Material injecting method and material measuring apparatus in injection molding machine

An injection molding method operable in conjunction with an injection molding machine having an injection screw which rotates and moves backward while a preset dosage of molding material is injected into the molding machine. As the injection screw moves backward its arrival at a predetermined point relative to a discharge nozzle is detected by a sensor which initiates stoppage of feeding of material into the machine. Further backward movement of the injection screw resulting from internal pressure in the injection cylinder is arrested by an adjustably locatable stopper, disposed behind the injection screw. The stopper is designed to be located at a position that produces molded products of consistent quality. Further, in accordance with a preferred embodiment of the invention, the stop position of the injection screw is also used as a starting point for the injection screw in a subsequent injection step during which the dosage of material is injected into a molding cavity.

TECHNICAL FIELD 
This invention relates to a material injecting method and a material 
measuring apparatus in an injection molding machine used for molding 
synthetic resin products. 
BACKGROUND OF THE INVENTION 
The first step in carrying out injection molding by the use of a molding 
machine in which an injecting cylinder is provided interiorly of a 
cylinder involves measurement of molding material required for one shot. 
Ordinarily, a screw is rotated by applying back pressure thereto, and the 
quantity of material is measured. When the screw is moved backward to a 
predetermined position, rotation of the screw stops. If a distance of the 
screw from a position at which the screw starts its rotation to a position 
at which the screw stops is constant, it is considered that a quantity of 
the measured material is also constant. In addition, since the screw is 
moved backward by internal pressure of the cylinder produced by the 
rotation of the screw, the position of the backward movement of the screw 
is electrically controlled, and the stopped position of the screw is 
established as a measurement-completion position, which position is used 
as a reference to control pressure, speed, retaining pressure, position 
and time etc. in the injection steps. 
However, even if the screw is immediately stopped by means of a device 
actuated by a signal from a measurement completion detector, internal 
pressure still remains in the cylinder, and therefore the screw is 
affected by the internal pressure. In the measurement completion position, 
back pressure applied when measurement is made is reduced down to such an 
extent that material is not discharged from the tip of a nozzle. Such a 
state is maintained until the subsequent injection step starts. Because of 
this, the screw continues to be moved backward by the internal pressure 
even after the screw has been stopped from its rotation. With this 
backward movement, the molten material around the screw enters into the 
front portion of the screw from a clearance between a check valve in an 
open state at the end of the screw and a valve seat, and measurement is 
naturally carried out even after completion of measurement and the 
quantity of material is increased. In addition, the internal pressure 
varies with the plasticized state and density of material around the 
screw, and variation in measurement after completion of measurement, that 
is, the amount of backward movement of the screw is not repeated. 
Actually, when a number of products are molded at a time, unacceptably 
molded products result. It has been found by tests conducted to find the 
accuracy of material measured under the fixed stroke in an accurately 
controlled condition, that an error in weight of .+-.1.5 occurs on 
average. 
The aforesaid error in weight is a weight % in which material of 
approximately 80% of measurement required for one shot of a mold is 
measured, and the total quantity thereof, that is, material is injection 
filled into the mold to the termination point of the injection stroke, and 
molded products which are insufficient in filling are repeatedly molded 
dozens of times under the same condition. 
It has been found by the present inventor that when normal material and 
regenerated material (that is material which has been used previously for 
molding and which has irregular particles therein) are respectively 
injected under the same condition the regenerated material experiences a 
great weight error. 
The present inventor has found from the foregoing that the variation in 
injection quantity not only results from the responsiveness during the 
closure of the check valve but is greatly affected by the density and 
plasticized state of material from a material inlet at the rear of the 
cylinder to the end of the screw at the front of the cylinder, whereby the 
quantity of molten material to be measured at the front of the screw is 
assumed to be varied; and the quantity of material injected on every shot 
cannot be quantitized on account of the above mentioned problems. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a new material 
injecting method in which the screw position is locked at a predetermined 
position after measurement to thereby eliminate irregularities in the 
quantity of injected material. 
It is a further object of the invention to provide a material injecting 
apparatus which can promptly detect a measurement completion position and 
a screw stop position, and can very easily perform establishment of these 
positions. 
For achieving the aforementioned objects, according to this invention, the 
backward movement of the screw after completion of measurement is stopped 
at a given position, said position being used as a start point of the 
injection step, and there is provided an apparatus which assures that the 
backward stop position of the screw is constant every measurement to 
render precise measurement of material possible. 
According to the aforementioned apparatus, a member rotatably mounted on a 
flange at the rear end of a screw is projected externally from a hole 
formed in the side of a cylinder, an end is inserted into a guide bar 
laterally mounted on the side of the cylinder, and a stopper for defining 
the backward movement of the member is movably mounted on the guide bar or 
the guide bar is formed into a screw shaft rotated by said member and a 
brake member is mounted on the screw shaft.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to the drawings, an injection cylinder indicated at 1 has a 
nozzle 2 at the end thereof and has a material charging hole 3 at the rear 
thereof. The cylinder 1 is interiorly provided with an injection screw 4 
which is rotatable and movable forward and backward. 
A ring-like check valve 5 urged by means of a spring is provided around at 
the extreme end of the screw 4 leaving a required spacing 5a. Reference 
numeral 6 denotes valve seat. The rear end of the screw 4 is connected to 
a ram 8 within a hydraulic cylinder 7 connected to the rear end of the 
cylinder 1, the ram 8 being axially movably connected to a rotational 
shaft 9. 
Reference characters A, B and C designate respectively, a detector provided 
at a measurement completion position, a detector for switching injection 
pressure and a detector provided at an injection stop position. They 
comprise respectively limit switches or proximity switches actuated by a 
member 10 mounted at the rear end of the screw 4, but detection at 
respective positions may be made by other electric means. 
