METHOD AND SYSTEM FOR MAKING HOLLOW PLASTIC ARTICLES

Abstract: An improved method and injection molding system are provided for making a hollow plastic article wherein plastic resin is injected into an article-defining cavity so that the cavity is loosely filled but not filled to capacity with the plastic resin and wherein the article is defined within the cavity. At this time, the plastic resin has an undulating surface. Then, pressurized gas is injected into the plastic resin in the loosely filled cavity to distribute the plastic resin in the cavity and convert the undulating surface into a surface defined by the cavity, thereby forming at least one gas hole within the distributed plastic. Preferably, the plastic resin occupies approximately 92 % to 99 % of the volume of the cavity immediately after injecting the plastic resin. Preferably, the plastic resin in the cavity is sensed by a sensor during plastic resin injection to generate a signal for use by a hydraulic control system of the injection molding system to stop linear movement of a screw ram of the injection molding system. The sensor may be an infrared sensor assembly including an optical fiber bundle which defines a portion of the cavity. Alternatively, the sensor may be a pressure sensor, which also may define a portion of the cavity. The sensor may also be either an ultrasonic sensor or an electromagnetic sensor.

Description: METHOD AND SYSTEM FOR MAKING HOLLOW PLASTIC ARTICLES Technical Field This invention relates to methods and systems for making hollow plastic articles and, in particular, to methods and systems for making hollow plastic articles utilizing pressurized gas. Background Art U.S. Patent No. 5,098,637, issued to Hendry, discloses a method and system for injection molding hollow plastic articles with pressurized gas which provides for displacement by the gas of a portion of plastic from the mold cavity into a flow-coupled spill cavity. This feature enables plastic articles of relatively greater dimensions to be successfully molded with the advantages of established gas-assisted injection molding techniques. U.S. Patent No. 5,110,533, also issued to Hendry, discloses a method and system to enhance the surface quality of a hollow plastic article by simultaneously injecting a charge of pressurized gas into a mold together with an injection of a second amount of plastic, but only after a substantial first amount of plastic has been injected into an article-defining cavity of the mold. In this patent, the charge of pressurized gas first prevents the flow of the first amount of plastic from stopping and then distributes the total amount of molten plastic resin in the article- defining cavity after the completion of the step of injecting the second amount of plastic. The method and system prevent imperfections on an exterior surface of the plastic article. U.S. Patent No. 4,923,666, issued to Yamazaki et al . , discloses a method of injection molding wherein molten synthetic thermoplastic resin is introduced into a mold cavity sufficient to fill completely the mold cavity. Then, pressurized gas is injected into the mold cavity. In this patent, the screw ram must have gone completely into the pack mode which takes up any compressibility of the plastic. Consequently, the only space for pressurized gas in the mold cavity is in the shrinkage of the plastic. U.S. Patent No. 5,558,824, issued to Shah et al . , discloses tooling and a method for gas-assist injection molding of plastics material into a hollowed part in which a mold chamber formed between stationary and movable mold tools is initially charged with an inert gas to provide a resistance force against the flow front of a short shot of plastic melt subsequently injected into the cavity and supplied with a pressurized assist gas. Sensors are provided for sensing the pressure in the short shot and the mold chamber. Summary Of The Invention An object of the present invention is to provide an improved gas -assist method and system for making hollow plastic articles while eliminating any hesitation marks by injecting plastic resin into an article-defining cavity when the cavity is loosely filled but not filled to capacity with the plastic resin such that the article is defined within the cavity. Another object of the present invention is to provide an improved method and system for making hollow plastic articles while eliminating the necessity for a spillover cavity, thereby eliminating the need to cut off gates, grind plastic, or carefully position such a spillover cavity. Yet, still another object of the present invention is to provide an improved method and system for making hollow plastic articles which are completely strain-free and wherein energy savings are obtained through the use of the method and system. Yet, a further object of the present invention is to provide an improved method and system for making hollow plastic articles which do not have sink marks and