Dosing and blending - getting the right mix Accurate and repeatable dosing and blending is crucial in the manufacture of compounds and masterbatches. A wide variety of equipment is available to ensure that extruders run efficiently. Jennifer Markarian outlines some of the systems on offer from machinery suppliers.

Nov / Dec 2004

The most important features of dosing and blending equipment for compounding are accuracy and repeatability, agree industry experts. In high throughput, mega-compounding extruders, residence time is a matter of seconds rather than minutes, making short-term dosing accuracy more critical. Accuracy is particularly critical for dosing minor ingredients at low continuous feed-rates. Proper sizing of dosing equipment to match the total throughput is also important. Components that are easy to clean and change minimize downtime and eliminate contamination. Equipment reliability and durability also minimize downtime. Versatility of dosing mechanisms and the ability to change mechanisms quickly is important to customers who may have a variety of pellet, powder, and regrind feedstreams.

The Posimax is manufactured by Colormax.

A feeder is used to dose a single component into the extruder or compounding equipment. A blender is used to measure and blend multiple components that are metered into the process as a single feedstream. Some see a new trend towards using continuous split feeding using loss-in-weight gravimetric feeders, in which each ingredient is fed separately, directly into the extruder throat. This arrangement eliminates a mixer or blender, making cleaning easier and reducing contamination, says Fred Hauptli, industry manager at the K-Tron Feeder Group. Feeders designed to meter minor ingredients at low rates are an important element for continuous split feeding. For example, K-Tron's new Micro Feeder Model MT12 accurately feeds rates from 20 to 2000 grams/hour, says the company.

Blending systems
In the past, processors had to choose between continuous, loss-in-weight gravimetric blending systems, which are very accurate and expensive, and volumetric blending systems, which have much lower capital cost but are less automated and less accurate. Since volumetric systems feed by volume, operators must recalibrate for material bulk density changes. Gravimetric systems feed by weight and so automatically compensate for bulk density changes. Continuous gravimetric systems have one to three load cells on each ingredient hopper that measure the loss in weight of the hopper as it feeds into the blender or compounding equipment. About 10 years ago, gain-in-weight gravimetric batch systems became available that offer the accuracy of gravimetric measurement with much lower cost.

In a gravimetric batch blending system, each ingredient is dispensed into a common weigh hopper with one or two load cells that measure the hopper's gain in weight. For example, the first ingredient is dispensed to the hopper and weighed, then the next ingredient is dispensed to the hopper and the gain in weight again measured. The batch from the weigh hopper is then fed to a mixing chamber that continuously feeds the extruder. Having fewer load cells dramatically reduces a blender's capital cost. Batch systems are also easier to set up and calibrate, says Gary Hovis, commercial specialist for blenders at Conair. Until recently, batch systems could not be used for throughputs higher than about 2268 kg/h (5000 lb/h). However, the new Maguire® MaxiBatch™ Weigh Scale Blender (WSB) can process throughputs up to 3629 kg/h (8000 lb/h). "Traditional thought is that you need a continuous feeder for high throughput, continuous extrusion. However, we do not know of any processes that properly sized batch systems can't handle," claims Pat Smith, vice-president of marketing and sales at Maguire Products Inc. Some are concerned batch blenders may not be able to weigh poorly flowing material accurately, such as regrind or long glass fibre (LGF) concentrate pellets that bridge or hang up in the system. Maguire's software system takes potential feeding problems into account, says Mr. Smith. The difficult-to-feed material is dispensed and weighed first, and other material amounts are calculated based on how much of the first material actually flowed into the weigh hopper. This calculation method also doses colorants and additives as percentages of natural resin, taking into account the amount of pre-coloured regrind, adds Mr. Smith.

"Over the last few years we have seen very high growth of batch gravimetric systems in the compounding industry as compounders have seen that batch systems are equivalent in accuracy to loss-in-weight systems," concludes Mr. Smith. Volumetric single-component feeders are still purchased. However, new installations of volumetric blenders have virtually disappeared from the industry, because gravimetric batch systems offer greater accuracy at comparable cost, say industry experts. Blending equipment suppliers also see a trend towards fabricators saving costs by blending colour or additives on-line rather than purchasing compounded material. "This is possible because of capable and affordable blenders available today," says Mr. Smith.

Dosing mechanisms

K-Tron’s BSP-150 feeder.

Many dosing mechanism types can be used to meter material into the blender or compounder. Dosing accuracy depends on choosing a mechanism that can accurately feed the material type at the desired throughput. Dosing accuracy also depends on the weighing system, which is affected by calibration, load cell resolution, and vibration. Because smaller load cells have tighter resolution, the smallest appropriate load cell should be used, explains Mr. Smith. The extruder motor or external events in the plant can be vibration sources affecting dosing accuracy. Maguire's system takes every reading twice to verify accuracy and eliminate discrepancies caused by vibration.

