Method of precision volumetric control of a moldable material in an injection molding process

A method of operating an injection molding machine having a pair of separable mold elements forming a mold cavity therebetween with means for opening and closing the mold elements with means for introducing a moldable material into the mold cavity at a first pressure and for exerting a second pressure on the moldable material as it cures in the cavity. The method controls the molded product and comprises the steps of closing the mold elements and introducing a moldable material into the mold cavity. The separation of the mold elements is measured during the injection of the moldable material and a predetermined separation of the mold elements is detected. Upon detecting the predetermined separation the pressure is changed from the first pressure to the second pressure and the separation of the mold elements is continued. Measurement of mold element separation is continued and the cessation of the increase in the separation of the mold elements is detected. A determination is made whether or not the cessation occurs within a predetermined separation window. If the cessation is not within the window a signal is generated to indicate that the cycle has exceeded the product limit. If the cessation is within the window then the cycle is continued and the second pressure is maintained until the moldable material has cured. The mold is then opened.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a method of controlling a moldable 
material in an injection molding process whereby the dimension, weight and 
stress in the molded article is closely controlled. 
2. Description of the Prior Art 
The production of consistent and uniform product by the injection molding 
process has been a long standing objective of the injection molding 
industry. This objective has become more difficult to achieve as more and 
more products are produced by this process which are ever increasingly 
complex and have ever tighter tolerances. The objective has been further 
complicated by the trend towards fewer and fewer operators associated with 
the injection molding machines, requiring greater automatic control of the 
process and apparatus. 
A variety of interrelated parameters of the material, machine, and mold 
must be accomodated by any automated control system for the satisfactory 
operation of an injection molding apparatus; among these parameters are 
the type of material being molded, the consistency of the plastic 
characteristics, the molding cycle time, the machine shot size, melt 
viscosity and temperature consistency, mold clamp pressure, and injection 
pressure, among others. lt has been found that, as each of these 
parameters varies during the operation of an injection molding machine, 
the product uniformity may suffer without constant operator attention. 
Many attempts have been made at automatically controlling the injection 
molding process to produce uniform and consistent product, yet none of the 
proposed solutions has gained widespread acceptance, at least partially 
due to the fact that the proposed solutions do not satisfactorily address 
all of the variables involved. 
Among the solutions proposed are those taught by U.S. Pat. Nos. 2,433,132 
and 3,976,415 and French Pat. No. 2,527,976 wherein the separation of the 
mold elements at the part-line is measured and the result utilized to 
change the machine from the injection phase to a pulsing of the injection 
ram to maintain the part-line separation constant during the packing and 
curing of the mold. This system has not been found to be feasible for the 
manufacture of small, precision parts since it is nearly impossible to so 
precisely control the pulsing of the ram to pump the microscopic amounts 
of material necessary to achieve the desired volumetric accuracy of such 
parts. 
Other proposed solutions are exemplified by U.S. Pat. Nos. 2,671,247 and 
3,859,400, which teach the control of the switch point of the injection 
molding machine by sensing the pressure within the mold itself. This 
system proposes sensing the pressure within the mold to shift the machine 
from the injection phase to the holding phase while the material cures 
within the mold. The measurement of the pressure within the mold does not 
satisfactorily reflect the multiple variables noted above. For example, if 
the material characteristics are held constant and the machine clamp force 
is allowed to vary, even if the pressure in the mold cavity remains 
constant, the part size produced will vary with the clamp force, 
increasing with reduced clamp force and vice versa. Similarly, with 
variations in material characteristics, if the viscosity of the material 
changes, as it may with changes in the injection temperature or with 
different batches of material, even if the amount of material injected 
into the mold is substantially constant, the varying viscosity will affect 
the resistance of the material to being forced into the mold cavity and 
thus the pressure transmitted into the mold cavity from the injection ram. 
Accordingly, it will be seen that measuring the pressure in the mold 
cavity will not truely reflect many of the variables that influence the 
process and the product. 
