Demand for rigid PVC equipment, which was remarkably good in 2006 and 2007, saw an overall drop in 2008, although some regions continue to grow, say industry experts. “We expect the global market for rigid PVC extrusion equipment to shrink considerably in 2008, and remain at that level in 2009. This is a remarkable set-back but from a very high level, as current demand is still higher than it was in 2005,” reports Rainer Kottmeier, managing director of Battenfeld Extrusionstechnik GmbH. The demand set-back is most pro­nounced in North America and Western Europe, where pipe and window pro­file demand fell with the building and construction slump. Processors here are struggling with weak demand and high costs. Pipe was not hit as hard as other areas, with demand continuing for infra­structure replacement and new products continuing to be developed, comments Tom Brown, director of sales and market­ing at Cincinnati Milacron. The rigid CPVC (chlorinated PVC) market has been a growing niche in the US, bol­stered by California legislation allowing CPVC to replace copper in hot water piping, notes Mike Irish, vice-president at Buss. Regions outside of North America and Western Europe have maintained stronger demand for rigid PVC. Some see Eastern European demand for window profiles continuing to increase, while others say that this high growth market is cooling. Equipment suppliers see new capacity being added in Latin America, which typically uses similar equipment to that used in North America. Profiles, and especially pipe, are also growing in China. The Chinese equipment market is served primarily by domestic low-cost suppliers and by international companies manufacturing in China. Suppliers must adjust their systems and part sourc­ing in order to make equipment that is competitive in this low-cost market, say suppliers.

 

Suppliers accept that this is a cyclical busi­ness, and they anticipate waiting out the next year for market demand to cycle back upwards. US plastics market economist Bill Wood predicts: “We may even see some turnaround in 2009. When there is a sense that the market is no longer going down, confidence will improve as business activity begins to stabilize, and equip­ment investment can follow.” He notes that next year’s upcoming NPE (National Plastics Exposition) in the USA is a big marketing opportunity that may boost equipment sales. Meanwhile, rigid PVC extrusion equipment manufacturers have continued to develop new technology – focusing on throughput, energy savings, and various ways to save material costs.

 

Buss is a leading equipment supplier for compounding rigid PVC pellets and for producing rigid PVC calendered film. Buss reciprocating kneader technol­ogy provides intense axial mixing that keeps temperature even and provides good dispersion with low shear. In 2001, Buss introduced the quantec Kneader, a four-flight, 10 L/D, reciprocating screw kneader optimized for PVC compound­ing. The two-stage system has separate sections for mixing and for creating pressure for pelletizing. The quantec can produce throughput up to 2.5 times greater than a comparably-sized three-flight kneader, resulting in investment cost savings. Compared to Buss’ estab­lished, three-flight kneader technology, the four-flight quantec has overlapping screw flights that improve conveying stability, create more shear surfaces per L/D, and optimize self-wiping, says the company. Liquid temperature control enables precise product temperature control. The feed zone, optimized for low bulk density powders, has a large, square inlet and greater pitch so that cram­mer screws are not necessary. “The Buss quantec Kneader compounds both rigid and flexible PVC at about the same high rates (up to 4536 kg/h or10,000 lbs/hr) due to its ability to control shear and temperature. Co-rotating twins typically compound rigid PVC at about half the rates of flexible PVC grades. Counter-rotating and conical twins are size and speed-limited and can be used only for lower-output compounding lines (less than 2041 kg/h or 4500 lbs./h).” explains Mr. Irish.

 

Recently, Buss introduced a special ver­sion of the quantec Kneader that is modi­fied to allow compounding of very high filler levels in PVC. The modified quantec has a 15 L/D for extended residence time and downstream feed capability so that some of the filler can be added after the material begins to plasticate. “Demand for this machine is coming from the Eastern European market, where compounders are adding up to 60% calcium carbonate in window sill profile formulations. Return-on-investment time for the equipment is very short,” reports Mr. Irish.

