Basic Knowledge of CNC Foam Cutting

Density, cell structure and softening temperature decide whether you get a clean edge or a torn one. The machine just follows the toolpath. Pick the foam first, then the cutting method.

Expanded polystyrene (EPS) is the cheapest option and the most forgiving on a hot wire. It is molded bead foam, so a wire slices it cleanly, but a rotary cutter tends to pull beads out of the surface. Use EPS for shape-finding and lost-foam patterns, not for a finished surface.

Extruded polystyrene (XPS) sits in a similar price band but has a closed cell structure, so it holds a sharper edge and resists moisture. It machines well on both a hot wire and a router. Thin walls are still fragile, and fine detail below about 1 mm will crumble.

Polyurethane (PU) is the workhorse for tooling boards and patterns. Tooling board grades are dense, dimensionally stable and machine like soft wood, which is why they hold a surface for vacuum forming and composite layup. Flexible PU foam is a different animal: it compresses under the cutter, so it needs a sharp bit, high rpm and a light chipload.

Polyethylene (PE) and EPP are tough, closed-cell foams used for packaging, buoyancy and impact parts. They cut cleanly with a router but tend to melt and string on a hot wire. If the part needs to survive handling, PE is usually the better choice than PS.

PVC and PET foams are denser again and behave more like a light plastic. They hold threads and inserts, take paint, and are a common choice for signage and structural mock-ups. Dust extraction matters here, since the chips are heavier and the fumes are harsher than on PS.

Hot-wire cutting pulls a heated resistance wire through the block. It is fast and cheap, and it leaves a sealed surface on PS foams with almost no dust. The catch is geometry: a straight wire cuts a ruled surface, so it cannot produce an undercut, a pocket or a sharp internal corner.

Four-axis hot-wire machines add a rotary axis and a tilting wire, which lets you cut tapered wings and two-sided profiles in one setup. That covers a lot of aerospace and architectural work. It still cannot cut a closed cavity.

CNC milling uses a rotating cutter on a three-, four- or five-axis machine. A ball-nose or flat end mill removes foam in layers and can produce almost any shape, including pockets, ribs and undercuts. It costs more per part and needs dust extraction, but it is the only route when the geometry has real three-dimensional detail.

A practical rule: if the part can be described as a swept profile, use a hot wire. If it has a cavity, a stepped face or a compound curve that wraps around, use a router. Mixed jobs often run both, with the wire roughing the block and the mill finishing the detail.

Cutting forces on foam are low, so the workholding matters more than the spindle power. Vacuum tables, double-sided tape and low-melt fixturing all work. Clamp pressure is the usual cause of a crushed edge on soft grades.

Foam moves. It expands with heat, absorbs moisture, and springs back after the cutter passes. Quoting metal tolerances on foam is a common mistake. A ±0.005 mm callout on a tooling board is not achievable in a stable way, and chasing it just adds cost.

For hot-wire cutting on PS, ±0.5 mm on a profile is a fair expectation, and wire lag on thick sections can push that to ±1 mm. On a router with a dense tooling board, ±0.1 mm to ±0.2 mm is realistic for a controlled shop environment. Flexible foams are looser again.

Surface finish matters as much as size. A hot wire leaves a glossy sealed skin. A router leaves a matte, slightly fuzzy surface that usually needs a primer or a sealing coat before paint or composite layup. Tell us which finish the part needs, because it changes the cutting method.

Inspection on foam is usually dimensional, not surface. We check critical features with calipers and a height gauge, and we can supply reports on request. If a foam part is a pattern for a mold, the mold cavity tolerance drives the foam tolerance, not the other way around.