The foam sheet just lays on the ground, the frame straddles it and is simply pulled along.
Although a slow cutting process, it did work well, shaving 1" out of 3" thick foam sheets that were 69" (175cm) long. One was 19" (48cm) large, the other 38" (96cm).
Unfortunately, polyurethane is a foam that's supposed to be cut mechanically, as using a hot wire releases toxic fumes and leaves a sticky surface:
5 days later the smell has subsided noticeably but is still faintly present. Will have to scrape the melted smelly & sticky residue. Wonder if there's any substance that can be sprayed on it to passivate or cure that ? Anyway, onward to getting a long knife...
Oh well, live and learn 😂 Can still use this jerry-rigged hot wire on other foam types.
Happy cutting / shaving / slicing to all !
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Construction
The frame was simply built out of scraps of wood held together with wood screws:
The wire is kept taught thanks to a tensioning arm. The pivot is just a wood screw.
2 alligator clips on each side of the wire ensure good enough connection for the current used:
Wire & Power
The NiChrome wire used is 14 AWG gauge (1.65mm diameter).
It is simply attached to the frame with a wood screw on one end, and tensioned around a 0.5" (12mm) spacer at the other end to avoid kinking (would make reuse difficult).
A clamp plier holds the wire's other end via friction against the wood. It provides an easy way to dial in the tensioning arm's angle, and also avoids kinking the nichrome wire.
The alligator clips only power the stretch of wire that does the cutting.
A Volteq HY2050EX 50V 20A variable DC power supply was used to find out the proper current / voltage settings: at 20A the wire was glowing red and grew longer under the pull of the tensioner, until failure. At 12A the wire could cut the foam but too slowly. 15A worked best in the end.
15V was necessary to drive the 45" (115cm) long piece of energized wire at 15A.
Results
Cutting was a slow process. With a clean wire the first ~5" (13cm) go fast, but as the melted foam starts burning away and coating the wire, the progress slows down considerably and requires more pulling force.
The steady progress rate was about 1/4" (6mm) per second for the narrow 19" piece of foam, and 1/8" (3mm) per second for the large 38" piece. It would surely go faster in much lighter foam types like Polystyrene.
However, the gases released during the cutting were horrible. Had to open doors and windows, and wear an N95 mask. Fortunately it was a windy day !
The cuts were pretty consistently straight / flat. The first few inches were very clean, as if cut with a sharp blade, but then blisters appeared:
Worse, while still dialing in the process, when 20A was used, melted gooey foam was left behind in the form of stripes, as can be seen in this picture:
2 days later these stripes were still sticky and smelling the most.
So for now I'll let the foam outgas in the open. Might take a couple of weeks for the smell to go away, if it ever does. Meanwhile, am in no hurry to put that stuff back in the camper van, the outgassing would likely be extremely unhealthy...
Might have to scrape off the blisters and stripes, or shave them with a long bread knife or something.
So... How to Cut Polyurethane Foam ?
Well, after this gooey experience, I learned that PU is pretty much the one foam that should not be cut with a hot wire 😂
To cut slices of PU foam, pros use machines with an endless blade that is held horizontally by a guide. Similar principle as a bandsaw whose endless blade goes around wheels:
For much shorter cuts, like through the thickness of a sheet up to ~1' (30cm), reciprocating single or double blades are typically used, like on an electric bread cutting knife or a foam cutter:
Me learned something. Was fun, will definitely make more mistakes from now on !
Series of blog posts describing the Ford Transit van's bed this mattress is for:
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