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]。来*自-优=尔,论:文+网www.youerw.com

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

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

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。

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: