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Structure, functions and applications of strand pelletizing machines Ulrich Kreuz Differentiation between strand pelletizing and Die face pelletizing Process Die face pelletizing Extruding through a circular Die. Cutting the emerging mass by rotating blades directly at the Die. Transporting and cooling pellets in a stream of liquid or gas. For liquid cooling: separating the pellet from the liquid and drying the pellet. Strand pelletizing with dry cut Extruding through a beam-shaped Die. Cooling the strands by liquid or gas. For liquid cooling: separating the liquid from the strands and pelletizing the strands. Strand pelletizing with wet cut Extruding through a beam-shaped Die. Cooling the strands by liquid. Pelletizing the wet strands with liquid fed into the cutting housing. Separating pellet from liquid and drying the pellet. Product Typical uses of Die face pelletizing Mass plastics with high throughput rates (> 20 t/h). Natural plastics (with little or no additives) with longer, continuous operations. Plastics that can be cooled with a gaseous medium after being cut at the Die. Typical uses of strand pelletizing Low-viscosity plastics that cannot be Die face pelletized. Highly filled and/or highly reinforced plastics. Plastics that require strand pelletizing (oval shape). Plastics that have to be extruded with low head pressure. Pellet form Die face pelletizing Differing machine types (underwater, water ring, air pelletizing), structure, process parameters and plastic materials being pelletized can result in widely varied pellet forms. With a Die face pelletizing machine, basically only the pellet length can be influenced by changing the speed of the cutting blades. Different pellet diameters require the use of Dies with different hole-diameters. Strand pelletizing The pellet is round and the shape is basically constant. Depending on plastic and operating parameters, the cross section can vary from a circle to an oval. The pellet diameter for most plastics can be changed by orientation. Orientation ratios of Die hole cross-section to strand cross section of 4:1 are possible.

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The pellet length is varied by changing the speed ratio of the feed rolls to the cutting rotor. System cleaning Die face pelletizing with liquid cooling The cutting dust created during pelletizing is picked up by the cooling medium (normally water) and has to be filtered out continuously; but ultra-fine particles still remain in the cooling circuit. This means that fast machine change-over for different colors is not possible because of the complex coolant cleaning procedure. Further problems with the cooling system are created by a large-volume Die head and pellet dryers. Strand pelletizing with dry cut No cutting dust contaminates the cooling medium and a pellet dryer is not required. The procedure involved in product and/or color change is restricted to cleaning the pelletizer and the Die head. Strand pelletizing with wet cut Apart from the simplified cleaning of the Die head, the same cleaning problems exist as for Die face pelletizing with water-cooling. Value for the money Die face pelletizing with water-cooling requires a far higher water flow rate for transport reasons and to avoid agglomerates than would normally be necessary to dissipate the heat from the pellets being cooled. A high-power water circulation system is therefore indispensable. Manual strand pelletizing is therefore more economical to purchase, and when used for suitable applications, it is also more economical to operate. The procurement costs for automatic strand pelletizing with dry cut and Die face pelletizing are about the same. The difference being more economical operating costs for strand pelletizing. The decision between "automatic strand pelletizing with wet cut" or "Die face pelletizing" is preferably based on the process involved. Die face pelletizing is more economical for large throughput rates. Strand pelletizing with manual strand feed Machine structure, fig. 1 Normally, strand pelletizing with manual strand feed consists of the following components: Strand Die Cooling trough Strand dryer Strand pelletizer

Fig. 1: Diagram to show machine structure

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Extrusion Die, fig. 2 Normal extrusion Dies consist of a Die head in which the molten plastic is diverted from a round or octagonal connection cross-section into a rectangular shaped discharge cross-section. The discharge angle is between 45 and 90°.

The Die plate with Die holes between 2 and 7 mm is bolted to the flow channel. The maximum number of Die holes is around 250 with a maximum Die width of approximately 1500 mm. Breaker plates and screens can be inserted between the Die head and Die plate for better distribution and filtration of the molten plastic. The heating system is electric with screw-on heater elements (cast iron or ceramic heater elements) with 1 to 4 temperature regulation zones, or by thermal oil. The designs of the Die body, flow channel and the Die plate have a decisive influence on pellet quality. A uniform pellet can only be achieved with a uniform strand flow. Cooling trough, fig. 3 Depending on use and manufacturer, water baths are available in many different designs with working widths of up to 2 m and in cooling lengths normally of up to 12 m.

Fig. 3: Water bath All water baths have two or more longitudinally adjustable spreader bars or rods with grooved or flat surface of stainless steel or plastic. Depending on application, the cooling water flows through the water bath in the opposite direction of the strand flow.

