where the di is the desirability defined for the ith targeted output and the wi is the weighting of the di。 For various goals of each tar- geted output, the desirability, di, is defined in different forms。 If a goal is to reach a specific value of Ti, the desirability di     is:

Ti —

the specimens for a critical quality targets including the warpage at the middle locations of the  edge  of  both  the  x-direction and the y-direction are included in this study。 The experiments were carried out on a computerized reciprocating screw injection mold- ing machine, model no。 OS-50CH, manufacturing by Outstanding Machinery MFG。, Co。, Ltd。 Taipei Hsien, Taiwan, with capability   of a  maximum  injection  pressure  of  240 MPa,  an  injection  rate  of

d hYi —Highi i    if     T Y High

Ti —Highi i

di ¼ 0 if    Yi = Highi

ð2Þ

70 cm3/s, and a maximum clamp force of 500   kN。

3。3。 Simulation and experimental parameters  design

For a goal is to find a maximum, the desirability is shown as follows:

di ¼ 0 if    Yi 5 Lowi

The  product  quality  produced  by  the  injection molding  is al-

ways affected by the process parameters  like  the  cooling  time, the  injection  pressure,  the  injection  speed,  the  filling  time,   the

di ¼ h Yi —Lowi   i  if    Lowi < Yi < High

ð3Þ

melting temperature, the ejecting pressure, the molding   tempera-

Highi —Lowi i

di  ¼ 1 if    Yi = Highi

For a goal to search for a minimum, the desirability can be defined by the following formulas:

di ¼ 1 if    Yi 5 Lowi

ture, the packing pressure, etc。 The effects of the molding variables on the physical and mechanical properties of thermoplastics have been studied by researchers (Chen et al。, 2008; Chien et al。, 2004; Ismail  &  Suryadiansyah,  2004;  Lin  et  al。,  2008;  Oktem  et       al。,

di  ¼ h Highi —Yi   i   if    Lowi  < Yi < High

ð4Þ

Highi —Lowi i

di ¼ 0 if    Yi = Highi

where the Yi is the found value of the ith output during optimization processes; the Lowi and the Highi are the minimum and the maxi- mum values of the experimental data for the ith output。 In the Eq。 (1), the wi is set to one since the di is equally important in this study。

The DF is a combined desirability function (Myers & Montgomery, 2002), and the objective is to choose an optimal setting that maxi- mizes a combined desirability function DF, i。e。, minimizes F(x)。

3。 Simulation  and  experimental procedures

3。1。 Material

A new heat-resistant polyamide, ‘‘PA9T” is a heat-resistant polyamide  derived   from  terephthalic  acid  and    nonanediamine