CA2083856A1 - Method and apparatus for gas assisted injection molding having precise control of injection and holding pressure with stepped holding pressure capability - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method and apparatus for gas assisted injection molding which will first inject a shot of molten plastic into a suitable mold, shut off the plastic flow into the mold, and either immediately, or after a delayed time, inject a suitable pressurized gas into the mold at a desired injection pressure. The pressure of the gas in the mold is then controlled by stepping up or stepping down the pressure in the mold in a series of steps to arrive at a second predetermined and precisely controlled pressure. The predetermined pressure is held for a desired time. The setting and holding of pressure is repeated as many times as desired, and is then stepped down to a final pressure. The mold is then opened, and the article is removed.

Description

;;3,742-015 ~ ~ $ ~3 Expree6 Mail: FB622002579 ~l~OD A7iO APPARA'l'ilS FOR GAS ASSI:5Tel) I:HJRCTlON
MQLDI~ E~ING PRi~~5E CO}~ROL OF ~Nl!ECT C~l_ AND IIIIQLDING PRI~
STI~PPI~ IOLD~Q~QBILIT~

~a =IQlY

The present invention relates to gas assi6ted injection molding and to a gas control 6yfitem for use in a Bas a6sisted iniection molding system. Mcre particularly, the invention relate6 to a method and apparatus for gas assisted injection molding which h~ the ability to in~ect ~ shot of molten plastic into on in~ection mold, ahut off the plastic flow, and in~ect a gas at a flrst predetermined and preci~ely control pressure into the melt, elther almo6t lnstantaneou~ly or after a delayed time. The pre6ent invention then has the ability to 6tep up and/or step down and/or hold the Bas under pre6~ure in an Qlmost infinite v~riety of patterns while the pla~tic i~ cooling in the mold, fin311y drrivlng at R step down or controlled vent pres~ure when the part i6 sufficiently cooled to be self-6upporting, followed by opening the mold and removing the article.

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~eSCRI~TlQN OF 2~E P~I0~ A~

The history of injection molding as a manufacturing procesa is relatively recent compared to 60me other manufscturing methods. In the infancy of lnJection molding, article~ were generally proauced by injectlng a molten plastic or resin material into a mold cavity, aDd letting the material cool to form a molded article, at which time the mold cavity wa6 opened and the article was relea6ed.

Afi the art of conventional injection molding advanced, attempts were made to make larger and larger parts. It became evident that there were many re6trictions on the ability of conventional injection molding in 60me of the6e application6. Some restriction~
which were found were the impossibility to match, while molding a part, different factor6 6uch a6 the presence of large, flat surface6, diverse and heavy wall 6ection6 and exceptionally long flow6 with the need for acceptable physical properties, good productivity and investment level6, and an aesthetically pleagiDg surface fini6h.

These con6iderations were fo~md to be part~cularly relevant in the electronic3 industry, where there was the need for enclosures for television sets, computers, printers and the like in the homa furnishlngs industry, where there wa~ a need for large parts for gArden chairs and tables, bathroom furnishings, and the like. Also presenting a problem were tools, handles and machinery components in general, and in the automobile industry in particular. A particularly aggravating problem was the tendency of sink mark~ to appear opposite large or thick rib sections. This led those skilled in the art to devi~e a method and apparatuG capable of generating a pres~ure inside the molded component in addition to the injection pressure exerted by the m&chine at the in~ection point.

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One solution that proved satlsfactory for so0e injection molded parts was the use of 6tructural foam6. Proces6es for fitructural foam molding date back to the 1960s. In the 6tructural foam molding proce66, a gas and polymer mixture is injected inside the pla6ticizing barrel or the nozzle of the injection molding machil~e. Once injected, the ga~ expands and form6 cavities which are generally closed cell6 in which ga6 i6 entrapped. The structural foam in~ecting proces6 reduces internal 6tresses and surface sink marks, and permits a reduction in the weight of the material used. However, for some applications, it was found that gas bubbleG could migrate to the surface and burst therethrough, resulting in the unsuitability of the foam process for certain parts. A1BO~ the cycle time i9 longer and special mold~ need to be uaed. Therefore, the foam injection molding process remailled unsatisfactory for many spplications, and those in the art continued their search to find a satisfactory method of injection molding for use on lnrge and/or complicated parts.

In order to solve the problemg in the in~ection molding art which could not be solved with structural foam molding and other methods, several in the art developed the proce6~ of gas assisted ln~ection molding. In gas agsisted in~ection molding, unlike ~tructural foam moldin~ the cavities formed in the molded component remain in connection wlth the injection point of the gas. In such a process, the mold is filled with molten plastic or synthetic res~n molding material which, in many processe6, i6 leBB than the volume which is nece6sary to fill the mold. The fllling of the mold is completed by the injection of a gas into the molding material contained in the mold. This re~ult6 in the formation of a cavity or cavities within the molding material. The pressure of the injection gas compre6se6 or pack6 the melt againat the mold walls during the curing state, resulting in the eliminfltion of 6ink marks oppofiite thicker ~ections or ribs and the like.

63,742-015 Many variations of the gas aggigted injection molding proceg6 are present in the prior art. For example, U.S. Patent Number 4,106,617 requlrea a "aprue break" for venting the gag out of the part before opening the mold. U.S. Patent Number 4,106,887 requires gas to be injected through a needle valve within a runner which ig movably mounted and engagable in the cavity Kate. U.S. Patent Number 4,140,672 provideg for the injection of viscou~ llquid which, at room temperature~
ig either a grease or solid.

U.S. Patent Number6 4,136,220 and 4,247,515 relate to the formation of a structural web type material. U.S. Patent Number 4,498,860 relatea to a movable ram a6sociated with the cavity gate which is used to cut off the 6prue. U.S. Patent Numberg 4,830,812 and 4,913,644 relate to cutting members within the mold body that are u~ed to vent the pressurized gas. V.S. Patent Number 4,923,666 relate6 to a method wherein the cavity ig fully packed with resin before gas injection, and gag cavitie6 are formed only in areas in which there i8 6ignificant ghrinka8e.

U.S. Patent Number 4,923,677 requires a gas venting pa6sage ~eparAte from either the resin or gas injection pasgages. U.S. Patent Number 4,943,407 require~ that gag injectlon be through the sprue buahing. U.S. P~tent Number 4,944,910 require8 a simultaneous lnjection of the resln and gag during the molding cycle, while U.S. Patent Number 4,948,547 requires that the injection ga~ i~ confined to the resln flow path, and that lt does Dot enter the artlcle defining area. And finally, U.S. Patent Number 5,0Z8,377 requireg a flow path at leaat partially defined by a movable member where~n the pre~sure of the molten pla~tic can be reduced in a controlled manner by moving the movable member prior to ga6 injection.

. q. ~ ~ , 63,742-015 Also, variou6 type~ of control systems have been developed in the prior art to control the prefi~ure of the injection gas. U.S.
Patent Number 4,824,732 requires a variable volume chamber for gas storage and control. The gas i6 injected at a controlled rate by reducing the volume of the variable volume chamber. U.S. Patent Number 4,855,094 require6 the lnjection of the gas at a predetermined pres~ure no greater than the resln in~ection pre6sure. U.S. Patent Number 4,9~5,191 require6 that both the resin and the ga6 be injected cimultaneou~ly for at lea6t a portion of the in~ection cycle. And U.S.
Patent Number 5,015,166 provides for a control ~ystem for ~upplying a vsriable volume of a 8a~ art at a predetermined pres6ure and maintaining that pre66ure at a aubstantially con~tant level. All of these patents perform generally saticfactorily, but leave one or more problems in the injection molding art. Thus, tho6e 6killed in the art continued their 6earch for a 6ati6factory gAS a~6isted injection molding method and apparatu~.

Applicants have found, QS more fully explained in co-pending Patent Applic~tion Serial Number 07/628,746, filed on December 17, 1990 and entitled "Method and Apparatus for GA8 Assiated In~ection Molding", that ~tartin8 a flow of in~ection ga8 during the time that a flow of molten material i~ ta~ing place hac caused the problem of clogging of the ga6 sup~ly p~8ge8 due to the molten material entering and clogging the supply pas~age during the lnitial injection of molten plastic. This ha~ involved costly down time and complicated apparatus to try and prevent the entry of the molten molding material into the gas ~upply ~ystem. In Patent Appl;cation Serial Number 07/628,746, which is ~pecifically incorporated herein by re~erence, lt i8 shown that ~t i~
po~sible to sequentially first introduce 8 moldlng material into the mold cavity, and then 0hut off the 6upply of plastic. Thereafter, pre~curlzed gas is introduced through the injection nozzle into the :' :

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interior of the mold cavity, producing a 6ati~factory 601ution to Lhe problems in the prior art.

