KR20000016660A - Method and system for making hollow plastic product - Google Patents

Abstract

PURPOSE: A method and a system are provided not to need to place a spillover hollow, grind plastic or cut a gate by removing the spillover while making a hollow plastic product. CONSTITUTION: A method has the steps of: injecting plastic resin into a product limited hollow(38) of a mold(32); preventing the plastic resin from injecting into the hollow before fully filling the plastic resin in the hollow, for limiting the product into the hollow; forming a gas hole inside a distributed plastic by injecting compressed gas inside the plastic resin in the loosely filled hollow and by distributing the plastic resin inside the hollow; forming a product by cooling the distributed plastic resin; discharging compressed gas from the gas hole after cooling the plastic resin; and removing the product from the mold.

Description

Method and system of making hollow plastic articles

U.S. Patent No. 5,098,637, issued by Hendry, discloses a method and system for injection molding hollow plastic articles with compressed gas that provides for the movement of a portion of the plastic by gas from the mold cavity into the flow-coupled spill cavity. . A feature of this patent is that plastic articles of relatively large dimensions can be successfully molded with the advantages of injection molding technology with gas.

U. S. Patent No. 5,110, 533, also issued by Hendry, discloses that only after the first amount of plastic has been injected into the mold-limited cavity of the mold, a certain amount of compressed gas is simultaneously introduced into the mold with the second amount of plastic being injected. A method and system are disclosed for improving the surface quality of hollow plastic articles by injecting. In this patent, a certain amount of compressed gas first dispenses the entire amount of dissolved plastic in the article confinement cavity after completion of the step of preventing the flow of the first amount of plastic from stopping and then injecting the second amount. This method and system prevents defects on the outer surface of the plastic.

US Pat. No. 4,923,666, issued by Yamazaki et al. Discloses a method of injection molding, wherein dissolved synthetic thermoplastic resin is introduced into a sufficient mold cavity to fully fill the mold cavity. Compressed gas is then injected into the mold cavity. In this patent, the screw ram must proceed fully into the pack mode to accommodate any compressibility of the plastic. As a result, only the space for the compressed gas in the mold cavity is involved in the shrinkage of the plastic.

U. S. Patent No. 5,558, 824 issued by Shah et al. Discloses a method and machine for injection molding plastic material into a hollow by gas, wherein a mold chamber formed between the stationary mold and the movable mold tool is initially inert. Filled with gas, it melts and is injected into this cavity to provide resistance to the forward flow of small shots of plastic to which compressed aid gas is supplied. Sensors are provided to sense the pressure in this small shot and mold chamber.

The present invention relates to a method and system for making hollow plastic articles, and more particularly, to a method and system for making hollow plastic articles using compressed gas.

1 is a schematic cross-sectional view partially showing an injection molding system that can be modified to obtain a method and system of the present invention;

FIG. 2 illustrates a variation of the injection molding system of FIG. 1 embodying the novelty of the present invention in which plastic resin is injected into an article defined cavity until the cavity is loosely filled but not as full as possible to allow the article to be defined within the cavity. Drawing to show;

FIG. 3 is a view similar to FIG. 2 after the compressed gas has been injected into an article confined within the article confinement cavity;

4 is a schematic cross-sectional view of a partially broken state to illustrate the total volume of plastic that may accumulate therein while showing the screw ram in a prior art screw ram barrel having an open check valve;

FIG. 5 is a view similar to FIG. 4 where the different positions of the screw ram are shown with the closed check valve state showing a low pressure filled from the article defining cavity;

6A is a partially broken cross-sectional view of a portion of an article defining cavity in which the plastic resin has a wave surface and the plastic resin flow is represented by an imaginary line;

FIG. 6B is a view similar to FIG. 6A after compressed gas is injected into the plastic resin to change the wave surface to a flat surface;

FIG. 7 is a similar schematic diagram of FIG. 4 showing the amount of accumulated plastic sufficient to loosely fill but not fully fill the article KEPCO cavity; FIG.

8 is a cross-sectional view of a plastic article that is loosely filled but not fully filled with article defining cavities represented by phantom lines and prior to injection of compressed gas;

9 is a cross-sectional view of the plastic article showing an inwardly facing state;

Figure 10 illustrates a pair of graphs for the percentage and pressure of plastic injected at various stages of the method of the present invention, the bottom graph showing the plastic pressure inside the cavity and the top graph the hydraulic pressure on the screw rams injecting the plastic. The figure which shows.