Another proposed method of controlling injection molding processes is that 
taught by U.S. Pat. No. 3,940,465 wherein the measurement of the 
separation of the part-line is utilized to control the cure time of the 
injection molding cycle. This type of control fails to reflect all of the 
variables, noted above, which must be accomodated to accurately control 
part weight and dimension. 
U.S. Pat. No. 4,135,873 teaches the measurement of the part-line separation 
and comparing the separation with a predetermined value and thereafter 
varying the injection pattern of the injection ram during the following 
molding cycle. This system does not provide control of the process on a 
real time basis, reflecting system conditions that are affecting the 
current cycle. Such a system merely reflects what occurred on the previous 
cycle, resulting in a tendency for the system to hunt rather than zero 
into a mode of operation which provides product consistency. 
U.S. Pat. No. 4,131,596 teaches the measurement of the part-line separation 
to reduce the mold clamping pressure upon the measurement of a 
predetermined separation to minimize any damage to the mold due to 
flashing of the material at the part-line. This, of course, does not 
contribute to the control of product weight and dimension. 
Japanese Patent Publication No. 11974 of 1978 discloses a method of 
controlling an injection molding machine wherein the part-line separation 
is measured and, upon reaching a predetermined reference separation, the 
machine is switched from a material filling mode to a dwelling mode. The 
mold separation is then measured and the maximum separation is determined. 
Thereafter, pressure during the dwell or curing phase of the mold cycle is 
controlled dependent upon the maximum separation reached to control the 
final mold separation value at the end of the cure time. Thereafter, the 
reference separation value for the switch point for the following cycle is 
changed to accommodate the variations in the machine operation detected 
during the first cycle. This system of control has the disadvantage that 
the switch point is determined by the preceeding cycle and thus does not 
reflect the conditions of the current cycle. This system of control 
thereafter attempts to adapt to the variations in the molding conditions 
existing during the current cycle by controlling the holding pressure 
during the cure phase of the cycle which can adversely affect part weight 
and density uniformity. 
Each of the foregoing control systems has either been too complex and 
expensive and/or has not provided the requisite control related to all of 
the variables acting upon an injection molding process. 
It has been found that the variables noted above are reflected in the 
molding process through the part-line opening which also reflects the 
consistency of the dimensions and weight of the molded product. During 
repeated molding cycles variations in the product from the molding process 
can range from under-filled mold cavities (short) to over-filled mold 
cavities (flashed). The aim point for the process will be somewhere 
between these two extremes to produce a product which meets the 
dimensional and weight tolerances established for that process and 
product. Further, it is known that mechanical and thermal strains are 
inherent in the molding process which are then transfered to the molded 
product, sometimes to the detriment of the dimensional stability and life 
of the product. The mechanical strains are produced by the clamping 
pressure necessary to hold the mold elements together against the force of 
the injection of the molten material. The thermal strains occur during the 
filling and packing of the mold with the high temperature molten material 
and its effect on the much cooler mold cavity walls, followed by the 
shrinkage of the material as it cures. 
Accordingly, the provision of an injection molding process which accurately 
controls the dimension, weight, and stress in a molded product without the 
requirement of constant operator attention would significantly enhance the 
productivity and applicability of the process, as well as minimize the 
strain on the molding apparatus and the residual stress in the molded 
product. 
SUMMARY OF THE INVENTION 
The present invention provides a method of operating an injection molding 
machine having a pair of separable mold elements forming a mold cavity 
therebetween with means for opening and closing the mold elements. Means 
is provided for introducing a moldable material into the mold cavity at a 
first pressure and for exerting a second pressure on the moldable material 
as it cures in the cavity. The method controls the molded article and 
comprises the steps of closing the mold elements and introducing a 
moldable material into the mold cavity. The separation of the mold 
elements is measured during the injection of the moldable material and a 
predetermined separation of the mold elements is detected. Upon detecting 
the predetermined separation the pressure is changed from the first 
pressure to the second pressure and the separation of the mold elements is 
continued. The cessation of the increase in the separation of the mold 
elements and a determination is made whether or not the cessation occurs 
within a predetermined separation window. If the cessation is not within 
the window a signal is generated to indicate that the cycle has exceeded 
the product limit. If the cessation is within the window then the cycle is 
continued and the second pressure is maintained until the moldable 
material has cured. The mold is then opened. 
