CA1150921A - Pressure generating device for injection moulding and use thereof - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Pressure generating apparatus for use in injection moulding at a high pressure or normal pressure may be a device attachable to any convenient part of an injection mould-ing machine, for example to the mould, the platens, or the tie bars. The apparatus operates independently from the normal pressure generating mechanism and acts on the content of the mould cavity only. The device comprises a pressure generating part, e.g. a hydraulic cylinder connected with a plunger which can bring the pressure inside the cavity to the desired level during an injection cycle and a valve to prevent flow back of melt from the mould cavity. The invention includes in-jection moulding machines incorporating the apparatus and methods of injection moulding using the apparatus.

Description

~5~9Zl This invention relates to pressure increasing apparatus for injection moulding machines, injection moulding machines inclu-ding such apparatus and to methods of injection moulding.

From the Swedish Patent Specification No. 401,129 it is pre-viously known that a substantial increase of the pressure in the moulding of thermoplastic articles by means of injection moulding has a number of positive effects on the properties of the article and at the same time provides certain technical process advantages. Thus, in the injection moulding of certain crystalline thermoplastic resins, especially polyethylene of high density, a remarkable increase of the stiffness and the breaking stress of the final product is observed when the pressure in the mould cavity in connection with the pro-duction of the article has been increased to 300 - 500 MPa.
At the same time a mould shrinkage decreasing with increasing pressure in a controllable way was observed. A number of other properties were also remarkably improved such as e.g.
the resistance against flow under stress. The internal stresses normally present in injection moulded articles also could be greatly reduced; at the h:ighcst pressures (500 MPa) used in said context, they could practically be removed. Said effect could be traced back to the instantancous soliclification of the melt in the cavity wh:Lch is obtalned .It a suE~icien~ly high pressure as a result o~ the :increase of thc glass transi-tion temperature or melt temperclture with the pressure. The previously mentioned increase in thc stlffness and breaking strength of the final product was especially pronounced for high molecular qualities of polyethylene of high density.
Thus, for test bars made of such a material modulus values up to 3400 MPa were observed while simultaneously the braking strength was as high as 130 MPa. In the interior of the article, the so-called core, said values were still higher, viz. 5000 MPa and 180 MPa for the modulus and breaking stress respectively.

In addition to these changes of properties there are a number "~

~5¢~
-- 2 ~

of technical process advantages in the use of high pressure in injection moulding. In this respect with reference to the above discussed instantaneous solidification of the melted material in the cavity there is the possibility of shortening the injection moulding cycle. This is a factor of primary importance to the economy of the process. Another advantage which should be pointed out in this context is the possibility to use melts in injection moulding which owing to too high a viscosity cannot normally be processed in this way.

For these reasons it is clear than an increase of the pressure in injection moulding represents an obvious improvement of the technique of today wherein the pressures normally used do not exceed 200 MPa.

However, the method hitherto used for providing the intended pressure increase in the mould cavity, i.e. to provide the screw of the injection moulding machine with a special in-jector, or to use a high-pressure injector working separately from the screw have hitherto not been wholly satisfactory.
Such injectors usually cannot be mounted on existing machines.
Instead it i5 necessary to use specially constructed machines.
Furthermore, such machines are not yet available on the market.

The most important disadvantage of devices of the type just mentioned is that the high pressure cause large stresses in comparatively large parts of the machine. These stresses must be considered in the design of the machines. In this respect the design of the cylinder and the screw/injector is especially critical. In adclition there are problems in the selection of material for these machine components which in addition to the considerable pressure also must stand a comparatively high working temperature.

An additional disadvantage of the type of existing machines

2~

which work with a high pressure injector mounted on the screw is the fact that the volume of the moulded part must be kept relatively small as compared ~ith the volume of the moulded parts possible with low pressure injectors. The reason for this is the area reduction between the screw and high pressure injector which is required for obtaining the intended pressure increase. This also restricts the volume of the plastic melt in the storage chamber before the screw/injector.

The object according to the present invention is to mitigate the disadvantages caused by the use of current injection mould-ing technique using high pressures.

Accordingly the invention provides pressure increasing appara-tus for an injection moulding machir-e cavity above normal injection moulding pressure during an injection cycle, the apparatus being attachable to the injection moulding machine and comprising pressure generating means independent from injection moulding pressure generating means of the injection moulding machine and adapted to act in the mould cavity and valve means coacting with the pressure generating means to pre-vent flow back of lnjectioll moulding material~ from the mould cavity on activation oE the pre~ssure generating means.