Die body

Flow channel

Die plate

Fig. 2: Extrusion Die

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Other possibilities for varying temperature control are water baths with partitions and possibly with several water inlets and outlets, or several water baths connected in series. The water depth is normally constant, but in special cases it can be considerably deeper in the intake area with a first spreader bar that can be adjusted to the bottom of the bath. The strand inlet can be either over the water bath or before it. For a strand inlet before the water bath, during start-up and in the case of breakage in the strand, the strands fall to the floor. The described arrangement makes the start-up easier and the production risk from strand upsets is reduced, but the time in which the strand is conducted through the air before being submerged in the water is longer than for a water bath positioned under the Die. A combination of both techniques consists of a water bath that can be adjusted manually or pneumatically in length so that it is positioned after the Die in the direction of flow at the start but under the Die during operation. Maintenance work at the Die can also be used for making longitudinal adjustments. Accessibility of the Die can also be achieved by a water bath that can be swiveled around a fixed point at the strand outlet side. The cooling water temperature is controlled either by an adjustable fresh water supply, or by a coolant circuit that can be arranged in the frame underneath the water bath under favorable circumstances. Strand drying, fig. 4

Blowing, vacuum or a combination of blowing and vacuum can remove the water adhering to the strands. The most effective method is combined blowing and vacuum. Blowing alone does achieve a satisfactory degree of drying but creates the problem of sprayed water. The vacuum method is not capable of creating a pointed, sharp jet of air so that this achieves less satisfactory drying than in the method described above. Pre-takeoff rolls Modern strand pelletizers require practically no pre-takeoff because the feed rolls of the pelletizer perform this task.

Blowing nozzle

Suction nozzles

Fig. 4: Strand drying

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Strand pelletizer, fig. 5

The pelletizers can differ greatly depending on make. Here are a few points that a modern strand pelletizer should offer:

Working widths 25 to 1000 mm.

Compact design (motor in lower frame, control enclosure with control panel integrated in frame).

Pellet discharge freely accessible from all sides (not in lower frame) so that different classifiers can be mounted.

Lower frame with integrated step so that the strands can be introduced at a comfortable height for the operator, and to give complete visual control of the pelletizer interior during cleaning and maintenance work.

Drive of the cutting rotor not as direct drive but via belts so that the speed adjustment range can be rated for the drive motor to run at optimum frequency.

Modular structure so that wear parts, e.g. rotor blades, bed knives and wiper blades can be used for different machine sizes.

Easily replaceable wear parts and replaceable pelletizer head.

Integrated sound-proofing. Preferred machine arrangement Water bath after the Die in the direction of extrusion, strand drying adjustable in length over the cooling trough, at least 3 m air gap between cooling trough and pelletizer, strands brought to a width which is the same as or smaller than the working width of the pelletizer at the latest 1 m before entering the pelletizer.

Fig. 5: Strand pelletizer

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Applications Strand pelletizing machines with manual strand feed are the most universal pelletizing systems available. The cooling length and thus the dwell time of the strands in the water can be adjusted simply to the corresponding product by removing the strand at various points. The water temperature can be controlled in several zones and the strand can be conditioned after passing the drying section by a corresponding air gap. After start-up, monitoring strand breakage minimizes staff requirements. Manual strand pelletizing is thus the ideal method for machines that process many different products, and for machines that run for a long time unchanged after start-up. Manual strand pelletizing is not suitable for processes with frequent strand breaks (e.g. highly filled and reinforced products), for frequent starts with high throughput rate (e.g. batch mode) and for very tough requirements regarding pellet uniformity (strand crossing and thus irregularities in the pellet are practically unavoidable with manual strand feed). Strand pelletizing with automatic strand feed General Strand pelletizing with automatic strand feed is the name given to processes where the pelletizing system starts and stops without manual intervention, and where broken strands are automatically re-threaded during the production process. Dry cut, fig. 6

When the strands are fed into the pelletizer without water between the strands and without water on the surface, this is referred to as "dry cut". The "dry cut" can be achieved with different machine configurations. The most common arrangement consists of a water trough, a strand drying section and the pelletizer.

Extrusion Die

Water trough

Strand drying

Pelletizer

Fig. 6: Diagram to show dry cut

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Water trough, fig. 7 The water trough normally consists of a trough on a slight angle with water flowing down it where the strands are cooled, transported and kept separate. In addition, the strand should be shaped to be round or slightly oval and straight. The strand must be cooled at least to a point where it can be fed safely into the pelletizer and no agglomerates are formed after pelletizing.