Further work by the Applicants since the filing of Patent Applicstion Serial Number 07/628,746 has shown that it is also pos6ible to eliminate problem6 and to provide a higher quality part by providing a method and apparatus which produces precise and careful control of the presgure of the injection gas during injection into the part and during the cooling process. The method and apparatu6 for preci6ion control of the injection gas is preferably used in Applicants' system, wherein the ga6 injection i~ not started until after the supply of molten resin or plastic into the mold is stopped. It should be understood, however, that such system can also be used to improve the perEormance of other prior art gas assisted injection molding systems made by others.

SUMMAR~ QF Tn rN~NT~~

To solve the problems in the prior art, and to provide for precise control of the pressure of the injection gas during injection~
and while within the molded part during the coolin~ proces6, a method and epparatus for ga6 assi6ted injection molding i6 provided which will fir6t inject a chot of molten plastic into a 6uitable mold, 6hut off the plastic flow into the mold9 inject a suitable gas at a predetermined and preclsely controlled pressure into the mold, immediately, or after a delayed time, step up or step down the pres6ure in the mold in a finite number of 8teps to arrive at a second predetermined and precisely controlled pressure, hold the second predetermined pressure for a desired time, repeat the 6ettin~ and holding of pressure as many times as desired, step down the pressure to a final predetermined pressure, and open the mold and remove the article.

63,7l~2-015 ~ ?~ ~2 In a first embodiment of the invention, R method is provided wherein a quantity of molten plsstic or other material i6 injected into a mold, the flow of molten material is 6hut off, and a ga6 i6 injected into the mold at a predetermined and precisely controlled presgure after a delay time (Tl 2 O). The method continues by 6tepping down the pressure over time to a second predetermined and preclsely controlled pres6ure, which i6 held for ~ second time (Tz > O). A final 6tep down of pres6ure to a final or controlled vent pres6ure is made, and the mold iE opened and the article is removed.

In a second embodiment of the present invention, a method ig provided whereby an smount of plastic less than is nece6sary to fill the mold is injected lnto a mold cavity. The flow of plastic into the mold is then 6hut off, after which a delay time may be introduced, if desired. The method continues with an in3ection of gas at a predetermined pres6ure, after which t!-e pressure of the gas may be repeatedly ~ncreaeed and/or decreased in finite steps, determined by the particular application, to produce a predetermined number of hold pres6ures during the cool down cycle, terminating with a step down or final vent pressure which i6 arrived at before the mold is opened and the article removed.

In &nother modlfication ~ the pre6ent invention, an apparatus ls provided for injecting a molten molding material into a mold cavity. Apparatus is provided to shut off the flow of material and to in3ect a gas, preferably nitrogen, at a predetermlned and precisely controlled pressure after a delsy time (Tl > 0~. Apparatus is further provided to step down the pressure to a second predetermined and precisely controlled pressure, and to hold the pressure in the part at the second predetermined pre6sure for a time greater than or equal to zero (T2 > O). The apparatus then has means to further step down the pressure to atmosphere, and to open the mold and remove the article.

6 3 ~ 7 4 2 -015 ;~ r-In a 6tlll further modification of the present invention, a source of gag or gas supply at a commercially available pre66ure i6 pas6ed through f~ ga6 booæter to increa6e its pre6sure to a pres6ure higher than desired for the injection molding proce6s. The ga6 i6 then filtered and p~s6ed through n conduit to a preasure regulator which ha6 a fixed volume ga6 re6ervoir immediately upstream therefrom. The pres6ure regulator then lower6 the gas pressure to a desired lnjection pre66ure and, at the appropriate time, the gas i8 pas6ed through a fast operatiDg valve lnto the mold. The flow of gas iB then shut off. A
computer, including a programmable logic controller (PLC), connected to the sy6tem, in addition to operating the other devices just de6cribed, operates a back pre~sure regulator to step up and atep down the gas pres6ure in the mold, as desired, until 6uch time as the part has cooled, the pres6ure has been lowered to a final vent pre6sure, and the mold is opened.

Thus, it is one of the objects of the present invention to provide a method of ga6 as6i6ted injection molding where the pressure of the injectlon gas can be closely controlled.

It is another ob~ect of the pre8ent invention to provide a method and apparatus for gas essisted in~ection molding where the pre6sure of the 8a~ in the mold during the mold cooling procea6 can be closely controlled.

Another object of the present invention i6 to provide a rnethod for gas a6sisted injectlon molding wherein a short shot of pla6tic molding materlal i6 introduced into the interior of a mold cavity, the flow of plastic molding material iB 6topped, a gas is injected into the interlor of the mold cavit~r to form a cav~ty or '~' '' . '~"' ' " '~
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63,742-015 3~i~
channel in the part being molded, and the pre~gure of the injectlon ga6 in the ga6 cavity is stepped up and/or held and/or stepped down, as needed, to provide a fini6hed part of optimum quality.

A 6till further object of the present invention i6 to provide a gn~ assi6ted in~ection molding proce66 where the clo6e control of gas pre6sure in the part being molded will 6erve to reduce molding stre6~ and warpage.

A still further object of the pre6ent inventlon ia to provide a gas a6si~ted injection molding proceg6 whlch provlde improved elimination of aink mark6.

A still further object of the preæent lnvention i6 to provide an apparatu6 for ga6 assisted in~ection molding which will inject a shot of molten plastic or synthetic re~in into the interior of a mold cavity, cut off the flow of molten plastic or 6ynthetic re6in, inject a gas into the interior of the molten plastlc or ~ynthetic re6in to form a cavity or channel in the interior thereof, control the prea6ure of the gas in6ide the part until 6uch tlme as the part ha6 cooled sufficently to be 6elf supporting, and then allow the mold to be opened and the part relea6ed.

Another object oE the preaent invention i~ to provide an -lmproved apparatu~ for injection mclding which will inject a 6hort ~hot of molding material into the interior of a mold cavity, cut off the flow of molding material, in~ect a gas at a first predetermned pressure into the mold cavity thereby forming a ga~ cavlty within the part being molded, and step up and/or 6tep down and/or hold the gas pres~ure in the mold a~ needed, dependlng upon the particular application, to provlde a molded part of optimum quality.

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63,742-015 5~
~ Further objectg and advantages of this lnvention will be apparent from the following de6cription and appended claims, reference being had to the accompanying drawing6 forming a part of the specification, wherein like reference character6 des~gnate corresponding part~ in the several views.

~RI~F D~S~RIPTI~ OF T~ D~ING~

Figure 1 is a flow chart showing the 6teps followed by the proce66 or method of the pre6ent invention.

Figure 2 i6 an illu6trative graph 6howing the change6 of pre6sure with re6pect to time used in the proces6 and apparatus of the pre6ent invention.

Figure 3 is a diagrammatic view of an apparatus embodying the present invention.

Figure 4 i6 a detailed 6chematic view of an apparatus embodying the present invention.

Flgureu 5, 5A, 5B, and 6 illu6trate computer flow charts 6howing the stepa which can be used by the computer or control means utilized in the present inventlon to control the procesa of the present invention.

Figure 7 i8 a diagrammatic view ~howing speciflc portions of the computer con601e, including the progr mmable logic controller, u3ed in the operation of ths present invention.

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63,742-015 Figure 8 is a view 6howing the card arrangem~nt for the programmable logic controller shown in Figure 7.

It is to be understood that the present invention is not llmited in its application to the details of construction and arrangement of parts illu6trated in the accompan7ing drawings, since the invention is capable of other embodiments, and of being practiced or carried out ~n variou6 way6 within the 6cope of the claim6. Also it is to be under6tood tbat the phraseology and terminology employed herein 18 for the purpose of dezcrlption, and not of limltation.

D~T~IL~D DeS~BI~TI~N 0~ 1 ~

In the method of the pre~ent invention, a full or 6hort shot of molten pla6tic is injected into the mold cavity. A short shot of molten resin or plastlc is to be understood to be an mount less than the volume of the mold cavity. Preferably an amount equal to about 60 to 90 volume percent oE the amount of plastic necessary to fill the mold i6 injected. While a full shot (i.~Q~, the amount nece6sary to completely fill the mold cavity) may be lnjected, as uaed ln 60me Ba6 66Gisted injection moldlng mschines, in the preferred embodiment of the present invention, ~ 6hort shot i~ used.

