Gravimetric blending: now more affordable than ever.

More molders are choosing the superior accuracy and self-calibrating convenience of gravimetric blenders as their cost has come closer to volumetric units.

Previously limited to compounding and high-volume extrusion applications, gravimetric feeding and blending equipment has now caught on big in injection molding and is gaining adherents in blow molding as well Equipment suppliers have been surprised at the rapidity and magnitude of the swing. In the late 1980s, gravimetrics accounted for about 4% of feeding/blending units sold. Suppliers estimate that 85-90% of blenders sold today are gravimetric. Of those, 75% are batch units and the remainder are continuous loss-in-weight types.

One reason for the shift to gravimetrics is that electronics prices have fallen faster than the cost of mechanical components for volumetric equipment. As a result, today's batch weigh blenders offer the advantages of high-end continuous gravimetric systems at prices approaching those of volumetric equipment--often for under $10,000.

Drew Schmidt, product manager at Foremost Machine Builders, points to today's less expensive control options: "Versus the standard, more costly PLC, we now have a proprietary, lower-cost microprocessor board." Though PLCs today can run multiple blenders, suppliers worry that a problem with the central PLC would take several blenders out of action. Thus, suppliers gravitate toward proprietary modular controls.

Another reason is tightened quality standards. Microprocessor controls for gravimetric blenders provide quality-control documentation on exactly how much recycle or additive was used in a given production run. The need for strict records on post-consumer content in bottles has brought blow molders to gravimetrics, says Dan O'Donnell, regional sales v.p. at AEC/Whitlock.

A related benefit of microprocessors is improved inventory control. "Having the ability to track usage of expensive materials has driven the use of batch weigh blenders in injection molding," says Rob Northrup, v.p. of sales at Process Control Corp. Volumetric blenders can have microprocessor controls as well, but they add cost. Many processors see little point in buying a less accurate volumetric blender for around the same price as a gravimetric unit.

None of this means that volumetrics are a dying breed, as suppliers are quick to point out. There are still some customers who want the lowest-cost equipment for non-critical applications--even if the saving is small versus a gravimetric unit. And at least one supplier acknowledges that volumetric systems with low-end controls can be less susceptible to radio-frequency interference.

FALLING PRICES

Continuous loss-in-weight feeding and blending equipment, which first appeared on the market in the 1980s, is designed to dispense many ingredients simultaneously. Such equipment has one or more load cells on each ingredient dispensing station. Some of these systems now sport price tags as low as $18,000-20,000, as compared with $25,000-30,000 or more previously. John Stacy, president of Inoex Technologies, says lower prices are driven by lower-cost controls and heavier competition among a greater number of sup" pliers. However, continuous gravimetric units still carry a price premium of roughly 50% over volumetric types.

Batch weigh blenders, which first appeared in the early 1990s, cost less than half as much as continuous gravimetric units while providing the same accuracy and other benefits, suppliers say. Batch units are also inherently better suited to discontinuous processes like injection or blow molding. Batch blenders use fewer load cells (typically two) and weigh all ingredients sequentially (one at a time) into a batch.

"Batch blenders have a price premium of about 10-15% over their volumetric counterparts, versus 30% five to six years ago," says HydReclaim president Bill Moller. "The economies of scale of building the batch blenders are far better, and the controls today are no more expensive than the microprocessors used on volumetric units."

Depending on the supplier and throughput requirement, most weigh blenders are selling in the ballpark of $8000-12,000. Suppliers such as Universal Dynamics and Maguire Products have brought out mini units for throughputs up to 200 lb/hr that start at under $7000 (see New Products section). HydReclaim's Moller reports that customers are now asking for gravimetric units to run as little as 10 lb/hr.

Conair Franklin product marketing manager Gary Hovis says gravimetric batch blenders cost at most 10% more than volumetric blenders and may carry no cost premium at all. He says weigh blenders with throughputs under 1000 lb/hr sell for around $8000-10,000, and those for 1000 lb/hr and up run $10,000-12,000.

AEC's O'Donnell places the current price differential between gravimetric batch blenders and volumetric units at about $2000. "You have volumetric units selling at $5000-8000 while comparable gravimetric batch blenders can be purchased for around $7000-10,000."

