Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
.~0()7~8
The present invention relates to a method of reducing
and controlliny internal stresses and improving mechanical pro-
perties of injection molded articles of thermoplastics, which
method is charac-terized by injection molding the thermoplastics
under in other respects normal injection molding conditions with
the use of high injection and holding pressures, suitably
exceeding 250 MPa, preferably exceeding 300 MPa, a suitable
range being for instance 300-500 MPa and up to 800 MPa. MPa=
10 Pa; lPa=lN/m ; lN=lkg.m/s where M=mega, Pa=Pascal, N=newton,
m=meter, kg=kilogram and s=second according to the International
System of Units.
The magnitude and distribution of internal stresses in
injection molded plastic products is the result of a complicated
interplay of a large number of factors, among which, as has been
discovered according to the present invention, the injection
pressure and the holding pressure, i.e. the pressure which pre-
vails in the mold cavity during the solidifica~ion o~ the plastic
melt, have been shown to play an important role~
The influence of the injection pressures, by which
herein is meant the injection and holding pressure, which do
not need to be equal but preferably both should reach values
above 250 MPa, on internal stresses in injection molded products
has not been clearly elucidated. On the whole, the variation of
the pressure within the range normally used in injection molding
appears to affect the properties o~ the moldings to a minor
extent only.
In order to measure the influence of the injection
pressure the volume proportion of oriented material as dependent
on the pressure has been determined; c~. Kantz, M R
- 2
~10(~7Z8
Intern. J. T'olymeric MatO 1974, 3, 245, Further, the
degree of orientation has been deterrnined and the re~ults
h~ve been presented in terms of a so-called orientation
stress; cf. Jensen, M and Whisson, R R, Polymer, 1973, 14,
193. Measured in the flow direction this quantity decreased
only to a minor extent as the pressure was raised. The
materials studied were polystyrene, polypropylene, polysulfone
and ~crylonitrile-butadiene-styrene-plastic (ABS)~ Similar
results have been reported also when using a hardness method
for measuring the combined action of internal stresses and
anisotropy; cfo Fett, T, Plastverarbeiter 1973, 24, 665.
The injection pressures used in these investigations were,
however, only moderate, generally lower than 200 MPa.
When the polymer melt solidifies in the mold,
; 15 internal stresses are frozen-in as a result of difference~
in the solidification rate between the surface parts and
the interior of the object~ Normally, this results in
compressive stresses at the surface and tensile stresses ln
the interior. Such effects are well-known and have been
analyZed both experimentally; cf. Fett, T, loc~ cit;
Menges, G and Wubken, G "IKV Kunststofftechnisches Kol-
loquium", 1972, 21; Alpsten, G "Residual Stresses in
l~ot-Rolled Steel Shapes"; Diss, R, Inst. of Technology,
Stockholm 1967; Knappe, W, Kunststoffe 1961, 51, 562;
and theoretically; cf. Knappe, W, loc. cit.
The present invention relate~ to a method of
reducing and controlling the lnternal stress level (~i)
; and improving the mechanical strength properties of
1~07Z8
injection molded articles of thermoplustic resins by
using hi~h injection and holding pressure~, preferably
exceeding 2~0-300 MPa.
Normally, in injection molding of thermoplastic
resin~ injection pres6ure~ of from about 50 MPa up to
about 150-200 MPa are used, the last-mentioned range
only being used very seldom.
AS wil~ be desc~d in yreater detail below, it has
~urprisingly been found that by increasing the injection
pressure it is possible to bring about a reduction in the
overall internal stress level of injection molded products
of thermoplastic resinsO Further, it has been found that
molded products having highly improved mechanical ~trength
properties can be obtained by injection molding at high
injection and holding pressures above 250 MPa. The increase
of the injection pressure which is necessary for achieving
said reduction of the overall internal stress level of
injecti~n molded thermoplastic articles i9 somewhat
dependent on the thermoplastic resin usedc As a general rule
it can be said that to achieve an essential reduction of the
internal stress level of injection molded thermoplastic
objects according to the present invention an injection
pressure exceeding 250-300 MPa must be usedO By means of
the method according to the present invention it is pos~ible
to produce injection molded thermoplastic parts which are
characterized by an essentially reduced tendency to mold
shrinkage, warping, crazing and cracking1 post-shrinkage
and time-dependent deformation and other negative effects
.~ J
11007~8
which are usual in articles injection molded with noLmal
injection pressures. These advant;ayes, which are obtained by
the method according to the invention, are very important in
the production of for instance articles with close tolerances.
