Title
Defect occurrence in water-assisted injection molded
products: definition and responsible formation mechanisms
S. Sannen1,2*, P. Van Puyvelde2, J. De Keyzer1,2
1
Cel Kunststoffen, Faculty of Engineering Technology (KU Leuven @ KHLim),
Wetenschapspark 27, 3590 Diepenbeek, Belgium
2
Soft Matter, Rheology and Technology, Faculty of Engineering Science (KU Leuven), Willem
de Croylaan 46, 3001 Leuven, Belgium
*corresponding author
1
Abstract
Defect occurrence in water-assisted injection molded
products: definition and responsible formation mechanisms
S. Sannen1,2*, P. Van Puyvelde2, J. De Keyzer1,2
1
Cel Kunststoffen, Faculty of Engineering Technology (KU Leuven @ KHLim),
Wetenschapspark 27, 3590 Diepenbeek, Belgium
2
Soft 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 predefined 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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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
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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
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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
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completely enclosed within the residual wall or connected to the formed hollow core by a socalled 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.
Re 
 v DH
(1)

Herein, ρ represents the density, v the average velocity, μ the dynamic viscosity of the water
and DH the hydraulic diameter of the pipe. In the reference experiment, presented in the
second part of this study, the Reynolds number is 62 300, which validates the existence of the
turbulent water flow. A pulsating progression of the water/polymer front gives rise to a
successive deposition of a high and low amount of polymer melt onto the wall. As supported
by the study of Polynkin et al. [8], this periodic and unequal deposition of polymer creates an
irregular residual wall.
void
The holes observed within the residual wall of the final product are subdivided in air- and
water-filled voids (Figure 3b). Both void types mainly occur at the outer side of curved
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sections. The air-filled voids are typically smaller and completely enclosed within the residual
wall. The larger water-filled voids are more spread out in the longitudinal direction of the
wall, where they are either completely enclosed or connected to the formed hollow core by a
micro-channel.
The air-filled voids seem to originate from volumetric shrinkage of the polymer melt
during secondary water penetration. During this phase, the polymer between the solidified
inner and outer wall is further cooled down. This temperature decrease is accompanied with
volumetric changes of the melt, which possibly leave holes enclosed within the residual wall.
This formation mechanism is expected because these defects are small, completely enclosed
in the wall and mainly occur at the outer side of a curve, where a higher amount of polymer
melt is present.
The water-filled voids appear to be formed by penetration of the water through the
solidified inner wall, which can occur during primary as well as secondary water penetration.
During primary water penetration, it seems that the water pushes through the solidified inner
wall at the water bubble front. At this point, water is ‘sprayed’ into the polymer melt, whereby
a part of the inner wall is ripped off. By the continuous water/polymer progression, the water
is subsequently displaced towards the wall while the solidified inner wall at the bubble front
recovers. This encapsulated water can further form steam, by local pressure variations or by
the release of the pressure at the end of the cycle. The force of the accompanied expansion in
combination with the still high temperature of the polymer in the wall can hence enlarge the
present void in the longitudinal direction. The voids which form this way are completely
enclosed within the residual wall. The existence of the described mechanism is seen in Figure
4. In this picture, the ‘skin’ at the start of the hollow core proves the presence of a torn part of
the penetrated solidified inner wall, which further resulted in the formation of the water-filled
void. Voids originating from the penetration through the solidified inner wall during
secondary water penetration on the other hand can be distinguished by the presence of a
micro-channel (visible in Figure 3b). After the water presses through the wall, it will penetrate
further into the wall where it compensates the polymer melt volumetric shrinkage. Here, a
pressure reduction/release can also cause steam to form, which enlarges the void in the
longitudinal direction. The voids, created during primary and secondary penetration, are thus
filled with water. It is however possible that this water evaporates and/or (partially) escapes
through the micro channel.
double wall
The presence of more than one hollow space in the intended hollow core appears at the start
as well as the end of the final product. From the experiments it is seen that double wall
generally forms at one of both locations, whereas the combination in the same product (as
observed in Figure 3c) only rarely occurs.
