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Quest for perfection.

Compounding today is a blend not only of material quality and cost effectiveness, but also a flexibility, response speed, and acute sensitivity to customer needs. The days of the custom compounder's being just a material "mix-master" are over. The continuing effort is to precisely control and optimize every aspect of the delivery and performance of the compounded products.

Compressing delivery time

Robert Sinuc, manager, Manufacturing Engineering, GE Plastics, cites increasing demands for delivery of smaller material batches in shorter times. The push is to eliminate the drag of inventory on the bottom line, to react faster to changing markets.

GE is targeting a goal of material delivery within 72 hours from time of order, compared to up to several weeks currently. "Such major time compression requires that the many elements in the cycle, from order initiation to delivery of compounded material, all must be analyzed and integrated more innovatively," Sinuc says.

With expansion of Just-in-Time (JIT) manufacturing, total predictability of compounded resin quality becomes even more critical. Consistent material release, based on rigid statistical process control (SPC) procedures, represents a fundamental cultural change. Customers operating on JIT systems want total, fast flexibility for adjustment of supplier schedules in response to their changes. Unnecessary infrastructure that inhibits response and adds costs is not acceptable.

Sinuc emphasizes that "there is no possibility of competing economically unless the process is in control, using standards that are critiqued to fit the customer's JIT requirements. A lot of redundant testing has been eliminated, based on the credibility established by ISO 9000 certification and SPC documentation of material batches. Predictability of how the compounded resin runs in the customer's molding machines, plus other factors that could affect the molder's competitiveness, also must be factored into the overall input."

Sinuc adds that "in the last year, we have made substantial improvements in the order-to-delivery cycle. Continued analysis of the entire process is essential for significant further gains."

Versatile combination

Berstorff Corp.'s new MSE Multi-Screw Reactor addresses the interest of typical compounders or small processors in expanding into custom-tailored reactions and "finishing" fields. Gene Stroupe, vice president, Compounding Division, says the versatile machine can be used as a reactor or mixer and/or a devolatilizer. The machine, in addition to the reactor/mixer section, includes ten intermeshing, self-cleaning screws in a cylindrical barrel that operate in a planetary motion under vacuum. All the screws are driven by a single motor. The machine permits use of a shorter design that generates less heat history and removes residual moisture or solvents from the melt, providing a high surface renewal self-cleaning reactor, with much more surface area than in twin screws of comparable size.

Stroupe says that market potential includes polycondensation of nylon and polyester finishing, and some special mixing applications for polyblends, where polymers of high viscosity ratios must be dispersed.

In another design, Berstorff's in-line compounding sheet line provides high flexibility for end-use requirements and eliminates the pelletizing step, so the polymer heat history is greatly reduced. For example, the flexible line for polypropylene sheet production, utilizing a Berstorff twin-screw compounding extruder fitted with two side-feed ports, allows the same machine to be used alternately for polypropylene with calcium carbonate, wood dust, fine talc, or glass fiber; laminates of these compounds on fabric; and coextrusion of barrier layers of polypropylene on these compounds. Edge trim from the sheet can be reused, and a pelletizer can also be fitted for general-purpose compounding. This allows one line to handle numerous applications, such as automotive panels, thermoformable TV dinner trays, and carpet backing.

Multiple demands

Among the changing expectations and demands on the part of compounders, according to Asmut Kahns, director of sales, Werner and Pfleiderer Corp., are gains in productivity; better, more consistent product quality; greater ease of maintenance and safer operation; smaller machine sizes; and more help in process technology.

Kahns confirms the move among major extruder manufacturers towards integrated data acquisition and control systems that can help compounders avoid downtime that would be needed for quality control checks.

Side venting, which can enhance productivity, is available in all WP extruders. The feature eliminates possible problems in conventional top venting, wherein volatiles might solidify, drop back into the extruder, and contaminate the product.