wherein a mold of the system is not overpacked and consequently there is no flashing. It is still another object of the present invention to provide an improved method and system for making hollow plastic articles whereby shot-to-shot weight variations are eliminated. In carrying out the above objects and other objects of the present invention, a method is provided for making a hollow plastic article in an injection molding system including a mold and a screw ram which injects plastic resin into the mold during linear movement thereof. The method includes the steps of injecting plastic resin into an article-defining cavity of the mold of the system and preventing further injection of plastic resin into the cavity when the cavity is loosely filled but not filled to capacity with the plastic resin so that the article is defined within the cavity. The method also includes the step of injecting pressurized gas into the plastic resin in the loosely filled cavity to distribute the plastic resin in the cavity and to form a gas cavity within the distributed plastic. The method finally includes the steps of allowing the distributed plastic resin to cool to form the article, venting the pressurized gas from the gas hole in the article after the step of allowing, and removing the article from the mold. Preferably, the step of preventing includes the step of stopping the linear movement of the screw ram. Also, preferably, the plastic resin occupies approximately 92% to 99% of the volume of the cavity immediately after the step of injecting the plastic resin. Still preferably, the plastic resin disposed in the cavity has an undulating surface during the step of injecting the plastic resin and before the step of injecting the pressurized gas and wherein the undulating surface is converted into a surface defined by the cavity after the step of injecting the pressurized gas. Again, preferably, the method includes the steps of sensing plastic resin in the cavity during the step of injecting t e plastic resin and generating a signal when the cavity is loosely filled but not filled to capacity. Further in carrying out the above objects and other objects of the present invention, an injection molding system for making a hollow plastic article is provided. The system includes a mold, a screw ram for injecting plastic resin into an article-defining cavity of the mold during linear movement thereof, and a control system for controlling linear movement of the screw ram. The system also includes means for preventing further injection of plastic resin into the cavity when the cavity is loosely filled but not filled to capacity with the plastic resin so that the article is defined within the cavity. Still further, the system includes means for injecting pressurized gas into the plastic resin in the loosely filled cavity to distribute the plastic resin in the cavity and to form a gas hole within the distributed plastic, the distributed plastic cooling within the mold to form the article. Finally, the system includes means for venting the pressurized gas from the gas hole in the article. Preferably, the plastic resin occupies approximately 92% to 99% of the volume of the cavity immediately after the plastic resin has been injected into the cavity. Also, preferably, the means for preventing includes a plastic-sensing sensor disposed in the mold for sensing the plastic resin in the cavity and coupled to the control system to generate a signal for use by the control system to stop linear movement of the screw ram when the cavity is loosely filled but not filled capacity to prevent further injection of the plastic resin. The advantages accruing to the method and system of the present invention are numerous. For example, hesitation marks are eliminated. Also, a spillover cavity within the mold may be eliminated. Completely strain- free plastic parts may be made since the article-defining cavity is not completely filled with plastic resin. When completely filled with plastic resin, strain marks typically occur and the potential for warpage is possible. Also, high gas injection pressure must be provided. Because there is no need to completely fill the article-defining cavity, there is no need to pack the article-defining cavity and thus energy savings can be realized in the method and system. Also, sink marks may be eliminated with more generous gas space than is allowed when the article- defining cavity is completely packed. Also, the method and system provide no flashing since there is no overpacking of the mold. Finally, the method and system eliminate shot- to-shot weight variations. The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. Brief Description Of The Drawings FIGURE 1 is a schematic view, partially broken away and in cross-section, illustrating an injection molding system which can be modified to obtain the method and system of the present invention; FIGURE 2 is a modified version of the injection molding system