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
1995-12-18
2001-01-30
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
C425S150000
Reexamination Certificate
active
06179600
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process as well as to a device for a hydraulic mass-drive, especially for the closing and opening of the clamping mechanism in injection molding machines, with a controllable pressure operated system.
The hydraulic drive is essentially based on the peculiarity of a liquid. The rough definition of a liquid is: A media in liquid state, which differs from others by the free movement of the constituent molecules and the low compressibility. In contrast to pneumatic controls, hydraulics are differentiated in that in praxis the hydraulic media is considered noncompressible, which represents a simplification for the mathematical evaluation and the design of a complete hydraulic system.
One of the tasks in injection molding and die-cast machines is to close and open the clamping unit. Since this movement has no direct influence on the process it should happen as fast as possible to improve productivity. This means a heavy mass (all moving parts of the clamping unit including the movable half of the mold) needs to be moved from point A to point B and returned by oil-hydraulics. The respective end positions are to be set with high accuracy.
For example, the German Patent No. 2 902 264 (corresponding to U.S. Pat. No. 4,375,747) proposes therefore to achieve the acceleration and deceleration of the clamp drive mechanism by a defined increase and decrease of the oil flow. By the use of several electronic control elements it is possible to avoid acceleration shocks. In detail a 4-way valve with 3 positions is arranged between pump and motor which can be controlled electrically and which has a so-called ramp assigned in the electrical control arrangement. The output signal of the ramp is amplified to control the directional valve (4-way valve). There is no consideration of any compressibility of the hydraulic fluid or any expansion of the drive components with increasing pressure in this case.
In die-cast and injection molding machines a resting or moving mass is driven by a plunger-cylinder unit, and is brought up to a certain velocity. For this purpose a throttle is increasingly opened to raise the acting pressure on the plunger, which drives the mass. The hydraulic fluid is not non-compressible, and also the components of the drive mechanism expand, especially the hydraulic fluid conductors, and therefore a certain part of the hydraulic flow input is absorbed capacitively by the drive system and does not cause a movement of the mass immediately. This capacitively absorbed hydraulic flow represents a compressed spring, which together with the mass is an oscillating device. The oscillating impulse increases with decreasing time for reaching the end velocity. Therefore, if one uses the set-point generator with a steep ramp between the various set-points, the system will respond with lowly damped oscillations, which means the desired velocity will only be reached gradually.
The best known method uses directional valves, which feed oil flow in the desired direction to a cylinder. These can only be used up to a certain velocity, since the acceleration is not controllable. More recent methods use so-called proportional—directional—valves. These control the valve opening in relation to the solenoid coil current. Even with this method the acceleration control is limited. In machine tools the typical acceleration period is approximately 6 times the swing period of the mass/spring oscillators (spring=elastic oil column), in order to reduce the time for operational movements. Typically, dependent on degree of dampening applied, vibration can occur already if the acceleration period is 3 times the swing period. Typical natural frequency for the clamping mechanism of a fully hydraulic injection molding machine, dependent on machine size is 3 to 8 Hz, which means usable acceleration ramps are in the order of 0.3 to 1.0 seconds. This results in a total loss time of 1 to 2 seconds per cycle.
According to the proposal of European patent No. 97275 (corresponding to U.S. Pat. No. 4,563,939) the inventors tried to improve the dynamic of the hydrostatic drive for die-cast or injection molding machines, in order to reduce the machine cycle time and/or to reduce the wear and tear of the machines. In this case the mass to be accelerated or decelerated is intermittently driven by hydrostatics with a correspondingly controlled valve arrangement, in which dependent on the direction of a change in a ramp angle, the oil flow increase or decrease is related to the compression volume of the system. The valve arrangement accordingly is controlled by a set-point generator, which provides a position or time dependent ramp with superimposed positive control impulse at the start of the ramp rise, or at the end of the ramp fall, respectively, and with a superimposed negative control impulse at the end of the ramp rise, or at the start of the ramp fall, respectively. This provided reasonable results in practical applications but in many cases, because of improper settings, improvement of the dynamic behavior is not achievable.
The objective of the invention is to reliably control the dynamic behavior with simple means, also at shortest acceleration periods, in order to reduce especially the machine cycle time.
SUMMARY OF THE INVENTION
The solution according to the invention is characterized, in that the movement of the mass in relation to a velocity profile and/or position profile (or at least a part thereof) is closed loop controlled, with the acceleration pressure for the mass as control medium.
In this new invention astonishingly the natural frequency of the spring/mass oscillator is no longer significant, rather is the limiting frequency of the pressure generator and control the system limitation. This limiting frequency for example is in the order of 20 to 50 Hz. With that markedly steeper acceleration, shorter actuation periods are allowable without any disturbing vibration forces. Further the system losses are much lower than in state-of-the-art systems. This reduces the energy consumption especially during fast cycles. Even if it is not explainable on a phenomenological basis, measurements have shown a total elimination of the previous pressure shocks due to oscillations even with the steepest ramps as required by the pressure system, apparently because the controlled pressure adjustment prevents the mass/spring oscillator behavior of the elastic oil column.
Further, the invention allows for a number of advantageous arrangements. Most preferred is the control of the acceleration pressure directly dependent on a selectable velocity and/or position profile. The acceleration pressure can be calculated as a function of time and via a comparison device it can be adjusted for selected velocity and/or position profiles. Advantageously an adjustable counter pressure will be maintained in the return line, this for the controlled deceleration of the mass, and preferably also in the return line the counter pressure will be calculated and adjusted as a function of a velocity and/or position profile. According to a further arrangement it is proposed, that in the startup phase, over a selectable time period a continually changing acceleration pressure is provided according to a selectable motion profile, where the motion profile in a velocity/position or velocity/time diagram initially shows an approximate linear rise, gradually changing to a constant value. Further a continually changing acceleration pressure can be provided during the acceleration and deceleration phase, where the motion profile in a velocity/position diagram is at least in the approximate shape of a trapezoid. For less demanding requirements for an exact motion profile it is possible that the motion profile for the mass is at least monitored in one but preferable two or more key positions, and compared with a selected motion profile with the differential provided as correction signals for the control of the following cycle. On this basis interference values can be eliminated adaptively.
The new invent
Siegrist Ronald
Stillhard Bruno J.
Baker & Daniels
Heitbrink Tim
HPM Corporation
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