The transport water speed must be high enough and the layer of water deep enough to accelerate the strands safely at start-up or after a strand break, and to transport them reliably. Trough length and cooling water temperature guarantee the maximum permissible pellet temperature for a given strand speed. A laminar flow of the transport water and correctly adjusted spray nozzles prevent the strands from sticking together. The main components of a water trough are:

Water supply table

Water trough

Spray nozzles

Vertical trough adjustment

Horizontal trough adjustment

Water manifold Water supply table The water supply table generates the laminar water flow and defines the thickness of the water layer with the discharge gap. The table can be smooth or grooved. It should be possible to swivel the table to the vertical plane. For the start-up, the transport water flow in the water table is increased by approx. 30% by a solenoid valve. Water trough The strands are guided from the water supply table to the water trough, which slopes from 2 to 10°. The trough is between 2 and 7 m in length, although troughs of up to 15 m cooling length are also available. The bottom of the trough must be smooth and flat to maintain the laminar water flow. If necessary, the water trough can also be covered. Spray nozzles The spray nozzles are arranged next to each other with an overlapping spray pattern in a spray bar that covers the entire width of the trough. Depending on the particular application, the droplet size of the nozzles can be adjusted from "mist" to "spray". The spray bars are

Fig. 7: Diagram to show water trough

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separated in longitudinal direction approx. 500 mm, with increasing separation in the direction of strand flow. The spray nozzles cool the strands at the surface (floating strands), prevent the formation of a hot water coat around the strand, keep the strands separate and also assume part of the transport function. Vertical channel adjustment – horizontal channel adjustment It should be possible to adjust the height of the complete water trough to optimize the distance between Die plate and water supply table. It should also be possible to adjust the position of the complete water trough to optimize the impact point of the strands on the water supply table. Water manifold The water manifold distributes the water coming from a central supply to the water supply table, start-up water and spray water. The individual flows can be regulated and can be displayed and/or monitored. Strand drying In the strand-drying phase, the largest quantity of water is separated out by gravity through a perforated plate or slotted screen, and is then returned to the coolant circuit. The surface water adhering to the strands and the water conducted between the strands is removed through slots by vacuum, blowing or a combination of both. In order to achieve residual surface moisture values of approximately 0.1%, the strand speed should not exceed 100 m/min. The effectiveness with which an air stream removes the water adhering to the strands depends on its force and the dwell time of the strand under this force. The force depends principally on the air speed. For noise reasons, air speeds exceeding 35 m/s should be avoided. Speeds of less than 25 m/s considerably decrease the drying efficiency. For economical reasons (air requirements,) the blowing and vacuum cross sections should not be rated too large. The slot width for blowing nozzles is between 2 and 5 mm, for vacuum nozzles between 5 and 15 mm. These parameters are capable of achieving good drying values for strand speeds of up to 100 m/min. Greater strand speeds must be expected to produce poorer drying values. To ensure that the machine starts up safely, it is advisable to switch off the vacuum and/or blowing air during the start phase. Extreme air streams could prevent strand transport and cause a false start. Pelletizer The strands are fed from the strand-drying phase into the pelletizer. In the "dry cut" version, the pelletizer corresponds essentially to a pelletizer for manual operation, with the exception of the enlarged strand intake chute and the driven upper feed roll. Both measures make it easier for the pelletizer to grasp the strands during the start up. The combination of the extruder height and the inclined water trough cause the pelletizer intake chute and discharge chute to be very low. Therefore, the pellet has to be raised for further processing (classifying) or the following machines have to be arranged even lower. To raise the pellets, normally a spiral conveyor or pneumatic transport is used. Typical applications Typical applications for the described machine are production lines with frequent color changes. Machines are available in working widths from 100 to 1000 mm. Throughput rates of up to 800 kg/h can be achieved at pellets temperatures between 70 and 120°C.

