How to convert multiple xls files to csv using powershell?
Try moving the code to launch and quit excel INSIDE your loop.
Yeah, it's slower that way, but it'll encourage Excel to clean up its temp files between each operation.
It won't be as slow as you think because Windows and COM are smart enough to keep Excel mostly in memory even after you quit for a few seconds so that the next time you create an Excel object it'll happen fast, exactly for situations like this.
Maybe you can implement a timer to wait a while? Or you have a look inside the direcotry while you script is running and then remove the temp files after each cycle (one xls file conversion).
I believe you can avoid the message box asking if you want to save by using $wb.Close($False) instead of $wb.Close($True). That's what I use normally and I don't get any message boxes.