The invention also includes an injection moulding machine including such apparatus.

The invention also includes a mcthod of :Lnjection moulding comprising firstly injecting melt into a mould cavity under a first pressure; secondly, applying a second pressure in the mould cavity independently of and greater than the first pressure; preventing flow back of melt from the mould cavity during application of the second pressure; releasing the second pressure immediately prior to opening the mould cavity; opening the mould cavity and ejecting a moulded article therefrom.

~5~92il Conveniently the pressure generating means comprise a hy-draulic cylinder and plunger.

Apparatus of the invention may be attached to conventional machines without any major reconstruction since the injection takes place in a conventional manner while the apparatus in-tended to increase the high pressure only acts on the mould part o the machine.

The pressure generating devices disclosed herein can easily be mounted on any mould constructed to stand the increased pressure without losing its normal function. This refers first of all to sufficient mechanical stiffness and strength.
A special advantage of the present invention is the possibility to move the pressure generating device according to need from one mould to another. Thus, it is not necessary to provide each mould intended for injection moulding at elevated pres-sure with a special pressure generating device but one can work with only one such device which according to need is mounted on different moulds. This is a further illustration of the flexibility which characterixes the new device.

An embodiment of the invcntion will now be described by way of example with reEerence to the drawings in which:-Figure 1 illustrates the principal mounting of the pressure generating device on the mould of a conventional injection moulding machine;
Figure 2 illustrates an injection moulding machine with a screw on the end of which a high pressure injector has been mounted in a conventional manner;
Figure 3 illustrates an injection moulding machine wherein the plastication cylinder and the high pressure injector have been placed separately in parallel with each other;
Figure 4 illustrates an injection moulding machine with a high pressure unit according to the present invention (closed mould);

~5~9;~1 - 4a -Figure 5 illustrates the same machine with a high pressure unit (open mould);
Figure 6 shows a pressure-time diagram for the injection mould-ing process with the device according to the present invention, Figure 7 illustrates the mould cavity with plunger and non-return flow valve in its upper position;
Figure 8 illustrates the mould cavity with plunger and non-return valve in its lower position;
Figure 9 illustrates the mould cavity with plunger and the auxiliary accumulator chamber turned 90 degrees;
Figure 10 shows a sequence diagram illustrating the injection moulding process when using a pressure device according to the prevent invention;
Figure 11 illustrates a control system for controlling the cavity pressure when using a pressure device according to the present invention.

; ., In orcler to illustrdte the more ad~alltaqeous construction and flexibility of the novel device l~hen used, as said properties are reflected in a comparison with machines of conventional construction, figures 1 - 3 illustrate on one hand the fundamental mounting of the novel device on the mould in a conventional injection machine (figure 1) and on the other hand an injection moulding machine of current type for use with elevated pressure wherein two different embodiments of the latter type have been included. In one case (figure 2) a machine provided with a screw on the end of which a high pressure injector has been mounted is illustrated while in the other case (figure 3) an injection moulding machine is illustrated wherein the plastication eylinder and the high pressure injector have been placed separately in parallel with eaeh other. It is to be noted that the novel device, such as schematically illustrated in figure 1, can be mounted on an optional injection moulding machine when the injection takes place in a conventional manner while the generation of the elevated pressure ta~es place separately from the other maehinery by means of a deviee attached to the mould.

The above outlinin(~ oE tlle prillcipal method of oper.tioll and advantages Or L~ e novcL dcvicc as conparecl to devicts of existin(l typ(? will now l)c fo]lowe(l, with reEererlce to the skeleton drawirl(ls in ~ig~ll^C`X ~ ;Illd 5, 0~ ;1 CICta.i.l.C(l description oE tl-le Ill(ocllanical const:rlctic)n oL lhe devicc concernecl whereitl l:llc~ press~lre in ~he ~oul(l cavity is generated with a llyclraulic cylindor lS a pressure gener~tillg part. 01` course, iE dt?sire-l, th(~ pressure generatincJ part call consist oE mcchanical dc~vices of other types which arc wcLl-known ~ sc to the ones skilled in the art.

The injection moulcling progress with the present method differs essentially in certain points from the conventional injection moulding cycle. The injection ~oulding progress now obtained can be divided into four steps (a - d).