Kahns also says WP is continually supplying new screw comporates. Recently introduced ZME and TME mixing elements offer efficient distributive and dispersive mixing, including low shear generation, flexibility of pitch angles, and self-wiping capability. Downtime-reducing, innovative screw and barrel materials are improving wear and corrosion resistance. Extruder barrel bores protected with a powder metallurgy alloy, either as a direct coating or as easily replaceable liners-inserts, are available for applications involving abrasive fillers and reinforcements. WP also fabricates screw elements by diffusion bonding a highly wear-resistant powder metallurgy alloy to a ductile-steel core, which ensures spline straightness and correct shape, and avoids distortion during hardening. Improved fit on the spline provides maximum contact surface, and minimizes stress concentrations and danger of cracking under high-torque operation.

Kahns adds that cost pressures often force compounders to upgrade or debottleneck existing facilities, instead of investing in new equipment. WP cites rate improvements of up to 60% in conversions from triple-flighted to two-flighted screw profiles.

A WP remote-control monitoring system facilitates troubleshooting of a customer's operations from the extruder supplier's facility, speeding diagnosis and solution without an in-plant visit.

Among improvements in the line of ZSK twin-screw compounding extruders are an easy-to-operate quick-disconnect mechanism on downstream attachments that replaces complicated and time-consuming fasteners; self-aligning pendulums that virtually eliminate frictional forces, as support stands for the extruder's process section; a new heat-insulation hood that enhances operator safety; electrical wiring and cooling water feed, located on opposite sides of the processing section, which enhances operation and safety; and a new sound-insulation hood that brings the machines well below OSHA requirements.

Urgency the rule

Robert H. Heinold, technical director, A. Schulman, Inc., stresses that JIT delivery requirements have stimulated unprecedented service. Schulman, for example, supplies a multitude of precolored plastics to a major Detroit molder of automotive trim parts. In an assembly plant with a computer interface, the parts are racked in the same color sequence needed for the car assembly line. "Obviously," Heinold says, "the failure of the compounder to provide perfect quality material 100% of the time would be intolerable. As a result, special planning, cooperation, and a sense of urgency permeate the entire organization."

Pressure for cost reduction can force the compounder to purchase larger volumes of raw materials at lower prices and store them for longer periods; the gamble is that the material prices will not fall substantially during the interval. With longer production runs essential to cost reduction, larger quantities of compounded material must often be produced, not covered by the initial purchase order, and then stored internally or in outside warehouses.

Also, Heinold says it is frequently not enough for the compounder to buy plastics with generic stabilization packages that enhance properties such as long-term heat-aging and weatherability. Customers now look to the compounder to formulate and process the products to optimize durability in more hostile environments. Throughout the part development and production cycle, the compounder is expected to be a working partner in material selection and specification resolution.

Importance of flexibility

Stephen E. Peterson, director of marketing, Farrel Corp. stresses the need for accurate record keeping of processing conditions and machine operating profiles, as well as assurance that the equipment that compounders buy will be flexible for processing varied products. Farrel's standard compounders have PLC control systems with a baseline capability to interface with a plant's data-acquisition system. "It is then a small step," Peterson says, "to fully automate the compounder and download formulations and operating conditions. The operating window of our compounders is also expanded through a combination of increased rotor speeds, new rotor-pair combinations, new geometry on existing rotors, and adjustable mixing-chamber inserts."

The nomenclature and sizes of Farrel's compounders now reflect the nominal capacities in kg/hr, instead of by bore diameters. "The new generation of personnel in the industry can not easily correlate the bore sizes to productivity," Peterson says.

Farrel indicates that there also has been a more pronounced engineering effort to supply a machine that does not leak powder, molten polymer, or lubricant. Grease packing has replaced flood lubrication of main mixer bearings on some series of compounders, and new polymer seals eliminate leakage. The need for better process control has also fostered improved hardware and better locations for measuring melt temperature.

To increase the operating hours on a production line, special rotors that change over in a few minutes, rather than six to eight hours, facilitate color changes, radical material change where a different rotor combination would be best, and rotor or chamber-barrel maintenance.

