Hot-wire foam cutter

A **hot-wire foam cutter** is a tool used to cut polystyrene foam and similar materials. The device consists of a thin, taut metal wire, often made of nichrome or stainless steel, or a thicker wire preformed into a desired shape, which is heated via electrical resistance to approximately 200 °C (390 °F). As the wire is passed through the material to be cut, the heat from the wire vaporises the material just in advance of contact.

The depth of the cut is limited only by the wire length. Width of cut is limited by throat, if any.

## Types of wire used

* Preformed, manually or with pliers.
* Tensioned, held tight by a frame with springs or (equivalently) by a bow. Many indicate a preference for single-strand stainless steel wire over nichrome wire, in diameters ranging from 0.008 in (0.20 mm) to 0.020 in (0.51 mm) for frame-tensioned cutters. Musical instrument strings and leader wire used for fishing lures are readily available and inexpensive. Commercial cutting equipment often uses the nickel-based alloy wire Rene 41, a high-strength alloy which maintains most of its tensile strength at cutting temperatures.

## Foams suitable for hot-wire cutting

All thermofusible expanded foams. This means those materials that melt at a relatively low temperature:

* Expanded polystyrene (blocks of expanded white balls).
* Extruded polystyrene (in blue, green, gray, pink or purple smooth plates, depending on the manufacturer).
* Polymethacrylimide low density rigid foam (Rohacell), excellent mechanical properties but very expensive.
* Flexible expanded polypropylene foams, ideal for shock protecting.
* Polyethylene foams, flexible and tear resistant.
* Custom materials, such as the foams used in flexible fuel tanks to reduce explosion risks.

## Types of cutters

### Preformed wire

A preformed wire is attached to a handle. The operator manually guides the wire through the foam to cut freeform shapes.

### Tensioned wire

#### Handheld

A short tensioned wire is attached to a handle. The operator manually guides the wire through the foam to cut freeform shapes.

#### Simple manual table

Hot wire foam cutting tables can use a vertical or a horizontal wire, usually with a horizontal table used as a guide or for securing the foam work piece.

A table with a vertical wire is mainly used by hobbyists to cut small and complex shapes such as letters. Only prismatic members can be cut on this type of table.

Tables with horizontal wires are heavily used in construction and packaging. The cutting wire is attached to tracks on either side of the table enabling it to be moved up and down making cuts very exact. This table type can also perform angular cuts.

#### Template-guided manual table

A horizontal-wire manual table that uses templates placed on each side of a foam block to guide a taut hot wire to form a prismatic or tapered shape. The hot wire, tensioned with a bow or by springs attached to a rigid frame, can be propelled through the foam by hand or with a cord and pulley mechanism connecting each end of the hot wire to a weight. This method is widely employed by hobbyists to produce foam cores for model airplane wings.

Template-guided cutter accuracy is highly dependent on operator skill at fixturing the foam and guiding the hot wire along the templates. Accuracy is also affected by the precision to which the templates are formed and proper assessment and handling of kerf.

#### Push through slicer

A vertical or near horizontal frame contains a set of evenly spaced parallel tensioned cutting wires. Massive blocks of foam are pushed through the frame on rollers or by gravity, thus slicing them into sheets that are sold as "bead-boards" for general use in construction work etc. The blocks are guided so little skill is needed by the operator. The frame holding the wires is often a replaceable and or adjustable unit so the spacing can be changed.

This allows a range of plate thicknesses and sizes to be produced without the cost of expensive molds for each size, as the blocks are made in standard "block form" machines that produce blocks of industry standard sizes (such as 0.5 x 1 x 2 m). To produce plates shorter than the usual length of the massive blocks, the blocks are simply pre-cut to the desired length before slicing. This method is a common way of making the EPS (Styrofoam/Styropor) sheets used in construction.

#### Computer numerically controlled

Computer numerical controlled (CNC) stages on each side of a block of foam guide a tensioned hot wire to form a prismatic or tapered shape. This is a four degree of freedom CNC system (one each X and Y on each side of the foam block).

Accuracy is affected by the precision of the stepper motors, backlash of drive systems, and stiffness of the races supporting the end effectors which hold the tensioned wire.

Some CNC hot-wire cutters offer lathe attachments, which can be used with either stretched or preformed wire. Lathe attachments allow cutting of spiral shapes, and are often used to create concrete forms for ornamental and structural items, such as planters and stairway balusters and newels.

## Workmanship considerations

### Wire length limits

The practical limit to the length of a tensioned cutting wire is the tendency of the wire to sag under the force of gravity, which requires higher tension for longer wires. Some CNC cutting machines place the ends of the wire above and below the workpiece, with the wire being mostly vertical, to counter this problem.

### Speed and temperature tuning

Cutting speed and wire temperature combine to determine the thickness and consistency of kerf. For a given wire temperature, an attempted motion in excess of the wire's ability to melt the foam can lead to wire drag and poor surface finish of the cut foam. Conversely, dwelling in the foam any longer than strictly required to melt the cut path can lead to excessively thick kerf. A nonuniform feed rate can lead to a kerf of varying thickness.

## Uses

* Signage
* Architectural solid modeling
* Props
* Prototyping
* Creating molds for casting concrete and plaster pieces
* Aircraft design
* Model aircraft wing construction
* Lost-foam casting
* Surfboard construction
* Recreational hovercraft construction