Gas control unit and process for gas-assisted injection molding

A gas control unit for a gas-assisted injection molding system, wherein a quantity of gas is introduced into a mold cavity in combination with a quantity of plastic material during the molding process. The gas control unit operates on the basis of regulating the mass of gas within the system, as opposed to the final hold pressure. Because the mass of the gas is constant within the gas control unit throughout an injection cycle, a preferred initial-to-final (hold) pressure ratio can be determined through mathematical computations for a particular molding system. As a result, once the required mass of gas has been isolated from a suitable source, releasing the gas to the mold cavity results in a predictable pressure. This capability permits the gas control unit to determine whether the gas has fully expanded and, therefore, whether the gas has properly filled the mold cavity so as to produce a properly molded article.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to control systems for regulating 
the flow of fluids. More specifically, this invention relates to a gas 
control system for regulating the flow of a gas into a mold during an 
injection molding process, wherein the quantity of gas injected is 
accurately controlled in a manner that permits statistical monitoring and 
evaluation of the molding process. 
2. DESCRIPTION OF THE PRIOR ART 
Within the plastic molding industry, it is well known to inject a 
pressurized gas along with a quantity of molten plastic, or melt, into the 
mold cavity during the molding process. Because the pressurized gas will 
flow along a path of least resistance, within the mold cavity the gas 
flows through the warmer, less viscous melt which is located away from the 
mold cavity surfaces. As a result, the gas generally flows through the 
approximate center of the flow path cross section, urging the melt into 
contact with the surfaces of the mold cavity. At the same time, the gas 
forms a hollow channel or void within the molded article during the 
molding process. 
The use of gas serves several useful purposes. By continuously applying 
pressure throughout the molding cycle, the gas maintains the melt in 
contact with the mold cavity so as to force the melt to assume the shape 
of the mold cavity and thereby reduce the tendency for surface flaws, sink 
holes and warpage. To achieve this aspect, the gas must remain pressurized 
until the melt has sufficiently cooled within the mold cavity. Another 
benefit is that the gas assists in forming long and narrow molded articles 
in that the gas rapidly urges the melt through the mold cavity, preventing 
the melt from prematurely solidifying prior to completely filling the mold 
cavity. Again, because the gas will tend to flow through the least viscous 
melt at the approximate center of the mold cavity, the melt will cool and 
solidify from the surface inward towards the center of the mold cavity. 
This effect yields another advantage in that the resulting hollow 
structure of the molded articles will, by nature, be lighter, using less 
plastic material for a given geometry and size. 
Numerous gas-assisted molding methods and systems have been suggested by 
the prior art. Examples include U.S. Pat. Nos. 5,039,463 to Loren and 
5,056,997 to Hayashi et al., each of which control the injection of the 
gas from the standpoint of controlling the gas pressure. Specifically, 
both Loren and Hayashi et al. attempt to perform the gas-assisted 
injection process by introducing and temporarily holding the gas at one or 
more predetermined gas pressure levels, or "hold" pressures, within the 
mold cavity until the plastic has sufficiently solidified. 
Though both Loren and Hayashi et al. each begin with a "fixed" volume of 
gas, the references teach that it is specifically the pressure which is 
regulated at a predetermined level during the injection process in order 
to accomplish the invention set forth therein. In essence, the volume of 
gas used by both Loren and Hayashi et al. varies during the injection 
process in that the hold pressure is regulated from a higher pressure, 
necessitating that some gas will be vented from the system to attain the 
preferred, lower hold pressure. For example, Loren teaches charging a 
receiver with a gas to an extremely high fixed pressure (14,000 psi), and 
then regulating the gas to a desired pressure level (6000 psi) before 
introducing it into the mold cavity. The desired pressure level can be 
sustained throughout the mold cycle due to the reservoir of gas available 
from the receiver. 
Alternatively, Loren teaches that a fixed volume storage system can be 
charged to 6000 psi by the receiver, after which the fixed volume storage 
system is shut off from the receiver. The 6000 psi gas within the fixed 
volume storage system can then be vented to the mold cavity, such that the 
mold cavity is initially pressurized to 6000 psi with the gas. The 
pressure is then gradually reduced to about 1000 psi by a regulating 
relief valve. The 1000 psi pressure is then held for the remainder of the 
cycle, during which the plastic melt solidifies. 
