Method for reducing cycle time in an injection molding machine

The method of the present invention relates to reducing cycle time in injection molding machines that are running large capacity molds, such as multiple cavity preform molds, and require a high volume supply of quality melt. Specifically, the present invention proposes using a continually plasticizing extruder to supply two melt accumulators which will alternately operate to inject the plastic material into the mold. To accomplish this, a rotary valve at the end of the extruder is controlled to fill the pots, and ball check valves are positioned to control flow direction and limit pressure in certain parts of the system. A shuttle valve between the shot pots and before the nozzle facilitates decompression.

FIELD OF THE INVENTION 
The present invention relates to a method of reducing the cycle time in an 
injection molding machine and more particularly to a method involving 
operation of two melt accumulators that are supplied by a continuously 
operating extruder. 
DESCRIPTION OF THE RELATED ARTS 
With a standard reciprocating screw injection molding machine, material is 
melted on the screw as the screw rotates and retracts in the barrel to 
accumulate a quantity of melt in front of the screw. When a sufficient 
amount of material is accumulated ("a shot"), the screw is moved rapidly 
forward (without rotation) to inject the melt straight into the mold. 
Plasticizing of the material must occur during the cooling and clamp/eject 
portions of the cycle since the screw cannot plasticize during fill, pack, 
and hold (see FIGS. 1 and 2a). This means that in order to plasticize the 
same amount of material in the same cycle time, the plastic must be melted 
in less time. The result is higher screw speeds which leads to higher melt 
temperature. Higher melt temperature can mean longer time required to cool 
as well as higher acid aldehyde (AA) levels and other adverse melt 
effects. Furthermore, since the screw retracts during plasticizing, the 
processing L/D changes causing temperature gradients in the accumulated 
shot. 
A single melt accumulator, reciprocating screw injection molding machine 
plasticizes the material on the screw in the same manner as the standard 
machine and then injects(transfers) the material into the accumulator. The 
accumulator then injects the melted material into the mold. This allows 
the screw to plasticize during the fill, pack and cool/hold portions of 
the cycle, since they are now performed by the accumulator (see FIG. 2b). 
The only time the screw is not plasticizing is when it transfers into the 
accumulator. This is an improvement over the standard machine since it 
allows for longer hold time and additional plasticizing time. Longer hold 
time allows for better cooling of the part while pressure is held on the 
part to stop it from shrinking away from the mold surface. Additional 
plasticizing time overcomes some of the disadvantages noted earlier. 
However, it would be a further improvement to avoid the interruption of 
plasticizing that when the screw is used to inject the plastic melt into 
.the accumulator. 
The prior art has also proposed the simultaneous use of two standard, 
reciprocating screw injection units to allow consistent processing of 
material where large capacity molds with relatively short cycles are 
involved. This approach allows one or the other of the injection units to 
be plasticizing material at any point in time, in effect providing 
continuous plasticizing. While providing a significant improvement in 
processing time as compared to a single unit, the use of two injection 
units doubles the cost of this part of the machine since every component 
is duplicated. Furthermore, some of the disadvantages associated with 
processing material by a reciprocating screw, such as a variation in L/D 
during plasticizing, are still present. Accordingly, a lower cost, 
equivalent process would be the most desirable solution. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of efficiently 
supplying quality melt to a large capacity mold. This is accomplished 
through a dual accumulator arrangement in combination with a 
non-reciprocating screw extruder to allow for quicker cycle times while 
increasing plasticizing time to improve melt and part quality. 
A comparison of the charts given in FIGS. 2a through 2c shows the 
differences between the present invention and prior art systems. The 
sequence to injection mold virtually any thermoplastic part is basically 
the same. There is fill, pack, hold, cool and eject. Obviously, the 
material must be plasticized before filling can occur; the most efficient 
way to accomplish this is to have the plasticizing of the material be 
independent of the molding of the parts. This allows the molder to 
optimize the plasticizing of the material as well as the conditions 
relating to molding of the part. 
The dual accumulator, non-reciprocating screw design of the present 
invention plasticizes material on a fixed extruder and directs the plastic 
melt to the accumulator that is not actively involved in the molding 
function ("the shot-building accumulator"). While that accumulator is 
filling, the other melt accumulator ("the injection accumulator") is 
operating to fill, pack and hold pressure on the mold. When the molding 
operation is finished, the injection accumulator is inactive for a short 
period while the clamp operates. By this time the shot-building 
accumulator has received a full charge of plastic melt and a rotary valve 
on the extruder shifts to divert the flow of melt to begin building a shot 
in the empty accumulator. The accumulators thus alternate so that one 
receives material while the other operates, allowing continuous 
plasticizing by the extruder. 
This design allows for continuous extrusion of the plastic over a fixed L/D 
screw without interruption, as opposed to the variable L/D and 
reciprocating action of the typical injection unit. The entire cycle time 
can be used to build the needed shot of plastic melt. The melt 
accumulators are free to fill, pack and hold the material in the mold 
until the parts are ready to be ejected. In addition, there is no need to 
relieve pressure for material transfer from the screw. 
Accordingly, the present invention provides an extremely efficient method 
for molding parts, especially those involving a large volume of material. 
It allows for better use of the screw and provides more consistent 
results. It can reduce RPM on the screw which can reduce AA and melt 
temperature. Full pressure can be maintained on the mold for almost the 
entire cycle, providing an opportunity for more efficient cooling and 
better packing for thick parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention involves a specific method of injection molding; as 
such, it will be described in connection with a particular type of 
injection molding machine. Since the general structure and operation of 
injection molding machines is well known, those aspects of the apparatus 
which are different or take on a new use with respect to the method will 
receive primary emphasis. 
