Patent Application
20090108498
This invention relates, in general, to drive units and is particularly, but not exclusively, applicable to hydraulic pumps and their use in injection molding machines (or the like). More specifically, the present invention relates to drives associated with twin screw extruders.
In today's world, energy efficiency is of considerable concern from the perspectives of both financial cost and environmental impact. With energy inefficient systems, a manufacturer's overhead costs are increased and their profit margins accordingly reduced. Injection molding system suppliers and their customers are therefore keen to see energy savings in new system design, especially in application involving large clamp tonnage machines (typically above 10000 kN) where component scaling means that component/sub-assembly masses and energy consumption are both significant.
Injection molding machines make use of hydraulic circuits and particularly hydraulic pumps to control machine functions, including clamp unit functions and melt injection functions. In this general regard, oil from a central oil reservoir is pumped around the injection molding machine by an electric motor, with this motor providing energy to multiple hydraulic pumps used in particular circuits for particular functions. One such system was supported in the Moduline G-Series of injection molding machines manufactured and supplied by Husky Injection Molding Systems Limited.
In European patent EP-B-1343622, an electric motor is connected to a hydraulic motor via a transmission device, such as a belt. As the electric motor turns a feed screw (to effect plasticizing in the injection barrel of an extruder), the electric motor is also used to power a hydraulic motor that is tasked to charge one or more accumulators. The stored hydraulic energy in the accumulator may then used: i) to stroke pistons to open and close clamp unit; ii) to generate clamp tonnage by controlling pressure in clamp pistons; and/or iii) to cause the injection of melt into the mold. Furthermore, a clutch mechanism may be provided between the electric motor and the feed screw such that the electric motor may be disengaged from the feed screw. During times of disengagement, the electric motor is permitted to drive continuously the hydraulic motor and therefore to charge the hydraulic accumulator. More specifically, the clutch mechanism allows the electric motor to remain “on” to charge the accumulator during the injection cycle.
In terms of screw/extruder operation and particularly screw rotation, current injection molding systems can use both hydraulic and electric motors that are dedicated to this sole function. Such screws may furthermore be subject to either continuous or discontinuous operation, and such screws may furthermore operate on a reciprocating or fixed axial basis. The extruder may, furthermore, support single or multiple screws. For example, in a compounding environment, it is usual to make use of an intermeshed, co-rotating twin screw extruder (TSE) to reduce internal melt stresses while producing homogeneity in the melt.
Furthermore, existing injection molding systems, such as in-line compounding systems, have complex control circuits associated with the electric or hydraulic screw drives. Such controls are generally housed in dedicated cabinets that are located proximate to a distal end of the injection unit (i.e. at the end of the feed screw remote from the nozzle). These cabinets hold dedicated power supplies and expensive frequency control equipment that cooperate to control operation and torque in the electric motor that causes rotation of the screw through (typically) a gear box and associated clutch arrangement. As will be appreciated, the clutch and gear box combination therefore provide a mechanism for regulating potentially damaging levels of torque generated in the motor. Additionally, these control cabinets are not insignificant in size and may typically have a footprint and volume of ˜1 m2 and ˜2 m3, respectively.
According to a first aspect of the present invention there is provided an injection molding machine comprising: a twin screw extruder including at least one rotating screw; a power pack including: a motor; a plurality of pumps each for effecting an injection molding function; and a hydraulic circuit coupling the motor to the plurality of pumps such that the motor can selectively provide energy to each pump to manage its respective molding machine function; a hydraulic motor drive coupled to the twin screw extruder to effect and control rotation of the twin screw extruder, the hydraulic motor drive connected into the hydraulic circuit such that the hydraulic motor drive selectively receives energy from the motor to cause controlled rotation of the twin screw extruder.
In a preferred embodiment, a variable displacement pump is coupled between the motor and the hydraulic motor drive, wherein the variable displacement pump is arranged, in use, to provide hydraulic energy to the motor at least during one period when one of the plurality of pumps is idle and thus not operational to support its respective injection molding function.
In a second aspect of the present invention there is provided a method of operating an injection molding machine comprising: a twin screw extruder; a power pack having a hydraulic circuit, a plurality of pumps each controlling an active injection molding function and a motor coupled to the plurality of pumps through the hydraulic circuit selectively to actuate each injection molding function; and a hydraulic motor drive connected to the hydraulic circuit and the twin screw extruder, the method comprising: drawing energy from the motor in the power pack to operate the hydraulic motor drive to cause rotation of the twin extruder, the energy provided during at least a period of time when at least one of the plurality of pumps is idle and thus not actively supporting its assigned injection molding function.
In a further aspect of the present invention there is provided a method of operating a twin screw extruder in a molding machine having a plurality of pump-controlled machine functions, the method comprising: in periods where the pump-controlled machine functions are inoperative, using available energy from a motor dedicated to the pump-controlled machine functions to energize a hydraulic motor drive to cause controlled rotation of the twin screw extruder.
