Modular feeders make compounding more flexible and accurate.

For custom compounders whose processing lines must be flexible enough to handle a variety of formulations and throughput rates and yet maintain high accuracy, modular feeders may be the answer. One such firm, Chemtrusion Inc. of Houston, partially credits the ISO 9002 registration it received in 1995 to the accuracy and repeatability of its feeders.

"Our business is uncertain from day to day. Modular feeders give us the ability to change very rapidly," says Scott Owens, Chemtrusion's president. Compounding difficult-to-feed materials and smaller runs are his specialties. The necessary flexibility includes ability to scale up quickly from lab runs to full production. Interchangeable parts of modular feeders allow Chemtrusion to handle a wide range of materials without having to stock a lot of different specialized feeders. "We can't afford to have equipment sitting around unused, but we can't afford to compromise quality because we don't have the right equipment. Modular feeding and controls let us configure our material-handling systems for optimum delivery into the process," Owens says. Chemtrusion also gets quick cleanouts and changeovers with the modular system.

Chemtrusion demands a lot of its feeders. On any given day, it compounds a range of materials like PP, PE, metallocene polyolefins, post-consumer reclaim, and TPOs with various processing aids, stabilizers, and other additives. Its raw materials come in pellet, powder, flake, granule, crystal, sphere, and liquid forms. These materials have different bulk densities, heat sensitivities, melt temperatures, and susceptibilities to bridging or blocking.

FLEXIBILITY IS THE KEY

Chemtrusion has three compounding lines all fitted with gravimetric feeders from K-Tron America, Pitman, N.J. Each line has a 92-mm Werner & Pfleiderer twin-screw extruder. Typical runs range from 5000 to 600,000 lb and throughput rates from 1000 to 3000 lb/hr. One big advantage of the modular feeders is a much higher turndown rate than standard feeders. K-Tron's modular K2 feeders have a turndown rate of 200:1, versus 20:1 for standard feeders. The result is that the K2 feeders' auger discharge screw can handle a wider range of throughputs. "A standard feeder only gives you a limited range of pounds per hour," Owens says. "You would have to shut down the line and replace the feeder when screw speeds change. But with the modular feeder you can change gearbox ratios without changing the feeder."

A K-Tron official says modular units (which are offered by other suppliers as well) now constitute 85-90% of the feeder market in compounding. This penetration has been aided by the disappearance of the price premium that once distinguished modular from non-modular units.

Chemtrusion has purchased four generations of K-Tron feeders since starting operations in 1987. Its most recent purchases include K2 modular loss-in-weight units. These have a quick-disconnect screw unit, so Chemtrusion can switch from a single-screw to a twin-screw without replacing the entire feeder. Twin screws are typically fully intermeshing and can act like a positive-displacement pump for sticky materials or like a valve to stem flooding of aerated materials.

K2 feeders can also interchange concave, auger, or spiral-type screws with fine or coarse flight pitches. The feeders can be easily removed from the hopper for cleaning.

Each modular feeder is suspended from three K-Tron weight-measuring devices called Smart Force Transducers (SFT). The transducers use a patented vibrating-wire technology to sense weight loss. The SFT sends a digital weight signal to the controller, eliminating the analog-to-digital conversion common to other feeders. Analog-to-digital signal conversion can reduce accuracy and add equipment cost, says Don Dunnington, K-Tron marketing communications manager. The SFT has a weighing resolution of 1 part per million, Dunnington says.

K2 feeders are said to be accurate to [+ or -] 0.025 to 0.01% at two sigma (standard deviations). The accuracy allows Chemtrusion to use the feeders as a Total Quality Management (TQM) device. "The most appropriate way to verify material quality is to verify feeder performance," Owens says. Because the feeders ensure that the proper amounts of ingredients are added to the process, post-production laboratory q-c testing can focus on factors not related to formulation accuracy, such as material rheology and physical properties, he notes.

COSTS AND SAVINGS

Owens says the modular gravimetric units initially cost four to five times as much as volumetric units but represent only about 15% of the cost of a compounding line. Modular gravimetric units reportedly pay for themselves in material savings (from not overfeeding expensive additives) and reduced labor and time expended on calibration. Another benefit is the ability of microprocessor controls to provide quality documentation.