CNC Foam Cutting Guide

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  How CNC foam cutting actually removes material
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  Foam grades and what each one can hold
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  What accuracy foam can and cannot hold
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  When foam is the wrong answer
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  Fixturing, CAD prep, and shop practice
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  Hot-wire vs routing vs 5-axis routing

Foam is cut on the same machine platforms as metal, but the cutting physics are different. There is no chip to shear. A tool or a wire pushes through a low-density cellular structure, and the cell walls either tear cleanly or collapse. The result depends on cell size, density, and how fast the tool moves. A 30 kg/m³ EPS block behaves differently from a 300 kg/m³ PU tooling board, even with identical feed rates.

Two families of equipment cover most work. Hot-wire cutters use a heated resistance wire, usually 0.3–0.6 mm in diameter, running at 200–500 °C to melt through EPS and XPS. The kerf is 0.5–1.5 mm wide and leaves a sealed edge. Routers use end mills, ball nose cutters, or foam-specific burrs at 6,000–18,000 rpm to carve PU, PE, EVA, and rigid tooling boards. Each method has a hard limit on geometry.

Hot-wire cannot cut a closed pocket or a sharp internal corner. It cuts through-and-through profiles, tapers, and airfoil sections. Routing removes material in three axes or more, so it produces pockets, ribs, and sculpted surfaces. On a 5-axis machine, a foam block can be shaped into a complex mold form in one setup. The trade-off is that routing tears the surface more than a wire does.

Heat matters more than most people expect. Wire temperature drifts as cutting speed changes, so a fast pass leaves a narrow kerf and a slow pass widens it. In routing, friction heat can melt the foam onto the cutter, especially with low-density PU. Air blast or a coolant mist keeps the edge clean. Feed rates that work at 100 kg/m³ will burn a 30 kg/m³ block.

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  Hot-wireThrough-cuts, tapers, airfoils; sealed edge, no pockets
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  RoutingPockets, ribs, 3D surfaces; torn surface, needs finishing
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  5-axis routingComplex mold forms in one setup, best for tooling boards
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  Heat controlSpeed and temperature must match density or the kerf drifts

Not all foam is machinable. Expanded polystyrene (EPS) is the cheapest option, with densities from 10 to 30 kg/m³. It cuts fast on a hot wire and is used for lost-foam casting patterns and large sculptural forms. It cannot hold a screw thread and compresses under light clamping. Extruded polystyrene (XPS) is denser and stiffer, with a smoother cut face, but it still dents easily.

Polyurethane (PU) tooling boards are the standard choice for dimensional work. Densities run from 100 to 300 kg/m³, and some grades are filled with glass or aluminum powder for higher stiffness. They machine like soft wood, hold ±0.1 mm on a good router, and accept primers and coatings. PU is the material to specify when the foam is a check fixture, a vacuum-forming mold, or a master pattern.

Polyethylene (PE) and EVA are flexible and tough. They resist impact and chemicals, which makes them useful for protective packaging, gaskets, and bumper cores. They are harder to cut cleanly because the material smears instead of fracturing. Sharp tooling and high spindle speed help. Polypropylene (PP) foam behaves similarly but is stiffer and more heat-resistant.

For high-temperature work, consider polyimide (PI) or PMI foam. These are expensive and usually machined thin. They hold shape at 200 °C or more and are used as core material in composite layups. In our shop, the practical split is simple: EPS and XPS for patterns and packaging, PU board for tooling and fixtures, PE and EVA for protection, and PMI only when the thermal requirement forces it.

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  EPS / XPS10–30 kg/m³, hot-wire, patterns and packaging
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  PU tooling board100–300 kg/m³, routing, fixtures and molds
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  PE / EVAFlexible, impact-resistant, protective parts
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  PMI / PIHigh temperature, composite cores, thin sections

Metal tolerances do not transfer to foam. A ±0.005 mm callout is meaningless on a 100 kg/m³ PU board because the material moves with humidity and temperature. The realistic window for routed PU is ±0.1 mm on a rigid fixture, and ±0.2 mm on larger parts where the block can deflect. Hot-wire cuts are looser, typically ±0.5 mm, because wire temperature and feed rate interact.

