Hotline CNC Basics for Foam and Wax Parts

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  What hotline CNC basics actually describe
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  Which materials hotline CNC can and cannot cut
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  Temperature, feed, and the kerf they produce
• 4
  Where the process earns its place
• 5
  Hotline CNC versus laser and milling
• 6
  Choosing a process by material and geometry

Hotline CNC is a wire-based cutting process. A thin resistance wire, usually nickel-chromium, is clamped between two arms and heated by current. A CNC motion system moves the wire or the worktable along a programmed path, and the wire melts the material as it travels. There is no spindle, no cutter engagement, and no chip load. The tool is a wire.

The cut happens by thermal softening, not by mechanical shear. Material inside the kerf reaches its melt or softening range and is pushed aside or vaporized, so the wire never fights the part for cutting force. That is why a 1.2 m tall foam block can be cut on a frame that would collapse under a milling cutter of the same reach.

Wire diameter sets the smallest inside corner you can leave. A 0.4 mm wire removes roughly 0.5 mm of material on each side once you add the melt zone. Design the drawing around the wire, not around the tool you wish you had.

The path is CNC-controlled, so the same G-code mindset applies. Two-axis cutting gives straight profiles and tapers. Four-axis cutting, with independent X-Y control at each end of the wire, adds ruled surfaces and twisted shapes. True 3D contouring is not part of the process.

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  Cutting forceNear zero, so the part needs no heavy fixturing
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  KerfWire diameter plus a melt allowance of roughly 0.2–0.5 mm
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  AxesTwo axes for profiles, four for ruled and twisted surfaces
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  Tool wearThe wire thins and eventually breaks; it is a consumable

The process only works on materials that soften or melt below the wire temperature and do not conduct heat away too fast. Expanded polystyrene, extruded polystyrene, polyurethane foam, EVA, and polyethylene foam all cut cleanly at moderate settings. Machining wax and pattern wax cut even better because the surface resolidifies without stringing.

Density drives the setting more than chemistry does. A 15 kg/m³ EPS block cuts at a lower wire temperature and a faster feed than a 60 kg/m³ structural foam of the same thickness. If your supplier quotes one temperature for every foam, ask what they set for your density.

Thermoplastics that melt into a sticky bead are a poor fit. ABS, PC, and POM tend to drag, weld back together behind the wire, or leave a rough edge. Carbon-fiber composite is worse: the resin melts, the fiber does not, and the wire pulls a fuzzy edge that no finishing step fixes.

Metals, ceramics, and glass are out of scope entirely. They conduct heat away from the kerf and the wire cannot reach cutting temperature at the interface. For those, the work moves to milling, turning, or EDM.

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  Good fitEPS, XPS, PU foam, EVA, PE foam, pattern wax
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  MarginalLow-density PVC foam, some syntactic foams
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  Poor fitABS, PC, POM, carbon-fiber composite
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  Not applicableAluminum, steel, titanium, glass, ceramics

Wire temperature typically runs from about 200 °C to over 500 °C, set by the current through the wire and limited by the alloy and diameter. Too low and the wire drags, leaving striations along the path. Too high and the kerf widens, the surface scorches, and thin sections collapse inward.

Feed rate and temperature trade against each other. On a 300 mm thick EPS block at 20 kg/m³, a common starting point is a wire around 0.4 mm at roughly 350 °C with a feed near 6–10 mm/s. Halve the feed and you can drop the temperature and still get a cleaner wall, but the cycle time doubles.

Kerf width is what you design around. Measure a test cut on the same material and density, then offset the CNC path by half the measured kerf. A nominal 0.4 mm wire on 30 kg/m³ EPS commonly lands between 0.6 mm and 0.9 mm total kerf.

Taper angle is limited by the machine, not the wire. Most four-axis frames handle 30° to 45° of taper on a 200 mm thick block. Push past that and the wire bows, so the taper drifts along the depth and the part no longer matches the model.

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  Wire diameter0.2–0.6 mm typical; thinner wire, tighter corners
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  Wall surfaceSmooth on wax and fine foam, slightly striated on coarse foam
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  ToleranceUsually ±0.5 mm on foam; tighter on wax with a fine wire
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  Test cutAlways cut one before trusting a kerf offset

The clear win is large, light, low-force geometry. Aircraft interior mockups, architectural massing models, film and stage set pieces, and packaging inserts are all cut on hotline frames because the block is cheap and the tool load is zero. A 2 m long profile that would need a gantry mill can be cut on a smaller frame.

The second win is speed on prototypes. A foam or wax pattern for a vacuum-forming tool or a sand-casting pattern can go from CAD to a cut block in hours. No cutter selection, no workholding design, no tool-path simulation for gouges.

The third win is low-density tooling. Pattern wax for investment casting and foam for composite layup mandrels both accept the process well, provided the surface is sealed or coated afterward. Bare foam and bare wax are not structural.

The limits are just as clear. Hotline CNC parts are not load-bearing, not dimensionally stable over time, and not suitable for any application where the surface must take a fastener. If the part must hold a thread, take a bearing, or survive a press fit, it belongs in a milled or molded material.

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  GoodMockups, massing models, set pieces, packaging inserts
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  GoodWax patterns and foam mandrels for downstream processes
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  AvoidThreaded holes, bearing seats, press fits, snap features
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  AvoidAnything exposed to heat, solvent, or long-term load

Laser cutting handles thin sheet well and cuts a wider range of materials, but it struggles on thick, low-density foam. The beam loses focus with depth, the kerf is not vertical on thick stock, and the smell and fume load are far higher than a hot wire. For a 300 mm foam block, the hot wire is usually faster and cleaner.

Milling gives you the tolerance and the material range. If the part must be aluminum, steel, or a filled engineering plastic, milling is the only option. But a large foam block on a mill needs real workholding, and the cutter will deflect at long reaches unless you slow down and take light passes.

The practical split is this. Rough shapes in low-density material go to hotline CNC. Anything that needs a tight tolerance, a structural material, or a finished surface goes to milling or turning.

A common workflow uses both. Hotline CNC cuts the foam or wax pattern, the pattern is used to make a mold, and the production parts come off a mill or a casting cell. Each process does the job it is good at.

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  Thick foamHot wire wins on speed and edge quality
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  Thin sheetLaser is often the faster option
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  Structural partsMilling or turning, no substitute
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  Combined workflowWire-cut pattern, then mold or machine the production part