Many compounders use manual or hydraulic slide-plate or slide-bolt screen changers because these technologies have been around a long time, are simple to operate, and have a relatively low capital expense. While manual and hydraulic screen changers have a reputation for seal leakage and associated high maintenance, suppliers point to improved designs that minimize leakage. For example, Jay Cassidy, sales manager at Berlyn ECM says that Berlyn’s screen changers reduce this problem by creating a positive seal with the bolting system rather than relying on polymer pressure. Maag Pump Systems Textron introduced a new seal alloy that can withstand melt temperatures up to 340°C (650°F).

 

Manual screen changers require shutting down the process to change, which might be adequate for screens that only need to be changed when the extruder will be shut down for other reasons, such as for cleaning between lots. Berlyn offers a quick-clamp system to speed the process of manual screen changing, says Mr. Cassidy. Xaloy introduced a pneumatic assist option for its manual screen changer that uses plant air to allow quicker and easier screen changes.

 

Hydraulic screen changers are popular because they do not require shutting down the line; they briefly interrupt the flow as the hydraulic system quickly inserts a clean screen. While die-face pelletizers can typically handle the brief interruption caused by a screen change, this interruption could cause the strand to break in strand pelletizing. How often screens are changed is another key in considering what system to use. “Hydraulic slide-plates can often do the job, but if you are changing screens every half-hour you’re putting a lot of wear-and-tear on your hydraulics,” says Mr. Cassidy. In this case, he suggests designing the filtration system to use a larger diameter adapter and breaker plate, which must be streamlined to avoid product degradation, or considering continuous filtration. When using a slide-plate screen changer for high-capacity compounding, extended-area filters offer greater filtration area by placing multiple, extended, screen cartridges on the flat screen plate, explains Kevin Tuttle, director of marketing at Xaloy. Xaloy’s technology provides the equivalent of a 29 inch (737 mm) diameter screen and can be used in processes up to 60,000 lbs/h (27,216 kg/h).

 

Various types of continuous melt filtration technologies, in which the screen is changed without interrupting the extrusion process, offer the potential for rapid payback of their often higher capital expense. “Compounders and processors are considering continuous systems in order to increase automation and improve their process consistency,” notes Bob Vogel at Cofit. Compounders moving to finer filtration in order to meet higher quality requirements often need more frequent screen changes, making a continuous system worth considering, adds Dana Darley of Extrusion Auxiliary Services. When calculating payback for the capital expense of a melt filtration system, suppliers recommend looking at labour cost, screen cost, and cost of lost material during downtime and start-up due to screen changes, as well as off-spec due to pressure variation. Continuous systems that avoid variation in the process can significantly reduce material lost through scrap and off-spec, notes Cofit’s Mr. Vogel. Other aspects to consider include flexibility needed in changing screen type, how much physical space the screen changer uses, and maintenance requirements. How much filtration is needed and how much contamination is in the incoming material will greatly affect the frequency of screen changes. For example, recycled material is likely to have high contaminant levels. When screens need to be changed more frequently, compounders are likely to find that continuous systems quickly pay for

themselves, say industry experts. In purchasing an extrusion system, customers tend to focus on throughput and capital cost of the system parts, but should be careful not to overlook what value can potentially be added with continuous technology, adds Kenn Konetski, sales manager at Entek Extruders.

 

A key advantage of continuous melt filtration systems is reducing or eliminating large pressure changes in the extruder. In discontinuous screen changers, the pressure builds as dirt begins to collect in the screen, spikes during the screen change, and drops when a clean screen is inserted, explains Gneuss (see Figure 1). Consistent pressure creates consistent material flow, which improves overall quality of compounded resin by reducing shear or temperature spikes that can cause degradation in shear-sensitive polymers, and by providing consistent flow to the pelletizing process. Strand pelletizers require very consistent flow to avoid strand breakage. Die-face or underwater pelletizers are less sensitive, although large pressure changes increase the chance for die freeze-off and misshapen pellets.

 

Continuous melt filtration system types include single- or dual-piston, continuous belt or ribbon, and multi-segment rotary disc. Large-area or candle filters are used in polymer production to provide very fine filtration that can even remove polymeric gels, but these are rarely used in compounding. Screen-bearing piston or bolt systems come in various designs for diverting the melt stream in order to bring a screen off-line for changing (see Figure 2). While one screen is off-line, all of the melt flows through the other screen. While these systems have a slight pressure change before the screen changer, pressure variations after the screen remain very low, explains Kreyenborg (see Figure 3). Piston systems may have increased residence time of polymer flow compared to other continuous systems. In a ribbon filter, a roll of screen is indexed through the melt in response to small pressure increases, resulting in very low pressure variation and a streamlined flow path. Similarly, rotary disc filters hold the screen in a round disc, which rotates to bring a clean screen into the melt (see Figure 4). Optimized flow channel designs minimize residence time in the filter. Self-cleaning rotary disc or piston systems use ‘back flush’, in which melt is forced back through the off-line screen to clean off collected material. Screens can then be re-used many times, depending on filtration fineness. Processors should consider self-cleaning for high levels of contamination or for more expensive, finer mesh screens in order to save screen costs, say suppliers.

