All-Fill Powder Filling Machines & Technology
Powder filling machines come in a wide variety of shapes, sizes, filling principles and degrees of technical sophistication. All-Fill are at the technological forefront in the design of vertical auger powder filling machines. This represents a 95% volume of powder filling machines sold, the remaining 5% comprising; vacuum fillers, cup fillers and gravity fillers.
The principle of the vertical auger filling machine is very simple.
The basic filling head comprises six primary components:
The lower parallel flights of the auger within the funnel are machined to a constant pitch so that each pitch has a precise volume. The auger drive rotates the auger at a constant speed to produce a continuous dosing flow. The agitation blade, separately driven and controlled, rotates in the opposite direction to the auger, de-aerating and homogenising the powder, and breaking any bridge which, in non-free flowing powders, tends to form. The agitation blade extends down to the throat of the funnel preventing rat-holing and cavitation and ensures that the auger flights are consistently filled. Start/stop signals to the auger drive control the number of revolutions.
If an auger with flights of perfectly even pitch, evenly filled with powder of constant bulk density and even particle distribution, rotates a precise number of revolutions, theoretically there would be zero deviation in dose from one cycle to another. In the majority of applications, where these conditions more-or-less apply, volumetric filling via flux vector inverter drive is entirely appropriate. Typically, accuracies in the range of ± 0.5% to 2% can be expected.
Where such factors as variations in bulk density and particle size/distribution, high cost, or criticality of dose demand closer tolerances, weigh-filling technology and alternative drives can be employed.
A gravimetric (weigh) filling system may be selected for a number of different reasons:
• Inconsistency of tare weight
• Criticality of fill weight
• Need to document/validate the actual fill weight filled
• Inconsistency of particle distribution/density and therefore volumetric accuracy
• Cost of product give-away.
The simplest weigh-fill option is to fill directly onto a weigh-cell. When filling semi-automatically, the weigh-cell can be used in a variety of ways:
• Single Speed/Single Shot (dynamic)
Least expensive, but the fill output can be compromised by the need to use an auger small enough to provide the required accuracy, or large enough to meet the required output.
• Bulk and Dribble (dynamic)
The auger can be run at high speed for the bulk fill, and slow speed for the dribble top-up fill. This provides much improved performance, but the accuracy can still be compromised by the reaction time when weighing dynamically.
• Bulk, Predict and Top-up (static)
Again, high speed bulk filling, but the auger is stopped just short of the target weight. A static weight is taken, and the top-up fill calculated and converted into a second volumetric dose. Although the top-up fill is volumetric, if the accuracy (perhaps ±2%) is applied to the top-up weight (typically 10% of the target weight), an overall accuracy of ±0.2% can be achieved.
Weigh-cells can be incorporated into automatic filling systems in many different ways:
Single Head, Gravimetric
In a single head system, a single weigh-cell placed directly under the filling head can be used in exactly the same way as when filling semi-automatically; “single shot”, “bulk and dribble”, and “bulk, predict and top-up”. The weigh-cell can tare weigh the empty container and provide weight data as required. If the reaction/settling time of the weigh-cell prohibitively restricts the production output, moving the weigh-cell downstream and filling volumetrically with feedback may be considered, with upstream tare weigh-cell as required.
Dual Head, Volumetric Bulk Fill with
Alternatively, two filling heads could be used, but instead of the two heads dosing onto two live weigh-cells to double the output, a bulk and top-up system could be preferred. This provides the optimum of speed and accuracy, the volumetric first head bulk filling quickly with a large auger, the second head making the gravimetric top-up fill with a more accurate smaller auger. Furthermore, the cost of one weigh-cell is saved.
Dual Head, Bulk and Predict
For even greater speed and accuracy, the weigh-cell can be moved between the two filling heads, turning it into a bulk fill, predict forward and top-up system with intermediate weighing. This offers the speed of a large auger bulk fill together with the accuracy of filling the top-up with a smaller auger, but without the loss of cycle time when filling and weighing sequentially. In this case, the weigh-cell would feed back to the bulk filling head to maintain the proportion of the bulk fill, feed forward to the top-up head, and predict the top-up dose needed, all three functions occurring concurrently. It could not however provide final weight data. If required, a second weigh cell downstream of the top-up head could feed back to the top-up head, and forward to any reject station. If the containers vary in their empty (tare) weight, a tare weigh station would be added upstream of the bulk filling head.
When filling volumetrically, the number of auger revolutions are selected/controlled via the HMI/PLC to provide the auger stop signal. A back-up timer is also featured.
The choice of system is dependent upon the speed required, batch size, changeover times and available budget. When filling semi-automatically, the upper speed limit is dictated by the time taken to manually handle the container. If one second (normally the minimum) is allowed for picking up the container, presenting it to the filling nozzle and putting it down again, a two second auger dose time will restrict the output to 20 fills per minute. Others factors will limit the output further, for example:
• Larger fill weights
• Slow auger rpm necessary to provide the required accuracy
• Tendency to damage the product
• Excessive dust
• Small container neck opening restricting the auger diameter
• Container handling difficulties e.g. bags/sacks etc.
When filling rigid containers automatically, the same restrictions apply. The speed may be further limited by the need to lift or vibrate the container at the point of fill. A simple pneumatic lift is used to provide a clean neck-entry fill, with neck location for narrow-neck containers. A hydraulic lift/lowering system provides the power and constant rate of descent required for bottom-up filling where the powder needs compacting/compressing to fit the full fill weight into the container. Vibration may be required to settle granular products.
