CA1059714A - Method of injecting plastic material using a plastic injection machine - Google Patents

Abstract

METHOD OF INJECTING PLASTIC MATERIAL
USING A PLASTIC INJECTION MACHINE

Abstract of the Disclosure A method of injecting plastic material into the cavity of a mold is disclosed which method is particularly useful for the injection of foam-type plastics and/or the injection into a mold cavity which contains certain fill pieces, inserts or the like, which may be moved or disturbed in their position by high pressure injection. The method includes the prepres-surization of the plastic material within the injection cylinder to a relatively low pressure, opening the nozzle valve on the cylinder at the conclusion of the very short time interval and injecting the plastic material at the low pressure over a second time interval to effect a controlled injection of the plastic material. After the material has been injected, a third time interval is established throughout which a substantially higher pressure is applied to the plastic material to set up the foam material, to provide a uniform homogeneity of the product, and to insure complete and full filling of the mold without dis-turbing any delicate inserts which may have been within the mold cavity.

Description

Docket 6057 ~g59~4 Back round of the Invention g ~ his invention relates to a method of controlling a plastic injection machine, and more particularly to a method which is suited for injection of foam plastic materials and/or injection into mold cavities which contain small or delicate inserts or the like which could otherwise be disturbed by high pressure injection.
Machines are known which rapidly inject plastic material at a high pressure and then shift to a low pressure or a low holding pressure, and examples of such are described in V. S.
Patents Nos. 3,784,657 and 3,632,246. Also, plastic foam has often been injected under relatively high pressure and then the injection machine has been shifted to a low pressure, per~
mitting the foam to set up. However, I have discovered that there is a particular advantage for certain types o~ molds and for foam plastics to be injected at a relatively low pre~sure followed by the application of a substantially higher pressure to the plastic material over a period of time to provide a homogeneous foamed product. The low pressure injection method has a particular advantage whe~e the mold itself includes inserts which may easily be dislodged.
In the U. S. patent of Trueblood, No. 3,436,793, assigned to the same assignee of this invention, there is described and claimed an arrangement by which the opening of the nozzle is oomewhat delayed until the plasticized material in the barrel is substantially pxessurized by the screw, to provide for the

-2-Docket 6057 ~597~L4 rapid filling and injection into the mold. While the apparatus shown therein has been highly success~ul, it does not deal with ~he problem of the dislodgment of the metal parts and inserts, or with the injection of filled or foam-type plastics, nor does it suggest the injection at low pressure followed by the appli-cation of a higher pressure.

Summary of the Invention Briefly, this invention provides a method for pressure injection of plastic material, particularly foam or filled abrasive plastics, in a two-pressure phase injection cycle.
; The method is also adapted to the injection of plastic materials into a mold cavity which contains fill pieces, inserts or the like, which could be moved or otherwise disturbed in their position by conventional high pressure injection. The method involves the operation of a ram-screw type of plastic injection machine and includes steps of prepressurizing the plastic ma-; terial within the injection cylinder to a relatively low value while the nozzle is held closed, opening the nozzle at the conclusion o~ a short time interval, maintaining this low pressure on the screw for a second time interval a~ter theopening of the nozzle valve to ef~ect a controlled injection of the plastic material under a correspondingly low pressure, then establishin~ a third time interval and applying and main-taining a high pressure to the screw to establish a correspon-~5 dingly higher pressure in the mold cavity. The third timeinterval during which the higher pressure is maintained has a duration which exceeds that of either of the first or second time intervals. This assures aomplete filling o~ the mold, assures a homogeneous foamed product, prevents shrinking of the plastic material, and initiates set-up of the plastic Docket 6057 ~0597~4 material within the mold. The method has particular advantage in the injection of filled plastics, such as iron or glass filled or the like, and results in decreased wear of the screw and mold parts due to abxasion as compared to conventional injection.
It is accordingly an object of this invention to provide a method of injecting foam-type and/or filled plastic ma~erials.
A further object of the invention is to provide a method of injecting mold cavities which'contain inserts or the li]~e.
These and other objects and advantages of the invention will become apparent from the following description, the accom-panying drawings and the appended claims.

Brief Description of the Drawings Fig. 1 is a plan view of portions of a pressure injection machine and mold used in the present invention;
Fig. 2 is an elevational view, partially in section, of the Fig. l machine, taken generally on line 2-2 thereof;
Fig. 3 is a diagram of a portion of the electric control circuit controllin~ the two pressure injection phasesi Fig. 4 is a diagram of a portion of the hydraulic system of the ~ig. l apparatus; and Fig. 5 is a diagram of another portion of the hydraulic system.

