Sheeting, gauging and cutting
Sheeting, gauging and cutting is the preferred method for forming dough pieces from developed or fermented doughs and sometimes for short doughs.
The function of the sheeter is to compact and gauge the mass of dough into a sheet of even thickness and at full width of the plant. It is necessary that there are no significant holes and that the edges are smooth and not ragged. Often the sheeter also enables the incorporation of dough returned from the cutter, known as cutter scrap, with fresh, or virgin, dough brought from the mixer.
Within the sheeter the dough is compressed and worked to remove air and it is inevitable that some stresses are built up in the gluten structure.
The new sheet of dough then passes to one or more sets of gauge roll pairs which reduce the thickness to that required for cutting. Usually the dough is carried between the sheeter and each of the gauge rolls on conveyors. Sometimes, having been reduced in thickness, the sheet is folded to form many laminations, before being further gauged to a final desired thickness. For simplicity of description laminating is described separately.
Each gauging station adds further stresses to the dough sheet and usually there is insufficient time for these stresses to be relieved completely before the next stage of processing is reached. The way in which the dough sheet is fed into the gauge roll affects the amount of stress put into the dough so careful and precise control is needed if variable stresses, which tend to persist all the way to the cutter and therefore may after the baked biscuit shape, are to be avoided.
The whole series of machines, from sheeter to cutting and finally to panner, which puts the dough pieces onto the oven band, is referred to as the "cutting machine". It will be appreciated that as the sheet of dough is made thinner in its progress from the sheeter to the cutter it must become longer, thus, each gauge roll and the subsequent conveyor runs faster than those before it.
Precise control of the speeds of the various machines and conveyors is essential for smooth running of a cutting machine. They are linked together and the speed ratios are cascaded such that if the speed of one machine is changed all those behind it are changed proportionally. Usually the speed cascade is achieved with electronic control but older machines had PIV (potentially infinitely variable) gearboxes to give similar but not such precise control.
Between the last, or final, gauge roll and the cutter, special provision is usually made to allow a variable amount of relaxation of the dough before the pieces are cut out. During this relaxation the dough shrinks and thus thickens so the thickness at which the sheet is cut, the main factor in determining the dough piece weight, is dependent on both the gap at the final gauge roll and the amount of relaxation allowed. However, the main reason for providing relaxation is to control the shape of the biscuit after baking. A dough sheet that is under tension at the time of cutting will produce dough pieces that show shrinkage in length in the oven and will tend to be thicker at the front and back as a result. If most of the tension is relieved before cutting the shrinkage thereafter will be less severe and thus the uneven thickness of the biscuits less noticeable also. By varying the amount of relaxation the length and shape of the biscuit can be controlled to a certain extent.
Cutting produces not only the outline of the desired shape and size, but also the surface imprint and docker holes. It is necessary to ensure that the dough piece adheres preferentially to the cutting web and not to the cutter. This adherence must not be too severe otherwise difficulty is experienced in transferring the pieces, without distortion, onto the next conveyor or the oven band. Between the cut pieces is a network of unwanted dough known as cutter scrap. This scrap is lifted away and returned either to the sheeter or, less commonly, to the mixer for re-incorporation with fresh dough. As the density, toughness, and maybe the fat content and temperature of scrap dough is often different from the fresh dough, it is important that its incorporation with the new dough be as uniform as possible. In some cases it is best to incorporate this scrap preferentially in the top surface of the new sheet and at other times in the bottom surface. Scrap dough nearly always gives process control problems, so its re-incorporation should be planned carefully.
It is possible to sheet, gauge and cut most short doughs as well as extensible cracker and semi-sweet types. The network of cutter scrap from hard doughs is a relatively low percentage of the whole dough sheet and it can usually be lifted and handled easily after the cutter. However, when cutting short dough the cutter scrap often presents problems. During baking, short dough pieces often spread to a larger size and this means that the spacing between the pieces at the cutter must be larger than for hard dough pieces. This results in a higher percentage of cutter scrap that must be handled and reused. Short dough should be "worked" as little as possible to achieve best quality biscuits so the handling of the cutter scrap when sheeting and cutting of short dough is of fundamental importance to process control. Also, short dough by its very nature, is difficult to lift in thin strips, so the handling, at the point where the cutter scrap is removed from the cutting conveyor, can be very critical and may require special techniques.