It is an object of the present invention to make a hollow plastic article and to remove any marks by injecting the plastic resin into the article confinement cavity when the article confinement cavity is loosely filled with the plastic resin but not fully filled, thereby allowing the article to be confined within the cavity. It is to provide an improved method and system by gas.

Another object of the present invention is an improved method and system that eliminates the need for a spillover cavity while making a hollow plastic article, eliminating the need to cut a gate, grind the plastic, or position the spillover cavity deeply. To provide.

It is a further object of the present invention to provide an improved method and system for making hollow plastic articles that are completely deformed and to obtain energy savings through the method and system.

It is also a further object of the present invention to provide an improved method and system for making hollow plastic articles that do not have recesses, wherein the mold of the system is not overfilled and consequently no overflow.

It is another object of the present invention to provide an improved method and system for making hollow plastic articles to eliminate shot to shot weight variations.

In carrying out the above and other objects of the present invention, a method is provided for making a hollow plastic article in an injection molding system comprising a mold and a screw ram for injecting the plastic resin into the mold during linear movement. This method involves injecting the plastic resin into the article confinement cavity and preventing further injection of the plastic resin into the cavity when the cavity is loosely filled with the plastic resin but not fully filled, thereby preventing the article from entering the cavity. Defining a step. The method also includes injecting compressed gas into the plastic resin in the loosely filled cavity to distribute the plastic resin within the cavity to form a gas cavity in the dispensed plastic. The method finally involves cooling the dispensed plastic resin to form the article, venting compressed gas from the gas holes in the article after forming the article, and removing the article from the mold. Include.

Preferably, preventing further injection of this plastic resin comprises stopping the linear movement of the screw ram.

Also preferably, the plastic resin accounts for about 92% to 99% of the volume of the cavity immediately after the injection of this plastic resin.

Further, preferably, the plastic resin disposed in the cavity has a wave surface at the time of injecting the plastic resin and before the injection of the compressed gas and the wave surface has a surface defined by the cavity after the step of injecting the compressed gas. Is switched to.

Again, the method preferably comprises sensing the plastic resin in the cavity during the step of injecting the plastic resin and generating a signal when the cavity is loosely filled but not fully filled.

Further, in carrying out the above and other objects of the present invention, an injection molding system for making hollow plastic articles is provided. The system includes a mold, a screw ram for injecting plastic resin into the mold defining cavity of the mold, and a control system for controlling the linear movement of the screw ram. The system also includes a means to prevent further injection of plastic resin into the cavity when the cavity is loosely filled with plastic resin but not fully filled so that the article is confined within the cavity. The system also injects compressed gas into the plastic resin in this loosely filled cavity to dispense the plastic resin into the cavity to form a gas hole in the dispensed plastic to cool the dispensed plastic in the mold. And means for forming the article. Finally, the system includes means for venting compressed gas from the gas holes in the article.

Preferably, the plastic resin accounts for about 92% to 99% of the volume of the cavity immediately after injecting the plastic resin into the cavity.

Preferably, the means for preventing further injection comprises a plastic sensor which is placed in a mold and connected to the control system to detect the plastic resin in the cavity so that the cavity is loosely filled but not filled to the maximum. When used for this system, a signal is generated to stop the linear movement of the screw ram, preventing further injection of plastic resin.

There are many advantages with the method and system of the present invention. For example, the mark is removed. In addition, the spillover cavity in this mold can be removed.

A completely deformed plastic part can be made because the article constraining cavity is not completely filled with plastic resin. When completely filled with plastic resins, there is a possibility of deformation and distortion occurring normally. High gas injection pressures should also be provided.

Since there is no need to completely fill this article confinement cavity, there is no need to pack this article confinement cavity so that energy savings can be realized in this method and system.

In addition, recesses can be removed with more gas space than is allowed when the article defining cavity is fully packed.

In addition, the method and system do not provide any overflow since there is no excessive packing of the mold.

Finally, this method and system eliminates the shot-to-shot weight change.

The above and other objects, features and advantages of the present invention will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in conjunction with the accompanying drawings.

Referring to Figure 1, an injection molding system, indicated at 10, is illustrated, which can be modified to carry out the method of the present invention, which can then be used to implement the methods and systems of the present invention. The system 10 comprises a plastic injection unit 12 containing a screwram, designated 14, which is disposed in the injection chamber 15 and threaded in the barrel of the unit 12. It has a screw tip 16 that is fastened. The check valve or ring 18 is also provided in the injection unit 12 in a conventional manner. The unit 12 also includes a nozzle blocking block 22 with a movable pin 22 to open and close the plastic flow passage of the unit 12. The heating band 24 is provided to keep the plastic resin in a fluid state. The injection head 26 is secured to the injection unit 12 by bolts 28 (only one shown). The nozzle 30 is also screwed to the rest of the injection head 26 in a conventional manner.