While controlling the gas-assisted injection process by regulating the 
pressure generally works satisfactorily, a significant disadvantage with 
such an approach is that there is no means provided by which an operator 
can determine whether the gas has actually reached or entered the mold 
cavity. As with the teachings of Loren, the gas pressure is both regulated 
and detected well upstream of the mold cavity. As a result, the gas supply 
line can unknowingly be obstructed downstream from the regulator or 
pressure sensor by such things as contaminants in the gas or by plastic 
within the sprue or gates which feed both the gas and plastic melt to the 
mold cavity. In a high capacity production process such as injection 
molding, parts resulting from the above defective process will typically 
go unnoticed until many parts have been produced, resulting in a 
substantial loss of time and a high scrap rate. 
It is obviously impractical to regulate the gas pressure downstream of the 
mold cavity during the injection cycle in an attempt to overcome the above 
shortcoming. While it is possible to provide a pressure sensor in direct 
communication with the mold cavity, such an approach is generally 
unacceptable from the standpoint of surface blemishes and defects created 
on the molded article. Even if attempted, the pressure sensor may likely 
relay a faulty reading due to being in direct contact with the liquid 
plastic and not the pressurized gas. Furthermore, such a pressure sensor 
would be exposed to cyclical high temperatures, which would have an 
adverse effect on the accuracy and life of the sensor. 
From the above discussion, it can be readily appreciated that the prior art 
does not disclose a gas-assisted injection system which is capable of 
immediately detecting when the gas has failed to enter and expand the melt 
within the mold cavity. Consequently, the prior art is unable to 
immediately notify a system operator or cause a system shutdown upon the 
occurrence of such a failure. 
Accordingly, what is needed is an uncomplicated system for injecting gas 
into a mold cavity wherein the system is able to immediately and 
continuously detect whether the gas has in fact entered the mold cavity 
and, as a result, whether the injection molding process is producing 
flawed parts from the standpoint of inadequate gas fill or pressure during 
the molding operation. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a gas control unit for 
a gas-assisted injection molding system, wherein a quantity of gas is 
introduced into a mold cavity in combination with a quantity of plastic 
material during the molding process. The gas control unit of the present 
invention is versatile in that its operational approach is applicable to 
practically any injection molding application, encompassing diverse sizes 
and geometries of mold cavities. Furthermore, the gas control unit is 
uncomplicated, relying upon flow components which are well known in the 
art. 
A feature which distinguishes the gas control unit of the present invention 
from the prior art is its ability to supply a predetermined mass of gas to 
the mold cavity, as opposed to a predetermined pressure. In that the mass 
of the gas is known, its volume and pressure can be determined at any 
given time through mathematical computations during the injection cycle. 
As a result, once the predetermined mass of gas has been isolated from a 
suitable source, its pressure will be primarily dependent upon the volume 
of the system. This capability permits the gas control unit to determine, 
through monitoring pressures, whether the gas has been fully injected 
within the mold cavity, which is necessary to produce a properly molded 
article. 
The gas control unit includes an accumulator for containing the 
predetermined mass of gas, a control valve upstream of the accumulator and 
a control valve downstream of the accumulator. The injection molding 
apparatus is placed downstream from the downstream control valve, and a 
pressure sensing device is located between the accumulator and the mold 
cavity, and more preferably near the accumulator. The accumulator is 
charged with a suitable gas, and preferably an inert gas such as nitrogen, 
from a high pressure source. 
The charging of the accumulator is regulated by the upstream control valve, 
which is closed after an appropriate mass of the gas has accumulated in 
the accumulator. During the charging operation, the downstream control 
valve remains closed to prevent flow to the mold cavity. Once the 
appropriate mass of gas is accumulated within the accumulator, the 
downstream control valve vents the mass of gas to the mold cavity such 
that the mass is distributed between the accumulator, the mold cavity and 
the flow passage therebetween. Once the gas has reached steady state, its 
pressure is compared to a predetermined pressure level to monitor and 
evaluate the injection cycle. 
