WO2009063287A2

WO2009063287A2 - Rotary pellet press

Info

Publication number: WO2009063287A2
Application number: PCT/IB2008/003035
Authority: WO
Inventors: Helmut Wagner; Bill Reschke
Original assignee: Hibar Systems Ltd.
Priority date: 2007-11-14
Filing date: 2008-11-06
Publication date: 2009-05-22
Also published as: WO2009063287A8; WO2009063287A3; HU0700731D0

Abstract

A rotary pellet press having an upper turret, a die turret and a lower turret, these turrets are guided for rotational movement around a common axis, a plurality of press tool assemblies, guided in axial direction and attached to the turrets so as to be evenly spaced in angular direction, and each press tool assembly comprises: - an upper press tool assembly guided in the upper turret, - a die guided in the die turret, and -a lower press tool assembly guided in the lower turret, wherein the pellet press further comprises: - a stationary upper cam arranged above the upper turret with a cam path defining the axial movement of the upper tool assemblies, and - a stationary lower cam arranged under the lower turret with a cam path defining the axial movement of the lower tool assemblies, the upper and lower cams define discrete angular sections where (a) filling, (b) pre- pressing, (c) final pressing, (d) pellet ejection and (e) pellet ejection complete operations are carried out, - a load cam assembly having a lower end that holds a load cam which is angularly arranged in the final pressing section above the upper press tool assembly and in that section it replaces the upper cam, the load cam is guided in axial direction and the load cam assembly generates and exerts a required axial pressure on the load cam and on the upper press tool assembly when being moved underneath, each of the upper press tool assemblies comprises at least one cam follower rolling along the cam path of the upper cam and of the load cam in the final pressing section, and each of the lower press tool assemblies comprises at least one cam follower rolling along the cam path of the lower cam.

Description

Rotary Pellet Press

Field of the Invention

The invention relates to a rotary pellet press, especially for preparing cathode pellets for high speed manufacturing of alkaline batteries.

The rotary pellet press is of the type which comprises:

- an upper turret,

- a die turret and - a lower turret, these turrets are spaced from each other, they have a common axis and being guided for rotational movement around the common axis,

- a plurality of press tool assemblies, all guided in axial direction and attached to the turrets so as to be evenly spaced in angular direction around the periphery of the turrets, wherein each of the press tool assemblies comprises: - an upper press tool assembly guided in the upper turret,

- a die guided in the die turret, and -a lower press tool assembly guided in the lower turret.

Background of the invention

Cylindrical alkaline batteries are widely used. These batteries can be manufactured at fairly low cost due to the lower cost of the raw materials used and the cell construction, which can be highly automated to allow for a low cost manufacturing process with minimal manpower requirements. Increasing manufacturing speeds ensure that manufacturing remains competitive in countries with higher labor rates.

Rotary presses, which are mostly used in the pharmaceutical industry for tablet and pill pressing from powder material, have been traditionally also used for alkaline battery cathode pellet pressing. For example, US Patent No. 4,057,381 describes a rotary press with a rotary driven matrix table for holding matrixes with boreholes, top and bottom dies and means of compressing moldable material inside the die with at least one profiled press roller. In US Patent No 4,793,791 a rotary powder compression molding apparatus is disclosed with a special powder trapping provision for leaking powder from the mortars to prolong continuous run times before the press has to be cleaned. US Patent No. 6,1 16,889 describes a rotary pellet press with improved absorption of the forces introduced into the pressure roller unit to result in a low-vibration and low-noise press, which is accomplished with elastic mounts.

In US patent 5,211,964 a rotary press is described used for pressing cylindrical nuclear fuel pellets, wherein movement of upper and lower punches is determined by upper and lower cams, however, in the press position respective pneumatic devices are pushing respective rollers, which exert a punching force onto upper and lower punches advanced into the punching positions between rollers resulting in a very high compression force for final pellet dimensions. Here, the displacement of both the upper and lower pressing rollers is measured, and the weight and dimension of the pressed pellets is also measured, whereby a control is provided to ensure the manufacture of uniform pellets.

While this is an advanced system compared to earlier rotary presses, the machinery disclosed is inappropriate for high output (over 100-200 pellets per minute) pellet production and would be too costly for production of low cost alkaline batteries. Also, the final pressing forces are exerted during a very short angular range of the rotation, when two opposing rollers get into contact, and the maximum pressure will therefore be momentary, while for manufacturing other pellets like those of the cathodes of alkaline battery cells optimum pressing requires that the pressing force being maintained through a longer period of time.

