where Pc=cavity gate pressure, psi; K1 =ramping slope, psi/s;

K2 =process gain, psi/% change in valve opening; τ=process time constant, s; and D=time delay, s。

In addition, the cavity pressure at the packing stage, based on various step changes in the servo valve opening, can be expressed by the power law:

PcðtÞ ¼ K。1−e−ðt−DÞ=τ 。 ð2Þ

where Pc=cavity gate pressure, Pa and K=process gain, Pa/% change in valve opening。

Taguchi Tabl e 1 Factors and levels for Tagu chi experim ent

Experiments Various P arameters

(One-factor-at-a-time Control factor Level 1 Level 2 Level 3

Analysis of Variance Experiments) A Melt temperature (°C) 230 240

(ANOVA) B Injection pressure (MPa) 70 75 80

C VP switchover position (mm) 6。24 6。12 6

D Cooling time (s) 10 15 20

E Packing time (s) 2 3 4

F Packing pressure (MPa) 75 80 85

G Mold temperature (°C) 60 70 80

H Injection velocity (mm/s) 80 90 100

Fig。 2  Schematic diagram for this study。 a Part layout in mold。 b

Dimensions of the lens

Therefore, the cavity pressure model of the near gate can be built。

3 Experimental setup

This study used the Taguchi method for the injection molding experiment in order to obtain the optimal parameter combina- tion of the lens form accuracy and then conducted experiments on the mounting height of the sensor and different levels of eight factors, according to the optimal parameter combination。 Runner and cavity pressure history profiles were obtained by the sensors, and the form accuracy of the lens was measured。 The relationship between the cavity pressure profile at differ- ent runner positions and lens form accuracy was thus established。 The overall procedure is as shown in Fig。 2。

3。1 Lens specifications

There were three sensors mounted in the secondary runner, tertiary runner, and cavity center of a four-cavity mold with a round runner system with an “H” patterned    geometrically