The mass of gas which is required for a successful injection cycle is 
determined by the needs of the injection system and, more specifically, 
the volume of the mold cavity and the pressure which is preferably held as 
the plastic melt cools and solidifies. This "hold" pressure will serve as 
the predetermined pressure against which all injection cycles are 
compared. By applying the ideal gas law, or any suitable adaptation 
thereof, the pressure which must be initially attained within the 
accumulator to produce the desired hold pressure can be determined. 
As noted above, the pressure at the end of the injection cycle is compared 
to the predetermined pressure so as to determine whether the gas has fully 
expanded throughout the injection system, including the mold cavity. It is 
preferable, but not necessary, that the gas control unit include a 
programmable logic controller or other suitable control device for 
evaluating the pressure at the end of the cycle. Using a logic controller 
permits the entire injection system to be coordinated to operate 
efficiently. In addition, the system can be adaptive in that compensations 
can be made for temperature and other transient influences. 
According to a preferred aspect of this invention, the gas control unit 
relies upon delivering a mass of gas through the injection system so as to 
permit the evaluation of the injection cycle based upon the final gas 
pressure at the end of the cycle. Knowledge of this pressure enables the 
gas control unit to immediately and continuously detect during subsequent 
cycles whether the gas has in fact entered the mold cavity and, as a 
result, whether the injection molding process has produced a good or 
flawed part from the standpoint of adequate gas fill within the mold 
cavity. 
In addition, the gas control unit can be adapted for use under differing 
modes of operation, whether it be primarily manual control by an operator, 
or automatic control during high volume production, in which the gas 
control unit can regulate the injection molding process with minimal 
attention from operators. 
Another significant advantage of the present invention is that the gas 
control unit is relatively uncomplicated, relying upon a minimal number of 
well known fluid control devices which are conventionally in a system for 
injecting gas into a mold cavity. However, the manner in which the devices 
are used together is entirely different in terms of the parameter being 
controlled, in that the mass of the gas is regulated at the beginning of 
the cycle, instead of pressure being regulated at the end of the cycle. 
In addition, the gas control unit can be made adaptive by incorporating a 
logic controller. By determining what final gas pressure is necessary to 
produce good parts, the system can accumulate the necessary mass of gas to 
produce the final pressure, while compensating for changes in temperature 
of the gas. 
Accordingly, it is an object of the present invention to provide a 
gas-assisted injection molding system for injecting gas into a mold 
cavity, wherein the system is able to immediately and continuously detect 
whether the gas has sufficiently filled the mold cavity as a method for 
determining whether the injection molding process is producing acceptable 
or flawed parts. 
It is a further object of the invention that the gas-assisted injection 
molding system operate on the basis of a predetermined mass of gas within 
the system by which gas pressure can be monitored to evaluate the 
operation of the molding operation. 
It is still a further object of the invention that the gas-assisted 
injection molding system be capable of providing a specific volume of gas 
to the mold cavity, which is delivered in conjunction with the need to 
fill the mold cavity with a specified quantity of plastic melt. 
It is another object of the invention that the predetermined mass of gas be 
determined on the basis of the desired hold pressure during which the 
molten plastic solidifies to form the molded part. 
It is yet another object of the invention that the gas-assisted injection 
molding system be readily adaptable in terms of sizes and geometries of 
mold cavities which can be used, fluctuating ambient temperatures, and 
various hold pressures and durations necessitated for a specific molding 
process. 
It is still another object of the invention that the gas-assisted injection 
system require a minimal number of fluid control devices so as to be 
relatively uncomplicated and thereby optimize its maintainability and 
cost. 
Other objects and advantages of this invention will be more apparent after 
a reading of the following detailed description taken in conjunction with 
the drawing provided.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the Figure, there is shown schematically a gas-assisted 
injection molding system 10 which includes a gas control unit 12 according 
to the present invention. The gas control unit 12 operates with the 
molding system 10 to produce plastic molded articles 21 which are 
characterized by having one or more hollow portions 20 formed within their 
cross section. 
