INJECTION MOULDING PROCESS USING HIGH PRESSURES

Abstract: Method in injection moulding of crystalline thermoplatics with an elevated injection pressure, the thermoplastics being injection moulded with high injection pressure and with a use of a low barrel temperature which is so close above the melting point of the thermoplastic material as possible whereby injection moulded parts of high modulus of elasticity and very high tensile strength are obtained. The use of a mould which is equipped with an auxiliary exit gate in the injection moulding process is also described and the influence of gate with different geometry.

Description: INJECTION MOULDING PROCESS USING HIGH PRESSURES It is known, from the Swedish Patent 7506828-8 and the US Patent 4,237,089, that injection moulding of thermoplastics, e.g. high density polyethylene with a high molecular weight, at elevated injection pressures, preferably in the range 300-500 MPa, may produce a highly significant improvement of the property profile of the moulding. When, using such with HDPE (type DMDS 2215, Unifos Kemi) , with a melt index value of 0-1 g/10 min. (MFI 190/2), and a density of 0,953 g/cm , the injection pressure was increased from 100 to 500 MPa, the modulus of elasticity increased from 1.0 to about 3.3 GPa, and the tensile strength from 50 MPa to 130 MPa. In this process, on the whole, conventional injec¬ tion moulding conditions were used, i.e. a mould tempera¬ ture of 30°C and a barrel temperature of 280 C, The improve- ment in the properties attained in this way were certain¬ ly large, implying a significant technical progress. It has now surprisingly, according to the present inven¬ tion, been found that it is possible to achieve further substantial improvements of the stiffness (modulus) and strength (tensile strength) of the injection moulded pro¬ ducts by lowering the melt temperature in the barrel (barrel temperature) before the melt reaches the mould, keeping the injection pressure and the mould temperature unchanged, It became apparent that the improvement of the modulus of elasticity and tensile strength is the larger, the lower the temperature of the melt. There . is , however, a lower limit for the barrel temperature, as this temperature obviously influences the viscosity of the melt. One has therefore to choose a melt temperature which is as close to the melting point of the polymer as possible. The lowest melt temperature is to be considered as that tempe¬ rature at which the viscosity of the melt has increased to the highest value, which - at the igh pressure being used - still allows for a good filling of the mould without local solidification. If the barrel temperature is too low, this OMPI implies that the melt may show local solidification before the mould is completely filled. The temperature as a rule is not more than 65 C above the solidification point of the melt. A temperature range often used is 30-50 C above the solidification point of the melt. The materials suitable to be used according to the present invention are crystalline thermoplastics which may be selected among the polyolefins, further polyoximethylene, poly(vinylidene fluoride), polyamides, and others. Among suitable polyolefins., high density -polyethylene (HOPE) with a high molecular weight deserves special mention.. It became further evident that, according to this invention, the use of a mould equipped with an auxiliary chamber (exit gate) results in additional improvements in both the modulus of elasticity and tensile stress in the injection moulding process according to the present invention. These additional improvements may be quite substantial, and the use of a mould cavity provided with, an auxiliary exit gate is therefore to be considered as an important and highly suitable embodiment of the present invention. Another important factor in carrying out the injection moulding process according to the present invention is the shape of the mould filling gate. The shape of this gate thus should be such that the melt is given a degree of pre-orientation, in that the gate is shaped with a continuously diminishing cross-section. On the other hand, this section may not be too small, in order to prevent too high heat dissipation effects which may counteract the desirable improvements in modulus and strength. It has thus, according to the present invention, become evident that a reduction cf the melt (barrel) temperature, when using an elevated injection pressure and a normal mould temperature, in a most tangingable and surprising OMPI way can improve the stiffness and strength of the moulded parts. The improvement in the properties of the moulded part is most likely due to a surprising combinedaction of the high injection pressure and the shear forces to which the melt is subjected during the filling of the mould cavity. Said shear forces are enhanced by an increasing viscosity of the melt. To underline the pronounced technical progress inherent in the method according to the present invention, it may be mentioned for the sake of comparison that the known engineering plastics with the highest stiffness and strength values (short time data), i.e. poly(ethylene terephtalate) (PETP) with 45% glass fibre (Rynite 545) and poly(phenylene sulphide) (PPS) filled with glass fibre (Ryton) , have modulus and tensile strength values of 14 GPa and 200 MPa, and 12 GPa and 150 MPa, respectively. Corresponding values for high pressure injection moulded HDPE (DMDS 2215) produced according to the present in- vention are 13 GPa and 260 MPa (1 mm thick test bar) . On the other hand, the same