of Figure 1 incorporating novel features of the present invention wherein plastic resin is injected into an article-defining cavity until the cavity is loosely filled and not filled to capacity such that an article is defined within the cavity; FIGURE 3 is a view similar to Figure 2 after pressurized gas has been injected into the article defined in the article-defining cavity; FIGURE 4 is a schematic view, partially broken away and in cross-section, of a screw ram within a prior art screw ram barrel with an open check valve and illus- trating the total volume of plastic which can be accumulated therein; FIGURE 5 is a view similar to Figure 4 wherein different positions of the screw ram are shown with a closed check valve to illustrate low pressure fill out of the article-defining cavity; FIGURE 6a is a view, partially broken away and in cross-section of a portion of the article-defining cavity in which the plastic resin has an undulating surface and plastic resin flow is illustrated by phantom lines; FIGURE 6b is a view similar to the view of Figure 6a after pressurized gas has been injected into the plastic resin to convert the undulating surface into a smooth surface; FIGURE 7 is a schematic view similar to the view of Figure 4 and illustrating the amount of accumu- lated plastic sufficient to loosely fill but not pack the article-defining cavity; FIGURE 8 is a sectional view of a plastic article prior to injection of pressurized gas and which loosely fills but does not completely fill the article- defining cavity indicated by phantom lines; FIGURE 9 is a sectional view of a plastic article which indicates tow-in thereof; and FIGURE 10 illustrates a pair of graphs of pressure versus percentage of plastic injected during various phases of the method of the present invention, the lower graph indicating plastic pressure within the cavity and the upper graph indicating hydraulic pressure on the screw ram to inject plastic. Best Mode For Carrying Out The Invention Referring now to Figure 1, there is illustrated an injection molding system, generally indicated at 10, which may be mod: f ed to carry out the method of the present invention and wherein, after modification, can be used to practice the method and system of the present invention. The system 10 includes a plastic injection unit, generally indicated at 12, including a screw ram, generally indicated at 14, disposed within a injection chamber 15 and having a threadedly secured screw tip 16 within a barrel of the unit 12. A check valve or ring 18 is also provided in the injection unit 12 in a conventional fashion. The unit 12 also includes a nozzle shut off block 20 having a movable pin 22 to open and close the plastic flow path of the unit 12. Heater bands 24 are provided for maintaining the plastic resin in its fluid state. An injection head, generally indicated at 26, is secured to the injection unit 12 by bolts 28 (only one of which is shown) . A nozzle 30 is threadedly secured to the rest of the injection head 26, also in a conventional fashion. The injection molding system 10 also includes a mold, generally indicated at 32, including a station- ary mold half 34 and a movable mold half 36 which together define an article-defining cavity 38. Both of the mold halves 34 and 36 include apertures 46 formed therethrough for cooling water. The injection head 26 is disposed within a dished-out portion of a first platen 40. The mold 32 also includes a sprue bushing 42 located within the mold half 34. A locating ring 44 locates the sprue bushing 42 within the mold half 34 relative to the first platen 40. The mold 32 also includes a tie bar 48 which has a tie bar bushing 50 positioned thereabout to permit a second platen 58 to reciprocate in a conventional fashion relative to the first platen 40. A knock-out frame assembly, generally indicated at 52, includes a pin 54 to knock-out a plastic article from the article-defining cavity 38 after molding of the plastic article. Mold clamp assemblies 56, which are preferably used on both halves of the mold 32, secure parts to the second platen 58 in a conventional fashion. Referring now to Figure 2, there is illustrated the system 10 of Figure 1 which has been modified to perform the method of the present invention, thereby resulting in the system of the present invention. The system 10 of Figure 2 (as well as the system 10 of Figure 1) includes a hydraulic control system 60 for controlling linear and rotary movement of the screw ram 14. Preferably, the hydraulic control system 60 includes therein a quick-acting electronic servo valve (not shown) such as one commonly employed in the alumi- num. casting art area. The servo valve is responsive to an electric control signal generated by a plastic-sensing sensor assembly, generally indicated at 62, and provided along the line 64 coupled to the hydraulic system 60. The assembly 62, which is threadedly secured within the mold half 36, includes a fiber optic bundle