Rotary Biscuit Moulding
Rotary moulding process
Rotary moulded biscuits
Rotary moulded biscuits are essentially produced from a soft dough, ranging from thin sandwiched products through all grades of shortcake to shortbread and including such distinctive lines as Digestive, Nice, Oreo, etc. which although their names are incised in the biscuit face and not embossed in relief, are actually soft dough biscuits.
Dough hopper
The level and distribution of the "kibbled" dough in the hopper can influence the dough piece weight.
As a guide, I would try to produce with a minimum quantity of dough in the hopper, but feeding the "kibbled" dough continuously into the "nip" between the two rollers, at a similar rate to the extraction rate. e.g. input = 15 kg per min. output = 15 kg per min.
Rotary moulder
The castellated forcing roll (1) takes the kibbled dough from the hopper and presses it against the moulding roll (2) to fill the engraved moulding roll impressions. Excess dough is removed by the scraper knife acting on the moulding roll surface and is then returned to the hopper by adhesion to the forcing roll in the form of a "blanket". The woven endless extraction web is pressed against the moulding roll by a rubber covered roll (3) and the dough pieces are extracted from the engraved impressions in the moulding roll.The extraction web terminated in a "nose piece" enabling the dough pieces to be transferred onto the panning web and then onto the oven band. By increasing the rubber roll pressure, the extraction web is pressed against the moulding roll, to create suction to extract the dough pieces from the impressions in the moulding roll. This can be done by transferring the dough pieces over a rotating spindle roll which is inserted between the extraction web and the panning web.
The rubber roll shore hardness is usually between 65-70, and the lower the value the softer the rubber, and the greater the tendency to dough piece “tailing”. Decomposition of the rubber also contributes to poor tracking performance of the extraction web. On the rotary moulding roll, each of the dies should be considered on its own merits for composition, geometry and surface finish and the type of dough consistency. The process control functions of a rotary moulder, results from a balanced setting of the adjustable parameters being the gap between the forcing and moulding rolls, the position of the scraper, the pressure between the moulding roll and rubber roll, the differential speed between the moulding roll and the extraction web.
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Rotary moulding process
By increasing the rubber roll pressure, the extraction web is pressed against the moulding roll, to create suction to extract the dough pieces from the impressions in the moulding roll. The extraction web terminates in a nose piece, enabling the dough pieces to be transferred onto the panning web. This can be done by transferring the dough pieces over a rotating spindle roll which is inserted between the extraction web and the panning web. The rubber roll shore hardness is usually between 65-70, and the lower the value the softer the rubber, and the greater the tendency to dough piece “tailing”.
Decomposition of the rubber also contributes to poor tracking performance of the extraction web. On the rotary moulding roll, each of the dies should be considered on its own merits for composition, geometry and surface finish and the type of dough consistency.
Design of the rotary moulder
The feature of the castellated forcing roll is to allow for a "blanket" of dough to adhere to the roll, but the depth of this "castellation" is excessive, as dough can be rotating on the roll for 15 – 20 minutes. It’s major function is to press the kibbled dough into the engravings of the moulding roll and not to keep recycling the dough within the hopper, entrapping air, toughening the gluten by "secondary" mixing. By staggering the pitch of the castellation, exerts a reduction in pressure onto the engraved moulding roll, contributing to "secondary" mixing and dough piece wedging. This is also evident dependant on the lateral pitch of the castellation, relative to the pitch of the engravings on the moulding roll. The extraction web is non-porous and there is an accumulation of dough layering on the web, commonly known as "tailing". This is dough that has been displaced to the trailing edge of the die.
Forcing roll
Normally fixed speed drives on rotary moulding machines, are based on a one to one gear ratio. As a consequence, this relative fast speed of forcing roll can be influencing the displacement of dough in the engraving. Fitting an inverter drive or changing the gear ratio on the forcing roll can eliminate the tendency for wedged shaped biscuits.
Scraper knife
The knife should be set to a precise reference datum, as per the instructions of the working manual supplied by the equipment manufacturers. This can be in the form of a feeler gauge and a T piece.
Type: hooked profile; roof top; or straight edged
Angle of knife; Position of knife; Static or Oscillating function
Engraved moulding roll
A square or oblong shaped product is more prone to wedging than round biscuits, unless the surface of the die has been engraved or surface treated. Surface treatment of the only the dies, e.g. first, sand blasting, then coating with Teflon, minimises the displacement of dough under pressure, thereby reducing the tendency for product wedging, this reduces the surface tension between the dough and the engraved mould. These results, in a reduction in rubber roll pressure and improved dough piece release from the die. The product appearance is enhanced and the shore hardness of the rubber roll is maintained for a longer duration.
