True "mandrel" benders are somewhat similar to "conventional" draw/pull through benders (like the JD2 stuff). They have a similar bending inner die/outer shoe design riding on the outside of the tubing. But they also have an inner mandrel fixture (think of a series of round metal spheres linked together) that is driven/pulled inside super thin wall tubing (like header J-bend kit tubing) on tight bends to help make the super thin tubing in the bend as "perfectly" round as possible.
If you already have a cheap HF pipe bender & some tubing on hand, you might as well try the sand in the tubing trick just for sihts & grins. Years ago one shop I rented had some dirt track guys in the next stall. The HF bender/sand deal was all they ever used on their stuff, sometimes it worked OK, & sometimes not so good. Usually if they used sand & it still collapsed the tubing it was because either they didn't pack the sand in good enough, or they were trying to bend the tubing at too great of an angle.
It should be noted that even a damn good manual/power "conventional" draw/pull through bender like the great JD2 stuff won't bend tubing 100% "perfect". There will always be some small/very minor amount of deforming of the tubing walls during bending (flattening on the outside of a bend & start/stop dimpling on the inside of a bend) with these die/shoe style benders. It might be hard to see sometimes, but it's always there. How noticeable the deforming is really depends on the tubing used (material, OD, and wall) the brand bender (some are just designed a little better than others) and how tight the bend is.
And they will also stretch the tubing wall a small given amount on the outside of a bend, the tighter the bend/greater the angle, the greater the possible stretch will be. Some bender brands are better on this aspect than others. With some brands the stretch is easy to see when the tubing bend is cut open, and some the stretch is so minor it's not really noticeable. The whole "wall stretch" thing on the outside of a bend is really a moot point anyway since now days tech inspectors for NHRA/IHRA/SFI certs aren't supposed to sonic test on the bends anyway, only the straight runs are supposed to be tested.
But even though the conventional benders aren't 100% "perfect", they are still 1000 times better than the cheaper HF style pipe benders.
I bought my JD2 bender somewhere around 1985-87 (IIRC) and it's still in damn good shape. The original 1 5/8" die/shoe it came with is a little worn, but still gets the job done.
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