Another essential advantage obtained by means of the method
according to the invention is that considerabl~ shorter cycle
times are required for the injection molding, the reason being
a substantial increase in thermal diffusivity of thermoplastic
materials with pressure.
A further advantage which is obtained by the method
described herein is that the mechanical properties, such as
breaking stress and modulus of elasticity of the injection
molded articles can be highly improved.
The method described herein is use~ul for producing
injection molded products of different thermoplastic resins which
normally are used for preparing injection molded products.
Examples of such thermoplastic resins are ole~in plastics, such
as polyethylene, of both low and high density type (LD- and HD-
type, respectively), polyethylene havin~ extra high molecular
weight, i.e. having a molecular weight above 200,000 and up to
1.5 million and higher, includin~ so-called ultra-hi~h molecular
weight polyethylene, polypropylene, polyethylene copolymers;
styrene plastics, such as polystyrene, styrene-copol~mers, for
instance styrene-acrylonitrile plastic ~SAN~ and acr~lonitrile-
butadiene-styrene plastic ~ABS~; acrylic plastics, such as poly-
methylmethacrylate (PMMA~; amide plastics; acetal plastics;
carbonate plastics; polyesters of thermoplastic type, such as
polyethylene or polybutylene terephthalate (PETp or PBTP);
cellulose plastics; vinyl plastics, such as vin~lchloride
plastics, for instance polyvinyl chloride (PVC~, copolymers of
- 5 -
.
~;
11()()7Z8
vinylchloride, etc., and othex thexmoplastic resins having so
high molecular weight that normally they can not be injection
molded.
According to the present invention, then, there is
provided in a method of injection molding a thermoplastic resin
under conditions of melt temperature, mold temperature, injection
time, holding time, and cooling time appropriate for said thermo- -
plastic resin, the improvement comprising carrying out the injec-
tion molding at injection and holding pressures from 250-800 MPa,
whereby the molded thermoplastic resin has reduced internal
stresses and an essentially reduced tendency to mold shrinkage,
warping, crazing and cracking, post-shrinkage and time-dependent
deformation.
Embodiments of the present invention will now be des-
cribed in greater detail and will be better understood when read
in conjunction with the following drawings in which:
Fig. 1 shows the internal stress as plotted against the
injection pressure for two dif~erent types of polyethylene, viz.
high density polyethylene (HDPE) and low density polyethylene
(LDPE).
Fig. 2 shows the internal stress plotted against the
mold shrinkzge.
Fig. 3 shows different curves obtained from differential
scanning calorimeter measurements on slices cut at different
distances from the surface of samples of injection molded high
density polyethylene having extra high molecular weight.
Fig. 4 shows the modulus of elasticity and the breaking
stress and the elongation at rupture plotted against the maximum
cavity pressure.
Fig. 5 shows the tensile properties of high molecular
- 6 ~
110~)728
weight HDPE and normal HDpE plotted against the injection
pressure.
In Fig. 6, the left part shows the cr-~stallinity
plotted against the injection pressure for high molecular weight
HDPE and normal HDPE, while the right part of said figure shows
the mold shrinkage plotted against the injection pressure for
the same materials.
Fig. 7 shows the internal stress plotted against the
injection pressure for high molecular weight HDPE and normal
HDPE, respectively.
~ 6a -
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~.; . J
07'~8
Fig. 8 shows the creep plotted against time
for injection molded samples of high molecular weight
HD~E and normal HDPE injection molded at 100 and 500 MPa.