Still further, the present invention provides a method of operating an 
injection molding machine having a pair of separable mold elements forming 
a mold cavity therebetween and arranged for separation along a part-line 
for the removal of the molded article. Means is provided for opening and 
closing the mold elements, and injection means is provided for injecting a 
moldable material into the mold cavity at a first pressure and for 
exerting a second pressure on the moldable material as it cures in the 
cavity. The method comprises the steps of closing the mold elements and 
measuring the separation of said mold elements during the closing of the 
mold. A predetermined separation of the mold elements is sensed during the 
closing of the mold and a first timing cycle of a predetermined time 
length is initiated in response thereto. The termination of relative 
closing movement between the mold elements is sensed and whether or not 
the termination occurs during the first timing cycle is determined. The 
mold elements are reopened if the termination does not occur during the 
first timing cycle. If the termination occurs during the first timing 
cycle, a moldable material is injected into the mold cavity at the first 
pressure. The separation of the mold parting line is measured during the 
injection of the moldable material and upon detecting a preselected 
separation the injection pressure is changed from the first pressure to 
the second pressure. The separation of the mold elements following the 
changing of the pressure is measured and the cessation of the increase in 
the separation is sensed. A determination is made whether the cessation 
occurs within a predetermined separation window and if not a second signal 
is generated to indicate that the cycle has exceeded the product limit. If 
the cessation occurs within the window then the cycle is continued and the 
second pressure is maintained until the thermoplastic material has cured. 
The mold is then opened and the cured product is ejected. 
Various means for practicing the invention and other features and 
advantages thereof will be apparent from the following detailed 
description of illustrative preferred embodiments of the invention, 
reference being made to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
One for of an injection molding apparatus 10 is illustrated in FIG. 1 and 
comprises a pair of pressure platens 12 and 14 arranged to carry a pair of 
mold elements 16 and 18, respectively. The mold elements are arranged to 
meet at a part-line 20 and form a mold cavity 22 therebetween, all in a 
manner well known in the art. Platen 12 and the mold element 16 associated 
therewith are stationarily arranged on the machine while platen 14 and 
mold element 18 associated therewith are movably arranged to be displaced 
along tie bars 24 and 26 between an open and closed position by a 
hydraulic cylinder 28. 
A plastic extruder assembly 30 is arranged to engage a gate 32 in the mold 
element 16 with an injection nozzle 34 at the outlet end of the extruder. 
The main portion of the extruder comprises an extruder barrel 36 having a 
rotating plasticating screw 38 therein which receives particulate material 
from a supply 40, and via heat and manipulation plasticates the material 
for injection through nozzle 34 into the mold cavity 22. To aid in the 
plastication of the material the extruder barrel is provided with encasing 
heater elements 42 and 44, in a manner well known in the art. The screw is 
rotated by a gear 46 driven by a motor, not shown, and is driven 
longitudinally to inject the molten material into the mold cavity by means 
of a hydraulic cylinder 48. The hydraulic cylinder 48 is provided with 
hydraulic fluid from a power source in order to drive the screw 
longitudinally during the injection process. The hydraulic fluid supply 
provides both a high pressure for the injection phase of the cycle as well 
as a low pressure for the holding phase of the cycle, as is well known in 
the art. One example of such a hydraulic supply comprises two separate 
sources of high, injection pressure 50, and lower, holding pressure 52 
connected by lines 54 and 56 to a control valve 58 which determines which 
pressure is supplied, by line 60, to the hydraulic cylinder 48. 
While a reciprocating screw injection molding machine is illustrated for 
the purposes of describing the present invention, it will be appreciated 
by those skilled in the art that other forms of injection molding machines 
such as plunger and transfer-compression molding machines may also be 
employed. 
A distance sensor 62 is mounted on the stationary mold element 16 adjacent 
part-line 20. A distance sensor target 64 is mounted on the movable mold 
element 18 in opposition to the sensor 62. The target may comprise an 
adjustable bolt or pin member 66 which is arranged to provide the target 
for sensor 62. The sensor and target are arranged to come into close 
proximity when the mold elements are closed and clamped but are carefully 
positioned so that at no time do they contact one another. The sensor 
element 62 may be of any type known in the art including capacitive, 
inductive, optical, or other type proximity sensor having a substantially 
linear output over a range from +10 volts to -10 volts representing a 
distance range of 0.020 inches. The proximity sensor 62 provides an analog 
output signal via line 68 to a central processing unit, or controller 70, 
the operation of which will be described herein below. The controller 70 
is arranged to provide an output signal via line 72 to actuate a portion 
of the molding apparatus, such a valve actuator 74, which is connected to 
valve 58. Thus, when controller 70 receives the appropriate signal from 
the proximity sensor 62, it provides an output to valve actuator 74 which 
switches the valve 58 from the high injection pressure 50 to the lower, 
holding pressure 52 to thereby control the cycle of the injection molding 
machine in accordance with the present invention. 
It has been found that when an injection molding machine is operated with a 
sensor sufficiently sensitive to accurately measure the part-line 
separation between the mold elements 16 and 18, that a characteristic 
time/displacement (separation) curve is generated for that machine. It has 
also been found that the part-line separation dimension represented by 
this curve reflects and integrates the multiplicity of variables operating 
on the molding machine during the current molding cycle. These variables 
include mold and machine rigidity, clamp pressure variations, friction and 
inertia in the mold clamping system, the machine shot size, melt viscosity 
and temperature consistency, the characteristics of the plastic being 
molded, and the characteristics of the mold and runner system employed. 
One example of such a time/displacement curve is illustrated in FIG. 3 and 
will be referred to in the following description of the operation of the 
present invention in conjunction with the logic chart illustrated in FIG. 
4-1 to 4-4. The curve represents the variation of the part-line separation 
with respect to time during a single molding cycle of an injection molding 
machine operating at a steady state condition after stabilization 
following start-up. As the empty mold elements begin to close, the 
part-line sensor will start to indicate the part-line separation as the 
mold elements approach each other. As the mold elements approach a 
predetermined separation S.sub.1, which is designated the Entry Threshold, 
for example a separation of 0.0095 inches, the high pressure clamp system 
on the injection molding press is disabled, actuating the low-pressure 
protection portion of the system. As the Entry Threshold is crossed by the 
continued closing of the mold elements the master counter/timer is 
actuated at T.sub.1 and a "bad part sort" output signal is disabled or 
turned off. As the mold elements continue to close, the part line 
separation reaches a "low pressure protection" separation S.sub.2 and the 
machine high-pressure clamp is enabled permitting the high pressure 
clamping of the mold so that the molten plastic material may be injected 
into the mold cavity. Thereafter, the timer reaches the mold closed offset 
point T.sub.2, and the part-line separation sensor is read to determine 
the actual measurement of part-line separation sensed after final closing 
and clamping of the mold elements at S.sub.3. The separation value 
S.sub.3, for example 0.005 inches, is then stored in the controller memory 
for use later in the program. After it is determined that the mold 
elements have been clamped together, the injection of the molten material 
into the mold cavity is initiated with the injection ram 30 operating 
under the high injection pressure 50. As the mold fills during the time 
from T.sub.2 to T.sub.3, the part-line separation value remains 
substantially constant at S.sub.3 until the molds have been filled and the 
injection ram begins to pack out the mold. At this time T.sub.4, the mold 
elements begin to separate along the part-line. When the part-line 
separation reaches a predetermined Control Point S.sub.4, a control signal 
is delivered to the valve operator 74, switching valve 58 from the high 
injection pressure 50 to the lower holding pressure 52. 
