Edited by Stephen J. Mraz
Even engineers in the know who appreciate foam for its wide range of characteristics and capabilities are often unaware how versatile it can be and how best to use it. And if you don’t know much about foam, working with it can be tricky — from selecting the right material for an application to die-cutting to meet the design. In most cases, working with foam requires partnering with a materials converter experienced in the varieties and capabilities of foam.
What is foam?
Foam is formed by trapping gas bubbles in a liquid or solid. Solid foams are open or closed cell. In open-cell versions, pores are connected to each other, forming a relatively soft network. Filled with air, open-cell foams make great insulators, like those typically used for insulating homes. However, fill an open-cell foam with water and its usefulness as an insulator plummets.
Closed-cell foams lack interconnections between pores and cells. They have higher compressive strength, thanks to their structure, and are usually denser than open-cell foams. They are also more expensive. In addition, closed-cell foam can be filled with specialized gases to boost its insulating capabilities. The closed-cell structure gives it more dimensional stability, lower moisture absorption, and more strength than open-cell solid foam. Closed-cell foam is also well known for its buoyancy, hence its widespread use in flotation devices.
Types of foam
Within these two broad categories, there are many variations including flexible, rigid, reticular, rate responsive, and syntactic — with new types of foam being continually developed. Here are some of their distinguishing features:
Flexible foams are useful when you need a material that bends, flexes, or absorbs force without damage or delamination.
Rigid foams have a matrix structure that gives them little or no flexibility. They can be sealed to prevent fluid absorption or air penetration for flotation, insulation, or gasketing.
Reticulated foam has had the window membranes of each cell removed, leaving only a skeletal structure. Foam can be reticulated by zapping or quenching. Zapping involves placing a bun of foam in a vacuum-pressure vessel and igniting a controlled gas flame. The flame passes through the foam, melting window membranes. In quenching, material converters get the same result by running foam through a temperature-controlled caustic bath. Quenching results in a rougher, more-etched skeletal structure that holds liquids better due to surface tension. However, quenching is not effective on polyether polyurethanes. Reticulated foam is well suited for gaskets, filters, and acoustical applications.
Rate-responsive foam feels soft when you apply pressure relatively slowly. However, if you hit or slap it, it becomes firm. The foam doesn’t act like a spring under load but instead relaxes, which makes it good for cushioning and seating applications.
Syntactic foam is a composite consisting of rigid hollow glass, carbon, or polymers microspheres held together by a metal, polymer, or ceramic matrix. The hollow particles make up more than half of the composite’s volume. This means the foam has less density, more strength, a lower coefficient of thermal expansion, and it resists compression. By changing the concentration of microspheres used and their wall thickness, converters can vary the properties of syntactic foam.
Memory foam, also known as viscoelastic polyurethane, was developed by NASA for Space Shuttle seats. It becomes more viscous at lower temperatures and also responds to heat — including body temperature — to become more elastic and pressure sensitive. This means it conforms to a (living) human body and makes it a good choice for use in mattresses.
Characteristics of foam
Thanks to innovations in chemistry and fabrication, foams can offer shape retention, water resistance or absorbency, porosity, density, and a host of other physical characteristics. When looking for just the right foam, engineers can specify:
Bulk density: The mass per unit volume for a material.
Tensile strength: The maximum stress needed to fail or break the material in a tension-loading test.
Elongation: The percent of deformation occurring during a tensile or other mechanical test.
Tear strength: Used to measure the tear resistance of foam rubber, elastomeric foam, and other thin and flexible foam materials.
Noise-reduction coefficient (NRC): Indicates a foam’s ability to absorb noise.
Thermal conductivity: Measures the linear heat transfer per unit volume through a material for a given temperature gradient.
Dielectric strength: Maximum voltage a material can withstand before electrical breakdown.
Flammability: The ability to reduce or slow flame spread or resist ignition when exposed to high temperatures.
How foams get used
Here’s a quick rundown of how different industries take advantage of foam’s properties:
Acoustics: Foam is often used as an insulator to form a barrier or isolator between sources of noise and vibration and people. It can also diffuse sound without too much attenuation.
Aerospace: Foam’s inherent lightness can be beneficial in aircraft construction. It is a good choice for thermal insulation and sound dampening when sandwiched between bulkheads. Because it baffles sound and suppresses explosions, it is also used to minimize potentially dangerous vapors created by fuel surging and sloshing in tanks and cells. It is also used for emergency flotation.
Construction: Foam is widely used as an insulator, as a core material for columns, blocks, and panels, and as structural panels.