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Description of the Preferred Embodiment With reference to the drawings, and more particularly to Figs. 1 and 2, there is illustrated a plastic injection machine lO substan*ially of the type shown in U. S. Patent No. 3,43~,793.
Machine lO, certain details of which are more particularly des-cribed in the above patent, includes a carriage 12 on which the machine is mounted for sliding movement back and forth along a guide rail 14, toward and away from,a split mold 15.

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It is understood that the parts of the mold 15 are clamped together and held closed with considerable force during injection. The clamp arrangement of my U. S. Patent

3,309,739 is particularly useful for this purpose.
Movement of the machine 10 along the rail 14 is effected by a hydraulic carriage cylinder 20. Adjustment stops 21 engage carriage stop rail 22 for exact adjustment of the forward carriage position, in order to seat the injection nozzle cap 25 properly into the mold nozzle seat 26.
A reciprocating and rotating plasticizing screw 30 is mounted within the cylinder barrel 33, to mix and force plas-tic material received from the hopper supply inlet 36 toward the nozzle cap 25. Screw 30 is rotated by a hydraulic motor and gear box 40, serving as a drive means, and is drivably coupled thereto by means of an intermediate driver member 41.
The driver me~b2r 41, in -turn, is rotatably supported in a thrust plate 43 by a shouldered bronze thrust bearing 45 having a shoulder 46 thereon. The shoulder 46 is supported by the thrust plate 43 and a boss 4~ on the driver member 41, so that axial force from the thrust plate 43 may be ~ransmitted through the shoulder 46 to the boss 48-, and from there to the screw 30.
A pair of injection cylinders 50 are mounted on either side of machine 10 and are connected by cylinder rods 51 to the thrust plate 43 for applying an injection force to the thrust plate, and from thence to screw 30. Cylinders 50 themselves are attached directly to carriage 12, and move therewith, so ; that injection cylinder rods 51 cause the thrust plate 43, hydraulic motor and gear box 40 (mounted on plate 43), driver 41, and screw 30 to move relative to the carr;age 12, and hence also relative to the cylinder barrel 33. The injection .

5_ Docket 6057 cylinders 50 are therefore able to drive screw 3~ axially of cylinder 33 to pressurize the plastic within the cylinder.
. Movement of the injection cylinder rods 51, thrust plate 43, hydraulic motor and gear box 40, driver 41 and screw 30 relative to carriage 12 and cylinder barrel 33 is guided by four guide pins 55 located ln pairs on either side .
of machine 10. The pins 55 are attached to boss members 57 supported on the carriage 12, and are slidably received in bushings 59 in the thrust plate 43, to allow the thrust plate, hydraulic motor and gear box, driver ~1, and so on, to slide backwards and forwards thereon with respect to carriage 12 and cylinder barrel 33.
An injection nozzle valve 65 is located at the end of cylinder barrel 33 adjacent nozzle cap 25~ The opening and closing of the injection nozzle valve 65 is controlled by crank arms ~7 which in turn are actuated by an injection nozzle valve ; cylinder 70. Movement of the crank arms 67 in one direction, .
by cylinder 70, opens the injection nozzle valve 65, while movement of the arms in the opposite direction closes the valve, 20. essentially as taught in my U. S. Patent No. 3,43~,793.
~ he preferred hydraulic circuits for operating the plastic injection machine 10 according to the method of this invention are illustrated in Figs. 4 and 5. As may be seen therein, carriage cylinder 20 and injection nozzle cylinder : ~5 70 are operated from a low pressure hydraulic source 80, and are controlled by a four-way solenoid-operated carriage cylinder control valve 81 and a similar injection valve control valve 82, respectively.
The injection.cylinders 50 and hydraulic motor 40 are .

