To get a flare radius of 35mm or more the "Heated PVC over a mold" method fails

A custom flare can be made by adding MDF rings to the outside of PVC pipe and using a profiled scraper to shape the flare with automotive body filler.

Cross section of a large custom flare

50mm flares before the body filler is added

A scraper is made from some thin sheet...

and then slid into the port and rotated to shape the body filler into the flare

This technique was used to produce large flares for turbulence testing at up to 31 m/sec. One disadvantage of this method is that you need a new scraper for each flare radius. Here's the eight I made for the tests:

In normal use, port compression considerations will force you to design for a lower velocity, where turbulence can be dealt with using flares smaller than these. However some require large flares when designing subs for automotive SPL competitions.

As it happens, I've helped someone out in this area, and the design is perfect to illustrate the maths.

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