FIG. 3 shows a material measuring apparatus in which the member or leg 10, 
rotatably mounted on a flange 11 at the rear end of the screw, projects 
externally from a hole portion 1a formed in the side of the cylinder, an 
end of member 10 is inserted into a guide bar 12 that is laterally mounted 
on the side of the cylinder, and a stopper means including a stop position 
means in the form of a stopper member 13a for defining the backward 
movement of the member 10 is movably mounted anywhere along an axial path 
between two end positions on the guide bar 12. 
Measurement of material is carried out by the rotation of the screw 4 while 
applying back pressure in a conventional manner. The screw 4 is moved 
backward by pressure within the cylinder produced by rotation thereof, and 
the rotation of the screw 4 is stopped at a position where the measurement 
completion detector A is actuated by the member 10 or the back pressure is 
reduced to complete measurement. 
After the rotation has been stopped, the screw 4 is moved backward by 
internal pressure of the cylinder, but the backward movement stops by 
abutment of the member 10 against the predetermined stopper member 13a. 
The amount of backward movement of the screw 4 after it stopped its 
rotation in the prior art embodiment was 3.5 to 5.0 m/m under the 
conditions of screw--38 .phi., back pressure--20 kg/cm.sup.2, measuring 
stroke--35 m/m and material--styrol, the amount of backward movement has a 
difference of .+-.1.5 m/m. Also, a difference in weight of the 
injection-molded product varies with the difference in the amount of 
backward movement, but in the case where the stop position is made 
constant by the stopper member 13a, that difference is in an allowable 
range in terms of precision. 
The stop position of the screw 4 is set by moving the position of the 
stopper 13a forward and backward at the time of injection adjustment 
carried out at the beginning of injection molding. This position is used 
as a start point of injection step to effect injection molding. The screw 
4 is first moved forward at low pressure (15 Kg/cm.sup.2), and the check 
valve 5 is closed. Then the member 10 actuates the injection pressure 
switching detector B thereby the injection pressure is switched to high 
pressure and to actuate the injection stop detector C, completing the 
injection step. 
In the material measuring apparatus shown in FIG. 4, the guide bar 12 is 
formed as a screw shaft 12a, and rotation of the screw shaft 12a may be 
braked to ensure that the backward position of the screw 4 constant. The 
apparatus comprises the rotatable screw shaft 12a juxtaposed at the rear 
portion of the screw on which the flange 11 is provided, the member 10 
provided over the screw shaft 12a and the flange 11, a stopper means 
including a braking system 13 provided on the end of the screw shaft 12a 
and a stop position means in the form of a material measurement detector D 
at the rear of and adjacent to the measurement completion detector A for 
actuating the braking system 13 to stop rotation of the screw shaft 12 and 
to stop the rearward movement of the screw 4 at a predetermined position. 
The screw shaft 12a is formed from a ball screw shaft to provide smooth 
rotation. The screw shaft is placed in the hole portion 1a formed in the 
side at the rear of the cylinder 1 and mounted on the side of the cylinder 
by means of bearings 15. 
The member 10 further comprises a nut portion threadedly engaged with the 
screw shaft 12a through a plurality of balls 16 received in threaded 
grooves and a portion fitted in the flange 11 to allow rotation of the 
flange 11, the member 10 being moved forward and backward together with 
the screw 4 while rotating the screw shaft. 
The above-described braking system comprises an electromagnetic brake, an 
electric servo-motor and the like, schematically shown at 13 and is 
operated to provide braking in response to an electric signal from the 
material measurement detector D. 
Next, the operation of the apparatus shown in FIG. 4 will be described. 
First, measurement of material is carried out by rotating the screw 4 while 
applying back pressure in a conventional manner. The screw 4 is moved 
backward by pressure within the cylinder generated by the rotation of the 
screw 4, and when the position of the member 10 is detected by the 
measurement completion detector A, the rotation of the screw 4 is stopped 
at that position by the electric signal from the measurement completion 
detector A and the back pressure is reduced to complete measurement. 
After rotation has been stopped, the screw 4 is naturally moved backward by 
internal pressure of the cylinder. However, when the position of the 
member 10 is detected by the predetermined material measurement detector 
D, the detector D is actuated to provide an electric signal for the 
braking system 13, and the braking system 13 is energized and actuated to 
stop the rotation of the screw shaft 12a. 
Thereby the backward movement of the member 10 is retarded and the screw 4 
stops at that position. This stop position is set by setting the material 
measurement position detector D at the backward position of the screw at 
which a molded product of highest quality was obtained. Said position is 
used as a start point of injection step to start the injection step. 
As described above, according to the present invention, the backward 
movement of the screw is completely stopped after measurement has been 
completed, said stop position is used as a start point of the injection 
step to remove the irregularity in quantity of injection caused by the 
backward movement of the screw after completion of measurement of the 
quantity of injected material. Therefore, the present invention has the 
following effects: 
(1) Injection molding conditions are easily provided. 
(2) True, fine and stabilized molding can be performed. 
(3) Size, weight, density and appearance of molded products are free from 
irregularities. 
(4) Even if a large quantity of regenerated and pulverized material are 
mixed, precise molding may be performed. Particularly, in the case of 
multi-cavity molding using expensive material such as engineering plastics 
and in the case where the runner is heavier than the molded products, even 
if the runner is broken into fragments for use, precise molding is 
possible, which is therefore economical. 
As described above, according to this invention, a material measuring 
apparatus for stopping the backward movement of screw after completion of 
measurement at a required position is mounted whereby injection molding of 
high precision in dimension may be carried out; and in addition, since the 
apparatus may be applied to a conventional injection molding machine, the 
present invention may be widely utilized as an optimum material injection 
method and material measuring apparatus.