BACKGROUND OF THE INVENTION High intensity batch mixers are known and consist of a cylindrical processing chamber in which rotating blades mounted on a drive shaft are driven by an electric motor to rotate at tip speeds usually at least about 20 meters per second.

Such mixers are used to mix or blend small pieces or particles of material which may vary in size typically from powder to larger than 1/2 inch and which comprise or include particles of thermoplastic material. These thermoplastic particles are transformed under heat created by the particle collisions with each other and with the wall of the mixing chamber under action of the high speed blades from a solid state through a softening stage to a flowable melted or molten state. In the softening transition stage between the solid and molten states, such softening particles convert the initial free flowing particles into a viscous mass the degree of viscosity of which depends upon the nature of the material being mixed. After reaching the molten state, at which point the viscosity of the mixture falls off rapidly, the material is further heated to the desired temperature and then discharged from the cylindrical processing chamber and delivered to a further processing step usually in the form of a molding operation.

Since the heating occurs as a result of the high speed particle collisions, provided the motor has the power to maintain blade rotation, it will be understood that as the amount of material, (batch weight), fed into the

processing chamber is increased so too will the number of particle collisions be increased. As a result, the rate of heating of the material will be increased and the cycle time to bring the material to the discharge temperature will be correspondingly reduced. In this heating process, the power to drive the blades during the particle softening stage rapidly increases to a high peak demand just prior to the batch reaching the flowable molten state. During this peak power demand, the mixer is subjected to vibrations which at times can become severe. The magnitude of this peak power input demand is dependent on the batch weight and the viscosity characteristics of the thermoplastic particles being brought to the molten state. Once these particles have reached the melting temperature, then the power demand falls off sharply.

Unless the motor has the capacity or power to meet the increased resistance to the blades caused by the softening particles and maintain blade rotation during this peak energy demand period prior to the mixture reaching the molten state, it will stall out stopping the mixer.

Therefore, in order to avoid damage to the equipment which would be caused by the stalling of the motor, an AMP meter is usually installed to monitor the energy consumption of the motor at all times during the entire blending and heating cycle. The AMP meter is connected to a disconnecting switch and shuts off the power to the electric motor when the demand is above the capacity of the motor before the motor is actually physically stalled.

In the past, to avoid this problem where demand of a full charge of material would exceed the motor's capacity, the charge of plastic, material or the material formulation has been reduced so that the mixer operates significantly below its capacity. Because of the fewer particles in the chamber, the mixing cycle is increased because the particle collisions are reduced requiring a longer time for the heating effect to occur to convert the particles through the softening stage to the molten state. Because of the reduced volume in the mixing chamber, the resistance to blade rotation will be

lowered reducing the peak demand on the motor so that it can continue to drive the blades without stalling.

With the reduced charge in the mixing cylinder, not only is the cycle time increased but at the same time the volume of the mixed or blended batch is reduced. The combined effect of these two factors is to significantly reduce the capacity of the mixer.

On the other hand, if an electric motor of a sufficient horse power capacity to provide the torque necessary to mix a full charge of high viscosity materials without stalling under peak power demand were to be provided, the cost of the motor and the associated equipment to handle such a powerful motor would require a large capital outlay. In addition, the powering of such a large motor would require a large capacity power source to supply the necessary motor current. This problem would, of course, be magnified if a plant were to operate a number of these large motor mixers.

SUMMARY OF THE INVENTION The present invention is directed to overcoming the problems of the prior art and to provide a high intensity mixer which can handle a full charge of particulate thermoplastic containing material regardless of their viscosity characteristics to thereby maximize mixer capacity or output per hour without requiring the use of excessively high horse power motors.

More particularly, the invention resides in the discovery that a flywheel or an arrangement of flywheels fixed to the drive shaft of a high intensity mixer and driven at normal mixing speeds can be used to provide the energy necessary to maintain blade rotation against the resistance of the softening particle mixture during the critical high demand period immediately prior to the mixture reaching its flowable molten state without overloading the motor. In this way, the mixer can be used to full capacity with a minimum cycle time without requiring the use of a motor of excessive capacity.

Moreover, the use of the flywheel kinetic energy to meet the peak power demand has been found to significantly reduce mixer vibrations thereby extending the life of the mixer.

For optimum balancing of the mixer operation and further minimizing mixer vibration preferably a flywheel is fixed to the drive shaft on either side of the mixing or processing chamber.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side elevational view partially broken away of a high intensity mixer incorporating a pair of flywheels in accordance with a preferred embodiment of the invention.

Figure 2 is an end elevational view partially broken away looking from the right hand side of Figure 1.

Figure 3 is an elevational view of the discharge door operating mechanism with the discharge door moved to the open position and showing a cross section of the lower half of the mixer cylinder with the mixer beads omitted.

DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION High intensity mixer systems are primarily used for the mixing and compounding of different thermoplastic raw materials with which may be included organic and/or inorganic fillers, reinforcements, colorants, etc.

The number and geometry of the mixer's processing blades, the volume of its chamber and the sizing of its motor determine the output of a

given raw material or raw material combination. For example, when the processing chamber of a typical high intensity mixer having a capacity of 8000 grams and driven by a 350 horse power motor at 1800 revolutions per minute is empty, the mixer idles at a power consumption of 55 AMPS. When a charge of 8000 gram of high density polyethylene (HDPE) with a Melt Flow Index ("MFI") of 6 (that is, a medium viscosity, injection molding grade of thermoplastics) is fed into the chamber the power consumption of the motor instantly increases to about 200 AMPS.

After 10 to 15 seconds, the power consumption of the motor increases first slowly as the material in the chamber starts to increase in temperature. Thereafter the power consumption increases rapidly when the material starts to soften or flux at about 160 to 170 degrees C peaking at about 850 AMPS for 1 to 2 seconds (which is about the upper safe limit for the 350 horse power motor), before it starts dropping back.

At the peak power consumption of 850 AMPS, the shaft of the high intensity mixer was observed to be shaking and vibrating considerably, before stabilizing again. When the material finally started melting at about 180 to 190 degrees C, the intensity of the vibrations of the shaft as well as the power consumption of the motor was observed to drop and continued to drop further as the viscosity of the material decreased with increased melt temperature.

Finally the material was discharged at 350 AMP power consumption, resulting in a material temperature of 220 degrees C. The total cycle time for a single batch was approximately 22 seconds and - based on the upper limit of a 8000 gram charges for HDPE with a MFI of 6 - the total output of the high intensity mixer was approximately 1309 kg/hr.

When the same amount of 8000 gram of HDPE with a Melt Flow Index of 0.6 (high viscosity, "fractional melt" blow molding grade thermoplastic material) was fed into the chamber, it first showed the same instant amperage reading of 200 AMPS during the first 10 to 15 seconds.

However, as soon as the material reached its softening flux-temperature of 160 to 170 degrees C, the amperage consumption of the motor first jumped dramatically up to a value of 1200 AMPS. The mixer commenced shaking violently and then abruptly stopped, as the motor was stalled. This shut down of the motor occurred at 17 seconds into the batch cycle.

However, when only 3000 gram of such HDPE with a Melt Flow Index of 0.6 was fed into the same high intensity mixer with the same 350 horse power motor, its amperage reading first stood at 150 AMPS for 25 to 30 seconds before the temperature increased to the softening or flux temperature of 160 to 170 degrees, at which point the motor amperage increased to a final peak of 850 AMPS for 1 to 2 seconds.

The shaft of the high intensity mixer under this 3000 gram charge was observed to be shaking considerably at its peak amperage consumption, but the intensity dropped along with the drop of amperage consumption, while the material temperature was further increasing.

When the motor reached an amperage reading of 350 AMPS, the material was discharged at a corresponding temperature of 230 degrees C.

The total cycle time for this single 3000 gram charge was 38 seconds, and, - based on the upper limit of 3000 gram of HDPE with a MFI of 0.6,- the total output per hour was decreased to only 284 kg per hour.

As hereinafter more fully explained, it was discovered that with the addition of a flywheel arrangement fixed to the mixer drive shaft, the 350 horse power motor could process a full 8000 gram batch of HDPE with a Melt Flow Index of 0.6 increasing the mixer output by more than some 400 per cent.

Referring now to the drawings, Figure 1 illustrates a high intensity mixer generally designated at 1 mounted on a subframe 2 from which the cylindrical mixing chamber 3 and the driving electrical motor 4 are supported by a frame 5 and brackets 6 respectively.

The motor drive shaft 7 through a suitable coupling 8 extend axially through the cylindrical mixing or processing chamber 3 to drive the mixer blades 9 attached to the drive shaft 7 by means of a hub 10.

The shaft 7 is supported on either side of the mixer chamber 3 by means of suitable bearings 11 mounted on the frame 5.

The charge of thermoplastic particles or mixtures of thermoplastic particles and organic or inorganic fillers, reinforcements, colorants and other materials is introduced through a hopper 12 which delivers the material to a feed screw 13 which feeds the material through an annular mixing chamber inlet 14 surrounding the hub 10.

When the material has reached the final molten state, it is discharged through a discharge outlet 15, Figure 3, at the underside of the cylindrical chamber 3 upon the opening of a mechanically actuated door 16. This door 16 pivots on an axis 17 through a mechanical linkage comprising pivot arm 18 actuated by a hydraulic cylinder (not shown) linked to a crank arm 19 carrying the door 16 by a toggle arm 20.

Mounted on the shaft 7 on opposite sides of the cylindrical mixing chamber 3 are flywheels 21 enclosed in housings 22.

