Extruding and Wire Cutting

Principles and techniques of extrusion and depositingadvertisehere

Extrusion and depositing constitutes some of the simplest means of making dough pieces. The technique involves the forcing of a soft dough through holes in a die plate. Where the dough is very fluid, and probably has been aerated in the course of mixing, it is known as a batter. Thus the dough is extruded rather than sheeted or moulded. Depositing is a form of extrusion so the mechanism of these two means of dough piece forming are not distinct from one another. However, the machinery that handles soft short doughs is normally different from that which deposits a dough that is so soft that it is pourable.

The firmest doughs are often wire cut and these may have a similar consistency to soft rotary moulder doughs. Wire cutting makes it possible to form pieces from more sticky doughs and dough containing coarse particles, such as nuts, chocolate chips or oatflakes, that cannot be successfully rotary moulded.

In extrusion and depositing the dough is forced through orifices having been pressurised either by means of rollers (short and soft doughs) or a pump (sponge batters). The two types will be dealt with separately.

General description of dough extrusion machines

Most machines basically consist of a hopper over a system of two rolls which force the dough into a pressure/balancing chamber underneath. The rolls may run continuously or intermittently and may be capable of a short period of reverse motion to relieve the pressure and cause a suck back at the dies or nozzles at the base of the pressure chamber. Thus dough can be forced continuously or intermittently out of the pressure chamber.

The machine spans the width of the plant and is usually situated over the oven band. In the case of certain driers, wire cut doughs and rout types (see below) which are subsequently cut into lengths before baking, the machine is over a normal canvas conveyor and not the oven band. Dough pieces formed on a conveyor may be spaced out as they are transferred onto the oven band.

The dies of the extruding machine are usually about 70 mm above the oven band, or take away conveyor, but it is possible to adjust this to suit. The size of the extrusion is determined by the size of the dies and the rate of extrusion is adjusted by the speed of the forcing rolls. The rate of the extrusion is affected by the consistency of the dough and the pressure in the chamber behind the die plate. The rate may also be affected by the head of dough in the hopper. Some deferential pressure in the chamber behind the dies across the machine may cause uneven extrusion but ways of compensating for this will be discussed below.

The formation of dough pieces by wire cutting, rout press and co-extrusion

3cg-Wire-cut-thumbGeneral arrangement of wire cut machine.

For wire cutting, the dies are about 70 mm above the band or conveyor, but provision is usually available to change this gap if necessary by raising or lowering the band. Dough is extruded through a row of dies (of any desired size or shape) and a frame, bearing a taut wire or blade, strikes across the base of the die holes "cutting" off the extruded dough at intervals. The dough pieces then fall onto the band or conveyor. The wire may cut in a forward (direction of the oven band) motion or more usually in the opposite direction. In any case, the cutting stroke is close to the die and the return stroke is lower, away from the dies so that it does not touch the dough which is continuously being extruded. The pieces may fall off straight or they may turn over before they reach the band. It does not matter whether they turn or not, but all must do the same all the time! It is here that difficulties are often encountered because sticky, coarse textured dough may not always fall as expected. Some control of the fall of the dough piece is possible by adjusting the height of the dies above the band and also by attention to the position of the wire as it passes through on the cutting stroke. Every effort must be made to achieve uniform position on the band so that the pieces will experience similar baking conditions.

Sometimes a blade with a sharp edge or even a fine serrated edge is used in place of the wire. Sometimes the wire or blade may be vibrated, in a horizontal plane, to improve the "cutting" action. The best conditions for a particular dough are, unfortunately, found principally by trial and error. Having found the good settings record them in terms of machine calibration.

The speed of wire cut machines is not high, rarely exceeding 100 strokes per minute, though the number of rows can be increased either by having a double row die, if the pieces are fairly small, or by having two machines synchronised to deposit alternate rows of pieces.

Dough extruding from a round die may be distorted somewhat as the wire or blade "cuts" across it. This is particularly common if the dough contains coarse pieces or dried fruit that drag on the wire. The shape of the baked pieces will be affected, but may not be as much as expected.

Wire cut dough pieces always have a rough surface which is not completely lost during baking. A "home made" look is accentuated by the fact that the outline after baking is often irregular. The nature of extrusion means that the dough pieces are thicker in the centre than at the edges. A constriction in the die can reduce this effect.

It is, of course, possible to have different die shapes across the band and even, by dividing the hopper, to use two or more different doughs at once. This allows variety production for packs of assorted biscuits, but the dough recipes and piece weights should be calculated carefully to achieve optimum baking of all types. Doughs with different formulations can be expected to bake differently.

When the machine is used in a continuous mode, without wire cutting of pieces, the products are called bar or rout cookies. The die plate is now usually inclined in the direction of the extrusion so that transfer of the dough ribbons onto the conveyor is as smooth as possible.