Commonly the dough sheet or dough pieces are garnished with sugar, salt, nut fragments, grated cheese, etc., or are given a wash of milk or egg before being baked. It will be appreciated that such applications must be made uniformly and if they are made before cutting they should not affect the performance of the cutter or have too much effect on the quality of the cutter scrap as it is reincorporated with fresh dough.
Sheeting and gauging
Dough feeding systems
The dough may be fed to the plant by placing it directly into a hopper above the sheeter. In other cases, so that the feed to the sheeter may be better controlled, and the incorporation of the cutter scrap be more uniform, dough is fed into a pre-sheeter. Typically pre-sheeters have two rolls and do not deliver a complete and cohesive sheet of dough. Instead they deliver pieces of dough onto a conveyor which passes to the hopper of the main sheeter. The pre-sheeter and the subsequent conveyor are normally started and stopped to maintain a constant level of dough in the sheeter hopper.
The main disadvantage of such a dough feed system is that the dough is exposed to the air for a longer time and may cool or dry.
Pre-sheeter and dough feed arrangement.
Sheeters are available with either two, three, or rarely four rolls. The two roll varieties are usually used as pre-sheeters; that is they meter the dough from a hopper as a rough or incomplete sheet to other machinery such as a rotary moulder or another sheeter at the head of the forming machine. The performance of pre-sheeters is usually not critical as they are not designed to produce a perfect sheet of dough.
Sheeters heading the "cutting machine" are nearly always of the three roll variety because, the configuration of the rolls is designed to compress and gauge the dough into an even full width sheet. The two top rolls are known as forcing rolls and then one side of these rolls plus the lower third roll constitutes the gauging facility. In order to draw the dough into the sheeter at least one of the forcing rolls has a rough surface in the form of fluting or grooving. If both forcing rolls have grooves a pattern will be given to one surface of the emerging dough sheet. Generally this is not desirable if the pattern is on the top of the sheet as the effect may persist to the cutter and then affect the appearance of the biscuit surface. The gauging roll always has a smooth surface.
The diagram above shows that there are front and back discharge types of three roll sheeters. The front discharge variety is preferred for an extensible dough but back discharge is required where the dough is weak and short and needs to be well supported and not bent as the sheet is transferred to the following conveyor.
Four roll sheeters are essentially three roll sheeters with an extra roll below the gauging roll. When the unit is being used as a sheeter the lowest roll is not used and is merely a support for the conveyor. The two lowest rolls act as a gauging pair when dough is fed from behind the sheeter, for example, from a laminator.
Two roll sheeters do not have the forcing or compression facility and consequently are apt to give sheets which are holey or have ragged edges.
Where cutter scrap is fed back to the hopper of the sheeter other problems may arise. If there is a continuous feed of fresh dough to the sheeter it is quite satisfactory to run the scrap into the rear or front of the hopper for incorporation. However, where the fresh dough is fed intermittently in large masses, provision must be made to allow a regular feed of scrap along with the fresh dough. This is usually done with the aid of a gap or small feed roll as shown below. The problem is that the amount of scrap or the way in which it is fed through the sheeter varies, so some adjustment of the gap or the height of the feed roll over the forcing roll is required - this is rarely provided and control is difficult or unsatisfactory.
The surface of the sheet as it emerges from the sheeter is of very great importance to both the baked biscuit surface appearance and often to the degree of lift obtained in the oven. It is usual that a rough, rippled or holey sheet surface cannot be satisfactorily "repaired" during subsequent gauging so the quality and integrity of the dough sheet from the sheeter is very important.
The dough sheet emerging from the sheeter is collected on a take-off conveyor which is driven by the sheeter and whose relative speed can be adjusted over a short range to ensure that the sheet lies well without being pulled from the sheeter. The conveyor takes the dough to the first gauge rolls.
Pairs of heavy steel rolls are used to gradually reduce the dough sheet thickness to that desired for cutting. Typically, there are two or three pairs though only one pair may be used for short doughs and more than three where very gentle reductions are necessary. As a rule of thumb the reduction in thickness at each gauge roll should be about 2:1, although ratios of up to 4:1 are commonly used. Obviously the greater the ratio the more work and stress is put into the dough.