Double wall at the start of the product seems to be formed by the penetration of water
through the solidified inner wall during primary water penetration. This mechanism is also
proposed for the formation of water-filled voids. For double wall, the mechanism is however
present in a greater extent, which is clear from the difference in severity between both defect
types. The penetration of the solidified inner wall by the water at the water bubble front is
larger for double wall. In this way, more water is sprayed in the polymer melt and
subsequently displaced towards the wall by the continuous water/polymer progression. The
higher amount of enclosed water gives rise to more steam and accordingly to a larger
expansion when the pressure changes, by local variations or by the release at the end of the
cycle. The wall is thus locally blown up so that the solidified inner wall is pushed into the
formed hollow core. The torn part of the penetrated solidified inner wall and the
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accompanying occurrence of double wall is observed in Figure 5. The comparison between
Figure 4 and Figure 5 clearly indicates that the extent of the penetration determines the
occurring defect type.
At the end of the product, the presence of more than one hollow space appears to originate
from the penetration of water through the solidified inner wall during secondary water
penetration. Double wall at this location is thus also formed in a similar way as seen for a
water-filled void. But for double wall, the formation mechanism is again present in a larger
extent, causing a more severe part defect. Here, a higher amount of water penetrates further
into the wall, after the solidified inner wall has been penetrated. Hence, more steam forms by
local pressure variations or the release at the end of the cycle. The accompanying expansion
subsequently pushes the inner wall into the formed hollow core. The penetration point
through the solidified inner wall, corresponding to the micro-channel as observed for void,
could however not be retrieved in the experiments. The absence of this point can be explained
by the subsequent expansion of the enclosed water or by the cut of the product along the
longitudinal axis.
Double wall at the start or end of the product should thus be a water-filled defect. But, as
described for water-filled voids, the water can vanish by evaporation and/or (partially) leakage
through micro channels.
no residual wall
The absence of a normal-shaped inner wall in combination with solidified polymer melt in the
intended hollow core occurs at the start, end or throughout the whole product.
The presence of no residual wall seems to originate from penetration of the water through
the solidified inner wall. The fact that there is no clear built residual wall indicates that the
penetration has to take place during primary water penetration, because the residual wall is
normally built during this phase (phase 3). It thus appears that during the primary water
penetration the water pushes through the solidified inner wall at the water bubble front, which
is similar to the formation of water-filled voids and double wall at the start of the product. For
no residual wall, the penetration at the water front is so large that the entire solidified inner
wall is torn apart. All the water hence passes through the penetration point, leaving the
complete inner wall behind. The further penetration of the water in the polymer melt is
therefore disturbed, which gives rise to an unstable water/polymer flow. Here, it is possible
that the water is sprayed into the polymer melt or exhibits a turbulent flow behavior. The
water and polymer are thus mixed up. Due to the existing instability, a new solidified inner
wall at the water bubble front is prevented to build up or is immediately penetrated again. In
this way, the final product consists partially or completely of an uneven inner wall, in which
the intended hollow core is filled with solidified polymer melt. In Figure 3d it is seen that at
some locations the wall is blown up by enclosed water and penetrated solidified inner walls
are left behind. Comparing no residual wall with a water-filled void and double wall at the
start of the product, it can be concluded that the proposed formation mechanism is here
present in a larger extent.
discussion
The different defect types and their accompanying formation mechanism, as observed in the
final product of the current experimental setup and discussed in the previous paragraphs, are
summarized in Table 6. Herein, void and double wall are further subdivided by their nature
and location respectively.
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The table and preceding paragraphs elucidate that the nature as well as the extent of the
responsible formation mechanism decides on the actual defect type. From the performed
experiments it is clear that the presence and extent of these mechanisms is controlled by the
water and polymer properties/conditions which exist during primary and/or secondary water
penetration. The nature and relation between these properties/conditions determines the force
exerted by the water as well as the resistance provided by the polymer. The
properties/conditions vary throughout the product and change with the process settings and
properties of the applied material. In this way, the different defect types occur at particular
locations (at the start, end, throughout or at the curved sections of the final product) and can
(dis)appear under influence of a parameter variation. Important water and polymer
properties/conditions and the influence of the process and material parameters on the
occurrence of part defects will be clarified in section 3.2, which is done within a pre-defined
reference experiment.
