When the Clean Air Act (CAA) was passed in 1990, it caused big changes to occur in the expanded polystyrene industry. The EPA classified pentane, one of the chief blowing agents used to expand EPS, EPP and EPE beads, as a volatile organic compound (VOC). Under new CAA regulations, pentane emissions had to meet strict reduction targets, beginning in 1993. These regulations have led to a number of innovations in the way EPS is manufactured and molded.

For many years, the American motto towards EPS expansion has been this: the simpler the better. For this reason, continuous pre-expansion systems were the systems of choice. But Pentane reduction regulations caused EPS manufacturers to take a new look at how they manufacture foam.

"The biggest development in EPS foam manufacturing in recent years has been the introduction of the reduced-Pentane bead," said Robert Coughanour of R.B. Coughanour Consulting of Hilton Head, S.C. "These beads emit less pentane but they also lack the expansion vibrancy of the higher-pentane beads. This development caused some difficulties for continuous expansion systems."

Continuous pre-expanders are just that: EPS beads are continually fed into one end of the system and the expanded bead comes out from the other. But continuous systems are designed to work with high-pentane beads.

"To achieve lower densities using reduced-pentane beads in a continuous system," said Coughanour, "operators may need to pass the beads through the system more than one time. The process can be time-consuming and inefficient, and achieving low density on a consistent basis can be very difficult."

Coughanour explains that many molders have solved the problem by making use of pentane emission systems that capture the pentane gas as it is released during the manufacturing process. The capture systems enable manufacturers to continue using high pentane beads. But other manufacturers have turned to a more-controlled pre-expansion process known as batch processing.

The Rise of the Batch Pre-Expander

Unlike continuous pre-expanders, batch pre-expanders expand beads one batch at a time. Beads are loaded into a sealed chamber, where they are exposed to controlled levels of steam and pressure, then they are released and another batch is processed.

There are four stages in the batch pre-expansion process:

1. The beads are conveyed into the pre-stage and dozing hoppers.
2. The beads then pass into a scaled pressure vessel, where they are exposed to controlled levels of steam and pressure.
3. After the beads are expanded to a specified density, they are sent into a conditioning stage.
4. The expanded beads are then passed through a screening and de-lumping process to eliminate any lumps of fused beads. The expanded beads (pre-puffs) are conditioned typically in storage for from two to 24 hours prior to molding.

"One advantage of a batch system is control," said Coughanour. "Batch systems use computer controls to give manufacturers precise command over steam temperature and pressure. This allows them to achieve reasonably low densities even when using reduced-pentane beads."

What Manufacturers Are Doing

Coughanour notes that the batch pre-expansion systems on the market offer many similarities in quality and design, but there are some interesting variations. One variation exists within the conditioning stage. Most manufacturers use a fluidized bed process to condition beads after pre-expansion. But Hirsch Maschinenbau of Glanegg, Austria employs a vacuum chamber process.

A fluidized bed system is a traditional method of conditioning beads. After pre-expansion condition, the beads pass into a bed in which a large blower causes air to percolate up through the beads. The air causes moisture to dry and causes the beads to stabilize.

"When the bead is freshly dumped out of the pre-expander, it is still hot and tender and subject to damage," said Ron Watkins, eastern region sales manager at Kurtz North America of Plymouth, WI. "Our fluidized bed drier takes the energy out of the bead and conditions it in an efficient manner. This makes the bead more durable and keeps it at a precise density."

Though Hirsch uses the fluidized-bed process for units designed for block molders, the company has created the vacuum chamber process for units used primarily by shape molders. This unit consists of an upper and lower chamber and eliminates the need for a fluidized bed.

"When beads pass out of the pre-expansion chamber," said Mark Clark, manager of North America for Hirsch, "they are transferred into the lower vacuum chamber where the temperature is lowered, treating the sensitive beads very carefully. The steam on the beads is vacuumed out. This allows the beads to cool and become very stable. This process is also very energy-efficient, and allows a closed-loop control, batch by batch."

Innovations in Shape-Molding Equipment

In addition to innovations in pre-expansion processes, there have been several innovations in shape-molding processes. These innovations have also been driven, in part, by the difficulty of working with reduced-pentane beads.

"Once the reduced-pentane bead has gone through the pre-expansion process, it is still more difficult to work with than the high-pentane bead in the shape-molding process," said Coughanour. "This is because there isn't much pentane left in the bead. The reduced-pentane bead, then, requires more steam and more control to achieve high-quality shape-molding."

To gain more control over the shape-molding process, manufacturers have integrated sophisticated computer systems into the process. Bill Hubbard, operations manager of Styrologic of Helena, AL explains:

"When it comes to shape-molding equipment, control has become the number one issue," said Hubbard. "It wasn't too long ago that shape molding was a manual process. But now we use touch-screen, WINDOWS-based monitors that are very easy for operators to use."

Hubbard also said that the computer-based systems allow Styrologic to design and execute sophisticated recipes to mold parts for Saturn cars.

"To create an aluminum motor block," said Hubbard, "we first shape-mold a precise EPS foam replica. The EPS version is then packed into sand. When we pour molten aluminum into the sand, the EPS vaporizes and the molten aluminum fills the void that the EPS was taking up. What remains is the aluminum motor block or other part."

Hubbard says that because the WINDOWS-based computer systems have allowed Styrologic to achieve high-precision EPS molds, they are able to bring savings and efficiency to their customers.

"We're able to mold the parts with less rough edges that require grinding, "said Hubbard. "We're able to mold parts with holes already in them, to prevent the need for boring after the iron or aluminum piece is cast. This not only improves the finished part, but also significantly reduces waste and labor for our customers. And all of those improvements can be traced to the improvement of the EPS shape-molding process."

Other innovations in shape-molding include the vacuum transfer mold process developed by Hirsch. Unlike most shape-molding processes, which continually heat and cool one molding tool, the Hirsch process uses two tools -- one is hot and one is cool.

"After the foam is heated by the first tool to cause fusing of the EPS beads, the parts are transferred to the second tool, where they are stabilized using a vacuum and air," said Clark. "Since the biggest users of energy in the shape-molding process are the tools themselves, the heat exchange process significantly reduces energy and water consumption, thus producing drier parts. It also provides one of the fasted molding cycles in the industry."

Continued Innovation

The EPS industry continues to respond to environmental considerations by improving efficiencies and minimizing water and energy consumption. Today, new technologies and innovations have revolutionized EPS expansion and shape-molding.

"When the pentane restrictions began in 1993, the industry was forced to go through a lot of change," said Coughanour, "Looking back, the reduced-pentane bead is the cause of many interesting innovations that have helped to make EPS expansion and molding a precise and efficient science."