Fi-Jure 6 illustrates said four steps in a pressure-time diagram.

a. The feeding of the mo~lld cavit~ takes place in a normal manner, i.e. the plastication screw presses the melt into 5 the mould cavity. Said function is controlled by the injection system of the machine. The pressure can be main-tained relatively low (20 - 100 MPa). Figure 7 illustrates -the mould cavity when the injection is complete.

b~ At the moment when the injection pressure is released 1~ the plunger starts movin~ into the auxiliary sprue accumu-lator. Said func-tion can be controlled by normally used control systems, which are described more in detail later on. Since the plun~er is brought down into the auxiliary sprue accumulator the pressure in the mould cavity will 15 be increased. The increase rate of the pressure is adjusted over a flow valve in the oil feed to the prcssure side of the hydrau:lic cylinder. Figure 8 illustrates the mould cavity after the application of the hi~h pressure.

A closer study oE the auxiliary spruc accumulator 2~ (FicJure 8) shows t,hat the pluncJer acts on an ar~a witl\ a diameter which is 1eL;S tllan tllat oF the auxiliary S})IUO
accumulator in order to prcvc!nt the application of the prcssurc C~ll tlle matcri.ll whicll a] rcacly has bccll so.lidi.t.:ied Erom tl~e wall of t:hc accumul.ltor alld ~nwards. 'L'hl1s, the 25 plun(~eY orll.y act~; on thc~ m.lterial wh:ich is still in ~1 melted con(:litiol1. As is shc)wn in fi(~urc 7b the allxili.lry sprue accumulal:or c.ln bc madc sli~htly tapcrcd (incrcasin~J
radlus towal-(ls thc bOttom of the accumulator) in ordcr to facilitatc the maintainance of a core correspondin~ to thc ~0 diameter of the pressure pin in melted condition for the whole motion path of the plunger. Fi~ure 7b also illustrates another modification which can be made in this case in order to prevent a premature solidification in the zone between the auxiliary sprue accumulator and the 9~
-moulded part. .his is providec by placing an immersion heater near to said transition zone between the auxiliary sprue accumulator and the moulded part.

In the case that the auxiliary accumulator chamber is completely heated, which means that the plastic does not solidify in the contact with the wall of the auxiliary sprue accumulator the plunger and the auxiliary sprue accumulator can have the same diameters.

An advantage obtained by means of diameter difference 10 between the auxiliary sprue accumulator and the plunger is that the risk of squeeze forces between plunger and cylinder which arise if plastic material is pressed there between is reduced. Said problem is reduced ~then the plunger works outside the sealing cylinder. Since this es~ecially is a 15 problem in the processing of filled plastics the present process shou]d also be well suited for such plastics.

Simultaneously ~lith the plunger a non-return flow valve moves downwardc, and prevents return flot-~ and that thc pres-sure propacJates in~o tlle Elastication cylillder. The non-20 return Elow valvc~ b:locks aftcr about two r~illLme~tc!r~-; o~
movement and then li~e~s l)locked durin~J the r~mainin~J
passage WiliC~ e l)lu~ el- arld r,on-retllrn ~Iow valve movc downwards. For mn(`llin(''; WhiC~Il are e~luipyed with .I shut-o~!
nozzle in the ron~ part o~ tllc.~ I~las~ic.-ltirlcl cylincler tllC' 25 return flow can be pleVOIlteCI wi th saicl no?~le.

c. 'l'he hy(irauli~ E~ressure ancl thus the pressure in the mOIIlCI cavity i~ kept constant during the remaining coolin time. Different pressure controlling system can be turned on during thi~ phase of the injection moulding cycle ~us: a,e.g.
30 vibrating injection or pressure control over a cavity pressure sensor which iS described in more detail later on.

d. Before the o~ening of the mould the hydraulic cylinder is relieved ancl the plunger returns to its starting ~5~

positiorl and then thc rmould is o~ened and the moulded ~art is ejected.

For machines having a hydraulic ejector system the movement of the plunger if desired can be taking care of by this 5 system. The hydraulic ejector system is ~laced between the movable platen and the clampi.ng system of the machine. The ejector pin from the ejec-tor system moves through a hole in the movable platen into the mould and thus, ean -take care of the movements of the ~lun~Jer. The plunger in this case 1~ will ~ork transversally to the parting line of the mould.
Figure 9 illustrates how the auxiliary sprue accumulator has been turned ~Q degrees so that the plunger worlcs via the hydraulic ejector system.

Furthermore, it is characteristic for the novel device 15 that i-t is possil~le to connect the eontrol system alld hydraulics over exiSti.ncJ eomF)onents sueh as e.~. core pul-lincJ equipment or Viil the holdin~ pressure whicll in ti~i!;
ease is not used i.n a convcntional manner.