3.2 Influence of process and material parameters
reference experiment
In Figure 6, a representative example of the defect occurrence within the reference experiment
(Table 5) is presented. From the picture it is clear that in this experiment double wall at the
start of the product occurs.
As explained earlier, this defect is thought to originate from the penetration of water
through the solidified inner wall at the water bubble front during primary water penetration,
which is followed by an expansion of the water enclosed in the wall.
The presence of the described formation mechanism and the corresponding defect depends
in general on the nature and relation between the water and polymer properties/conditions
during water penetration. Here, the relation determines the penetration through the solidified
inner wall as well as the extent of the subsequent expansion. At the start of the product, the
presence of double wall can be attributed to the high velocity and/or low temperature of the
water at the beginning of water penetration. These conditions give respectively rise to a high
force of the water and a more brittle solidified inner wall, which both can induce the proposed
formation mechanism.
water temperature
Changes in defect occurrence under influence of the water temperature variation within the
reference experiment are seen from the comparison of Figure 6 and Figure 7a. This
comparison indicates that for an increased water temperature, from 20 to 50°C, double wall at
the start of the product disappears and small air-filled voids at the last curved section start to
develop.
The observed variation in defect occurrence for an increased water temperature can be
attributed to the lower cooling capacity of the warmer water, during the primary as well as
secondary water penetration. The slower cooling results in a higher extensibility of the
developed solidified inner wall. This property prevents the water from penetrating through the
inner wall at the water bubble front during primary water penetration. Double wall at the start
of the product is thus no longer formed. In addition, the slower cooling gives, for the applied
semi-crystalline material, rise to a more quiescent crystallization. During secondary water
penetration, the polymer melt between the solidified inner and outer wall hence undergoes
higher volumetric changes. As a consequence, small air-filled voids are being formed. These
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voids are enclosed in the residual wall at the outer side of the last curved section. At this
location, the heated water and large amount of accumulated polymer melt between the
solidified inner and outer wall result in high volumetric shrinkage, which cannot be
compensated by the applied holding pressure.
melt temperature
For a decrease of the melt temperature within the reference experiment, from 250 to 200°C,
double wall at the start of the product is prevented. This observation is clear from the
comparison of Figure 6 and Figure 7b.
The observed influence of melt temperature on the presence of the part defect can be
attributed to a variation of the polymer properties during primary water penetration. A lower
melt temperature is accompanied with a more strong and stiff solidified inner wall behind
which the polymer melt exhibits a higher viscosity. Both properties hinder the penetration of
the water through the solidified inner wall at the water bubble front. The formation of double
wall at the start of the product is hence avoided.
It should be remarked that a change of melt temperature could be accompanied with a
variation in the obtained thickness of the residual wall. This variation is possible when the
viscosity changes dominate the building of the residual wall, as explained in an earlier study
[20]. This remark is important because a variation of the thickness can have an influence on
the occurrence of part defects as well. The residual wall thickness determines the amount of
accumulated polymer melt between the solidified inner and outer wall. This amount of melt is
decisive for the volumetric shrinkage and penetration through the solidified inner wall during
secondary water penetration so that the accompanying defects possibly form. For the applied
melt temperature variation in the current reference frame it is however seen that the thickness
of the residual wall does not change.
water holding pressure
The comparison of Figure 6 and Figure 7c points out that an increase of the water holding
pressure within the reference experiment, from 50 to 220 bar, gives rise to a more severe
double wall at the start as well as water-filled voids at the end of the product.