As is conventional, the molding system 10 includes an injection nozzle 14 
for injecting molten plastic, or melt, into a mold 16. The mold 16 defines 
a mold cavity 22 by which the final geometry of the molded article 21 is 
determined. The injection nozzle 14 is supplied with a quantity of plastic 
from a suitable source, which is conventionally a hopper 18. The hopper 18 
will generally supply the injection nozzle 14 with plastic pellets which 
are plasticized, or melted, within the barrel of the injection nozzle 14 
prior to being injected into the mold cavity 22. 
During the operation of the gas-assisted injection molding system 10, a 
quantity of gas is injected into the mold cavity 22 along with a specified 
quantity of melt. The gas most often chosen for molding operations is 
nitrogen, which is preferable, in that nitrogen is inert to plastic 
compounds. Once the gas and melt are injected into the mold cavity 22, the 
melt will cool to form the molded article 21. While being injected, the 
gas will establish itself centrally within the mold cavity 22, forming the 
hollow portion 20 within the molded article 21. The gas forces the melt 
against the mold cavity 22 to ensure that the final molded article 21 
acquires the internal geometry of the mold cavity 22. At the same time, 
the molded article 21 advantageously acquires a hollow cross section as a 
result of the hollow portion 20 defined by the gas, thereby reducing the 
weight of the molded article 21 and the quantity of plastic necessary to 
form the molded article 21. 
At the completion of the mold cycle, the gas is vented from the mold cavity 
22 and the molded article 21 is ejected from the mold cavity 22 by 
ejection pins (not shown), in a conventional manner. 
According to the present invention, the gas control unit 12 operates to 
introduce a predetermined volume of gas into the mold cavity 22 in 
combination with a specified quantity of plastic melt. The volume of gas 
required is derived from the simple relationship: 
EQU V.sub.gas =V.sub.cavity -V.sub.plastic 
wherein V.sub.gas is the volume of gas required within the mold cavity 22, 
V.sub.cavity is the internal volume of the mold cavity 22, and 
V.sub.plastic is the desired quantity of plastic for the molded article 
21, taking into consideration the strength, rigidity, etc., required of 
the molded article 21. 
To provide this specific volume of gas, the gas control unit 12 includes an 
accumulator 26 of a type well known in the art. The principal 
characteristic of THE accumulator 26 is that it must have a sufficient 
capacity to house the gas prior to the gas being vented to the mold cavity 
22. More specifically, the accumulator 26 is pumped to a pressure to 
contain a predetermined mass of the gas which is sufficient, at a 
preferred "hold" pressure level, to fill a volume V defined by: the 
accumulator 26; V.sub.gas for the particular mold cavity 22 and molded 
article 21; and a passage between the accumulator 26 and the mold cavity 
22. The gas contained within the accumulator 26 remains entirely within 
the gas control unit 12 during the injection operation and, according to 
the operation of the gas control unit 12, the hold pressure is not 
directly regulated. In fact, the hold pressure is determined strictly by 
the amount of gas initially stored within the accumulator 26 at the 
beginning of a molding cycle. 
Typically, hold pressures are less than about 6000 psi, and vary with the 
particular molded article 21 being produced. Because the gas control unit 
12 operates with a specific mass of gas, attaining the desired hold 
pressure after the gas is released from the accumulator 26 to the mold 
cavity 22 requires several preliminary molding cycles with close 
inspection of the molded article 21 to determine whether proper expansion 
has occurred. As will be explained below, the gas control unit 12 of the 
present invention substantially automates this process. 
Theoretically, by knowing the volume of gas necessary to fill the 
accumulator 26, mold cavity 22 and the passage therebetween at the hold 
pressure, the mass of the gas which must be accumulated in the accumulator 
26 prior to the injection cycle can be determined according to the ideal 
gas law: 
EQU m=PV/RT, 
wherein m is the mass of the gas; P is the desired hold pressure at the end 
of the molding cycle; V is the combined volume of the accumulator 26, 
V.sub.gas, and the passage between the accumulator 26 and the mold cavity 
22; R is the ideal gas constant for the gas; and T is the gas temperature. 
While the ideal gas law is often sufficiently accurate for an estimate of 
gas behavior, it will be readily appreciated by those skilled in the art 
that more accurate computational methods are known and can be employed 
within the teachings of this invention. 
