Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Reexamination Certificate
1999-06-04
2001-12-04
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
C264S040500, C264S328100, C425S145000, C425S149000
Reexamination Certificate
active
06325954
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for controlling an electric injection unit and, more particularly, to an injection speed control procedure for timing the change of the injection speed of an injection molding machine which carries out an injection process having a plurality of different injection stages to inject a molten resin into the cavity of a mold.
2. Description of the Related Art
FIG. 5
is a diagram of assistance in explaining a conventional injection molding method, in which the position S of an injection screw with respect to a reference position or time from a reference time point is measured on the horizontal axis, and the injection speed V of the injection screw or injection pressure P is measured on the vertical axis. The injection process represented by
FIG. 5
comprises an injection phase S
1
for injecting a molten resin into a cavity formed in a mold by advancing the injection screw and changing the injection speed V of the injection screw stepwise at a plurality of stages, and a dwelling phase S
2
for holding the injection pressure for molding after the completion of the injection phase by changing the injection pressure P stepwise at a plurality of stages.
Generally, in the injection phase S
1
, a pressure control valve included in a hydraulic circuit is set for a high pressure, and the opening of a flow control valve is varied according to the position of the injection screw with respect to a reference position or time elapsed from the start of an injection molding process so that the speed of the injection screw changes stepwise at a plurality of stages with the distance of movement of the injection screw or time elapsed from the start of the injection molding process. When filling up the cavity with a molten resin, the flow control valve is regulated to control the speed of a piston fitted in an injection cylinder, i.e., the injection speed of the injection screw.
After the cavity has been filled up with the molten resin, the opening of the flow control valve is adjusted to a relatively small opening for the dwelling phase S
2
. In the dwelling phase S
2
, the opening of the pressure control valve is regulated to apply a predetermined pressure constantly to the molten resin filling up the cavity.
Generally, desired injection speed is changed stepwise at time points where the measured position of the injection screw coincides with predetermined injection speed changing points to control the injection speed in the injection phase S
1
.
FIG. 6
is a diagram representing such an injection speed control procedure, in which desired injection speeds V
1
, V
2
, V
3
and V
4
are given at the start of the injection process and at speed changing points
1
,
2
and
3
, respectively.
When injecting the molten resin into the cavity, a screw tip, i.e., a check ring, for preventing the back-flow of the molten resin on the front side of the injection screw closes at different time points in different shot cycles.
The conventional injection control method sets beforehand changing points where injection speed and injection pressure are changed. Therefore, the actual strokes of the injection screw in each of the stages for the set injection speeds and the set injection pressures are reduced if the closing operation of the check ring is delayed.
An injection region SA representing an amount of work done in a shot cycle A in which the check ring closes at an early time point, which is indicated by a continuous line A in
FIG. 6
, in the injection process is greater than an injection region SB representing an amount of work done in a shot cycle B in which the check ring closes at a late time point, which is indicated by a broken line B in
FIG. 6
, in the injection process.
Thus, in a shot cycle in which the check ring closes at a late time point, the actual amount of work for injection is reduced even if the set injection time and set injection speed are not changed. Such an injection process affects the weight and the quality of the products; that is the variation of the time point when the check ring closes causes the variation of the weight and quality of the products.
A prior art control method previously proposed in JP-A No. 8-66944 determines a change time point when an injection phase is to be changed for a dwelling phase on the basis of a measured pressure of the hydraulic fluid or the molten resin. This prior art control method determines the change time point on the basis of only the pressure measured at one time point and does not take into consideration the amount of work done in the injection process. Accordingly, this prior art control method is unable to solve satisfactorily problems causing the variation of the weight and the quality of products.
Possible causes residing in an injection molding machine and causing molding faults due to such an unstable injection process include, in addition to the variation of the time point when the check ring closes, the change of the density of the metered resin, the variation of the metered volume and the variation of the injection speed.
Various control methods giving considerations to such problems have been proposed. An injection molding machine disclosed in JP-A No. 8-66944 carries out one of those prior art control methods. This injection molding machine is provided with a position sensor for measuring the position of an injection screw, a pressure sensor, and a speed calculating device for calculating the derivative of an injection stroke measured by the position sensor with respect to time or an injection screw speed measuring device. In this injection molding machine, the injection phase is divided into a plurality of stages and the injection screw is advanced at different injection speeds at different stages, respectively. In this injection molding machine, a reference pressure for deciding the closing operation of a check ring is set at an optional point in an injection phase. The difference between a position of the injection screw at a moment when the injection pressure reaches the reference pressure and a position of the same at a moment when the injection pressure reaches the reference pressure in a predetermined reference shot cycle is calculated and stored. The injection speed is changed when the injection screw reaches the reference injection speed changing points before the injection pressure reaches the reference pressure, and the injection speed is changed when the injection screw reaches injection speed changing points determined by correcting the initial injection speed changing points by using the difference after the injection pressure has reached the reference pressure.
This control method is able to suppress the adverse effect of the irregular closing characteristic of the check ring to the least extent. However, since the effects of the change of the density of the metered resin, the variation of the metered volume and the variation of the injection speed on the condition of the molten resin are not taken into consideration in devising this control method, this control method is not necessarily satisfactory in the effect of perfectly solving the problems attributable to the unstable injection process due to the different conditions of the molten resin in different shot cycles.
Another injection molding machine proposed for the same purpose in JP-B No. 2-39973 measures the hydraulic pressure applied to an injection ram and the injection speed of an injection screw, the product of hydraulic pressure and injection speed measured from the start of the injection process to the termination of the dwelling phase is integrated with respect to time, the integral thus obtained is compared by a comparator with values in a predetermined reference range set by a reference range setting device and, if the integral is outside the reference range, a signal is generated.
An injection pressure monitoring system proposed in JP-B No. 4-57490 to monitor a condition of an injection process on the basis of time taken by an injection molding machine
Iimura Fukio
Kato Fumiyuki
Koike Jun
Sasaki Kiyoshi
Tamaki Masahiro
Heitbrink Jill L.
Toshiba Kikai Kabushiki Kaisha
Winthrop Pillsbury
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