While there are numbers of rotary presses available, increasing the manufacturing speed of alkaline battery production above a few hundred pellets per minutes has its unique problems and requires equipment that can operate with high efficiencies to avoid jams and downtime. Cathode pellets for alkaline batteries are difficult to produce due to unique characteristics of cathode powder granulate such as abrasiveness, moisture contents, stickiness, compressibility and lubricity. A problem lies in that the powder from which the pellets are pressed not precisely uniform, i.e. particle size, size-distribution and humidity can vary. Such variations may affect the pressing quality and the rigidity and size of the pellets made. Tubular battery cathode pellets require very high pressing forces in the range of 2000 - 4000 kg/cm² on the pellet to form good, strong pellets. Further, for high speed production systems cathode pellets require increased pellet strength to avoid pellet breakage and downtimes. Still further, the rotary press should have a manufacturability at a lower price point compatible for the production of alkaline batteries.

Therefore, there is still a need for improved pellet presses that can operate at high speed with maximum efficiencies and are capable of running difficult to process powder granulate compositions for alkaline batteries.

Object of the Invention

The primary object of the invention is to provide for a rotary pellet press capable of operating at high speeds (above a few hundred pellets per minute) with high efficiencies and uniform pellet quality. A further object is to provide for an efficient control of the pressing parameters that ensures uniform pellet properties even if the characteristics of the granulate powder slightly vary in time.

Summary of the invention

According to the present invention a rotary pellet press has been provided which comprises:

- an upper turret, - a die turret and

- a lower turret, these turrets are spaced from each other, they have a common axis and being guided for rotational movement around the common axis,

- a plurality of press tool assemblies, all guided in axial direction and attached to the turrets so as to be evenly spaced in angular direction around the periphery of the turrets, wherein each of the press tool assemblies comprises:

- an upper press tool assembly guided in the upper turret,

- a die guided in the die turret, and

-a lower press tool assembly guided in the lower turret, wherein according to the invention the pellet press further comprises: - a stationary upper cam arranged above the upper turret with a cam path defining the axial movement of the upper tool assemblies, and -A-

- a stationary lower cam arranged under the lower turret with a cam path defining the axial movement of the lower tool assemblies, the upper and lower cams define discrete angular sections where (a) filling, (b) pre- pressing, (c) final pressing, (d) pellet ejection and (e) pellet ejection complete operations are carried out,

- a load cam assembly having a lower end that holds a load cam which is angularly arranged in the final pressing section above the upper press tool assembly and in that section it replaces the upper cam, the load cam is guided in axial direction and the load cam assembly generates and exerts a required axial pressure on the load cam and on the upper press tool assembly when being moved underneath, each of the upper press tool assemblies comprises at least one cam follower rolling along the cam path of the upper cam and of the load cam in the final pressing section, and each of the lower press tool assemblies comprises at least one cam follower rolling along the cam path of the lower cam.

A preferable embodiment comprises a weigh cam assembly arranged under the lower press tool assembly when being in angular position corresponding to the filling section, the weigh cam assembly holds a weigh cam that replaces the lower cam in the filling section and defines the vertical position of the lower press tool assembly, the weigh cam is guided for axial displacement, and the weigh cam assembly comprises a vertical position adjusting means and a vertical position sensor for positioning the lower press tool assembly to finely adjust the weight of the pellet.

In a preferable embodiment the load cam assembly comprises a hydraulic piston and a position sensor sensing the axial position of the load cam, and the load cam assembly generates the required pressure by hydraulic control and maintains a predetermined position of the load cam.

In an alternative embodiment the load cam assembly comprises an indirect hydraulic drive acting through a spring-biased plate connected to the load cam, the load cam assembly comprises a load cell and/or a position sensor to enable adjustment of either a predetermined pressure acting on the load cam or a predetermined displacement of the load cam. Uniform pellets will be obtained if the position information of the load cam assembly controls the weigh cam assembly to correlate the required pressure profile in the final pressing section with corresponding pellet weight.

It is furthermore preferable, if the cam followers both of the upper tool assembly and lower tool assembly are rollers extending out in lateral direction from the associated assembly and guided along the upper and lower cam paths, respectively.

The guided movement can be realized if the upper tool assembly and lower tool assembly both comprise additionally a respective yoke assembly with a roller means pivoted around a shaft normal to the axis of the assembly and arranged centrally at the free end of the associated assembly, the upper roller means rolls along the load cam, and the lower roller means rolls along the weigh cam.

The pellet manufacturing rate can be increased if the upper and lower cams with the associated operational sections are arranged periodically around the turrets in such a way that the number of repetitions is a small integer, preferably 2, 3 or 4, and the number of the press tool assemblies cannot be divided with this integer number.

The pellet quality increases and the maximum pressure can be decreased if in the final pressing section the profile of the load cam and the lower cam define a substantially constant pressure by keeping the same distance between the two ends of the assemblies.