Referring now to Figures 1 and 2, the step~ used in the method of the present invention can be understood. It zhould be under6tood that the numbers applied to the boxeg 6hown ln Figure l will correspond to the numbers applied to certain portionz of the pres6ure ver6ua time graph shown ln Figure 2. An injection molding apparatus of a type well known in the art, which may be 6uch as u6ed in the in~ection molding apparatus previously de6crlbed, or in Patent Appllcation Serial Number 07/628,746, will inject a full shot (not ~hown) or a short shot _ . .
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63,742-015 ~3~

of molten plastic (6tep 200) during the time zero to tl ~hown il~ the graph of Figure 2. At time tl, the 6upply of plagtic or other molding material is shut off (step 210). After the plastic flow is terminated and following a delay time (Tl = (t2 - tl) in ~igure 2), a ga6 at a desired injection preBsure i8 injected into the plaetic material in the mold cavity (6tep 220). The delay time i8 generally in the range of about 0 to 60 6econds. The gas pressure within the mold will increase until time t3, at which point the gas pres6ure is equal to the fir~t predetermined and preci6ely controlled pressure. The actual injection duration (i e., t3 - t2 in Figure 2) i6 usually in the range of 0 to 60 6econd6 and more often in the range o~ just a few 6econd6.

Next, the pres6ure in~ide the mold cavity is stepped down or lowered durine a time TSD (i~e., t4 - t3 in Figure 2) in a predetermined 6eries o~ steps to be more fully explained hereinafter (step 230). The time TSD is also generally ln the range of 0 to 60 second6. At time t4, the presaure i6 held at the second predetermined and preci6ely controlled pressure for a time T2 (~lQ~, t5 - t4 in Figure 2) (6tep 240). At time t5, the gas pressure is stepped down to a final pressure or controlled vent pres6ure (step 250), which is usually atmospheric pre66ure or 61ightly above atmospheric pressure, after which the mold can be opened and the article removed (step 260).

The apparatus to be described hereinafter is capable of, or can readily be modified to be capable of, stepping down (or, if desired, stepping up) the pressure in as many a6 100 steps or more using a ~eries of timers in a programmable logic controller. More or less steps may be used, depending upon the particular application, and the full capacity of the apparatu6 does not have to be u~ed in every application.

Generally, it i~ preferred that the number of steps in which the pressure is dropped is in the range of about 10 to 25 steps, and more 63,7~2-015 ~ 5~

preferably about 20 steps. It i6 well within the acope of the present invention to provide an apparatus which can provide for over 20 step~, if needed or desired.

It should be under6tood that the pressure versus time graph of Figure 2 is an illustrative example only, and that depending on the specific application, many other pregsure/time relation6hips may be suita~le and even preferred. For example, it iB posaible that the pressure can be increased or 6tepped up for some step~ during the time period TSD before beinB lowered in a atepwiae fashion to the aecond predetermined pres~ure. Or, if desired, the pressure can be lowered in a 6tepwise fashion from the second predetermined pressure to the final venting pressure. As one gkilled in the art will realize, many otber pressure/time patterna can be employed within the acope and spirit of this invention.

It should be further under~tood that the method and apparatus of the pre~ent invention are usable with any of the aforementioned 8as ns6isted injection molding ~y6tems, wbether the plastic i~ injected into the mold cavity through an injection pln, ~hrough a runner, or through a nozzle.

Also, the method and apparatu~ of the present inventlon are usable whether or not the in~ection of the gas starta after the flow of molding material i8 completely shut off, or while some of the moldine material i6 ~till flowlng into the mold or mold cavity. Generally, however, it i6 preferred that a gaa a~6~sted injectlon molding ~ystem i6 used where the plastic and the preasurized gas are both ~njected into the mold cavity through a nozzle. It is also generally preferred that the pre6surized eaa la injected after the flow of molding material ha~
been 6hut off. More preferred iB a gas assisted injection molding ,~ .

63,742-015 ~ 3~J~

system wherein the nozzle contains a valve mean6 for controlling the flow of both pla6tic and pres6urized ga6. Suitsble nozzle6 with valve means include tho6e described in U.S. ~atent Application Serial Number 07/628,746 filed on December 179 1930, U.S. Patent Application Serial Number 07/714,117 filed on June 12, 1991, and U.S. Patent Application Serial Number 07/714,118 filed on June 12, 1991, all of which are hereby incorporated by reference. As one 6killed in the art will realize, the ga6 control sy~tem of the present invention can also be used with other gas delivery sy~tem6. One such other gas dellvery system is described in U.S. Patent Application Serial Number 07/724,044 filed July 1, 1991, which i6 hereby incorporated by reference. In thi6 la6t listed Patent Application, the pressurized gas is introduced through either the nozzle or the 6prue bu6hing using a cone-shaped checX valve which prevent6 the flow of molding material back lnto the pressurized ga6 passageways.

Referrin8 now to Figure 3, a diagrammatic view of an apparatus embodying the construction of the present invention is ~hown.
A 8a6 6upply or 60urce of injection gas 31 is placed in fluid communication with a gas booster 33 to raise the pre6sure of commercially available gaæ supplies to a pre6sure higher than a desired ln~ection pre66ure. A ga6 booster 33, which may be such a6 the AG~-62/152C ~a6 booster manufactured by Ha6kel Incorporated of Burbank, California, will increase the injection ~a6 preg~ure ~rom the co~mercinlly available pre66ure of the gas supply or source of in~ection gas (i.e., about 500 to 2,500 p8i), to a 6ignificantly higher pressure, preferably about 8,000 to 13,000 p6i, and more preferably about 12,000 to about 13,000 p6i.

Down6tream of the ga6 boo6ter 33 ig a fixed volume ga6 re6ervolr 39 which, in the preferred embodiments of the pre6ent invention, ha6 a volume of approxlmately 10 to 100 cubic inchefi, and prefersbly about 15 to 25 cubic inches, and mo6t preferably about 20 63,742-015 cubic inches. The combination of the high pressures generatetl by the gas booster 33 and the fixed volume reservolr 39 perform an important function in the preferred embodiment of the present invention, a6 will be explained in detail hereinaf ter in connection with the de6cription of Figure 4. - ;

DownstreQm of the fixed volume re~ervoir 39 is a pressure regulator 40 which may be the same as the ER serie~ of electronic pre66ure regulators manufactured by the TestCom Corporation of Elk River, Minnesota, or lts equivalent. The use of an electronic regulator is preferred to Hllow computer control of the sy6tem. The purpose of the pressure regulator i8 to reduce the pres6ure of the injection gas from a pressure hlgher than a preferred or de0ired injection pressure to the first predetermined or desired in~ection pres6ure. In the preferred ambodiment of the lnvention, the pres6ure wlll be reduced from the gas-boo6ted pre6sure of approximately 12,000 to 13,000 psi to the preferred or desired injection pressure of about 1,000 to 6,û00 psi.
The actual injection pres6ure selected will depend, in large part, upon the plastic u6ed, the mold cavity ~ize, and related parameter6; such a selection of the appropriate injection pressure i~ within the skill in the art of ga~ a6sisted in~ection molding.

A f irst pre66ure transducer 41 downætream and in f luid communlcation with the pressure regulator 40 provides feedback to the pres6ure regulator via computer 42 to enable the æystem pre66ure to be maintained at the desired value. The pres6ure transducer 41, the pressure regulAtor ~0, 6nd the computer 42 form n fir~t control loop (LOOP 1) for controlling and ad~u6tlng the gas preæ6ure to the first predetermined and preci6ely controlled pressure. This first predetermined and preci~ely controlled pressure is the initial pressure of the ga6 upon injection into the molten plaætic withln the mold 30.

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;, , , 63,742-015 Important to the preferred embodiment of the method and apparatu6 of the present invention i6 the Ea6t operating valve 44 located between and connected to the pressure regulator 40 and the mold 30. Since, ln the preferred embodiment of the invention, a method of gA5 a6si6ted injection molding iB utili~ed where a short shot of-pla6tic i6 fir6t injected into the mold, the plagtic flow into the mold is 6topped, and the pregsurized or injection ga6 i6 in~ection into the mold cavity, the speed at which the gas reache6 the pre66ure nece66ary to break into the hardening reBin iE important. In larger parts, or for difficult to mold materials, if the break-through or preferred injection pressure i6 not reached quickly, it may become difficult or impo6sible to inject tbe ga6 into the plastic within the mold.