With knowledge of the mass of gas which must be accumulated, the required 
pressure to which the accumulator 26 must be charged can also be 
determined by the ideal gas law: 
EQU P=mRT/V, 
wherein P is the initial pressure within the accumulator 26; m is the 
previously calculated mass of the gas; R is the ideal gas constant for the 
gas; T is the gas temperature; and V is the volume of the accumulator 26. 
From the above, the following fundamental parameters are now known: the 
required mass of gas (though possibly based on a preliminary hold 
pressure), and the initial and hold pressures for the gas. The remaining 
variables for the ideal gas law--volume and temperature--are highly 
repeatable within a particular injection apparatus once the molding system 
10 reaches a continuous, steady-state operation. Therefore, once a 
suitable hold pressure has been determined for the particular process, all 
that is necessary to successfully operate the molding system 10 is to 
properly charge the accumulator 26 with the required mass of gas. 
Because the gas control unit 12 operates with a constant mass of gas, and 
the temperatures and volumes for the system will remain substantially 
constant under normal operating conditions, only pressure remains as a 
variable under the ideal gas law. As a result, the gas control unit 12 can 
operate solely on the basis of the ratio between the hold and initial 
pressures for a particular molding operation. Because the gas control unit 
12 operates with a constant mass of gas, according to the ideal gas law: 
EQU P.sub.1 /P.sub.2 =(V.sub.1 /T.sub.1)/(V.sub.2 /T.sub.2) 
this relationship allows for two general modes of operation for the gas 
control unit 12. The first mode involves estimating the initial pressure 
needed to acquire the desired hold pressure and, during several injection 
cycles, making manual adjustments to the initial pressure until the 
desired hold pressure is achieved. The second mode involves designating a 
hold pressure and allowing a computer, during several injection cycles and 
mathematical iterations, to determine the required initial pressure which 
will attain the designated hold pressure. 
The fluid control devices necessary to perform the above operations are as 
follows. To charge the accumulator 26, the present invention requires only 
a suitable high pressure source for the gas, such as a booster 24. 
Typically, the booster 24 will deliver gas to the accumulator 26 at a 
pressure of about 10,000 to about 15,000 psi. A flow control valve may be 
placed between the booster 24 and the accumulator 26 to permit accurate 
control of the gas flow rate. However, the preferred embodiment does not 
contemplate the use of such a flow control valve. Immediately upstream of 
the accumulator 26 there is a normally-closed on-off valve ("upstream 
valve 30"). The upstream valve 30 enables the passage between the booster 
24 and the accumulator 26 to be rapidly and reliably closed once the 
proper mass of gas has been accumulated. 
Downstream from the accumulator 26 is a second normally-closed on-off valve 
("downstream valve 36"). The downstream valve 36 is operable to both 
accumulate the gas within the accumulator 26 and vent the gas to the mold 
cavity 22. Pressure gages 34 and 38 on either side of the downstream valve 
36 enable an observer to quickly determine whether or not gas has been 
vented to the mold cavity 22. 
A pressure transducer 32 senses the pressure of the gas accumulated within 
the accumulator 26. The pressure transducer 32 operates to monitor the 
pressure within the accumulator 26 so as to determine when the accumulator 
26 has been charged with the required mass of gas, i.e., the initial 
pressure for the injection cycle. The pressure transducer 32 also detects 
the hold pressure at the end of the injection cycle, as an indication of 
gas volume based upon the presence of the known mass of gas which was 
accumulated at the beginning of the injection cycle. It is preferable to 
measure the hold pressure at the end of the cycle near the accumulator 26 
in that positions further downstream from the accumulator 26 may be 
unstable during the injection cycle because of the gas motion and sudden 
changes in pressure. 
As previously noted, over time, the temperature of the gas within the gas 
control unit 12 will stabilize near ambient temperature. In addition, the 
gas temperature within the mold cavity 22 will rapidly attain a 
temperature substantially close to the melt temperature. These conditions 
will be highly repeatable once the molding system 10 reaches a 
steady-state operating condition. Accordingly, it is not necessary that 
temperature be measured continuously. However, if desired the gas control 
unit 12 can be controlled to compensate for temperature effects. If more 
accurate determinations are desired, a temperature transducer may be 
directly used by the gas control unit 12. 