For appropriate weight control in the filling sections, the upper and weigh cams have profiles which make the die fill until a predetermined depth with a granulated powder material from which the pellet will be made, then raises the lower press tool assembly by a predetermined extent to remove any superfluous amount of the powder from the die.

The most preferable field of use is in case when the pellets are hollow cylindrical pellets used for cathodes of cylindrical alkaline cells. By means of the rotary pellet press according to the invention uniform pellets can be made in high production rates and with minimum cost on the equipment, wherein wear and abrasion is low and there is no or only reduced danger of vibration and noise.

Description of the drawings

The invention will now be described in connection with preferable embodiments thereof, wherein reference will be made to the accompanying drawings. In the drawing: Fig. 1 shows a 45° vertical section through the die turret of the rotary press;

Fig. 2 shows the stages of forming a pellet during a press cycle;

Fig. 2a shows an enlarged view of the pellet forming stages;

Fig. 3 shows a section view of lower and upper tooling assembly in stage e of Fig 2; Fig. 4 shows the weigh cam assembly in the filling position;

Fig. 5 shows the load cam assembly in the pressing position;

Fig. 6 shows the direct hydraulic cylinder assembly in the pressing position;

Fig. 7 shows the top view of the rotary press with the press tool assemblies and two load cam assemblies at 90°and 270° positions; Fig. 8 illustrates a flow chart for constant height mode operation; and

Fig. 9 illustrates a flow chart for constant pressure mode operation.

Description of the preferred embodiments

The rotary pellet press shown in elevation sectional view in Fig. 1 and top vie in Fig. 7 comprises a gear driven frame-fixed shaft assembly 1 , a lower turret 2, which is bolted to the shaft assembly 1, a die turret 3, and an upper turret 4, which is bolted to the top portion of the shaft assembly 1. The turrets 2, 3 and 4 are fixed together by means of locating and tie bars 5 and rotate around the vertical axis. The die turret 3 holds a number of dies 6 depending on the output required that are arranged equidistant around its 360°. The lower turret 2 holds a number of lower press tool assemblies 7 and the upper turret 4 holds a number of upper press tool assemblies 8. As shown in detail in Fig. 3, each lower press tool assembly 7 comprises a lower punch 9, a lower punch holder 10, a lower bushing 1 1, a core rod 12 and a core rod support pin 13, which fixes the core rod 12 in position. Each upper press tool assembly 8 comprises an upper punch holder 14, an upper bushing 15 and an upper punch 16. The upper and lower press tool assemblies 7 and 8 are illustrated in an enlarged section view in Fig. 3. During the rotation of the turrets 2, 3, 4 these press tool assemblies 7, 8 are guided in axial direction and move towards and away from each other and press cathode granulate in the boreholes of the dies 6. The vertical movement of the lower and upper press tool assemblies 7, 8 is facilitated by means of respective cam followers 17, 18, which follow the profile of stationary lower and upper cams 19, 20. Auxiliary cam followers 17a and 18a are arranged on the assemblies for constituting anti-rotation guides to prevent the press tool assemblies 7, 9 from rotating around theirs own axes. Each press tool assembly 7, 8 has a respective yoke assembly 21 installed at the extreme end thereof, which acts (as it will be explained in detail later) as an individual pressure roller at the pellet pressing stage. FIGs. 2 and 2a show the stages of forming a pellet 100 during a press cycle that takes place during a 180° rotation of the turrets 2-4, including the following cycles: (a) filling, (b) pre-pressing, (c) final pressing, (d) pellet ejection and (e) pellet ejection complete. The term "pellet" is used in this specification to describe the article obtained by pressing cathode granulate powder 100a during the aforementioned cycles, that has an outer diameter, an inner diameter for the hollow center and a height. In the filling cycle (a), as shown in the enlarged views of Fig. 2a, the cathode granulate powder 100a is introduced from a stationary fill frame (not shown) located above the die turret 3 with a small gap. As the turret 3 turns towards position (a), the empty die 6 travels under the fill frame. As it passes under the fill frame, the lower punch 9 is moved downwards guided by cam profile 19a to the filling position and the cathode granulate powder 100a will fall into the void space of the die 6. The core rod 12 is fixed in position by the core rod support pin 13 and it essentially flushes with the top of the die 6 during the whole pressing cycle. After the die 6 has been filled with cathode granulate powder 100a, the lower punch 9 is moved upwards slightly to push some granulate powder 100a back out of the die 6 and a scraper (not shown) removes the excess granulate from the top surface of die 6. The amount that the lower punch 9 moves up is determined by means of a weigh cam assembly 22 as illustrated in FIG. 4. The assembly 22 has obtained its name by its function, i.e. it determines the amount of the granulate powder 1 Ia used for making the pellet 100, and thus it adjusts the weight of the pellet 100. The weigh cam assembly 22 is a stationary device, and the lower press tool assembly arrives in the position shown in Fig. 4 in a portion of the filling cycle (a) when the granulate powder 100a has been filled in the die and it determines the extent of the aforementioned slight upward motion of the lower punch 9. Of course, the path of the stationary lower cam 19 is interrupted in the angular section where the weigh cam assembly 22 is arranged, and the vertical position of the lower punch 9 is defined in this angular section by the weigh cam assembly 22. The weigh cam assembly 22 has an electric motor 23, a screw drive 24, a spline shaft

