Corrugated pipe: multi-wall takes off.

Dual-wall corrugated pipe is growing larger and running faster. Even smaller sizes are finding new markets. Forming integral bells in-line is catching on. Now, look out for annular triple-wall pipe!

The U.S. market for multi-wall corrugated plastic pipe is growing by 20-25% a year, riding a wave of product innovation. Technology for extrusion dies, corrugating molds, and cooling is racing to keep up. Once a sleepy, inbred market niche, corrugated polyethylene pipe has been shaken awake in the past two or three years, as processors launched significant new products and faster, more economical ways of making them. For example, dual-wall pipe goes up to 60-in. diam. today and is heading for even larger sizes. The first annular triple-wall products are invading sanitary sewer markets. New in-line belled ends are reducing pipe and installation costs.

The storm-sewer market is the key to double-wall corrugated growth. "The drainage market is already 95% owned by corrugated PE, so growth has to be in storm sewers," says Fred Fedosoff, executive secretary of the Corrugated Polyethylene Pipe Association in Toledo, Ohio.

In large sewer pipe, plastic is muscling in on concrete pipe, helped by PE's higher chemical and abrasion resistance plus ease of installation. A 20-ft length of PE pipe with integral bell and spigot saves installers $1000/day compared with using 8-ft lengths of concrete conduit, says Hancor Inc., a major PE pipe maker in Findlay, Ohio.

In smaller diameters, the competition is PVC. The first PE triple-wall pipe is trying to break into an even newer market - sanitary sewers for homes. Gaining speed

The largest dual-wall pipe today is 60 in. - but not for long. Hegler Plastic GmbH in Germany is also currently developing molds for up to 72 in. pipe. And Corma's newest "pulsating" corrugator (see below) is designed for making up to 9.8-ft diam. pipe.

"Pipe gets bigger all the time, and line speeds get faster every day," says Keith Cousineau, sales and customer-service representative at Cullom Machine Tool & Die. Eighteen-inch double-wall pipe today runs at 4.7 ft/min vs 3.5 ft/min two years ago, Cullom says. Meanwhile Unicor Plastic Machinery and Corma Inc. guarantee machinery for 50-60 ft/min of 4-in. dual-wall corrugated.

New techniques for both air and water cooling are essential to improving outputs in the new large diameters. Corma's new "Super Cooling" technology maintains cooling as the plastic shrinks away from the mold by blowing chilled air between the pipe and mold. One-meter PE pipe shrinks 3%, creating a 15-mm gap where chilled air can circulate, Corma says. "Super Cooling" is used on two-thirds to half the travel of molds in contact with the pipe. Thus, mold blocks close to the extruder stay warm enough to give the pipe a smooth surface. The first installation of Super Cooling is at Francesville Drain Tile Corp. in Francesville, Ind. President Steven Overmyer confirms that it has increased output.

Unicor, which water cools with spray and/or internal mold channels, introduced a new "direct-cooling" methods for larger diameters. "Direct" cooling means hollowing out the entire mold block and flooding it with chilled water. This technique is used commercially on two 48-in. lines, one at a pipe producer in Iran and the other at Quail Piping Products Inc., Magnolia, Ark.

Unicor also has a patent pending on a high-speed, "closed-circuit" form of direct cooling, which Unicor calls "permanent cooling." This has been installed on a high-speed UC-75 corrugator for pipe up to 3-in. diam. The 42 F chilled-water supply is underneath the ground plate that holds the rails on which the mold blocks travel. Cooling water is sucked into the molds as they travel over the rails. Drawing water in with negative pressure avoids sealing problems, and there is no need for hose or plug fittings on the 81 fast-moving mold blocks.

Corrugator makers are also extending mold lengths and moving molds independently rather than at fixed intervals on a chain. Independent movement lets blocks return faster to the extruder and increases the time molds are in contact with pipe by 40%, Corma says.