It i6~ therefore, preferred that the fast operating valve of the pre6ent invention be fully opened from its normally clo6ed po6ition in about l/lOth to 3/lOthc of a gecond ~or fa6ter). This require6 careful 6election of the valve for the present invention. One valve which has been found to be especially 6atisfactory is the "SNO-TRI~"
Model SS-410~FP-C valve, which ig manufactured by the SNO-TRIR Company of Cleveland, Ohio. Another suitable value is a Whitey Model SS-H83P54-31C available from Whitey Company of ~ighland Heights, Ohio;
ba~ed on the manufacturer'6 fipeCifiCatiOn~, thiB YalVe iB alfiO expected to be e~pecially sati8factory. The fa~t operating valve 44 should open and very quickly 6upply fully pres6urized injection ga6 to the mold 30.
A second pre6sure transducer 46 is connected to, and in fluid communication with, the fa6t operating valve 44 down6tream thereof, and i8 electrically connected to the computer 42. A back pre6sure regulator is also connected to, and in Eluid communication with, tlle fast operating valve 44 and the mold 30. The back pres6ure regulator 45 i6 electrically connected to the pre6sure tranaducer 46 via the computer 42, thereby providing a second control loop (LOOP 2~ for control of the 63,742-015 ;~

gas pressure in the mold cavity after the flow of plastic or resin molding material is stopped and the pres6urized gas has been injected.
Thus, the second control loop i6 used to control the pres6ure of the gas within the mold during the pre~sure step down procedure, the holding pressure procedure, and the venting procedure (steps 230, 240, and 250, respectively, in Figure 2).

- ~eferring now to Figure 4, a scbematic dlagram repre~entative of a preferred embodiment of the present invention i5 shown. This preferred embodiment compriseg four basic part~: (1) an iniection molding Islachine 25, (2) a gas booster system 22, (3) a gas injection consol 24, and (4) a computer ay6tem 42. The injection molding machlne 25 has a plastic source, hopper, or source of molding material 27, (~, thermoplastic, resin, or the like) communicating with an injection rarn 50. Injection ram 50 plaaticizes, heats, or otherwise tran6forms materials contained in the hopper or source of plastic 27 into a molten resin or plastic molding ms-erial which is trsnsmitted by conduit 51 to the mold 30 which contains a mold cavity 28. Pressurized gas of a predetermined pressure i6 in~ected lnto the molten resln and the mold cavity 28 through eas inject~on point 60. The prer6ure of the injected ~a6 is controlled as de~cribed in detall beloe".

A source of injection gas or gas supply 31, which may consi~t of a plurality of gas cylinder6 (not shown) or a bulk gas atorage system (not shown) i5 provided as part of the gas booster sy6tem Z2. The injection gas preferably is nitrogen because it is inert and relatively inexpen6ive, but other ga6es (e.g., air, argon, carbon dioxide, Imd the like) may be used, if desired.

The gss supply 31 i6 in fluid communication9 through gas valve 53, with a fir6t or inlet filter 32 for removlng impurltie6 ln the -17~

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. .
: i . . ' , 63,742-015 ga6 6team which might damage the ga6 booster 33. Conventional ga6 filter6 may be uaed (e.~., model number 28728-l manufactured by the Haskell Manufacturing Company~. Gas filter 32 is connected to the inlet 29 of the gas boo6ter 33.

A 6upply of compressed air i6 used to drlve the gas booster 33. The compre66ed air 6upply 47, which may be building air 6upply or directly from a compres60r, i6 connected through air filter 54, normally open 6hut off valve 55, and regulator 56, to the gas boo~ter 33. Air pre~6ure gage 57 allows the operator to ob6erve the alr pre6sure delivered to the ga6 boo6ter.

The compre66ed alr 6upply 47 i6 al60 used to operate the firat pilot 6witch 34 and the second pllot 6witch 35 by meanfi well known in the art. Instruction6 for the connection of first pilot 6witch 34 and second pilot 6w~tch 35 are normally 6upplied by the manufacturer of the ga6 boo6ter 33. In the preferred embodiment of the invention, the fir6t pilot 6witch 34 i8 a decrea6ing pilot 6witch which will deactivate the ga6 booster 33 if the pre6sure of the gas 8upply 31 to the boo6ter fall6 below about S00 p6i. ~eactivation of the gas boo6ter 33 in 6uch ca6es i6 designed to prevent damage to the gas boo~tsr. The fiecond p~lot switch 35 is an lncrea~ing pllot 6witch which i6 set to vent excess pres6ure lf the pressure of the ~as exltin~ the 8a6 booster ;s greater than a ~et value.

The outlet 36 of the ga6 booster is connected to both the 6econd pilot switch 35, the pre6aure relief valve 37, the manual relief valve 43, and the 6econd gas fllter 38. The preesure r~lief valve 37 i8 uaed, in combination with the 6econd pilot 6witch 35, to vent any exce66 pre06ure generated by the ga6 booster 33. A ~uitable prea6ure relief valve 37 16 available from Haskell Manufacturlng Comyany afi model number 63,7~2-015 ~3~S~

15700-25. The relief valve 37 should automatically release exce6s pre6sure from the 6ystem if the second pilot 6witch 35 fail~ to prevent the gas booster 33 from gsnerating pre6sures in excess of the generally preferred 12,000 to 13,000 psi range. In the preferred embodiment of the pre6ent invention, the 6econd pilot 6witch 35 and the relief valve 37 are 6et at about 500 psi higher than the desired gaG boosted pressure (e.g., if the ~a6 booster i~ to deliver a pre~sure of 13,000 p8i, the ~econd pilot switch 35 and the relief valve 37 would be set at 13,500 psi). The manual relief or bleed valve 43 i~ also provided as a safety feature which allow6 the operator to quickly vent or depre~surize the 6y~tem in the event of a malfunction or to vent or depres6urize the sy6tem eO perform maintenance.

The second filter or outlet filter 38 16 in fluid communication downætream with the fixed volume reservoir 39. Downstream of re6ervoir 39, the pre~sure regulator 40 reduces the pressure from the ga6 booster 33 to a desired injection pressure of about lO00 to 6,000 psi. The pres~ure regulator 40 iB al60 connected by 6uitable electrical connections to the electro pneumatic controller 58 which form~ part of the pressure reeulator 40. First pressure transducer 41 ia provided downstream of the pressure regulator 40, and ~n fluid communication therewith. The first pre6sure tranaducer 41 is al~o electrically connected to the programmable logic controller (PLC) 80 of the computer or control means 42. A flrse control loop (LOOP 1) is establi6hed between the PLC 80, the first pressure tranaducer 41, and the pressure regulator 40 (and its electro pneumatic controller 58) to control the pressure of the inJection gas to a predetermined and precisely controlled o~ desired injection pres6ure. The electro pneumatic controller 58 is cormected to and driven by a 60urce of low-pressure nltrogen (generally about 170 p6i) for control purpo6es, as is well known in the art.

63,7~-015 ~ ~ $ ~

The preasure regulator 40 i6 connected to the fa6t operating valve 44, 6uch ag the SN0-TRIK or Whitey fast operating valve6 described above (or their equivalent), which will remain normally clG6ed until the 6y6tem is ready to inject gas into the mold 30. The fa6t operating valve 44 is electrically colmected to the P~C 80 to receive a 6igual therefrom st the appropriate in~ection time. When the signal 1B
received, the fa6t operating valve 44 open~ and injects, via gas line ~2, n quantity of pre6surized ga6 into the re6in or plastic contained in mold cavity 28 until a ir6t predetermined and preci6ely controlled pre~sure i6 reached. Since the fast operating valve 44 will remain open until the desired pressure within the mold cavity 28 i~ reached, the actual time the valve remait~s open (tt3 - t2~ in Figure 2) will vary depending upon the application, and may even vary from cycle to cycle becau6e of 6uch variable~ as temperature and preasure. It i~ generally preferred that the gas line ~2 between the fa6t operating valve ~14 and the injectlon point 60 be as short a6 po66ible to allow injection of the pre6surized 8as a6 quickly as pos6ible. The desired pressure wlll be initially programmed lnto the PLC 80 by the operator depending on the application, and then will be controlled by the control means 4Z
thereafter.