Surface finish follows the same logic. A sharp router leaves a visible scallop pattern, and the depth depends on stepover. A 6 mm ball nose at 0.3 mm stepover gives a smooth surface on PU. At 1.0 mm stepover, the ridges are obvious and need sanding. Hot-wire leaves a sealed, glossy edge on EPS, but the surface can show wire drag marks if the feed is uneven.

Moisture is the hidden variable. PU and PE absorb water from humid air, and a block that measures 500.0 mm in the morning can measure 500.4 mm after a rainy week. For tight work, let the stock stabilize in the shop for 24 hours before the final pass. Seal the finished part if it will sit in a damp environment. A coating also protects the foam from handling damage.

Inspection is mostly dimensional. We check critical features with calipers and a height gauge, and we scan sculpted surfaces against the CAD model when the customer asks. Foam does not need the same documentation as a metal flight part, but for tooling boards we still record the raw material batch and the final dimensions. Reports are available on request.

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  Routed PU±0.1 mm on rigid fixtures, ±0.2 mm on large blocks
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  Hot-wire±0.5 mm typical, depends on speed and temperature
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  Stepover0.3 mm for smooth finish, 1.0 mm needs sanding
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  MoistureLet stock stabilize 24 hours before final pass

Foam is a poor choice when the part carries structural load. A 300 kg/m³ PU board is stiff enough for a mold, but it will not replace an aluminum bracket. If the part needs to hold a thread, resist abrasion, or survive repeated assembly, specify metal or a composite. Foam is also a bad fit for thin walls under 1.5 mm, because the material crushes during clamping and the wall deflects.

Tight internal corners are another limit. A router cutter has a radius, so a square internal corner is impossible without a secondary operation. Hot-wire cannot cut a pocket at all. If the design has deep narrow slots or undercuts, the geometry may need to change before cutting starts. We review the model and flag these features during the free DFM check.

Volume matters too. Foam is cheap per kilogram, but machining time is not. A large sculpted block on a 5-axis router can run for hours, and the cost is mostly machine time. For simple 2D profiles, hot-wire is fast and inexpensive. For one prototype, foam is often the fastest path to a physical form. For 500 identical parts, injection molding or casting wins.

Fire safety is a real concern. EPS and XPS burn and release dense smoke. Hot-wire cutting must run with extraction, and the shop needs a clean area free of ignition sources. PU dust from routing is also combustible. We use dust collection on every foam job and keep the cutting area separate from metal machining. This is not a detail to leave to the last minute.

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  Load-bearingUse metal or composite, not foam
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  Square internal cornersCutters leave a radius; redesign or add a secondary op
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  High volumeFoam loses to molding and casting above a few hundred parts
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  Fire riskExtraction and dust collection are mandatory

Foam is light, so it moves. A vacuum table is the cleanest way to hold a flat block, provided the surface is sealed or a sacrificial sheet is used. For sculpted parts, we cut a negative nest in a waste board and drop the block into it. Double-sided tape works for small pieces. Clamps are a last resort because they dent the material and distort the part.

CAD prep is straightforward. Send a solid model, not a surface mesh, and include the finished dimensions. Foam has no standard stock size, so we cut a block to the bounding box plus 10–20 mm of allowance. If the part has a parting line or a draft angle, model it. If the foam is a pattern for casting, add the shrink allowance before sending the file. We can advise on the value.

Tool selection follows density. For PU board under 200 kg/m³, a two-flute carbide end mill with a high helix angle clears chips well. For denser boards, use a compression cutter to avoid fuzz on the top edge. For EPS, a hot wire is faster than any cutter. We keep foam-specific burrs for EVA and PE because standard end mills pack the material instead of cutting it.

Cycle time depends on the part. A 300 × 300 × 100 mm PU block with a simple 3D surface takes 40–90 minutes on a 3-axis router. A complex 5-axis form can run 4–8 hours. Hot-wire cuts are minutes, not hours. If your schedule is tight, the geometry and the process choice matter more than the machine. We quote both routes when the part allows it.

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  WorkholdingVacuum table, negative nest, or tape; avoid clamps
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  File prepSolid model, finished dimensions, shrink allowance if casting
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  ToolingHigh-helix carbide for PU, compression cutter for dense board
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  Cycle time40–90 min for simple 3D, 4–8 h for complex 5-axis