Buyers' guide to gravimetric controls.

Buyers' Guide to Gravimetric Controls

As gravimetric feeders become more widely recognized as a way of controlling average extrudate thicknesses, the variety of gravimetric systems to choose from is rapidly expanding. Gravimetric feeders control the average thickness of a product by monitoring the weight of resin per unit time being fed into an extruder. A computer compares these values to a setpoint and makes needed adjustments to screw speed and/or hauloff to maintain a constant flow rate (see PT, Sept. '88, p. 23).

Although the operating principles of most of these units are basically similar, there are two key questions that buyers should consider before purchasing: Is it better to control screw speed, hauloff--or both? And what are the relative advantages of batch vs. continuous types? Our buyers' guide to available units gives insight into these issues and also tells how easily these models are integrated into existing line controls, for what applications they are best suited, and cost.

HAULOFF VS. SCREW SPEED

Gravimetric feeders can either be tied into the hauloff to control weight per length, or screw speed to control pounds per hour--or both, depending on the process and whether the end product is sold according to weight or length. For simple monolayer lines, hauloff is often an acceptable way of controlling output. One advantage of hauloff control, according to Robert De Palma, product development manager of Inoex Inc., Cranston, R.I., is that it responds quicker than screw speed, which must be adjusted in small increments. The hauloff, he adds, can essentially ignore the extruder yet correct for it, as in the case of a dirty screen pack.

On the other hand, coextrusion or processes with critical downstream operations such as corrugated tubing, vinyl siding, or extrusion coating, require screw-speed control. Rob Northrup, marketing manager for Process Control Corp., Atlanta, points out that screwspeed control tends to maximize output in these applications.

However, about 80% of PCC's coextrusion installations are tied to both the screw and hauloff. One advantage of such "ramping," or simultaneous control of screw and hauloff, is elimination of the reaction time between hauloff and extruder if only one were controlled, thus maximizing good product, says Northrup.

BATCH VS. CONTINUOUS

Although batch-type weighing units still are used widely in blending operations, several manufacturers now favor continuous loss-in-weight feeders for extrusion control. Batch systems, which use two hoppers and a level switch, have more mechanical parts than loss-in-weight feeders and make rate calculations only once per batch--far less often than continuous systems, according to Dana Darley, product manager for Luwa Corp., Charlotte, N.C.

The more frequent rate calculations of continuous feeders are said to permit tighter control loops to be maintained. Luwa's loss-in-weight feeder, for example, operates on a [+ or -] 0.5% "deadband," or acceptable error limit; Darley says a continuous control feeder typically will make speed adjustments every 10-20 sec. The real edge continuous-type gravimetric controls have over batch shows during speed changes involving new set-points (e.g., when changing materials) rather than during steady-state operation, he adds.

Other manufacturers say batch-type feeders still will have a place in extrusion control in the future. Inoex, for example, introduced a continuous feeder at K'89 (see PT, Jan. '90, p. 76) but has not phased out its batch-type system. Ron Beaudoin, v.p. of Inoex, says that accuracies are generally slightly higher on batch systems, which weigh in small batches and use a tighter class of load cell. Batch weighers, which reportedly do not experience bridging, are also said to be better suited for working with reclaim.

CATCHING ON WITH OEMs

Despite gravimetric controls' high profile at K'89, some suppliers claim that interest among U.S. extruder OEMs has so far been slack. Dana Darley, Luwa's manager of extrusion products, says that few U.S. extruder manufacturers, in contrast to their European counterparts, have developed algorithms that truly incorporate gravimetrics into an extrusion line control system. The result, he adds, is that a gravimetric control will not integrate easily, and may be redundant, resulting in extra cost for the end user.

Gloucester Engineering is one extruder manufacturer that has integrated gravimetric control into its own software to modify screw speeds. The advantage, according to a company spokesman, is simplified overall control of the extrusion line. NRM-Steelastic also is reportedly taking a "systems" approach to gravimetric control. Sales manager Peter Anderson says the company's ATS control system already incorporates gravimetrics. In the last few months, Windmoeller & Hoelscher, Lincoln, R.I., which developed a gravimetric control jointly with Plast-Control GmbH of West Germany as part of W&H's Optifil system, reportedly has replaced that gravimetric control with a similar one of its own design, based on W&H software.

