Otto Rothlin
K-Tron (Switzerland) Ltd

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Conveying and feeding of bulk materials and liquids, process and control technology

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Q: What are the differences between loss-in-weight feeders and ‘normal’ gravimetric feeders? What kind of powders/particles would you recommend them for?

A: At K-Tron we use the term 'Gravimetric' to cover all feeding and dosing processes where loss-in-weight or weigh-belt technology is utilized. Most process materials can be handled volumetrically or gravimetrically, decisions are generally based on:
- Cost of equipment, where volumetric is lower cost than gravimetric.
- Need for automation - refilling or data logging, for example.
-Cost of ingredients - low to high value materials.
- Importance of the ingredient to the finished product.

Q: We have a number of questions on pneumatic conveying, specifically transferring ABS powder using air.

1. What is the explosion limit for ABS powder?
The dust hazard will vary according to particle size but ABS has a fairly low energy required for ignition, typically between 5 and 50 mJ. The actual composition of your ABS grade is needed to determine the full characteristics, for example if it is already compounded with flame retardant additive. A true value can be obtained from specialists such as Burgoynes.

2. How can we minimize dusting when filling a silo?
Reducing the amount of air used to convey the powder will reduce the separation and dust in the silo. This can be achieved by dense phase transport, either positive pressure or vacuum.

3. What should the design pressure for the silo be? We transfer ABS powder to an atmospheric silo.
Silo design is varied and construction codes depend on site location. An atmospheric silo will not tolerate the vacuum levels used for conveying and large area vents are needed to ensure the structure remains atmospheric.

4. Which one is better for filling a silo - using a cyclone and a dust collector or using a dust collector alone?
Silo design differs. In general, silo filling is performed by directly loading the vessel through a tangential inlet and a positive pressure system. In this case a large area vent filter is required, with reverse jet cleaning to prolong filter life. When filling by vacuum we recommend using a vacuum separator with integrated filter and a large, quick acting dump valve.

Q: There are two ways of manufacturing talc-filled polypropylene compounds. Firstly, by mixing all the ingredients in a mixer and then pouring the mixture into the extruder. Alternatively, compounding can be done by using gravimetric feeders and feeding the polypropylene, talc and additives separately. Which is the better system and why?

A: This is a common dilemma. Traditional batch processing can be cost-effective and reduces some of the need for automated control systems that are required for continuous dosing. The pre-mix can be manually verified for the correct ingredients prior to carrying on to the next phase. However, the disadvantages include the lack of repeatability from pre-mix to pre-mix, which leads to variable end product quality. In addition, there are risks associated with segregating and handling the pre-blend including where it is stored and how it is transported to the process. For example, vacuum transporting of mixed powders over moderate distances can cause more dense powders to separate from lighter ingredients. Occasionally with certain ingredients it is necessary to add slightly more of one additive to ensure that there is sufficient to function properly in the final product, which can prove costly. Between campaigns the pre blender will need to be cleaned to prevent product contamination and this step can add time to changeover routines.

When adding the additives separately on a continuous basis it is possible to monitor repeatability and performance online from the data provided by the gravimetric system. The main component in the recipe can be used as a master rate and if this fluctuates a correctly-performing gravimetric system can adjust the individual additive dosing rates to maintain the correct ratios in the recipe. Gravimetric feeders are typically refilled automatically from bulk silos/daybins directly into the feeder, removing operator involvement and reducing dust. Dedicated feeder streams ensure rapid product changeover times without the need for time consuming cleaning routines.

Q: We need to feed a mixture of nylon and rubber strands. The strands are thin and the mixture has a very low bulk density. The material is reclaimed from nylon tyre cord and owing to the properties mentioned present problems in feeding. What should we consider?

A: For such applications we would recommend a feeding solution that uses a vibratory tray as the metering device. Attention needs to be given to the width of the tray to allow a large hopper outlet diameter to prevent bridging of the fibres. In similar applications when feeding fibres we have used a unique tray design that promotes flow into the tray and then aligns the strands using a comb-like insert. As with all bulk material applications the solution depends very much on the feedrate required, accuracy demanded, refill type proposed and other relevant site conditions. Any theoretical solution should also be proved to work using testing facilities. Most leading suppliers in this field should be able to offer free-of-charge performance tests in well-equipped laboratories.

Q: An additive masterbatch producer is using LDPE, calcium carbonate and two additional additive powders in a recipe. Can all the ingredients be fed into a twin screw extruder in one go?

A: Compounders usually feed twin screw extruders in starved mode. The most accurate and economic equipment to feed carrier, filler and additive ingredients into the extruder are Loss in Weight (LIW) feeders. This means that all the ingredients are fed continuously in the right proportion into the starved intake zone of the extruder.

The most important factor is the selection and sizing of the right feeder for a specific ingredient and its throughput. A wide variety of different models are available. For example, these include a single-screw feeder, vibratory feeder or Bulk Solid Pump (BSP) for granules and free flowing powders, belt feeder for waxy products with low melting points and a twin-screw feeder for sticky powders.

LIW feeders are arranged around the intake. The line throughput is the sum of the actual feed rate of each individual feeder. To achieve the best results a fast responding control system and a high resolution weighing system is required. This is extremely important during start-up, set-point changes and shut-downs. In order to maintain a steady high product quality and avoid off-spec materials all the feeders involved must react almost instantly and with the same hysteresis to guarantee the proportions as defined in the recipe.

Sometimes, particularly with low density fillers, it is not possible to feed everything at once into one inlet. In order to overcome this situation the fillers can be split and fed into a second inlet. Abrasive reinforced materials, such as glass fibres are fed downstream into the melt for two reasons. Firstly, this is to avoid wear in the extruder and secondly to protect the fibres from mechanical damage. The fibres are fed gravimetrically into a side feeder and then pressed in starved mode directly into the melt.

Traditionally LIW feeders are associated with solids. In the same manner LIW feeders can also be used for liquids. The liquids are sprayed via nozzles into the extruder.

Q: When operating a loss-in-weight feeder, why is optimized feeder refill so important?

A: Operation of loss-in-weight feeders requires periodic refilling. Consequently, because weight-based control during this brief but critical refill phase is not possible, maintaining temporary feeder accuracy becomes a concern. The solution to this problem requires determining the relationship between system weight and material density within the feeder’s metering zone.
During loss-in-weight hopper refill, sensed weight cannot be used to control feedrate. While some systems freeze the screw speed until the refill is complete, a better approach is to use refill technology that memorizes recent gravimetric performance throughout the whole period of the decreasing weight cycle.

In order to ensure the highest possible feeder accuracy throughout the refill phase, leading loss-in-weight feeder suppliers offer a solution based on previously collected gravimetric reference values. This enables the metering speed to be gradually adjusted during refill to counterbalance precisely the effects of variations in material density occurring within the metering zone as the hopper weight increases.
By using this software-based feature, gravimetric feeding accuracy during the refill cycle can be maintained. As a result, once the refill is complete, the return to gravimetric operation is smooth and without any disturbance from oscillations.

This feature is strongly recommended for use with fine, compressive or fluidizing powders. If density variations are minimal, for examples with granules, then this feature can be deactivated. The reference for screw speed will then be a calculated average value.