Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Using sonic – supersonic – or ultrasonic energy
Reexamination Certificate
1998-12-03
2001-02-20
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Using sonic, supersonic, or ultrasonic energy
C264S069000, C264S328170, C264S349000, C366S078000, C366S100000, C425S174200, C425S207000, C425S564000, C425S587000
Reexamination Certificate
active
06190601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for controlling a screw injecting apparatus such that vibrations are given to a molding material to thereby decrease its viscosity, as well as to such a screw injecting apparatus.
2. Description of the Related Art
A typical prior screw injecting apparatus is shown in section in
FIG. 9
hereof. The screw injecting apparatus
100
comprises a heating cylinder
101
, a screw
102
received in the heating cylinder
101
rotatably and movably back and forth, an injecting cylinder
103
for moving the screw
102
back and forth, and a hydraulic motor
105
for rotating the screw
102
by means of a piston rod
104
of the injecting cylinder
103
. In the injecting apparatus thus arranged, plasticizing-metering phase, waiting phase and injecting phase as described are carried out:
Plasticizing-Metering Phase: Raw molding materials are fed from a hopper
107
into the heating cylinder
101
during. rotation of the screw
102
and heated by the heating cylinder
101
while being transferred toward a discharging nozzle
108
by rotation of the screw
102
. By friction heat arising from the transfer and heat transmitted from the heating cylinder
101
, the molding materials are plasticized and kneaded. The screw
102
is pushed back rightwardly by a counter force of a molten material accumulated around the tip of the nozzle
108
. The amount of the molten material is measured by metering the retreating stroke of the screw
102
.
Waiting Phase: After completion of the metering, the hydraulic motor
105
and the screw
102
are held inactive until the molten molding material becomes ripe for injection.
Injecting Phase: By the action of the injecting cylinder
103
, the screw
102
is advanced at one stroke to cause the molten molding material accumulated forwardly of the screw
102
to be injected through the nozzle
108
into a mold not shown.
It is important to fill up cavities of the mold before solidification of the molten molding material therein progresses. Thus, the faster the injection speed becomes, the better. To speed up the injection, one may propose (1) to make the injection cylinder more pressurized, (2) to make the injection cylinder have an increased diameter, (3) to melt the molding material at an increased temperature, or (4) to decrease the viscosity of the molten molding material without relying on temperature increase.
The proposals (1) and (2) lead to up-sizing of the apparatus, thereby increasing the cost of production.
In the case of the proposal (3), the viscosity decreases by increase of the melting temperature. However, as can be seen from p-v-T (pressure-volume-temperature interrelation) characteristics, the density decreases as the temperature increases. Thus, volume variation becomes large when the temperture is changed from a molding temperatue to a normal temperature. To compensate for the large volume variation, a large pressure becomes necessary. In a normal injection molding process, pressure control is effected through a gate, whereby the pressure distribution of the resin within the cavities of the mold becomes non-uniform. This non-uniformity becomes more significant when the pressure is increased. Thus, the proposal to increase the melting temperature of the molding material. Rather, the temperature of the molding material should be as low as possible so that the temperature difference between the molding material and the mold can be kept to a minimum.
Consequently, the proposal (4), that is, to decrease the viscosity of the molten molding material without relying on temperature increase, has been seriously noted by researchers. The present inventors have continuously researched to realize the proposal (4) by means of mechanical measures.
FIG. 10
hereof graphically shows the results of such inventors' research. As can be readily appreciated from the figure, the inventors have found during the research that the viscosity of the resin material significantly changes when the material is vibrated by a vibration frequency in a given range.
More specifically, vibration frequencies are shown in herz (Hz) along the horizontal axis while the material viscosity is shown in poise along the vertical axis. PMMA (poly(methyl methacrylate)), a typical resin material, was imparted various vibrations while maintaining it at 240° C. While it exhibits the viscosity of 126×10
3
poise at the frequency of 0 Hz, the PMMA exhibited the viscosity of 65×10
3
poise at 5 Hz, 14×10
3
at 30 Hz, and 9×10
3
poise at 55 Hz.
While it exhibits the viscosity of 63×10
3
at the frequency of 0 Hz, PC (polycarbonate) exhibited 26×10
3
poise at 15 Hz, and 15×10
3
poise at 40 Hz.
The research has thus revealed that it becomes possible to satisfactorily decrease the viscosity of the resin material by vibrating the material at the frequency ranging from 5 Hz to 40 Hz, desirably at 15 Hz or more.
A technique for applying vibrations to a molding material during injection thereof is known from, for example, Japanese Utility Model Laid-Open Publication No. SHO-63-197113 entitled “SHAPING INJECTION MOLDING APPARATUS WITH SHAKING CAPABILITY”. The known apparatus includes an ultrasonic oscillator mounted in a heating cylinder thereof and an ultrasonic wave generator disposed remotely from the heating cylinder for ultrasonically vibrating the ultrasonic oscillator. In response to a high frequency signal, the ultrasonic wave generator generates a ultrasonic wave for vibrating the ultrasonic oscillator. The resulted ultrasonic vibrations are applied to a molten resin material being injected, thereby fully filling up finely-patterned portions and complex-shaped portions of a molded product.
Ultrasonic waves have a frequency above about 20 kHz which is too high for humans to hear. The ultrasonic wave generator is designed to produce such waves. This and the above-described arrangement bring about the following problems:
(a) it is likely that fine bubbles be formed in the molten material, because an extremely high vibration frequency is applied to the molten material, thereby suddenly changing the pressure of the molten material;
(b) there remains a fear that the effect of the vibrations or shaking may not reach deepest portions of the cavities of the mold, because the injecting phase is finished too quickly; and
(c) being disposed within the heating cylinder, the ultrasonic oscillator will present a bar to the flow of the molten material.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for controlling a screw injecting apparatus, which does not give rise to the problems (a)-(c).
Another object of the present invention is to provide a screw injecting apparatus in which the inventive method can be performed.
According to one aspect of the present invention, there is provided a method for controlling a screw injecting apparatus having a heating cylinder and a screw slidably and rotatably received in the heating cylinder and designed to carry out a plasticizing-metering phase in which a predetermined amount of a molten material is accumulated forwardly of a tip of the screw by plasticizing and kneading a raw molding material via the screw and the heating cylinder, a waiting phase in which the screw is held inactive for a period between completion of the metering and subsequent injection of the molten material, and an injecting phase in which the molten material is injected, which method comprises the step of causing the apparatus to vibrate the molten material at a predetermined low frequency axially of the screw during the plasticizing-metering phase.
By vibrating the molten material at a low frequency during the relatively long plasticizing-metering phase, the viscosity of the molten material can be reduced. Further, since vibrations are generated axially of the screw, sufficient vibrations can be applied to the tip of the molten material.
According to a second aspect of the present invention, there is provided
Heitbrink Jill L.
Merchant & Gould P.C.
Nissei Plastic Industrial Co.
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