N.B. Do not coat the whole surface area of the moulding roll, only the engraved dies, as past experience by suppliers coating the whole surface of the roll, have damaged the scraper knife and increased dough slip at the nip between the moulding roll and scraper knife.
Heating the surface of the moulding roll, can promote or accelerate the tendency for fat bloom, due to the heat liquefying the fat: oil phase on the surface of the dough piece. Under pressure from the knife and rubber roll, oil can be expressed from the dough, contributing to dough piece slip in the die.
Cleaning the moulding roll
The moulding roll is normally cleaned with compressed air, unless there is an accumulation of dough in the engraved die, and a firm bristled nylon type brush, should clean the roll effectively.
Rubber extraction roll
Parallel alignment of rubber roll to moulding roll is critical for side to side for weight control of dough pieces
Shore hardness of the rubber roll
The rubber pressure roll, may be of a dual shore hardness, i.e. relatively soft in the inside and firm on the outside. If of a single composition, then check the shore hardness against supplier’s specification, e.g. 65. If excess pressure is applied to the rubber extraction roll to aid dough piece release for a long duration, then in conjunction with oil ingression from the dough, there will be some softening of the rubber. You should measure the shore hardness around the circumference and across the width, taking readings in several positions on the surface of the roll.
Conditioning of the extraction web
The extraction web can be porous or non-porous, but should be an endless web, with no joints. Conditioning of this a new extraction web with steam rather than water is a more effective way of treating the web, which is normally applied uniformly across and around the complete circuit of the extraction web. This may only be required just prior to start up of production and thereafter, the oil and water continuously expressed from the dough maintains the adhesive properties of the web. When water is applied, it may not be absorbed uniformly into the web and can result in differential tension across the web, influencing product registration at the point of printing application.
Factors contributing to the "tailing" of the dough piece
Bronze engraved moulding roll
Bronze engraved moulding roll: Teflon coated
Engraved bronze moulding roll - plain finish
Design and machine surface finish of the engraved die
As the rotary roll is rotating in conjunction with the extraction web, there are two forces competing with one another.
The smooth flat surface area of the engraved die is creating a suction force and the dough piece, whilst the extraction web is creating suction in an opposing force. This surface tension is a major factor in poor extraction properties, and alternative surface finishes to engraved moulding roll, i.e. machine engraved bronze finish, or sand blast finish, assist in overcoming this problem.
Engraved rotary moulding roll - plain finish
Uneven- wear of the Teflon (P T F E) coating
This uneven wear of the Teflon coating, illustrates the effect of dough ‘slip’ displacement under pressure, during the moulding process, contributing to ‘tailing.’
Pitch of the castellated forcing roll relative to the engraved moulding roll dies
Pitch of engravings: lateral / longitudinal i.e. land-surface relative to the Castellated Forcing Roll contributes to lateral and longitudinal dough piece "wedging", due to the forward pressure being exerted to the dough being forced into the moulds. This is of major concern if the biscuits are to be process moulded in chocolate, as my experience in this, resulted in an over-usage of 3% chocolate.
The course of action recommended was to "stagger" the layout of the castellations, which allowed to dough to be relieved of the pressure, resulting in biscuits of a uniform geometry, and savings of 3% chocolate usage. As chocolate is one of the most expensive ingredients in the biscuit industry, the monetary value saving per annum was considerable.
