Note: Descriptions are shown in the official language in which they were submitted.
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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
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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~
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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.
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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);
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- 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).
921
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~
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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
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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.
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