The combination of the high pre6sure provided by the gas booster 33 and the f~xed volume re6ervoir 39 should prevent, or ~t lea6t minimize, 6ignificant gas pre~sure drop when the fa6t operating valve 44 opens. Aa noted sbove, the volume of the fixed volume reaervoir 39 i8 normally in the range of about approximately 10 to 100 cubic inche6, and preferably about 15 to 25 cubic inche6, and most preferably about Z0 cubic inche~. The volume of the fixed volume ga6 reservoir 39 6hould be large relative to the cumulative volume of the as60ciated gas line6 or passagewaya between the ga6 booster 33 and the mold cavity 28 60 that, for example, when the first control loop call6 for additional pre66ure , ~ . " --~ , . . .................................................... .

:

63,742-015 or the fast operating valve 44 i8 opened, there i6 only a relatively small chang~ in volume7 and therefore pre6sure, experienced by the pressurized 8a6 in the fi~ed volume reservoir. Thus, as one skilled in the art will realize, if the volume of the associated gas passage ways is increased (e.~., by increasing the length of the pas6age ways ~etween components~ or if a part with large volume gas channel6 i~ to be made, it may be necessary to increase the volume of the fixed volume reservoir 39. Limiting the pre~6ure drop in thi6 manner will allow for pres~ure control with only minimal "over shooting" or "under shooting" of the "6et" or de6ired pre6sure.

The second pressure transducer 46 and the back pressure regulator ~5 are connected to each other via the PLC 80 to ~orm a second control loop (LOOP 2). Thi6 control loop control6 the preHsure in the mold cavity 28 during the step down and pre6aure holding step6. This control loop can also be used to increase the pre6sure, if de6ired, in the mold cavity 28 (i.e., 6tepping up the pre6sure). If the pres~ure is to be increa6ed, the computer 42 6ignAls the fa~t operatillg valve 44 to open, thereby increa6ing the pressure ln gaR line 62. Thia 6econd control loop is also used for controlled venting (250 in Figure 2). In controlled venting, the pressurized gns i8 vented from the mold cavity 28, via g&6 pa~agew&y 62, through the back preBBure regulator 45.
Normally, the ga6 16 vented to atmo6pherlc pre6sure at the completion of the molding cycle. In such a ca6e, the pres6ure within the plastic molded part will be at atmo6pheric pressure. In 60me ca6e6, however, it msy be de~irable to have re6idual ga~ pre6sure within the mold plastic article. In such cases, the ga8 6hould be vented to the desired re6idual pressure (ç~g~, 20 to 50 psi) u6ing the cecond control loop and then, while holdin~ the de~ired pressure, in~ecting a ~mall quantity of resin to "seal" the molded plastic part 80 it can retain a positive gaa preH~ure when, after the "plastic seal" ha6 hardened, the molded part i8 removed from the mold.

.. . . ..
', 63,742-015 In order to begin the injection cycle lt i6 nece88ary, primarily for 6afety rea60ns~ to determine whether the injection ram 50 ha6 come home and whether the safety 8ate 48 has closed. In accomplish this, a ram proximity switch 81 i~ connected through first relay 83 to the central proce66ing unit ~CPU) 79 of the computer 42. A gate cloged proximity switch 82 18 connected through second relay 84 to CPU 79.
When relays 83 and 84 both ~end the appropriate signals to CPU 79 the injection cycle can begin. The safety gate 48 is a plexigla6s or 6imilar shield that encloses the injection ram 50 and the mold 30 during the actual injection process, thereby 6hielding and protecting the operator in case of any sudden or ~mexpected pres~ure release or blow-out during the injection molding cycle.

The computer or control means 42 include6 a power supply 86 connected to an operator interface terminal 87 which iB connected to the CPU 79. In the preferred embodiment of the invention, the operator interface terminal 87 i6 the mlniature operator lnterface terminal or "MINI OIT" manufactured by GE Fanuc Automation North America, Inc. of Charlotteaville, Virglnia. The operator interface terminnl 87 ig connected via CPU 79 to the pro~rammable lo~ic controller 80, ~hich also may be such as that made by GE Fanuc Automation North America. It 19 preferred that the operstor interface 87 i6 a "touch sen6itive" CRT unit for ease of control, although other devices 6uch as hardware switches, keyboards, and the llke can be u6ed. The arran8ement of the programmable logic controller and various cards needed for the operatlon of the control means i6 shown in Figures 7 and 8. If de61red, a printer 88 may be connected to the PLC 80 for prlnting out operator report6. As tho6e skilled in the art will reallze, many commercially avallable computer systems or computer components can be used as the control mean6 42.

'`~

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~, 63,742-015 ~,S~3~
Referring now to Figures 5, 5A, 5B, and 6, there are shown flow charts generally illu6trating computer software procedures which can be used to operation the apparatu~ and method of the pre6eut invention. A6 those skilled in the art wlll realize, many other computer flowchart6 a6 well a6 many different computer programs or set6 of computer software instructions could be uaed to preform e66entially the 6ame or very 6imilar 6tep6 illustrated in theee flow charts.

In an preferred embodiment there i~ provided an automatic mode of operation and a manual mode of operation. The automatic mode i6 used for normal operation6. The manual mode i6 used for determlning variou~ operat10nal parameters and for diagno6tics and 6y6tem evaluation6 (~g~, clearing ga6 or plast~c lines aasociated with the mold 30). In either mode of operation, the operator mu6t fir6t enable the 6y6tem (block 280), and then enter ~block 290) the de6ired values for injection pre~6ure and the duration OI' time at each pre66ure fox all of the lndividual 6teps by which the injection or fir6t deslred and preci6ely controlled pre6sure 18 stepped down to the second or sub6equent predetermined desired pressure (auto mode~ or the gas pressure (manual mode). The programmable logic controller 80 provide~
for the ~tepping down to the 6econd predetermlned and precisely controlled pressure in a plurality of individual ateps (in a most preferred embodiment, about 20 steps are used). As discus6ed above, more or fewer than 20 6teps may be used if desired, and by virtue of the fast opening valve 44, the pressure inside the mold cavity 28 may be 6tepped upwardly a~ well aa downwardly during the pres6ure 6tep down cycle (i~Ç~, during time TSD in Figure 2).

Referrin8 now to Figure 5A, the enable subroutine (bloc~

280) 18 further descr1bed. Ualng a touch aensitive 6creen or CRT (i~
a preferred operator interface terminal 87) provided on tlle control 63,7~2-015 ~$ ~

means 42, the logo or enable 6creen will fir6t preaent itgelf to the operator w~th a mes~sge "touch to proceed" (block 281). After activating the "ennble" screen, a safety check screen will appear (block 282). The 6afety check6 will vary depending on the particular proceas being u~ed. Generally the6e 6afety checks will involve an evaluation of various temperatures and pre~sure~ throu~hout the injection molding 6yatem to determine if they are within acceptable limita. Both the safety checka and the safe operating limit6 can be, if deaired, modified by a computer programmer.

The operator will next be required to enter a ~ecurity code or pas6word ~block 283). Normally two level6 of securlty are provided.
Level I, the lowest security level, simply allow~ the sy6tem to run without the ability to modify or change the operational parametera.
Level II allow~, in addition to running the sy~tem, an operator to modify an e~isting set of operational par~meters or to set up a new set of operational parameters. If desired, additional security level6 could be provided. An operator entering Level I 6ecurity automatically bypas6e~ the "enter value~" subroutine (block 290) and proceed~ directly to the selection of the approprlate mode of operatlon ~l.e., automatic or manual operation). For Level I operation it is necesaary that the valuea called for in the "enter values" subroutine have already been entered and remained stored within the computer memory or on a machine readable device ~uch as a computer disk or punched tape. For the Level II operator, the "enter value" aubroutine can be bypasaed if the desired operating parameters are already available in the ~ystem. (The flow charts do not illu~trate bypassing the "enter value" subroutine in order to simplify the flowchart6. Such conventional programming techniques are well within the ~kill of the art.) 63,742-015 2q~3~

Operating parameters can be modified or entered initially into the computer 6ystem through the "enter value" 6ubroutine 2gO by a Level II operator. Three ~alues or parameters are required ln the auto mode for each 6tep in the stepwl6e reduction of the pressure and for the holding pre66ure indicated in Figure 2 by the numeral6 230 and 240, respectively. (As oDe 6killed the art will reallze, the holding pres6ure or ~tep 240 in Flgure 2 is of the same character as the step~
230 in that figure e~cept that the length of time at the holding pre~sure 18 much longer. Similarly, the venting cycle 250 can also, if de6~red, be treated a6 ju6t another step or other 6teps. Thu6, the parameters a6sociated with the holding conditlon 240 will be of a 6imilar nuture and will be inputted in the same way a8 the parameter6 a660c~ated with the step~ 230. And 6imilarly for parameter6 a660ciated with controlled venting 250.) The three required values include (1) the step number (block 284), (2) the total time at pres6ure for each 6tep (block 285), and (3) the pre66ure for each 6tep (block 286). The loop in the "enter value"
6ubroutine 290 will be repeated a6 many times a5 i8 nece6sary to 6et up the proce6s (1~, for N total 4tep~ the loop will be repeated N
time6). For each 6tep, the operator will fir~t enter step number N
(block 284), then the time ~"atep time") that the pre~sure i5 to be held for that step (block 285)9 and lastly, the pre6sure ("step pre~6ure") for a part~cular step (block 284). The step number N i8 an integer which identifie6 the order of the variou6 step3. A66uming there are twenty step~, N will be equal, in turn as each data entry loop i6 completed, to 1, 2, 3 . . . 20. Data entry will contlnue through block 237 and lt6 associated loop back to block 284 until the appropriate data ha6 been entered for all N step6.

