You mean a completely solid plastic model... then yes, fill density will do it. Set it to 1 as this is 100%. Takes a lot of plastic, but if it isn't too big a model you can get away with it. If you fill the bed (a 6x6x6 model) you will probably go through close to half a spool and it will take forever, but if you need the strength.
That being said, you can save time and plastic by printing a 50% infill and still have pretty good strength. If you're trying to use it for a casting, you definitely don't want 100% infill as that would be more plastic to have to melt away.
Don't get me wrong, if you need to print at 100% for your needs, do so, but the alternative do exist.