3D puff — sometimes called foam or puff embroidery — is the raised-letter finish you see on premium caps and streetwear. Done right, it makes ordinary letters look sculpted and expensive. Done wrong, foam pokes out of every edge and the finished cap looks like a mess.
The technique is deceptively simple: a layer of EVA foam is placed on top of the fabric, and the machine stitches over it. The foam gives the letters vertical relief. The trick is that the file has to be digitized specifically for foam — you can't just add foam to a flat file and expect it to work.
The stitch order is called 'cap-and-cover.' Perimeter first: the outline is stitched at full density to lock the foam in place. Then the cover pass fills the interior. If you reverse this order, the foam moves before it's anchored and you end up with fuzzy edges.
Density has to increase. Foam is thicker than fabric, so more thread is needed to cover it. Standard satin density (0.4mm) becomes 0.3–0.35mm for puff.
Foam thickness matters. 2mm foam works for finer designs; 3mm for chunky block letters. Anything thicker requires a specialized machine and is rare outside of headwear specialists.
Not everything should be puff. Very small text (below about 6mm) doesn't have room for the foam layer. Fine detail collapses under the added height. Keep 3D puff for the bold elements — logos, initials, big letters — and use flat embroidery elsewhere on the design.