Fig. ~ shows the pressure course in the mold as
compared to the hydraulic pressure during the injection
and holding periods for polyacetal.
Figo 10 shows the pressure course in the mold
as compared to the hydraulic pressure during the injection
and holding periods for polyethyleneterephthalate.
Fig. 11 shows stress-strain curves for polyacetal
samples prepared at different injection pressures.
In Fig. 12, the left part shows the modulus of
elasticity and the yield and breaking stresses plotted
against the injection pressure for polyacetal samples,
while the right part of said figure illustrates the same
parameters for polyethyleneterephthalate.
In Fig. 13, the left,part shows the values for
the elongation at rupture and at-yield plotted against
the in-jection pressure for polyacetal, while the right
part of said figure shows the same parameters for
polyethyleneterephthalateO
Fig. 14 shows the stress-strain curves for
polyethyleneterephthalate samples at different hydraulic
pressures of the injection molding machine.
Fig~ 15 shows the mold shrinkage plotted against
the injection pressure for polyacetal and for polyethylene-~
terephthalate.
Fig. 16 shows the crystallinity plotted against
the injection pressure for polyethyleneterephthalate at
' a mold temperature of 30C. and 130C., respectively.
)728
1~` in;:l~ ly, I'`i'. 1'1 ShoW9 the mold shrinkage plotted
agains-t 1~1e injection pressure for injection molded samples
of low density polyethylene, hi6h density polyethylene and
polypropylene, respectively.
As stated above, the use of high injection pressures
for reducing the internal stress level of injection molded
thermoplastic articles also re~ults in improvements of
other properties of the injeetion molded object~, for
instance an increase of the yield stress or/and breaking
stress and a reduction of the mold shrinkage,
The invention is illustrated by means of the
following specific examples which describe embodiments of
; the invention but which are not intended to limit the
invention in any respect.
EX~MPLES 1- ? .
Experirnents were carried out with samples of poly-
ethylene of both low density type (~D-type) and high density
type (I~D-type). The following materials were used: LDPE
(BASF, Lupolen~1800 M), density 0.916-0.918 ~ cm3, melt
index 6-8 ~10 rninu-tes (r~FI 190/2 16); I~DPE (Hoechst,
Hostalen GC 12600), density Oo960 ~ cm3, melt index 7
10 minutes (MFI 190/2 I6).
The injection molding of the samples at varying
injection pressures wa~ performed using a modified injection
molding machine of conventional type (Engel 500/250 AS~.
This machine was equi~ped with a special screwO The ~ain
feature of this screw was Q plunger (diameter 30 mm) at
its end, the molten polymer flowing through a central bore
in the plungerO ~ackflow of the melt during injection into
the mold was prevented by a non-return v~lve. In this way
*Trade~ark
8.
728
inJec~io~l ~)rc~ure~ varying between 100 MPa and 500 MPa
could be ~taine-l. The holding pressure was identical with
the in~ection pressure.
The conditions in the injection molding process
are shown in the following table I.
The method u~ed for determinin~ the internal stress
values, the ~i-value~, was a stress relaxation method which
has been previously described, cf. Kubat, J and Rigdahl, M,
Intern. J. Polymeric MatO 1974, 3.
The stress - strain and relaxation experiments were
carried out at 22 + 0.5C.
The relationship between the internal stress
parameter ~i and the injectlon pressure i~ shown in Fig. 1
for the two types of polyethylene, HDPE and LDPE, respecti~ely.
The ~i-value changes from comparatively large negative values
to rather small positive ones. It is to be noted that one can
cause the shrinkage to disappear completely at a certain
pressure. It can also be seen that for HDPE the ~i-value
at 100 MPa is larger (negative) than that for LDPE. From
said figure it can be clearly seen that by a ~uitable
~ choice of the injection pressure the ~i-value can be reduced
; to zero.
