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Project
Water-assisted injection molding (WAIM): studying the effect of material properties.
Water-assisted injection molding (WAIM) produces hollow or partially hollow products, potentially having a higher quality and a lower process cost when compared to products produced with other and earlier plastic processing techniques. Since the development of WAIM is relatively recent (1998) [1, 2], the knowledge of the process is restricted. A critical assessment of available literature (Chapter 2 of this thesis) points out that there is a lack of fundamental understanding of the water and polymer behavior during the process, leading to an unclear and unexplainable influence of process and material parameters on product quality.
This thesis studies both the building of the residual wall thickness and the formation of part defects, because the thickness of the wall as well as the possible presence of defects in the final product are important quality characteristics. The accompanying insights into the water and polymer behavior reveal the principle mechanisms behind the process so that the influence of process and material parameters on product quality is elucidated.
The building of the residualwall thickness is investigated with experimental observations: in addition to the residual wall thickness measurements, the water volume flow rate, water and melt pressures are monitored during the process. Themeasurement and corresponding interpretation of these signal progressions lead to renewing insights regarding the water and polymer behavior. Based on this understanding, a physical model is derived with which the stress between the polymer chains can be determined. This stress induces a water channel with a certain radius and consequently influences the behavior during water penetration. In Chapter 4 of this thesis, the model is elucidated and linked to experimental data. The model is furtherused to predict the influence of important process and material parameters on the building of the residual wall. The proposed prediction is subsequently related to the observed variations in volume flow rate, pressure and residual wall thickness. Both the qualitative and quantitative application of the model on the signal variations points out that the model is able to explain the experimental observations physically and accordingly to describe as well as predict the building of the residual wall thickness in a qualitative way.
The formation of part defects is studied in experiments in which a process and/or material parameter variation is applied. With a qualitative defect analysis on the produced products, a clear definition of the occurring defect types in the current experimental setup is formulated: irregular residual wall, void, double wall and no residual wall. To these defects, responsible formation mechanisms are further unambiguously designated. The occurrence of the defects is herewith related to water and polymer properties/conditions existing during water penetration. The new and more complete insights into the different defect types and their accompanying formation mechanisms are presented in Chapter 5 of this thesis. The derived definitions and mechanisms are further used to explain observed variations in defect occurrence for altering process and material parameters in a pre-defined reference experiment. It is found that the proposed definitions and mechanisms are able to explain the experimental observations andaccordingly to describe the formation of part defects.
Numerical simulations on WAIM are performed with the only commercially available software package which is able to simulate the process: Moldex3D. The simulation results are validated and assessed with experimental data to support and supplement the insights into the building of the residual wall thickness and the formation of part defects of Chapter 4 and 5 respectively. The comparison between both results, as presentedin Chapter 6 of this thesis, indicates similarities as well as dissimilarities, whereby the latter can be related to the presence of model and/or numerical errors in the simulation software.
This thesis studies both the building of the residual wall thickness and the formation of part defects, because the thickness of the wall as well as the possible presence of defects in the final product are important quality characteristics. The accompanying insights into the water and polymer behavior reveal the principle mechanisms behind the process so that the influence of process and material parameters on product quality is elucidated.
The building of the residualwall thickness is investigated with experimental observations: in addition to the residual wall thickness measurements, the water volume flow rate, water and melt pressures are monitored during the process. Themeasurement and corresponding interpretation of these signal progressions lead to renewing insights regarding the water and polymer behavior. Based on this understanding, a physical model is derived with which the stress between the polymer chains can be determined. This stress induces a water channel with a certain radius and consequently influences the behavior during water penetration. In Chapter 4 of this thesis, the model is elucidated and linked to experimental data. The model is furtherused to predict the influence of important process and material parameters on the building of the residual wall. The proposed prediction is subsequently related to the observed variations in volume flow rate, pressure and residual wall thickness. Both the qualitative and quantitative application of the model on the signal variations points out that the model is able to explain the experimental observations physically and accordingly to describe as well as predict the building of the residual wall thickness in a qualitative way.
The formation of part defects is studied in experiments in which a process and/or material parameter variation is applied. With a qualitative defect analysis on the produced products, a clear definition of the occurring defect types in the current experimental setup is formulated: irregular residual wall, void, double wall and no residual wall. To these defects, responsible formation mechanisms are further unambiguously designated. The occurrence of the defects is herewith related to water and polymer properties/conditions existing during water penetration. The new and more complete insights into the different defect types and their accompanying formation mechanisms are presented in Chapter 5 of this thesis. The derived definitions and mechanisms are further used to explain observed variations in defect occurrence for altering process and material parameters in a pre-defined reference experiment. It is found that the proposed definitions and mechanisms are able to explain the experimental observations andaccordingly to describe the formation of part defects.
Numerical simulations on WAIM are performed with the only commercially available software package which is able to simulate the process: Moldex3D. The simulation results are validated and assessed with experimental data to support and supplement the insights into the building of the residual wall thickness and the formation of part defects of Chapter 4 and 5 respectively. The comparison between both results, as presentedin Chapter 6 of this thesis, indicates similarities as well as dissimilarities, whereby the latter can be related to the presence of model and/or numerical errors in the simulation software.
Date:26 Oct 2009 → 31 Aug 2014
Keywords:Water-assisted injection molding, Hollow plastic parts, Rheology, Polymer processing, Material properties, Molecular weight, Molecular weight distribution
Disciplines:Other engineering and technology
Project type:PhD project