Usually the pair of gauge rolls are mounted vertically one above the other. The adjustment of the gap is by moving one roll and sometimes this is the upper, sometimes the lower.
All gauge rolls should have instruments indicating the gap setting so that the machine can be changed or the settings recorded with accuracy. Often the gauges do not correspond well with the gap, but this is only a question of engineering maintenance for calibration.
Claims are made for benefits using rolls of various diameters ranging from about 150-300 mm, and also for different surface finishes. Both aspects are related to the tendency for the dough to stick to the roll as it emerges.
Dough emerging from a gauge roll is always of a slightly greater thickness than the gap it has come through. This is as a result of the elastic properties of the dough ("spring") and also because some extrusion, as compared with rolling, occurs through the nip.
Scrapers are provided to aid in the release of the dough from the rolls and also to keep the rolls clean.
Both sheeter and gauging rolls locate together between flanges fixed on the sides of one of the rolls. The flanges allow the dough to fill the roll right to the edges, thus ensuring a full width sheet without a ragged edge. Normally the flanges are part of the lower roll but as this makes it difficult to bring the take away conveyor very close to that roll, development has now meant that the flanges are fitted onto the upper roll instead. There is a difficulty that release of the dough at the flanges can be a problem so when upper flanged rolls are used there is a greater tendency for the dough to follow this roll rather than the lower.
Since tensions in the dough should be minimal and as constant as possible, it is necessary to maintain a full sheet by allowing a slight loop in the dough on the feed side of the gauge rolls and to have a similar loop at the discharge side. If the dough is pulled away from the rolls, tensions are produced that are worst at the edges. The optimum conditions are shown below.
Short doughs are very non-elastic so careful attention must be given to the scraper position and the proximity of the take-off conveyor to prevent strong curvature at the discharge side of the gauge rolls. Thus, it is here that the top flanged roll configuration has maximum potential advantage.
When machining harder or tight doughs it is frequently found that the dough is slightly thicker in the centre of the sheet than towards the edges. This is because some flexing of the rolls occurs under load. Before requesting engineering fixes for this problem see if it is possible to arrange that there is less reduction at the gauge roll by arranging that the sheet coming to it is thinner.
Dough piece cutting
Dough relaxation arrangements
The reasons for dough relaxation have been outlined above. Usually special provision is made for this only between the final gauge roller and the cutter. It may, however, be desirable to relax the dough more often in puff or other laminated types.
Relaxation of dough before cutting is only applicable to hard doughs. Short doughs have should have no significant shrinking after sheeting and gauging.
The dough is relaxed by allowing time and also giving facility for limited shrinkage in the direction that the dough is travelling. In some older cutting machine plants the conveyor to the cutter was very long and the dough was slightly overfed onto it from the final gauge roll so that low ripples were formed. Before the sheet reached the cutter the shrinkage had absorbed the ripples so that a smooth sheet was available for cutting. The plant length required for this was rather long, especially on high speed lines, so an "intermediate web" is now the more usual method to allow relaxation before the cutter. Dough is overfed onto this intermediate web to form ripples which may be quite large. From the intermediate web the dough is fed onto the cutting web. The speed of the intermediate web is adjusted so that the ripples in the dough are just taken out so that a smooth sheet is presented to the cutter.
This arrangement is reasonably satisfactory but it can frequently be noticed that this rippling causes local transverse lines of stress, particularly in dry doughs, that are not completely removed before cutting. Also, it is easy to neglect the plant adjustment so that the dough is pulled off the intermediate web onto the cutting web thereby introducing some stress again.
Older biscuit plants always employed reciprocating cutting machines. These use heavy block cutters which stamp out one or more rows of pieces at a time. The equipment needs to be strong and incorporates a swinging mechanism so that the dough sheet travels at constant speed and the cutter drops, moves with the dough, then rises and swings back before dropping again. Adjustment is provided to allow heavy or lighter cutting as the block falls. If the cut is too heavy there will be more damage to the cutting web, reducing its useful life.
