STL vs STP

Hi
I need a theoretical or practical answer to a question please...
I understand that new rapid prototyping methods like 3d printing have made prototyping much more quicker and cheaper. I am of the understanding that designers use tools like 3d prototyping to design out features and flaws and then when finally happy, flip the same file into production.
Given it's not easy (at all) to actually convert a 3d printing STL file to a production die casting STP format, how do designers and companies make that jump?
Have I got it wrong in that companies don't actually design in STL for speed and cost and then convert the same file to STP? Or do designers prototype in STL and then when finally happy, redraw the whole piece in STP (but potentially introduce news flaws)?
Thanks
P.s. I'm using PTC Creo Direct Elements, not Autocad per dropdown box... I can't get to box to scroll to PTC
4 Answers

i think u take things from a real complexe view, it's just easy as Mark Burke said, just Save As and then u choose your format, not just STL but all formats are availible.
i use SolidWorks and Catia, so if i want to send a file to someone using Creao for example, i give STP or STEP (just different names but same usability)
if u mean other thing then please be exact in your question

Thanks for the info guys.
My problem is the native format that Creo Direct Elements saves in is .pk2 which doesn't seem to be used elsewhere. No option for STP.
...So I feel stuffed :[
I'll check the other file formats tonight and post then here in case anyone has a idea of how to handle the source file to get to a clean STP file.
Thanks vm all... so far...

I am a Solidworks user and not Creo but in Solidworks both .STP and .STL are secondary output files. I only create an stl file if I am going to print an object. I use .STP files if I have to send the file to someone using a different software. Converting .STP to .STL is as simple as saving a file type. Converting .STL to .STP or any other modifiable file type is a pain and not worth the hassle in my opinion. I hope this helps.

Thanks Mark...
...so per original question, do you usually use STL formats to rapid prototype, or some other format? Just trying to work out what the industry norm is.
Are there freelance designers out there that can re-create STL models into a more universal format?
I'm a lone home inventor with no big budget, so when I saw Creo downloadable for free, I've been using that blissfully unaware of the STL to STP issue :/