Connected to the passage which connects the downstream valve 36 to the mold 
cavity 22 is a vent line which vents the gas to atmosphere after the 
molding cycle is complete. The vent line includes a normally-closed on-off 
valve ("vent valve 40"), a flow control valve 42 and a muffler 44. The 
vent valve 40 remains closed other than when it is venting the gas within 
the mold to atmosphere. 
To coordinate and automate the operation of the gas control unit 12, a 
programmable logic controller 48, or other suitable control device, is 
preferably included within the molding system 10. The controller 48 
preferably communicates with the booster 24, the upstream valve 30, the 
downstream valve 36, a temperature transducer, the pressure transducer 32, 
the vent valve 40 and the injection nozzle 14. 
The controller 48 can be employed to either command the booster 24 to 
operate until the accumulator 26 has been fully charged, and thereafter 
shut down, or the controller 48 can simply monitor the booster 24 to 
ensure that it is operating properly and supplying gas according to 
requirement. The injection nozzle 14 can be monitored and controlled from 
the perspective of signalling the beginning of the injection process and 
detecting the end of the injection process, at which time the controller 
48 can command the molded article 21 to be ejected from the mold cavity 
22. 
Primarily, the controller 48 serves to synchronize the operation of the 
on-off valves 30,.36 and 40 to properly route the gas through the gas 
control unit 12 cycle. The initial and final (hold) pressure signals 
received from the pressure transducer 32 permit the controller 48 to 
determine the charging of the accumulator 26 and to evaluate the initial 
pressure requirements on the basis of the resulting hold pressure. The 
controller 48 is, therefore, capable of automating the second of the two 
previously-described modes of operation. Specifically, an operator can 
select a preferred hold pressure with knowledge of the mass of the gas 
required for the application and allow the controller 48 to initially 
estimate, make the necessary adjustments, and eventually attain the 
required initial pressure over several injection cycles. Afterwards, the 
gas control unit 12 will operate on the basis of charging the accumulator 
26 to the required constant mass of gas at the initial pressure and 
signalling an error if the hold pressure significantly deviates from the 
originally selected hold pressure. 
By regulating each of the above devices with the controller 48, the 
operation of the gas control unit 12 essentially becomes automatic. A 
typical cycle will entail the controller 48 sensing the end of the 
previous injection cycle, and then closing each of the valves 30, 36 and 
40. Gas can then be delivered to the accumulator 26 by opening the 
upstream valve 30, while the downstream valve 36 and the vent valve 40 
remain closed. As set forth above, once the controller 48 has determined 
the initial pressure necessary to attain the desired hold pressure, the 
upstream valve 30 can be commanded by the controller 48 to remain open 
until the pressure transducer 32 detects the required initial pressure. At 
this time, the upstream valve 30 will close and, after a suitable system 
delay where desired, the downstream valve 36 will open to vent the gas to 
the mold cavity 22. In doing so, the predetermined mass of gas will become 
distributed between the accumulator 26, the mold cavity 22, and the 
passage therebetween. The gas will quickly stabilize and its final hold 
pressure will be sensed by the pressure transducer 32. 
Alternatively, the gas can be released in pulses to the mold cavity 22 by 
cycling the downstream valve 36 through a series of successive venting 
steps until all of the mass of gas has been substantially vented to the 
mold cavity 22. Such a procedure may be desirable under particular 
circumstances, such as when different areas of the mold cavity 22 are to 
be filled at different times. Each pulse may be programmed to deliver a 
predetermined mass of gas, in that the pressure remaining within the 
accumulator 26 provides a continuous indication of the volume of gas 
remaining in the accumulator 26. In addition, the controller 48 can be 
programmed to anticipate each level of gas to be delivered to permit the 
downstream valve 36 to close slightly in advance of the desired level in 
situations where the response time of the downstream valve 36 is 
inadequate. 