GUIDE TO AVAILABLE SYSTEMS

Following are key features of some currently available gravimetric extrusion control systems.

* Luwa's new Smarthopper, a dedicated, scaled-down gravimetric control is said to easily integrate into an OEM's own control system. The Smarthopper is a simple, non-expandable, continuous gravimetric feeder that controls a single extruder through the puller--not the screw. For the end user, the price is relatively low, beginning at under $9000 for the smallest hopper size (0.25 cu ft).

The end-user version of Smarthopper is designed for raw-material management and control of the extrusion process by monitoring the rate of material consumption of the extruder. It calculates the required line speed for control of the weight per unit length of the finished product. Data is provided on pounds per revolution, total material used over an extended period of time, amount of material in the hopper, and any alarm conditions.

The OEM version of Smarthopper communicates the information to the main extruder console; like its end-user counterpart, the control hopper does not control screw speed directly. A Smart Gravimetric Additive Feeder--part of the OEM package--does adjust feeder screw speed and flow rate by comparing actual flow rate to a downloaded additive flow rate setpoint.

A key feature of the Smarthopper is that it is designed to be "supervised," or controlled by an main plant computer, according to Darley. An RS485 serial link (standard on the OEM unit; optional on the end-user model), based on the SPI communications protocol, permits downloading of data to a PC, which can then perform data-logging, inventory control, and other data-manipulation functions.

Later this year, Luwa--which acquired Buhl Automatic of Guelph, Ontario, earlier this year--will also bring to market an intermediate-priced ($15-18,000) Buhl gravimetric feeder positioned between the Smarthopper and Buhl's full-featured PPC 6020 gravimetric control. The more sophisticated single-extruder, non-expandable unit will control both the screw and hauloff, and will include data-logging and trending capabilities.

* Process Control's Gravitrol, a continuous-type gravimetric weigh unit, is said to be capable of controlling monolayer to seven-layer processes. Weight-per-length accuracy is said to be [+ or -] 0.5%.

The weigh hopper of the unit is enclosed in an outer canister to protect the load cell from interferences that may throw off readings. Seven different sizes of weigh hoppers are available from 1/3-20 cu ft.

The Gravitrol unit includes one-touch command buttons for ease of use. It is available with a standard remote communications package as well as optional remote supervisor package. Setpoints and recipes can be downloaded from a supervisory computer. The unit can be made to monitor only or to control weight, length, or both. During setup, any number of parameters can be locked out to permit different operating configurations. The control panel includes 40 user-definable data fields to display operating parameters. It also includes 24 user-programmable alarm fields.

For monoextrusion, the Gravitrol can be made to monitor either screw speed or hauloff. For coextrusion, the system can control both screw speed and hauloff simultaneously. The automated ramp-up to full speed is said to eliminate scrap during start-up.

About 80% of Gravitrol systems sold are working on coextrusion lines. Blown film, cast film and sheet, and pipe account for the highest number of installations. A Gravitrol system for a three-layer blown film line, including hardware and computer controlling extruders and line speed, plus communications for a supervisory controller, would cost about $50,000.

* Inoex offers its basic Saveomat gravimetric weigher in either batch or continuous versions. The batch system, which is used for monolayer control and usually adjusts line speed, makes its weight calculation at the end of each batch, typically on 10-15 sec cycles. Continuous weighers take readings four times/sec and can make subtle adjustments in screw speed.

One advantage of the Inoex line of gravimetric control is modularity. The company's various systems are all based on its basic Saveomat feeder. The Aurex 90 F system for blown film combines gravimetric mass-throughput recording with capacitive film-thickness measurement. Similarly, the Aurex 90 R system for pipe extrusion is combined with an ultrasonic wall-thickness gauge. In both systems, information from the gravimetric controls can be used to recalibrate the gauges if there is a density change in the material. The system for pipe can also be retrofitted with a thermal centering device. Prices of the Aurex system typically run from $47,000-84,000, depending on options.

