Bending PET foam

I am endeavoring to make a new Bimini top using PET foam and glassing both faces.
Have built a male former/jig and fixed PET foam in place, the foam (20mm) takes the gentle curves with ease however will not take the tight radius fore and aft.
I believe the way to assist the radius is to cut groves in the sheet to allow bending, my query is how deep should the cuts be and should they be done with a skill saw or a carton cutter blade. I then presume the would be filled with epoxy glue or bog.
Any advise would be greatly appreciated

You might try heat on a scrap piece.

I have built boats with Airex , and about 140F it is easy to bend , and will retain the new shape after it cools.

I haven't worked with the foam but have bent wood and pvc flooring by cutting the concave side.
I Usually have gone approx 3/4 of the thickness to get enough flex but in all cases I do several test pcs to determine depth and spacing to get the radius desired. Spacing can be as or more important than depth and will depend on kerf width. More thin cuts vs fewer wide will give a smoother curve.
Experiment a little even with scrap wood of same thkness as a start then confirm w foam if foam scrap is in short supply.
With pvc flooring I was able to get tight radius 90* bends with combination of cutting back side and heating w a heat gun.

From Armaform website:

"thermoformability: Due to its thermoplastic nature, ArmaForm Core is well-suited for thermoforming. 3D-shaped or double curved sandwich panels are possible without cutting the foam and thus eliminating core stress concentration and increased resin consumption."

Being a thermoplastic material a heat gun should do the trick.

Done it. Kerf cuts using a circular saw with a thin kerf blade, leave a 1/8th inch outside skin, and space the kerfs roughly 1/4 inch apart. practice on scraps to get your curve right, then transfer the pattern to your finish stock. Fill the kerfs with glue to help hold and strengthen them. I use a batten lightly clamped to the foam to use as a guide. Gentle pressure and keep the saw moving so the foam doesn't melt.