It is crucial to understand the different types and the advantages/disadvantages of screw trough,
agitator, and the screw itself. Many loss-in-weight feeders incorporate a screw as the feed device
due to its wide feed-range capability.
Each selected feed device has a range of available screws with different diameters and pitches.
Each feeder supplier provides a range of models capable of providing feed rates over the spectrum
of normal applications. The feed device is comprised of several components.
The screw trough is a critical component of a screw feeder. It converges dimensionally from the inlet,
attached to the extension hopper, and into the screw. It transitions the mass of dry ingredient
contained in the feeder from the larger-dimension extension hopper into the smaller-dimension screw.
Most dry ingredients (powders, granules, pellets, fibers, and flakes) do not flow reliably through
converging transitions. There is friction between the particles and an interlocking tendency that
restricts or stops the flow. When the flow stops it is called a “bridge.”
All dry ingredients have a minimum bridging dimension. Flow stops and a bridge forms at this
cross-section dimension. The reducing cross-section dimensions in the screw trough eventually
reach the bridging dimension. An agitation device is incorporated in the screw trough in order to
prevent the bridge from forming.
Flexible walled/external paddle and internal stirring are two common types of agitation devices.
The screw trough is constructed of a flexible material, normally polyurethane, in a wedge shape on
two sides and near vertical on the other two sides. Movable metal paddles contact the two
wedged-shaped sides and “massage” them, pushing the sides in and out continuously.
This movement causes an unstable support for the dry ingredient particles. A restriction movement
followed by a void movement causes any bridges that are starting to form to collapse. Gravity acts
on the particles to move them down and eventually into the screw.
Comparison of feeder components for external paddle agitation and internal stirring agitation
The screw trough is constructed of metal, normally stainless steel, and is curved in a constant
radius to accommodate the close movement of rotating, agitating blades. The agitator blades
move through the dry particles on each side of the screw, which is positioned directly below the
agitator blades. Movement is down into the screw on one side of the screw trough and up on the
other side. This blade movement prevents bridging in the screw trough.
The shape of the screw trough is critical. A loss-in-weight feeder’s main purpose is to feed the
ingredient accurately at the desired setpoint. The weight-feedback control requires that each screw
flight be filled with the same weight of ingredient. Repeatable-weight filling of each screw flight is
achieved by a properly designed agitation device (flexible-walled with external-paddle massage
or with internal stirring).
Also, there must be sufficient screw flights exposed openly in the bottom of the screw trough
so that complete and repeatable filling occurs before the ingredient is enclosed in the screw tube.
The best screw-trough shape is a wedge, rather than a spherical or conical shape at the bottom
of the screw trough, in order to expose the required length of screw to fill properly.
The flexible-walled/external-paddle-massaged screw trough, available from several suppliers,
has drawn more interest. One important reason for this is that the movement of the flexing
screw trough directly contacts and massages the ingredient in the screw trough on both
sides of the wedge surfaces, from the top inlet to the top of the screw.
Since the stirring agitator has a circular motion, there are two dead zones (no agitation) at
the top of the screw trough. The screw trough is sized to reduce the volume of the dead zone.
As a result, the flexible-walled/external-paddle-massaged screw trough typically has a larger
volume and inlet dimension than the internal-stirring screw trough. Since the extension
hopper is mounted directly above the screw trough, the bottom dimension of the extension
hopper is typically the same as the inlet to the screw trough.
Loss-in-weight feeder extension hoppers are commonly sized (rule of thumb) to
contain 4 minutes of storage at the maximum feed rate/minimum bulk density of the ingredient.
The extension hopper often can be supplied with vertical sides, which provides floorspace
advantages over conical extension hoppers. Also, there is no requirement for additional
stirring agitation in the extension hopper, since the sides are vertical (no converging).
Generally, a screw feed device with flexible-walled/external-paddle-massaged screw trough
is a first choice for loss-in-weight feeders. However, there are situations where the stirring agitator is
required: cohesive ingredients and ingredient temperature. Cohesive ingredients form a solid
mass when exposed to compression. Some are so sensitive that the force of gravity acting on their
own head mass can cause them to solidify. These ingredients are difficult, sometimes impossible, to
feed reliably without excessive maintenance. Testing is highly recommended at feeder manufacturers,
who offer several types of feed devices. Stirring agitators may have the best chance of success in such
cases. Polyurethane becomes too soft at ingredient temperatures above 50 C (122 F), requiring
a metal screw trough.
The screw is the component of the feeder that provides the feeding. The agitation only ensures that
the screw flights fill uniformly. Screws are selected to achieve maximum and minimum feed rates.
The screw diameter and pitch are sized to achieve the feed rate at an optimal screw rotational speed.
At too low a screw speed the “pulsing flow” may cause an undesired variation in the final product.
Too high a screw speed may cause incomplete filling of the screw.
There are three common screw types: spiral screw, blade screw and twin concave screw.
The single-spiral screw is the most common selection. It is “open” for the ingredient to flow
freely into the flights of the screw. It has a smaller surface area that helps reduce adhesion of
the ingredient, which changes the screw volumetric geometry. Ingredients are transferred with
low shear. On the negative side, aerated ingredients can easily flood past the screw flights.
The single-blade screw offers more resistance to aerated ingredients, but because
there is more surface area, sticky ingredients can adhere, reducing the volumetric geometry.
Blade screws are often preferred for heavy powders with bulk densities of more than 80 lb/ft3.
Twin concave screws can provide superior performance for poorly flowing powders. Twin concave
screws are co-rotating, intermeshing, and self-wiping. The powder flows around the flights of
both screws. The negative effects of some adhesive powders are reduced by the self-wiping
of the two screws. Twin concave screws are solid and the screw flights have a low volume compared
with spiral and blade screws. The shear on the powder is high, particularly in the screw tube.
The screw tube is typically short to reduce negative effects of high shear.
Twin concave screws are ideally suited to powders with feed rates below 5 ft³/hr. The cross-section
of the two screws is wider than an equivalent single screw, reducing the tendency to bridge in the
transition from the screw trough into the screw. Also, screw speeds for twin concave screws can be
higher than single screws for the same feed rate because of the larger inlet and low volume of the
screw flight. Higher screw speeds reduce the negative effects of pulsation.
Source: Plastics Technology