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Strand conditioning, fig. 8 One essential variation to the explained dry-cut pelletizing machine consists of adding a strand conditioning phase. The machine then consists of:

Die head

Water trough

Strand drying

Temperature compensation section (strand conditioning)

Pelletizer The liquid cooling section, strand drying and pelletizer elements are as described above. Temperature compensation section The temperature compensation section is 2 to 5 m long and arranged between strand drying section and pelletizer. It lengthens the dwell time after strand drying until the strand enters the pelletizer, and usually consists of a transport belt to guarantee safe strand conveyance. The transport belt can be horizontal or at an ascending angle of up to 15%. The ascending transport belt already eliminates the disadvantage of the low strand intake height described above for conventional dry cut. Standard pelletizers with pelletizer discharge heights of approx. 800 mm can be used. The pellets fall directly onto a classifier. Pellet transport devices such as spiral conveyors or pneumatic transport are no longer required. Temperature homogenization If the pelletizer machine has no temperature compensation section, this results in large differences of temperature across the strand cross section. The outer edge has approximately the temperature of the cooling water, from 40 to 80°C. In extreme cases, the core can still be molten with temperatures between 220 and 300°C. The strand strength differs considerably according to the differences in temperature. If the plastics being processed are glass-fiber reinforced, then the glass fiber are only firmly embedded in the outer edge where it is cut through neatly during the cutting procedure. The fiber in the core "float" in the carrier material and can change position during the cutting procedure, becoming erect or even being pulled out at the cut surface. This results in an unclean pellet cut. Use of a temperature compensation section homogenizes the temperatures across the strand cross section: the edge warms up and the core cools down. This means that the strength of the strand is approximately the same throughout. Integrated glass fibers are firmly embedded throughout and are cut through neatly during the cutting procedure. This results in a clean pellet cut.

Fig. 8: Diagram to show strand conditioning

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Apart from the desired temperature homogenization, two other positive process side-effects occur: the pellet temperature is reduced by 10 to 30°C and the surface residual moisture is lowered to values less than 0.1 weight %. The described effects result in more favorable cutting conditions: the firmly embedded glass fibers reduce abrasion at the cutting tool, and the completely solidified strand and the dry cutting chamber reduce chemical corrosion of the cutting tool. Advantages There are three advantages for the user in a temperature compensation section: the improved cut quality results in better pellet quality, the service life of the cutting tools is increased and there is no need for intermediate transport of the pellets resulting in no additional wear parts (pneumatic transport) to be cleaned. Typical applications Typical applications for dry pelletizing with strand conditioning are lines for glass-fiber reinforced plastics, master batch systems and color lines. Cooling belts, fig. 9 The described dry pelletizing machine can be equipped with a pair of cooling belts instead of a cooling channel. Cooling belts are used when the strand needs to be grasped mechanically because water transport alone is no longer sufficient, for example in the case of very soft or very brittle plastics. Cooling belts can be operated horizontal or in ascending arrangement, with a clear gain in height for strand intake into the pelletizer compared to the cooling channel.

Die head

Cooling belts

Strand drying

Pelletizer

Fig. 9: Diagram to show cooling belts

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Water-free pelletizing, fig. 10

Automatic strand pelletizing with dry cut is supplemented by water-free pelletizing. The strands are transported on a guide plate. The specific shape of the guide plate gives the flowing air a transporting and carrying component. There is no contact between strand and guide plate so that there is no wear on the guide plate. Typical applications are water-soluble plastics and highly filled thermoplastics with good cooling behavior. Wet cut – horizontal design, fig. 11

When the strands are conducted into the pelletizer with the cooling liquid between the strands and the adhering cooling liquid on the strand surface, it is referred to as "wet cut". The machine consists of:

Guide plate

Guide plate channel

Pelletizer

Pneumatic pellet transport

Fig. 10: Diagram to show water-free pelletizing

Fig. 11: Diagram to show wet cut, horizontal design

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Water trough Pelletizer Pelletizer after-cooling section with agglomerate trap or start-up filter and water separator Pellet dryer Wet cut granulating machines can have a horizontal or vertical design. Horizontal strand pelletizing Water trough The water trough essentially corresponds to the same component of the dry cut machine, with a cooling length between 1.5 and 7 m. The strand must be cooled down to such an extent that it can be safely fed into the pelletizer and not deformed by the feed rolls. Water separation Water trough and pelletizer are connected together but can be detached. A stream of water flows down the water trough, which corresponds to approximately 6 to 8 times the mass of the strands. This water stream is separated off primarily by gravity before the strand enters the pelletizer. This separation can take place either at the end of the channel or in the pelletizer intake chute via perforated or slotted bottom plates. The separated water can both be returned directly to the coolant circuit or into the pelletizer discharge and used together with the water kept in the pelletizer as cooling and transport water for the after-cooling section. The water is then conveyed from the pellet dryer to the coolant circuit. Direct separation of the cooling water before the pelletizer is advisable when the water temperature in the after cooling section is to be kept as low as possible, and/or the water flow should be no more than 3 to 4 times the quantity of pellets. It is advisable to introduce the cooling water into the pellet discharge when the heated water can be used in the cooling channel because the pellet has acquired a high water temperature. One further advantage to this kind of water system is that no water return connection is necessary. However, this is countered by the disadvantage of higher pre-drainage costs before the pellet dryer. Pelletizer A wet cut pelletizer is essentially different from a dry cut pelletizer. The cutter housing must be waterproof and the bearings of the feed rolls and the cutting rotor should be sealed against water and steam. The cutting tools must be abrasion-proof and corrosion-resistant. Suitable materials for the bed knife are stellite and ceramic, for the rotor, stellite, powder-metallurgical steels and hard metals. The strand intake chute can be designed with integrated channel water separation. The pellet discharge can be horizontal instead of having a slope of approx. 40°C as is the case with the dry cut pelletizer, because the pellet is discharged by the transport water. It is advisable to spray the cutting rotor to prevent pellet adhesion. In hot water operation (water temperature > 50°C), it is advisable to install a ventilator in the soundproofing hood, which surrounds the pelletizer cutting housing. Pelletizer cooling section The pelletizer after-cooling section is connected to the pelletizer discharge, which is usually rectangular, by means of a rectangular to round adapter leading into a pipe measuring 4 to

