The good, the bad, and the future of structural foam

Many businesses are cyclical, but the wild market undulations of structural-foam molding seem to grow more violent as the technology approaches its 40th anniversary.

Boom: It?s 1972, and Russ LaBelle is so convinced of the promise of low-pressure structural foam, he starts Wilmington Machinery in Wilmington, NC to serve the area?s growing furniture industry. He creates a multistation indexing machine to mold furniture legs and spindles.

Bust: OPEC shuts off the petroleum spigot, and as LaBelle says, ?all those dreams were dashed by the oil embargo. The furniture industry did an about face on plastics.?

Boom: Computers create an entirely new market for structural foam with housings made from polyphenylene oxide (PPO), notably Noryl from GE. Business is good enough to prompt LaBelle and Wilmington to introduce a medium-sized machine utilizing a T-arrangement in which one injection unit could feed molds in two clamps.

Bust: By 1983, led by Japan?s growing presence in electronics and its desire for a wet look in plastics vs. the dull swirl of structural foam, computers switch to straight injection. The ?80s continue to be slow, and the only major foam applications, pushed by Rubbermaid, are carts, mop wringers, and other custodial products, which are molded on large machines Wilmington doesn?t create.

Boom: Material handling explodes in the early ?90s with business coming from pallets and collapsible bins among others. Many are destined for the automotive industry, which uses the containers to carry components through assembly systems. Resin around $.28/lb helps push the expansion.Bust: Peaking with the rest of the plastics industry in the late ?90s, structural foam then falls in conjunction with the overall manufacturing economy.

Faced with overbuilt capacity and record-high resin prices, structural foam remains in a relative funk, but LaBelle and others pushing new applications, materials, and technologies, think it could be poised for another surge.

Foam?s Emergence

Originally the property of former chemical giant Union Carbide, low-pressure structural foam molding technology was initially developed in the late ?60s by that company as a means to market its polystyrene. Interested shops essentially built their own machines, separately purchasing an extruder, a press, and a license from Union Carbide, which also threw in a technical package, including melt accumulator, manifold, and nozzles. Structural-foam machines resemble straight injection presses in how the clamp and injection unit are situated, but similarities end there. The process uses an extruder to melt resin, and then an inert gas and/or chemical blowing agent is introduced, lowering the material?s density by 10% to 20%. This allows material to flow easily into the mold, which can be made from aluminum due to lower injection pressures. The resulting part has a high stiffness-to-weight ratio, making for relatively lightweight, rugged structural parts. The parts have a distinct surface appearance caused by gas breaking through the surface. This has been accepted and even desired in North America, with the rough surface associated with strength, and some even mistaking it for the presence of reinforcing fibers.

That ruggedness of the surface can be mitigated by newer processes, including external gas molding, which uses gas introduced from the injection side of the clamp to force the part?s exterior surface against the cavity wall; gas counterpressure, in which the mold is sealed to contain the gas or foaming agent within the resin so it doesn?t break through the melt-flow front; or structural-web molding, in which pressure is mitigated by having the mold close in two stages.

Whatever the process, structural foam uses large amounts of resin, making it susceptible to material price swings. Wilmington?s Richard Morgan explains that the smallest shot size on its machines is 25 lb, with some going over 300 lb. In those quantities, even small upticks in price are felt quickly, making structural foam less competitive against the metal, wood, and reinforced straight-injection applications it aims to replace. Helping combat those disadvantages and in response to the high resin prices of the last three years, Wilmington and others who serve the industry, such as Uniloy Milacron (Cincinnati, OH), are targeting material and machine technology developments to make structural foam more competitive and push it into new markets.

Wilmington is currently working with Incoe (Troy, MI) on a process that would backmold laminate material and give the weight, strength, and cost benefits of structural foam but cover the plastic surface with vinyl or leather. In the process, a laminate would be placed into a mold, the foam would be shot behind it, and then, in combination with external gas molding, the part would pack, with the laminate adhering to the resin substrate. Potential markets include automotive, where the strength-to-weight benefits of structural foam make it attractive for fuel efficiency purposes and the laminate provides a clean surface finish.

On the machinery side, Wilmington has increased extruder torque and built more robust manifolds and nozzles that can process 100% regrind. LaBelle says the process has created parts with better properties than virgin resins that have melt-flow rates of 6 to 8 g/10 min. Higher flow-rate resins have traditionally been used because machines didn?t have the ?oomph,? according to LaBelle, to run off-spec resin, or resin with high levels of fillers such as flax, wood, or talc. Before the end of the year, the company also hopes to introduce a system that allows the accumulators, nozzles, and manifold to be cleaned out faster during mold changes. As it stands now, tool change on a 60- to 70-nozzle mold can take 30 hours.On the gas front, LaBelle is searching for alternatives to nitrogen, saying the gas, although cheap, isn?t entirely friendly to polymers. LaBelle was around when attempts to use pentane (flammable) and freon (harmful to the ozone) failed, but he says Wilmington?s made progress with another gas: carbon dioxide.

Normally, walls in a structural-foam part are relatively thick, with the foam creating a 15% reduction in density and a flow length from the gate of 15 inches. Using CO2, Wilmington has been able to double that flow length and the density reduction, in some cases achieving parts that are 1-inch thick with density reductions of 60% to 80%. This could allow the use of stiff fractional-melt materials and open up the opportunity for applications in flotation devices, or even floating piers.

Foam?s Future

If these advances, and others, are successful, structural foam could move into boom territory again, especially if it overcomes issues with flame retardancy and cost that keep its Holy Grail?pallets?largely out of reach. ?There are challenges [with pallets],? Wilmington?s Morgan says, ?but it?s kind of like beer and PET; once it goes, there?s a huge volume there.? In January 2004, the Integrated Pollution Prevention & Control initiative, which affects the European Union, China, and Australia, among others, decided that wood pallets must be treated and marked, raising their cost and hassle, while potentially opening up the market for plastics once again.

Given the ups and downs he?s seen with structural foam, LaBelle takes a long view on any developments, but his overriding sentiment remains optimistic. ?I thought structural foam all but died back in the 1980?s,? LaBelle says, ?but I think it?s just a process waiting for the next big application.?