Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
  • B29C45/17—Component parts, details or accessories; Auxiliary operations
  • B29C45/26—Moulds
  • B29C45/27—Sprue channels ; Runner channels or runner nozzles
  • B29C45/2701—Details not specific to hot or cold runner channels
  • B29C45/2708—Gates
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
  • B29C45/0001—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor characterised by the choice of material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
  • B29C45/17—Component parts, details or accessories; Auxiliary operations
  • B29C45/26—Moulds
  • B29C45/2669—Moulds with means for removing excess material, e.g. with overflow cavities

Definitions

• thermoplastics e.g. high density polyethylene with a high molecular weight
• elevated injection pressures preferably in the range 300-500 MPa
• HDPE type DMDS 2215, Unifos Kemi
• 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.
• suitable polyolefins high density -polyethylene (HOPE) with a high molecular weight deserves special mention..
• the shape of the mould filling gate is another important factor in carrying out the injection moulding process according to the present invention.
• 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.
• 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.
• 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.
• 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.
• Figure 1 shows the modulus of elasticity of the moulded parts as funtion 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
• 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.
• 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.
• 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.
• 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 ( ⁇ "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.
• 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.
• 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.
• OMPI injection moulded HMWPE-test bars with varying thickness.
• 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 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.

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• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Mechanical Engineering (AREA)
• Injection Moulding Of Plastics Or The Like (AREA)
• Moulds For Moulding Plastics Or The Like (AREA)

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• 1982
  • 1982-05-19 EP EP19820901638 patent/EP0091903A1/de not_active Ceased
  • 1982-05-19 WO PCT/SE1982/000182 patent/WO1982004009A1/en not_active Ceased

Non-Patent Citations (1)

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