Temperature effects injection moulding

Figures 18.29 and 18.42 show the effect of temperature on the falling weight impact strength for compression moulded and injection moulded specimens, respectively, of a range of materials. The reduction in impact strength with decrease in temperature is particularly marked with the injection moulded samples of the two otherwise higher impact resistant types, PP copolymers and ABS terpolymers. The superior impact strength at low temperatures of injection moulded HOPE is clearly illustrated.

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

A study was made of the effect of density of injection moulded, thin-walled, integral foam products made of propylene copolymers, containing 20% PE, on their low-temperature properties. Results are discussed and evaluated. 5 refs. 

The blends (PET plus core-shell elastomers) were compounded using a co-rotating twin screw extruder. The contains of particules ranged from 7 to 21 % in mass. Test bars were injection moulded in a second step. In order to produce compound of different levels of crystallisation, the mould wall temperature was varied. Truly amorphous mouldings could not be obtained. However, for mould temperature of 5°C only a small amount (less than 8 %) of crystalline phase was observed. These mouldings will be considered as amorphous, since their continuous phase is amorphous. Conversely, a wall temperature of 145°C made it possible to reach the maximum level of crystallisation (approximately 30 % in mass). Finally, no evidence for a significant crosslinking effect due to reactive nodules was found. 

In the cooling stage of processing, the product must completely solidify before the mould can be opened. Figure A.4 in Appendix A shows the dimensionless temperature profiles at different times since the start of cooling. Products cooled from both sides, such as injection mouldings, cool faster than products of the same thickness (extruded pipe, blow moulded and thermoformed) where one side is effectively thermally insulated. The temperature profiles are given in terms of the Fourier number, a dimensionless time defined by Eq. (A.26) as... 

Discuss how increases in the mould wall temperature and the melt injection temperature would decrease the orientation in an injection moulding. What adverse effects would these changes have on the production costs ... 

Intermediate - Temperature Relaxations. Secondary relaxations in the glassy state at temperatures intermediate between those of the a- and P- relaxations have been reported, but workers disagree as to their nature, location and origin. Confusion arises in part from a failure to recognize the existence of two separate processes. Krum and MUller [19] observed an intermediate relaxation only for injection-moulded or cold-drawn polycarbonate samples. Since the magnitude was diminished by annealing and the loss was not detected in fully annealed samples, they concluded that the intermediate process is a non-equilibrium effect associated with residual stresses. 

M Thompson and J R White, The effect of a temperature gradient on residual stresses and distortion in injection mouldings , Polym Eng Sci 1984 24(4) 227-241. 

In order to evaluate the combined effect of typical injection molding pressures and temperatures, a new equipment was designed as a natural extension of the previously described appwatus. A standard Negri Bossi NB25 injection moulding machine was used as a source of molten polymer supplied at a pre determinable and maintainable pressure at which the polymer can be injected into a preheated mould cavity. 

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