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Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding
G Model
CIRP-1835; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding M. Sorgato a, D. Masato a, G. Lucchetta (2)a, L. Orazi (2)b,* a b
Department of Industrial Engineering, University of Padova, Italy Department of Sciences and Methods for Engineering, University of Modena and Reggio Emilia, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser micro machining Injection moulding Wall slip
In injection moulding, high pressure is required to fill the mould, due to the viscosity of thermoplastic polymers, the reduced thickness of the cavity and the low mould temperature. In this work, significant pressure reduction was achieved inducing the slip of the polymer melt over the mould surface by means of Laser-Induced Periodic Surface Structures (LIPSS). In particular, the slipping of molten PET was investigated as a function of nano-structuring orientation and injection velocity. The results demonstrate that LIPSS parallel to flow induce strong wall slip of the polymer melt, allowing a maximum reduction of the injection pressure of 23%. © 2018 CIRP. Published by Elsevier Ltd. All rights reserved.
1. Introduction Precision manufacturing of plastic parts by injection moulding has recently been driven down in size and weight, especially for packaging and electronic applications. Decreasing the main thickness of injection moulded parts is an effective driver for cost saving and overall environmental impact reduction, due to lower resource consumption and shorter cycle times [1]. However, in injection moulding, high pressure is required to fill the mould, due to the viscosity of thermoplastic polymers, the reduced thickness of the cavity and the low mould temperature [2]. Significant reduction of the injection pressure could be achieved by inducing the slip of the polymer melt over the mould surface [3]. The resulting drag decrease would allow the design of thinner parts without compromising the achievement of complete filling of the mould cavity. In injection moulding, for a non-slip polymer/wall interface, the non-isothermal melt flow is characterized by an increase of the wall shear stress for increasing shear rates [4]. However, when the wall shear stress exceeds a critical value, the onset of the wall-slip phenomenon has been experimentally observed [5]. The slip mechanism, in the case of polymer melts, occurs within the first monolayer of macromolecular chains adsorbed at the wall. Under flow conditions, the adsorbed macromolecules are pulled by the entanglements with those in the bulk, and they are stretched in the flow direction. For increasing shear stress, the macromolecules in the bulk progressively disentangle and slip over those adsorbed at the mould surface [6]. Low surface energy coatings can suppress polymer adsorption and promote slip, but they have limited
* Corresponding author. E-mail address: [email protected] (L. Orazi).
longevity [7]. Recently, laser ablation has been used to affect the slip of polymer melts by varying the surface roughness. However, all the proposed nano-patterned surfaces resulted in a decrease of wall slip and a consequent raise of injection pressure [8]. Laser Induced Periodic Surface Structures (LIPSS) are well known surface structures generated by ablation [9]. Recently, the use of new, reliable and efficient ultra-short laser sources has allowed significant improvement of their production rate. Hence, an increasing number of applications have been developed using sub-micron ripples to modify the tribological, wetting and adhesion properties of surfaces [10]. LIPSS have also been used to treat the surface of injection moulds in order to produce hydrophobic plastic parts [11]. However, to guarantee high replication of the sub-micron features the mould surface temperature has to be higher than the polymer glass transition temperature (for amorphous polymers) or melting temperature (for semicrystalline ones) [12]. In this work, the effect of differently oriented LIPSS on the melt flow resistance of polyethylene terephthalate (PET) was experimentally investigated. The slippage of polymer melt over the cold, nano-structured cavity surface was modelled to understand the effect of the laser treatment on the filling pressure. 2. Experimental 2.1. Mould design The cavity considered for this analysis is an open flow channel characterized by a length of 47.5 mm, a width of 6 mm and three alternative thickness values of 1, 1.45 and 1.9 mm. The modular mould assembly was designed to mount inserts with different surface nano-structures, on both its moving and fixed half (Fig. 1). The thickness of the channel was varied using different front plates on the moving side insert.
https://doi.org/10.1016/j.cirp.2018.04.102 0007-8506/© 2018 CIRP. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
G Model
CIRP-1835; No. of Pages 4 M. Sorgato et al. / CIRP Annals - Manufacturing Technology xxx (2018) xxx–xxx
2
Fig. 1. Schematic representation of the moving half of the mould.