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10 m in length. If the pipe is half-full, the transport takes place according to the open channel principle. To guarantee a reliable flow, the quantity of water used must be at least three to four times the quantity of pellets, and the gradient in the pipe should not be less than 1°. The adapter piece can be equipped with a trap cage for any agglomerate produced during the start-up. A start-up filter can be integrated in the pipe to convey the pellet/water mixture produced during the start-up phase in a perforated container. Pre-drainage should be provided before the pellet dryer. Pellet dryer Dryers are used to accelerate the pellets mechanically and let them fall onto sieves or rings, such as centrifugal dryers and impact ring centrifuges, or dryers that accelerate the pellets pneumatically and let them fall onto slot sieves or fluidized bed dryers. Horizontal strand pelletizing systems with wet cut are available in working widths from 200 to 1000 mm. Depending on the thermoplastic, the system can operate at strand speeds of up to 250 m/min, achieving throughput rates of up to approximately 12,000 kg/h. The ideal application for this kind of machine are natural plastics or plastics with low levels of additives, e.g. PET, PBT; PA, PC, ABS, PP. Wet cut, vertical type, fig. 12 In principle, the automatic strand pelletizing procedure with wet cut is the same in horizontal and vertical form. Vertical pelletizing works with cooling lengths between 0.6 and 1.5 m, i.e. with far shorter cooling lengths than horizontal pelletizing. Water trough The water trough (strand guide) and pelletizer form a unit. In contrast to the horizontal machine, the strand guide is deeply grooved from the strand takeover point to the strand intake into the pelletizer. Each strand has a separate guide groove so that the strands can be packed much more densely than on a horizontal system, but have to be resorted after the start in some cases. The vertical system is much more compact than a horizontal system,

Fig. 12: Diagram to show a wet cut system, vertical type

Die head

Strand guide

Pelletizer

Re-cooling section

Pellet dryer

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but also requires a far greater height between the lower edge of the nozzle and the working platform. Normal heights are between 2000 and 3600 mm. Vertical pelletizing systems are available in working widths from 300 to 900 mm working with throughput rates of up to 7500 kg/h. Application The ideal applications for machines of the described type are reactor operation in batch or continuous process with brittle plastics, which should be treated on short cooling sections in order to avoid dust. (For example: GPPS, HIPS, PMMA, and SAN.) Strand pelletizing, Scheer type Design principles of the SCHEER pelletizers, fig. 13 Geometry The geometric arrangement of the feed rolls in relation to the cutting tools has a decisive influence on the cutting quality of a pelletizer.

Fig. 13: Diagram to show the design principles of SCHEER pelletizers The following prerequisites have to be fulfilled, which contradict each other in some cases:

The clearance (dimension A) between the nip point of the two feed rolls and the cut off point of the fixed blade (bed knife)/cutting rotor (shear blade) should be as short as possible to minimize the unguided length of the strand.

The clearance (dimension B) between the edge of the scraper and the edge of the bed knife should be as large as possible for maximum stability in the design of the bed knife holder and scraper.

The feed rolls should be offset (angle ) so that the theoretical progress of the strand impacts the scraper before it reaches the edge of the bed knife. This arrangement means that the strand contact on the bed knife is pre-tensioned and stabilized. If the offset of the rollers is too large resulting in too much pre-tension on the strand, brittle strands can break.

The cutting edge of the bed knife should be underneath the middle of the rotor to achieve a "pulling" cut.

The bed knife should be slanting (angle ) to have the minimum possible influence on the trajectory of the pellets.