The Coex 90 system controls both hauloff and up to eight extruders to 0.3% accuracy layer-thickness control. According to De Palma, the system shows tighter accuracies when used on a monolayer blown film line as well. Price for a two-layer system is about $55,000.

Inoex v.p. Ron Beaudoin claims that gravimetric extrusion controls can be coupled with a volumetric blender and still achieve acceptable thickness control. "A lot of people have a misconception that they have to go to gravimetric blending to get accuracy," he says. In his opinion, a gravimetric blender may not be needed for extrusion control if a line is already getting acceptable product with a volumetric blender. "If properties are accurate with volumetric blenders, then all you have to do to control weight per foot is put a gravimetric weigher under it."

* Harrel Inc., Norwalk, Conn., uses its gravimetric controls as part of a closed-loop control system. For pipe, it controls ID, OD, and lb/ft. Controls use two feeders in tandem: a volumetric device feeds the gravimetric weigher. Holton Harris, president of Harrel, claims that the configuration gives better accuracy because the size of the charge remains relatively constant. The feeders, which weigh in small batches, flood feed to the extruder. The unit controls throughput by adjusting the speed of the screw. The microprocessor control for the gravimetric weigher can be expanded to control the entire line, according to Harris. Cost is in the $25,000-30,000 range.

* HydReclaim Corp., Fenton, Mich., offers a loss-in-weight gravimetric control that can adjust either hauloff or screw speed. The company claims accuracies of [+ or -] 0.5%. Price of a typical single-layer line for blown film is about $15,000, says Richard Hatherley, general sales manager.

* Plast-Control GmbH of West Germany is now being represented in North America by Control and Metering, Chicago. It offers a gravimetric control system incorporating a continuous weigh hopper, hauloff-speed sensor, and DR3000 average-thickness and extrusion-line controller for under $20,000.

ADAPTING BLENDING FOR CONTROL

A couple of manufacturers have adapted gravimetric blending technology for thickness control.

* K-Tron Vertech, Pitman, N.J., has taken advantage of its technology for weighing and blending to develop a blender and software that can be expanded to control the entire extrusion line, according to Bruce Lowden, national sales manager. The system operates in either feedback or feed-forward modes, both on a continuous real-time basis. In the feedback mode, the blender feed rates are paced to the extrusion process. If the level in the blender changes, the sensor will adjust the setpoint to the blender to bring the level up or down. In the feed-forward mode, the sensor provides a signal to the extruder, and by varying the rpm, operates the extruder based on gravimetric throughput.

The Graviblend system, as it's called, uses an ultrasonic level sensor claimed to provide continuous real-time level measurement. Each hopper sits on three Smart Force transducers, which are said to have a higher (1,000,000:1) resolution than typical analog load cells. In addition, the SFT load cells are reportedly drift-free, and need to be calibrated only once during installation. The Graviblend can be interfaced with the Genicon supervisory package that can be configured for various control strategies.

* Polynetics, Inc., Atlanta, whose president, Dean Ward, sees the function of the blender changing from an auxiliary device to the centerpiece of extrusion control, is offering a gravimetric blender that weighs each of the materials going into a blend ratio, then uses the sum of those to control the downstream process. The Wynner blender integrates with the extruder immediately because it doesn't use an auger to feed in the main ingredient. It reportedly starve feeds the additive while flood feeding the primary ingredient, ensuring that each flight of the screw gets a constant percentage of additive. The material going through the hopper is weighed through a loss-in-weight system, and the blender is tied to the extruder drive and/or hauloff. It can interact with up to six other blenders for control of up to seven layers. Cost of a three-layer system is $15,000-$20,000.

PHOTO : The Smarthopper from Luwa Corp. is a dedicated, economical gravimetric control for

PHOTO : monolayer lines that controls hauloff speed.

PHOTO : The Gravitrol system from Process Control Corp. consists of weigh hopper, microprocessor,

PHOTO : and various input devices, and can be expanded to control up to seven extruders on the

PHOTO : same line.