25 and a profiled weigh cam 26 bolted to the spline shaft 25. A proximity sensor 27 is provided for feedback of the distances traveled by the screw drive 24. By adjusting the position of the weigh cam 26, the final amount of granulate powder 100a that is to be

5 pressed into a pellet 100 can be controlled. Raising the vertical position of the weigh cam

26 will cause more granulate powder 100a to be pushed back out of die 6 and the fill amount of the granulate powder 100a that will be pressed into a pellet 100 will be reduced. Conversely, lowering the weigh cam 26 position down will cause an increase in granulate powder 100a retained in the die 6 and the fill amount of the granulate powder

10 100a that will be pressed into a pellet 100 will be increased. Since an electric motor drive is provided in the weigh cam assembly 22, adjustment of the fill volume to achieve the target weight for the cathode pellets can be automated by means of suitable control software.

Referring now again to Figs. 2 and 2a, after the filling cycle (a), the lower punch 9

15 moves an amount downward to lower the granulate powder 100a level when the upper punch 16 enters the die 6. In the pre-pressing cycle (b), the lower and upper punches 9, 16 are moved towards each other by means of a suitable profile of lower and upper cams 19, 20 and 20a. These profiles have been schematically illustrated in Fig. 2. The granulate powder 100a is formed in this pre pressing cycle (b) into a slightly compressed

20 pellet and the entrapped air is removed through the slight gaps that are present from the clearances of the tooling. In the final pressing cycle (c), the lower and upper punches 9, 16 are driven towards each other. In this cycle (c) the upper cam 20 and 20a is interrupted and a load cam 29 arranged at the lower end of a load cam assembly is arranged above the upper tool assembly 8 to generate pressing forces of several thousand

25 pounds that compress the cathode granulate powder 100a trapped in the dies 6 at the final pressing stage. Again, the entrapped air is removed through the slight gaps that are present from the clearances of the tooling. The travel of the lower punch 9 is determined by the fixed cam 19. The pressing forces can be generated by different means.

In FIG. 5, a spring loaded load cam assembly is shown, which controls in the press

30 cycle (c) the travel of the upper punch 16. The load cam 29 that forces the upper tool assembly 8 down is spring loaded at this pressing position. Compression spring 35 is preloaded with a predetermined force in order to exert a corresponding known pressure on the cathode granulate powder 100a to form the pellet 100. The known force is generated by means of hydraulic pistons 36, which compress the compression spring 35. The force on the compression spring 35 can be regulated by a pressure gauge (not shown). This arrangement is also called an indirect hydraulic pressure assembly, as the hydraulic pressure is exerted onto the compression spring 35, rather then the load cam 29 directly.

In FIG. 6, an alternative embodiment to that shown in Fig. 5 is illustrated, this is a direct hydraulic loaded load cam assembly, which controls the travel of the upper punch

16. Instead of the load cam 29, a similarly profiled load cam 39 that forces the upper tool assembly 8 down is loaded by means of a direct hydraulic pressure at the pressing position. Direct hydraulic piston 41 is preloaded with a predetermined force in order to exert a known pressure on the cathode granulate powder 100a to form the pellet 100. The known force is generated by means of the hydraulic piston 41, which can be regulated by a pressure gauge (not shown). This arrangement is called a direct hydraulic pressure assembly, as the hydraulic pressure is exerted directly onto the load cam 39.

In both load embodiments designed according to FIGs. 5 and 6, a dwell period is required at the final pressing position to allow air to release and to obtain constant pressing. This is achieved by means of a suitable profile on the cams 19 and 29 or 39 to provide a flat pressing period. The cam followers 17, 18 facilitate the vertically guided movement of the lower and upper punch holders 9, 16.

In the pellet ejection cycles (d) and (e), the pressed pellets are ejected by means of the lower punches 6, which are moved upwards by means of the cam followers 17 and the suitable cam profiles of the cams 19, 20 to eject the pellets. The ejection forces are high in the first portion of the ejection travel and reduced due to slight tapers on the dies 6 and the core rods 12. When the lower punch 6 will be flushed with the top of the die 6, the pellets are stripped and conveyed away from the press to the next step in the alkaline cell production process, which is pellet insertion into battery cans.

