolding which sounds like you are looking for large parts. Because of the high cost of rotomolding tooling I would recommend looking into fiberglass construction for dimensionally large short run parts. If you can find a local CNC shop you can have foam bucks machined and then take them to a fiberglasser to create a run of parts or one-offs. DIY fiberglassing is also very possible on a reasonable budget.
For most molding processes you will need to get familiar with draft angles, part lines, minimum wall thickness, sprue placement, etc. These are things that will have an impact on your parts during the design phase.
Very generally speaking on manufacturing, it is good to get into the habit of defining your budget, how big the parts you want to make are, how many you need and what material you want them in. As you answer those questions the better fabrication methods for a part will become more obvious.
Final (and probably unpopular) piece of advice; I would stay away from buying a CNC machine or other large equipment until you know exactly what you need. If your goal is to make finished prototypes and products then it's usually better to spend your time understanding the various manufacturing methods vs learning to be a machine operator for a specific process.
Good luck!