Being the primary factor by which the molding cycle is judged, the desired 
hold pressure can be compared to the actual hold pressure sensed by the 
pressure transducer 32. Any deviation in excess of the desired hold 
pressure will indicate that the gas has not properly expanded the molded 
article 21 within the mold cavity 22. In the same way, any deviation below 
the desired hold pressure will indicate that there is a system 
malfunction, such as inadequate boost pressure from the booster 24 or a 
malfunction in one of the valves 30, 36 and 40. A tolerance limit can be 
defined above and below the desired hold pressure which can be the basis 
for either sending an error signal to a control panel or, particularly 
where there is a substantial deviation, shutting down the system. 
As a final step in the operation of the molding system 10, the downstream 
valve 36 is closed and the vent valve 40 is opened to vent the gas within 
the mold cavity 22 to atmosphere. The flow control valve 42 modulates the 
rate at which the gas will be vented, while the muffler 44 reduces the 
noise level which results from the release of high pressure gas to the 
atmosphere. 
From the above, it can be seen that a significant advantage of the gas 
control unit 12 of the present invention is that the gas control unit 12 
permits the evaluation of the injection cycle ergo, the molded article 
based upon the final hold pressure at the end of the cycle. Knowledge of 
the resulting hold pressure enables the gas control unit 12 to immediately 
and continuously detect whether the gas has in fact completely entered the 
mold cavity 22. As a result, the injection molding process can be 
continuously evaluated using statistical methods to ascertain immediately 
whether a flawed part has been produced. 
In addition, the gas control unit can be adapted for use under differing 
modes of operation. Specifically: 
(1) The gas control unit 12 can be operated entirely manually, with an 
operator estimating and manually setting the initial pressure within the 
accumulator 26. The resulting hold pressure need not be measured or used, 
though part quality may suffer under mass production conditions if the 
feedback available in the form of the hold pressure is not utilized to 
improve or maintain process quality. 
(2) Alternatively, an operator may estimate and set an initial pressure 
(stored mass) for the accumulator 26. Once, after several attempts and 
readjustments are made to the initial pressure setting, a successful 
injection cycle has been achieved, the controller 48 can calculate the 
preferred hold-to-initial pressure ratio (i.e., the initial-to-final 
volume-to-temperature (V/T) ratio) for the molding system 10. The 
controller 48 can then evaluate subsequent cycles by comparing their 
initial-to-final V/T ratio with the preferred initial-to-final V/T ratio. 
This mode is particularly suitable for high volume production, in which 
the gas control unit 12 can regulate the injection molding process with 
minimal attention from an operator. 
(3) As a final mode of operation, an operator can program the controller 48 
to achieve a preferred hold pressure based entirely on molding 
requirements. The controller 48 can then make all initial assumptions, 
including system volume and temperature, and then estimate the initial 
pressure required within the accumulator 26 to produce the desired hold 
pressure. Thereafter, the resulting hold pressure can be compared to the 
preferred hold pressure, and the controller 48 can make self-adjustments 
to the initial pressure within the accumulator 26 until the desired hold 
pressure is achieved, at which time all of the relevant parameters are 
stored to memory. 
This mode is particularly useful when controlling an initial run by which 
the requirements of a particular part or mold cavity geometry are being 
evaluated for mass production. The operational mode of the gas control 
unit 12 can then be switched to the second mode described immediately 
above for continuous operation. 
Though providing the capabilities described above, the gas control unit 12 
is relatively uncomplicated, relying upon the operation of a minimal 
number of fluid control devices. In addition, only a limited amount of 
information is needed to control a molding operation. Once the system 
desired hold pressure is known, the controller 48 call manage all of the 
mathematical computations necessary to determine the initial pressure to 
which the accumulator 26 is charged. This is possible because the mass of 
the gas within the gas control unit 12 remains constant during a given 
injection cycle. This is contrary to the prior art, in which the gas 
pressure is regulated upstream of the mold cavity to maintain a desired 
hold pressure in the mold cavity. 
While the invention has been described in terms of a preferred embodiment, 
it is apparent that other forms could be adopted by one skilled in the 
art. For example, different mathematical algorithms could be used to 
evaluate the molding system. In addition, different flow control devices 
could be substituted for those described. Accordingly, the scope of the 
invention is to be limited only by the following claims.