The design of the rotary press according to the present invention allows for an automatic, software controlled operation with adjustments being made based on measuring the deflection of the load cam, which is an indirect measure of pressing force.

The regulation made in the pressing cycle (c) will now be described referring again to

FIGs. 5 and 6. Load cam assembly 28 of Fig. 5 comprises the load cam 29, a spline shaft 30, a coupling connection 31, a load cell 32, a bottom spring plate 33, a top spring plate 34, the compression springs 35, the hydraulic pistons 36 and fittings 36a for hydraulic fluid inlet. A proximity sensor 37 is provided for feedback of the deflection of the bottom plate 33 in the final pressing position, which is an indirect measure of the applied pressing force. In this embodiment, the actual force exerted on the granulate powder 100a is measured by means of the load cell 32, which is mounted in line with the load cam 29. Under normal operation, the compression spring 35 will deflect slightly when the final pressing is performed. Therefore, two control parameters, the deflection as measured by the sensor 37 and the force measured by the load cell 32 are available for a software controlled operation of the press, which allows for automatic adjustments to control and achieve the desired target values.

Software controls can adjust two parameters: (1) the granulate powder fill level in cycle (a) and (2) the pressing force applied in the cycle (c). Therefore, during operation of the rotary pellet press and after suitable calibration routines, the rotary pellet press can run in automatic mode, wherein these parameters are adjusted automatically to achieve target values or to keep these parameters within predetermined target ranges. The adjustment is required because the granulate size of the granulate powders used, may change in time, or other parameters, like humidity, composition, temperature, etc. of the granulate powder can also change gradually during a longer manufacturing period. If no regulation was used, the final pellet dimensions and weight would not be consistently the same.

The rotary pellet press with the load cam assembly 28 can be operated in one of two modes: ( 1 ) a constant height mode and (2) a constant pressure mode. For the constant height mode, the load cam pressure is adjusted so that normally no deflection on the sensor 37 occurs except in case of an overload. The pressing forces on the load cam 29 are measured by means of the load cell 32, and in automatic run mode the weigh cam 26 is continuously adjusted to maintain the pressing forces in the target range. The target range has an upper and lower limit, which can be defined in the control software. Pressing force measurements are performed on each tool, but the average of all tools for one revolution is typically computed to test against the target range. In a preferable embodiment thirty five tool pairs are used in the rotary pellet press. The number of measurements taken for the average is defined in the control software and can be changed as required, but is preferably between 35 and 105. If the average pressing force measurement falls within the predetermined range, no adjustments to the weigh cam position is done. If the average pressing force measurement falls outside of these limits, the control software will automatically make small adjustments to the position of the weigh cam 28 by means of the electric drive 24. If the average pressing force measured is too high and it exceeds the upper limit, the weigh cam 28 will be moved upwards slightly to reduce the fill level of the granulate powder 100a in the die 6, which will reduce the pressing force. Incremental adjustments are being done until the average measured pressing force falls within the expected range again. If the average pressing force measured is below the lower limit, the weigh cam 28 will be moved downwards slightly to increase the fill level of the granulate powder 100a in the die 6, which will increase the pressing force. Incremental adjustments are being done until the average measured pressing force falls within the expected range again. Fig. 8 illustrates this constant height mode operation in a logic flow chart. In the constant pressure mode the pressing forces are adjusted to a target value and the load cam 29 is allowed to deflect to provide essentially constant pressing forces. The deflection is measured by means of the sensor 37, and in automatic run mode the weigh cam 26 is adjusted to maintain the same deflection range thereby keeping the pellet height within a target range. The deflection target range has an upper and lower limit, which can be defined in the control software. Deflection measurements are performed on each tool, but the average of all 35 tools for one revolution is typically computed to test against the target range. The number of measurements taken for the average is defined in the control software and can be changed as required, but is preferably between 35 and 105. If the average deflection measurement falls within these limits, no adjustments to the weigh cam position is done. If the average deflection measurement falls outside of these limits, the control software will automatically make small adjustments to the position of the weigh cam 26, by means of the electric drive 24. If the average deflection measured is too high and over the upper limit, the weigh cam 26 will be moved upwards slightly to reduce the fill level of the granulate powder 100a in the die 6, which will reduce the deflection. Incremental adjustments are being done until the average measured deflection falls within the expected range again. If the average deflection measured is below the lower limit, the weigh cam 26 will be moved downwards slightly to increase the fill level of the granulate powder 100a in the die 6, which will increase the deflection. Incremental adjustments are being done until the average measured deflection falls within the expected range again. Fig. 9 illustrates this constant pressure mode operation in a logic flow chart. In the alternate embodiment of a direct hydraulic load assembly 38 shown in Fig. 6 comprises the cam 39, a spline shaft 40 the hydraulic piston 41 and hydraulic fluid inlet ports 41a. A sensor 42 is provided for feedback of the deflection of the spline shaft 40 in the final pressing position, which is an indirect measure of the applied pressing force. The hydraulic piston 41 is preloaded with a predetermined force. The rotary pellet press with direct hydraulic load assembly 38 can only be operated in a constant pressure mode ' as this configuration always requires a deflection for applying the pressing force. The constant pressure mode requires that the load cam 39 deflects to ensure that pressure is applied. The pressing forces are adjusted to a target value and the deflection is measured by means of the sensor 42. In automatic run mode the weigh cam 26 is adjusted to maintain the same deflection range thereby keeping the pellet height within a target range. The deflection target range has an upper and lower limit, which can be defined in the control software. Deflection measurements are performed on each tool, but the average of all 35 tools for one revolution is typically computed to test against the target range. The number of measurements taken for the average is defined in the control software and can be changed as required, but is preferably between 35 and 105. As long as the average deflection measurement falls within these limits, no adjustments to the weigh cam position is done. If the average deflection measurement falls outside of these limits, the control software will automatically make small adjustments to the position of the weigh cam 26, by means of the electric drive 24. If the average deflection measured is too high and over the upper limit, the weigh cam 26 will be moved upwards slightly to reduce the fill level of the granulate powder 100a in the die 6, which will reduce the deflection. Incremental adjustments are being done until the average measured deflection falls within the expected range again. If the average deflection measured is below the lower limit, the weigh cam 26 will be moved downwards slightly to increase the fill level of the granulate powder 100a in the die 6, which will increase the deflection. Incremental adjustments are being done until the average measured deflection falls within the expected range again. Fig. 9 illustrates this constant pressure mode operation in a logic flow chart.

