Heat Molded Port Flares

PVC pipe can be heated with a hot air gun and pushed onto a mold to produce a port flare

To make the mold, stack MDF disks of decreasing size and then cover with automotive body filler.
From thinner material, cut a disk of the desired flare radius to use as a scraper.
Draw a "stop" line on the baseboard where you want the port to stretch to.
This method is suitable for port flares up to 25mm or 40mm radius depending on pipe diameter

90mm port flared out to an overall diameter of 140mm

The mold design for an 86mm port

For more photos, plus all the maths, see the Mold details page

Some heat molding Tips

Adding an extra disk on the top which is the same size as the last one makes it easier to keep the pipe upright
Heat the end of the pipe and place it over the mold. Continue heating with the pipe in place. This will help to stop the pipe from kinking.
Occasionally take the pipe off the mold and apply heat to the inside of the pipe. Heating the mold as well reduces splitting.
Keep the pipe upright as you work. Reheating later to "square up" increases the risk of kinking. Setting up a temporary jig is well worth the effort.
Use a plug and circle jig to clean up the outside edge by 2mm to get a perfect finish. This requires the extra 2mm shown in the drawing. When flaring, if the edge looks OK, stopping at the required diameter reduces the chance of splitting.
Buy your pipe before making your mold and jig to avoid a bad fit due to production tolerances, and changes in manufacturer
For a 25mm flare, you will lose about 20mm in length - 15mm due to the changed geometry and about 5mm due to distortion of the PVC depending on the wall thickness. A 40mm flare will cost 30mm in length and a 50mm flare will cost 38mm in length Measure a practice piece before and after flaring. You need to know this if you are flaring both ends.
I've saved the best tip till last. Less heat more pressure.

Walk-through of flaring thin walled pipe

Thin-wall PVC has additional tendency to buckle and to split. The solution is not to overheat and to work quickly. Here's a summary of flaring 90mm pipe.

With pipe not on mold....
One pass of the heat gun around the mold to preheat - For 90mm pipe, each pass should take 30 - 40 seconds only
Two passes the heat gun around outside of pipe
Two passes the heat gun around inside of pipe

Place Pipe onto mold

One pass the heat gun around edge of pipe
Push down and twist pipe - small amount of stretching

One pass the heat gun around pipe at centre of curvature - aim is to avoid overheating outside edge which will cause splitting
and to prevent heat transfer up the pipe which will cause buckling
Push down and twist pipe - about 1/2 stretch

One pass the heat gun around pipe at centre of curvature
Push down and twist pipe - about 3/4 stretch

One pass the heat gun around pipe at centre of curvature
Final push down and twist pipe - full stretch
Keep mild downward pressure on pipe until it cools

Thanks to Paul Spencer on the diyAudio forum, who pointed me to Dan Cramer's project where a pair of 100mm ports are flared using this method

Sizes achieved so far...

See the list of pipe sizes for exact internal diameter and PVC thickness

Nominal diameter (mm)	Flare radius (mm)	Notes
90	20	No problems
90	25	Care needed to avoid bulges and splitting
100	20	No problems
100	25	Prone to splitting - one good flare for every three tries
150	25	No problems
150	30	No problem - could probably do 35mm
225	40	No problem - tried 50mm but problems with splits and bulges

One of the 86mm test ports.

I wanted no joins in the port, which required fitting the MDF rings before molding the second flare

100mm flared port

Another beautiful one-piece port - this time it's 100mm internal diameter made for an acquaintance.

For a one-piece, you usually make the inside flare a little smaller so you can install it after the box is finished.
If the inside is not flanged, you can just stop a bit early in the stretching process.
If you are going to flange, you'll need a second, smaller mold so that the flare finishes at 90 degrees to the pipe.
A 20mm inside flare will give you a 5mm wide surface to glue the port into place.

The smaller inside flare will "chuff" a bit earlier than the outside flare. Because it only "chuffs" on the intake cycle, the inrush of air into the box can mask this. Some tests I did recently found that for normal ports, the inside flare can be about 5mm smaller in flare radius

A temporary jig was used to hold the pipe upright whilst still allowing it to be turned and removed from the mold.

The mold is secured to the table by screws, and a spirit level was used get everything square.

Another trick is to use a jig to mark the port length after doing the first flare.

For short ports, this compensates for any slight angle problems with the first flare.

When flaring large diameter pipe, use a lever for additional force, and some foil to keep the heat in.

When you come to mount your port, you may find that, despite using a jig, the flange is not exactly sitting flat.

Use the heat gun to apply a small amount of heat to the section that is sitting up, then use a flat piece of board to hold it down until it cools.

If you go to the extra trouble of making a one-piece port, it looks fantastic, however a two-piece also has some advantages:

You can install the port after the box has been built and finished
It is easier to flange the inside end of the port - AES preprint 4855 recommends flanging to reduce even harmonic distortion
The inside flare can be the same size as the outside flare
If you get a split or a buckle when making the flare, you only waste one end rather than the whole port.
A two-piece port will however need a PVC joiner

How do these 25mm DIY flared ports compare to commercial ports, which have a 30mm flare?
	Cheaper. Once you have the molds, you can make as many as you want
	Don't have to be imported
	Can be made one-piece
	Less mounting options
	Since the flare goes right to the edge of the port, there is no room for screws, so these ports have to be glued into place. They look best if they're rebated flush with the mounting surface
	Slightly lower usable velocity. According to Flare-it:
	For a 3 inch (75mm) port, reduce speed by 13%
	For a 4 inch (100mm) port, reduce speed by 11%
	The 6 inch diameter can be flared to 30mm, which will equal the performance of commercial flares
	The 10 inch diameter can be flared to 40mm. I'm the only one that can actually sell you a set

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Heat molded port flares