, ' , ., ~'; , .

63,742-015 ~3~
After completlon of data entry or lf the data entry subroutine wa6 bypassed, the operator must 6elect between the auto mode (blocks 300, 330, and 340 in Figure 5 and all of Figure 6) and the manual mode (blocks 310, 350, 360, and 370 in Figure 5).

The auto mode of operatlon (block 300) ls used for normal operation of the gas assisted injection molding 6y~tem and is illustrated in Figure 6. After beginning the auto mode (block 330), it mu6t fir6t be determined if the safety gate 48 i8 clo6ed (block 380).
If the 6afety gate is open, the auto mode cannot begin and the sy6tem will exlt fro~n the auto mode (block 340). If the safety 8ate i6 clo6ed (block 380), the injection cycle can begin (block 390) thereby beginning the injection of the molten or plastic resin. Once the injection of molten or plastic resin is complete (i.e, the injection ram is at the home po6ition) (block 400), the injection cycle for injection of the pressurized ga~ begins. Thus, two condition6 m~6t be met before pressurized g~8 injection can begin: (1) safety gate i6 closed (block 380) and (2) in~ectlon of the molten or pla6tic resln iB complete (block 400). Although not shown on the flow chart, the gate closed proximity switch 82 forms an interlocking sy6tem such that, if at any time during the injection cycle (for elther recin or pres6urized ~as lnjectlon), the ~afety g~te 48 opens the i~jection cycle will abort and the sy6tem wlll exit the auto mode (block 340). The ram pocition proximlty switch 81 does not form a 61milar interlocking system. Thus, once the re6in injection is complete, the ram 50 can begin it's recovery ~nd prepare for the next in~ection cycle, during the pressurized gas lnjectlon cycle.

Once the injection of molten or plastlc resin is complete (block 400), the pressurlzed gas injection cycle iB inltlated. Flrst the "step reglster" 18 initialized (i.e., ~he step number N i6 ~et equal to 1) (block 410). Then the "tlmer" is set equal to the "step tlme" for ~tep .
.': ~ ' .

63,742-015 ~ f~

N (block 420) and the "auto pre6sure" 18 Bet equal to the "6tep pre66ure" for step N (block 430) (for the first time through the loop --block6 4Z0 to 480 -- N will equal 1). The "~tep time" and the "~tep pre6fiure" for each 6tep N ure the value6 entered in the "enter values"
6ubroutine 290. If N equal6 1 (i~., the fir6t 6tep) (block 440), the fast operating valve 44 i~ opened (block 450) to inJect ga6 into the molten or pla6tic resin at the first predetermined and precisely controlled pressure (equal to the "6tep pressure" for 6tep 1~. The fast operating valve will automatically clo~e once the pres6ure reaches the first predetermined prea6ure~ Although not shown in the flow chart, a delay time Tl (see Figure 2~ between the completion of the injection of the plastic (block 400) and the initial injection of pres6urized ga6 (block 450) can be in6erted if de6ired. After the ~ast operating valve is opened (block 450), the pre6sure 16 held at the "6tep pressure" lsvel for the "6tep time" duration (block 460). After holding at the de6ired pre66ure for the de6ired time (block 460), the 6y6tem determines if the pre66urized gas injection cycle i6 complete (block 470). lf Lhe cycle ia not complete, the "6tep regi6ter" i~ incremented by 1 (i.e., N = N +
1) (block 480) and the æystem return6 to block 420 to begin the ga6 injection cycle once ~gain for the next step N. The ga6 injection cycle repeat6 it6elf exactly as described for the fir~t cycle (N = 1) except that block 460 (opening the fa6t operating valve) i6 bypa6sed. In other word6, the "timer" and "auto pre~sure" value6 are 6et for the appropriate step N (block6 420 and 430) and, ~ince N i~ not equal to ]
in block 440, the aystem proceed~ directly to block 460 where the "6tep pre6sure" i6 maintalned for tlle "6tep time."

The ga~ in~ection cycle or loop (blocks 420 through 480) i6 repeated for the desired number of 6tep6 whereby the pre6sure in the mold cavity is reduced to the second predetermined and preci6ely controlled pre~sure (240 in Figure 2). Thi6 second predetermined and :

63,742-015 precisely controlled pressure is usually -- but is not required to be --the la~t 6tep in the cycle or loop In 60me in6tance6 it may be de~irable to further reduce the pres6ure in a stepwi6e fa6hlon after the 6econd predetermilled and preclsely controlled pre66ure. In other word6, the pre6sure can be further reduced in a stepwise fashion to the final venting pressure u6ing thi6 8as in~ection cycle or loop. A~ one gkill in the art will realized, thi~ gas in~ection cycle or loop can easily be adjusted by varying the input values in the "enter values" subroutine 290 to obtain almost endle~s variations or patterns for stepping up or stepping down the pressure in the mold cavity. As noted above, one such psttern is ~hown in Figure 2. Once the ~a6 in~ection cycle or loop i8 compl&te (i~Ç~. the last step N ha6 been carrled out), the 6ystem will exit the auto mode via block6 470 and 340.

IE the manual mode of operation tblock 310) is ~elected, an operator can manually ln~ect gas at a single pressure level (i e., the "manual pressure" of block 350). No reF.in is injected into the mold cavity 28 during the manual mode of operatlon. As noted above, the manual mode can be used for determining various operational parameter6 and for diagnoatics nnd system evaluations. Perhaps more importantly9 the manual mode can be used to clear plugged lines in or associated with the mold 30 and mold cavity 28. The operator first ~ets the manual pre~aure deslred (block 351) by entering thnt pressure via the touch sensitive screen lnput device 87. (Although not shown in the flow ch~rt, the computer software could ea~ily be modified BO thqt the manual pressure could be entered via the "enter value" subroutine Z90.) Once the manual presHure has been set, the operator, again u6ing the touch sensitive screen lnput device 87, manually activates or opens th& fast operating valve 9 thereby blowing pressurized ga~ at the manual pres~ure into the mold cavity 28 (block 360). Ga6 is injected until the operator deactivates or closes the fast operating valve ~block 370). After the ~' ~ ., . ;'.
'- .' ~ ' ' 63,7~12-0l5 2.`~,~$~

faet opening valve is clo6ed, the 6ystem exit6 from the manual mode.
This manual in~ection cycle can be repeated as often as desired or needed to, for example, clear the lines.

After exiting either the auto mode (block 340) or the manual mode (block 370j, the 6y6tem return6 to the main trunk line (Figure 5).
From thi6 point, the 6ystem can return to the auto mode (block 300) or the manual mode (block 310) or the 6y6tem can be turned off (block 320). The Iy6tem can be 6tarted up or re-enabled through the "enable sy6tem" gubroutine 280. A~ one 6killed in the art will realize, the flow chart in Figure 5 could ea6ily be modified, if de6ired, go that, for example, the "enter valueg" ~ubroutine 290 could be entered from the auto mode (block 300) without having to turn the sy6tem off and then back on. Many other variation6 could be made in the flow charts and the 6y6tem using conventional flowcharting and programming skills and techniques to obtain a computer software package to operate and control the ga6 control system of this invention.