The use of flat surfaces will not limit the applicability of the method to free-form mould cavities. Algorithms and procedures for laser texturing of free-form surfaces was already demonstrated in literature and 5 axes laser texturing systems are nowadays commercially available. 2.2. Mould surface treatments and characterization Mould inserts were treated using ultrashort laser nanopatterning, allowing the generation of LIPSS on the steel surface. Before the laser treatment, mould inserts were ground to obtain surface roughness (Sa) smaller than 0.3 mm. This preparation of the surface is compatible with the manufacturing process usually adopted for injection moulds of preforms. A femtosecond laser source (Coherent HyperRapid NX), operating at the fundamental harmonic of 1064 nm with pulse duration of about 8 ps, was used to create ripples on the cavity surfaces. The generated beam was deflected by a scanner (Raylase Focusshifter CS) equipped with a 160 mm focal length lens. The focused spot, measured with a beam analyser, presented a diameter of about 30 mm. The setup allows control of the polarization plane direction, thus control of LIPSS orientation with respect to the direction of the polymer melt flow. The periodic nano-structures were created on both sides of the mould cavity using a repetition rate of 1 MHz, scanning speed of 2500 mm/s and a step of 10 mm between scan lines. The laser mean power was set to 16 W, resulting in pulse energy of 16 mJ. The surfaces of different mould inserts were treated in two orthogonal directions to obtain topographies with ripples aligned in the flow direction (parallel LIPSS — P) and transversal to the flow direction (transversal LIPSS — T), respectively. The morphology of the nano-structures was evaluated using Scanning Electron Microscopy (SEM — FEI, Quanta 400). Their regularity was characterized considering their local orientation, which was evaluated using the Dispersion of the LIPSS Orientation Angle (DLOA) method [13]. This procedure is implemented in the open-source software ImageJ and is based on the Riesz Filters structure tensor analysis from the Orientation Distribution plugin. For each analysis, the half width at half maximum of the angle distribution was extracted to obtain the DLOA. The topography was characterized using Atomic Force Microscopy (AFM — Veeco Digital Instruments, CP II). The instrument was equipped with a silicon sharp tip, which had curvature radius of 6 nm, cantilever length of 125 mm, width of 30 mm and thickness of 2 mm. For each LIPSS treatment, both mould inserts were characterized in three 10 mm 10 mm areas along the flow direction.
The effects of different LIPSS orientation and cavity thickness were characterized by monitoring the cavity pressure drop in injection moulding experiments, conducted using a micro injection moulding machine (Wittmann-Battenfeld, MicroPower 15). The surface temperature of the cavity was set and controlled using a chiller directly connected to the mould cooling circuits and two thermocouples, one for each mould half. Two piezoelectric pressure transducers (Kistler, 6182C) were used to measure the pressure during cavity filling at 5.5 and 40 mm from the injection location, respectively. The piezoelectric charge signals were acquired, processed and logged using a charge amplifier (Kistler, 5039A), a 16-bit analog input module (National Instruments, NI 9205) and a software specifically coded in Labview. The acquisitions were performed with a number of samples of 10 at a rate of 80,000 s1 (i.e. with a time step of about 0.1 ms). The acquired signals are characterized by a linear growth of the pressure during cavity filling. When the flow front reaches the open end of the cavity, the two pressure values stabilize to a steady-state value, due to the equilibrium between heat convected by the injected polymer melt and that remove by conduction through the mould. The effect of the laser treatment on the flow resistance was analysed considering the pressure drop in the channel, which was evaluated as the difference between the steady-state values of the pressure signals acquired in the two locations. 