对热流道系统注塑工艺英文文献和中文翻译(5)

菜单

PP 170 7.5 7.5

200 15 17.64

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.