Plastification and injection device for an injection molding machine

A plastification and injection device for an injection molding machine includes a plunger rod on the pressurized piston coupled to a spindle on which the valve body of a directional control valve is guided axially. The spindle is axially rotatable and is coupled to a stepping motor controlled in accordance with the injection pressure and/or the injection rate. The shaft of a hydromotor driving the plastification and injection screw is guided by means of another directional control valve, the valve body of which is guided so as to slide axially on the shaft. And, this shaft is further coupled to another stepping motor. This, upon completion of the plastification process, the direction of rotation of the hydromotor is reversed in order to close the backflow and, because of the interaction of the other stepping motor and the other directional control valve, the screw moves axially by a predetermined amount to close the backflow.

This application relates to U.S. Ser. No. 463,112, filed Feb. 2, 1983, and 
based on German application No. P 32 03 810.0-16. 
BACKGROUND OF THE INVENTION 
This invention relates to a plastification and injection device for an 
injection molding machine, the device having a drive which, during 
plastification, causes the extruder screw to rotate about its central 
axis, and having a pressurized injection piston and cylinder unit 
controlled by a directional control valve for axially shifting the screw 
for injecting plasticized material into the machine. 
The plastic or plasticized material as herein referred includes 
thermoplasts, duroplasts, and elastomers/rubber. 
The general type of plastification and injection device to which the 
invention relates, is known in construction and operation. The device 
usually includes a screw housing having an outlet nozzle and an inlet for 
plastic material granules, and the plastification and injection screw is 
mounted within the housing with its tip end spaced from the outlet to 
define a chamber for the plasticized material. A hydromotor is usually 
provided for rotating the screw about its central axis for plasticizing 
the granular material fed into the housing, and a pressurized piston and 
cylinder unit is provided for axially shifting the screw for injecting the 
plasticized material into the space between the mating molding parts of 
the molding machine. During this operation, at the end of the piston 
suction stroke within the injection cylinder, the pressurized working 
fluid discharged from the injection cylinder is subjected to a so-called 
back pressure and, to fabricate the molded article, the injection cylinder 
is pressurized such that the screw, now operating as a plunger, moves 
forwardly and at the same time injects plasticized material into the 
injection mold. After the mold is filled, a holding pressure is applied to 
take into account the cooling of the molded article which takes place 
under thermodynamic conditions. 
Compensation must be made for the shrinkage of molded articles on cooling. 
This was heretofore carried out by maintaining a sufficiently high holding 
pressure, so that through a slight axial displacement of the screw 
material could be injected into the mold. However, such control mechanisms 
are usually cumbersome and uneconomical, and it has been found that 
shrinkage of the mold article can thereby be compensated only incompletely 
since the operating temperature of the injection molding machine and, 
thereby, the viscosity of the plasticized material to be injected, 
fluctuates. 
Another known technique in producing accurately sized and error-free mold 
articles is to minimize the measurement and weight variations of the 
molded articles so that, during the changeover from injection pressure to 
holding pressure, discontinuities in the actual pressure variation versus 
a pre-programmed pressure variation do not occur. Such is disclosed in 
West German Pat. No. 25 33 303. 
As will be seen, the present invention adopts the principle of a 
hydraulically rigid feedback loop in accordance with the principle of an 
electro-hydraulic linear amplifier. Hydraulic components operating in 
accordance with this principle are known from the publication of Hartmann 
and Lammle KG, "Electrohydraulic Linear Amplifier LVS," 10th edition, 
1977. However, these components have heretofore only been employed for the 
exact positioning of machine components that can slide on machine tools, 
for example. 
And, plastification and injection devices are disclosed in DE-AS No. 25 41 
733 and DE-OS No. 21 55 130 in which an axially non-returning is provided 
within the screw housing between the tip of the screw and the annular 
space surrounding the screw. The ring forms a back flow stop with the 
screw together with a retaining ring. The axial movement of the non-return 
ring is delimited on one side by an adjacent conical shoulder of the screw 
tip having crown grooves, and on the other by the leading edge of the 
screw thread. During plastification, the ring bears on the conical 
shoulder of the screw tip, and the plasticized material flows into the 
plastification chamber downstream of the screw tip through an annular gap 
defined between the ring and the conical shoulder as well as through the 
grooves of the conical shoulder beyond the screw tip. During injection, 
the non-return ring bears sealingly against the end face of the screw 
thread and prevents plastified material from flowing back along the lands 
or ridges of the screw. 
When fabricating injection-molded precision articles, extremely small 
quantities of weight variations are induced, partly because of the fact 
that when the injection pressure is being applied, the small amounts of 
plastified material flow back from the area of the screw tip through the 
open annular gap between the lands or ridges of the screw and the inner 
wall of the screw housing. Since the viscosity of the plastified material 
depends, among other things, the temperature, it can be seen that 
continually fluctuating amounts of material, however small, flow back and 
deliberately affect the precision parts. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a plastification and 
injection device for an injection molding machine, the device having an 
hydraulic control mechanism such that when molding precision parts 
adequate compensation is made for shrinkage of the material. 
In accordance with the invention, a piston rod on the piston of the 
pressurized piston and cylinder unit is threadedly coupled with a 
rotatable spindle which is rotated by a stepping motor coupled therewith 
in accordance with the injection pressure and/or the rate of injection, 
and a directional control valve has its valve body mounted on the spindle 
for axial movement with the spindle upon its rotation to control piston 
movement. 
Thus, the stepping motor rotates the spindle. At the same time, the valve 
body of the directional control valve is moved correspondingly, thereby 
allowing a larger or smaller open passage from the pressurized source, so 
that the piston is correspondingly pressurized and the plunger rod 
executes an appropriate movement. This, in turn, results in a 
corresponding movement of the valve body, in an opposite direction, which, 
in turn, is counteracted by the stepping motor. Thus, a hydraulically 
strong feedback loop arises, with the result that the movement of the 
injection piston, as well as the screw, is controlled with any desired 
degree of precision, which is dependent upon the accuracy of the spindle. 
This technique makes it possible to switch over, during injection, from a 
speed-sensitive to a pressure sensitive control as the stepping motor 
receives the appropriate control demands. 
A hydromotor is provided for rotating the injection screw, and another 
directional control valve is coupled thereto and is operated by another 
stepping motor which is controlled as function of a given injection 
pressure/holding variation, or in accordance with a given speed variation 
of the injection piston or of the screw, for axially rotating the screw in 
an opposite direction upon completion of the plasticizing process so as to 
prevent any backflow of the plasticized material from the plasticizing 
chamber. The valve and the stepping motor which control the axial position 
of the screw effect a simultaneous axial shifting of the screw upon 
rotation thereof in such opposite direction. 
In accordance with another feature of the invention, the hydraulic control 
for axially shifting the screw includes a stepping transmitter or coder 
for sensing the actual values of the distance travelled by the screw or by 
the piston of the injection cylinder. This transmitter is operatively 
coupled with the spindle. Therefore, with such arrangement, even after a 
temporary power failure, the particular position of the screw or of the 
injection cylinder piston can be determined without having to move the 
latter to extreme position first in order to redefine the zero position. 
A further feature of the invention includes the provision of a pressure 
sensor for the injection cylinder, which senses the actual values of the 
pressure. Thus, it is possible to compare the desired value with the 
actual value of the pressure as a function of the distance travelled, so 
that with a small device, a feedback loop is provided for the injection 
and holding pressure.