or probe 66 disposed in the mold half 36. A surface of the probe 66 helps to define the article-defining cavity 38. The fiber optic probe 66 transmits infrared radiating radiated by the molten plastic resin as it fills the article-defining cavity 38 to an infrared sensor 68 of the assembly 62. The sensor 68 provides the control signal to the hydraulic system 60 along the line 64. In another embodiment, the assembly 62 may comprise a pressure sensor assembly which also helps to define the article-defining cavity 38. Alternatively, the assembly 62 may comprise an ultrasonic or electromagnetic sensor assembly. In Figure 2, the pin or nozzle shut off valve 22 is open to allow the plastic resin to flow from the injection unit 12 into the mold 32. Also, the check ring 18 is in its closed position. The amount of plastic in the article-defining cavity 38, as illustrated in Figure 2, is enough so that the cavity is loosely filled but not filled to capacity and so that an article 70 is defined in the mold 32. When the plastic-sensing sensor assembly 62 sees either pressure (in the pressure sensor case) or a predetermined infrared light change (in the infrared sensor case) , the sensor assembly 62 emits the control signal along the line 62 to the hydraulic system 60 to the quick-acting electronic servo valve located therein to cause the servo valve to instantaneously drop all pressure of injection provided by the screw ram 14 so that no more plastic resin goes into the article-defining cavity 38 even if a small amount of plastic is still in the injection chamber 15 of the unit 12. Referring now to Figure 3, there is illustrated a completed, hollcw plastic article 70 formed within the cavity . 38 of the mold 32 after pressurized gas has been injected into the plastic resin to distribute the plastic resin in the cavity 38 to thereby form at least one gas hole such as gas holes 72 within the distributed plastic. The pressurized gas may be injected into the plastic resin through the nozzle 30, in the runner system or a pin disposed in one of the mold halves 34 and 36 after the pin 22 has moved to its blocking position. Typically, the hydraulic control system 60 of the present invention stops movement of the screw ram 14 before it reaches its extreme leftmost position within the chamber 15 to prevent packing of plastic resin within the article-defining cavity 38. As shown in Figures 2 and 3, the screw ram 14 is not in its extreme leftmost position. Alternatively, the system 10 can work without the sensor assembly 62 through experimentation with the system 10 wherein the hydraulic system 60 senses either time or distance that the screw ram 14 linearly moves to loosely fill yet not completely fill the article-defining cavity 38 to capacity. Referring now to Figure 4, there is illustrated by arrows 76 the maximum extent of travel of the screw tip 16 of the prior art wherein, because of packing, high strains are introduced to the resulting moldings which causes warpage and dimensionally incorrect moldings . Referring now to Figure 5, there is also illustrated the prior art screw ram 14 and its positions from the starting position illustrated in Figure 4. The position of the screw ram 14 within the unit 12, as indicated by solid lines from the position of Figure 4, represents a low pressure fill of the article-defining cavity 38. Arrows 78 indicate further movement or stroke of the screw tip 16 to loosely fill the article- defining cavity 38. Arrows 80 indicate movement of the screw tip 16 from the solid line position indicated in Figure 5 to a position of high pressure packing of the article- defining cavity. This utilizes a cushion of plastic to pack the cavity of the prior art. Typically, the screw ram 14 never reaches the front of the screw ram barrel. This volume is the total volume of the plastic needed to make the molding. Arrows 82 indicate very high pressure exerted by the prior art on the screw ram 14 by the hydraulic control system 60 in order to take up shrinkage as it occurs . Referring now to Figure 6a, in combination with Figure 6b, a flow of plastic resin 84, as indicated by solid lines in Figure 6a, together with subsequent break out fronts, as indicated by phantom lines 86 in Figure 6a, are illustrated. As is well known, due to surface drag of the plastic on the cavity and core walls in accordance with Bernoulli's principle, the exterior plastic flows slower than the interior plastic. When the plastic is moving into an extreme portion 88 of the article-defining cavity 38, the break out fronts 86 progressively get closer and closer to the fiber optic bundle 66, thereby causing the fiber optic bundle 66 to transmit a greater and greater amount of thermal radia- tion to its infrared sensor 68 (not shown in Figure 6a) As further illustrated in Figure 6a, as the plastic flows within the portion 88 of the article- defining cavity 38, undulations are formed within the portion 88 to obtain an undulating