Thereafter, because of the finite lag in the signal activating the valve 58 
and the injection ram responding to the change in pressure, as well as 
other inertial factors in the operation of the machine, the part-line 
separation will continue to increase until it reaches a maximum S.sub.6. 
At that point the material in the mold will begin to cool and shrink and 
the part-line separation will fall back to approximately the initial mold 
closed separation S.sub.3 during the curing or cooling phase of the 
molding cycle. Thereafter, the mold will be opened and the part ejected 
and the sensor will indicate that the part-line separation has exceeded 
the Exit Threshold S.sub.8. When the maximum part-line separation value 
S.sub.6 is detected and measured, the part-line value is calculated by 
subtracting from the maximum part-line separation S.sub.6 the mold closed 
separation value S.sub.3 giving a part-line separation value for each part 
produced by the mold. Inasmuch as the mold closed separation value is 
determined for every cycle, variations in the performance of the machine 
is accommodated by rezeroing the mold closed value for every cycle of the 
machine. 
Examples of other part-line separation curves occurring on the illustrative 
machine are also illustrated in FIG. 3 as dotted and dash-dot lines. These 
curves illustrate variations in the part-line that may occur because of 
variables acting upon the overall system. For example, should some minor 
change slightly reduce the clamping pressure during a given molding cycle, 
the maximum part-line separation might increase as illustrated by the 
dotted line. So long as the increase in part-line separation does not 
exceed the operator-selected maximum separation S.sub.7, then the part 
produced will still be considered acceptable. Likewise, should the mold 
clamp force, as an example, be increased during a mold cycle, the final 
part-line separation would be reduced as illustrated by the dash-dot line. 
Again, so long as the final part-line separation is above the 
operator-selected minimum separation S.sub.5 the part will be considered 
acceptable. On the other hand, should the maximum part-line separation 
achieved fall outside the maximum and minimum part-line separation values 
selected by the operator, then a "bad part sort" output signal will be 
generated by the controller which may be utilized by the system or by the 
operator to identify or remove the out-of-specification part. 
The process is initiated after the sensor has been installed in the molding 
machine and the controller wired to the appropriate controlling portion of 
the machine. The injection molding machine is then operated by a competent 
operator to produce acceptable parts. During this operation the controller 
is set to the "monitor mode" whereby part-line separations are measured 
and monitored with appropriate data being retained in the controller 
memory. Appropriate adjustments are made to the process by the machine 
operator to achieve satisfactory molded parts. As the satisfactory parts 
are identified, the part-line separation measurements made for those 
satisfactory parts are utilized to determine the desired predetermined 
control point as well as the maximum and minimum part-line separations to 
be used for controlling the process. At the same time, the appropriate 
time offsets are also being selected by the operator according to the 
particular characteristics of that individual machine. As soon as 
sufficient data has been collected to ensure the operator that the 
sampling is representative, and the values have been set into the 
controller, the controller may be switched to the "control mode" wherein 
it commences the control of the injection molding process. 
Referring now to FIG. 3a wherein a part-line separation graph similar to 
that of FIG. 3 is illustrated, the effect of varying the selection of the 
control point is illustrated. For example, should a lower control point 
S.sub.4 ' be selected, occurring at an earlier time T.sub.4 ', the 
resulting part-line separation following the control point would be as 
represented by the dotted line and would result in a lower maximum 
part-line separation S.sub.6 '. As illustrated, should such a lower 
control point be selected, the part produced would still fall within the 
preselected maximum/minimum and thus fall within the acceptable part 
quality window with a finite reduction in the material utilized to produce 
the acceptable part. This provides an incremental saving in the quantity 
of material required to produce satisfactory parts. At the same time, it 
will be noticed that the cycle time could also be shortened, if desired, 
while still producing satisfactory parts. On the other hand, if a control 
point S.sub.4 " is selected which is greater than that previously 
utilized, the resulting part-line separation following the control point 
would be represented by the dash-dot line and the maximum part-line 
separation S.sub.6 " would be increased. Thus, the operator has the option 
of increasing or decreasing the average part size and weight by simply 
adjusting the preselected control point. 