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The gap between the lower feed roller and bed knife holder increases continuously to the bottom and thus prevents strand deposits.

SCHEER has combined the required geometric prerequisites for feed rolls and cutting tools into a good compromise and can therefore guarantee flawless cutting quality. Stiffness Deflection of the cutting rotor should be kept to an absolute minimum so that the cutting gap between cutting rotor and bed knife can be kept between 0.02 and 0.08, depending on requirements. Given the same lengths and same load on the cutting rotor, the deflection depends only on the moment of inertia and thus the diameter of the rotor. The cutting rotor chosen by SCHEER with a diameter of 200 mm, for example, bends only half as much as a cutting rotor with a diameter of 165 mm. In order to obtain uniform tension on all strands and a constant narrow gap between the scraper and lower feed roll, the deflection of this roll should also be kept to a minimum. For example, the diameter of 70 mm or larger chosen by SCHEER means that the lower feed roll only bends half as much as a roll with a diameter of 60 mm. SCHEER has deliberately chosen the larger dimensions so as not to impair the required close clearances through mechanical stability of individual components. The great stiffness of the rotating parts in a SCHEER pelletizer reduces vibrations, thus increasing the service life of the cutting tools and reducing noise emissions. Thermal stability As already indicated, cutting gaps must be reduced to a clearance of 0.02 mm. The machine tool industry demands air-conditioned rooms for precision manufacturing. For a pelletizer, it is taken for granted that the closest tolerances are obtained in a temperature range of more than 100°C. For this reason, SCHEER has performed comprehensive cutting gap measurements at changing temperatures. The surprising result is that the cutting gap changes cannot be covered with expansion calculations when the relevant parts are not symmetrical. Asymmetrical parts can result in deformation, which counteract or amplify linear expansion. SCHEER has therefore consistently designed all parts relevant to the cutting gap to be as symmetrical as possible. The cutting gap changes resulting from differences in temperature (e.g. cutting housing to cutting rotor) are essentially compensated for by using specifically selected materials with expansion coefficients between 0.5 and 20 x 10-6 l/K. The entire package of geometric design and specific choice of materials results in a machine with a stable cutting gap. Materials SCHEER is constantly trying to improve the cutting tools as prime pelletizer components. Corrosion and abrasion play a decisive role in the service life of a cutting rotor. Optimization of the cutting material is demonstrated by a corrosion examination, fig. 14.

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Weight loss after 16 h test in boiling acetic acid Weight loss g/m²h

5,9

205

122109

162

6,3

144

86

0

50

100

150

200

250

1 2 3 4 5 6 7 8

Probe Nr.

Sample no. Steel grade Weight loss g/m²h

Holding temperature °C

Hardness in HRC

1 1.4112 5.9 400 58.0

2 1.2591 205 400 58.7

3 CPMT440V 122 400 58.2

4 PM16.5%Cr 109 400 59.1

5 PM16.5%Cr 162 520 60.9

6 PM20%Cr 6.3 400 58.1

7 PM20%Cr 144 520 60.9

8 PM19.5%Cr 86 400 61.0

Fig. 14: Diagram and table to show corrosion examination in various materials. The experiment clearly shows that a compromise has to be made between hardness and corrosion resistance. Materials with weight losses under 10 g/m²h are normally absolute corrosion-resistant. Materials with around 100 g/m²h weight loss are used for lighter corrosion. Materials with more than 200 g/m²h weight loss can be used for negligible corrosion.

Sample no.

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Comparison of pelletizer designs with shaft fastened in bearing assemblies on one or two sides. Strand pelletizers series SGS-E (shaft fastened in bearing assemblies on one side), fig. 15 Strand pelletizers in series SGS-E have the main components such as cutting rotor, lower and upper feed roll, bed knife holder and strand intake chute fastened on just one side, resulting in unsurpassed accessibility, unrestricted scope for visual inspection at one glance, and outstanding possibilities for part cleaning.

The condition shown in the right-hand picture can be achieved effortlessly in less than two minutes without requiring any tools. The undisputed advantages of having the main components fastened only on one side are in turn contradicted sometimes by the objection that it is more stable to fasten the components on two sides. When comparing the two designs: pelletizers with the main components fastened on one side and pelletizers with the main components fastened on both sides, the latter design appears superior at first glance. Presuming that the geometric conditions (diameter and length) are identical, the pelletizer with main components fastened on both sides is capable of taking higher loads. The following analysis attempts to find out: a) How large are the deflections in actual fact?

b) Do the deflections have a practical influence on the cutting gap, or are the advantages

of pelletizers with main components fastened on one side primarily to be found in improved handling, cleaning and accessibility?

