Table 1.

Factors and step values of central composite design.

Factor	Meaning	Unit	$-\mathit{\alpha }$	$-1$	$0$	$+1$	$+\mathit{\alpha }$
${\mu }_i$	inner coefficient of friction	-	0.35	0.44	0.58	0.71	0.8
$d_p$	particle diameter	mm	1	1.61	2.5	3.39	4
$D_B$	barrel diameter	mm	35	45	60	75	85
$h_e/D_B$	channel depth	-	0.1	0.12	0.15	0.18	0.2
$t/D_B$	screw pitch	-	0.7	0.82	1	1.18	1.3
${\phi }_G$	groove angle	°	50	60	75	90	100
$v_0$	peripheral speed	m/s	0.11	0.25	0.45	0.65	0.79
$p$	backpressure	Bar	10	110	255	400	500