Guidance on Die Sizing

Rich,

I'm not sure what you are looking for in the way of tolerance or surface finish, but for this extrusion I would suggest some sort of "contact" sizing to obtain the best dimensional control and cosmetic appearance.

Die size could be 2x the tube OD with a 1.05 DDB and some internal air size control or vaccum assist.

Fractional melt is pretty stiff, so you;ll have enough melt strength, especially as you'll be on a small extruder because of line-speed limit, and thus have good control of melt temperature.
You will have to balance (a) die dimensions, (b)the swell that is natural when a crystalline material exits the die, and (c) the 20% shrinkage from melt to solid. Usually drawdown takes care of this balance but will not guarantee the 1:10 ratio you are trying to get between wall and diameter.
If you already have a die in that "ballpark," try it and see how much off you are. It may work, especially as you have die temperature and die-to-water distance to play with. (I assume you are running at an angle down into a water trough = free extrusion. (A sizing sleeve and vacuum box would make the whole operation easier as the tube would "inflate" from the pressure differential to proper diameter, and drawdown would take care of thickness.)
I would normally make the die oversize, as drawdown is easier than inflating to control diameter, but that risks orientation and splitty tube under certain service conditions such as cold temperature and stretching over a metal connector.
Wire people sometimes use a conical die with adjustable pin so the gap can actually be varied, which gives you yet another degree of freedom.
Get some quotes from die makers and see what they suggest, and why. Anyone who says it MUST be a certain ratio or diameter should also specify the material (frac-melt covers a lot of ground), melt temp and line speed.
Maybe someone else will have an opinion, too. I know the rules, but spend more of my time with sized extrusion and film/sheet.
ALGriff, Forum Moderator