HDPE-grade, injection moulded using conventional pressure and temperature conditions, has modulus and strength values of 1 GPa and 50 MPa, res¬ pectively. A comparison of the above modulus and strength values show ' s clearly the large and unexpected improvements in these properties, attained under injection moulding conditions according to the present invention. The known engineering plastics mentioned above, possessing high stiffness and strength, are also significantly more expensive than the crystalline thermoplastics moulded according to the present invention, which constitutes another important advantage of this invention. In the attached figures, Figure 1 shows the modulus of elasticity of the moulded parts as funtion of the barrel temperature; OMPI - Figure 2, the tensile strength of the moulded parts as function of the barrel temperature; Figure 3, the relationship between the melting point and the modulus of the moulded parts; Figure 4, a mould with gate, cavity and the auxiliary exit chamber which can be used in performing the injection moulding process according to the pre¬ sent invention; and Figure 5, the modulus and the strength as function of the thickness of the moulded test bars, with and without the use of the auxiliary exit chamber. We have performed a series of experiments using HDPE with a high molecular weight as a crystalline thermoplastic material. The HDPE-grade used was DMDS 2215 (Unifos Kemi) , melt flow index 0.1 g/10 min (MFI 190/2), density 0.953 g/cm . The injection moulding was carried out with a nominal injection pressure of 500 MPa at a mould tempera¬ ture of 30 C, while different barrel (melt) temperatures were used in the various experiments. The results are summarized in figures 1 and 2 , showing the modulus and the tensile strength, respectively, as function of the barrel temperature used. From Figure 1 it is evident that a reduction of the barrel temperature from 250 C to 170°C resulted in an increase in the modulus from 4.2 to 8 GPa, which is a highly significant improvement. From Figure 2 follows that a reduction of the barrel temperature from 250 C to 170 C resulted in an increase of the tensile strength from 110 MPa to c. 200 MPa, a highly significant improvement as well. These property improvements are pro¬ bably related to the formation of new crystal modifications of the high density polyethylene material. These structures appear to be induced by a combined action of the high pressure and the substantial shear deformations during the filling of the mould. The new structures have a some¬ what higher melting point than polyethylene with normal structure. When the melting point of the high strength OM structures, which may be interpreted as a measure of the degree of perfection of these structures, is plotted versus the modulus of elasticity, a rectilinear relation¬ ship is obtained, cf. Figure 3. The structures associated with the highest modulus values also have the highest melting points. The polyethylene grade used should contain a certain fraction of high molecular weight material, that is to say, its melt flow index has to be sufficiently low. It has, according to the present invention, also become apparent that, when the injection moulding operation is carried out with a mould cavity attached to an auxiliary exit-gate, unexpected additional improvements in the modulus of elasticity and tensile strength can be attained. This is based on the fact that the improvement in the mechanical properites is due to the formation of structural elements different from the spherulitic structures normally encountered. The exact character of these elements, and the mechanism of their formation, are not known in detail at present. It can, however, be expected that the formation of such structures, enhanced by the combined action of shear and high pressure, is facilitated along the entire length of the moulded part when an auxiliary exit gate is placed at the farther end of the part. Such an auxiliary chamber fulfills the task of homogenizing the shear field within the part. The results obtained show that this brings about unexpected improvements of both the modulus and the tensile strength, The use of the auxiliary exit chamber also improves the mechanical ' parameters for injection moulded parts with increasing thickness (up to 6.0 mm). An increase in thickness for high pressure injection moulded test bars results otherwise in a substantial reduction of both the modulus and the tensile strength. This can thus be counteracted by using the auxiliary exit chamber. OMPI An important factor when using the present injection moulding procedure is also the geometry of the gate. The gate is thus supposed to produce a pre-orientation of the melt; the gate should have a continuously diminishing cross-section which, on the other hand, should not be too small in order to avoid excessive heat dissipation effects. We have carried out a number of experiments showing the improved results which can be obtained using the auxiliary exit chamber, as well as experiments shewing the role played by the geometry of the conventionel gate. The thermo¬ plastic material used was the same HDPE-grade as descirbed above. The injection moulding machine was a conventional machine from Sund-Akesson AB; the geometry of the mould (gate, cavity, and auxiliary exit chamber) follows from the attached figure 4. The nominal injection pressure used was always 500 MPa, and the mould • emperature 30°C. The barrel (melt) temperature was 190 C, which was a suitable barrel temperature for attaining a sufficiently high production rate. When the