 

For direct calendering of rigid PVC, Buss recommends using a one-stage quan­tec kneader with the compounded and degassed melt going through an intermit­tent cutting device to produce pieces fed directly to the calender rolls. Compared to a three-flight (11 L/D) kneader, the four-flight (15 L/D) kneader significantly reduces start-up time and cleaning time, reports the company.

Counter-rotating twin-screw extruders dominate pipe and profile production. Extruder manufacturers put much ef­fort into designing screws to fit a given application and formulation. “Screw design is the heart of the machine,” says Mr. Reisenhofer, Cincinnati Extrusion’s European product manager for profiles. For example, highly-filled pipe formu­lations require screw geometries with higher compression rates to supply more mechanical energy for plasticizing. PVC-based wood or natural fibre composites have their own screw design requirements for degassing and encapsulating fibres. Even within an application, every custom­er’s formulation differs, and screw designs must be modified to get the best output.

 

Window profile processors in particular are looking for increasingly high throughput, and extruder manufacturers have answered the call with record outputs of 1000 kg/h. One of these is KraussMaffei’s new KMD 133-32/P parallel counter-rotating TSE, introduced at K2007. The extruder has a longer processing unit (32 L/D), which helps thoroughly plasticize the dryblend even at high outputs. The new extruders have improved wear protection and a new screw design with self-regulating, internal screw temperature control. Battenfeld’s BEX 2-135V plus TSE, introduced in 2007, can reach 1000 kg/h, and uses a direct drive for greater energy efficiency. Cincinnati Extrusion says that its Argos 135-28D TSE, combined with a 5-cham­ber profile tool, set a world record for output with 1004 kg/h during a recent demonstration. High throughput processes typically use double-strand extrusion, in which one extruder feeds two profile tools. “Today, the benchmark for twin-strand is 5-6 metres/minute, but insiders expect continuous improvement in tooling and calibration will allow line speeds of 8-10 m/min within the next few years,” says Mr. Kottmeier. Output greater than 1200 kg/hr will be needed to meet this line speed, predicts Battenfeld (see Figure 1).

 

While increasing output continues to be a primary goal, energy savings are becoming a greater priority. “Significant energy savings are realized by switching from DC to AC, by insulating barrels and by using internal screw cooling,” notes Mr. Reisenhofer. He describes Cincinnati Extrusion’s IC^® Intra­cool (see Figure 2) as a closed temperature control system that transfers heat from the metering section to the feed section.

Material cost-saving trends

 

In addition to saving costs by optimiz­ing throughputs and energy use, proces­sors are looking at saving material costs any way they can. For example, Brown sees a trend towards gravimetric feeders, which allow tighter control than volu­metric feeders. Krauss-Maffei Berstorff sees strong demand for its QuickSwitch inline pipe dimension-change system, introduced in 2003 for polyolefins and in 2006 for PVC pipe. QuickSwitch helps processors save costs by allowing diameter changes from 90 to 160 mm without stopping the production line, improving capacity utilization and order turnaround times, says Annette Beierling, head of ex­trusion product management for Krauss­Maffei Technologies. There is a trend towards using high fill levels to save mate­rial costs, but these can be a challenge to feed and process. KraussMaffei notes that its 36D parallel, counter-rotating TSE are successful in producing highly-filled PVC wastewater pipe with up to 60 per cent chalk (calcium carbonate). The extruders have a special material feed, weighing, and metering system and a stuffing unit above the extruder feed throat.

 

PVC processors make good use of regrind, typically putting 10-15 per cent back into the process. “Using up to 25 per cent regrind doesn’t require a special twin-screw design, but greater percentages may require different flow channels in the feed zone to accommodate larger particles. Regrind can also process differently be­cause it has a different heat history,” notes Mr. Brown. Mr. Irish at Buss notes that, in addition to using internal recycle such as injection moulded sprues and runners, some customers purchase post-industrial PVC from calendered film operations, such as scrap left after punching credit cards from PVC sheets. Recycling of post-consumer (PCR) PVC is done today, but generally by specialized companies that use extruders with special wear protection.

