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Weigh Filling (Gravimetric)
Weigh filling machines and equipment may be selected for a number of different reasons:
| 1. | Inconsistency of tare weight (as discussed above) |
| 2. | Criticality of fill weight and need to document/validate same |
| 3. | Inconsistency of particle distribution and therefore volumetric accuracy. |
| 4. | Cost of give-away |
The simplest weigh fill option is filling directly onto a weigh cell.
A weigh cell can be utilised to fill in a variety of ways:
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speed/single shot:- Least expensive but the speed can be compromised
by the need to use an auger small enough to provide the required accuracy,
or the accuracy compromised by an auger large enough to meet the required
speed.
Bulk and dribble:- The auger can be run at high speed for the bulk fill and slow speed for the dribble top-up. Much improved performance but the accuracy can still be compromised by reaction/settling time of the weigh cell. Bulk, predict and top-up:- Again, high speed bulk filling but the auger is stopped just short of the target weight, the weigh cell allowed to settle and the top-up calculated and converted into a volumetric dose. Although the top-up is volumetric, if the volumetric accuracy (typically 1%) is applied to the top-up weight (typically 5%), an overall accuracy of + 0.05% can be expected. |
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Weigh cells can be used in conjunction with a single head automatic filler in different ways to different purposes. 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, bulk predict and top-up, and 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 might have to be considered, with upstream tare weigh cell as required. |
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Alternatively, two 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 would be preferred. This provides the optimum of speed and accuracy from a live gravimetric system; the first head bulk filling quickly with a large auger, the second head making the top-up with a more accurate small auger. Furthermore, the cost of one weigh cell is saved. |
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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. This offers the speed of large auger bulk fill together
with the accuracy of predicting forward and volumetrically filling the
top-up with a smaller auger. In this case the weigh cell would feed back
to the bulk filling head to maintain the proportion of the bulk fill but
could not provide final weight data. To provide this final weight data,
a second weigh cell is required downstream of the top-up head which could
feed back to the top-up head and forward to any reject station. A tare
weigh station could be added upstream of the bulk filling head if required.
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Gravimetric filling at high speed is normally always carried out on rotary machines with check-weighing systems after a volumetric fill. Most volumetric systems can be linked to a downstream check-weigher or weigh cell. This check-weigher/weigh cell can perform three important functions; it can provide weight data, reject filled containers for under/over weights and/or monitor trends in powder bulk density. Such trends, generally caused by stratification of fines and larger granules during bulk storage/handling, cause fill weights to increase/de-crease pro-rata. This data can be fed back to the filling head to compensate by increasing/de-creasing the dose to suit. In the case of intermittent filling this is done by increasing/decreasing the number of revolutions. When filling continuously it is achieved by changing the auger rpm.
When filling powders into glass bottles, the variation in bottle weight one to another is often greater than the upper and lower reject limits for the fill. If 100% weight validation is required it is necessary to weigh the empty and filled bottles and deduct the tare (empty) weight from the gross (filled) weight to arrive at the net weight of fill. The use of free standing check-weighers introduces the problem of maintaining control of the containers during the weighing and filling processes and therefore of referencing the gross weight of each bottle with the tare weight of that 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 rotary turret, registration being mechanically guaranteed by the turret starwheel system.
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With such systems, speed is restricted by the reaction/settling time of the weigh cells, although multiple filling heads and weigh cells can be incorporated to double/treble/quadruple the output. Where higher speeds still are required, a mechanically registered intermittent tare and gross load cell system integrated into the infeed and outfeed starwheel assembly of a continuous motion rotary turret provides the ideal solution. |
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