The observed changes in defect occurrence under influence of the water holding pressure
are unambiguously related to the level of the water pressure applied during secondary water
penetration. With the higher pressure, the water is able to penetrate through the solidified
inner wall. Here, the water intrudes further to compensate the occurring polymer volumetric
shrinkage. By pressure variations/release at the end of the cycle, the enclosed water possibly
forms steam, which enlarges the defect in the longitudinal direction. In this way, water-filled
voids at the end of the product are developed. These voids mainly occur at the end because
here the heated water gives rise to a less strong and stiff solidified inner wall behind which the
polymer melt exhibits a higher volumetric shrinkage as well as lower viscosity. The presence
of a micro-channel, which connects the void with the formed hollow core, is also seen in
Figure 7c. Furthermore, it is assumed that the more severe double wall at the start of the
product is also related to the secondary water penetration, whereas this type of defect
normally forms during primary water penetration. This reasoning can be explained by the fact
that the amount of water, which is already enclosed in the wall during primary water
penetration, is further supplemented during secondary water penetration: with the higher
holding pressure, the water is able to penetrate through the solidified inner wall so that the
wall during this phase is further filled with water. The higher amount of water gives rise to
more steam and accordingly to a larger expansion, by pressure variations or the release at the
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end of the cycle. The wall is hence more blown up so that a bigger hole at the start of the
product is formed.
presence/absence of nucleating agents in the polymer melt
The solidification behavior of the polymer melt is in this experiment modified by the addition
of nucleating agents, converting PP 515A into PP 515AN. These agents serve as nuclei on
which nucleation and subsequent crystallization takes places. It is known that this
heterogeneous crystallization under quiescent conditions in general results in a higher
crystallization temperature, possibly an increased degree of crystallinity and a higher amount
of smaller spherulites [21]. The thermal properties of PP 515A and PP 515AN are present in
Table 4, in which a higher crystallization temperature and slightly increased degree of
crystallinity for PP 515AN is observed. The described variation in crystallization behavior by
the addition of nucleation agents is accompanied with a modification of the mechanical
properties of the material. The nucleated materials are more strong, stiff and exhibit an
increased extensibility, which is attributed to the higher amount and smaller size of the
formed spherulites [22]. During processing on the other hand, materials are typically
subjected to large deformations and high temperature changes within a short time frame. For
the WAIM process, the crystal lamella hence orient in the direction of flow instead of folding
into the aforementioned spherulites. This orientation mainly occurs at the solidified inner and
outer wall, where high shear and fast cooling rates are present. The appearance of these
oriented lamella lead to an increased strength and stiffness of the material in the direction of
flow. When nucleating agents are added, highly oriented structures such as ‘shish-kebab’ are
able to form in the inner and outer wall, because their energy barrier to develop is lowered by
the addition of the agents [23]. The presence of these structures thus leads to a further
improvement of the material its strength and stiffness in the flow direction.
Within the reference experiment, the addition of nucleating agents gives rise to air-filled
voids in or around curved sections, whereas double wall at the start of the product is
prevented. This finding is seen by the comparison of Figure 6 and Figure 7d.
The observed variations in defect occurrence by the addition of nucleating agents can be
attributed to the above-described differences in solidification behavior. The earlier start of
crystallization and the possible presence of highly oriented structures for the nucleated
material give rise to a thicker solidified inner wall which is more strong, stiff and extensible.
These properties prevent the water from penetrating through this inner wall at the water
bubble front during primary water penetration. Double wall at the start of the product is thus
no longer able to form. In addition, the increased degree of crystallinity for the nucleated
material is accompanied with higher volumetric changes of the polymer melt between the
solidified inner and outer wall. As a consequence, air-filled voids enclosed within the residual
wall are being formed. Here, the voids are mainly present in or around curved sections. At the
thicker region of these curves, the higher amount of accumulated polymer melt results in a
larger extent of the volumetric shrinkage, which is not compensated by the applied water
pressure. The presence of the increased crystallization degree is visible by the more intense
white color in the middle of the residual wall (Figure 7d), especially in and around the curved
sections.
discussion
The influence of the process and material parameters on the nature and location of part defects
observed within the reference experiment of the current experimental setup, are summarized
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in Table 7. The responsible formation mechanism and accompanying water and polymer
properties/conditions are herein indicated for each parameter variation.