In reciprocating cutting there are two basic procedures used depending upon whether merely cutting or embossing and cutting is required. Where simple cutting and dockering is needed, as for most crackers and hard sweet types, the cutting edges, docker pins and any types of decorative patterns are mounted on a base plate and a spring loaded ejector plate is located to move vertically around the fixed parts. When the cutter drops onto the dough the ejector plate is pushed back and as the cutter is lifted away the ejector pushes the dough to ensure it stays on the cutting web and does not stick to the cutter. If there is a tendency for the dough to stick to the ejector it is necessary to either lightly dust the dough sheet with flour, or to effect a little drying with a blast of air before the cutter.
When cutting and embossing is required, as for some short dough types, the ejector plate is replaced with an embossing plate. This plate is held back as the cutter drops then the plate is dropped to a predetermined position to press a deep pattern into the dough pieces. The cutting edges are then lifted away followed by raising of the embossing plate. This arrangement means that the dough pieces are firstly determined (which is important for controlling dough piece weight) then the surface relief is imprinted without loss of any dough by extrusion. It is unusual to have docker holes right through this type of dough piece, but if they are required, they are incorporated in the pattern cut in the embossing plate.
Reciprocating cutters thus require much heavy mechanism with many moving parts that need good lubrication and maintenance. They are often noisy, especially if the plant is run at high speed. Speeds of up to about 180 cuts per minute can be achieved, usually less with embossing cutters than cutting only types. It is however unusual to run these cutters at more than 100 cuts per minute. Two or more rows can be cut per drop of the cutter, but the more rows there are at once, the wider and heavier the cutter block becomes.
With the development of longer ovens (higher line speeds) and wider plants, it was necessary to consider improvements in the cutting arrangements. Rotary moulders have largely replaced embossing cutting and rotary cutters are now used very widely for most other types.
Rotary cutters are of two types, those that employ two rolls, one immediately after the other, and those with only one roll. Dealing with the two roll type firstly, the principle is that the dough sheet, on a cutting web, is pinched between engraved rolls (mounted in series) and a rubber coated anvil roll(s).
The first cutting roll dockers the dough, prints any surface pattern or type and thereby pins the dough onto the cutting web. The second roll is engraved with only the outline of the biscuit and cuts out the piece leaving a network of scrap (which has not been pinned down) as with the reciprocating type of cutter. There is a facility to adjust the rotational position of one roll relative to the other so that correct synchronisation between pattern and outline cut is possible. The pressure between each of the cutting rolls and the anvil can be adjusted independently but there is normally a quick release arrangement which lifts both rolls and maintains their relative positions so that production can easily be interrupted and resumed.
It will be appreciated that there is a fine adjustment required for the height of the first roll to make sure that the dough is dockered and pinned down sufficiently, but the pressure must not be too great, otherwise displacement of the dough both backwards to form a wedge and sideways affecting the thickness of the eventual dough piece, may occur. The dough sheet thickness and hence the piece weight should be controlled at the final gauge roll not at the cutter. It is also important that the docker pins are not too long otherwise the cutting web may be damaged.
The rolling action on the dough means that rotary cutters are not satisfactory for embossing short dough as is possible with reciprocating cutters. The formation of deeply patterned short dough pieces is much better done with a rotary moulder.
The cutting rolls are driven so the speed relative to the cutting web can be adjusted slightly to affect the length of the cut piece and also to assist release of the dough from the cutter. Setting the optimum relative speeds of the cutter rolls and the cutting web is very critical. If the cutter speed is not right severe damage can be done to the cutting web.
A single roll rotary cutter achieves both dockering, pinning and outline cutting with only one roll. In most cases this works well and there is a saving in capital equipment, but there is a strong tendency to lift the dough piece from the cutting web because the pinning down facility is not independent of the cutting pressure. Cutting rollers are expensive so one should not have two where one would do, but the frustration and disruption to production caused by malfunction leads one to advise that the chances of difficulty which may occur from using a single roll type are significantly avoided by using the two roll system.
Rotary cutting rolls are usually fabricated in bronze or gunmetal and modern automated engraving techniques allow high precision of shape and thickness of the patterns over the whole roll surface. Moulded plastic "cups" which are attached to a roll surface are also a standard now. The moulded cups are produced from a single die so they are all identical. The cost is not much different for the fabrication of a cutting roll with cups compared with an engraved roll but as it is possible to have an unlimited number of spare cups from the same mould, savings in the event of wear or damage are very favourable.