Advantageously, the present invention provides increased energy efficiency achieved from costs savings arising from the elimination of idle losses in the pump power pack. Furthermore, the present invention reduces investment costs, since no dedicated electric (rotational drive) motor for the plasticizing unit needs to be supplied in the injection molding system. Moreover, with the elimination of a dedicated motor for the screw/extruder, its associated power supply and frequency controller (used to control the speed and torque of the electric motor) can also be eliminated, thereby saving additional costs and space (in terms of elimination of the dedicated control cabinet and a corresponding reduction in overall machine length/height).
The timing of operations in the molding machine is therefore orchestrated such that a common motor within a power pack can effectively and advantageously provide contiguous or sequential support of multiple system functions, including extruder rotation, through a common but system-extensive hydraulic circuit.
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
a is timing diagram that shows relative timing of hydraulically controlled machine functions in a prior art system having a reciprocating screw (RS) operating a sequential plasticizing process;
b is timing diagram that shows relative timing of hydraulically controlled machine functions in a prior art system having a reciprocating screw (RS) operating a simultaneous plasticizing process;
c is timing diagram that shows relative timing of hydraulically controlled machine functions in a prior art system having a shooting pot machine configuration operating a discontinuous plasticizing process;
d is timing diagram that shows relative timing of hydraulically controlled machine functions in a prior art system having a melt-buffered compounding machine configuration operating a continuous plasticizing process; and
Referring to
A hydraulic power pack 28 includes an electric motor 30 that provides energy to a multiplicity of hydraulic pumps 32-38 that are located within a hydraulic circuit 40, including a reservoir 42 from which (and to which) hydraulic fluid, e.g. oil, is drawn and delivered. Specific ones of these hydraulic pumps 33-38, each responsive to the control of a machine controller 44, operate to support a variety of conventional molding machine functions, including: i) stroking of the moving platen; ii) injection; iii) clamp and lock functions within the clamp unit 20; and iv) integrated in-mold functions, e.g. the operative control of core ejectors.
The machine controller 44 also orchestrates operational control of additional machine functions, e.g. continuous or discontinuous operation of the extruder and also power supply and general operation of the electric motor 22 associated with the extruder 12. In the latter respect, the machine platform 10 further includes dedicated power and frequency control circuitry 46 that operates to control accurately the rotation and translation of the screws in the extruder 12. Additionally, a power converter 48 is associated with the electric motor 22.
On occasion, the electric motor 22 has (in the prior art) been substituted by a dedicated hydraulic motor that is and remains additional/auxiliary to the motors in the power pack 29.
A schematic representation of a hydraulic-extruder drive 100 according to the present invention is shown in
Referring briefly to
From a fluid return perspective, hydraulic fluid is returned from the hydraulic motor drive 104 to the reservoir 42 via a tank return valve 130. Preferably, the tank return valve 130 is a preloaded check valve because the supply and return lines (in the hydraulic circuit 40) need to be pressurized (when the hydraulic motor 104 stops) to eliminate short-term shifting of the hydraulic motor's pistons due to gravity. In addition, inclusion of the preloaded circuit will smoothly bring the pistons into position during start up of the TSE 102. The variable pump 120 therefore controls oil volume flow to the hydraulic motor drive 104.
In terms of operation, this can best be understood by contrasting
In a typical large tonnage molding cycle of about 50 seconds (see
Injection, hold and mold movement functions consume approximately 50% of the total cycle. Consequently, during the remaining part of the cycle (i.e. cooling and part take-out), the pumps perform no function and thus conventionally run “idle”, whereby about 20%-25% of the available/nominal installed power (of the electric motor 30) is nevertheless consumed. The reason for running the pumps idle is to provide low flow and appropriate pressure to the various valves within the hydraulic system. Therefore, in terms of the power pack 28 and operation of its motor 30, energy from the motor 30 has hitherto resulted in continuous loading of the associated pumps that each independently control, for example, clamp force, hold, mold motion, injection, etc.
In terms of plasticizing, this can clearly be achieved in continuous and discontinuous processes in a machine configuration having a shooting pot or a melt buffer; this machine configuration is typical for in-line compounding applications. An alternative machine configuration is that realized with a reciprocating screw (or RS unit).
The following table highlights the periods over which plasticizing occurs in these various machine configurations.
Turning now to
As will now be appreciated with particular regard to
For simultaneous process (sometimes referred to as parallel processes), it may be necessary to increase the size of the motor in the power pack to accommodate combinations of certain processing functions, e.g. plasticizing and mold movement. However, in all case, the system benefits from increased efficiency since wasted energy through idle losses is eliminated. In fact, the larger the motor (associated with progressively larger tonnage machines), the greater the energy saving because the percentage power consumption associated with nominal idle losses in the prior art remains generally constant.
As will be appreciated, control of the various pumps and functions is typically achieved through a processor-controlled closed loop mechanism.
In one particular embodiment, the gear box of the system of
It will, of course, be appreciated that the above description has been given by way of example only and that modifications in details may be made within the scope of the present invention. For example, while the preferred embodiment has been described in relation to an in-line compounding environment, the principals of the present invention are more widely applicable to any sized machine (especially injection molding machines) in which dedicated extruder drives are historically used side-by-side with hydraulic pump circuits that are themselves driven by motors. Indeed, the present invention finds general application in injection molding technologies, irrespective of whether, for example, the injected material is a plastic, plastic composite or metal (as in the case of thixomolding technology).