barrel temperature is lowered ttoo 117700°CC, aann aaccceptable production rate is sometimes diffi- cult to attain. The mechanical parameters modulus of elasticity (E) , and tensile strength (σ"L) were determined using an Instron tensile tester (model 1193) according-to ASTM D 1638. The temperature was 20 _ 0.5 C, the deformation rate 20 mm/min. The test bars can be injection moulded with or without the auxiliary exit chamber, as shown in Figure 4. The thickness of the test bars could be varied between 1 and 6 mm. The shape of the gate could be varied as shown. Most cf the experiments were carried out with that shape of the gate which produced the best results, i.e. the gate with a rectangular cross-section (gate III) . Figure 5 shows the modulus (E) and the tensile strength OMPI (cr . as function of the thickness of the test bars moulded using gate III with and without the auxiliary exit chamber. For thin test bars, high values of the modulus and strength are obtained (maximum values 11 GPa and 260 MPa, respecti- vely) ; when the thickness is increased, these values are reduced significantly (about 3 GPa and 70 MPa, respective¬ ly, at a thickness of 6 mm) . This reduction appears to be associated with the relaxation of the high modulus/high strength structures due to a lower degree of supercooling for the thicker parts, as well as to less intense shearing of the melt during mould filling. It is important to note that the modulus increases by 1-1.5 GPa for all parts, irrespective of thickness, when the mould cavity is connected with the auxiliary exit chamber (exit gate) . The use of such a chamber is thus most important, relatively seen, for the thicker parts. The use of the exit chamber also results in a significant improvement of the tensile strength. For parts with a thickness less than 4 mm, the improvement is about 30%, while it is less for thicker parts, for example from 60 to 70 MPa for a part with a thickness of 6 mm. The influence of the gate geometry on the mechanical properties o_f -the test bars (moulded with the exit chamber) having a thickness of 1.5 mm is shown in table I. In these experiments, the barrel temperature was the same as earlier, i.e. 190 C, and the mould temperature 30 C. The best results (E = 10 GPa and-,cr = 250 MPa) were obtained with the gate having a rectangular cross-section (gate III), while the gate having the shape of a long capillary with a small diameter (gate I) gave parts with the lowest values of E and cf _ (6.5 GPa and 180 MPa, respectively). The results of the experiments described above show that providing the mould cavity with the auxiliary exit chamber has a positive influence on the stiffness (modulus of elasticity) and the tensile strength of high pressure . OMPI ~ " injection moulded HMWPE-test bars with varying thickness. Especially for parts with large wall thickness, where the reduction in the level of the mechanical parameters may be appreciable, is the use of the exit chamber a highly suitable means ' to counteract such an unwanted reduction. The use of the exit chamber also produces a more homo- . geneous distribution of the mechanical parameters along the length of the part. The influence- of the gate geometry on the results obtained is probably associated with a certain orientation of the melt in the gate area, giving parts with a higher stiffness and strength, cf. the results obtained with gates III and IV in table I. Depending on its shape, the gate thus appears to give a pre-orientation of the melt, a preferable shape being a continuously diminishing cross-section. The gate should, on the other hand, not be to narrow or too long, as this results in a deterioration of the mechanical parameters, cf. gate I and II in table I. This deterioration is related to lower pressure levels during the moulding cycle, and to an increased heat dissipation when the melt is forced through such narrow channels. The dissipated heat may influence the high modulus structures adversely, thereby leading to lower modulus and strength values. TABLE I "& Q *£

Claims: CLAIMS 1. A method in injection moulding of crystalline thermo¬ plastics with the use of an elevated injection pressure of at least 250 MPa for producing injection moulded parts of substantially improved modulus of elasticity and tensile strength characterized in that the thermoplastics are injection moulded with a high injection pressure and with a use of a low barrel temperature which is so close above the melting point of the thermoplastic material as possible which temperature however at the high pressure being used still allows for filling of the mould cavity without local solidification whereby injection moulded parts having very high modulus of elasticity and very high tensile strength are obtained. 2. A method according to claim 1 characterized in that the injection moulding is carried out with the use of a mould equipped with an auxiliary exit gate, 3. A method according to any of claims 1 or 2 characterized in that the injection moulding is carried out with an injection moulding machine which is provided with a gate which has a continously diminishing cross-section which however is not so small that the heat dissipation disturbs the temperature balance, whereby the melt is pre-oriented. 4. A method according to any of the preceding claims characterized in that HD-polyethylene having high molecular weight is injection moulded.

Inventors: KUBAT JOSEF, MANSON JAN ANDERS, RIGDAHL HANS MIKAEL
Applicants: KUBAT JOSEF, MANSON JAN ANDERS, RIGDAHL HANS MIKAEL
Application year: 1982
Main classification: B29F1
IPC codes: B29C45