David Cosner, v.p. of sales and marketing at Universal Dynamics (Una-Dyn), says the firm now offers a gravimetric model for upwards of 150 lb/hr that is less expensive than some of its volumetric blenders.

CONTINUOUS VS. BATCH

Continuous loss-in-weight blenders lend themselves best to continuous processes like compounding and extrusion. Batch weigh blenders, though strongest in injection and blow molding, have also picked up some extrusion applications. "They [batch units] are typically used for shorter runs--be it sheet, profile, or film--where you want to change dies or colors and where blending is done off the feed throat," says Una-Dyn's Cosner.

Northrup of Process Control recommends continuous loss-in weight blenders over batch units for all but the least critical extrusion applications. The difference is not in metering accuracy, he says, but rather in improved blend homogeneity obtained with continuous systems.

FEEDERS STAY VOLUMETRIC

Feeders without blenders are widely used in compounding, extruder throughput control, and low-volume colorant addition in molding and extrusion. Unadorned feeders account for roughly 15% of the entire feeding/blending market, suppliers estimate, and 85-95% or more of them remain volumetric. HydReclaim's Moller says this is mostly because gravimetric feeders still cost two and half times more than volumetric feeders, even though prices have come down by half since the late 1980s.

Gravimetric feeders are unchallenged by volumetrics in one particular niche--extruder throughput control. Suppliers such as Process Control, HydReclaim, Inoex, Plast-Control, and LCI recommend gravimetric control for critical applications such as coextrusion.

High-end gravimetric feeders, such as those offered by K-Tron or Acrison, carry price tags of $15,000-20,000. They're ruggedly constructed to meet the needs of compounders and are used primarily for hard-to-handle powders that tend to pack, bridge, or flood. K-Tron marketing communications manager Don Dunnington adds that gravimetric feeders are valuable whenever a material tends to vary in bulk density.

Outside of compounding, many sup pliers say low-cost gravimetric blenders offer a better solution than a plain feeder. Still, suppliers concede that there is sizable demand for volumetric feeders among extruders that need to meter minor ingredients and among injection molders with short runs. "If they are looking only for a low-cost, simple, quick solution to put a small amount of colorant or additive in their process, volumetric feeders are a good economic option at a current cost of around $2000," says Hovis.

Gravimetric pellet feeders cost around $4500. They have found a niche in high-output film and profile extrusion where expensive additives are used, according to HydReclaim's Moller. "In commodity film and profile applications where additives are lower cost, they still use volumetric units."

Northrup sees a few molding and extrusion applications for gravimetric feeders (rather than blenders). "One example is PET, where you don't want to mix the color before the material goes into the dryer, so you put the color feeder right at the throat of the extruder."

Cosner sees gravimetric feeders as an option for molders who need to add one ingredient--e.g., regrind or color. He says components should be similar in size and bulk density and warns that homogeneity could be compromised.

RELATED ARTICLE: Two Ways to Save with Gravimetrics

When properly calibrated, volumetric blenders can be just as accurate as their gravimetric counterparts, says Conair Franklin product marketing manager Gary Hovis. Volumetric units must be calibrated frequently by hand. Gravimetric blenders self-calibrate automatically on every cycle, compensating for material bulk-density changes.

Hovis says volumetric calibration can take up to 15-20 min for a single blender, depending on the number of ingredients metered. For example, if blender calibration takes 15 min and two material changes are required per day per machine, and the machines run five days a week, this amounts to 2.5 hr per week of downtime per machine, or about 125 hr/machine annually. Depending on the cost of machine time, the dollar saving can be significant, as shown in the chart (facing page). The chart shows only machine-time savings and does not reflect savings in man-hours.

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The other key savings opportunity with gravimetrics comes from more precise metering of costly additives. According to Hovis, many processors overfeed critical ingredients to compensate for the variability of systems using volumetric blenders. "Typically what happens is that settings are increased to maintain production of acceptable parts. This excess additive/color usage is always a minimum of several percent more than what is actually required to achieve acceptable parts," he says. This waste is compounded by feeding variations and calibration errors to which volumetric units are prone.

For a rough check on whether you have an "over-coloring" problem, Hovis says calculate the amount of color concentrate required for your product and compare it to the actual amount consumed. The difference is what can be saved by employing a gravimetric blender (see chart on facing page).