Title
Defect occurrence in water-assisted injection molded products: definition and responsible formation mechanisms
S. Sannen1,2*, P. Van Puyvelde2, J. De Keyzer1,2
1Cel Kunststoffen, Faculty of Engineering Technology (KU Leuven @ KHLim), Wetenschapspark 27, 3590 Diepenbeek, Belgium
2Soft Matter, Rheology and Technology, Faculty of Engineering Science (KU Leuven), Willem de Croylaan 46, 3001 Leuven, Belgium
*corresponding author
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Abstract
Defect occurrence in water-assisted injection molded products: definition and responsible formation mechanisms
S. Sannen1,2*, P. Van Puyvelde2, J. De Keyzer1,2
1Cel Kunststoffen, Faculty of Engineering Technology (KU Leuven @ KHLim), Wetenschapspark 27, 3590 Diepenbeek, Belgium
2Soft Matter, Rheology and Technology, Faculty of Engineering Science (KU Leuven), Willem de Croylaan 46, 3001 Leuven, Belgium
*corresponding author
ABSTRACT:
This study starts with the definition of the different defects that occur in water-assisted injection molded products to which subsequently responsible formation mechanisms are unambiguously designated. It is seen that the four different defect types in the current experimental setup ‒ irregular residual wall, void, double wall and no residual wall ‒ are either formed by other mechanisms or by the same mechanism of which the extent decides on the actual defect type. The current insights into the occurring part defects are used in the second part of this study to explain the influence of process and material parameters on the defect occurrence in a reference experiment. The presence as well as the extent of a formation mechanism is here further linked to the water and/or polymer properties/conditions which exist during water penetration. The water and melt temperature, water holding pressure and the presence of nucleating agents in the polymer melt were therefore varied within the pre-defined reference setting. The influence on the nature and location of the part defects was herewith investigated with a qualitative defect analysis. It is found that the proposed definitions and mechanisms are able to explain experimentally observed changes in defect occurrence physically, with which the existing unclearness in literature can be elucidated as well.
keywords: part defects, injection molding, processing, thermoplastics, water-assisted
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Text
1 Introduction
In plastic processing industry, water-assisted injection molding (WAIM) is one of the latest innovations [1]. This technique is developed in 1998, in the Institute of Plastics Processing in Germany [2, 3], in order to produce hollow or partially hollow products. Within the process, the melt injection step is followed by the injection of high pressurized water into the core of the already formed product. The development of this molding technique was made as variant on the well-known gas-assisted injection molding process (GAIM), in which gas is used to core out the product. Both techniques are able to produce tube and rod-like products, complex products with thin and thick sections and sheet like products with reinforcing ribs, which are present in various branches of industry.
Improved product characteristics and lowered production costs are the main advantages of both GAIM and WAIM, in their production of (partiality) hollow products, when compared to conventional injection molding processes. The use of water (WAIM) instead of gas (GAIM) leads to a further enhancement of some product characteristics and reduction of process costs, which is principally related to the physical properties of water. More details on the specific advantages and disadvantages of both techniques are already extensively documented in earlier studies [4, 5].
Despite the fact that WAIM is the most suitable technique to produce (partially) hollow products, up to now, a global implementation fails to occur. Besides the more complicated process setup and control, WAIM has to deal with the unrestrained occurrence of unwanted part defects in or at the residual wall of the final product. Figure 1 displays the most common defect types as observed in literature, to which here their most frequent designation is allocated. The presence of these defects influences main product characteristics such as mechanical properties and available flow section, of which the importance depends on the actual application. Since to date no clear solutions exist to prevent these defects, WAIM is still seen as an uncontrollable and unpredictable technique.