This single head in-line machine is filling oral suspension antibiotic powder into narrow-neck glass/plastic bottles (a common pharmaceutical macro-dosing application) typically 5g to 80g volumetrically at between 15 and 30 containers per minute.
Doubling the number of filling heads has the effect of virtually doubling the output. Containers are indexed two-at-a-time, with each head filling 100% into alternate containers. Further increases can be made by upgrading to four head or multi-head systems.
The most frequently chosen high-speed solution is the continuous motion rotary machine where containers are fed into a rotary turret possessing 12 or more transfer funnels. Powder is dosed, not directly into the containers, but into the funnels which travel around the turret above the containers, providing the time required for powder transfer. Container jogging or air pulse ensures thorough transfer.
Volumetric fillers can be linked to a downstream check-weigher or weigh-cell to perform three important functions; provide weight data, reject under/over weights, and compensate for trend changes in powder bulk density. Such trends, generally caused by stratification of fines and larger granules during transport/bulk handling, cause fill weights to increase/decrease pro-rata. This trend data can be fed back to the filling head for automatic compensation by increasing/decreasing the auger rpm.
Tare and Gross Weighing
When filling oral dry suspensions into glass bottles, the weight variation from one bottle to another is often greater than the upper and lower limits for the fill. If 100% weight validation is required, it is necessary to weigh the empty and filled bottles, and deduct the empty (tare) weight from the filled (gross) weight to determine the nett weight. Using free-standing check-weighers would introduce the problem of maintaining registration during the weighing and filling processes, cross-referencing the gross weight with the tare weight of the same bottle. At slower production rates this potential problem is addressed by choosing an indexing rotary system whereby the tare and gross weigh stations are incorporated into the intermittent-motion rotary turret, registration being mechanically guaranteed by the turret starwheel system.
Where higher speeds require a continuous-motion rotary solution, mechanically registered intermittent tare and gross balance systems integrated into the infeed and outfeed starwheel assemblies of the turret provide the 100% nett weight validation required.
The size of the auger and funnel is determined by a number of factors:
• fill weight
• dosing rate
• container neck opening.
The smaller the auger/funnel, the slower the dosing rate but greater the accuracy. Conversely, a larger auger will deliver product more quickly but less accurately. Where a wide range of fill weights are required, more than one auger/funnel set may be needed to achieve the necessary combination of speed and accuracy. Generally, a ratio of 5:1 for each set may be considered appropriate. Accordingly, if a range of weights from 10g to 1 kg+ is required, three tooling sets would probably be needed; one small set for weights of 10g to 50g, a second set for 50g to 250g, and a large set for 250g to 1.25 kg.
All-Fill’s auger/funnel tooling (and hopper) is no-tools removable for ease/speed of change-overs and clean-downs.
A choice of three drive configurations is available to suit the application (and budget). Servo drives offer the best performance with high power/torque and precision stopping position. Directly coupled inverter or clutch/brake drives offer lower cost alternatives.
Before we look at the different types of tooling for different product characteristics, we first need to identify the type of product we are handling. There is a simple (but not completely foolproof) test for evaluating the flow characteristics of powders. If, when you insert a pencil into the top of a container full of power and then remove it, the hole immediately collapses in on itself, then the powder would be deemed to be free flowing. If, however, a perfect impression of the pencil remains, then it is deemed non-free flowing. Partial collapse might indicate a semi-free flowing powder.
Non-Free-Flowing Powders (e.g. talc)
This diagram shows the tooling set-up where the auger is flush with the bottom of the funnel and the agitation blade extends into the throat of the funnel. The auger featured shows larger diameter “over-flights” in the hopper section. This pushes the otherwise reluctant powder downwards to the funnel throat where the agitation blade feeds the flights of the smaller diameter parallel section. The agitation mode can be controlled so that it stops between fills to avoid overworking and compacting the powder. If the powder is highly aerated coming into the hopper, the agitation mode can be changed to “continuous running” to assist with removal of the air. A larger hopper volume will aid de-aeration and homogenisation by allowing greater agitation/residence time.
Semi-Free-Flowing Powders (e.g. instant coffee powder)
A delicate or granular product might be damaged if large diameter compressive auger over-flights are used within the hopper section. In instances like this, an auger with parallel flights would be selected. Furthermore, a semi-free flowing powder might tend to dribble after the auger has stopped. In such cases, it is necessary to create a slight back pressure to retain the powder in the funnel by fixing a “drip washer” to the bottom of the auger, or adding a lip or crosswires/gauze to the bottom of the funnel. The correct solution varies with each powder.
Free-Flowing Granular Products (e.g. table salt, granulated sugar)
When free-flowing (granular) products are being filled, a method of cutting off the powder flow is required. The usual method is to use a spinner disc; a saucer-shaped disc attached to the end of the protruding auger, designed to retain the product flow. All powders have an angle of repose, the angle at which a collapsing heap of powder will come to rest rather than continue to spread outwards; the (top enclosed) angle for small smooth spherical beads for instance being much greater than for granulated sugar. The gap between the bottom of the funnel and the spinner disc is determined by the powder dosing rate, i.e. the gap needed to permit unrestricted product flow. The spinner disc is sized to prevent the powder trickling over the edge of the disc once the auger has stopped, taking into account this gap and the powder’s angle of repose. As the spinner disc throws the powder outwards, a simple collection cone funnels it down to an appropriate size. The spinner disc approach achieves a perfectly clean fill without expensive shut-off valves and complicated pneumatic/moving parts.