Technical data: Pelletizer type SGS 200-E Drive power: 7.5 kW Take-off speed: 60 m/min No. of shear blade teeth 32 Shear blade diameter 200 mm Shear blade length 200 mm Pellet length 3 mm

SGS 100-E with open cutting chamber SGS 100-E with open cutting chamber without strand intake shaft

, without strand intake shaft.

Fig. 15: Strand pelletizers series SGS-E (one-sided fastening)

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The cutting force is presumed to be the line load. The maximum occurring cutting force thus reaches 1,146 N. In order to ascertain the elastic deformation under this load, it is necessary to take a detailed look at the fastening. The fastening used by SCHEER with pre-tensioned taper roller bearings in O-arrangement can be considered to be a fix clamped, freely salient support. (fig. 16) The stated dimensions and loads result in a maximum deflection fm of approx. 0.001 mm using the formula stated in fig 16. This deflection of 0.001 mm is negligible. Pelletizers with the main components fixed in a bearing on one side are available in the working widths of 50, 100 and 200. The handling advantages and almost unlimited possibilities for adjustments, thanks to a list of standard versions, makes this series ideal for all products with throughput rates of up to approximately 1500 kg/h. Strand pelletizers in series SGS-L (shaft fastened in bearing assemblies on both sides, fig. 17)

fm = F x l³

8 x E x I E = elasticity module I = inertia module

l

F

f m

Fig. 16: Diagram to show the bearing fastening

Strand intake

Pellet discharge

Step

Drive

Fig. 17: Strand pelletizers in series SGS-L (shaft fastened in bearing assemblies on both sides)ng)

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A few special features of the SCHEER pelletizers are listed below: The pelletizer discharge gives free access on all sides. Various classifiers can be mounted and cleaned without any problems because the classifier does not protrude into the lower frame. The pelletizer can be operated in three instead of just one direction. A step is integrated in the frame on the strand intake side, so that cleaning and maintenance work can be performed at a pleasant working height. Full visual inspection of the pelletizer interior is possible without any problems. The cutting rotor driven by a toothed belt, results in the following advantages:

Possibilities for adjusting the speed range to optimum frequencies. Standard production speed at approx. 50 – 60 Hz. The motor runs in the most favorable range. Normally, operation directly from the mains is possible.

Increased torque at the rotor shaft, i.e. higher motor power, for normal operation.

Higher flywheel effect at the rotor shaft, i.e. increased motor power, for short-term overload.

Compact design with small outer dimensions, as the motor does not lie in the extension of the motor shaft.

The pelletizer can be opened without the use of tools. The patented feed roller is unique and can be swung away from the bed knife holder, again without needing any tools, for ideal cleaning of the gap between bed knife holder and lower feed roll. This minimizes machine down times and cleaning times. (fig. 18) The complete control and drive cabinet is integrated in the lower frame. The soundproofing hood consists of a solid, stainless housing (aluminum) lined with sound absorption material. Here SCHEER has developed a highly resistant material with glass fibers laminated into it. The surface of the material is smooth (no perforated sheet or similar) which prevents pellet deposits in the soundproofing hood.

Fig. 18: Swivelling lower intake roller

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The modular design of the pelletizer makes it possible to use many different drives and controls to modify structural heights, intakes and discharges, and to specify cutting materials, feed roll materials and pellet lengths while still using a standard machine. The rotor blades and bed knives of a pelletizer, SGS 100-E (100 mm working width) for example, can also be used in a pelletizer SGS 1000 (1000 mm working width). The pelletizers in an automatic strand granulating system can also be used as manual strand pelletizers without any problems. And vice versa, after changing the base frame and intake area, a manually operated strand pelletizer can also be used in an automatic strand pelletizing system. Pelletizers with the main components fixed in bearing assemblies on both sides are available in the working widths from 200 to 1000 mm with throughputs of up to 10,000 kg/h. Several pelletizers that have been operating for more than 25 years bear witness to the incredible stability of this "work horse". Automatic strand feed with SCHEER Water trough pelletizing In systems with good water guidance, i.e. laminar transport water stream from the water feed point over the whole cooling length, correct rating of transport water speed and layer thickness together with spray nozzles, which can be adjusted in angle and strand flow direction with suitable formation of the water droplets, the water trough presents no problem. Some differences between the SCHEER water trough design and other makes are described below. In unconventional water trough pelletizing with parallel water trough, the number of strands is limited by the trough width. The normal division (clearance) of the strands is 10 mm (minimum approx. 8 mm). This means, for example, that 60 strands can be run in water troughs with a working width of 600 mm. A pelletizer can easily take up strands with a division of 5 mm. For a pelletizer with a working width of 600 mm, this means that 120 strands can be run. It can be easily concluded that in conventional, parallel strand pelletizing the pelletizer only works to 50% capacity. The use of a tapered water trough means that the SCHEER series WS-KT can be twice as wide as the corresponding pelletizer in the strand intake area. For machine WS-KT 600, this results in an initial width of 1200 mm with the possibility of running 120 strands. This in turn also means that the system WS-KT can run at about twice the throughput rate of a WS. As a result, much smaller pelletizers can be used for a given throughput rate, with lower costs for purchasing and spare part stocking. Design of a WS-KT water trough for size 600 Width in the intake area: 1200 mm Parallel length of the section 1200 mm wide: 1.5 to 3 m Tapered constriction of the water trough Taper angle 3 to 7° Length of the Tapered section 2 to 3 m Width in the strand discharge area: 600 mm Length of the strand discharge area: 0.5 m

