Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
1999-08-23
2002-08-06
Gordon, Paul P. (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S033000, C700S045000, C264S040500, C425S149000
Reexamination Certificate
active
06430466
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the regulation of a clamp assembly in a injection molding machine and, more particularly, to a closed loop system for controlling the pressure of a clamp assembly to optimize production efficiency in an injection molding machine.
Injection molding machines are used to mass produce plastic parts by automated injection of plastic into a mold that is opened and closed by a clamp assembly. At the end of an injection molding cycle, the clamp assembly is retracted, and the part is ejected. The next cycle is initiated by motion of the clamp from its retracted position to a position where the mold is nearly closed. During the cycle the mold machine should not be subjected to excessive pressure and forces generated by parts left behind in the mold, or incorrect positioning of the mold. In the event that parts are left behind nor incorrectly positioned in the mold the machine must be stopped and the piece removed. This causes production delay which impacts production efficiency.
More particularly, a typical injection molding process utilizes a injection molding machine
10
, illustrated in
FIG. 1
, wherein plastic pellets (not shown) are melted and forced into a mold
20
, by a clamp assembly
113
that opens and closes mold
20
during the mold injection cycle.
The clamp assembly
113
comprises a hydraulic cylinder which is mechanically coupled to a piston
27
. Clamp assembly
113
further comprises a stationary platen
13
which is mechanically affixed to a mold
20
, and a moving platen
12
, which is coupled to piston
27
. Piston
27
is adapted to hydraulically traverse from a clamp closed position to a clamp open position in an injection molding cycle by variable force applied to piston
27
. A hydraulic cylinder
11
is movably coupled to piston
27
. The variable force applied to piston
27
by the fluid in hydraulic cylinder
11
may be controlled by a control unit
28
.
The injection molding process comprises four successive stages. The first stage called “plastication” comprises steps wherein the plastic pellets are pushed forward from a hopper
17
through a barrel
21
towards a nozzle
14
by a rotating screw
22
while being heated in barrel
21
by electric heater bands
23
surrounding barrel
21
. The second stage called “injection,” occurs when the plastic is pushed through nozzle
14
into a mold
20
by clamp assembly
113
. The third stage called “packing,” occurs when mold
20
is packed with the molten plastic. The fourth stage called “cooling,” occurs when mold
20
is cooled to solidify the plastic part therein. After the completion of the solidification and cooling stages, piston
27
is retracted and the part is ejected. The injection molding machine
10
is thus ready for the process to be repeated in the next cycle.
It is desirable to protect the injection mold machine
10
during motion of piston
27
by “smoothing” the response of the clamp assembly cycle during clamp closure of piston
27
so as to prevent damage to the machine from plastic parts, remaining in the mold after having been jammed in mold
20
during the previous cycle. It is also desirable to control the stroke of piston
27
during the injection molding cycle so as to minimize the duration of the clamp cycle.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for controlling the force and velocity of a piston within the clamp assembly of a injection molding machine. The control unit comprises the following elements: a cascade compensator, wherein the cascade compensator is adapted to generate a force error signal having lead and lag compensation based on the desired force minus the actual force of the piston; an estimator coupled to the clamp assembly, wherein the estimator is utilized to estimate the position of the servo valve and to generate a corresponding actual force signal; a velocity and position controller coupled to the clamp assembly and the estimator, wherein the velocity and position controller is adapted to generate a velocity error signal based on the piston velocity signal; and an auto selector coupled to the cascade compensator, wherein the auto selector is adapted to select between the velocity error signal and the force error signal so as to generate the valve position command signal.
The present invention additionally employs a method for determining the velocity profile of the piston of a clamp assembly, wherein the velocity profile having a ramp-up interval, a constant speed interval, ramp-down interval; and a final touchdown interval. The method comprises the following steps: determining the maximum displacement x
f
of the servo valve; selecting a final touch down velocity v
s
of the piston; choosing a ratio &agr; of the piston ramp-down interval to the servo valve displacement “x
v
” during the ramp-up interval, selecting a desired maximum piston velocity (v
max
) during the ramp-up interval and constant speed interval; and determining servo valve displacement x
u
from simulations or estimate from previous cycles.
REFERENCES:
patent: 3857658 (1974-12-01), Muzsnay
patent: 4917540 (1990-04-01), Recalde
patent: 4917840 (1990-04-01), Harada et al.
patent: 5182716 (1993-01-01), Stroud, III et al.
patent: 5680315 (1997-10-01), Rimondi et al.
patent: 5786999 (1998-07-01), Spahr et al.
patent: 5792483 (1998-08-01), Siegrist et al.
patent: 5911924 (1999-06-01), Siegrist et al.
patent: 6108587 (2000-08-01), Shearer et al.
Breedlove Jill M.
General Electric Company
Gordon Paul P.
Thompson John F.
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