(2) With the results of CFD simulation on WAIM cavity filling process, the coupling relationship between load pressure and flow rate is found approximately linear and a linear model on load characteristic is built.

(3) The system is uncontrollable due to the derivative elements, and through the model-based  feedback, the system can be transformed to a type 0 system. Further more, the steady-state error can be reduced and the disturbance resisting capacity can be enhanced by closed-loop   control of load pressure with integral compensation. The linearization and large variation of flow  rate  have negligible effect on the low frequency control. In addition, high melt viscosity leads to large oscillation and instability.

(4) Based on the feed-forward compensation for the water hydraulic proportional pressure relief valve, the experimental results show that the max steady-state error of slope injection pressure and maintaining pressure are separately 5.8% and 0.7%. The developed linear controller meets the requirements of water injection pressure control.

(5) Pressure impact and overshoot occur on switch of on-off directional valve. The pressure oscillation caused by switch of directional valve can be suppressed through increasing the flow resistance at the inlet of the accumulator or replacing by a new directional valve with gentle open process.

References

[1] KNIGHTS M. Water injection molding makes hollow parts faster, lighter[J]. Plastics Technology, 2002, 48(4): 42–47.

[2] MICHAELI W, BRUNSWICK A,  POHL T.  A comparison  of gas and water injection moulding of hollow articles by fluid-assisted injection[J]. Kunststoffe Plast Europe, 1999, 89(9): 18–20.

[3] LIANG J C, LI Y, ZHOU D H, et al. Analysis of  diffusion mechanism between gas and melt in gas-assisted injection molding[J]. Journal of Materials Processing Technology, 2007, 187: 685–689.

[4]   MICHAELI  W,  JUNTGEN  T,  BRUNSWICK  A.  WIT-en  route to

series production: first industrial application of the water injection technique[J]. Kunststoffe Plast Europe, 2001, 91(3): 37–39.

[5] LIU S J, LIN S P. Factors affecting the formation of fingering in water-assisted injection-molded thermoplastics[J]. Advances in Polymer Technology, 2006, 25(2): 98–108.

[6] LIU S J, CHEN Y S. Water-assisted injection molding of thermoplastic materials: effects of processing parameters[J]. Polymer Engineering and Science, 2003, 43(11): 1 806–1 817.

[7] PROTTE R, BANGERT H, COOPER C, et al. Water-assist injection molding — an   innovative   process   technology   for  productivity

improvement: developments in processing, equipment and materials[C]//Proceedings of the 61st Annual Technical Conference of SPE, Nashville, TN, USA, May 4–8, 2003. Brookfield: SPE/ANTEC, 2003: 404–408.

[8] LIU S J, LIN C H. An experimental study of water-assisted injection molding of plastic tubes with dimensional transitions[J]. Journal   of

Reinforced Plastics and Composites, 2007, 26(14): 1 441–1 454.

[9] LIU S J, CHEN Y S. The manufacturing of thermoplastic composite parts by water-assisted injection-molding technology[J]. Composites Part A: Applied Science and Manufacturing, 2004, 35(2): 171–180

[10] BACKÉ W. Water- or oil-hydraulics in the future[C]//Proceedings of the Sixth Scandinavian International Conference on Fluid Power, Tampere Finland, 1999: 51–65.

[11] SIUKO M, PITKÄAHO M, RANEDA A, et al. Water hydraulic actuators for iter maintenance devices[J]. Fusion Engineering and Design, 2003, 69(1–4): 141–145.