Process Control has developed a patented DSP (Digital Signal Processing) weighing technology to filter out electrical noise and vibrations. Short-term dosing accuracy is becoming increasingly important with the trend towards higher throughput extruders.

Feeders must match the set-point on a 5 second basis rather than the traditional minute basis, notes Robert Barnett, global vice-president of sales and marketing at K-Tron Feeder Group. Obtaining rapid accuracy is challenging and must take into account variations in material flow and exterior vibrations. K-Tron's system maintains this accuracy by sampling 112 times per second and filtering the data to eliminate vibration effects, says Mr. Barnett. In addition to accuracy, the ability to change dosing mechanisms quickly as materials change is important for compounders. Many suppliers offer feeders with modular designs to allow quick dosing mechanism changes. K-Tron recently introduced a Quick Change/Quick Clean T35/S60 feeder that allows swapping of a twin screw and single screw configuration, says the company.

The mechanism of Motan’s Gravicolor blender

Micro dosing with the Gravicolor

 

Augers
Single and twin screw horizontal auger feeders are used in both loss-in-weight and batch gravimetric systems. Powders such as wood flour and talc require auger feeders. Various stirring, vibrating, or pulsing devices can be used to prevent or remove bridging. Regrind containing fluff or stringy material, which presents the biggest challenge to most feeder types, can be fed with an auger using starve feeding or a crammer, notes Daniel Saigh, national sales manager for Motan Incorporated.
A drawback of auger feeders, particularly single screws, is that they produce an inherently uneven output or 'pulsing' due to screw flight position. Pulsing decreases feeder accuracy, especially at low rates. The software for Maguire's batch gravimetric system accounts for pulsing at the weigh hopper, notes Mr. Smith. The Feeder Group of K-Tron International introduced a new screw speed modulation algorithm that further improves accuracy for loss-in-weight gravimetric feeders when running at low feed rates with screw speeds below 60 rpm.

Screwless
The industry is seeing movement away from conventional, horizontal augers towards other feeder types for pellets and free-flowing granules, says Terry Fahlenbock, president of Brabender Technologie, Inc. Several 'screwless' alternatives to auger feeders have been introduced for use with materials that do not feed well in augers or as a solution to the problem of pulsing.

The new Maguire® MaxiBatch™ Weigh Scale Blender.

Brabender's ScrewDisc uses a downward flowing vertical screw with a flow disc for pellet feed rates from 2.3 - 1361 kg/h (5 to 3000 lb/h). Brabender's RotoTube, a rotating cone loss-in-weight feeder, feeds 30 g/h to 4.5 kg/h (10 lb/h), says Mr. Fahlenbock.

Motan's dosing cone has a two-sided valve that is forced both open and closed, allowing for better flow of challenging materials. Dosing cones work well with tacky materials and regrind that are difficult to feed and tend to bridge in slide valve or auger feeders, says Mr. Saigh.

The ACS Group offers a volumetric disc-type feeder in which material falls into pockets and through holes in a rotating disc. This works well for pellets and free-flowing powders, providing high accuracy for volumetric systems, says Keith Larson, product manager at the ACS Group, which includes the companies AEC, Colortronic, and Sterling.

K-Tron's Bulk Solids Pump™ (BSP) technology, introduced in early 2004, uses positive displacement to feed free flowing pellets or granules with a very uniform, accurate discharge, says the company. In the BSP, material fills up a chamber or feeding duct, then 'locks-up' and rotates to the discharge point. The new BSP 150 has four feeding ducts for feed rates of 45 to 4500 dm3/h (1.6 to 160 ft3/h).

Colormax, a division of K-Tron International, introduced the Posimax B200, for rates of 1 to 100 kg/hr, and the Posimax B300, for rates of 5-500 kg/hr. Posimax feeders use the positive displacement principle that is used in K-Tron's BSP.

Vibrating trays are also used for materials such as long glass fibres, which would be broken up in an auger feeder. Vibratory feeders are generally used for higher throughputs and are not accurate below 2-5%, says Steve Buckley, vice-president of project engineering and marketing at Process Control Corporation.

Slide gates
Batch gravimetric systems commonly use slide gates to dispense material. Horizontal slide gates are used for high volume, free flowing material. Maguire's patented vertical slide gate, used for lower volumes, has an outer and inner tube - pushing the inner tube down allows material to flow through. Various vertical slide gate designs are available to handle different material types and compensate for jamming tendencies, explains Mr. Smith. For example, a double acting air cylinder can be used to both extend and retract the inner tube under pressure, preventing jamming of material such as wedge-shaped regrind.