Taking the reference setting as starting point, double wall at the start of the product disappears
for a higher water temperature, lower melt temperature and the addition of nucleating agents.
These parameter variations imply a change in the mechanical properties of the solidified inner
wall, its thickness or the nearby polymer melt viscosity. With these new properties/conditions,
the penetration of water through the solidified inner wall at the water bubble front during
primary water penetration is prevented. Although, the higher water temperature and addition
of nucleating agents are accompanied with larger volumetric changes of the polymer melt so
that air-filled voids in or around curved sections are being formed. On the contrary, double
wall at the start of the product becomes larger when a higher holding pressure is applied. This
higher pressure enhances the penetration of water through the solidified inner wall during
secondary water penetration, by which water-filled voids at the end of the product start to
develop as well.
From these results it is seen that under influence of a parameter variation it is possible that
a particular defect type disappears while another type starts to develop. This observation
makes clear that the influence of a single parameter as such cannot be determined. Since all
parameters of the experimental setup interact, the starting point will decide the amount and
the type(s) of the part defect(s) that (dis)appears when a parameter variation is applied. The
use of other experimental setups among the studies available in literature, hence explains the
contradictory results on the single influence of the process and material parameters on the
occurrence of part defects. For example, an increase of melt temperature is in the results of
Polynkin et al. [8] and Mulvaney et al. [10] accompanied with more voids, whereas in the
results of Liu and Lin [13] for the higher melt temperature less fingering is observed. The
importance of a reference experiment against which the influence can be defined is herewith
underlined. In addition, the presence as well as the extent of the responsible formation
mechanisms is within this study related to the water and polymer properties/conditions
existing during water penetration. Compared to literature, this relation reveals new insights
into the formation of part defects. Although, the exact value and consequently the most
crucial property/condition for the occurrence of a formation mechanism is not known. For this
purpose, the analysis of part defects should be done with more profound techniques compared
to the applied qualitative defect analysis.
4 Conclusion
In the first part of this study, the current insights into the occurring part defects within the
final product of the used experimental setup were presented. These insights consist of a clear
definition of the different defect types and responsible formation mechanisms which are
unambiguously designated. In the second part of this study, process and material parameters
were varied in a pre-defined reference experiment to investigate their influence on the nature
and location of the occurring part defects, for which a qualitative defect analysis was applied.
The proposed definitions and mechanisms for the different defect types were herewith used to
explain the observed changes in defect occurrence under influence of the applied parameter
variation.
Irregular residual wall, void, double wall and no residual wall are the four different defect
types that occur in the final product of the current experimental setup, which differ in nature
and/or location along the formed or intended hollow core. The mechanisms responsible for
12
Text
the formation of these defects either 1) change or 2) have a variable extent in accordance with
the occurring defect type. The presence as well as the extent of a mechanism depends on the
nature and relation between the properties/conditions of the water and polymer existing
during water penetration. These properties/conditions determine both the force exerted by the
water and the resistance provided by the polymer, which vary with the location in the product
as well as the applied process and material parameters. The new and more complete insights
when compared to literature originate from earlier and systematic experiments on the
formation of part defects, which enable a proper evaluation of the nature and formation of the
occurring defect types.
A variation of the water and melt temperature, water holding pressure and the presence of
nucleating agents in the polymer melt, within the pre-defined reference setting, reveals that
the current insights into the occurring part defects are able to explain the observed changes in
defect occurrence. The proposed definitions and mechanisms are hence valid to describe the
formation of part defects. In addition to literature, decisive water and polymer
properties/conditions responsible for the presence of the formation mechanisms and
accordingly the accompanying part defect, could herewith be allocated. The definition of the
reference frame is herein required since the nature and interaction of all parameters
determines the amount and the type(s) of the part defect(s) that (dis)appear under influence of
a parameter variation. In this way, the influence of a single parameter as such cannot be
determined. The influence of other parameters or similar parameters within another reference
frame should thus be further explored.