The injection molding system 10 also includes a mold 32 comprising a fixed half mold 34 and a movable half mold 36 that together define an article defining cavity 38. Both molds 34 and 36 include through holes 46 formed for cooling water.

The injection head 26 is arranged inside the outwardly concave portion of the first platen 40. The mold 32 also includes a sprue bushing 42 located in the half mold 34. The positioning ring 44 positions the sprue bushing 42 in the half mold 34 relative to the first platen 40.

The mold 32 also has a tie bar bushing 50 positioned circumferentially to allow the tie bar 48 to cause the second platen 58 to reciprocate in a conventional manner with respect to the first platen 40. Include.

Squeezing frame assembly 52 includes pins 54 to squeeze the plastic article out of the article defining cavity 38 after molding the plastic article.

Mold clamp assembly 56, which is preferably used on both halves of the mold 32, secures the parts to the second platen in a conventional manner.

Referring to FIG. 2, the system 10 of FIG. 1 is shown modified to implement the method of the present invention resulting in the system of the present invention. This system 10 of FIG. 2 includes a hydraulic control system 60 that controls the linear and rotational movement of the screw ram 14 (as well as the system 10 of FIG. 1). Preferably, this hydraulic control system 60 includes therein a fast acting solenoid servovalve (not shown) as is conventionally employed in the aluminum casting art.

The servovalve is responsive to an electronic control signal generated by the plastic sensing sensor assembly 62 and provided along a line 64 coupled to the hydraulic system 60. Assembly 62 screwed in half mold 36 includes an optical fiber bundle or probe 66 disposed in half mold 36. The surface of the probe 66 helps to define the article defining cavity 38. The optical fiber probe 66 transmits infrared radiation emitted by the dissolved plastic resin to the infrared sensor 68 of the assembly 62 as the dissolved plastic resin fills the article molding cavity 38. This sensor 68 provides a control signal along the wire 64 to the hydraulic system 60.

In another embodiment, this assembly 62 may be comprised of a pressure sensor assembly that also helps to define the article defining cavity 38. Optionally, this assembly 62 may be comprised of an ultrasonic sensor assembly or an electromagnetic sensor assembly.

In FIG. 2, the pin or nozzle shutoff valve 22 is open to allow plastic resin to flow from the injection unit 12 into the mold 32. In addition, the check ring 18 is in a closed state.

As shown in FIG. 2, the amount of plastic in the article confinement cavity 38 is sufficient such that the cavity is loosely filled but not fully filled so that the article 70 is formed in the mold 32.

When the plastic sensing sensor assembly 62 detects a pressure (if it is a pressure sensor) or a predetermined infrared change (if an infrared sensor), the sensor assembly 62 is connected to a line 64 to the hydraulic system 60. Control signal is then sent to the internally actuated fast acting solenoid servovalve which causes the servovalve to momentarily drop all the pressure of the injection provided by the screw ram 14, so that a small amount of plastic is still present in the unit 12. Although in the injection chamber 15, no further plastic resin enters the article confinement cavity 38.

Referring to FIG. 3, after the compressed gas is injected into the plastic resin to distribute the plastic resin into the cavity 38 to form at least one gas hole such as gas hole 72 in the dispensed plastic, the mold 32 Illustrated is a finished hollow plastic article 70 formed in a cavity 38. Compressed gas may be injected into the plastic resin through the nozzle 30 in a runner system or pin disposed in one of the half molds 34 and 36 after the pin 22 has moved to the blocking position.

Typically, the hydraulic control system 60 of the present invention stops the movement of the screw ram 14 before the screw ram reaches the leftmost position in the chamber to prevent packing of the plastic resin in the article confinement cavity 38. . As shown in FIGS. 2 and 3, the screw ram 14 is not in its leftmost position.

Alternatively, the system 10 can be operated without the sensor assembly 62 as a result of experimenting with the system 10, where the hydraulic system 60 is the time at which the screw ram 14 moves linearly or The distance is sensed to loosely fill the article confinement cavity 38 but not to fill it fully.

Referring to FIG. 4, the maximum length of the movement of the prior art screw tip 16 is shown by arrows 76 which results in moldings which introduce high deformations due to packing and cause warping and dimensionally inaccurate moldings. have.