, ,i";, , , , . .:

Claims (45)

1. A method of controlling the pressure of an injection gas which has been injected into plastic resin contained within a mold cavity, said method including the method steps of:
(a) providing a quick operating valve downstream and in fluid communication with a source of the injection gas and upstream and in fluid communication with the mold cavity;
(b) providing a back pressure regulator downstream of the fast operating valve in fluid communication therewith;
(c) providing a pressure transducer downstream of the fast operating valve and in fluid communication therewith;
(d) connecting a control means to the fast operating valve, the pressure transducer, and the back pressure regulator, thereby forming a control loop; and (e) utilizing the control means with the control loop to control the pressure of the injection gas within the mold cavity.

2. A method of adjusting and controlling the pressure of an injection gas during injection and packing of fluid resin in a mold cavity, said method including the method steps of:
(a) providing a control means electrically connected to a first control loop and a second control loop, wherein the first control loop allows the pressure of the injection gas to be adjusted and controlled during the injection of the injection gas in the fluid resin in the mold cavity and wherein the second control loop allows the pressure of the injection gas to be adjusted and controlled during the packing of the fluid resin in the mold cavity;

63,742-015 (b) providing a pressure regulator downstream of and in fluid communication with a source of injection gas, wherein the pressure regulator is electrically connected to the control means within the first control loop;
(c) providing a first pressure transducer downstream of and in fluid communication with the pressure regulator, wherein the oressyre transducer is electrically connected to the control means within the first control loop;
(d) providing a fact operating valve downstream of and in fluid communication with the pressure regulator and the first pressure transducer, wherein the fast operating valve is upstream of and in fluid communication with the mold cavity and wherein the fast operating valve is electrically connected to the control means;
(e) providing a back pressure regulator between and in fluid communication with the fast operating valve and the mold cavity, wherein the back pressure regulator is electrically connected to the control means within the second control loop; and (f) providing a second pressure transducer downstream of and in fluid communication with the back pressure regulator, wherein the second pressure transducer is electrically connected to the control means within the second control loop.

3. The method defined in claim 2, and including the additional method steps of:
(a) boosting the pressure of the injection gas to a pressure higher than a desired injection pressure at a point upstream of the pressure regulator; and (b) utilizing the first control loop to lower the pressure of the injection gas to the desired injection pressure.

63,742-015

4. The method defined in claim 3, and including the additional method step of:
(a) storing a fixed volume of the injection gas at the pressure higher than the desired injection pressure and at a point upstream of the pressure regulator to help prevent a significant pressure change when the fast operating valve is operated.

5. A method of gas assisted injection molding with precise control of gas pressure during and after injection of the gas into a injection mold containing plastic, said method including the method steps of:
(a) providing a gas source;
(b) connecting a gas booster in fluid communication with the gas source to increase the pressure of the injection gas to a pressure higher than a desired injection pressure;
(c) providing a fixed volume reservoir downstream of and in fluid communication with the gas booster;
(d) providing a pressure regulator downstream of and in fluid communication with the fixed volume gas reservoir to reduce the pressure of the injection gas from that provided by the gas booster to the desired injection pressure;
(e) providing a first pressure transducer downstream of and in fluid communication with the pressure regulator to measure the pressure of the injection gas;
(f) providing a fast operating valve downstream of and in fluid communication with the first pressure transducer and the first pressure transducer to control the flow of the injection gas into the injection mold, wherein the fast operating valve is connected to and controlled by a computer;

(g) providing a back pressure regulator downstream of and in fluid communication with the fast operating valve;

63,742-015 (h) providing a second pressure transducer downstream of and in fluid communication with the back pressure regulator the fast operating valve;
(i) connecting the computer to the first pressure transducer and the pressure regulator to control the gas pressure during injection of the gas into the injection mold; and (j) connecting the computer to the second pressure transducer and the back pressure regulator to control the gas pressure of the gas in the injection mold after injection of the gas into the injection mold.

6. A method of forming a molded article, which method comprises:
(a) injecting molten thermoplastic material into an injection mold cavity;
(b) injecting a pressurized gas at a first pressure into the molten thermoplastic material contained within the injection mold cavity, thereby forcing the molten thermoplastic material to conform to the shape of the injection mold cavity;
(c) stopping the flow of pressurized gas into the injection mold cavity;
(d) controlling the pressure of the pressurized gas in the injection mold cavity for a predetermined period of time whereby the pressure is lowered from the first pressure to a second predetermined and precisely controlled pressure;
(e) holding the pressurized gas at the second predetermined end precisely controlled pressure for a predetermined time;
(f) venting the pressurized gas from the injection mold cavity to atmosphere; and (g) removing the plastic article from the injection mold cavity.

63,742-015

7. The method defined in claim 6, wherein the pressurized gas is nitrogen.

8. The method defined in claim 7, where the first injection pressure is about 1000 to about 6,000 psi.

9. The method difined in claim 6, wherein the method step of controlling the pressure of the pressurized gas in the injection mold cavity includes the method step of:
(a) stepping up and stepping down the pressure in a finite number of steps while lowering the pressure to the second predetermined and precisely controlled pressure.

10. The method defined in claim 6, wherein the method step of controlling the pressure of the pressurized gas in the injection mold cavity includes the method step of:
(a) stepping down the pressure in a finite number of steps while lowering the pressure to the second predetermined pressure.

11. The method defined in claim 9, wherein the finite number of steps is between about 10 to 25.

12. The method defined in claim 10, wherein the finite number of steps is between about 10 to 25.

13. A method of forming a molded plastic article, which method comprises:
(a) injecting a short shot of molten thermoplastic material into an injection mold cavity;
(b) shutting off the flow of molten thermoplastic material into the injection mold cavity;

63,742-015 (c) injecting a pressurized gas at a first predetermined and precisely controlled pressure into the molten thermoplastic material contained within the injection mold cavity, thereby packing the molten thermoplastic material into the injection mold cavity;
(d) stopping the flow of pressurized gas into the molten thermoplastic material within the injection mold cavity;
(e) allowing the molten thermoplastic material to cool and hardened within the injection molding cavity while:
(i) reducing the pressure of the pressurized gas within the injection mold cavity from the first predetermined and precisely controlled injection pressure to a second predetermined and precisely controlled pressure in a finite number of steps, (ii) holding the pressure within the injection mold cavity at the second predetermined and precisely controlled pressure for a predetermined time, (iii) reducing the pressure within the injection mold cavity to a final vent pressure, and (iv) venting the pressurized gas from the injection mold cavity to atmosphere; and (f) removing the molded plastic article from the injection mold cavity.

14. The method defined in claim 13, wherein the pressurized gas is nitrogen.

15. The method defined in claim 14, wherein the short shot of molten thermoplastic material is about 60 to 90 volume percent of the amount of material needed to completely fill the injection mold cavity.

63,742-015

16. The method defined in claim 15, wherein the first predetermined and precisely controlled pressure is about 1000 to about 6,000 psi.

17. The method defined in claim 16, wherein the finite number of steps is from about 10 to 25.

18. A method of controlling the pressure of a gas, said method including the method steps of:
(a) providing a gas supply;
(b) increasing the pressure of the gas from the gas supply to a pressure higher than a predetermined pressure;
(c) supplying the gas at a pressure higher than the predetermined pressure to a fixed volume reservoir and a pressure regulator;
(d) utilizing the pressure regulator in conjunction with a first pressure transducer and a computer to regulate and lower the pressure of the gas to the predetermined pressure;
(e) supplying the gas at the predetermined pressure to a fast operating valve;
(f) supplying the gas exiting the fast operating valve to a back pressure regulator, a second pressure transducer, and a mold cavity after the mold cavity is fully or partially filled with a molding material; and (g) utilizing the back pressure regulator in conjunction with the second pressure transducer, the fast operating valve, and the computer to control the pressure of the gas in the mold cavity as the molding material in the mold cavity cools.

19. A method of controlling the pressure of an injection gas in a mold cavity wherein the pressure in the mold cavity can be changed in 63,742-015 a stepwise fashion from a first predetermined pressure to a second precisely controlled and predetermined pressure in a finite number of steps, said method including:
(a) providing a control system having means to accept data entry;
(b) inputting data into the control system for each step, wherein the data includes a step number, a pressure to be maintained during each step, and a time the pressure is to be maintained for each step;
(c) utilizing a transducer to measure the actual pressure within the mold cavity during for each step, wherein the transducer is electrically connected to the control system whereby the actual measured pressure for each individual step can be entered into the control system;
(d) comparing the pressure to be maintained for each step number with the actual measured pressure for that step number; and (e) changing, if necessary, the actual pressure for each individual step to the pressure to be maintained for that step by utilizing a back pressure regulator in fluid communication with a fast operating valve and a source of gas.