Modifications for niches

Charles Martin, national sales manager, American Leistritz Extruder Corp., confirms the trends of the last few years towards modifying existing polymers to meet niche needs, and the demand for more smaller twin-screw extruders to produce small-volume, high-value products. The company's cost-saving Micro-18 mm multimode unit, the world's smallest full-torque modular twin screw, helps develop new prototype compounds. Evaluations can be run with samples as small as 1 lb. In development settings, it is not unusual for prototype samples to cost hundreds, and sometimes thousands, of dollars per pound to prepare.

A new quick-change screen assembly, integrated into Leistritz's 18-mm to 70-mm twin screws, is accessed by a swing bolt system. The integral design eliminates two front-end adapters (extruder/screen changer and screen/changer die), which minimizes the "dead" area, improving quality for sensitive materials such as nylons and urethanes. A spare breaker plate permits quick screen changes, typically in less than two minutes.

Leistritz has introduced a PC-based data-acquisition and control package so that the twin-screw extruders and related auxiliary equipment communicate with an Opto-22 controller via RS-422 serial communication links and/or industry-standard analog signals. By using a separate computer module, the user takes advantage of the logic associated with the discrete devices. For example, the PID (proportional integral and derivative) algorithms developed over the years for temperature controllers are still utilized. Information is transmitted via RS-422 signals, including setpoint, deviation from setpoint, percentage, and cycle times for heating and cooling.

Cost-effective ingenuity

Proper choice of the mixing equipment, while sometimes taken for granted, can be critical for success. George Niznik, vice president, R&D, LNP Engineering Plastics, cites an example of a 20% glass-filled polypropylene, utilizing a polyethylene concentrate containing brominated flame retardant and antimony oxide additives, in a single-screw extruder. A customer's request for a change to 30% glass content resulted in poor dispersion of the flame retardant, sporadic test values, and nonuniform surface appearance (attributable to the 10% less resin content) when processed in the single-screw machine.

The first attempt to solve the problem was to use an outside source of flame-retarded polypropylene, which solved the processing problems but involved paying a 30% premium for the 70% outside-produced portion of the blend. In any event, although LNP successfully dispersed the material and glass in the single-screw extruder, the grade of flame-retarded polypropylene also lacked the tensile and flexural strength, and elongation, achievable with the original polypropylene selected for the application.

Instead, LNP now compounds the material in-house in a twin-screw extruder. The glass is fed into the extruder downstream rather than introducing the total mix upstream, as in the single-screw extruder. Niznik says that even a double pass in the single-screw is not as effective as a single pass in the twin-screw machine. "Although there is still a premium for the twin-screw extrusion, the dual-feed-zone approach with the twin screw brought us back to the original material cost structure for the product," Niznik says, "where we were buying low-cost, better performing polypropylene and adding the flame retardant and other additives ourselves.

"We are constantly upgrading our processing capability for difficult-to-disperse blends, often with lower resin content. The single-screw extruder, however, is still a cost-effective workhorse for the simpler, more straightforward resin systems."

Detailed paper trail

The compounder must integrate all components into a highly sophisticated process, but at a price that is competitive. Arthur F. Dellheim, president, Adell Plastics, Inc., says that today's custom compounders seldom have dedicated extrusion lines and must have the versatility to respond quickly to a large variety of projects and new parameters, including highly filled and reinforced compounds, multifunctional concentrates, and reactive extrusion. The varied projects often require different combinations of extruder configurations and auxiliary equipment.

Dellheim asserts that since it is impossible to purchase the data-processing equipment to meet the diversity of modern-day compounding requirements, the small- and medium-sized custom compounder must instead provide in-house programming and data-acquisition capability to be sure that every operation is monitored and documented.

At Adell, all screw configurations for twin-screw extrusion are printed from a computer program, including the capability to store critical parameter information for a given process. Where required, a layout of the process is printed and presented to production, including what equipment is required, how it is to be positioned, the conditions of processing, and all other data necessary to ensure accurate and complete repeatability. The computer-generated plant order also includes a continuous improvement section designed to help Adell reach its goal of process optimization.