Referring to FIG. 5, the positions of the screw ram 14 of the prior art and the screw ram from the starting position shown in FIG. 4 are shown. The position of the screw ram 14 in the unit 12 as shown by the solid line from the position in FIG. 4 represents the filled low pressure of the article confinement cavity 38. Arrow 78 represents another movement or stroke of screw tip 16 that loosely fills the article defining cavity 38.

Arrow 80 shows the movement of the screw tip 16 from the solid line portion shown in FIG. 5 to the position of the high pressure packing of the article confinement cavity. This uses a cushion of plastic to pack the cavity of the prior art. Typically, this screw ram 14 never reaches the front of the screw ram barrel. This volume is the total volume of plastic needed to make a molded part. Arrow 82 represents a very high pressure acting on the screw ram 14 by the hydraulic control system 60 as prior art to accommodate shrinkage.

Referring now to FIG. 6A in conjunction with FIG. 6B, a plastic resin 84 as represented by a solid line in FIG. 6A, with a sequential, fronted portion as shown by an imaginary line 86 in FIG. 6A. The flow of is illustrated. As is known, due to the Bernoulli principle, the outer plastic flow is slower than the inner plastic due to the surface resistance of the plastic on the cavity and the core wall. When the plastic is moved into the distal end 88 of the article confinement cavity 38, this recessed front portion 86 gradually approaches the optical fiber bundle 66, so that the optical fiber bundle 66 has a greater amount of heat. The yarns are transferred to an infrared sensor 68 (not shown in FIG. 6A).

As illustrated in FIG. 6A, as the plastic flows in the portion 88 of the article confinement cavity 38, a wave portion is formed in the portion 88 to obtain the wave surface 90 of the plastic resin. As represented by one of the front edges 86 shown in FIG. 6A, when this plastic resin reaches an experimentally determined position, the signal generated by the infrared sensor reaches a predetermined level to reach the hydraulic control system. This control signal is provided to the servovalve within 60 to stop the screw ram movement. Typically, when the plastic resin occupies about 92% to 99% of the total volume of the article confinement cavity 38, it signals from the sensor to the hydraulic control system 60 such that the wave surface 90 of the plastic resin confines the article confinement cavity. It is desirable to stay within (38). In this way, completely deformed parts can be made. Thus further injection of plastic resin into the article confinement cavity 38 is prevented.

Compressed gas is then injected into the plastic resin in the loosely filled article confinement cavity 38 to distribute the plastic resin to the cavity 88 to form a gas hole 92 in the dispensed plastic resin. In this way, the leading wave surface 90 of the plastic resin is converted to a conventional smooth surface defined by the article defining cavity 38 after the step of injecting the compressed gas.

This dispensed plastic resin is then cooled to form an article and the gas from the gas aperture 92 is expelled from this article by any of a number of conventional methods.

Finally, the article can be removed from this mold 32 through the use of a squeezing pin 54 that can also be used to supply compressed gas into the article confinement cavity 38.

Referring to FIG. 7, the amount of accumulated plastic is shown by arrow 94 to the extent that the article confinement cavity 38 is loosely filled but not fully filled. The dashed line indicates the leftmost position of the screw tip 16. When the screw ram 14 is moved forward, all accumulated plastic between the arrows 94 is injected into the mold 32 under low injection pressure. This is just enough to overcome pressure drop through the gutter, runner system, and gate of the mold 32. There is little or no pressure in the article confinement cavity 38 until the cavity 78 is filled.

Referring to FIG. 8, an article 70 confined within the article confinement cavity 38 is illustrated, where the plastic resin loosely fills the article confinement cavity 38 but does not completely fill it. As can be readily appreciated, the article 70 includes a boss 96 and a rib 98. This molded article or article 70 defines a finished article but has a relatively wavy surface 90 that emerges from the mold dividing line.

9 illustrates a deformation inwardly of the molded article 70 in the leg 100, where shrinkage occurs to float the dividing line of the mold 32.

Referring to FIG. 10, the percentage of plastic injected into the cavity 38 along the horizontal axis and the injection pressure along the vertical axis are illustrated. This particular example is for polypropylene with 8 to 10 dissolution rates and 3 to 4 mm wall thickness.

Line segment 102 represents the hydraulic pressure on screw ram 14 that initially injects plastic into mold 32 from point 3000 to point 6000 which is about 6000 psi. The hydraulic pressure of this vertex is reached in plastic packing mode. This hydraulic pressure remains on the plastic until the gate hardens in the mold 32. In the example shown, line segment 106 is at about 5500 psi. In line segment 108, the hydraulic pressure disappears before rotating the screw ram to accumulate plastic for the next molding.