The advantages of the rotary pellet press of the present invention are the longer dwell period that can be achieved in the final pressing position with suitable cam profiles; cams provide a more cost effective design over pressure wheel designs; cams avoid the lateral sliding forces on conventional mushroom punches while sliding in an arc over the pressure wheels; the cam profile design avoids various transition plates and allows a more compact arrangement to perform the pressing and the ejection operations; the cam followers avoid the sliding friction encountered with prior art mushroom punches and provides smoother running of the pellet press. The most significant advantages come from the segmentation of the cam profile, wherein in the pressing position (c) a controlled load cam 29 or 39 is used and in filling cycle (a) the weigh of the granulate powder 100a can be changed, allowing thereby a pellet- weigh and pressure regulation that provides highly uniform pellets even in case of using granulate powders with slightly varying properties.

In a preferable embodiment, the pellet press has thirty five press tool assemblies 7, 8 installed around the circumference of the turrets, with two pressing positions per revolution. This press provides e.g. a pellet production output rate of 1260 pellets per minute at a die turret speed of 18 revolutions per minute. Normally, three pellets are assembled into one alkaline cell cathode; therefore, one such pellet press can support an alkaline cell production rate of up to a maximum of 420 cells per minute. For higher speeds, two such pellet presses are operated in parallel for up to a maximum of 860 cells per minute. For even higher speeds, the diameter of the main turret has to be increased to accommodate more press tool assemblies 7, 8. The number of the press tool assemblies 7, 8 installed on the main turret is always an uneven number to prevent that two tool assemblies are in the pressing position at the same time for presses with 2 pressing positions for one revolution. This is illustrated in Fig. 7, which shows that the two pressing position are provided at 90° and 270°, respectively, using two of the direct hydraulic load assemblies 38. The angular length of the pressing cycle (c) is smaller than the angular distance between two neighboring assemblies. In Fig. 7 in the first half circle the individual cycles (a) to (e) of Figs 2 and 2a have been shown. Similarly, for a pellet press with three pressing positions per revolution, the number of the tool assemblies should not be dividable by 3, to prevent simultaneous pressing.