2.4. Experimental approach The shear rate of the polymer melt was experimentally varied by modifying the cavity thickness (t) and the injection speed (Vinj). The latter was varied from 200 to 600 mm/s. The other process parameters were fixed for all the experiments (Table 1). To ensure the stability of the injection phase, 20 cycles were performed before the acquisition of the cavity pressure. For each moulding condition 10 acquisitions were collected, one every 5 cycles. Table 1 Processing conditions for the experiments. Parameter
Unit
Value
Mould temperature Melt temperature Injection speed
15 300 200, 300, 400, 500, 600
C C mm/s
3. Modelling For a straight, rectangular channel having length L, width w, and thickness h, with the assumptions of a fully developed steady state laminar flow with no-slip on the wall, the apparent shear rate and real shear stress in the slit model are given by [3]:
g_ wðappÞ ¼
6Q 2
wh
ð1Þ
and
t wðrealÞ ¼
wh DP 2ðw þ hÞ L
ð2Þ
where Q is the volumetric flow rate and DP the pressure drop. The Graetz number (Gz) was calculated for each experimental run to evaluate the ratio between the heat convection along the flow direction and the heat conduction in the transverse one: 2
uh aL
2.3. Injection moulding setup
Gz ¼
The polymer used is a semi-crystalline polyethylene terephthalate (PET, Cepsa PET SR08), which has a Melt Flow Index (200 C — 5 kg) of 6. The material was dried for 8 h at 180 C before moulding, to achieve residual humidity smaller than 50 ppm.
where u and a are the average velocity and the thermal diffusivity of the polymer melt, respectively. Gz can be used as a measure of the anticipated thickness of the skin layer. Higher values of Gz indicate the formation of thinner skin layers and no skin develops
ð3Þ
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
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CIRP-1835; No. of Pages 4 M. Sorgato et al. / CIRP Annals - Manufacturing Technology xxx (2018) xxx–xxx
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if Gz > 100 [14]. This latter condition is satisfied for all experimental runs except the one conducted at Vinj = 200 mm/s and t = 1 mm. Therefore, in the calculation of the shear stress at wall, which neglects the thickness of the skin layer, this specific data point was not considered. For a steady-state, laminar, incompressible non-Newtonian flow under wall slip, the slip velocity, us, which is assumed to be solely a function of the wall shear stress, can be calculated from the following expression [15]:
g_ wðappÞ ¼ g_ wðappÞ;s þ
6us h
ð4Þ
where g_ wðappÞ;s is the apparent shear rate corrected for the effect of slip, which was determined by calculating the deviation of the flow curve affected by wall slip from the no-slip flow curve [6]. 4. Results and discussion 4.1. LIPSS characterization The topography of the nano-structured mould inserts was initially evaluated using SEM, showing good overall regularity of the ripples (Fig. 2). Average values of dispersion of the orientation angle of 48 for insert T and of 26 for insert P were calculated using the DLOA method, as shown in Fig. 3. This indicates overall better regularity for the ripples in insert P compared to insert T. In fact, the grinding texture in insert T is perpendicular to the LIPSS direction, leading to higher presence of bifurcation points.
Fig. 2. SEM micrographs of the mould surfaces with the differently oriented nanostructures.
Fig. 4. Experimental measurements of cavity pressure drop for the different LIPSS orientation as a function of injection speed and cavity thickness.
