The lower parallel flights of the
auger within the funnel are produced to a constant pitch so that,
when filled, each pitch has a precise and consistent volume. The
auger drive rotates the auger at a preset speed to produce a continuous
flow of product. The actual filling head comprises of the primary
elements illustrated below. By moving the mouse pointer over the
names below the illustration, elements within the illustration will
change colour to identify each particular part.
1. Hopper containing the product to be dosed
2. Auger rotating vertically through the hopper and
funnel to discharge product.
3. Funnel from hopper through which the lower section
of the auger doses.
4. Scraper blade to assist powder feed by bringing
product into the flights of the auger.
5. Auger drive mechanism. |
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The scraper blade rotates in the opposite direction
to the auger and with the exception of the DigiFill range, is independently
driven. Its purpose is to prevent any build-up that generally will
form when running non-free flowing powders. The scraper blade extends
right down to the neck of the funnel, ensuring that the auger flights
are fully filled with product, and preventing cavitation (cavities
that can other wise form within the product resulting in poor filling
accuracy).
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Non-free flowing powders
The auger illustrated is fitted with 'over-flight',
this can be either a separate detachable component to provide ultimate
flexibility, or can be part of the actual auger for dedicated applications. An over-flight assists the flow of
non free flowing products by providing compression into the
auger flights, down to the neck of the funnel where the scraper
blade feeds the flights of the parallel section of the auger. |
Intermediate Powders
With intermediate products, an auger with parallel
flights would be normally be selected and no over-flight fitted,
as compression could well cause unacceptable damage to the product.
These types of product often have the tendency to dribble from the
auger once it has stopped and therefore require back pressure to
be applied by use of a lip or grid attachment to the bottom of the
funnel.
Free flowing powders
Free-flowing powders require a positive means of cut-off
to be fitted to the end of the funnel. This can be in the form of
a scissor cut-off unit where two horizontal scissor blades close
at the end of the funnel immediately a discharge has completed,
or a single / double flap attachment (also fitted to outlet of funnel).
Either method ensures a clean cut-off at the end of each discharge
and prevents product dribble / seepage when the machine is not it
a filling cycle. In general, the flap type cut-off is used when
the filling machine is mounted above a bag making machine (form,
fill & seal machine), allowing the auger and funnel to be extended
down through the bag forming tube, and the scissor cut-off is used
in applications where the auger filler is dosing into ready made
bags / containers.
Spinner plate & Cowl
An alternative tooling option that usually would be
fitted for handling particle type products is the Spinner plate
& Cowl illustrated below. Attached to the end of the auger
is a spinner plate (disc), this is a saucer shaped disc 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 angle for
small smooth beads would be much greater than, for example,
vermicelli. The distance between the bottom of the funnel and
the spinner plate (the gap needed to allow unrestricted product
flow) is defined by the product dose rate. The spinner plate
is sized to prevent the powder dribbling over the rim of the
disc once the auger has stopped, taking into account this gap
and the powder's angle of repose. As the spinner plate throws
the powder outwards, a simple collection cowl funnels it to
a size suitable for the aperture of the container to be filled.
The use of a spinner plate results in a perfectly clean fill
without the requirement for any additional cut-off devices. |
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Tooling Size
The size of the auger and funnel is determined
by a number of factors: fill weight, discharge rate, accuracy and
container neck opening. The smaller the auger/funnel, the slower
the discharge rate but greater the accuracy. Conversely, a larger
auger delivers more quickly but less accurately. Where a wide range
of fill weights are required, more than one auger/funnel set may
be supplied to achieve the required combination of speed and accuracy.
Drives
Clutch/brake drives offer a combination of high accuracy
and torque at a budget price. The down side is that clutch/brake
drives require periodic maintenance and have less features relating
to epitomized control that is on offer from direct drive alternatives.
Direct drive closed loop a.c. motor and servo systems offer the
best in terms of technical performance and flexibility, with completely
maintenance free heads and a high level of control in terms of acceleration
/ deceleration and auger speed.
Volumetric Filling
In volumetric filling mode, the auger will rotate
a consistent amount each time a discharge takes place and providing
that the product being dispensed has a stable bulk density, i.e.
a measured volume of product always weighs the same, then accuracy
will also remain consistent from discharge to discharge. Volumetric
filling machines are often linked up to automatic check-weighers
placed down stream of the filler, these can provide a feedback signal
that increases / decreases the discharge should the dose weights
start deviating due to density fluctuation.
The discharge amount is determined by the number of
auger revolutions for a given set of tooling, and can be set / displayed
to 1000th of a revolution of EasiFill machines. On ServoFill machines
this increases to a control accuracy to 4096th of a revolution.