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawing wherein like reference characters refer to like 
and corresponding parts throughout the several views, the plastification 
and injection unit is generally designated 1 and comprises an elongated 
screw housing 2, and an injection screw 3 mounted within this housing. An 
inlet 4 is provided on the housing through which granules of plastic 
material are fed as in any known manner. And, the housing has an outlet 
nozzle 5 through which the material, when plasticized, is injected into 
the space between mating molds of an injection molding machine (not 
shown). Tip end 6 of the screw, in a retracted position, is spaced from 
the outlet nozzle of the housing so as to define a plasticizing chamber 7. 
The screw is rotated about its central axis in a direction which causes the 
granules of plastic material to be plasticized and fed into chamber 7. 
Rotation is effected by means of a hydromotor 8 which rotates a gear train 
operatively coupled with the screw. And, the plastification and injection 
screw is connected, through a driven gear of the drive train, to a 
hydraulically operable piston 9 which operates within an injection 
cylinder 10 extending from the rearward end of screw housing 2. The piston 
head of the injection piston defines together with the cylinder opposing 
pressure chambers 11 and 12 respectively interconnected through pressure 
lines 13 and 14 with a source (not shown) of hydraulic working fluid. A 
4/2 directional control valve 15 is provided for pressurizing and venting 
the chambers when the valve body of this control valve is moved into an 
appropriate position. 
An elongated spindle 16 is threadedly connected with piston 9 by means of a 
piston rod 17 extending from the piston and a connecting arm 18 mounted on 
the rod, and an internally threaded collar 19 on the arm being in threaded 
engagement with an externally threaded portion 20 of the spindle. 
Otherwise, a toothed bar rod or a toothed belt may be substituted for the 
spindle, with a suitable mating element provided in lieu of collar 19. The 
valve body of directional control valve 15 is guided so as to shift 
together with the spindle within its housing during spindle rotation to 
thereby control the axial movement of the pressurized piston and the 
screw. 
And, a stepping motor is coupled to the spindle for rotating the spindle in 
accordance with the injection pressure and/or the rate of injection, to 
cause axial movement of the piston. This is illustrated schematically by 
means of a pair of accumulators 22, 23 and a switch 24. Furthermore, an 
angular coder 25 is coupled to the shaft of motor 21 and indicates the 
actual value of the path travelled by piston 9. Moreover, pressure chamber 
11 is connected to a pressure sensor 26 which indicates the actual value 
of the injection pressure and of the holding pressure. 
The valve body of another multi-directional control valve 27 is arranged to 
slide on shaft 28 of hydromotor 8 within its valve housing. The shaft 
extends out through the valve housing of control valve 27 and is coupled 
to a stepping motor 29. Axial movements of the valve bodies of both 
directional valves 27 and 15 is carried out according to the principle of 
an electro-hydraulic linear amplifier of the type disclosed in the 
aforementioned publication. 
It has been found that reverse rotation of the screw on completion of the 
plastification process and prior to initiation of the injection process, 
stops backflow of the material and practically reduces it to zero. Thus, 
stepping motor 29 is controlled in such a manner that backflow is closed 
upon completion of the plastification process by rotating the screw in an 
opposite direction and with a simultaneous axial displacement of the screw 
in a forward direction by an amount which corresponds to the closing path 
of the backflow. This is detected by the interaction of stepping motor 29 
and directional control valve 27. 
From the foregoing, it can be seen that screw 3 performs two functions: 
first, it serves to plasticize the granular material; and second, it 
serves as a plunger for injecting the mold with the plasticized material 
from chamber 7. On switching over to the first from the second function, 
and during injection, care must be exercised to prevent the plasticized 
material from flowing from chamber 7 along the screw threads or between 
the annular space surrounding the screw threads. The present invention 
prevents backflow automatically, quickly and precisely. 
Obviously, many modifications and variations of the present invention are 
made possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.