CRITICAL PARAMETERS AND RISK ANALYSIS
Key Areas in Controlling the Rotary Moulding Process
Dough Consistency
Variation in dough consistency
Dry and crumbly or soft and cohesive
Influence on dough piece weights
Influence on transfer properties of moulded dough pieces
Influence on product dimensions
Influence on product texture
Dough Standing Time
Dough standing time: Influence on the release properties of the dough from the engraved mould
Dough standing time effect on the hydration rate of flour and sugar
Dough consistency crumbled or dry or soft
Kibbler
Dough feed system: sheeted or kibbled
Dough distribution
Effect on the re-compaction of the fragmented dough
In the Hopper
Feed rate of dough per: input = output
Hopper Level
Auto /manual control
Effect on weight control
Secondary mixing
Gluten development
Laminating dough and air incorporation
Geared Rolls
Relative speed of geared rolls
Feed Roll
Variable or fixed speed
Gap
Moulding Roll
Composition: Bronze / Teflon Coated / Plastic Inserts
Rpm’s
Surface speed / Relative roll speed
Pitch of engravings: lateral / longitudinal i.e. land- surface
Pitch between engravings
Pitch of engravings: staggered or in line
Composition
Surface finish smooth / engraved / sand blasted /
Surface temperature
Hot water / steam circulation / Hot air impingement
Compressed air impingement
Mould description and dimensions
Mould pattern staggered / in-line
Moulding roll diameter
Geometry of engraving: radius/ angles of relief
Rubber Roll
Shore hardness
Hard or soft
Pressure
Scraper Knife
Fixed or oscillating
Position: effect on dough piece weight and shape
Extraction Web
Conditioning
Texture
Tension
Speed
Rotary moulded process/product faults
Rotary moulded shortcake biscuits: Blisters
Rotary moulded shortcake biscuits: internal texture with blisters
Rotary moulded shortcake biscuits: hollow bottoms
Rotary moulded shortcake biscuits: no hollow bottoms
ROTARY MOULDED BISCUITS BAKED ON A STEEL BAND
Hollow bottoms (cavitation) and blisters
The common denominator contributing to "hollow bottoms" or "blistering" is the processing of rotary moulder biscuits baked on a steel band. The major factor contributing to "hollow bottoms" are associated with the particle size of the "kibbled" dough, plus the volume of dough being fed into the hopper of the rotary moulder, plus the speed of the forcing roll, relative to the moulding roll. The dough is subjected to secondary mixing developing the gluten) and entrapping air within the dough, in the rotary moulding hopper, between the forcing roll / moulding roll / and scraper knife, due to the relative speed of the forcing roll and moulding roll.
The air is randomly incorporated within the dough pieces, and the dough pieces expand during the baking process in the oven, creates "blisters" or for those baked on a steel band contribute to "hollow bottoms". Dough temperature was also important in obtaining good dough piece release from the rotary moulder, with an increase in the extraction web speed helping to reduce wedging and tailing. However, the major factors effecting “hollow bottoms” were associated with the particle size and the volume of dough being fed into the hopper of the rotary moulder.
The relative speed of the feed roll and moulding rolls.
The surface finish of the engravings on the moulding rolls.
The operating parameters of the rotary moulder. i.e. Position of the forcing roll and knife and also the relative speed of the extraction web. If the product is rotary moulded it produces a denser textured biscuit, than if it were rotary-cut, where the additional water plays an important role in biscuit aeration. The docker pins of the rotary moulder die were not going completely through the extracted dough piece, and this caused the biscuit to “blow up” and stick to the oven band. Replacement tapered docker pins were used to improve the release properties from the moulder (replacing straight pins). Sugar particle size is critical for not only spread but also definition of the design.
"Tailing"
"Tailing" is associated with the design and surface finish of the engraved moulds, the position of the scraper knife and forcing roll relative to the moulding roll and the speed of the extraction web. The dough is expressed from the trailing edge of the engraved die and is preventable by an engraved die that has been designed correctly and with the desired surface finish on a bronze roll, with good distribution of the Emboss and Docker Pins.Excess dough is being expressed around the circumference of the dough piece due to the pressure applied by the rubber extraction roll. This may be due to a change in shore hardness of the rubber, created by oil expression from the dough and wear. "Dell" ring shells and other plastic type moulds require a rough textured surface to prevent "tailing" but all too often this has not been overlooked.
Plain engraved rotary moulding roll
The following is an example of a "plastic" engraved mould where the dough could not be extracted from the mould, until action was taken to roughen the surface, highlighting the "lack of understanding between the product and the process". The surface finish on the engraved plastic is too smooth and does not "grip" the dough as it is being compressed into the engraving. This will have the effect of changing the density of the dough piece.
ROTARY MOULDED BREAKFAST BISCUITS
Does the product match the features on the packaging?
Rotary moulded breakfast biscuits
This is what it could look like:
Rotary moulded breakfast biscuits
Note the difference in quality of the engraved design features of this product compared to the previous illustration, where both products have inclusions of Whole Wheat Grains and Fruit
The layout of these types of engraved moulds should be lateral to the flow of the process, to prevent stress cracking, as the dough pieces are transferred from the extraction web and onto both the panning web and oven band.
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