Extruding stage in biscuits production is one of the simplest ways of making dough pieces. This involves forcing soft short dough through holes in the die plate. In cases where the dough is very fluid and has probably been aerated during the mixing process, it is referred to as batter. As such, this dough is extruded rather than moulded or sheeted.
The firmest dough pieces are typically wire cut and may be of a similar consistency to the soft dough used in the rotary moulder. Wire cutting allows for the formation of pieces from the more sticky dough as well as dough that contains coarse particles such as nuts, oat flakes or chocolate chips, which can be formed successfully in the rotary moulder. During extrusion, the dough will be forced through orifices that have been pressurized either by means of a pump for sponge batters or rollers for soft and short dough.
Dough is typically extruded through a row of die of any desired shape or size and a frame, bearing a taut blade or wire, which strikes across the base of the die holes, cutting off the dough that has been extruded at intervals. The pieces of dough then fall onto the oven band or the conveyor. The cutting stroke is done close to the die, while the return stroke is typically lower and away from the dies. This ensures that it does not touch the dough that is being extruded continuously. The pieces may fall off straight or may be turned over before reaching the oven band. It does not matter whether or not the dough pieces turn. What’s of vital importance is that all the dough pieces must do the same thing all the time.
Extruders are dough extrusion machines which basically consist of a hopper over a system of two rolls which forces the dough into a balancing/ pressure chamber underneath. The rolls may run intermittently or continuously, and may have the capacity for a short period of reverse motion. This allows for the relief of pressure and results in a suck back at the nozzle or dies located at the base of the pressure chamber.
The machine spans the width of the plant being typically positioned over the oven band. Where certain dies, rout types and wire cut dough are cut subsequently into lengths prior to baking, the machine is situated over a normal canvas conveyor, as opposed to the oven band. Pieces of dough that have been formed on the conveyor may be spaced out during transfer onto the oven band.
The dies of the extruding machine are approximately 70mm above the take away conveyor, or oven band, although it is possible to adjust their positioning to suit the specific production needs. The extrusion size is determined by the size of the dies, while the extrusion rate can be adjusted by the speed of the forcing rolls. The extrusion rate is affected by the dough consistency, and the pressure in the chamber behind the die plate. This rate may also be impacted by the head of the dough inside the hopper. Further uneven extrusion may be caused by some deferential pressure inside the chamber, behind the dies, across the machine.
Wire cut and rout press extrusion machines come in a relatively simple design. The pressure of the dough inside the pressure chamber behind the dies is maintained and achieved through the friction of the dough on top of the feed rollers. This frictional force will alter if the consistency of the dough, both in terms of stickiness and softness changes. General control over the extrusion rate and subsequently the weight of the dough piece is achieved through the alteration of the feed roller speed. This will in turn result in a reduction or increase in pressure of the dough in the chamber beneath them.
At the sides of the machine, there are drag forces that act upon the dough. It is therefore not uncommon to find that pressure across the pressure chamber is not uniform.
Photo: Rheon Automatic Machinery
Also, more dough tends to be extruded from the dies at the center of the machine, as opposed to those located at the sides. While it is very difficult to compensate for this by making the dies of different aperture, adjustable restrictors are typically provided for purposes of effecting a certain degree of control over the extrusion rate, and in turn the weight of the dough piece. These comprise of plugs that can be screwed into the sides of the die hole. As they are screwed in, this causes a reduction in the size of the die.
Balancing the weights of the dough pieces across the band is a complicated and often frustrating task. When the machine is started up, pressure builds in the chamber to a steady state. But when the machine is stopped, this pressure will fall slowly through the dies by continued extrusion. Therefore, the effects of adjusting the die restrictors can only be checked after running the machine for a few minutes. It is normally impossible or unsafe to make adjustments to the restrictions while the machine is running. Moreover, if the dough consistency varies, either due to dough age within a batch or between different batches, this may affect the extrusion variation from the dies.
Uniformity of the weight of the dough piece is therefore not a major factor in extrusion machines. The most practical engineering design that has been introduced for the improvement of weight control is providing vertical divider plates within the pressure chamber. These are designed to prevent lateral dough flow and more or less eliminate the hopper side drag effects. Ideally, there should be a divider plate that is related to every single or each pair of die holes. As this machine is designed to accept die plates with any size and number of die holes, this means that the divider assembly needs to be changed to suit every die plate.
The batter size deposits are controlled by the length of time that the dies are kept open. There is typically a variation of deposit weight across the band, although this generally is less problematic for the dough extrusion machines.