Fig. 4. Conversely, when they are transverse to the polymer melt flow, LIPSS have negligible effect on drag reduction. However, the effect of the treatment varies with cavity thickness. In particular, significant average reductions of 23% and 7% were observed with cavity thickness of 1.45 and 1 mm, respectively. A negligible effect was observed for cavity thickness of 1.9 mm. These results indicate that the polymer flow in the thinner cavities is affected by wall slip. Indeed, the decrease of pressure drop with increasing flow rate observed with insert P, shown in Fig. 4, clearly indicates that the rise of slip velocity with flow rate prevails over the frictional forces in the flow. The smaller effect of the parallel ripples on drag reduction observed, despite the higher shear stresses, with a cavity thickness of 1 mm, indicates that the interactions at the polymer/tool are stronger compared to the 1.45 mm thick geometry. In fact, the higher cavity pressure leads to higher adsorption of the macromolecules at the mould surface, which results in lower slip velocity. The negligible effect of LIPSS on the melt flow resistance for the thicker cavity, suggests that with a thickness of 1.9 mm the shear stresses are too low to promote the onset of wall slip. Moreover, this indicates that the laser treatment and the generated nanostructures do not modify the thermal boundary condition at the polymer/mould interface. 4.3. Wall slip
Fig. 3. Normalized distribution of the orientation angle for inserts T and P.
Using the deviations from the no-slip flow curve (Eq. (4)), the slip velocity values associated to the mould surface treated with parallel LIPSS were calculated for cavity thicknesses of 1.45 and 1 mm. Fig. 5 shows the calculated slip velocities as a function of the shear stress at wall for the mould insert with parallel LIPSS at
The AFM measurements showed that ripples on Insert T and P are characterized by similar dimensions. In particular, an average periodicity of 780 nm for the nano-structures was measured, with a standard deviation of 60 nm. The average depth of the troughs was of 220 nm with a standard deviation of 50 nm. Thus, the grooves on mould topography are characterized by an aspect ratio of about 0.3. 4.2. Drag reduction The pressure drop results at different injection speed show that LIPSS effectively reduce the melt flow resistance when the nanostructures are aligned along the flow direction, as shown in
Fig. 5. Slip velocity of PET as a function of wall shear stress and mould cavity thickness for the LIPSS parallel to flow.
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
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M. Sorgato et al. / CIRP Annals - Manufacturing Technology xxx (2018) xxx–xxx
different values of cavity thickness. The two sets of data are shifted and differently inclined due to the different conditions of pressure and shear rate. The linearity and the high values of both slip velocity and shear stress at wall indicate that, for both values of cavity thickness, the melt flow is in a strong-slip regime [16]. In order to understand the peculiar influence of LIPSS parallel to flow on the onset of strong wall slip, it is important to consider the physical mechanisms at the origin of the slip at the wall, as presented in the scientific literature [6]. For high molecular weight polymer melts (as PET), the wall slip phenomenon occurs within the first monolayer of macromolecules adsorbed at the wall. In fact, these chains form loops and attach to the wall at several sites along their backbone. These chains are also entangled, at several sites depending on the entanglement density, with those in the bulk. For increasing shear stress, the macromolecules at the wall deform and stretch in the flow direction. Above a critical shear stress value, which depends on the density of adsorbed chains and on their capability to deform under flow, the macromolecules in the bulk suddenly disentangle from those absorbed at the wall. This leads to the onset of strong slip at this newly formed polymer/ polymer interface [8]. In injection moulding, the surface of the cavity is maintained at temperature values much lower than the polymer melting point and the macromolecules that contact the mould during filling are not able to enter the LIPSS troughs [12]. Hence, in the case of a wall surface nano-structured with ripples, the macromolecules are adsorbed near the crests (Fig. 6(a)). According to this mechanism, the untreated surface is characterized by higher density of adsorbed chains (Fig. 6(b)), and thus by higher polymer/wall interface interactions. Therefore, the untreated mould surface is characterized by higher value of the critical shear stress for the onset of strong wall slip. However, the different orientation of LIPSS on mould inserts does not modify the density of adsorbed chains. Both nano-structured surfaces, compared to the untreated mould, are characterized by less interaction with cavity walls due to the presence of absorption points on ripples peaks only. Under strong flow conditions, transversal ripples obstruct the deformation of the macromolecules adsorbed on ripples crests by counteracting their bending (Fig. 6(c)). Conversely, parallel LIPSS allow higher deformation of the adsorbed chain loops due to the lack of support given by the near empty troughs, as schematized in Fig. 6(d). Hence, drag reduction
Fig. 6. Schematics of macromolecules adsorption on (a) ripples crests and on (b) the untreated surface. Points of adsorption at cavity wall and entanglement with bulk chains are represented with blue and red dots, respectively. (c) Support effect of the transversal LIPSS and (d) bending mechanism of chain loops in troughs for the parallel LIPSS.
