Hybrid injection molding machine

Details Hybrid injection molding machine Hybrid injection molding machine

2000-07-05

2001-10-09

Heitbrink, Tim (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Control means responsive to or actuated by means sensing or...

Feed control of material en route to shaping area

Reexamination Certificate

active

06299427

ABSTRACT:

This invention relates generally to injection molding machines and more particularly to the power transmission arrangement used in a general purpose injection molding machine.
INCORPORATION BY REFERENCE
Our prior patents listed below are incorporated herein by reference so that details related to the control systems illustrated therein need not be repeated in detail in this patent. The materials incorporated by reference herein do not, per se, form the present invention.
1) U.S. Pat. No. 5,513,115 to Richards et al., issued Apr. 30, 1996 and entitled “Clamp Control for Injection Molding Machine”;
2) U.S. Pat. No. 5,493,503 to Richards et al., issued Feb. 20, 1996 and entitled “Clamp Control for Injection Molding Machine”; and
3) U.S. Pat. No. 5,456,870 to Bulgrin, issued Oct. 10, 1995 and entitled “Barrel Temperature State Controller for Injection Molding Machine”.
BACKGROUND OF THE INVENTION
Any evaluation of a drive mechanism, whether hydraulic, pneumatic or electric, must first consider whether the drive is functionally acceptable for its intended use. Once that fundamental consideration has been addressed, other characteristics of the drive train, such as the purchase price, the performance/reliability/maintenance costs and the operating (energy) costs must be evaluated.
An injection molding machine performs a molding cycle which includes the general steps or phases of clamp, inject, recover and eject. A general purpose injection molding machine must have the flexibility to perform the molding cycle for a wide variety of plastic materials, and more importantly, for a wide variety of molding applications. Limitations in the drive trains are oftentimes addressed, today, by complicated mold designs and extensive runner systems. For example if the machine can not deliver a long travel stroke at very fast speeds or high acceleration rates, an expensive multi-branch runner system for a mold with multiple feeders and gates is typically designed so that the injection stroke can be shortened and the injection speed slowed. In this manner an inferior drive train can be made to “work” and, most times, the machine end user is not aware of the limitations, even in competitive bidding situations.
Further complicating an evaluation of the drive trains are the tremendous improvements recently made in the control art which are able to mask or compensate to some extent for inherent weaknesses in a drive train. For example some control techniques developed in the precise, relatively low power/low torque machine tool and robotic industries have been literally copied into heavy duty injection molding machines exerting forces measured in tonnages. On the other hand, because improved control techniques are available for all drive trains and since the control is an inherent part of the drive train, itself, it simply must be evaluated along with and as part of the drive train.
Injection molding machines have traditionally been operated with hydraulic systems as their primary source of motive power. The most important step in the molding cycle, the injection step, is inherently suited to a fluid drive train system. A fluid drive train system using fluid pressure hydraulicly coupled to a prime mover has an innate ability to directly correlate to the movement of the injection molding material into the mold cavity. While relatively recent developments in velocity profiling have now standardized injection ram control, the hydraulic pressure exerted on the “ram” by the hydraulic drive directly corresponds to the pressure of the melt in the mold and is important for controlling or establishing the velocity profiling. Fundamentally then, a hydraulic system provides a direct measure of what is happening in the mold, whereas other drive systems, specifically an electric drive, can only provide an indirect measurement. This distinction becomes significant when considering mold packing. A hydraulic drive can easily maintain a packing pressure through direct pressure sensing. An electric drive has to use separate transducers to measure melt pressure (importantly affected by its position in the mold) and has to switch to torque control (and the torque has to be correlated to pressure which is not necessarily linear because of slip in the drive) at slow or zero velocity where torque pulsations from the motor can adversely affect the molded part. In addition, there is a non-linear translation of forces before static friction is overcome and the mechanical coupling engages in the electric drive train. That static friction can be variable.
There are also fundamental differences between electric and hydraulic drive mechanisms in the speed and speed control during injection. Each drive train has some advantages and disadvantages in this regard. However, for reasons discussed below, a hydraulic drive can linearly move the screw faster over a longer stroke than an electric drive.
In this regard, it must be noted that electric drive injection molding machines have been present in various forms for a number of years and have recently been promoted for general purpose injection molding machine applications to which this invention relates. Screw translation in an electric drive machine is typically accomplished by a ball screw coupling and recent developments in ball screw couplings have rendered such systems acceptable for a wide variety of plastics and plastic applications. Nevertheless, electric motors employing mechanical drive couplings (such as ball screw couplings) while initially quicker, cannot provide the rapid acceleration characteristics of a fluid drive train. As the science of molding plastics continues to evolve, the velocity profile spectrum of a fluid system will remain superior to that achieved by the mechanical couplings of electric drive machines. This feature coupled with the direct pressure sensing/control concept previously discussed provides hydraulic drive systems functional advantages over electric drive machines. Again the discussion is limited to general purpose injection molding machines which must possess a wide range of operating characteristics. Certain molding applications requiring relatively slow or medium speed injection strokes are adequately handled by electric drives. This is especially true with the advances in the control technology. However, the advances in control technology also apply to hydraulic drives and the consideration again reduces to the fundamental distinctions between the drive trains.
A similar functional analysis can be likewise applied to the clamping step of the molding process. Hydraulic clamp drives function like a press in that a high pressure is exerted to generate a high tonnage force and the valve simply closed to lock the mold halves together at a high pressure while pump pressure is released. The electric drive can only function in a similar manner by being left “on” rotating at “zero” velocity with maximum torque. This produces undesirable motor effects effectively limiting the electric drive to “toggle” clamp applications where the zero velocity characteristics of the motor are less noticeable.
Apart from functional considerations, when the other decision factors mentioned above are considered, hydraulic drives have advantages and disadvantages. Hydraulic drives are relatively inexpensive. They are tried and proven drives suitable for performing all the steps of a molding cycle and they have proven themselves rugged and reliable over the years. Further, in almost all instances, hydraulic fluid arrangements are present at the molding facility, i.e., setting and pulling cores. Any injection molding machine, whether electric or hydraulic, therefore must have the ability to handle hydraulics. Since the facilities use hydraulics and the machines must interface with hydraulic arrangements to perform the molding cycle, the environment is suited to hydraulic drive trains on the machine.
There are disadvantages however. Hydraulic oil is subject to dirt and contamination and requires filtering and maintenance and, in addition, has a potential for oil leakage. In a

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