In conclusion: with the new and more complete insights when compared to literature,
defects occurring in the final product can be defined and subsequently explained with
responsible formation mechanisms. The principle mechanisms behind the occurrence of part
defects are thus enclosed within the proposed definitions and mechanisms, leading to a more
fundamental understanding of the WAIM process. Experimentally observed changes in defect
occurrence can hence be explained physically and contradictions in literature can be
elucidated. In this way, products free from defects can be produced, which gives rise to a
more controllable as well as predictable WAIM process.
5 Acknowledgments
This work on WAIM was a corporation of Cel Kunststoffen (research group KU Leuven @
KHLim) and Soft Matter, Rheology and Technology (research unit of KU Leuven). The
authors would like to thank Sabic® for supplying and compounding the polymer grades, the
Agency for Innovation by Science and Technology (IWT) for funding the research by a Ph.D.
grant and the European regional development fund (ERDF) for providing financial support.
vragen/bedenkingen:
doorgehaalde tekst
= deze kan eventueel worden verwijderd maar dient eventueel ter verduidelijking?
inleiding
= paragraaf 3
Jozefien: ‘deze kan worden verwijderd’
Sofie: ‘deze dient eventueel als algemene inleiding om het probleem (= defecten) te
kaderen’
13
Text
= verklaringen voor de tegenstrijdige resultaten uit de literatuur was eerder toegevoegd aan
de 4de paragraaf
The current unclearness concerning the occurring part defects can, in addition to the low amount of available research, be attributed to
the differences between the performed studies. The studies usually use another experimental setup and apply different
visualization/quantification methods. The interaction between the components of the applied setup (material, process, mold design)
therefore has an influence on both the nature and formation of part defects. Among the performed studies other types and amounts of
defects can hence form, so that the defect type and corresponding formation mechanism are differently evaluated.
= dit staat normaal gezien over de gehele tekst weergegeven!
= ok of de verklaring laten staan aan het einde van de 4de paragraaf?
conclusie
= inhoud: te gelijkend/te herhalend met de ‘discussie’ en ‘inleiding’?
eventueel:
 de discussie gaan beperken tot de eerste paragraaf (en daarbij de titel veranderen
naar samenvatting) en de resterende informatie gaan verwerken in de
eindconclusie
 de eerste paragraaf van de eindconclusie laten vallen
6 References
[1]
Michaeli, W. In Injection Molding: Technology and Fundamentals, Hanser, 2009.
[2]
Liu, S. J. International Polymer Processing 2009, 24, 315-325.
[3]
Michaeli, W.; Brunswick, A.; Pohl, T. C. Kunststoffe 1999, 89, 62-65.
[4]
Sannen, S.; Keyzer, J. D.; Puyvelde, P. V. International Polymer Processing 2011, 26,
551-559.
[5]
Sannen, S.; Munck, M. D.; Puyvelde, P. V.; Keyzer, J. D. International Polymer
Processing 2012, 27, 602-616.
[6]
Michaeli, W., Institut für Kunststoffverarbeitung: Aachen 2009.
[7]
Gründler, M. Internationales Kunststofftechnisches Kolloquium, Aachen, 2010, pp 1623.
[8]
Polynkin, A.; Bai, L.; Pittman, J. F. T.; Sienz, J. Polymer Processing Annual Meeting,
Salerno (Italy), 15-19 June 2008.
[9]
Daios, D. In Metalurgy and Materials Engineering, Katholieke Universiteit Leuven
Leuven 2002, p 51.
[10] Mulvaney-Johnson, L.; Brown, E.; Coates, P.; Polynkin, A.; Bai, L.; Pittman, J. F. T.;
Sienz, J.; Brookshaw, B.; Vinning, K.; Butler, J. Polymer Processing Society Annual
Meeting Salerno (Italy), 15-19 June 2008.
[11] Liu, S. J.; Lin, C. H. Journal of Reinforced Plastics and Composites 2007, 26, 14411454.
[12] Liu, S. J.; Lin, S. P. Advances in Polymer Technology 2006, 25, 98-108.
14
Text
[13] Lin, K. Y.; Liu, S. J. Polymer Engineering and Science 2009, 49, 2257-2263.
