Converting mesh files to solid formats is a complex task.
While converting files from mesh to solid you should look for accuracy, number of patches, file size and many other variables that should be taken in consideration based on the use you will give for the solid file.
I have compiled some thought as a starting point for this research, make sure your STL/OBJ/PLY (mesh) file has a decent quality. Meshmixer/Meshlab can do a validation on the file quality.
You will CONSIDERABLY increase your chances by having a watertight model. If the model does not have a closed mesh, you will most likely have a hard time/fail converting it.
There are 4 ways to do it:
1 - Using free software
This is a valid solution for a simple shape models. Here are some tutorials available on internet:
2 - Professional software solution
This is probably the best solution for companies with a high conversion volume and other CAD/CAM demand. The best solutions I have found:
3D Systems Geomagic Wrap
Power Surfacing (SolidWorks plugin)
3 - Hiring a professional freelancer
Higher chance to get a decent converted file for an affordable price. Try to look for the feedback/customer review before hiring someone. I have seen professionals charging as cheap as US$25 per model/conversion. You can check on these options (there are many others):
4 - Online conversion platform
There are solutions with instant delivery, and other that takes some days to process the file. I found solutions varying from free online conversion to expensive prices. There are prices as low as US$1 per file. You can check on these platforms (there are others on Google):
Have in mind that, the higher is the model complexity (shape), the harder is the model to convert.
If trying to conver a 3D scanned model, make sure to have a closed mesh and a high density mesh definition. Don’t expect to convert a model 3D scanned using a smartphone to anything decent.
During your Google research, you can also use the term “model surfacing”.