In the experiments performed in this research, it is noticed that warpage generally  occurs  on  long-side wall of the injected part, after taking it out from the mould. Therefore, the warpage definition is described by taking length-wise warpage  into  consideration,  as  shown  in Fig. 8. Ten days after the experiments, the related measure- ments were performed at three points of the part and the amount of single-side warpage was  calculated  by  using the following equation:

Single-side warpage : W  ¼ ½ðW 1  þ W 3Þ=2 — W 2]=2 ð3Þ

A comparison of the minimum warpage rates for ABS and PP polymers in the case of using both runner systems vs. pressure at the stages of process temperature is pre- sented in Figs. 9 and 10. In these figures, the measurement points are presented on the horizontal axis, and the values of single-side warpage were presented on the vertical axis. These figures show that increasing process temperature leads to increasing warpage generally. The process temper- ature and injection pressure providing minimum warpage occurrence were realized at 260 °C/60 MPa for CRS and

225 °C/90 MPa  for  HRS  when  using  ABS  polymer, and

170 °C/120 MPa for CRS and 200 °C/40 MPa for HRS when using PP polymer.

6. Effect of runner system on sample   weight

Saint-Martin et al. [9] studied the effect of holding pres- sure, mould and melt temperatures and injection speed on the density of the injection-moulded part and the voids rate inside the part. They found that the hydraulic holding pres- sure level is the most relevant parameter. When the hydrau- lic pressure level increases, the polymer pressure inside the

1474 A. Demirer et al. / Materials and Design 28 (2007) 1467–1476

Comparison of the Minimum Warpages for ABS

225 ˚C, CRS (95Mpa) 245 ˚C, CRS (70Mpa)

260 ˚C, CRS (60Mpa) 280 ˚C, CRS (50Mpa)

225 ˚C, HRS (90Mpa) 245 ˚C, HRS (65Mpa)

260 ˚C, HRS (40Mpa) 280 ˚C, HRS (30Mpa)

0.225

0.200

0.175

0.150

0.125

0.100

0.075

0.050

0.025

0.000

1 2 3

Measurement Points

Fig. 9.  Comparison of the minimum single side warpages for ABS vs. temperature and pressure when using both HRS and CRS.

Comparison of Minimum Warpages for PP

170 ˚C, CRS (120Mpa) 200 ˚C, CRS (70Mpa)

260 ˚C, CRS (45Mpa) 170 ˚C, HRS (80Mpa)

200 ˚C, HRS (40Mpa) 260 ˚C, HRS (30Mpa)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1 2 3

Measurement Points

Fig. 10.  Comparison of the minimum single side warpages for PP material vs. temperature and pressure when using both HRS and CRS.

cavity decreases more slowly, the shrinkage compensation becomes more efficient, and the voids rate goes to zero level. They reported that the mould and melt temperatures also affect parameters, while the injection speed is non-sig-

nificant. As voids can lead to stress concentrations  and early failure of the part, decreasing the void rate in injec- tion-moulded part (i.e., increasing the density and  weight of the part) is crucial. In the case of using HRS, the dis-