260 12。5 20

4。 Shrinkage evaluation

It has been reported that the primary influential factors on shrinkage of an injected part are both the magnitude and  duration  of  exerted  packing  pressure。  The     excess

shrinkage on the remote regions of the part from the sprue is largely attributed to the reduced effect of the packing pressure at the outer regions。 There is a direct relationship between orientation and shrinkage due to the fact that molecular chains are oriented in line with the flow direction under the effect of friction and elongation。 This effect is more pronounced at the  outer  zones  of  the  part  where the material sets relatively faster。 Therefore, the sample produced by injection moulding experiences more shrink- age in the direction of flow [7]。

The shrinkage of injected part is defined by taking its bottom surface’s length and width into consideration as shown in Fig。 5。 The x-direction shrinkage in length and the y-direction shrinkage in width are defined by the shrinkage of Lp—x and Lp—y segments, respectively。 The thickness of the bottom surface of the part is 2 mm, and the length and width of this feature in the mould cavity are Lm—x = 93。81 mm and Lm—y = 59。79 mm, respectively。 The  residual  stresses  of  the  samples  produced  in      the

1472 A。 Demirer et al。 / Materials and Design 28 (2007) 1467–1476

Fig。 5。  Definition of shrinkage on bottom view of the injected part。

experiment were allowed to relax by keeping them in the room temperature for 10 days and then the magnitude of shrinkage was measured。  The  percentage  of  shrinkage was calculated by using the following  equations:

Length-wise shrinkage rate (%) ¼ 100 · ðLm—x — Lp—x Þ=Lm—x     ð1Þ

Width-wise shrinkage rate (%) ¼ 100 · ðLm—y — Lp—y Þ=Lm—y ð2Þ

The calculated shrinkage rates in length and width for both runner systems vs。 injection pressure at the process temperature of 225 °C for ABS and 170 °C for PP are pre- sented in Figs。 6 and 7, respectively。 According to these fig- ures, it was observed that the shrinkage rate decreased with increasing injection pressure for both runner systems。 This point is also stated by previous researches in the literature [4–6]。 Liao et。 al。 [5] reported that the packing pressure is the most important process parameter for shrinkage, because it becomes effective during cooling down period whereby the material starts。 Under the condition that the packing pressure is high, the polymer can be squeezed into the cavity to reduce and even the   shrinkage。

For HRS and CRS, the average shrinkage  rates  in length and width for ABS and PP polymers are presented in Table 4。 These results showed that using HRS decreases the shrinkage rates for both of the polymers in comparison with CRS。 It is interpreted that this shrinkage-decreasing effect of HRS is resulted from more influential packing stage due to late solidification of the gates, lower  heat losses and better fluidity of the molten plastic。 In addition, using HRS makes the adaptation of central gate location possible in multi-cavity moulds。 This shortens flow length, decreases pressure loss and contributes to achieving more influential packing stage。

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。