is higher with the parallel ripples due to the lower shear stress required to disentangle the macromolecules in the bulk from those adsorbed at the wall. 5. Conclusions This work focused on the effects of laser-induced periodic surface structures on the filling flow resistance in injection moulding of PET. The results from this study showed, for the first time in the literature, that periodic nano-structures aligned along the flow direction can significantly reduce the critical shear stress for the onset of strong wall slip in injection moulding. Low mould temperature and sub-micron periodicity are essential conditions to restrain macromolecules, which contact the mould during filling, from entering the LIPSS troughs and, therefore, to decrease the density of adsorbed chains. Moreover, LIPSS orientation plays a fundamental role in triggering strong wall slip: LIPSS parallel to flow allow higher deformation of the adsorbed chain loops, due to the lack of support given by the empty troughs, promoting the disentanglement of the macromolecules in the bulk. The obtained maximum pressure reduction of 23%, due to the high values of slip velocity, could allow the design of thinner plastic parts. The consequently lower resource consumption and shorter cycle times would bring significant cost saving and reduced environmental impact.
References [1] Lucchetta G, Bariani PF (2010) Sustainable Design of Injection Moulded Parts by Material Intensity Reduction. CIRP Annals—Manufacturing Technology 59 (1):33–36. [2] Lucchetta G, Bariani PF, Knight WA (2006) A New Approach to the Optimization of Blends Composition in Injection Moulding of Recycled Polymers. CIRP Annals—Manufacturing Technology 55(1):465–468. [3] Lucchetta G, Masato D, Sorgato M, Crema L, Savio E (2016) Effects of Different Mould Coatings on Polymer Filling Flow in Thin-wall Injection Moulding. CIRP Annals—Manufacturing Technology 65(1):537–540. [4] Zhang N, Gilchrist MD (2012) Characterization of Thermo-rheological Behavior of Polymer Melts During the Micro Injection Moulding Process. Polymer Testing 31(6):748–758. [5] Masato D, Sorgato M, Babenko M, Whiteside B, Lucchetta G (2018) Thin-wall Injection Molding of Polystyrene Parts with Coated and Uncoated Cavities. Materials & Design 141:286–295. [6] Hatzikiriakos SG (2012) Wall Slip of Molten Polymers. Progress in Polymer Science 37:624–643. [7] Sanchez-Reyes J, Archer LA (2003) Interfacial Slip Violations in Polymer Solutions: Role of Microscale Surface Roughness. Langmuir 19:3304–3312. [8] Ebrahimi M, Konaganti VK, Moradi S, Doufas AK, Hatzikiriakos SG (2016) Slip of Polymer Melts over Micro/nano-patterned Metallic Surfaces. Soft Matter 12:9759–9768. [9] Bonse J, Höhm S, Kirner SV, Rosenfeld A, Krüger J (2017) Laser-induced Periodic Surface Structures—a Scientific Evergreen. IEEE Journal of Selected Topics in Quantum Electronics 23(3):1–15. [10] Orazi L, Gnilitskyi I, Pavlov I, Serro AP, Ilday S, Ilday FO (2015) Nonlinear Laser Lithography to Control Surface Properties of Stainless Steel. CIRP Annals— Manufacturing Technology 64(1):193–196. [11] Wu PH, Cheng CW, Chang CP, Wu TM, Wang JK (2011) Fabrication of Large-area Hydrophobic Surfaces with Femtosecond-laser-structured Molds. Journal of Micromechanics and Microengineering 21(11):115032. [12] Bekesi J, Kaakkunen JJJ, Michaeli W, Klaiber F, Schoengart M, Ihlemann J, Simon P (2010) Fast Fabrication of Super-hydrophobic Surfaces on Polypropylene by Replication of Short-pulse Laser Structured Molds. Applied Physics A 99:691– 695. [13] Gnilitskyi I, Derrien TJY, Levy Y, Bulgakova NM, Mocek T, Orazi L (2017) Highspeed Manufacturing of Highly Regular Femtosecond Laser-induced Periodic Surface Structures: Physical Origin of Regularity. Scientific Reports 7(1):8485. [14] Bay RS, Tucker III CL. (1992) Fiber Orientation in Simple Injection Moldings. Part 2: Experimental Results. Polymer Composites 13:332–341. [15] Mooney M (1931) Explicit Formulas for Slip and Fluidity. Journal of Rheology 2:210–222. [16] Mhetar V, Archer LA (1998) Slip in Entangled Polymer Melts. 1. General Features. Macromolecules 31:8607.
