Rotary Moulding

The principles of rotary moulding to form dough pieces

(Click the images to enlarge)

3cf-RM-plant-thumbTypical Rotary Moulding Plant     

A rotary moulder is a machine commonly in use for producing biscuit dough pieces from short doughs. The dough is forced into moulds which are the negative shape of the dough pieces complete with patterns, name, type and docker holes. The excess dough is scraped off with a knife bearing upon the mould and thereafter the piece is extracted onto a web of cotton canvas or other fabric.

Although short doughs maybe sheeted, gauged and cut with an embossing type of cutter the advantages of moulding are,

  • it is not necessary to form and support a dough sheet,
  • difficulties and control of gauging are eliminated,
  • there is no cutter scrap dough which must be re-cycled.

The last is of great significance as short dough toughens as it is worked and gauged. In a rotary moulder all the dough has a similar history, there is no cutter scrap that has to be re-incorporated.

The shape of the mould allows a much more intricate pattern outline than cut dough and can give hollow centres to the pieces if required. Large or very small dough pieces can be moulded but there can be extraction difficulties if very thick dough pieces are moulded.

The rotary moulding machine

3cf-RM-with-letters-thumbCross section through a typical rotary moulder

Roll A is known as the forcing roll. It is usually made of steel and has deep castellations, in various patterns, designed to hold a blanket of dough. The roll is driven so that dough from the hopper (H) is drawn down into the nip against roll B. Roll A may or may not be adjustable in a horizontal direction. Roll B is the moulding roller. Typically it has a similar diameter to roll A but it has a smooth surface into which are engraved, or inset, moulds to form the shape of the dough pieces. Typically the roll is made of bronze or gunmetal (a malleable alloy of copper, tin, zinc and sometimes lead) which is suitable for delicate engraving. However, if plastic insert moulds are made the roll may be of steel. In both cases the roll, which is the same width as the biscuit plant, is usually a tube mounted on an axle. It is important that this tube and axle is rigid and does not flex under the pressure of dough created in the nip with the forcing roll. The moulding roller is driven as shown so that dough is forced into the moulds in the nip. Its position is fixed.

Bearing on the moulding roll is a blade of steel known as the scraper (D). The tip of this blade is below the centre line of rolls A and B where maximum dough pressure is exerted in the nip. Dough which has been forced into the mould is sliced off and the excess runs down the scraper and is pressed into the blanket of dough which adheres to the forcing roll. The scraper knife may be adjusted in its position on the moulding roller but the ways and means of achieving this varies in different moulders. Ideally it should move tangentially with the surface of the moulding roll.

Roll C is the extraction roll. It has a thick rubber coating over a steel centre and around it passes the extraction web (E). By adjusting the position of this roll in a vertical direction the extraction web can be pressed against the moulding roller. It is driven in the direction shown and the dough pieces are pulled out from the moulds onto the extraction web. The hardness of the rubber on the extraction web is fairly critical and with time and use this changes. The rubber coating will have to be replaced at intervals to maintain optimum efficiency of the rotary moulder.

The dough pieces are carried to the nose piece where they are peeled off and panned on to either the oven band or some intermediate web. In order to aid the smooth transfer of dough pieces from the extraction web sometimes a thin wire or a small diameter metal shaft is used near to the web nose piece to prevent the dough piece going round with the web.

On its return to the extraction roll the web passes over a web cleaning scraper (F) which removes any remaining traces of dough. The tension of this web is adjustable. The web is seamless so easy removal of the extraction roller is necessary in the design. The extraction web will need to be replaced at regular intervals. The life of this web depends on the pressures needed to run a dough and also on whether the moulds have docker pins.

To form different biscuits it is merely necessary to move the scraper away from the moulding roll and thence to exchange the latter with a different one. This change is straightforward and quick, but as the rolls are heavy, lifting tackle is always required. Great care must be taken not to knock or drop the moulding roll while it is being moved as it is expensive and the metal being relatively soft is easily damaged.

The position of the scraper tip is typically between 3-11mm below the axis of rolls A and B.

The scraper knife warrants some further consideration. In order to slice the dough the blade should be as sharp as possible, but as there is a great pressure of dough in the nip, the tip of the blade should not be so thin that deflection towards the moulding roller can occur otherwise it will cut into the metal. The scraper is sprung so that the tip always runs against the roller otherwise dough may pass behind it tending to force it away and it will engage quickly with the forcing roll and cause much damage. As some flexing of the scraper knife tip is inevitable, the docker pins in the mould must be fractionally lower than the level of the mould edge otherwise they will be damaged by the blade.

When the height of the scraper blade is to be changed it is important that it is moved as nearly as possible in a tangential direction relative to the moulding roll surface.

The formation of a dough piece

Dough is placed in the hopper and the machine is started. Dough trapped at the nip is churned and worked to force it through the nip. This churning may toughen the dough, but the toughening effect is less if the dough from the mixer has been allowed to stand for at least thirty minutes before use. The level of dough in the hopper should be maintained at a minimum so as to reduce pressure differences at the nip and excessive working of the dough, and also to minimise the chance of bridging of the dough. The dough is typically of a slightly firmer consistency than would be the case if the dough was to be sheeted.

The dough is pressed against the forcing roll and also into the moulds on the moulder. The scraper knife slices off the dough level with the top of the mould and presses the excess against the forcing roll to form a blanket which revolves with the forcing roll.

The dough piece then passes to the point where extraction is achieved. This is the place where greatest difficulties can occur. For example, the dough piece may stick preferentially to the mould and therefore not be extracted or there is a squeezing action that causes the piece to be wedged in cross section and some dough extruded behind the piece in a "tail" on the extraction web.

The surface of the extraction web must be sufficiently rough or sticky to effect good adherence of the dough piece, but the adhesion must not be so good that subsequent peeling at the transfer off the web is difficult. The surface and type of web is important and many have been tried. A thin web is usually not rough enough and a thick web will not go round a sharp nose piece. A sharp nose piece is needed to cause a peeling away of the dough pieces. The internal surface of the moulds is important in both shape and smoothness with regard to ease of removal of the dough piece. If the mould edges are too steep or the pattern too deep or intricate, extraction will be difficult.

Extraction is effected by the web being pressed against the dough held in the mould. The slightly soft surface of the extraction roller allows the web to be pressed into the mould. This has two effects. Firstly, the dockers pass right through the dough to the web and, secondly, a slight excess of dough is extruded outside the limits of the mould. The rolling action means that the dough is extruded mostly at the rear of the piece and this forms a "tail".

Dough feeding to a rotary moulder

Feeding dough to a rotary moulder is more critical than to a three roll sheeter. There are two important considerations. Firstly the level of the dough in the hopper of the moulder should be kept at a minimum to avoid bridging and this involves supplying the dough in small pieces. Secondly because the moulding roll is expensive and made of a relatively soft metal great care must be taken that no pieces of metal pass into the hopper with the dough.

Thus dough fed to a rotary moulder typically comes through a pre-sheeter, which is of two roll design. It passes to the hopper of the moulder on a full width conveyor and then passes through a kibbler before it falls into the hopper. The kibbler is a set of rotating fingers that break the dough into pieces of no more than 50mm cube. The conveyor from the pre-sheeter passes through a metal detector and there is a rejection arrangement so that any dough found to contain a metal fragment is taken away before it can reach the moulder and damage the moulding roll or the knife.

There is typically a dough level sensor in the hopper of the moulder and the pre-sheeter and conveyor are started and stopped to meter the dough to the moulder.