The extent of shrinkage was determined by measuring
the distance between two marks along the flow direction in
the mold and the corresponding distance between the replicas
of the3e marks left on the molded samples. The shrinkage
value S was calculated from;
.,. ~
)7Z8
a - a
~~s = am
where am and as denote the distance between the points in
the mold and on the sample, respectively.
~ig. 2 shows the relationship between internal stress
and the mold shrinkage. It follows from this figure that
the lower the absolute value of ~i~ the lower is also the
shrinkage.
When discussing the results obtained, one should
keep in mind the complexity of the various *actors influencing
the residual stress distribution in an injection ~olded
specimen. In the first place such stresses are not
homogeneousO Normally, their distribution forms a pattern,
the characteristics of which depend on processing and
material parameters. For specimens of the type used here
one usually finds relatively high compressive internal
stresses in the surface layers and weak tensile stresses in
the interior.
The ~i-values stated above are thus to be considered
as average values of the various layers of the sample. As
the average ~i-level is evaluated from certain parameters
of stress relaxation curves, it is to be assumed that also
these parameters in their turn are averagesO Thus, the
course of the stres relaxation is the result of a super-
position of relaxation processes in the different layers of
the specimen having different ~i-valuesO
An analysis of the relaxation curves and the overall
values obtained from them shows the general influence
of the injection pressure on the properties of the molded
sampleO The first result to be noted is that the residual
10.
.. . . .. . .. . . . ...
11~)S 728
compressive stress obtained in normal injection molding
practice is reduced by increasing the pressure. At the
highest pressures used this compressive stress is reversed
into a weak tensile one. Thus, it appears possible to
reduce the average ori-value to zero by an appropriate rise
in the injection pressure; cf. Fig. 1.
~he mechanism behind the appearance of an internal
stress distribution in an injection molded specimen has
been previously discussed; cfo Knappe, W, Kunststoffe 1~61,
51, 562. In the present context it may suffice to say that
these stresses are due to a ~emperature gradient during
cooling. The outer layers solidify in the initial stage of
the cooling process. Owing to differences in specific
volume between melt and solid compressive stresses are
frozen into the solidified surface layers when the interior
of the specimen becomes solid. ~or balance reasons weak
tensile stresses prevail in the interior.
The method according to the present invention
of reducing internal stresses in-injection molded articles
by increasing the injection and holding pressures to a high
level can probably be theoretically explained in the
following manner, but the invention shall not be restricted
in any way by said theory. It is known that the melting
point of a polymer is relatively sensitive to pressure,
an increase of about 20C. per 100 MPa having been found
~or polyethylene; cf. Matsouka, S, J. PolO Sci. 1962, 57,
581 and Osugi, J and Hara, K, ~he Review of Physical Chemistry
of Japan 1966, 3-6, 28. Increasing the pressure on the melt
in the mold is thus equivalént to an overall increase of
728
m~ )om~t o~ ~he polyrner. In pri~lciyle, when
~le mol~l lla~ b~n filled ~nd the peak pressure is reached
the whole cavity content can be caused to solidify
simultaneou~ly. In normal molding the solidification
(crystallization) takes place when the temperature in
different parts of the mold pa3ses a critical value (Tm)~
The important thing to note i9 that this critical temperature
is reached at different times in different parts o~ the
specimen. Contrary to this, when using high pressures the
crystallization can take place ~imultaneou~ly in the whole
of the ~pecimen. It i8 thus possible to ascribe to the
inj ection pressure the role of a crystalliza~ion regulator,
a role not taken advantage of hitherto in the production of
6tress-free moldings.
~rom a closer look at Fig. 1 it can be ~een that
the injection pres~ure corresponding to 0-level of Gri
iB approxima-tely the pressure by which the melting points
of HDP~ and LDPE are raised to a temperature equal to that
of the melt leaving the cylinder; cf. Matsouka, S J, loc. cit.
and Osugi, J and ~ara, K, loc. cit.