A recent project, for example, consisted of compounding eight components. An Adell in-house program, devised with the engineering department, joins a weighing device with a computer. The stored formulation is recalled by use of bar-coding, and at the end of the weighing cycle, the computer prints a document that attests to the weighing accuracy.

Dellheim emphasizes that to meet today's needs for sophistication and accuracy, the custom compounder's in-house capabilities, including a strong process engineering group and computer support, must be used to augment what cannot be obtained cost-effectively on the open market.

Chemically coupled glass

Chemically coupling glass fiber to thermoplastic resins, notably polypropylene, by reactive extrusion, has played a major role at Thermofil, Inc. The company cites recent benefits in cost/performance where a polypropylene window-cable housing replaced 40% glass fiber/mineral 6/6 nylon at 40% less material cost; a pool-pump housing replaced glass-reinforced polyphenylene oxide at a 20% reduction in material cost; and a seat pan for an office chair edged out glass/mineral polyester. In all cases, the level of coupling and efficiency of the chemical reaction were critical to end-product success.

Jeff Neupauer, director, Sales and Marketing, says Thermofil is continually challenged to maintain glass-fiber length, through downstream feeding, and to adjust melt flows to higher ranges to meet new demands. Ensuring uniform melts through entire lots requires precise metering of flow enhancers; SPC; monitoring by computer/digital readouts; and in-house screw designs. With excellent dispersion the goal, traditional fillers and single-screw technology are, in many cases, no longer enough. Application solutions now often require twin screws, in combination with sub-micron particle-sized minerals. A recent thin-wall, continuous compounded HVAC application required superhigh filler dispersion, unique screw design, and multiport feeding.

Thermofil recently opened a 26,000 ft2 "state-of-the-art" technical center, which includes automated testers of flexural, tensile, and Izod impact strength. The company says they are the first such testers in the U.S. Another automated unit evaluates flexural loading at elevated temperatures. The instrumentation features robotic specimen positioning, testing and retrieval, and computerized data recording.

Electrically heated Kneaders

Buss-America has supplemented its MDK Kneaders with a new APC (Advanced Performance Compounder) line featuring electrically heated barrels for use with polymer formulations above 600|degrees~F. Chris Case, product manager, Engineering Plastics, says that "with temperature constraints imposed by liquid oil heating removed, we can now compete with twin-screw compounders over a greater temperature range."

New gear box models enable the Kneader to attain higher speeds (up to 500 rpm) to process materials of varying viscosities and achieve throughputs up to 30% higher. Case says that customers scaling up their operations often look to continuous compounding, rather than batch processing, to replace antiquated lines. For a new highly engineered material under development, for example, the Kneader accurately replaces a batch formulation with continuous production, down to the most minute weight and volume of ingredients.

A reverse flight element now permits low-pressure compounding of highly filled formulations in a dynamic mixing zone, replacing a static restriction barrier offered previously. The design creates a turbulent mixing zone with higher pressures, which is less aggressive with low viscosity materials and more aggressive with high viscosity materials. Additionally, refinements in screw and barrel design now enable customers to meet strength and impact requirements in a single compounding machine that can be easily converted with changing formulations. The machine's highly dispersive mixing characteristic enables compounders of toughened thermoplastics and thermoplastic elastomers to maintain larger particle size, yet distribute the properties thoroughly within the compound.

Averting handling problems

"The call for specialized products has prompted us to invest in mid-size extruders to gain production flexibility," says Kirk Walker, technical superintendent of Hoechst Celanese Corp.'s Specialty Products Facility. At higher loadings, Kirk indicates, handling problems encountered with powdered and fibrous materials that do not flow well have been averted to a great extent through use of segmented-barrel, twin-screw compounders and by installation of sophisticated loss-in-weight gravimetric feeders, with vertically agitated hoppers on each feeder. Forced or gravity feeding at multiple locations in an extruder is often essential because of material handling characteristics with higher loaded products. Side feeding has been shown to reduce wear, minimize melt temperature, and maximize system performance. To accommodate the different flow properties of resins with large amounts of filler, such die-plate features as hole size and land length have been customized to maintain production rates. Pelletizing of highly loaded resins has been improved with ministrand baths that enable custom cutting of the material while it is still very hot.