The effect of such control is illustrated in the graphs shown in FIGS. 5 
and 6, wherein the part weight measured during a series of molding cycles 
is plotted. In the upper portion of FIG. 5, the part weight measured on 
parts produced with the control aspect of the present invention turned off 
is illustrated and shows a wide variation in part weight achieved. In the 
lower portion of FIG. 5, the control feature of the present invention has 
been employed and is controlling the molding machine. This shows a 
significant reduction in variations in part weight. The frequency and 
percentage of total parts falling within a particular part weight category 
is then illustrated for the same parts as illustrated in FIG. 5 in FIG. 6. 
It is readily apparent that the part weight consistency of the controlled 
machine output is significantly improved. The average size can also be 
slightly reduced since the assurance of consistency and repeatability of 
part size and weight provided by the present invention allows for the 
reduction of mean-part weight. The operator need now only use that amount 
of raw material required to achieve the smallest statistically acceptable 
product resulting in additional material savings. for a material savings 
while still remaining above the minimum part weight determined for the 
uncontrolled operation of the machine. 
The various timing offsets (e.g., T.sub.2 and T.sub.3 in FIG. 3) provided 
by the present system permits an operator to select the appropriate timing 
intervals appropriate to that particular machine. Moreover, the timing 
offsets also function as blanking signals to limit the reading of the 
part-line separation sensor to the appropriate period in the cycle. Thus, 
any vibration or jitter present in the machine that occurs during the 
blanked portions will not provide spurious part-line separation signals 
that could adversely affect the overall machine control. 
Further, the present invention provides a number of protective features for 
the molding machine. One of these is the low pressure protection portion 
of the program whereby the controller disenables the machine high pressure 
clamp system until the satisfactory closing of the mold is assured. Thus, 
if a part from a previous cycle has been retained between the mold 
elements, the machine is prevented from clamping down on this retained 
part and possibly damaging the molds. Similarly, the system is adapted to 
sense any plugged gates which will prevent the filling of the mold. Still 
further, by the system provided, "oil canning" of the molds can still be 
accommodated while providing measurement of part-line opening to determine 
part weight. 
Still further, the use of the present invention can be helpful to a machine 
operator to achieve machine start-up after shut-down, such as for changing 
molds. Typically, it has been found that utilizing the present invention 
initial machine start-up time has been reduced by up to 80%. Also, it has 
been found that with the present invention it is possible to mold 
satisfactory parts at lower clamp pressures than previously required for a 
given machine. This not only reduces operating costs for the machine, but 
also extends the life of the machine and permits the manufacture of some 
parts on lower tonnage machines than were previously thought possible. 
Accordingly, the present invention provides method and apparatus for 
controlling an injection machine and process by using the measurement of 
the separation of the mold elements as a verification of achieving product 
quality. The control and measurement of the part-line separation assures 
part completion as well as part uniformity and quality. The present 
invention provides verification that all of the variable parameters in the 
molding machine and process are combining to achieve the specified part. 
Because of the improved product quality provided by the present invention, 
reductions in part rejects of as much as 98% have been achieved. This 
results in improved costs by minimizing the amount of material regrind 
necessary as well as the improved quality raw material from reduced 
regrind. With a reduction in rejects also comes a reduction in labor. 
Still further, older molding machines are capable of producing higher 
quality products with reduced labor increased flexibility permitted by 
frequent mold and process changes while still providing the requisite 
product quality. 
The invention has been described in detail with particular reference to a 
presently preferred embodiment, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.