Double Your Throughput

In a dual strand set-up, there’s one extruder, two dies, and dual downstream extrusion equipment. A “Y” block is used to split the melt before it enters the dies.

You get double the throughput, without the cost of buying a second extruder.

“An extruder usually has more output capacity than is being used,” Barry Reynolds, vice president of sales at Lachine, Que.-based downstream equipment manufacturer CDS, said. “The limiting factor has been the capacity of the downstream equipment.”

Any good quality extruder can be equipped with dual strand downstream equipment, but usually a twin-screw extruder is used, since this is the typical choice to run high volumes of polyvinyl chloride (PVC) powder.

According to dual line manufacturers, the major advantages of the downstream equipment are:

* Maximized throughput.

* The ability to run two applications or two different applications simultaneously.

* Significant savings in floor space.

* Quicker changes of tooling.

The technology is primarily designed for high-output PVC profile applications, such as window frames, siding and fencing panels, but is also available to process wood-plastics composite (WPC), as well as pipe.

“You can run two different profiles on a dual line, but they should be close in volume and geometric shape to balance the output of the two dies,” CDS’ Reynolds said. “If a co-extruder is used, you can put a capstock on one or both strands. It’s possible to run pipe on dual line equipment, but the maximum pipe diameter would be four to six inches.”

There are actually two types of dual downstream equipment: twin strand and dual strand. Both have a dual haul-off system, but in twin strand, both strands are fitted on the same calibration table. On a dual line, each strand has its own calibration table, providing for higher flexibility and a larger process window.

The main factor to consider when looking at investing in dual line is having a large volume of identical, or similar, profiles. “When counting with an output rate of 500 kg/hr., with the profile weight being one kg/m, the capacity of a twin strand downstream line will be 3 to 4 million meters per year, which the sales department has to place in the market in the first place,” equipment supplier Greiner Extrusiontechnik, of Nussbach, Austria, advised.

GROWING MARKET ACCEPTANCE

Dual strand technology has been around for a while, but it’s only recently become popular.

Currently, two European companies supply the majority of dual line equipment: Greiner and Technoplast. Both are headquartered in Austria, but have tooling facilities in the U.S. In the past five years, Greiner has installed over 25 dual lines in North America, while Technoplast has shipped approximately 30 in the same period.

CDS commercialized its own dual line in early 2005. Over the past two years, Brunswick, N.J.-based Silver Line Windows, a major extruder of plastic window components, has ordered ten dual lines from CDS. Extrusion manufacturer Vinyl Visions Inc., of Wilmington, Ohio, also installed a similar CDS system in early 2005 to produce accessories for fence, railings and posts. In Canada, meanwhile, CDS has sold dual strand systems to a company in Alberta running WPC deck boards, and has a few burgeoning customers in the Toronto area.