To demonstrate the effect of the automatic control adjustments of the pellet press of the present invention, the rotary pellet press with 35 press tool assemblies 7, 8 installed on the press and two pressing positions per revolution was subjected to a number of experimental test runs. Cathode granulate powder for these experiments has been produced in a conventional manner by blending the following cathode raw materials: 89.6 wt. -% EMD (electrolytic manganese dioxide), 6.5 wt.-% graphite, 3.4 wt.-% aqueous potassium hydroxide solution (36.9wt.-% KOH) and 0.5 wt.-% polyethylene powder as binder in a suitable blender followed by roller compaction of the blended cathode powder mix in a standard cathode powder compactor HB910 available from Hibar Systems Limited. The resulting cathode granulate powder was measured by sieve analysis and had a particle size distribution of 0.41% on Mesh 16 (1.18mm opening), 13.70% on Mesh 20 (850μm opening), 52.39% on Mesh 40 (425μm opening), 20.08% on Mesh 60 (250μm opening), 8.32% on Mesh 100 (150μm opening) and 5.07% in the pan (less than 100 Mesh particles). The moisture content of the cathode granulate powder used in the experiments ranged from 2.5% to 3% as measured by a standard moisture balance (i.e. Sartorius) for a 1Og granulate sample at 150°C until constant weight is maintained. In the first experiment, the rotary pellet press with the load cam assembly 28 was setup to run at a nominal speed of 900 pellets per minutes or 12.8 revolutions per minute. The nominal pellet height target was set at 0.565 inches and the nominal weight target was 3.45 grams. A pressing force of 3300 pounds was required to form a pellet that met the nominal target values. The surface area of the pellet in the tool assembly was 0.742 cm². (die inner diameter 0.522 inches, core rod outer diameter 0.355 inches), therefore, the applied force to form the pellet was 4447 pounds/cm² or 2022 kg/cm². Fifty (50) random pellet samples were taken for pellet height and weight measurements. Next, without any other changes, the speed of the rotary press was reduced to run at a lower speed of only 6 revolutions per minute (rpm) and 50 random pellet samples were taken for pellet height and weight measurements. Next, without any other changes, the speed of the rotary press was increased to run at the speed of 18 revolutions per minute, and 50 random pellet samples were taken for pellet height and weight measurements. Table 1 shows the results of this experiment. The coefficient of variance for the 50 pellet samples was calculated by the standard deviation of the samples divided by the sample average and expressed in percent.

TABLE l

Press Average Pellet Average Pellet Coefficient of Variance for

Speed, rpm Weight, g Height, inches Weight Measurement

6 3.509 0.574 0.73% 12.8 3.435 0.566 0.76% 18 3.419 0.566 0.75%

As can be seen from Table 1, without changing any other parameter than pellet press speed, the pellet weight increases with lower speed and decreases with higher speed.

In the second experiment, the same lower and higher press speeds were evaluated, but this time, the press was allowed to automatically adjust itself to meet targets by means of built in software controls in constant height mode at a constant pressure force of 3300 pounds. Table 2 shows the results of this experiment.

TABLE 2

Press Average Pellet Average Pellet Coefficient of Variance for Speed, rpm Weight, g Height, inches Weight Measurement

6 3.449 0.569 0.70%

12.8 3.436 0.568 0.76%

18 3.461 0.571 1.14%

As can be seen from Table 2, the auto-adjust feature of the pellet press allows for maintaining better weight control of pellets over a wide speed range of the pellet press. It also shows that at the high speed setting of 18 rpm, the coefficient of variance was slightly increased and the pellet height is slightly higher.

In the third experiment, only the high press speed of 18 rpm was evaluated, but this time, the press was allowed to automatically adjust itself to meet targets by means of built in software controls including automatic pressing force adjustments in the range of

3300 to 5000 pounds, which calculates to a force 2022 to 3063 kg/cm². Table 3 shows the results of this experiment.

TABLE 3

Press Average Pellet Average Pellet Coefficient of Variance for

Speed, rpm Weight, g Height, inches Weight Measurement

18 3.444 0.567 0.73% As can be seen from Table 3, in full auto-adjust mode of the pellet press, even at maximum pellet press speed, an excellent pellet weight and height control can be achieved. It should be noted that for this experiments the pellet press had to be stopped to be able to take samples. The samples were taken after a few minutes at the set speed to allow for the auto-adjustments to take effect. If the pellet press runs in continuous mode, the coefficient of variance will be even better than measured in this experiment and lower than 0.5% was achieved in continuous production mode.

In the fourth experiment, the rotary pellet press had a direct hydraulic assembly 38 instead of the load cam assembly 28 and was setup to run at a nominal speed of 900 pellets per minutes or 12.8 revolutions per minute. The nominal pellet height target was set at 0.565 inches and the nominal weight target was 3.45 grams. Surprisingly, only a pressing force of 2100 pounds or 1286 kg/cm² was required to form a pellet that met the nominal target values with the direct hydraulic load. Compared to the indirect hydraulic load cam assembly 28, 1200 pounds less pressing force was required with the direct hydraulic assembly 38. It is believed that the lower pressing force needed is a result of less friction losses due to the direct pressure transfer. As a consequence, the lower pressing forces will yield a longer tool life and less maintenance, which will increase production yield with this embodiment. Table 4 shows the results of this experiment. Data are the averages for 140 consecutive pellet samples for the pellet press running in auto-adjust mode.

TABLE 4

Press Average Pellet Average Pellet Coefficient of Variance for

Speed, rpm Weight, g Height, inches Weight Measurement

12.8 3.448 0.564 0.72%

As can be seen from Table 4, the pellet press achieved excellent pellet weight and height control with lower pressing forces applied.