20. The method defined in claim 19, wherein the number of finite steps is between about 10 and 25.

21. The method defined in claim 20, wherein each step results in a reduction of pressure.

22. A gas assisted injection molding system, said system including in combination, (a) apparatus for injecting a fluid molding material through an injection flow path into a mold cavity;

63,742-015 (b) apparatus for interrupting and stopping the flow of fluid molding material in the flow path;
(c) a gas supply for injecting a pressurized gas into the flow path and therethrough into the fluid molding material within the mold cavity once the flow of the fluid molding material has been interrupted and stopped;
(d) apparatus to control the pressure of the pressurized gas within the mold cavity as the fluid molding material in the mold cavity cools and hardens; and (e) apparatus to vent the pressurized gas from the mold cavity.

23. The system defined in claim 22, wherein the apparatus for injecting the fluid molding material includes an injection molding machine communicating with the injection flow path.

24. The system defined in claim 23, wherein the apparatus for injecting a pressurized gas into the flow path includes a gas supply and a fixed volume reservoir in fluid communication with the flow path and wherein the fixed volume reservoir is located between the gas supply and the mold cavity.

25. The system defined in claim 24, wherein the apparatus to control the gas pressure within the mold cavity includes a fast operating valve connected between and in fluid communication with the fixed volume reservoir and the mold cavity.

26. An apparatus for producing a molded article of plastic material, said apparatus including:
(a) an injection molding machine nozzle for introducing a supply of fluid plastic material into a mold cavity;

63,742-015 (b) a means for introducing a supply of pressurized gas through the injection molding machine nozzle and into the fluid plastic material within the mold cavity;
(c) means to control the injection molding machine nozzle whereby the flow of fluid plastic material into the mold cavity can be stopped and the flow of pressurized gas into the mold cavity can be started when desired;
(d) means to control the pressure of the pressurized gas while it is being introduced into the mold cavity;
(e) means to control the pressure of the pressurized gas after it has been introduced into the mold cavity;
(f) means for venting the pressurized gas from the mold cavity; and (g) means for opening the mold cavity and removing the molded article.

27. The apparatus defined in claim 26, wherein the means for introducing a supply of plastic material includes:
(a) an injection molding machine having a screw ram communicating with the injection molding machine nozzle.

28. The apparatus defined in claim 27, wherein the means for introducing a supply of pressurized gas includes:
(a) an interruptable passageway provided in said injection molding machine nozzle and in communication with the source of pressurized gas;
(b) a valve means interposed between the source of pressurized gas and the interruptable passageway to control the flow of pressurized gas; and 63,742-015 (c) a control means connected to the valve means to control the valve means.

29. An apparatus for introducing an injection gas into the interior of a mold cavity, said apparatus including, in combination:
(a) a source of injection gas;
(b) a gas booster in fluid communication with the source of injection gas;
(c) a pressure regulator in fluid communication with the gas booster;
(d) a fast operating valve in fluid communication with the pressure regulator; and (e) a mold cavity in fluid communication with the fast operating valve.

30. The apparatus defined in claim 29, wherein the gas booster has an inlet and an outlet, where the inlet of the gas booster is in fluid communication with the source of injection gas and the outlet of the gas booster is in fluid communication with the pressure regulator and wherein a pressure relief valve and a fixed volume gas reservoir are located between and in fluid communication with the outlet of the gas booster and the pressure regulator.

31. The apparatus defined in claim 30, wherein the apparatus further includes:
(a) a first pressure transducer located between and in fluid communication with the pressure regulator and the fast operating valve.

32. The apparatus defined in claim 31, wherein the apparatus further includes:

63,742-015 (a) a back pressure regulator located between and in fluid communication with the fast operating valve and the mold cavity; and (b) a second pressure transducer located between and in fluid communication with the fast operating valve and the mold cavity.

33. The apparatus defined in claim 32, and further including a control means to control the pressure regulator, the back pressure regulator, and the fast operating valve.

34. The apparatus defined in claim 33, wherein the control means includes a computer utilizing a programmable logic controller.

35. The apparatus defined in claim 34, and including a first pilot switch connected to the gas booster to shut off the gas booster if the source of injection gas falls below a predetermined pressure.

36. The apparatus defined in claim 35, and including a second pilot switch to vent excess pressure from the gas booster if the pressure of the source of injection gas is increased by the gas booster above a predetermined pressure.

37. The apparatus defined in claim 36, wherein the volume of the fixed volume gas reservoir is approximately 15 to 25 cubic inches.

38. The apparatus defined in claim 37, wherein the control means is electrically connected to the pressure regulator and the first pressure transducer to establish a first control loop to control the injection pressure of gas as it is injected into the mold cavity.

39. The apparatus defined in claim 38, wherein the control means is electrically connected to the second pressure transducer and the back 63,742-015 pressure regulator to establish a second control loop to control the pressure of the injection gas inside the mold cavity after the injection gas has been injected into the mold cavity.

40. A gas pressure control apparatus for controlling the pressure of an injection gas during injection into a mold cavity, said apparatus including in combination (a) a source of injection gas connected to, (b) a gas booster connected to, (c) a fixed volume gas reservoir connected to, (d) a pressure regulator connected to, (e) a first pressure transducer connected to, (f) a fast operating valve connected to the mold cavity.

41. The apparatus defined in claim 40, and further including apparatus for controlling the pressure of the injection gas after injection into the mold cavity, said apparatus further including in combination:
(a) a second pressure transducer connected to the fast operating valve and the mold cavity; and (b) a back pressure regulator connected to the fast operating valve, the second pressure transducer, and the mold cavity.

42. The apparatus defined in claim 41, and further including:
(a) a computer electrically connected to the pressure regulator, the fast operating valve, and the first pressure transducer to establish a first control loop for controlling the pressure of the injection gas during injection of the injection gas into the mold cavity.

43. The apparatus defined in claim 42, and further including a computer being electrically connected to the back pressure regulator, 63,742-015 the fast operating valve, and the second pressure transducer to establish a second control loop for controlling the pressure of the injection gas within the mold cavity after injection of the injection gas into the mold cavity.

44. The apparatus defined in claim 43, wherein the computer further includes:
(a) an operator interface terminal;
(b) a power supply;
(c) a first relay;
(d) a second relay;
(e) a central processing unit; and (f) a programmable logic controller.

45. The apparatus defined in claim 44, and further including:
(a) a ram position proximity switch connected to the first relay; and (b) a gate closed proximity switch connected to the second relay.
47. A gas assisted injection molding system, said system including, in combination:
(a) a first apparatus for plasticizing a quantity of injection molding material, the first apparatus including an injection ram for plasticizing the molding material, and an injection machine nozzle communicating with the injection ram for delivering a quantity of molding material;
(b) a mold cavity in communication with the injection machine nozzle to receive the molding material;
(c) a second apparatus for injecting a pressurized gas into the mold cavity after the flow of molten plastic has been stopped;

63,742-015 (d) a source of pressurized gas connected to the second apparatus;
(e) a gas booster in fluid communication with the source of pressurized gas;
(f) a pressure regulator in fluid communication with the outlet of the gas booster; and (g) a fast operating valve in fluid communication with the pressure regulator and the mold cavity.
48. The system defined in claim 47, and further including a gas reservoir in fluid communication with the gas booster and the pressure regulator.
49. The system defined in claim 48, and further including:
(a) a first pressure transducer located between and in fluid communication with the pressure regulator and the fast operating valve.
50. The system defined in claim 49, and further including:
(a) a second pressure transducer located between and in fluid communication with the fast operating valve and the mold cavity;
and (b) a back pressure regulator located between and in fluid communication with the fast operating valve and the mold cavity.
51. The system defined in claim 50, and further including:
(a) a control means connected to the pressure regulator, the first pressure transducer, the fast operating valve, the second pressure transducer, and the back pressure regulator to control the 63,742-015 pressure of the pressurized gas during and after injection into the mold cavity.

52. The system defined in claim 51, wherein the control means includes:
(a) a ram position proximity switch connected to the control means; and (b) a gate closed proximity switch connected to the control means.

53. The system defined in claim 52, wherein the control means further includes:
(a) a central processing unit;
(b) a power supply connected to the central processing unit;
(c) an operator interface terminal connected to the central processing unit; and (d) a programmable logic controller connected to the central processing unit.

54. The system defined in claim 53, further including a printer connected to the programmable logic controller.

55. A plastic article produced by the method of claim 1.

56. A plastic article produced by the apparatus of claim 22.