surface 90 of the plastic resin. When the plastic resin has reached an experimentally determined position, as indicated by one of its break out fronts 86 indicated in Figure 6a, the resulting signal generated by the infrared sensor reaches a predetermined level to provide the control signal to the servo valve within the hydraulic control system 60 which stops screw ram movement. Typically, it is desired to provide the signal from the sensor to the hydraulic control system 60 when the plastic resin occupies approximately 92% to 99% of the entire volume of the article-defining cavity 38 so that the undulating surface 90 of the plastic resin is present within the article-defining cavity 38. In this way, completely strain-free parts can be made. Further injection of plastic resin into the article-defining cavity 38 is then prevented. Pressurized gas is then injected into the plastic resin in the loosely-filled article-defining cavity 38 to distribute the plastic resin in the cavity 88 and to form a gas hole 92 within the distributed plastic. In this way, the prior undulating surface 90 of the plastic resin is converted into a typically smooth surface defined by the article-defining cavity 38 after the step of in]ecting the pressurized gas. Thereafter, the distributed plastic resin is allowed to cool to form the article and the gas from the gas hole 92 is vented from the article in any one of numerous prior art methods . Finally, the article is removed from the mold 32 such as through the use of the knock-out pin 54 which may also be utilized to supply pressurized gas into the article-defining cavity 38. Referring now to Figure 7, there is illustrat- ed by arrows 94, the amount of accumulated plastic sufficient to loosely fill but not completely fill the article-defining cavity 38 to capacity. The dashed lines represent the furthest leftmost position of the screw tip 16. During such movement of the screw ram 14 forward, all of the accumulated plastic between the_ arrows 94 is injected into the mold 32 under low injection pressure. That is, only enough to overcome the pressure drop going through the sprue, the runner system, and the gate of the mold 32. There is little or no pressure within the article-defining cavity 38 until the cavity 78 is full. Referring now to Figure 8, there is illustrated an article 70 defined within the article-defining cavity 38 wherein the plastic resin loosely fills but does not completely fill the article-defining cavity 38 As can be readily seen, the article 70 includes a boss 96 and a rib 98. The molding or article 70 generally defines the finished article but has the relatively wavy or undulating surface 90 as well as "lift-off" from the mold parting line. Figure 9 illustrates tow-in of the molding 70 at legs 100 wherein shrinkage occurs to "lift off" the parting line of the mold 32. Referring now to Figure 10, there is illus- trated injection pressure along a vertical axis and percentage of plastic injected into the cavity 38 along a horizontal axis. The particular example is for 8 to 10 melt index polypropylene and 3 to 4 mm wall thickness . Line segment 102 indicates hydraulic pressure on the screw ram 14 to initially inject plastic into the mold 32 from approximately 3000 psi up to point 104, which is approximately 6000 psi. This peak hydraulic pressure is reached during the plastic packing mode . At line segment 106, the hydraulic pressure is maintained on the plastic until the gate has frozen off within the mold 32. In the example shown, the line segment 106 is at approximately 5500 psi. At line segment 108, the hydraulic pressure has decayed prior to screw ram rota- tion to accumulate plastic for the next molding. Line segment 110, indicates plastic pressure in the article-defining cavity 38 during initial injection of the plastic into the article-defining cavity 38. The difference in pressure between the line segments 102 and 110 is due to restriction in the sprue 42, runners, gate, etc. plus drag in the screw area. Due to surface drag of the plastic on the cavity and core walls and the beginning of solidification of the plastic in contact with the cooled mold surfaces, no matter how slight, plastic pressure in the article-defining cavity 38 slowly increases as the volume of plastic in the article-defining cavity 38 increases. Plastic pressure increases from 0 psi within the article-defining cavity 38 at 0% plastic injected to approximately 150 psi at 90% plastic injected. At point 112, which is approximately 200 psi and 94% of the cavity 38 is filled, plastic flow is stopped as described above and pressurized gas is injected into the article-defining cavity 38. Line segment 114 represents gas hold pressure of approximately 300 to 1000 psi during part solidification. As previously mentioned, this pressurized gas is then vented to the atmosphere before the finished article is removed from the mold 32. While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.