Micro-dosing
Obtaining high accuracy is more difficult for low volume feedstreams. Various feeders have been developed for tight tolerances at low throughputs. Maguire's patented Micro-Pulse™ technology features a vertical valve in which the dispense tube is modified by a baffle restricting material flow. In Process Control's slide gate mechanism, the distance the gate is opened can be adjusted, allowing the gate to open and close in just milliseconds. Motan's Gravicolor 30 utilizes a patented, vertical micro-dosing screw. These systems are designed to meter 2 to 3 pellets at a time. K-Tron Feeder Group's MT-12 is designed to deliver 20 to 2000 grams/hour on a continuous basis.

Liquid feeders
Liquid additives, such as colour, silanes, and flame retardants, can be pumped into either volumetric or gravimetric feeders for compounding. A traditional control system for a liquid feeder uses a flow meter to measure flow rate, which is controlled by a pump or flow valve. While flow meters are adequate for many processes, the loss-in-weight principle can more accurately control very low rates or materials that experience viscosity changes, says Mr. Barnett. Depending on the additive, processors may use tanks with heaters or mixers in order to keep the liquid under controlled conditions, he adds.

The Colorblend Duo from ACS Group.

Liquid colour use seems to go back and forth as processors try to take advantage of its low cost but may be discouraged by cleaning difficulties. Liquid pigment systems typically contain between 70-80% solids in an inexpensive mineral oil-type carrier, compared to colour masterbatches, which contain 50% or less solids in a polymer carrier. Although the mineral oil lowers cost, spills are messy. A 'Pump-in-a-Drum'™ system marketed by Riverdale Color has a pump inside the colour drum so the customer does not need to open the drum, eliminating dripping and spilling. After it is empty, the sealed container is sent back to the colour supplier, eliminating drum disposal. "With these two main deficiencies being addressed, we see companies beginning to make the transition to liquid colour," says Mr. Smith. Liquid colour offers an extra advantage in PET processing, because solid PET colour concentrates require drying.

Developments and trends in feeder control systems
User-friendly software and systems with improved reliability continue to be important features for dosing and blending control systems. Many of today's software packages allow users to print documents that certify blend components or archive data for later investigations. In addition, material usage tracking improves inventory management. A trend over the last few years has been to mount controllers for individual feeders on the feeder rather than in central control panels. Controllers can then be pre-wired at the factory, saving installation costs. Processors more frequently want to tie individual feeder control systems into a central controller. Equipment suppliers must provide a variety of interfaces to allow their blending systems to communicate with existing, centralized plant systems. Suppliers are taking communication a step further by connecting blender microprocessors to the Internet. All Process Control's blender controllers are available with an embedded web server. Motan introduced its WEBpanel web-based controls at the 2004 K-Show and Maguire expects to have Internet capability by 2005. "This will allow us to look at blender problems real-time via the Internet. It is a huge step forward for customer assistance," says Mr. Smith.

A new method for controlling the overall compounding or fabrication process through blender or feeder control is starting to catch on, says Mr. Saigh. The traditional method for controlling an extrusion process is to fill the ingredient hoppers to a set level, measure the extruder output, and adjust the extruder speed to gain the desired output or gauge. Since the gravimetric blender is the most accurate measuring instrument in the system, material usage can be measured and used to control the end product throughput. For fabricated parts, greater end product accuracy translates to higher quality and material cost savings, notes Mr. Saigh. Systems such as Process Control's Gravitrol System and Motan's new GraviMaster control system use this principle of throughput or yield control. Maguire's new LineMaster AC™ system, an extension of its LineMaster™ yield control technology, combines a gain-in-weight gravimetric blender with a loss-in-weight hopper.

"The high-speed responsiveness of the LineMaster AC system makes our low-cost yield control applicable even in extrusion lines where there are frequent ramp-down/ramp-up cycles, such as for film roll changeovers, or where inconsistencies in extrusion rate pose especially great problems, such as in co-extrusion," says the company.

Contacts:
ACS Group: AEC
Tel: +1 630 595 1060
Website: www.aecinternet.com

Brabender Technologie Inc.
Tel: +1 905 670 2933
Website: www.brabenderti.com

Colormax Ltd.
Tel: +44 1952 292 101
Website: www.colormaxltd.com

Colortronic
Tel: +1 856 312 9600
Website: www.colortronic.com

Conair
Tel: +1 412 312 6000
or +44 1753 215 100
Website: www.conair.com

K-Tron Feeder Group
Tel: +1 856 589 0500
Website: www.ktron.com

Maguire Products Inc.
Tel: +1 610 459 4300
Website: www.magire.com

Motan Incorporated
Tel: +1 269 685 1050
Website: www.motan.com

Process Control Corporation
Tel: +1 770 449 8810
Website: www.process-control.com

Riverdale Color
Tel: +1 732 376 9300
Website: www.riverdalecolor.com

Schenck Process Systems
Tel:+44 161 975 1800
Website: www.schenk.co.uk

Sterling
Tel: +1 414 354 0970
Website: www.sterlco.com