CA1100728A - Method of reducing internal stresses and improving the mechanical properties of injection molded thermoplastic resins - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method of reducing internal stresses in and improving the mechanical properties of injection molded articles made of thermoplastic resins is disclosed wherein under in other respects normal injection molding conditions thermoplastic resins are injection molded at high injection and holding pressures within the range 250-800 MPa.

Description

.~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 .
~;

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.

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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.

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 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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llU~)7~8 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 (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 22.

``` llV~)728 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.) 24.

, :

Mold shrinkage Injection Holding Cooling Cylinder plates time (s) t _ (s) time (s) temp. (C~) LDPE .1 14 30 180-220 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.

Claims (7)

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

1. 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 thermoplastic resin, the improvement comprising carrying out the injection 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.

2. The method according to claim 1 wherein the pressures are from 300-500 MPa.

3. The method according to claim 2 wherein the thermo-plastic resin is selected from the group consisting of olefin plastics, styrene plastics, acrylic plastics, amide plastics, acetal plastics, carbonate plastics, polyesters, cellulose plastics and vinyl plastics.

4. The method according to claim 1, wherein polyethy-lene having a molecular weight of 200,000 to 1,500,000 is used as the thermoplastic resin, whereby highly improved mechanical properties of the injection molded articles are obtained.

5. The method according to claim 1 wherein by adjust-ing the injection and holding pressures properly the mold shrink-age of the molded part can be reduced to zero.

6. The method according to claim 3 wherein the thermo-plastic resin contains at least one crystalline phase and the crystallinity is changed in volume or structure by the injection and holding pressures resulting in improved mechanical strength properties of the injected molded resin.

7. The method according to claim 6 wherein the resin is selected from the group consisting of olefin plastics, amide plastics, acetal plastics, polycarbonate plastics and polyalky-lene terephthalates.