 

Coextrusion allows processors to save material costs by using either recycled or foamed PVC in a core layer. Ms. Beierling says that for foam core wastewater pipe, KraussMaffei’s standard PVC pipe extrud­ers are equipped with a special mixer on top of the inlet that ensures the foaming agent is well-dispersed in the dry blend. “Pipe head design is the critical part of a foam core extrusion line,” adds Ms. Beier­ling, noting that KraussMaffei introduced a new, larger multilayer pipe head in December, 2007. In window profiles, co­extrusion is used for profiles with regrind cores covered with virgin, sometimes pigmented, surface layers. KraussMaffei’s profile extruders are designed for flexible, compact set-up, such as ‘piggybacking’ one of KraussMaffei’s new, short conical twin-screw extruders on top of one or two, longer, parallel TSEs to save space on the shop floor. This set-up can be used for either single-strand or double-strand extrusion.

 

Cincinnati Extrusion’s Argos 173, introduced in 2007, is designed for high-output (greater than 1800 kg/h or 4000 lb/h) PVC sheet used in North America for trim boards. The machine has 173 mm screws, a redesigned low-shear screw, internal screw cooling, and air-cooled barrel.

 

KraussMaffei’s KMD 184-32/PL, introduced in 2007, is designed for 800 – 2000 kg/h (1760-4400 lb/hr). At a customer’s facilities, the extruder dem­onstrated over 2000 kg/h in PVC sheet production, says the company.

Cincinnati Milacron offers a 172 mm screw in a parallel counter-rotating TSE. Besides this very large extruder, Mila­cron expanded its product line with an 84 mm TC84 conical, counter-rotating extruder, slightly larger than its TC80. Milacron is also developing a 75 mm, parallel counter-rotating TSE, smaller than its existing 93 mm extruder. The new extruder, targeted for small diameter pipe and custom profile, should be avail­able in the first quarter of 2009.

 

Randcastle Extrusion Systems has devel­oped a mixing element that dramatically improves single-screw extrusion of PVC pellets and allow rigid PVC dryblend to be effectively transported and mixed in a single screw extruder, claims the company, which introduced the mixer and presented its findings at the Society of Plastics Engi­neers’ ANTEC 2008. In a study using a 25 mm, 36:1 L/D, 5 hp single screw extruder with three mixing elements, Randcastle’s researchers processed rigid PVC pellets at 96 rpm without excessively high melt temperatures. “This is unheard of, because rigid PVC has historically been limited to 30 to 35 rpm on single screw extruders. Usually, rigid PVC will burn at higher speeds,” comments Keith Luker, president of Randcastle. Randcastle also processed dryblend at 180 rpm without a vacuum hopper and crammer feeder, which are normally needed to feed powdered dry­blend into a single screw extruder.

 

These results are possible because the op­erating principle of the Elongator mixing element is fundamentally different from that of other single screw mixing elements or conical counter-rotat­ing twin screw extruders, with some simi­larities to co-rotating twin screw extruders, says Luker. Rather than using shear, the Elongator generates elongational flow to create dispersive mixing. The Elongator is similar to Randcastle’s Recirculator mixing section with axial flutes, but in the Elonga­tor the axial flutes have been twisted into a spiral, preventing upstream axial flow that could potentially cause degradation in thermally sensitive resins like PVC. The mixers operate at low pressure, and vents can be placed over them.

 

Randcastle recently received its first order for a commercial-size, 4.5 inch screw for PVC. Luker sees great potential for the Elongator in processing rigid PVC dryblend. He says: “No twin screw or any other type of mixer has the low price and low maintenance of a single screw extruder. That’s what we’re offering.”