Preferably, the method of the present invention is applied to an injection 
molding machine 10, as shown in FIGS. 4 and 5. The general configuration 
of the molding machine 10 is typical, including a conventional clamp unit 
12, and a specially designed injection unit 14, both of which are mounted 
on an elongated support or base 16. 
The components of the injection unit 14 are uniquely integrated to 
implement the present molding method. Specifically, the primary elements 
are an extruder 18, and two accumulators 20. The extruder 18 is intended 
for continuous plasticizing and, therefore, has a non-reciprocating feed 
screw 30 (see FIG. 3). Material is supplied to the extruder in any 
convenient manner, such as by a hopper 24. The rotational power for the 
screw 30 is also provided in a conventional manner, as by an electric 
motor 26 or other motive force, connected to a gearbox 28 that drives the 
screw 30. Since the movement of the screw 30 is rotational only, the drive 
system is greatly simplified over the injection units having a screw which 
must also reciprocate. 
The accumulators 20 are preferably identical in construction since they are 
used alternately for the same functions as will be more fully described 
later. Accordingly, specific descriptions of the features, construction 
and capabilities apply equally to both accumulators 20. Each accumulator 
20 is essentially a variable volume reservoir by virtue of a cylindrical 
barrel 32 and a hydraulically actuated piston 34 that moves linearly 
within the barrel 32. The relative size of the barrel 32 and piston 34, as 
well as the stroke of the piston 34, will vary according to the quantity 
of melt required to=fill the mold. In the constriction of melt 
accumulators, it is desirable to configure the end-shapes of the barrel 32 
and piston 34 in a way that minimizes the amount of resin remaining in the 
barrel 32 when the piston 34 is fully extended, as will be more fully 
discussed later. 
At the outlet of the extruder 18 is a rotary valve 36 having outlet ports 
38 and 40 that connect to accumulators 20 and 22, via suitable conduits 42 
and 44. At a suitable point between the outlets 38 and 40 of the valve 36 
and the inlets 39 and 41 to the accumulators 20 and 22, ball check valves 
46 and 48 are provided to control the direction of the flow through 
conduits 42 and 44. The check valves 46 and 48 prevent a pressure increase 
in the extruder 18 when one of the accumulators is activated to inject 
plastic and maintain pressure on the mold during pack and hold. The outlet 
of each accumulator is connected to a shuttle type (double-acting) ball 
check 50 that also serves as a junction for the outlets 52 and 54 of the 
accumulators 20 and 22, uniting the resin to flow to supply a single 
nozzle 56. The shuttle valve 50 prevents the material that is being 
injected into the mold by one accumulator from pressurizing the other 
accumulator and impeding the resin transfer from the extruder 18. 
A cycle of operation of the injection molding machine 10, incorporating the 
method of the present invention will now be described. Referring to FIG. 
2c, and beginning at the point in time where the clamp unit 12 has just 
operated to close the mold that will receive the plastic melt, several 
things are happening concurrently: (1) the clamp 12 maintains pressure to 
hold the mold closed; (2) the injection accumulator 20 is operating to 
inject plastic melt into the mold; (3) the extruder 18 is plasticizing 
material into plastic melt; and (4) the rotary valve 36 is positioned to 
transfer the melted resin into the shot-building accumulator 20. While 
functions (1), (3), and (4) continue, the injection accumulator 20 
initiates pack, then hold to maintain the proper pressure on the mold. 
When the injection accumulator 20 reaches the "hold" portion of the cycle, 
it has emptied itself of material. More specifically, the injection of 
plastic melt is accomplished by applying sufficient force to move the 
piston 34 rapidly forward in the barrel 32. The melt is thus forced to 
flow through the outlet 52 of the injection accumulator 20, on through the 
shuttle valve 50 and nozzle 56, then into the mold. This approximate point 
in the cycle can be identified by the configuration shown in FIG. 3. The 
piston 34 in the shot-building (lower) accumulator 20 is retracted to 
provide room for the accumulated melt in the barrel. The piston 34 in the 
injection (upper) accumulator 20 is fully forward in the barrel 32, having 
completed the injection function. 
As part of the injection process, it is highly desirable to avoid "dead" 
spots in the material flow path where plastic melt can remain stationary 
through repeated cycles, allowing it to degrade, possibly later mix with 
good material and be injected to form a poor quality part. Accordingly, a 
mating configuration between the end of the piston 34 and the outlet of 
the barrel 32 will minimize the amount of material remaining in the 
accumulator 20 after the shot is completed. 
After sufficient hold/cool time, the pressure held by the injection 
accumulator is released, the clamp 12 operates to open the mold, eject the 
part(s), then re-close to begin a subsequent cycle. As noted earlier, 
simultaneously with the operation of the injection accumulator, the 
shot-building accumulator receives material from the continuously running 
extruder 18. Accordingly, the next molding cycle is identical except that 
the accumulators 20 switch functions; i.e., the rotary valve 36 shifts so 
that the accumulator that previously injected (and is empty) now receives 
material from the extruder 18, enabling the other accumulator that now has 
a full charge of melt to initiate the fill, pack and hold sequence. 
While the invention has been illustrated in some detail according to the 
preferred embodiment shown in the accompanying drawings, and while the 
preferred embodiment has been described in some detail, there is no 
intention to thus limit the invention to such detail. 0n contrary, it is 
intended to cover all modifications, alterations, and equivalents falling 
within the spirit and scope of the appended claims.