[14] Liu, S. J.; Lin, S. P. Composites: Part A 2005, 36, 1507-1517.
[15] Hua, Z.; Yinglong, C.; Zengmeng, Z.; Huayong, Y. Chinese Journal of Mechanical
Engineering 2012, 25, 430-438.
[16] Michaeli, W.; Lettowsky, C.; Grönlund, O. Kunststoffe/Plast Europe 2005, 95, 87-91.
[17] Michaeli, W.; Brunswick, A.; Pfannschmidt, O. Kunststoffe Plast Europe 2002, 92, 9498.
[18] Yang, J. G.; Zhou, X. H.; Niu, Q. International Journal of Advanced Manufacturing
Technology 2012, 67, 367-375.
[19] Liu, S. J.; Chen, W. K. Plastics, Rubber and Composites 2004, 33, 206-266.
[20] Sannen, S.; Munck, M. D.; Puyvelde, P. V.; Keyzer, J. D. 1st SPE EUROTEC
Conference, Lyon, 4-5 July 2013, pp 455-460.
[21] Bernland, K. M., ETH Zurich: Zurich, 2010.
[22] Ehrenstein, G. E. Polymeric Materials: Structure, Properties, Applications; Hanser:
Munich, 2001.
[23] Zheng, G. Q.; Jia, Z.; Liu, X.; Liu, B.; Zhang, C.; Dai, K.; Shao, C.; Zheng, X.; Liu, C.;
Cao, W.; Chen, J.; Peng, X.; Li, Q.; Shen, S. Polymer Engineering and Science 2012,
52, 725-732.
15
Figure captions
a)
b)
c)
d)
Figure 1: Visualization of the most commonly observed types of part defects within the crosssection of the final product, as observed in literature. These defects are generally designated
as a) irregular wall [8] b) fingering [11] c) void [10] and d) double wall [7].
Overspill
Core
R15
R50
Ø25
R25
R75
Melt inlet
Water inlet
Figure 2: Front view of the mold cavity used in the WAIM experiments in this work, with
indication of dimensions (in mm) and mold components.
16
Figure captions
inlet
a)
inlet
b)
inlet
c)
inlet
d)
Figure 3: Visualization of the different defect types which occur in the longitudinal crosssection of the products produced in the current experimental setup. The defects are defined as
a) irregular residual wall, b) void, c) double wall and d) no residual wall.
17
Figure captions
Figure 4: Visualization of a water-filled void at the start of the product, which is formed by
the penetration of water through the solidified inner wall during primary water penetration.
Figure 5: Visualization of a double wall at the start of the product, which is formed by the
penetration of water through the solidified inner wall during primary water penetration.
Figure 6: Defect occurrence in the reference experiment.
18
Figure captions
a)
b)
c)
d)
Figure 7: Defect occurrence for which a) the water temperature (from 20 to 50°C), b) the melt
temperature (from 250 to 200°C), c) the water holding pressure (from 50 to 220 bar) and d)
the presence of nucleating agents in the polymer melt (from PP 515A to PP 515AN) is
changed in comparison to the reference experiment.
19
Table captions
Polymer melt behavior
Water behavior
Viscosity increase under influence of water Flow stability [15]
cooling [8, 12, 14, 15]
Volumetric shrinkage [9, 12, 14]
Nature of the flow (viscosity mismatch with the
polymer [2, 13, 11], tearing the polymer [16])
Pulsating flow [9]
Enclosures in the wall, which possibly evaporate
[8, 9, 10]
A push through and penetration into the wall,
possibly flowed by evaporation [6, 7, 13, 16]
Vapor formation [6, 7]
Table 1: Summary of the mechanisms existing in literature to explain the formation of the
different types of part defects.
