Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Patent
1988-02-10
1989-06-27
Silbaugh, Jan H.
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
318632, 364476, 425135, 425149, 425166, 264 405, B29C 4576
Patent
active
048428019
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a switching control method which can smoothly effect transfer from a pressure holding step to a metering and kneading step in an injection-molding machine employing a servomotor as an injection shaft driving source.
An injection-molding machine comprises an injection shaft driving source for axially driving a screw, and operates in such a manner that the screw is driven for forward movement to inject molten resin into a mold. After the molten resin is injected into the mold, a hold pressure, which lower than the injection pressure and which acts in the same direction is as the injection pressure applied to the screw. Typically in an injection-molding machine which employs a servomotor as an injection shaft driving source and which comprises a numerical control unit including a servo circuit for driving the servomotor, a move command sufficient that the tip of the screw reaches a tip of a heating cylinder is sent to the servo circuit to generate the hold pressure. That is, the screw is driven for forward movement by axial driving force in accordance with the move command and, subsequently, the screw is retained at an axial position short of the cylinder tip position by a cushion amount, where the axial driving force balances with the pressure of the molten resin in the mold. This is the hold pressure that is applied to the resin. During a subsequent metering and kneading step, axial pressure, i.e., back pressure lower than the hold pressure and acting in the backward direction is applied to the screw while rotating the latter, to effect metering and kneading.
At transfer from the pressure holding step to the metering and kneading step, the molten resin slightly expands in volume due to a differential pressure between the hold pressure. The back pressure, and the screw moves rearwardly in response to this differential pressure. At this time, an error amount equal to an added value of the cushion amount and the amount of rearward movement of the screw is accumulated in an error register of the servo circuit. As the error amount exceeds the register capacity, the error register overflows and, in response thereto, the typical numerical control unit generates an alarm and operates so as to interrupt operation of the injection-molding machine. It is inconvenient that the injection-molding machine stops in operation during execution of the usual injection molding cycle. Additionally, as is publicly known, even if transfer is made to the metering and kneading step after follow-up of the error register at completion of the pressure holding step so as to bring the error amount accumulated in the error register and equivalent to the cushion amount, to "0", similar inconvenience is caused if the amount of rearward movement of the screw is large.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to prevent overflow of an error register within a servo circuit at transfer from a pressure holding step to a metering and kneading step, to thereby prevent interruption of operation of an injection-molding machine.
In order to achieve the above-mentioned object, a method of controlling switching from pressure holding to metering and kneading, according to the invention, in an injection-molding machine which employs a servomotor as an injection shaft driving source for axially driving a screw and which drives the servomotor through a servo circuit having an error register to apply back pressure during injection, during pressure holding and during metering and kneading, comprises the steps of reducing an error amount accumulated in the error register to such a predetermined value that the register does not overflow at completion of a pressure holding step, switching a value of torque limit acting on the servomotor from a set value for the pressure holding to a set value for the metering and kneading, reducing the error amount of the error register to a value equal to or lower than the predetermined value over a predetermined period of time from e
REFERENCES:
patent: 4524313 (1985-06-01), Kuno et al.
patent: 4710119 (1987-12-01), Otake
patent: 4718841 (1988-01-01), Kiya
patent: 4734025 (1988-03-01), Kawamura et al.
patent: 4755123 (1988-07-01), Otake
patent: 4759705 (1988-07-01), Otake et al.
Kamiguchi Masao
Umemoto Hiroshi
Fanuc Ltd
Heitbrink Jill L.
Silbaugh Jan H.
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