Abstract: In order to obtain the minimum values of the total residual wall thicknesses (RWT) and the inner melt RWT, response surface methodology (RSM) was used to optimize the processing parameters. A pipe with a 20 mm diameter and a 200 mm length was chosen as a test case. High density polyethylene (PE-HD) and polypropylene (PP) were used as the skin layer and the inner layer plastics in the experiments, respectively. Based on single factor experiment main influence factors of the total RWT and the inner melt RWT of O-WACIM were obtained. The result of a Plackett-Burman experiment exhibited the key influence factors of the total RWT and inner melt RWT of O-WACIM. Based on a Box-Behnken experiment and a RSM analysis and optimization, the optimized processing parameters to get the minimum values of the total RWT and the inner melt RWT were obtained: the water pressure 7.5 MPa, the water injection delay time 2 s, the inner melt temperature 215 ℃. Under the optimized conditions, the predicted total RWT of Design-expert was in good agreement with the experiment results. The research indicated that RSM could be used to optimize the processing parameters to obtain the minimal total RWT and inner melt RWT of O-WACIM pipes.

Key words: water-assisted co-Injection molding, overflow, response surface methodology, residuall thickness