The occurrence of part defects in the final product is a well-known problem within the WAIM process. Nevertheless, the available literature and corresponding knowledge concerning the nature and formation of these defects is limited [6]. Most studies restrict their report to isolated findings [7, 8, 9, 10, 11] and the few available systematic studies [12, 13, 14], merely performed by the research group of Shih-Jung Liu, contain furthermore contradictory results. As a consequence, there is no clear definition for the occurring defect types. Their different shapes as presented in Figure 1, are among the available studies freely named, so that similar types have other names and vice versa. In addition, the mechanisms responsible for the formation of part defects are not unambiguously defined [6, 13]. As seen from the summary in Table 1, these mechanisms are in general resolved into the water and/or polymer melt behavior during water penetration. However, the explanation of a defect type changes in and among the available studies. Moreover, the influence of process and material parameters on the occurrence of part defects, provided in Table 2 and 3 respectively, is only qualitatively and for some parameters even contrarily determined. The accompanying explanations to elucidate the observed influences are in addition not profoundly and explicitly defined. These explanations change with and within the performed studies, in which the earlier described mechanisms of both the water and polymer melt behavior are disorderly applied. It can thus be concluded that the principle mechanisms behind the occurrence of part defects are not (fully) understood [8, 10, 13]. Nevertheless, to be able to produce products free from defects, a fundamental understanding of these mechanisms is essential.
In the research towards the nature and formation of part defects in the final product, experiments with a variation in process and/or material parameters are performed. The products, which are produced this way, are subsequently evaluated with a qualitative defect analysis. With this relatively simple analysis, a proper evaluation of the defect nature and location is possible, which is among the studies available in literature omitted by the use of various analysis techniques. With this systematic experimental approach, a clear definition of the occurring defect types in the current experimental setup is formulated and responsible formation mechanisms are herewith unambiguously designated. These renewing insights, as presented in the first part of this study, disclose the principle mechanisms behind the occurrence of part defects and thus contribute to a more fundamental understanding of the WAIM process. In the second part of this study, the influence of process and material parameters on the occurrence of part defects is investigated in a pre-defined reference experiment. A reference against which the influence can be determined is required because the nature and interaction of all parameters (mold, process, material) hinders to define the influence of a single parameter as such, which the studies in literature try to determine. The observed changes in defect occurrence under influence of the applied parameter variations are here explained with the proposed insights into the different defect types and their accompanying formation mechanisms. The presence as well as the extent of the mechanisms is here further linked to the water and polymer properties/conditions existing during water penetration. In this way, the validity of the current insights to explain experimental observations physically and accordingly to describe the formation of part defects can be demonstrated.
2 Experimental
2.1 Method
In the first part of this study, the occurring defect types in the final product of the current experimental setup are clearly defined and unambiguously explained with responsible formation mechanisms. These insights are derived from earlier and systematic experiments on the formation of part defects, in which process and/or material parameter variations were applied in combination with the undermentioned qualitative defect analysis.
The current insights into the nature and formation of part defects are used in the second part of this study to explain the influence of process and material parameters on the occurrence of these defects. The water and melt temperature, the water holding pressure and the presence of nucleating agents in the polymer melt are selected as varying parameters. These parameters are chosen since their influence on the occurrence of part defects is expected based on the current insights and are furthermore unambiguously related to the water and/or polymer properties/conditions existing during water penetration. The level of each parameter is altered within a pre-defined reference setting, of which the values are presented in Table 5. For each new parameter setting, five products are produced. The products are evaluated with a qualitative defect analysis. This implies that the five specimens of each experiment are cut along their longitudinal axis and that their cross-section is subsequently controlled upon the nature and location of the occurring part defects with a visual inspection. This qualitative evaluation is rather simple and gives sufficient information on the defect characteristics to properly conclude on the type and responsible formation mechanism.
2.2 Setup
The experiments in this work were carried out on the mold cavity presented in Figure 2, which is equipped with an integrated hot runner system. The cavity forms a tube with a circular cross-section and four different curved sections, having a diameter of 25.0 mm and a flow length of 615 mm.
The melt injection was done with a Kraus Maffei CX130-1000 injection molding machine, which has a maximum clamping force of 130 ton. The injection unit exists of a screw with a diameter of 50 mm and a maximum dosing stoke of 220 mm. With this unit, the injection rate and specific injection pressure are limited to 157 cm³/s and 2304 bar respectively.
