Spray foam in 3D printing: How PU foam seals FDM parts and makes them more stable

Closed-cell, self-expanding PU foam has long been standard in construction. A YouTuber named Alex is now using the material for a rather unusual purpose: he fills cavities in FDM-printed components to make them denser and more pressure-resistant. This is particularly interesting for 3D-printed boats or housings with electronics that are in constant contact with water.

The starting point is a well-known problem with FDM printing. Due to the layered construction, small gaps remain between the layers and in the infill. Even with thick walls and many perimeters, the parts are often not completely sealed. PU foam can help here because it forms closed cells after curing and is very good at keeping water out.

For the tests, Alex designs test specimens with a filling port for the spray foam and several relief openings. After foaming, the material cures for 24 hours, and excess foam is then removed mechanically. Previous tests show that the interior of the components should be moistened before foaming so that the moisture supports the chemical reaction of the foam. In addition, each larger part needs enough relief openings, otherwise the expanding foam can burst the wall structure.

In a long-term test, filled and unfilled PETG cubes are placed in a water bath for around 28 hours. Some moisture enters the material itself, but none of the cubes fill with water. Tensile tests show little difference between filled and unfilled samples. Under pressure, however, the resistance to buckling increases by around 25 to 70 percent, depending on the geometry.

The insulating effect of the foam is also measurable. In another experiment, Alex insulates a printed box with PU foam and compares it with an unfilled container. The ice cubes in the foam-filled box last almost two hours longer.