Automotive: Foam makes up acoustical insulation, window seals, fascia, bumpers, lightweight body panels, fuel baffling, seat cushions, air and fluid filtration, and shock and vibration dampening.
Electronics: Foam can be found in EMI/RFI shielding, static/ESD control, and electrical insulation. It can also serve as a thermal barrier between electronic components. Low-outgassing versions, like RTV foam, work well in potting, back filling, and encapsulating.
Filtration: Open-celled foam makes good filters for air or liquids. Pore size determines particle size and flow rate.
Industry: Foam is used for shock and vibration dampening, as well as in making tooling, modeling, and making foundry and mold patterns.
Shipbuilding: Closed-cell foam’s buoyancy makes it useful for life jackets, as well as for seals, insulation, lightweight panels, fuel baffling, cushioning, and air and fluid filters for marine vessels.
Medicine: Foams are often found in products used for wound care and filtration, as well as patient positioning and support.
Packaging: Foam’s impact and protective qualities make it a good choice to absorb mechanical energy that can damage objects and structures during shipping and use. Polyethylene foam’s is used extensively in packaging because of its resilience.
Varying a foam cell’s structure changes its physical characteristics, letting engineers tailor foams for an application. There are a host of processes used to alter a foam’s structure. They include:
• Laminating: Adding a protective outer layer of Mylar, Tedlar, or Tyvec to increase a foam’s resistance to dirt, dust, oil, and abrasion.
• Flame laminating: Adds a film, fabric, or barrier materials to foams without adhesives.
• CNC machining: Can create complex cavities in closed-cell foams.
• Water-jet cutting: Cuts all types of rigid foams and follows computer-generated patterns to form complex shapes and cavities without distorting the foam’s edge or creating thermal stresses.
• Die cutting: Cuts most types of foam.
• Kiss cutting: Cuts foam pieces to size. Pieces of foam can then be rolled up on a nonstick liner.
• Laser die cutting: Cuts adhesive and nonadhesive foams to tolerances of ±0.005 in.
• Profiling and hot-wire cutting: Cuts all types of foam and can consistently turn out complex shapes
• Thermoforming: Forms foam into designs — simple or complex — using heat.
• Slitting: Cuts foams and foam tapes into precise, thin widths.
|Foam composition and chemistry |
Foams can be made of plastics and polymers, metal and metal alloys, ceramics, elastomer or rubber, thermoplastics, and thermoset and crosslinked materials. The available chemical formulations include:
• Polyurethane or PUR resins
• Plastic or elastomer-based silicone
• Vinyl and polyvinyl chloride
• Ethylene copolymer (EEA, EVA, EBAC)
• Expanded polystyrene (EPS and Styrofoam)
• Expanded polyethylene (EPE)
• Latex foam (which typically doesn’t contain VOC solvents)
• Natural rubber and sponge
• Polyimide, typically adhesive or sealant based on polyimide resin
• Polypropylene (EPP)
What’s up with foam tape?
Important foam-tape properties include tack and shear. Tack is measured in pounds/inch of width and indicates how much force it will take to remove the tape once it is applied to a surface. Shear is measured in minutes to failure, and indicates how strong an adhesive is when force is parallel to the bond surface.
Foam-tape adhesives are typically based on rubber or acrylic. Rubber-based versions have high tack values, lower shear values, are inert (the bond doesn’t get stronger over time), and are not recommended for UV exposure. They do not perform well on plasticized (low-surface-energy) surfaces, and often cost less than acrylics. In contrast, bonds of acrylic-based adhesives increases over time.
While not completely air or watertight, closed-cell foam tape works as gaskets, preventing gasses or liquids from escaping. For example, it is often used to between a door and its frame to eliminate drafts and to create airtight seals around windows.
As a mounting tape, closed-cell foam with adhesives on both sides can bond irregular surfaces, filling in gaps between the materials. Foam tape also works as a glazing or flashing tape.
Foam tape is widely used in construction to attach interior wall panels, on cars and trucks for gasketing and mounting interior and exterior panels and trim, for sealing the edges of solar panels of industrial applications that need both joining and sealing.
Selecting the right converter
The right converters can help engineers select the most appropriate foam or tape for a particular application, identify the best source for these materials, and pick out the most appropriate and cost-efficient fabrication process to create the finished part. They should offer design assistance, including determining part configuration and other technical considerations, make prototypes for testing, and set up short or long production runs. They should also help ensure quality control from raw material to finished piece, and make sure the finished part will work easily in the assembly process.