Control of dough piece weight and shape
The weight of dough pieces, which is fundamental to the production of good biscuits or crackers, is determined by the thickness of the dough sheet at the moment of cutting. To increase the dough piece weight the sheet leaving the final gauge roll must be increased and vice versa. As has been explained, some thickening of the dough sheet may occur as the dough relaxes before cutting.
In the case of rotary cutting care should be taken to avoid excessive cutting pressure by the first of two cutting rolls otherwise dough will be displaced into the scrap and a lighter dough piece will be produced. It is not a good idea to control the dough piece weight with the rotary cutter.
The length of a dough is controlled principally by the amount of relaxation before it reaches the cutter. A very small amount of adjustment is possible by changing the relative speed of the cutter roll to the cutting web speed. This may cause sticking of the dough piece in the cutter or lifting away from the cutting web.
Cutter scrap dough handling
Ideally the network of dough not included in the cut pieces will be adhering less firmly to the cutting web than the pieces because it has not been pressed down by the cutter. It can therefore be lifted and gently pulled upwards onto a scrap return conveyor. If the dough is very weak it may be necessary to use supports to aid the lift onto this scrap return web. These supports are called "fingers". It will be appreciated that they can only be used where the dough pieces are in straight lanes with clear lines of scrap where the fingers can be located. Scrap fingers cannot be used where the pieces are cut in a staggered arrangement as is usual for round or oval biscuits.
The scrap dough, having been lifted, is carried away on a full width conveyor whose speed can be controlled. Much of the success of good scrap dough removal relies on controlling the speed of the take away web.
The scrap dough may be returned as a full width network to the front of the sheeter hopper, or it may be collected and sent back on a narrow conveyor to be distributed across the back of the sheeter hopper or, less commonly, back to a bin or to a mixer. Whichever is the arrangement, it is important to ensure that this scrap dough is spread evenly in the sheeter to optimise a good distribution in the new dough. It is worth watching that edges of the new sheet are not rich in scrap as this will have an adverse effect on the quality of biscuits produced.
A particular problem with scrap reincorporation with fresh dough can occur when the full width of the dough sheet is lifted from the cutting web as can occur at start up or when cutting problems are encountered. If the amount of scrap dough involved is high it is better to collect the scrap in a bin and not to return it to the sheeter. Use of scrap dough collected in this matter is a matter of company policy.
If the quantity of scrap is normally high a two roll sheeter may be used to make a more or less continuous layer of scrap dough which is feed beneath the fresh dough from the main sheeter. The advantage of this arrangement is that the scrap can be metered and its placement is assured.
Scrap dough is always of different consistency from the fresh dough. This may be because it is cooler having been exposed as it has passed along the cutting machine. In some cases the scrap dough is taken through a heated tunnel in a humid atmosphere as it is returned to the sheeter. Theoretically this is very desirable but attention must be taken to ensure that mould growth is not allowed to develop in this warm area. Frequent inspection and cleaning are required.
Transfers of dough pieces
It is not usual to run the cutting web right through to the oven band. Normally there is a transfer onto a panning web. If the dough pieces are to be garnished (see Dough Piece Decoration) it is on this web, or another intermediate conveyor, that the garnishing or surface washing takes place.
The finished dough pieces are transferred (panned) onto the oven band. The panner unit has several functions. It allows an acceleration of the pieces off the cutting web which may be useful if some spatial separation, etc. is required and it is usually capable of swivel so that it can follow the line of the oven band as this tracks slightly from side to side. In this way the dough pieces are always deposited away from the extreme edges of the oven band.
The transfer of pieces from the panner to the oven band must be adjusted carefully so that the spacing is optimum and they lie down smoothly. Badly panned pieces may be distorted in shape after baking. The nose piece of the panning web should be as sharp as possible and as near as possible to the oven band.
Spacing of the dough pieces onto the oven band is a critical process control point. If the pieces are spaced too widely the oven is not being used to optimum capacity. Ideally the spacing should be as close as possible but the spaces in the line of travel should be equal to the lateral spacing that was determined by the cutter.