Another e~fect which probably contributes to the
reduction of the 0ri-level on increasing the pressure i9 a
decrease in the thermal fihrinkage occurring in the vicinity
of Tm; cf. Matsouka, S J, locO cit; and an incre~se in thermal
diffusivity reducing temperature gradients in the solidifying
partO
The influence of the injection ~nd holding pressure
on the Cri-level has been illustrated above, using LDPE and
HDPE as exarnples. For other crystalline polymers the
melting temperature is shifted in a similar way, e~g. for
polypropylene ~ increases from about 175C, at atmospheric
12.
C~`''~
V7;;; 8
pressure to about 245Co at a pressure of 220 MPa;
cf. Baer, E and Kardos, J ~, J. Pol. Sci. 1965~ A-3, 2827,
and for polyamide 6 and polyoxymethylene a change in
melting temperature of 38Co and 44Co per 100 MPa,
respectively, in the pressure range 0-200 MPa has been
reported; cfo Katayama, Y and Yoneda, K, Review Of the
Electrical Communication Laboratories 1972~ 20~ 921 and
Starkweather, H K, J. Phys. Chem. 1960, 64, 410. The role
played by the increase in the melting temperature with
pressure-is, on the other hand, not restricted to
crystalline polymers. It is known that the corresponding
critical temperature for amorphous polymers, i.e. the
glass transition temperature, also rises when the pressure
is increased, e.g. for polystyrene, PVC and PMMA a shift
of Tg Of 32Co~ 16C. and 29Co per 100 MPa, respectively,
has been determined; cfo Billinghurst, P R and Tabor, D
Polymer 1971, 12, 101. As this increase per 100 MPa is
of the-same order of magnitude as that in Tm for crystalline
polymers the effect of increasing injection pressure for
reducing the overall internal stress level appears to be
a generally useful method of reducing and controlling
internal stresses in injection molded articles of thermoplastic
resins of both crystalline and amorphous type.
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In ~his experiment high molecular weight IIDP~
(DMD~-221') ~upL)lied by Unifos Kerni AB) with a density of
0.953 ~ cm3, Inelt index (MI21) 1 ~10 minutes was used.
The polyethylene wa~ injection rnolded with a modified
injection Inolding machine of the same type as used in the
previous examples (Engel 250/500 AS).
The following molding conditions were used:
Melt temperature:250-280C.
Mold temperature:30C.
Injection time:6 seconds
llolding time:15 seconds
Cooling time:5 seconds
Sarnples were injection molded at pres3ures
varying from 100 to 490 MPa.
The inJection molded specimens were small tensile
test bars with a gauge length of 25 mm and a thickness of
1.5 rnm. During the cycle the hydraulic pre~sure and the
pre3sure within the mold were recorded. The mold pressure
~ was measured with a pressure tran3ducér (Colortronic 407)
via a du~ny ejection pin.
Thin slices (30 ~m) of the specimens, cut with~
microtome, were measured in a differential scanning
calorimeter (Pe-rkin Elmer* DSC 2). The slice3 were cut at
different distances from the surface of the samples. The
accuracy of the DSC-measurements wa~ ~ 2C.
The mechanical properties of the tensile test
samples ~ere determined using a conventional tensile tester
(Instron*mode~ 1193). The strain rate was 20 mm/minute
*~rad~ark
728
(1.3~ 5 1). The tangent modulus (E), tensile strength
at bre~k (~ ) and elongation at rupture (~B) were determined
according to ASTM DG38.
The res~ults of the DSC-measurements are ~ummarized
in Fig. 3u 'rhe curve~ ~hown relate to a maximum cavity
pressure of 100, 300 and 490 MPa, respectively, for samples
taken at varying distance from the surface of the molding.
As can be seen from the figure, the cur~es ~or the 100 MPa
sarnples have a non~l appearance, indicating a T~-value at
128C. Only the sample taken 350 ~m from the ~urface
exhibited a small shoulder at T~ Tm ~indicated by an arrow
in Figo 3).