Process parameter
Part defects
Water injection delay time (s)
Water volume flow rate (l/min)
Water pressure (bar)
Water holding time (s)
Water holding pressure (bar)
Water temperature (°C)
Short shot size (%)
Melt temperature
Mold temperature
↓
↓
↓
[12, 14]
[17]
[13]
↓
[9, 17]
↓
[13]
↓[12, 13, 14]
↓
[13]
↓
[13]
↑
[13]
↑
[18]
↑ [12, 14, 18]
↑ [6, 7, 13]
↑
[12, 14]
↑ [8, 10, 14]
↑ [9, 12, 14]
↑: an increase of the process parameter leads to more part defects
↓: an increase of the process parameter leads to less part defects
Table 2: Summary of the qualitative influence of process parameters on the occurrence of part
defects within WAIM products, as reported in literature.
Material parameter
Part defects
Shear viscosity (Pa s)
Surface energy (J/m²)
Temperature dependence of viscosity (1/K)
Shrinkage
Filler
↓
↑
↑
↑
↑
[15, 17]
[6, 7, 16]
[8]
[9, 12, 14, 17]
[9, 19]
↑: an increase of the material parameter leads to more part defects
↓: an increase of the material parameter leads to less part defects
Table 3: Summary of the qualitative influence of material parameters on the occurrence of
part defects within WAIM products, as reported in literature.
20
Table captions
Material
Melt temperature Tm (°C)
Crystallization temperature Tc (°C) at
PP 515A
PP 515AN
167
121
167
131
110
1660
0.34
115
1680
0.32
10°C/min
Heat of crystallization ΔHc (J/g) at 10°C/min
Zero-shear viscosity η0 (Pa s) at 190°C
Power law index n (-) at 190°C
Table 4: Thermal and rheological properties of PP 515A and 515AN, which are used in the
WAIM experiments in this work.
Parameter
Phase 1
material
melt temperature (°C)
Phase 2
injection delay time (s)
Phase 3
volume flow rate (l/min)
water temperature (°C)
Phase 4
time (s)
holding pressure (bar)
Reference
Variation
515A
250
515AN
200
2.5
/
30
20
/
50
3.0
50
/
220
Table 5: Reference and variation value of the process and material parameters in the
performed WAIM experiments.
Defect
Mechanism
Irregular residual wall (start/throughout)
Unstable primary water penetration, consisting of
turbulent water flow and/or pulsating
water/polymer front
Volumetric shrinkage
Penetration through the solidified inner wall (primary
or secondary penetration), possibly followed by
expansion of water enclosed in the wall
Penetration through the solidified inner wall (primary
penetration), followed by expansion of water enclosed
in the wall
Penetration through the solidified inner wall
(secondary penetration), followed by expansion of water
enclosed in the wall
Penetration through the solidified inner wall (primary
penetration), possibly followed by expansion of water
enclosed in the wall
Void: air-filled (curved sections)
Void: water-filled (curved sections)
Double wall: start
Double wall: end
No residual wall (start/end/throughout)
21
Table captions
Table 6: Summary of the mechanisms responsible for the formation of the four different
defect types in the final product of the current experimental setup, in which they are further
subdivided by their nature and/or location.
Experiment
Defect
Mechanism
Water/polymer
properties/conditions
Reference
Double wall
Penetration through the solidified
inner wall during primary water
penetration, followed by
expansion of water enclosed in
the wall
Volumetric shrinkage
High water velocity
Brittle solidified inner wall
(start of the
product)
Water
temperature
Void
(20 → 50°C)
(last curved
sections)
Melt Temperature
/
/
Double wall
Penetration through the solidified
inner wall during secondary
water penetration, followed by
expansion of water enclosed in
the wall
Volumetric shrinkage
(250 → 200°C)
Water holding
pressure
(50 → 220 bar)
(start of the
product)
Voids
(end of the
product)
Nucleating agents
Voids
(unfilled → filled)
(in or around
curved sections)
Tough solidified inner wall
Large polymer melt
volumetric changes
Strong and stiff solidified
inner wall
High polymer melt
viscosity
High water pressure
Strong, stiff and tough
solidified inner wall
Large polymer melt
volumetric changes
Table 7: Summary of the influence of the process and material parameters on the nature and
location of part defects within the pre-defined reference experiment, for which the responsible
formation mechanism and the accompanying water and polymer properties/conditions are
indicated.
22