In another embodiment, the pellet press has 53 press tool assemblies 7, 8 and will produce 1500 pellets per minute at 14.15 revolutions per minute with 2 pressing positions per revolution. This configuration can support the production rate of 500 cells per minute for 3 pellets per cell designs with a single press; no need to for tandem press operation to support the high production speeds. In yet another embodiment, the pellet press has 53 press tool assemblies 7, 8 and will produce 2160 pellets per minute at 13.58 revolutions per minute with 3 pressing positions per revolution. This configuration can support the production rate of 720 cells per minute for 3 pellets per cell designs with a single press; no need to for tandem press operation to support the high production speeds.

Claims

1. A rotary pellet press comprising,

- an upper turret, - a die turret and

- a lower turret, said turrets being spaced from each other, having a common axis and being guided for rotational movement around said axis,

- a plurality of press tool assemblies being guided in axial direction and attached to said turrets so as to be evenly spaced in angular direction around the periphery of said turrets, each of said press tool assembly comprising:

- an upper press tool assembly guided in said upper turret,

- a die guided in said die turret, and

-a lower press tool assembly guided in said lower turret, characterized by comprising, - a stationary upper cam arranged above said upper turret and having a cam path defining the axial movement of said upper tool assemblies, and

- a stationary lower cam arranged under said lower turret and having a cam path defining the axial movement of said lower tool assemblies, said upper and lower cams defining discrete angular sections where (a) filling, (b) pre-pressing, (c) final pressing, (d) pellet ejection and (e) pellet ejection complete operations being carried out,

- a load cam assembly having a lower end holding a load cam, said load cam being angularly arranged in said final pressing section above said upper press tool assembly and in said section replacing said upper cam, said load cam being guided in axial direction and said load cam assembly generating and exerting a required axial pressure on said load cam and on said upper press tool assembly when being moved underneath, each of said upper press tool assemblies comprises at least one cam follower rolling along said cam path of said upper cam and of said load cam in said final pressing section, and each of said lower press tool assemblies comprises at least one cam follower rolling along said cam path of said lower cam.

2. The rotary pellet press as claimed in claim 1, further comprising a weigh cam assembly arranged under said lower press tool assembly when being in angular position corresponding to said filling section, said weigh cam assembly holding a weigh cam replacing in said section said lower cam and defining vertical position of said lower press tool assembly, said weigh cam being guided for axial displacement and said weigh cam assembly comprising a vertical position adjusting means and a vertical position sensor for positioning said lower press tool assembly to finely adjust the weight of said pellet.

3. The rotary pellet press as claimed in claim 1, wherein said load cam assembly comprises a hydraulic piston and a position sensor sensing the axial position of said load cam, said load cam assembly generating said required pressure by hydraulic control and maintaining a predetermined position of said load cam.

4. The rotary pellet press as claimed in claim 1, wherein said load cam assembly comprises an indirect hydraulic drive acting through a spring-biased plate connected to said load cam, said load cam assembly comprises at least one of a load cell and a position sensor, to enable adjustment of either a predetermined pressure acting on said load cam or a predetermined displacement of said load cam.

5. The rotary pellet press as claimed in claim 2, wherein said position information of said load cam assembly controlling said weigh cam assembly to correlate required pressure profile in said final pressing section with corresponding pellet weight.

6. The rotary pellet press as claimed in claim 1, wherein said cam followers both of said upper tool assembly and lower tool assembly being rollers extending out in lateral direction from the associated assembly and guided along said upper and lower cam paths, respectively.

7. The rotary pellet press as claimed in claim 6, wherein said upper tool assembly and lower tool assembly both comprise additionally a respective yoke assembly with a roller means pivoted around a shaft normal to the axis of the assembly and arranged centrally at the free end of the associated assembly, the upper roller means rolling along said load cam, and the lower roller means rolling along said weigh cam.

8. The rotary pellet press as claimed in claim 1, wherein said upper and lower cams with the associated operational sections being arranged periodically around said turrets in such a way that the number of repetitions being a small integer, preferably 2, 3 or 4, and the number of said press tool assemblies cannot be divided with said integer number.

9. The rotary pellet press as claimed in claim 1, wherein in said final pressing section the profile of said load cam and said lower cam defining a substantially constant pressure by keeping the same distance between the two ends of said assemblies.

10. The rotary pellet press as claimed in claim 2, wherein in said filling sections, said upper and weigh cams having profiles making said die fill until a predetermined depth with a granulated powder material from which said pellet will be made, then raising said lower press tool assembly by a predetermined extent to remove any superfluous amount of said powder from said die.

1 1. The rotary pellet press as claimed in claim 2, wherein said pellets being hollow cylindrical pellets used for cathodes of cylindrical alkaline cells.