An increase of the ma~imum p-value in the mold
to 300 NIPa results in a new clearly-developed melting peak
for all the samples inveqtigated, and particularly for
the sample t~ken 350 ~m under the surface~ At 490 MPa, a
further increase in the intensity of this new peak can be
seenO Again the 350 ~m-sample peak is markedly higher when
compared with samples cut at 50 and 600 ~m from the surface,
respectively. At the 350 ~m-depth, the bulk of the melting
now seems concentrated to the 137C-level, but e~en at
600 ~m the higher melting peak is more intense than at
50 ~m.
The occurrence of a high pre~sure phase, melting
at 137C., was associated with rather marked changes in
the mechanical propertie~ of the moldings. ~ig. 4 shoW9
the modulus of elasticity and the breaking stress CB ~ld
elongation ~B a~ a function of the maximum ca~ity press~re.
Both the modulus and ~B increase markedly with this
pre~sure. At ~90 MPa the value of ~B reaches the notably
1~ . '
)728
high level of about 120 MPa. Parallel with this increase,
~B falls from 15 ~ at 100 MPa to 5 ~ at the highest
pressure. The samples showed no tendency to cold-drawing,
independent of the pressure.
The internal stress level o~ the samples,
measured using a stress relaxation technique, decreased
sharply with increasing pressure.
The results obtained show that increasing the
molding pressure above 300 MPa is associated with the
appearance of a new PE-phase showing a DSC-melting peak
at 137C. This phase appears to be concentrated to the
well-known second layer of injection molded parts.
During the filling of the mold, relatively high shearing
forces occur. This effect is due to an increase in melt
viscosity with pressure. The reason behind this is partly
a reduction o~ the free volume, partly a substantial
increase in the melting point (about 20C~ per 100 ~Pa).
It can be supposed that the shearing forces are
especially intense close to the first solidified layer
at the cavity walls. This could in turn be related to the
excessive occurrence of the new oriented phase in the
second layerO In this connection, the formation of extended
chains during capillary extrusion of IIDPE may also be
mentioned. Even though there is limited direct e~idence
for this, it seems plausible to suppose that this second
melting peak is associated with the occurrence of extended
chain like structures in the moldings - among other things
the Tm-value agrees with literature dataO
~he small shoulder in the DSC-curve exhibited by
the sample molded at 100 MPa, taken from 350 ~m depth~
17.
. _ ,
ll~V7~8
could be due to formation of less perfect extended
chain-like crystals, having a lower melting point than
the more perfect ones.
Substantial changes in the properties of injection
molded HDPE-parts may thus be obtained by increasing the
cavity pressure above 300 MPa. From the DSC-measurements
it can be seen that the barely discernible shoulder in
the DSC-curves occurring above the normal melting point
can be converted into a distinct maximum which, at the
highest pressures used, markedly exceeds the height of the
normal Tm-peak. Further, it can be seen that the increase
in the amount of the high melting phase is accompanied
by marked changes in the mechanical parameters of the
moldings. It appears plausible to assume that the phase
with the higher Tm-value, io e. 137C., is associated
with the occurrence of extended chains or similar structures.
Comparative experiments were also carried out
using high molecular weight HDPE (superstrength) and normal
HDPE which were injection molded at pressures within
the range 10~0-500 MPa and a cylinder temperature of
250-280C. The yield properties obtained, which were
measured, are shown in Figs. 5, 6, 7 and 8, where the
black points and squares represent high molecular weight
HDPE, while the unfilled rings and squares represent normal
HDP~. From Fig. 5 it can be seen that substantial improve-
ments of the yield p-operties, such as tensile modulus
and tensile strength, are obtained for high molecular
weight HDPE, when using high injection and holding
pressures up to 500 MPa. Fig. 6 shows how the crystallinity
18.
110(~72~
alld t~le mo~(i s~rinklge irlcrease and decrease, respectively,
when higher pressures are used. Fig. I shows how the
internal stress is reduced when using higher injection
and holding pressures from 100 to 500 MPa. Finally, in
Fig. 8 such a yield property as creep i9 plotted against
time for molding samples injectlon molded at 100 and 500 MPa,
respectively, for normal l~DPE and high molecular IIDPE,
respectively.