Line segment 110 represents the plastic pressure in the article confinement cavity 38 upon initial injection of the plastic into the article confinement cavity 38. The difference in pressure between the line segment 102 and the line segment 110 is due to regulation at the spout 42, runner, gate, etc., which adds resistance to the screw portion.

Due to the surface resistance of the plastic on the cavity and the core wall and the initiation of solid solution of the plastic in contact with the cooled mold surface, the plastic confinement in the article confinement cavity 38, however small, may be It gradually increases with increasing volume of the plastics at. The plastic pressure increases from 0 psi in the article confinement cavity 38 to 0 150 psi in 90% plastic injected in the 0% plastic injected. At point 112 the plastic is filled to about 200 psi and 94% in the cavity 38 and the plastic flow stops as the compressed gas described above is injected into the article confinement cavity 38.

Line segment 114 exhibits a gas holding pressure of about 300 to 1000 psi upon partial solidification. As described above, this compressed gas is then discharged into the atmosphere before the finished article is removed from the mold 32.

While the best mode of carrying out the invention has been described in detail, those skilled in the art to which the invention pertains will recognize various modification designs and embodiments for carrying out the invention as defined by the following claims.

Claims (17)

A method of making a hollow plastic article in an injection molding system comprising a mold and a screw ram for injecting the plastic resin into the mold during linear movement. Injecting plastic resin into the article confinement cavity of the mold of the system, Preventing further injection of plastic resin into the cavity when the cavity is loosely filled with plastic resin but not fully filled so that the article is confined in the cavity, Injecting compressed gas into the plastic resin in the loosely filled cavity to dispense the plastic resin in the cavity to form a gas hole in the dispensed plastic, Cooling the dispensed plastic resin to form an article, Cooling the dispensed plastic resin and then draining the compressed gas from the gas holes in the article, and Removing the article from the mold. The method of claim 1 wherein said preventing step comprises stopping the linear movement of the screw ram. The method of claim 1 wherein the plastic resin comprises about 92% to 99% of the volume of the cavity immediately after the step of injecting the plastic resin. The plastic resin disposed in the cavity has a wave surface at the time of injecting the plastic resin and before the injection of the compressed gas and the wave surface is defined by the cavity after the step of injecting the compressed gas. The surface is converted to a surface. 2. The method of claim 1, further comprising the step of sensing the plastic resin in the cavity during the step of injecting the plastic resin and generating a signal when the cavity is loosely filled but not fully filled. . 6. The method of claim 5, wherein said sensing comprises sensing infrared radiation in the cavity during the step of injecting the plastic resin. 6. The method of claim 5 wherein said sensing comprises sensing the pressure caused by the plastic resin in the cavity during the step of injecting the plastic resin. An injection molding system for making hollow plastic articles, Mold, Screw ram for injecting plastic resin into the article confinement cavity of the mold during linear movement, Control system to control the linear movement of this screw ram, Means for preventing further injection of plastic resin into the cavity when the cavity is loosely filled with plastic resin but not fully filled so that the article is confined within the cavity, Means for injecting compressed gas into the plastic resin in the loosely filled cavity to distribute the plastic resin to the cavity to form gas holes in the dispensed plastic resin to cool the dispensed plastic resin in a mold to form an article, and Injection molding system comprising means for venting compressed gas from a gas aperture in the article. The injection molding system of claim 8, wherein the plastic resin comprises about 92% to 99% of the volume of the cavity immediately after the plastic resin is injected into the cavity. 9. The control system of claim 8, wherein the preventing means comprises a plastic sensing sensor disposed in the mold and connected to the control system for sensing the plastic resin in the cavity such that the cavity is loosely filled but not filled to the maximum. Injection molding system, characterized in that it generates a signal for use by stopping the linear movement of the screw ram to prevent further injection of plastic resin. The injection molding system according to claim 10, wherein the sensor is an optical sensor. An injection molding system according to claim 10, wherein the sensor is an infrared sensor. 13. The injection molding system of claim 12, wherein the infrared sensor comprises a fiber optic bundle defining a portion of the cavity. An injection molding system according to claim 10, wherein the sensor is a pressure sensor. The injection molding system of claim 14, wherein the pressure sensor defines a portion of the cavity. The injection molding system according to claim 10, wherein the sensor is an ultrasonic sensor. An injection molding system according to claim 10, wherein the sensor is an electromagnetic sensor.