Kirk comments on the reliance on twin-screw machines to provide the needed dispersion because of the difficulty of economically making highly filled compounds on a single-screw machine. "We find that two-lobe supercompounder machines make more consistent products with shear-sensitive resins because the maximum shear rates at available screw speeds are lower and less sensitive to small temperature changes than in three-lobe machines. Two-lobe machines are also easier to scale up because they have more consistent shear rates and screw volumes."

Preventive maintenance protocols ensure that machine clearances are within predetermined maximum allowable gaps in high shear areas, such as the transition zone and melting areas.

Safety data

Compounders of custom-formulated plastic compounds and color concentrates must now generate Material Safety Data Sheets (MSDS). This has become a formidable task: The number of chemical ingredients lists in the thousands; the suppliers in the hundreds; and the compounds, matched colors, and concentrates in the thousands. The task is further complicated by the use of "compounded" ingredients as raw materials.

Stuart Daniels, environmental affairs coordinator, Teknor Apex Co., says that in order to facilitate the task of generating accurate MSDS, the company has established databases that contain OSHA Hazard and EPA reporting data for all ingredients, and developed computer programming that utilizes the company's on-line compound formulation system to track the levels of hazardous and reportable chemicals in each ingredient, preblend, masterbatch, color concentrate, and ultimately, each finished compound produced at any of Teknor Apex's ten manufacturing locations.

The computer program identifies the type of product, selects the appropriate MSDS format, and then compares the hazardous/reportable totals versus the OSHA/EPA standards to select the proper hazard/reporting text. The two are then combined and the MSDS is printed. The MSDS is generated automatically for the client at the time of initial order entry, and updated annually for each item.

Fast feedback

The positive impact on quality of a highly developed JIT program also derives from the rapid and accurate information feedback that the procedure inherently requires. Bertram Lederer, president, Teknor Color Co., says that as the JIT operations tune in, because the quantity of material on hand is sharply reduced, the procedure lends itself "to much faster, detailed communication between molder and vendor if a material problem is encountered. Responses are then much more immediate and thorough."

Closing the loop

Bay Resins, Inc., extends its service, notably with larger OEMs and molders, with what it refers to as "closed-loop recycling" of its customers' scrap accumulation. The scrap is analyzed at the Bay Resins plant and regenerated to satisfy the original material specification by addition of prime resin, additives, and lubricants. The compound then is returned to the customer with a certification of the revised material's properties. The result is a win-win situation for everyone; the customer is not faced with the expensive disposal of the scrap and the environmental concerns, and, according to Bay Resins, is saved substantial amounts in resin costs.

Expanded on-site capability

BASF Corp., Polystyrene Business Group, is the first polystyrene supplier in North America to implement an ignition-resistant compounding line on the same site as its resin manufacturing facility. Located at the company's Joliet, Ill., plant, the line enables the company to quickly respond to customer demands, have better control of product quality, and conduct evaluations for development of new ignition-resistant polystyrenes, according to Paolo Racanelli, R&D manager.

Shipping base polystyrenes to off-site facilities for ignition-resistant compounding can take days or weeks to complete, depending on a compounder's availability, production capacity. and capabilities. Instead, BASF can process the base material immediately, control the entire cycle from resin production to compounding, and evaluate the end product before shipping.

Pressure for perfection

"Providing quality compounding services on a contract basis always has been a challenge," says Stan Jakopin, president, Advanced Polymer Compounding Co. "Today, however, with new awareness of what quality really means, there is an ever-increasing pressure to strive for perfection, cost-effectively and in compliance with constantly changing environmental regulations."
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Title Annotation:Compounding; plastic mixing
Author:Wigotsky, Victor
Publication:Plastics Engineering
Date:Feb 1, 1993
Previous Article:U.S. manufacturing: economic set or subset?
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