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Applications for system WS-KT All products requiring a cooling length of min. 5 m or that can be processed with a cooling length of 5 m, e.g. PC, PET, PBT, PB, ABS and PP. Unsuited products: GPPS, SAN, PMMA, PA 6. The tapered water trough is available for wet and dry cut pelletizing in pelletizer widths of 300 to 1000 mm. Throughput rates of up to 10000kg/h are possible for dry cut and up to 20000 kg/h for wet cut. Start-up mode The start-up and shutdown device is of eminent importance for operating an automatic strand pelletizing machine. A differentiation is made between inserting and taking off devices. In the inserting system, at the start-up phase the strands fall past the water trough after being cut off and are either inserted into the trough by an adjustable section, or the whole trough is moved under the extrusion nozzle. In the taking-off system the strands are diverted to the floor. In the start-up phase after being cut off, they are guided into the water trough by an adjustable plate. (fig. 19)

The advantage of the inserting chute is that the strands fall freely on the floor in the non-operating phase. The taking off chute protects the cooling trough from uncontrolled splashing polymer in the event of a gas outbreak, generates a partially cooled tangle of strands instead of an cooled start-up cake (in highly gassing products, the partially cooled tangle of strands generates far fewer emissions than the un-cooled start-up cake) and when sufficient space is available, allows for the use of a wet cutting mill for direct processing of the taken off strands. SCHEER provides both techniques. When it comes to cutting the strands, a differentiation is made between cutting in the direction of strand flow and transverse to the direction of strand flow. When cutting in the direction of strand flow, all strands are cut through at the same time. This produces a thick start-up piece that can cause intake and cutting problems in the pelletizer. When cutting transverse to the direction of strand flow, the strands are cut through successively resulting in a start-up piece resembling panpipes, which can be taken into the pelletizer and cut without any problems. (fig. 20) SCHEER only uses systems cutting

Fig. 19: Diagram to show a start-up device, taking-off system

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transverse to the direction of strand flow to guarantee safe and reliable start-up of the system.

Component design: SCHEER produces a separate system component for every stage of the procedure: the water trough for strand transport and cooling, a suction/blowing nozzle combination with integrated pair of take-off rolls for strand drying, a transport belt for strand conditioning and a pelletizer for strand cutting. The listed components can be combined together and form a modular system that can be used to create different pelletizing systems. Execution details The SCHEER water troughs are fully adjustable horizontally and vertically during operation. This means that the water trough can be adjusted to many different operating conditions for ideal introduction of the strands into the trough. As explained in the section "dry cut", the strands need a longer dwelling time under the blowing/suction nozzles to achieve good drying values. The longer drying sections needed for this purpose cannot be overcome merely with flexible strands at the start or, in the event of the strand breaking, with the transport force of the water alone. SCHEER is the only manufacturer to produce a drying section with integrated pre-takeoff. The rolls of the pre-take-off and the feed rolls of the pelletizer are the same. Automatic strand feed with belts. The large majority of polymers being pelletized can be processed in water trough pelletizing systems. Special products such as high fill plastics (70-80% filling), plastics with extremely heavy fillings, very high viscosity plastics with extreme swelling behavior and very low viscosity plastics (under 800 mPa s) cannot be transported adequately in a trough. For such applications, SCHEER offers a belt drying system that can be used together with the other components of the SCHEER modular system instead of the water trough.

Fig. 20: Diagram to show the start-up device, working transverse to the direction of strand flow