 

Cofit recently introduced a new design for its AP AutoPurge automatic, self-cleaning, dual-channel, continuous screen changer, which uses valves to divert flow to one filter while the other is being cleaned. In the new design, material flows in a straight line, avoiding dead spots, and the orifice is enlarged to enable handling high contaminant levels. Cylindrical screens with three to four times the surface area of flat screens are able to catch more contamination. Cofit recommends the AP system for heavily contaminated material such as reclaim or for large volume, long-run production. For compounders performing frequent changeovers, Cofit recommends its F-series continuous screen changer that uses valves for manual flow-diversion during screen changes. Speciality compounders and masterbatch producers often use manual screen changers, says Mr. Vogel, who adds “F-series screen changers offer advantages of a continuous system, but still have good access for cleaning.” Cofit’s Autoscreen ribbon filter technology is recommended for long runs with low to medium contamination. This technology is well-suited for film, pipe, and sheet production, says Mr. Vogel.

 

In 2007, Gneuss introduced next-generation versions of its SFXmagnus and RSFgenius continuous, rotary disc filtration systems. The improved models offer significantly reduced energy consumption, and the new heating design promotes an even melt temperature, says the company. The new versions also have improved controls and an even more compact design. The pneumatic drive system has a lower investment cost compared to the previous hydraulic system. New sizes of the RSFgenius are designed for larger extruders in the range of 8 inches (203 mm) diameter. Monika Gneuss, vice-president of sales and marketing, explains that the systems are not ‘one size fits all’ but are fit for different applications. She explains: “We recommend the self-cleaning RSFgenius for long-run processes or those with high contamination levels or fine filtration requirements. The SFXmagnus is often used for processes with infrequent – perhaps weekly – changeovers, while the KSF is useful for processes with frequent changeovers.”

 

Kreyenborg recently introduced a melt reservoir system combining a backflush, piston-type screen changer with a melt pump in order to provide constant pressure and flow volume. This system, as well as Kreyenborg’s V-type screen changer introduced in 2004, are intended for direct extrusion processes, notes the company. In compounding going to pelletizing, Kreyenborg recommends its K-SWE (circular screens) or LK-SWE (oval screens) two-channel automatic screen changers for virgin material with low contamination levels.

Kreyenborg’s four channel screen changers have two screens on each piston, doubling the available filtration area to allow high throughput. The K-SWE-4K-75 four channel screen changer is designed to keep 75% of the screen area available during screen changes. This reduces pressure variation and improves the process stability compared to the K-SWE with two channels, which is important for sensitive applications like fibre spinning, sheet extrusion, and strapping tape production. Kreyenborg recommends its backflush screen changers for high contamination levels, to increase the lifetime of screens (see Figure 5).

 

PSI Polymer Systems Inc. introduced an oval breaker plate option for both its continuous (CSC) and hydraulic slide-plate (HSC) screen changers that allows a more streamlined flow than a round breaker plate. The design allows the screws in a twin-screw extruder to extend closer to the breaker plate to reduce residence time.

 

Screen changers restrict flow and increase pressure and shear rate to varying extents depending on the design. For shear-sensitive polymers in particular, suppliers work with their customers to balance filtration requirements with adequate polymer flow. “Removing smaller contaminants requires finer screens, which increases shear and the chance for degradation with certain polymers,” notes Xaloy’s Mr. Tuttle. He explains that an alternative is to increase the screen area, but too large of a screen diameter can result in divergent and convergent melt stream transitions that minimize uniform polymer flow. “With a clear understanding of the application and the processors objective, an optimal screen changer can be selected,” adds Mr. Tuttle.

PVC is particularly difficult to filter because it is corrosive when exposed to the atmosphere and is very sensitive to heat and dwell time. PVC, especially rigid PVC, can crystallize and burn with even slight temperature increases or if the resin encounters any hang-up or dead-spot areas. A streamlined flow path through the screen changer and adapter is very important. At K2007, Maag Pump Systems introduced a new generation of FSC slide plate screen changers that address the challenge of processing PVC or other sensitive polymers. “A channel guide keeps the melt flow constant, and the new contour of the screen mounting plate enables the screw tip to convey up to just before the screen,” explains the company. This FSC also has a heatable housing to ensure temperature consistency. Cofit’s new F/PVC screen changer, designed for flexible PVC processing, incorporates modifications to the internal flow channels and filter assemblies that provide unrestricted flow. In addition, the internal flow channels eliminate contact with the atmosphere, reports the company.