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operated from a high pressure hydraulic source 85. The appli-cation of the high pressure source to the circuit is controlled ; by a valve 86. A self-centering, closed-center three-way valve 87, when in a first or "recharge" position, controls the delivery of the high pressure hydraulic fluid to the hydraulic screw motor and gear box 40, to recharge cy3inder barrel 33 with a new shot of plastic material in conventional fashion. In the center or neutral position, high pressure fluid is not delivered to any function.
In the third or "injectl' position, fluid pressure is delivered to the injection cylinders 50 through either a high pressure regulator 91 ox a low pressure regulator 92. Delivery of hydraulic fluid from the regulator 91 is controlled by a valve 94, while delivery from the regulator 92 is controlled by a valve 95.
The electric solenoid portions of the hydraulic circuits as shown in Figs. 4 and 5 are controlled by the simplified cir-cuit diagram shown in Fig. 3~ Only so much of the circuit dia~ram of the hydraulic injection machine is shown as is neces-sary to describe the method of the inventionO With reference toFig. 3, a cycle timer Tl is shown, and only the contacts thereof pertaining to the present invention are described. The circuit also includes an injection nozzle valve timer T2, a first or low pressure timer T3 and a secona~or high pressure timer T4. Con-trol relays CRlA and CR9 are also included.
Diagrammatically shown is a limit switch LS9 which istripped or actuated in the forward position o the carriage, signaling that the nozzle 25 has come to rest in the nozzle seat 26 of the mold. Also shown is a shot-size limit switch LS8, ~ ~ocket 6057 ~059'7~.4 which is adjustably mounted on the machine as hereinafter ; described for determining the quantity of plastic material : which is mixed and placed at the extruding end of the barrel 33. The switch LS8 is tripped with the rearward movement of the screw when a predetermined shot size has been achieved.
The cycle timer Tl determines the overall length of the injection cycle. It is energized by the closing of the,limit switch LS9 as signaling that the carriage has reached its forward position. Power from a main power source 100 is 10 applied through closed contacts TlA to the 'inject" solenoid 102 of valve 87 so that fluid under high pressure from the source 85 is applied simultaneously to the control valves 94 and 95.
The fir.st or low pressure timer T3 controls the length of time that hydraulic fluid under pressure i5 applied through the regulator 92 to the injection cylinders 50. The second or high pre.ssure timer T4 is operated frorn a normally open con-tact T3B of timer T3 a,nd controls relay CRlA to control the length of time that fluid under higher pressure from the regu-; 20 lator 91 is applied to the injection cylinders 50.
The nozzle timer T2 controls the delay in the openingof the nozzle valve 85 following the application of fluid pressure to the cylinders 50. This delay enables the desired .injection pressure to build up in the cylinder barrel 30 ahead of the screw. The relay CRlA controls the valves 91 and 92 and thus effects the transfer of pressure control from low to high pressure. An ON/OFF switch 108, in normal operation, is positioned in the ON position, thus connecting timers T2 and 8-- .

Docket 6057 ~5971~
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T3 to the source 100 of power through the forward limit switch ~S9. The control relay CR9, during the injection cycle, is de-energized and thus its normally closed contact applies power to the solenoid of the hydraulic pressure valve 86, permitting pressure from the manifold 85 to be applied to the circuit.
However, relay CR9 is energized by the shot size limit switch LS8, upon the movement of the screw to a predetermined rearward position, thus disabling the hydraulic circuit by de-energizing valve 86 and permitting the fluid pressure to be vented to the tank. This is the idle condition of the machine.
In the operation, it is assumed that the switch 108 is moved to its "ON" position and relay CRlA is operated. The carriage 12 is brought forward on its track until the rail 22 rests against the adjustable stops 21, and the nozzle 25 is ~ properly seated. The forward movement: is accomplished by the application of fluid pressure to the cylinder 20 by actuation of the carriage solenoid 81 of Fig. 5. The circuit for control-ling the solenoid to bring the carriage forward at the beginning of a cycle and to return it to its retracted or rest position at the conclusion o~ the cycle is conventional with screw-type plastic injection machines and is not incorporated in Fig. 3.
The limit switch LS9 signals the forward position of the injec-tion cylinder ready for injection and is tripped by an adjust-able cam 110 supported on a rod 111 as shown in Fig. 2.
The closing of the forward limit switch LS9 applies power from the source 100 to the timers Tl, T2, and T3 and also applies power through timer contacts Tl~ to the "inject" sole-noid 102 of valve B7. The operation of valve 87 applies fluid under relatively low pressure to the injection cylinders 50 Docket 6057 ~59~73L~a through the solenoid valve 95 which has been energized through contacts CRlA2 of control relay CRlA~ The injection nozzla valve timer T2 is started at the same time r but the opening of the nozzle valve 65 by the nozzle solenoid 82 is delayed for a predetermined short period of time by reason of the open timer contacts T2A. When timer T2 times out, which may be in the order of from .5 to 3 seconds, for example, the contacts T2A
are closed, thus applying power to the solenoid valve 82 and operating the nozzle cylinder 70, permitting the plastic material to be injected into the mold cavity.
The second or high pressure control valve 94 is main-tained in a de-energized position by reason of the now open ; ; contacts CRlAl.
LS9 also applies power to the first or low pressure timer T3 and when it times out, the contacts T3B are closed to initiate the operation of the high pressure timer T4. At the same time, power i8 broken through contacts T3C to the control relay, thus interrupting power to the pressure valve 95 through contacts CRlA2, and applying power to the valve 94.
High hydraulic pressure will be maintained ~n the screw for the remaining portion of the injecting cycle, the duration of which is now controlled by timer T4. When timer T4 times out, contacts T4A interrupt power to the injection solenoid 102, returning it to its neutral position, and to the nozzle cylinder solenoid 82, causing the nozzle valve to be closed.
The cycle timer Tl times out and applies power through contact TlB and the normally closed contacts of limit switch LS8 to the recharge solenoid 104 of valve 87, thus applying power to the hydraulic motor 40 to form a new charge of plas-tic material in the barrel 33. The screw 30 thus retracts Docket 6057 ~059~L4 until limit switch LS8 i5 operated, interrupting power to therecharge solenoid 104, stopping the screw motor 40 and at the same time applying power to control relay CR9. This interrupts power to the hydraulic pressure valve 86, removing the hydraulic pressure and placing the system in a stand-by condition.
The duration of the operation of the timer T3 is suffi-cient to insure complete filling of the mold cavity with the foam plastic material without dislodging any inserts. In a typical example, the mold cavity of a 5 oz. capacity may be filled with plastic material in 5 seconds at an initial low pressure of 200 pounds per square inch. After this time, the injection force is transferred to a relatively higher pressure by the timing out of timer T3 and the initiation of timer T4. Timer T4 maintains a pressure which may, for example, be in the order o 800 psi to compact the foam plastic material to assure complete filling of the mold and to make up any shrinkage which occurs in the plastic ; material. This may, for example, be in the order of 10 seconds.
It is understood that the precise setting of the timers is depen-dent upon the capacity of the mold, the complexity of the mold ca~ity, the desired injection pressure used, and the type of material being used. - ~
The invention is suitable for use with a wide variety of structural plastic foaming materials, and thus may be used to mold foam items from low and high density polyethylene, polypro-ylene, styrene, ABS, vinyl, and nylon, when combined with com-mercially available foaming agents. These are often known as "expandable" plastic materials. In addition, the method may be used for the injection of urethane foams, both soft or flexible and rigid foams. The final pressure, as defined by the setting of valve 94 and by the setting of timer T3, also determines the density of the structural foamed product.