For cleaning the mixing or processing cylindrical chamber 3 may be formed of two half sections hinged together at 23, Figure 2.

As explained the output, in terms of kilograms per hour (kg/hr), of a high intensity mixer of a given geometry is essentially determined by the volume capacity a given processing or mixing cylindrical chamber can hold, the form and particle size in which raw materials are fed into the mixer, and the nature of the raw material itself. Provided always that the motor has the power to drive the mixer when loaded to capacity.

When a full charge of high viscosity materials such as semi- crystalline polyethylenes and polypropylenes are introduced into the mixing or processing chamber, eg. chamber 3 through the inlet 14 by the screw feed 13, the peak energy requirement during the softening process becomes exceedingly high for a period of about 1 or 2 seconds well beyond the capacity of the driving motor normally employed. However, in accordance with the invention it has been found that rather than having to decrease the input charge and thus the mixer output capacity this power gap between motor capacity and mixer energy demand may be supplied by flywheel kinetic energy through a flywheel arrangement attached to the drive shaft.

Further, in situations where the material has a lower viscosity and the motor can handle a full charge at a peak power consumption, it has been found that the use of a flywheel will allow the motor to operate at a lower peak power consumption saving motor wear and tear while at the same time eliminating or substantially eliminating mixer shaking and vibration. It will be understood that the mass and diameter of the flywheels will be selected according to the size of the motor, the size of the processing chamber and the nature of the material which is to be mixed or homogenized.

For example, using a high intensity mixer having a 8000 gram capacity processing chamber driven by a 350 horse power motor and attaching 600 pound flywheel with an outer diameter of 3 feet to the drive shaft, the idle ampere draw at 1800 revolutions per minute was about 67 AMPS compared to a draw of 55 AMPS at this idle speed without the flywheel.

When a full 8000 gram charge of high density polyethylene with a Melt Flow Index of 0.6, that is, a material having a high viscosity, was introduced into the mixing chamber, the amperage reading increased instantly to 260 AMPS. However, after 10 to 15 seconds when the material was reaching its softening or fluxing temperature of 160 to 170 degrees C the peak power consumption of the motor was only 750 AMPS and dropped with increased material temperature. When the material was finally discharged at a

amperage reading of 360 AMPS, the batch had reached a temperature of 230 degrees C. The cycle time was only 23 seconds giving an output capacity of 1252 kg/hr.

During even the peak energy consumption of 750 AMPS, the drive shaft showed only minor vibrations.

Without the flywheel and straining the motor to its maximum power consumption of 850 AMPS, the mixer could only handle a charge of 3000 grams of the same high density high viscosity polyethylene with a Melt Flow Index of 0.6. With this 3000 gram charge, the total cycle time was 38 seconds resulting in a total output of only 284 kg/hr.

Even at this low output, the mixer without the flywheel was subject to violent shaking and vibration.

While a single flywheel can be employed as described in the example given, preferably, for maximum balancing of the operation a flywheel is arranged on each side of the mixing chamber such as the flywheels 21 on each side of the mixing chamber 3 as illustrated in Figure 1. These flywheels 21 are selected, in the case of the example given, namely a mixer having an 8000 gram capacity driven by a 350 horse power motor, to provide the same kinetic energy to the drive shaft 7 as the single 600 lb. 3 foot diameter flywheel described above. As before, the flywheels 21 act to limit the current drawn by the motor during the peak demand period to a safe level under a full 8000 gram charge of high viscosity material in the mixing chamber as it reaches its softening or fluxing temperature of 160 to 170 degrees C.

Thereafter the power demand drops off as the charge reaches the melt temperature of about 180 to 190 degrees C on the way to the final discharge temperature of the order of about 230 degrees C.

It will be understood that the invention, through the use of flywheel kinetic energy to maintain blade rotation of a high intensity mixer during the

peak power demand of the mixing cycle, opens up a wide range of mixer designs not heretofore possible.

Thus, for instance in the case of a mixer with an 8000 gram capacity mixing chamber, the motor size can be reduced from 350 horse power by increasing the flywheel size. Alternatively, with a larger flywheel arrangement having a larger kinetic energy, a 350 horse power motor can drive a mixing or processing chamber having a capacity greater than 8000 grams.

Again, it will be appreciated that both the capacity of the mixer may be increased, and the size of the motor decreased, with the provision of the appropriate flywheel arrangement to provide increased kinetic energy needed to maintain blade rotation.

Further with the utilization of flywheel kinetic energy in accordance with the invention, it has been found possible to mix materials comprising or containing very high viscosity thermoplastic particles or pieces which previously it has not been practical to mix in a high intensity mixer.

It will be understood that the invention contemplates all such variations in flywheel arrangements, selected in accordance with the mixer capacity, motor size, and the nature of the thermoplastic material being mixed, and that such selections may be made without departing from the scope of the appended claims.