EXAMPLF.S 5-6.
In these examples the variations of the properties
of the polyacetal (POM) and polyethyleneterephthalate (PETP)
within the pressure range 100-500 MPa were investigated.
The same injection molding machine was u~ed in the present
examples as in the previous examples.
The mechanical properties were determined in
the same manner as in the previous examples with a tensile
tester (Instron*model 1193) and with a strain rate of
20 mm/minute (1.3-10 2 s 1). From the stress-strain curves
the yield and breaking stress values (~S~B)~ the corresponding
elongation values (S~l~) and the modulus of elasticity were
determined.
The materials used were:
PETP (unmodified): Arnite*A04900 from AkY.o P~astics,
relative viscosity 1.8-2Ø
POM: Hostaform*C 9021 from ~loechst AG, density 1.40 ~cm3,
melt index (M~`I 190/2) 9 ~10 minutes.
q'he molding conditions for preparing the different
~amples are su~mariæed in the following table II.
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RESUITS
In principle, the injection and holding pressures
during the injection course can be calculated from the
oil pressure of the hydraulic system in the injection
molding machine. It is true that a prerequisite for
this is that the melt remains liquid during the injection
and -that any pressure losses can be neglectedO In the
present experiments, the pressure conditions in the mold
correspond to the oil pressure of the hydraulic system
only for POM. For PETP, the maximum internal mold pressure
only was about 300 MPa at a hydraulic pressure of 500 MPaO
In Figo 9 the pressure course in the mold as
compared to the hydraulic pressure during the injection
and holding periods is shown for POMo The conformity is
comparatively good, especially as to the injection pressure~
The corresponding curves for PETP are shown in Fig. 10.
Already at 100 MPa the internal mold pressure (continuous
line) decreases more rapidly than the hydraulic pressure
(dashed line)~ Then the maximum pressure attainable in
the mold for PETP can not exceed a value of about 300 MPa~
Furthermore, the passing to holding pressure can not be
seen in the curve for PETP.
PROPERTIES OF THE SAMPLES
The variation of the properties with the injection
pressure used will now be described.
The essential course of the stress-strain curves
for the POM-samples prepared at different pressures can be
seen from Fig. 11. The values for the modulus of elasticity
and for the yield and breaking stress calculated therefrom
21.
7~8
are shown in the left part of Fig. 12. According to
said fi~ure, a comparatively smaller, approximately linear
increase with the pressure occurs. The resu1ts as to
elongation at rupture are quite different~ An increase
of the pressure to 500 MPa causes a considerable increase
of the elongation at rupture; cf, the left part of Figo 13
However, the ~s-values are practically lndependent of
pressure at a mold temperature of 30Co
The general shape of the stress-strain curves
for the PETP-samples is shown in Fig. 14~ The values of
E, ~S and ~B calculated therefrom are shown in the right
part of Figo 120 ~s with the POM-samples, the variation~
of said parameters are comparatively small also for PETP.
The mold temperature, i.e. 30C. and 130Co ~ respectively,
has only as~ight influence. The elongation at yield
remains practically unaltered. For the elongation at rupture9
on the other hand, there is a considerable difference
between cold ( 30Co ) and warm (130C~ ) mold, cf. the right
part of Fig. 13.
The dependence of the mold shrinkage on pressure
can be seen from Fig. 15. For POM, there is a monotone
decrease with increasing pressure from about 1.6 to 0.8 %.
(Mold temperature 30C.)
For P~TP, as expected, the shrinkage course is
dependent on whether the solidification in the melt takes
place in the amorphous or crystalline state, which can be
affected by adjustment of the mold temperature. The
difference in the pressure dependency of the shrinkage
resulting therefrom is clearly shown in Fig. 15. With a
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cold mold (30C.) the shrinkage decreases for the
initially amorphous molding from 0~25 to -0.3 5~ at the
highest pressure value. At a mold temperature of 130C.
solidification o PETP takes place in crystalline state
and the shrinkage is higher.