~ e~ pressu:re pt.~l-i.OC:I now c.ln bc slll.)stnlnt.Lall.y extenclecl -20 wi.thout any c~tn~n!;iorl Or t;he cyclc~ timc - si.nce the other routines of the l'laC~ lC, c~il)cci.a1.l.y the fc(!dirl~ oE new matcrial l.n tho scrcw C;ln t--l~;C ~ cc wi.t:hou~ tho ne,~(l of clny r~r~s~;~lr~ clc~lc~n~t~ tl~ loUL~ vi.~y (I;'i ~11~(l 1 () ) .

25 'rhc prcssurc conLrol call accoL-ding to tlle noveL Inel.h~!(l t~
r~ ce cluring tllc wllole soli.clil-:i.cation process. I~ a (Icc(l--~ael~ froll~ t~ C~ Vit~ r?r~ s~ or ~ac]i t~ r(~!.;sur(:~
cylinder is m-lde ~ very aCCIIrate compcnsation oi the shrinrac~e in thc? mould cavity is Drovided. This ena~les

3~ the production of moul.ded parts with high requirements as to si~.e tolerances in an essentially simplified manner (l'igure 11).

z~

lurt11ermore, it ici 1>oisiL~le to subject the content of the rnould cavity to a vihra-ting pressure, a treatment which accorcting to G. ~1enges, "9 Kunststofftechnisches Kolloquim des IKV in Aachen (1978) page 8, can result in an im~roved property profile of tne part, esneclally as to its dimension accuracy. Furthermore, vibrating pressure in hiqh pressure injection moulding can give rise to larger and more favourable shearinq forces which have proved to facilitate the formation of extended ehains in the injeetion moulded part. Although deviees qeneratinc3 a pulsating pressure in the r~ould are previously ~nown the combination of such pulsations with an elevated pressure level represents an especially desirable treatment of the material enclosed in the mould cavity. Also the fact tllat 15 now only said material is subjected to such vibrations, in contrast to the method proposed in other eonneetions to c3enerate the vibrations via the move1nent of the scrc~, means an apparent advantaqe.

Finally, the device accc)rdinc3 to thc prcscnt inventic)n 20 provides very inl~ortLlnt ceonornical ac1va1ltaqes. ~ iLe a conventional in-jcction moulcl i n-l nl~ ll:i ne c~c nstrllcted for prc~ss~ es ~Ip tc) ~-~)() r11~ (>~ v(- l v(~ 3 i. ~ t of t~l~ or~ L 5().()()() ~J.',. ~ ;(c)~ o .l1~ i moulclin~J maclll.nc~ wor li i rl~J wi t ll llc~r ma l pl-e~;~;nre t llc co~
25 Eor a (1evicc~ lc(.:~ lce witl~ the ~-I-o~:Cllt ~ lvon~:i.oll .;a;
only aE-ou~ one to1ltl~ or ~;aic1 s11rll. 1n i1)itc~ of t1~c incor1sider~l)]e (al)it.l1 dCelll;llld ~I'i C()lll~.lr~ Lo C~ll-l-C`rlt met1~ocls the llOVC`l pl0(-eS'; i.'; n~ore I lC.';:il1l.(.` not on.ly l'i t:o tl1e possi1ility oL conll)ininq it wi~h cu1-rent injection 30 mo\1l(linc3 nlac11incry l)ut .1lio ~s to the manner in whic11 the elevated pressure in the moulcl is qencrated and controllcd durinq an ir1jcction moukting cyclc.

Of course it is al~io possible to ma~e use of the nressure qeneratinc3 device in injection mouldinq with cor~on 35 pressure, i.e. prc?ssure of up to a~out 200 r~1Pa. In this 9z~

conte~t the advantages stated above in injection moulding with higil pressure which is due to the fact that the pressure genera-ting device works separately from the plasticating screw are obtained, such as the possibility to extend the pressure time without any extension of the cycle time, the possibility to provide pressure control during the whole solidification course, the possibility of subjecting the contents of the mould cavity to vibrating or pulsating pressure etc.

... _, .... .

Claims (27)

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

1. Pressure increasing apparatus for an injection moulding machine cavity above normal injection moulding pres-sure during an injection cycle, the apparatus being attachable to the injection moulding machine and comprising pressure generating means independent from injection moulding pressure generating means of the injection moulding machine and adapted to act in the mould cavity and valve means coacting with the pressure generating means to prevent flow back of injection moulding materials from the mould cavity on activation of said pressure generating means.

