Flow simulation lightens huge pilings.

Uneven melt flow and excess material usage bedeviled the maker of huge plastic marine pilings until computerized flow simulation pointed the way to a successful die design. The die was designed by Czech-based software supplier Compuplast Canada, Mississauga, Ont., for Seaward International of Winchester, Va., which coextrudes the pilings with a solid outer skin over a foamed core.

Composed entirely of recycled PE (mainly detergent bottles), pilings measure up to 16 in. diam. and 100 ft long. They are reinforced with rebar that is continuously pultruded of fiberglass and thermosetting polyester and then encapsulated in-line with extruded PE. The entire continuous production line is some 300 ft long.

Get the flow right

"Our original die design relied on valves to control flow within the die cavity," says William Jarvis, production manager of Seaward's Engineered Plastics Div. "With the old die design we saw variations in flow from different parts of the die on the order of 5:1."

"The major problem was the foam core," says Russell Gould, president of RG Associates, Des Plaines, Ill., who consulted on Seaward's die problem. The skin and core have very different viscosities, as does the core before and after foaming. The foam core is most viscous exactly when it has to flow evenly around the reinforcing rods. If the foam doesn't flow evenly, too much material is used, adding to costs. To compensate for thickness variations in the skin, Seaward also made the skin thicker than otherwise needed, adding to resin cost.

To fix the flow-control problem, Seaward consulted RG Associates, which in turn brought in Compuplast to simulate and analyze die and flow-channel designs. Compuplast used its Flow 2000 extrusion simulation software, which came out in a Windows version last fall. Flow 2000 uses finite-element methods to model pressure drop, flow velocity, and shear in multiple layers through the thickness of the melt stream. The program analyzes the coextrusion for interfacial stability and other characteristics. It has eight modules for different types of dies, and for the extruder, flow analysis in two- or three dimensions, and a materials database. Compuplast performed the analysis on a contract basis, but Flow 2000 is also for sale or license. Licenses cost from $2500 to $15,000 per module.

Proof is in the pilings

The die designs developed for Seaward "combine many flow channels of different kinds that go in many different directions," says RG's Gould. "The square shapes were the hardest," notes Compuplast v.p. John Perdikoulias.

The simulation was accurate enough that the new die worked well on the first try. Uniformity of foam and skin layers was so much improved that Seaward was able to cut the skin thickness by 40%. "That's a tremendous improvement," says Gould.