Some recent die improvements are aimed at improving output and product quality. An example is Unicor's flat "disk" die that is only 25% as long as a conventional dual-wall die. It's intended for diameters over 18 in. The big advantage is ease of adjusting the die gap, Unicor says. Flow channels are so short that less adjustment is needed to account for internal thermal expansion. The die is also much lighter and requires less energy to heat.

Corma's latest die advance is a new spider shape that overlaps strips of plastic, thus hiding spider lines and making stronger pipe, Corma says. Corma will demonstrate a 48-in. die with the new spider on its Pulsating Corrugator at an open house in June.

When Corma's new Pulsating Corrugator (U.S. patent #5,744,091 issued April 28, 1998) goes commercial, it will be the first machine that can handle molds of varying lengths, including in-line bells, Corma says. Its three pairs of aircooled molds close sequentially, powered by electric motors, and then advance with the pipe on a moving table. After traveling a certain distance, the molds (which have Super Cooling) open all together and the table returns to the extruder. Indexing takes 4-5 sec, depending on pipe diameter and mold length.

Belling goes in-line

In-line forming of an integral bell on one end of a double-wall pipe and a spigot on the other end was a major advance when Hegler introduced it in 1993. Since then, it has caught on as an alternative to spin welding an injection molded bell onto the pipe. Several North American pipe makers have adopted in-line bell forming - including Hancor, Francesville, Ideal Drain Tile Ltd. (London, Ont.), and ADS Advanced Drainage Systems Inc. (Columbus, Ohio).

However, ADS also welds bells onto pipe that require pressure-seals (Canada requires 15-psi ratings for sanitary sewers). ADS uses an inline spin-welding system from Big 'O' Inc. Paul Yaremko, pipe product manager for Big 'O,' says integrally formed bells can stretch enough to leak, but welded bells won't.

Note that Hegler's patented inline belling process is the only one that vacuum forms a double-thick bell from both inner and outer parisons. Other in-line belling systems use just the outer parison to make the bell and cut away the inner one. Drossbach offers such a system, for example.

Triple walls & other news

Triple-wall PE pipe has existed for decades, but was previously made using spiral-wound solid ribs between separately extruded skins. That produces a relatively heavy pipe and requires multiple operations. Now, both lighter and stronger tri-wall pipe with a "true" corrugated structure (similar to cardboard) is starting to be produced by a more efficient annular in-line extrusion technique.

The first of this new triple-wall pipe was commercialized last year by both Quail Piping and ADS. Both use Unicor UC250 lines. Quail, makes triple-wall annular pipe in 4-, 6-, and 8-in. sizes for sanitary sewer applications. ADS makes 4-in. triple-wall drainage pipe.

Meanwhile, Big 'O' has developed an experimental system for making annular triple-wall pipe and also has come up with an automatic welder to put bells on the pipe.

Drossbach and Cullom have adapted their dual-wall corrugators to add a third smooth wall outside with a crosshead die. Cullom recently developed a 4-in. triple-wall system for Total Containment Inc. in Oaks, Pa. It is making gasoline containment tubing with an inner barrier layer of Shell Chemical's Carilon polyketone resin.

Hegler holds patents on applying vacuum in the crosshead die to draw the smooth outer tube against the corrugated ridges of the center layer to ensure bonding. Other approaches apply vacuum in a tank downstream from the crosshead die. The latter approach adds more equipment and also makes it difficult to bell in-line. Hegler's vacuum die allows in-line belling of triple walled pipe, merging all three walls into the solid bell. Hegler has practiced this technique in its own pipe production in Germany for three years.

Corma's president and CEO Manfred Lupke says the next development will be to use vertical corrugating-type equipment with sequentially closing molds to make discrete parts of varying cross section, such as automotive ducts, without any flash. Output could be higher than blow molding, but tooling cost would also be higher. An advantage may be that this process could use resins that don't have sufficient melt strength to hang a conventional blow molding parison.