2. Apparatus as claimed in claim 1 in which the pressure generating means comprises a hydraulic cylinder and plunger.

3. Apparatus as claimed in claim 1 in which the pressure generating means comprises a hydraulic cylinder and a plunger adapted to pressurize the mould cavity by entry of the plunger into an auxiliary melt holding chamber com-municating with the mould cavity.

4. Apparatus as claimed in claim 3 in which the plunger is adapted to pressurize the injection moulding cavity through a cylindrical portion of the auxiliary holding chamber having a diameter greater than the diameter of the plunger.

5. Apparatus as claimed in claim 1 including a pressure sensor to sense the pressure in the mould cavity.

6. Apparatus as claimed in claim 2 in which the hydraulic cylinder and plunger are adapted to exert a pulsating pressure.

7. Apparatus as claimed in claim 1 in which the pressure generating means is adapted to produce a pressure of between 200 and 1000 MPa in the mould cavity.

8. In an injection moulding machine, apparatus to increase pressure in a mould cavity thereof, said means comprising:
pressure generating means independent from injection moulding pressure generating means of the injection moulding machine and adapted to act in the mould cavity and valve means coacting with the pressure generating means to prevent flow back of injection moulding materials from the mould cavity on activation of said pressure generating means.

9. Apparatus as claimed in claim 8 in which the pressure generating means comprises a hydraulic cylinder and plunger.

10. Apparatus as claimed in claim 8 in which the pressure generating means comprises a hydraulic cylinder and a plunger adapted to pressurize the mould cavity by entry of the plunger into an auxiliary melt holding chamber com-municating with the mould cavity.

11. Apparatus as claimed in claim 10 in which the plunger is adapted to pressurize the injection moulding cavity through a cylindrical portion of the auxiliary holding chamber having a diameter greater than the diameter of the plunger.

12. Apparatus as claimed in claim 8 including a pressure sensor to sense the pressure in the mould cavity.

13. Apparatus as claimed in claim 9 in which the hydraulic cylinder and plunger are adapted to exert a pulsating pressure.

14. Apparatus as claimed in claim 8 in which the pressure generating means is adapted to produce a pressure of between 200 and 1000 MPa in the mould cavity.

15. An injection moulding machine including apparatus to increase pressure in a mould cavity thereof, said apparatus comprising:-pressure generating means independent from injection moulding pressure generating means of the injection moulding machine and adapted to act in the mould cavity and valve means coacting with the pressure generating means to prevent flow back of injection moulding materials from the mould cavity on activation of said pressure generating means.

16. An injection moulding machine as claimed in claim 15 in which the pressure generating means comprises a hydraulic cylinder and plunger.

17. An injection moulding machine as claimed in claim 15 in which an auxiliary melt holding chamber is provided communicating with the mould cavity and the plunger is adapted to pressurize the mould cavity by entry into said chamber.

18. An injection moulding machine as claimed in claim 17 in which the auxiliary chamber has a cylindrical portion of diameter greater than the plunger into which por-tion said plunger fits.

19. An injection moulding machine as claimed in claim 15 including a pressure sensor to sense the pressure in the mould cavity.

20. An injection moulding machine as claimed in claim 16 in which the hydraulic cylinder and plunger are adapted to exert a pulsating pressure.

21. An injection moulding machine as claimed in claim 15 in which the pressure generating means is adapted to produce a pressure of between 200 and 1000 MPa in the mould cavity.

22. A method of injection moulding comprising:
firstly injecting melt into a mould cavity under a first pressure;
secondly, applying a second pressure in the mould cavity independently of and greater than said first pressure;
preventing flow back of melt from the mould cavity during application of said second pressure;
releasing said second pressure immediately prior to opening the mould cavity;
opening the mould cavity and ejecting a moulded article therefrom.

23. A method as claimed in claim 22 in which said second pressure is between 200 and 1000 MPa.

24. A method as claimed in claim 22 in which the second pressure is generated by a hydraulic cylinder and plunger.

25. A method as claimed in claim 22 in which the second pressure is generated by a hydraulic cylinder and plunger acting on the mould cavity by entry of the plunger into an auxiliary melt holding chamber communicating with the mould cavity.

26. A method as claimed in claim 22 including the step of sensing the second pressure whereby shrinkage of said moulded article can be controlled.

27. A method as claimed in claim 22 in which said second pressure is pulsating.