Let feeders, metering equipment help slash your resin costs

Greater accuracy in feeding and metering can mean significant cost reduction for processors. If only it were that simple. But Gary Hovis, product sales manager for blenders & feeders at The Conair Group Inc., says that accuracy in feeding and blending is a “rather ambiguous concept, where even the definition of the word “accuracy” can vary.

Hovis notes that published accuracy data “varies widely because there are different ways of calculating accuracy. Accuracy claims can range from ±0.5 to ±0.05%, depending on whether you are talking about weights as a percentage of the entire batch or as a variance from set point.”

Additionally, maintaining accuracy becomes particularly tricky when trying to dispense very small amounts of a critical ingredient, such as foaming agents, blowing agents or even very critical colorant. “However, accuracy in feeding and metering is creating an interest among processors in part because material prices are so high. Additive and color concentrates tend to track resin pricing because in most cases they are compounded using a base resin as the carrier,” Hovis says.

An interesting side effect of high virgin resin pricing, notes Hovis, is that he sees more processors looking to use scrap, not only what they generate in-house but also that which they can purchase on the secondary market, which offsets having to use all-virgin material. “That's something we see being done more and more to offset their costs and meet environmental objectives. Therefore, having a more accurate means of putting that regrind in is critical. If they overfeed regrind, it can alter the physical properties of the product and create problems. With limits on the maximum percentage of regrind, a device such as a gravimetric blender becomes a very important tool: not only can you more accurately control the material recipe, but processors also can document how much regrind and other components they're actually using.”

For metering very small quantities of certain additives, Hovis notes that the TrueBlend gravimetric blenders from Conair offer a precision control mode that gives priority to the weight of one minor component. “The normal dispensing order of these blenders starts with regrind, then natural/virgin and then color or additive, with the color/additive being done last so that it can be calculated to the proper percentage of the already weighed natural resin,” Hovis explains. The precision mode reverses this sequence to give priority to the critical minor ingredient. The regrind remains first in the sequence, since it is always a percentage of the total batch and includes color and/or additive. With precision mode selected, the minor ingredient is dispensed second, and weighed as accurately as possible, then the natural/virgin resin is adjusted accordingly for any variation of the critical ingredient. Accuracy is increased without having to split pellets.

Another supplier, Colortronic North America, has rolled out its new synchronized, digital dosing system, Colorblend M, designed to precisely feed plastic additives (either in pellet or powder form) to any plastics processing machine. Precision dosing of additives is accomplished using a correctly sized dosing disc in combination with a special DC drive motor and an operator-friendly control. This combination allows for additives to be dosed in sync with the processing machine and the flow of the main component material. When the additive and main material hit the processing screw, they are mixed at the right proportion, according to the inputted recipe.

Keith Larson, marketing and sales manager for Colortronic North America Inc., says that the company's Colorblend M's rotating feeding disc mounts horizontally and feeds material more consistently. “The disc pockets all have the same amount of material in them as opposed to screw-type feeders that might feed a lot one time and a little the next, resulting in greater accuracy at low rates,” he explains.

Gravimetric versus volumetric? It's not that simple

Larson says that one system—gravimetric feeding versus volumetric feeding—isn't necessarily better than the other; the choice is application-dependant. “We use feeders and blenders together depending on what we're doing, for greater accuracy especially for very small additive amounts,” he explains.

A lot of processors are jumping on the gravimetric feeder bandwagon primarily because of lower costs, and auxiliary companies are promoting these. “We've all come out with lower cost gravimetric feeders,” says Larson, “but those are not necessarily more accurate.”

Larson says that selecting a feeder requires understanding the minimum and maximum continuous running rates of the process is very important when trying to size any kind of feeding equipment. “Low rates on start-up are not an issue, but if you plan to run a broad range of continuous output rates, you need to take all of them into account when sizing a feeder,” he says. Most feeders can easily handle a turn-down range of 10:1 between the highest and lowest rates, and some designs have a broader range. Changing the feed screw or disc will usually cover the entire range, but it must be considered up front when specifying the feeder.

Also, consider the ratio of “cycle time” to “recovery time” of the shot in injection molding applications, because the screw is only rotating and feeding additive material during the recovery phase of the molding cycle. “If you assume a five second recovery and a 15-second cycle time, the feeder needs to feed the entire amount of the additive, with the virgin material, in one-third of the time. So the feeder needs to be sized for three times the actual feed rate,” he says. “This is important to understand or the feeder will be undersize, and will not achieve the desired cycle time.”

“Pulsing” during feeding—overfeeding one time, underfeeding the next—can be a problem in many older style feeders, notes Conair's Hovis. Conair's TrueFeed gravimetric feeder has a patented rotating metering cylinder coupled with a 0-200-rpm stepper motor, which accurately dispenses process ingredients, even at rates as low as 0.02 g/second when using small particle concentrates. “This new metering technology eliminates the uneven delivery or 'pulsing' that is associated with older auger and pocket-type feeders,” said Hovis. “While pulsing doesn't affect the total amount of material delivered by a feeder over a long period of time, it can upset the accuracy of mixes that depend on a consistently low rate of additive or color in shot-to-shot applications.”

He continues, “To compensate for erratic feeding due to the pulsing effect of augers, processors often over-feed, setting rates higher to ensure that enough colorant is always dispensed. With some ingredients costing up to $50/lb or more, using even a small amount more than necessary can be quite expensive. On the other hand, with the digital precision of the TrueFeed, and the more accurate metering tube, processors can be certain that just the right amount of color or additive is dispensed.”

The bottom line, says Hovis, is, “Processors are trying to cut costs more than ever and looking everywhere for savings. That means even in lean times, processors are still willing to spend money for a good feeding/metering system that blends better or colors better, because they can realize the payback.”

Colortronic's Larson notes that today's gravimetric blenders have come down in price so that volumetric blenders are “almost a thing of the past.” And, there are many very good gravimetric blending systems available to processors today. “Just be sure to consider the types of material you are using, and what is important to your process,” he recommends. “The key is selecting the system that is the most cost effective solution for your operation.”