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~OS97~14 The conventional practice of injecting foam has been to inject at a very rapid and fast ra~e of injection, requiring the use of a relatively high pressure on the injection screw, followed by very low pressure or no pressure at all to allow the plastic material to expand out into the cavity and find its own way into the cavity. However, the present invention departs from such conventional teachings and injects at low or moderate pressure followed by the application of higher pressure after - the filling of the cavity to assure a homogeneously foamed product and a~ filled cavity. By injecting conventional plastic materials under relatively low pressure throughout a time as determined by a timer T3,-the relatively delicate inserts or parts which may be held within the mold cavity are not dislodgèd or disturbed in their position, and the application of the relatively higher pressure throughout the timer interval defined by the timer T4 assures full and complete filling of the complex mold cavities about sùch inserts.
Glass may be added to a plastic to provide certain stre.ngth characteristics while iron may be added to provide a magnetic characteristic. However, these and other fill mater-ials may cause wear on the screw, the barrel, the mold gates, cavities and the like. The low pressure injection method has the advantage of reducing the wearing of the screw structure and the wear on the mold cavities, the nozzle valve and the gates which otherwise occurs in the injection of glass or iron filled plastic materials.
While the method herein described constitutes a prefer-red embodiment of this invention, it is to be understood that the invention is not limited to this precise method, and that changes may be made therein without depar~ing from the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

The method of injecting plastic materials in the cavity of a mold particularly useful for the injection of foam-type and filled plastics, and for the injection into a mold cavity containing fill pieces, inserts or the like, which may be moved or disturbed in their position by conventional high pressure injection, employing a ram screw type of plastic injection mach-ine having a controllable nozzle shutoff valve at the injection nozzle and having valve means for selection of low and high injection pressures, comprising the steps of:
a) pressurizing the plastic material within the injec-tion cylinder by applying a low pressure to the injection screw while holding the nozzle valve closed for a predetermined short time interval, b) opening the nozzle valve at the conclusion of said short time interval, c) maintaining said low pressure for a second time interval after the opening of said nozzle valve to effect a con-trolled injection of the plastic material, d) establishing a third time interval commencing at the conclusion of said second time interval, and e) applying and maintaining said high pressure to such screw throughout said third time interval, said third time inter-val being substantially longer than either of said first or second time intervals to prevent shrinkage and to initiate setup of the plastic material within the mold.