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In the case of using CRS, reduction in shrinkage rates requires impractically high working pressures. For exam- ple, CRS results in low shrinkage rate at 170 °C for PP polymer, but it requires 120 MPa injection pressure. Same shrinkage rate can be provided at much lower injection pressures when compared with HRS.
5. Warpage evaluation
In general, it can be said that the warpage is caused mainly by residual stresses inside the part and uneven thickness in various directions. Progressive narrowing of the cross-sectional area caused by setting of melt as layer on cold cavity surfaces leads to increasing stresses in the direction of the flow, thus causing different shrinkage rates. Consequently, higher shear stresses on the material and more molecular orientation will be expected, which may contribute to warpage. The mould and melt temperatures, design of the injection moulded part and cooling system, the higher ratio of the length to the thickness in an injected part, packing pressure and time, and gate type, dimension and location can be listed as the most influential factors on the warpage occurrence [4,8].
The cavity is maintained at a constant pressure for pack- ing stage when filling is nearly completed. Packing pressure
Shrinkage Rates at 225˚C for ABS
0.700
0.675
0.650
0.625
0.600
0.575
0.550
50 60 70 80 90 100 110 120
Injection Pressure (MPa)
Fig. 6. Shrinkage rates in length and width vs. injection pressure at 225 °C for ABS polymer.
A. Demirer et al. / Materials and Design 28 (2007) 1467–1476 1473
Shrinkage Rates at 170˚C for PP
1.95
1.90
1.85
1.80
1.75
1.70
60 70 80 90 100 110 120
Injection Pressure (MPa)
Fig. 7. Shrinkage rates in length and width vs. injection pressure at 170 °C for PP polymer.
Table 4
Average shrinkage rates in length and width for ABS and PP polymers
Shrinkage rates (%)
ABS (at 225 °C) PP (at 170 °C)
CRS, in length 0.603 1.828
CRS, in width 0.673 1.852
HRS, in length 0.598 1.790
HRS, in width 0.671 1.827
Fig. 8. Definition of warpage on top view of the injected part.
is used to fill the remaining volume of the cavity and to compensate for shrinkage in cooling stage. An appropriate packing pressure and time can reduce the shrinkage of the injected part and warpage caused by uneven shrinkage. Therefore, the packing pressure and the packing time are the most important process parameters for reducing the shrinkage and warpage. This point is highlighted by some other researchers [2–5]. When using HRS on multi-cavity moulds, nozzle-gate can be located symmetrically for each
cavity, and weakening tendency of packing pressure on outer regions from the gate can be reduced by decreasing the ratio of flow length to the thickness.