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. As the wire is passed through the material to be cut, the heat from the wire vaporises the material just before it would have made contact with it, creating a smooth cut.

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

Cutting speed is a critical matter to control for optimal results: if the wire moves too quickly, it can drag in the foam and result in a less-smooth finish. If it moves too slowly, the foam will melt further away from the wire. Similar issues can arise if the feed rate is not uniform.

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 .008 inches to .020 inches 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.

imple 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.

Computer Numerically Controlled

Computer Numerically 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.

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

External links

* [http://www.8linx.com/cnc/cnc.htm Build Hot Wire CNC Foam Cutter from parts available at the local hardware stores]
* [http://epp540.com/index.php?option=com_content&task=view&id=89&Itemid=43 Digital hotwire foam cutter]
* [http://nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm Build your own hotwire foam cutter]
* [http://www.spacemodeling.org/new/how_to/Foam_Cutter.htm Build your own hotwire foam cutter from Home Depot Parts]
* [http://www.vatsaas.org/rtv/construction/HotWireCutter.aspx Large easy-to-build hotwire foam cutter]
* [http://www.rcgroups.com/links/index.php?id=4063 Build your own CNC hot-wire foam cutter]
* [http://www.instructables.com/id/EK8Q92824BEP286S6S?ALLSTEPS Build your own hot-wire foam cutter for $30]
* [http://www.rcgroups.com/forums/showthread.php?t=436753&referrerid=40731 Discussion of merits of various tensioned wires]