Smoke Generated from Hot Knife & Hot Wire Cutting Processes

Hot knives, sometimes known as foam cutters, thermo cutters, hot wire cutters (same concept, but different configuration), or hot rope cutters, are used for a variety of professional and hobbyist applications.

Some examples include architectural modeling, maritime rope cutting, sculptural art, film and theatre stage construction, foam product packaging, advertising sign and display making, insulation shaping, and industrial applications such as cutting electrical boxes, utility hole passes, and plumbing channels.

Some hot knives are created for use on particular materials, but in general, they work well on a variety of materials, including foam, nylon rope, synthetic fabric, wax, rubber, plastic, and more.

Heat Can Create Smoke & Fumes
It is important to remember that heat can affect solid material properties and create hazardous by-product in the form of smoke and fumes. The PRO-1 Hot Knife, for example, reaches up to 750º F.

The Operating Instructions and Specification Sheet that is included with the PRO-1 Hot Knife provides this warning: “Use proper respiratory protection when working with hazardous materials.”

In order to understand the hazardous attributes of the product you’re using, you should request the Material Safety Data Sheet (MSDS) from the product manufacturer and read it carefully.

Smoke Generated from Foam Cutting
If you’ve ever used a hot knife to cut Polystyrene Foam (often referred to as the trademarked name Styrofoam™), you know that a dense and dark smoke is typically produced.

Extruded and Expanded Polystyrene Foam are two popular types of foam used for professional and hobbyist processes. Hazards can range from product to product; some foams may even have flame-retardant materials that off-gas particularly dangerous chemicals when heated. Here are two examples of foam MSDS: Atlas EPS Expanded Polystyrene and Dow Chemical’s STYOFOAM™ 1.00 x 24″ Square Edge Extruded Foam Insulation. Some MSDS specifically discuss toxic fumes that can be released in processes like wire cutting and other heat-related operations.

Rope-Cutting Processes

Another popular use of hot knives or hot wire cutters is for rope cutting. Sentry Air has received several inquiries, especially from workers in the maritime industry, regarding fume control solutions for these processes. When rope (e.g. nylon rope) is cut with a hot knife, smoke and fumes are typically released. Customers often complain about the extremely unpleasant odors caused by the melting rope. This is a combination of particulate (in the form of smoke and fume).

Fume & Smoke Control: Consider Local Exhaust Ventilation

In many of the polystyrene MSDS that we have read, local exhaust ventilation is recommended under the “Exposure Controls/Personal Protection” section. *You should refer to the MSDS of your unique product to determine what engineering controls are recommended and what exposure limits are in place by health and safety officials. In some cases, a respirator may be recommended. Typically, exposure controls and personal protection is covered in Section 8 of the MSDS.

We have listed some local exhaust ventilation solutions below that may help reduce particulate and fume exposure to workers:

Filter Media. All of the fume extraction solutions that you see below are equipped with filtration media. Media is selected based on the type and amount of materials being cut by the hot knife. Typically a HEPA filter is used to collect hazardous particulate. This filter is up to 99.97% efficient on capturing particles 0.3 microns and larger. Generally, an Activated Carbon filter is also used to adsorb any volatile organics that may be emitted during the cutting process. Even an impregnated Acid Gas filter may be employed for certain materials. *A Sentry Air Systems Applications Specialist will gladly take a look at your product’s MSDS to ensure filter compatibility and efficiency for your particular application.

Under-the-Table Mounted Configuration. Because many rope-cutting operations are performed on a benchtop or tabletop, we often recommend an under-the-table mounted fume extraction solution. In the configuration pictured below, a Model 300 Fume Extractor [Model # SS-300-MS] is mounted beneath a workbench. There is a 4″ diameter (6″ diameter also available) opening on the work surface to accommodate the inlet of the fume extractor. As the operator cuts the rope above the opening, the negative-pressure fan pulls the smoke away from their respiratory zone and into the filtration system. The Model 300 is capable of air volume up to 350 CFM. Models with greater air volume are also available.

This configuration may also work for small-scale and relatively stationary foam-cutting operations.

DownDraft Bench. A similar concept with a larger work area is the downdraft bench configuration. The model pictured below [Model # SS-440-DDB] provides a 40″ x 22″ perforated work area. Overall table dimensions are 48″ W x 30″ D x 35″ H. Typically, a Model 400 [up to 700 CFM] or Model 450 [up to 950 CFM] fume extractor is utilized for this configuration.
Cutting foam with a hot knife on a downdraft bench [Model # SS-440-DDB]. See Video below, under “Portable Floor Configuration with Flexible Arm”.
Portable Floor Configuration with Flexible Arm. If your workspace set-up is not conducive to under-the-table mounting, there are additional configurations that can work just as efficiently. The Model 300 Portable Fume Extractor is equipped with heavy-duty casters and a portability handle; work environments with multiple cutting stations may find the easy transport of this model beneficial to their operation. The flame-retardant, self-supportive flex arm is 68″ long and can easily reach standard benchtop heights.

This configuration is well-suited for foam cutting operations. The flexible fume extraction arm can be positioned at a variety of points on large pieces of foam.

Cutting nylon rope with a hot knife.
Utilizing the Model 300 Portable Fume Extractor for smoke extraction.
Shelf-Mounted or Wall-Mounted Configuration. An additional option, that utilizes elements of both the mounted fume extractor and the portable model, is a shelf-mounted configuration.

The base unit that houses the fan and filtration system can be set atop a shelf while a 4″ diameter opening in the shelf accommodates the flexible arm. This configuration can also be mounted directly to the wall with a Sentry Air mounting kit.

If you’d like additional information for smoke and fume control during hot knife or wire cutting processes, please give us a call at 1.800.799.4609, email us at, or fill out this simple online form to receive additional information from a Sentry Air Applications Specialist.