For rout cookies the dies are typically of asymmetric shape to give a smooth base and ribbed upper surface. The ribbons of extruded dough are usually guillotined into short lengths before baking, but, alternatively, some form of cutter may be used on the oven band after baking. Depending on the character of the dough in terms of coarse ingredients and consistency the bar cookies may have smooth or rough edges and surface.

It is possible to elaborate on the bar cookie by arranging twin pressure chambers under separate hoppers so that one dough is extruded within the other. This co-extruding arrangement is used in the production of, for example, fig bars (Fig Newtons). The fig (or other centre fill material) paste is fed from one hopper and dough from the other (see below). The filled tube so produced may be cut before or after baking as described above.

3cg-Co-extruder-thumbGeneral arrangement of filled bar forming machine

The disadvantage of this simple type of co-extruder is that it is difficult or impossible to have a cutting device that seals the filling within the dough when the pieces are cut. The Rheon company of Japan has produced some innovative machines capable of co extrusion with cutting systems that seal, or encapsulate, the filling within the outer dough. The cutting and sealing action is achieved either with specially shaped rotating discs or with iris cutters made of a plastic material. The machines are capable of handling a wide range of materials such as jam, minced meat, or very sticky dough but the outer dough must always be fairly soft in texture. Co-extruded products like this allow the use of fluid centre fillings like jam and fat based cream.

The formation of dough and batter pieces by depositing

If the dough consistency is soft, smooth and almost pourable it is possible to produce deposited rather than wire cut dough pieces. The die plate is replaced with a set of piping nozzles. These nozzles are cone shaped and may have patterned ends to give strong relief to the extruded dough. Individual deposits are achieved by raising and lowering the oven band to coincide with intermittent extrusion which is achieved by intermittent movement of the feed rolls behind the nozzles. When the band is raised to be close to the nozzles, dough is forced out and spreads on the oven band. At the end of the extrusion the band is allowed to fall back and the deposit breaks away from the nozzle. The break away may be encouraged by a reversing action of the feed rollers which stops the feed to the nozzles or even produces a suck back. It will be appreciated that by using variable times and mechanisms which twist the nozzles it is possible to produce fingers, swirls, circles and other shapes of dough pieces. By synchronising a second depositor with the first, it is possible to deposit jam (or jelly) or another dough on or within the deposit made by the first.

Deposited doughs are commonly very rich in fat and very short in texture.

There are a number of biscuits that are made from foamed egg batter, deposits of this batter are made directly onto the oven band from a pipe over the band. The batter is pumped through the pipe and is extruded through small die holes which are opened and closed at intervals.

Dough piece weight control from extrusion machines

Wire cut and rout press extrusion machines are relatively simple in design. The pressure of the dough in the pressure chamber behind the dies is achieved and maintained by the friction of the dough on the feed rollers. As will be appreciated this frictional force will change if the dough consistency (both in terms of softness and stickiness) changes. The general control of the rate of extrusion and therefore the dough piece weight is achieved by altering the speed of the feed rollers which then cause an increase or reduction of the pressure of the dough in the chamber beneath them.

There will be drag forces on the dough at the sides of the machine. It is therefore common to find that the pressure across the pressure chamber is not uniform and more dough tends to be extruded from the dies at the centre of the machine than from those at the sides. It would be very difficult to compensate for this by making the dies of different aperture but to effect some control of the rate of extrusion, and therefore the dough piece weight, adjustable restrictors are usually provided. These consist of plugs that can be screwed into the side of the die hole. As they are screwed in the size of the die reduces.

Balancing the dough piece weights across the band is a complicating and often frustrating task. When the machine is started the pressure builds up in the chamber to a steady state, when the machine is stopped this pressure slowly falls by continued extrusion through the dies. Thus the effects of adjustments to the die restrictors can only be checked when the machine has been run for a few minutes. (It is usually not possible or safe to make restrictor adjustments while the machine is running.) Furthermore, if the consistency of the dough varies, either within a batch due to dough age or between batches the variation in extrusion from the dies may be affected!

Dough piece weight uniformity is thus not a strong point with extrusion machines. The most useful engineering design that has been introduced to improve weight control is the provision of vertical divider plates within the pressure chamber. These prevent lateral flow of dough and more or less eliminate the hopper side drag effects. Ideally there should be a divider plate related to each or each pair of die holes. As the machine is designed to accept die plates with any number and size of die holes, this means that the divider assembly should be changed to suit each die plate.

The size of batter deposits is controlled simply by the length of time that the dies are held open. There is often a variation of deposit weight across the band but this is generally a lot less of a problem than for dough extrusion machines.