Claims: What Is Claimed Is; 1. A method for making a hollow plastic article in an injection molding system including a mold and a screw ram which injects plastic resin into the mold during linear movement thereof, the method comprising the steps of : injecting plastic resin into an article- defining cavity of the mold of the system; preventing further injection of plastic resin into the cavity when the cavity is loosely filled but not filled to capacity with the plastic resin so that the article is defined within the cavity; injecting pressurized gas into the plastic resin in the loosely filled cavity to distribute the plastic resin in the cavity and to form a gas hole within the distributed plastic- allowing the distributed plastic resin to cool to form the article; venting the pressurized gas from the gas hole in the article after the step of allowing; and removing the article from the mold. 2. The method as claimed in claim 1 wherein the step of preventing includes the step of stopping the linear movement of the screw ram. 3. The method as claimed in claim 1 wherein the plastic resin occupies approximately 92% to 99% of the volume of the cavity immediately after the step of injecting the plastic resin. 4. The method as claimed in claim 1 wherein the plastic resin disposed in the cavity has an undulat- ing surface during the step of injecting the plastic resin and before the step of injecting the pressurized gas and wherein the undulating surface is converted into a surface defined by the cavity after the step of injecting the pressurized gas. 5. The method as claimed in claim 1 further comprising the steps of sensing 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 filled to capacity. 6. The method of claim 5 wherein the step of sensing includes the step of sensing infrared radiation in the cavity during the step of injecting the plastic resin. 7. The method of claim 5 wherein the step of sensing includes the step of sensing pressure caused by the plastic in the cavity during the step of injecting the plastic resin. 8. An injection molding system for making a hollow plastic article, the system comprising: a mold; a screw ram for injecting plastic resin into an article-defining cavity of the mold during linear movement thereof; a control system for controlling linear movement of the screw ram; means for preventing further injection of plastic resin into the cavity when the cavity is loosely filled but not filled to capacity with the plastic resin so that the article is definedwithin the cavity; means for injecting pressurized gas into the plastic resin in the loosely filled cavity to distribute the plastic resin in the cavity and to form a gas hole within the distributed plastic, the distributed plastic cooling within the mold to form the article; and means for venting the pressurized gas from the gas hole in the article. 9. The system as claimed in claim 8 wherein the plastic resin occupies approximately 92% to 99% of the volume of the cavity immediately after the plastic resin has been injected into the cavity. 10. The system as claimed in claim 8 wherein the means for preventing includes a plastic-sensing sensor disposed in the mold for sensing the plastic resin in the cavity and coupled to the control system to generate a signal for use by the control system to stop linear movement of the screw ram when the cavity is loosely filled but not filled to capacity to prevent further injection of the plastic resin. 11. The system as claimed in claim 10 wherein the sensor is an optical sensor. 12. The system as claimed in claim 10 wherein the sensor is an infrared sensor. 13. The system as claimed in claim 12 wherein the infrared sensor includes an optical fiber bundle which defines a portion of the cavity. 14. The system as claimed in claim 10 wherein the sensor is a pressure sensor. 15. The system as claimed in claim 14 wherein the pressure sensor defines a portion of the cavity. 16. The system as claimed in claim 10 wherein the sensor is an ultrasonic sensor. 17. The system as claimed in claim 10 wherein the sensor is an electromagnetic sensor.

Inventors: HENDRY JAMES WATSON, HENDRY, JAMES, WATSON
Applicants: HENDRY JAMES W, MELEA LTD, HENDRY, JAMES, WATSON, MELEA LIMITED
Application year: 1998
Main classification: B29C45
IPC codes: B29C45