The auger drive motor is fitted with a precision encoder to measure
rotation.
Setting up product discharge parameters is a breeze
with DigiFill/E, EasiFill and ServoFill machines where an operator
simply selects the setup screen, enters the number discharge turns
and conducts a sample discharge. The sample discharge is then weighed
with the result along with the required weight entered into the
system by the operator. The control then calculates and stores the
required auger rotation to achieve the required weight.
In applications requiring high speed filling into rigid containers
intermittent dosing can be too slow, therefore all G Webb Automation
auger fillers can also be operated in a continuos dosing mode. In
this instance the auger filling machine would generally be fitted
above a rotary container handling machine (RP125/RP760/RP1400 Rotoplant).
Here containers are transferred via scroll into an infeed star-wheel
and subsequently into a continuously rotating turret. The turret
is effectively a carousel with a number of pockets, each of which
are fitted with an overflow chute with a precision machined splitter
ring above. The turret speed is set to the required line speed and
the auger rotation speed adjusted to achieve the correct amount
of product per container, the auger will continue to discharge all
of the time that the turret is rotating with containers present.
The precision splitter ring divides the actual dose into equal portions
as the turret rotates, and a vibratory track fitted beneath the
containers applies vibration through the container and overflow
chute above to settle the product within the container prior to
it exiting the turret. A Check-weigher can be integrated down stream
to provide density correction, and in this case increase / decrease
the auger speed. Filling speed in excess of 300 containers per minute
can be achieved on continuos dosing systems.
Weigh Filling (Gravimetric)
Weigh filling machines operate by dynamically weighing
the product as it enters the bag / container during actual discharge.
These machines are suitable for discharge weights ranging from a
few grams up to as much as 50Kg and are used for the following reasons;
1. Product has inconsistent density and therefore
is not suitable for volumetric filling.
2. Requirement for filling machine to provide printed reports /
statistics on fill performance.
3. Expensive product where product giveaway must be minimized.
Manual Presentation Systems.
The basic weight controlled auger filler usually is
fitted with either a bag support or container platform, that is
fitted with a precision loadcell. In the case of systems with a
bag support platform, an automatic bag gripper can be included to
secure the bag in place during the filling cycle.
On commencing a fill, the system will tare off the
bag / container weight and then discharge until the required weight
has been achieved. The auger runs for the first part of the fill
cycle at a fast speed changing to a slower speed for the final part
of the fill. Both fast and slow auger speeds are user configurable
as is the speed change set-point.
'Product-in-flight', the amount of product between
the end of the auger when stopping and the product already in the
bag / container is automatically measured at the end of the filling
cycle, and is then taken into account by the system adjusting it's
cut-off point to ensure that the correct amount of product is dispensed
each time.
Semiautomatic / Fully automatic systems.
A weighcell can be placed within conveying systems
directly under the filling head operating in exactly the same way
as when filling manually. It is possible to tare weigh the empty
containers prior to dosing to achieve true nett weight filling.
For an increase in speed, moving the weighcell downstream and filling
volumetrically with feedback can be considered, with upstream tare
weigh cell as required. This system configuration is subject to
product density fluctuations not being erratic.
Bulk Fill & Top-up Systems
Another alternative is the 'Bulk fill & Top-up'
system configuration with two filling heads. In this instance the
container is transferred beneath the first auger filler that discharges
volumetrically approximately 80% of the required weight, optional
container vibration can be used at this station to assist settling
the product if required, the container is then transferred beneath
the second filler where it is lifted clear of the conveyor belt
by a grid mounted on a weighcell. The second filler then discharges
the final 20% fill via a smaller auger under direct control of the
weighcell, thus maintaining overall accuracy regardless of density
variations. The system is optimized so that both filling operations
take the same overall time, thus providing maximum line speed.
Program Forward Systems
For even greater speed and accuracy, the weighcell
can be moved between the two filling heads turning it into a bulk
fill, program forward & top-up system. This offers the speed
of large auger bulk fill together with the accuracy of program forward
and volumetrically filling the top-up with a smaller auger. In this
case the weighcell between the filling heads feeds back to the bulk
filling head to maintain the proportion of the bulk fill as well
as feeding forward the individual discharges required once the container
arrives at the top-up filler. A second weighcell is required downstream
of the top-up head which provides density check & correction
feedback to the top-up filler, can display and produce hard copy
reports of filling statistics, and also police the line by rejecting
any out of tolerance weights. A tare weigh station can be added
prior to the bulk filling head if required when filling containers
with an irregular tare weight, a good example of this would be glass
jars which tend to vary in weight significantly.
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