For the PETP-samples injection molded at a mold
temperature of 30Co, an increase of the density from
1.3315 to 1.3476 g/cm37 i.e. 0.016 g/cm3, is obse~red in
the used pressure rangeO At a mold temperature of 130C.
the density increases from 1.3589 to 1.3704 ~/cm3, i.e.
00022 g/cm3. The increase of the density in both cases
is characterized by a plateau between 300 and 400 MPa (30C.)
and between 200 and 300 MPa (130C.), respectively.
By means Of literature data for the densities
Of amorphous and crystalline phases, respectively, of
PETP, cfo van Krevelen, D W, Properties Of Polymers,
~sevier Publ. Co. 1972, page 49; Thomson, A B and
Woods, D W, Nature 1967 (1955), page 78; and de PO Daubeny,
R; Bunn, C B and Brown, C J, Proc. Roy. Soc. (~ondon) Edo A,
226 (1954), page 531, corresponding values of the crystal-
linity and their variation with the pressure were calculated.
From Fig. 16 it can be seen that at a mold temperature of
30C. the crystallinity increases from about 1 to 15 ~,
while with the warm mold (130C.) the corresponding values
are 24 and 33 ~0, respectively. For the POM-samples~ mold
temperature 30C., the variation in density is only
0.001 g/cm3j similarly the variations of the crystallinity
were negligibleO
Thus, the results obtained show that the mechanical
23.
_ _ _ , . . , _ _ _ . _ _ . . . .. . . .
properties, such as modl~lus of elasticity and yield
and bre~king stress as ~'J"ll as the elongation at yield-
only vary to a minor extent within the pressure range
used, i.e. 100-500 MPa. On the other hand, the elongation
at rupture increases consideraoly with the injection
pressure, both when using POM and PETP. For PETP, said
increase extends o~er the whole pressure range for the
hydraulic pressure used, e~en if only a maximum of about
300 MPa was measured in the mold.
The increase of the elongation at rupture seems
to be related with variations in the crystalline phase
since it appears mainly for the POM-samples and for the
PE~P molded at a mold temperature of 130C. and thus
crystalline samples.
It was noted that PE~P injection molded in a cold
mold (20-30C.) at normal pressures (about 100 MPa) was
amorphous and translucent, while on increasing the pre~sure
to 300 MPa and above the crystallinity of the same samples
became ~ully developed.
EXAMP~ES 7-10.
- In these experiments different polyethylene
materials and polypropylene were injection molded at
pressures between 100 and 500 MPa. ~he same injection
molding machine was used as in the previous examples.
The molding conditions used in preparing tensile bars and
mold shrinkage plates can be seen below:
MO~DING CONDITIONS
~ensile Injection Holding Cooling Cylinder
bars time (s)time (s) time (~) tempO (C.)
LDPE - 1 14 10 200-240
HDPE 10 5 8 200-240
PP 1 18 10 200-230
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,
:
Mold
shrinkage Injection Holding Cooling Cylinder
plates time (s) t _ (s) time (s) temp. (C~)
LDPE .1 14 30 180-220
IIDPE 1 20 20 190--2 50
PP 1 20 20 200-230
The mold temperature was always 30C.
The following materials were used:
MI (g/10 minO) Densit~ (g/cm3)
I.DPE 7 0 . 9 2
HDPE 7 0.96
PP 3 0.90
The variation of the mold shrinkage for ~DPE, HDPE
and PP can be seen from Fig. 17. For ~DPE the shrinkage --
decreases with increasing pressure ~rom l.ô % to -0.20 ~o. .
Said decrease is especially pronounced between 100 and Z00 MPa
and between 400 and 500 MPa. For HDPE, the shrinkage decreases
comparatively steeply to 300 MPa and then takes the form of
a plateau at the highest pressure, which plateau corresponds
to a negative shrinkage of about -0.5 %o At about 250 MPa
the shrinkage is zero.
For HDPE, a small inflexion appears in the
density-pressure curve in the pressure range in which
extended chain crystallization normally begins.
25.