The water was injected into the core of the already formed product with the Engel MW 30/200 volume flow rate water injection unit. The flow rate is herein restricted to 30 l/min with a maximum pressure of 220 bar. In the mold cavity, an hydraulic-operating pulling injector is radial positioned with respect to the tube, having a diameter of 8 mm.
With the aforementioned equipment, the full-shot process with overspill cavity was performed. In this process variant of WAIM, the initial mold cavity is first fully filled with polymer melt (phase 1). A core located at the end of the mold cavity is subsequently pulled back, so that the initial cavity is enlarged with a so-called overspill cavity (Figure 2). After a certain time delay (phase 2), water is injected into the core of the already formed product (phase 3), which is defined as the primary water penetration. The hot polymer melt is hence transported towards the unfilled overspill. At the end of the primary water penetration, the water within the formed product is held under pressure (phase 4), in order to further cool down the product and to compensate the accompanying shrinkage. The further penetration of the water into the polymer during this phase is defined as the secondary water penetration.
2.3 Material
Because of its wide range of properties and large process window, polypropylene is a commonly used material in different injection molding processes. In the parameter variation applied in the second part of this study, the polypropylene grade PP 515A from Sabic® was used. The influence of the solidification behavior on the occurrence of part defects was here investigated by filling the material with nucleating agents, creating grade PP 515AN. The thermal properties of both materials were determined with a differential scanning calorimeter (TA instruments DSC Q2000), in which the materials were heated and cooled at a rate of 10°C/min. The rheological properties were measured with a parallel plate (TA AERES-2K) and capillary rheometer (CEAST Smart Rheo 2000 twin bore). The most important properties of both materials are summarized in Table 4.
3 Results and discussion
3.1 Current insights into the occurring part defects
definitions
In the experimental setup used in this work, four different defect types occur in or at the residual wall of the final product. These defects are presented in Figure 3.
The first defect type consists of an uneven inner wall, which is seen at the entrance of the product in Figure 3a. This defect is designated as irregular residual wall.
The presence of holes within the residual wall is a second type of part defect. This defect, as observed in Figure 3b, is defined as void. From the picture it is clear that voids are either completely enclosed within the residual wall or connected to the formed hollow core by a so-called micro channel.
The intended hollow core of the final product can consist of more than one hollow space, which are separated from each other by a solidified wall (Figure 3c). This third defect type is designated as double wall.
A last type of part defect is defined as no residual wall, which is presented in Figure 3d. Within the product, an uneven and damaged inner wall exists and the hollow core is disturbed with solidified polymer melt.
From the previous pictures and their accompanying definitions it can be concluded that the four observed defect types differ in nature and/or location along the formed or intended hollow core. In this way, the occurring part defect influences final product quality in a more or less extent, of which the importance depends on the actual application. It can further be remarked that the indicated defect types do not completely correspond to those presented in literature (Figure 1). In this work, no residual wall is added, whereas fingering is not observed. The described difference can be attributed to the applied experimental setup: within another geometry it is possible that other defect types are formed, whereas with the use of another visualization technique an identical defect type can be differently evaluated.
formation mechanisms
The mechanisms responsible for the formation of the different defect types are described below. Herein, the mechanisms explain the nature as well as the location of the occurring defect types.
irregular residual wall
An uneven inner wall principally occurs at the start (Figure 3a), but sometimes persists throughout the whole product. This irregular residual wall appears to be formed by an unstable water/polymer flow during primary water penetration. This instability can be ascribed to 1) the presence of a turbulent water flow as well as 2) a pulsating progression of the water/polymer front. Within a turbulent water flow, high frequently changing vortices as well as secondary flows (perpendicular to the flow direction) occur, causing an uneven solidified inner wall. The existence of this flow type can be determined with the dimensionless Reynolds number, for flows in pipes.