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
CIRP-1835; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding M. Sorgato a, D. Masato a, G. Lucchetta (2)a, L. Orazi (2)b,* a b
Department of Industrial Engineering, University of Padova, Italy Department of Sciences and Methods for Engineering, University of Modena and Reggio Emilia, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser micro machining Injection moulding Wall slip
In injection moulding, high pressure is required to fill the mould, due to the viscosity of thermoplastic polymers, the reduced thickness of the cavity and the low mould temperature. In this work, significant pressure reduction was achieved inducing the slip of the polymer melt over the mould surface by means of Laser-Induced Periodic Surface Structures (LIPSS). In particular, the slipping of molten PET was investigated as a function of nano-structuring orientation and injection velocity. The results demonstrate that LIPSS parallel to flow induce strong wall slip of the polymer melt, allowing a maximum reduction of the injection pressure of 23%. © 2018 CIRP. Published by Elsevier Ltd. All rights reserved.
1. Introduction Precision manufacturing of plastic parts by injection moulding has recently been driven down in size and weight, especially for packaging and electronic applications. Decreasing the main thickness of injection moulded parts is an effective driver for cost saving and overall environmental impact reduction, due to lower resource consumption and shorter cycle times [1]. However, in injection moulding, high pressure is required to fill the mould, due to the viscosity of thermoplastic polymers, the reduced thickness of the cavity and the low mould temperature [2]. Significant reduction of the injection pressure could be achieved by inducing the slip of the polymer melt over the mould surface [3]. The resulting drag decrease would allow the design of thinner parts without compromising the achievement of complete filling of the mould cavity. In injection moulding, for a non-slip polymer/wall interface, the non-isothermal melt flow is characterized by an increase of the wall shear stress for increasing shear rates [4]. However, when the wall shear stress exceeds a critical value, the onset of the wall-slip phenomenon has been experimentally observed [5]. The slip mechanism, in the case of polymer melts, occurs within the first monolayer of macromolecular chains adsorbed at the wall. Under flow conditions, the adsorbed macromolecules are pulled by the entanglements with those in the bulk, and they are stretched in the flow direction. For increasing shear stress, the macromolecules in the bulk progressively disentangle and slip over those adsorbed at the mould surface [6]. Low surface energy coatings can suppress polymer adsorption and promote slip, but they have limited
* Corresponding author. E-mail address: [email protected] (L. Orazi).
longevity [7]. Recently, laser ablation has been used to affect the slip of polymer melts by varying the surface roughness. However, all the proposed nano-patterned surfaces resulted in a decrease of wall slip and a consequent raise of injection pressure [8]. Laser Induced Periodic Surface Structures (LIPSS) are well known surface structures generated by ablation [9]. Recently, the use of new, reliable and efficient ultra-short laser sources has allowed significant improvement of their production rate. Hence, an increasing number of applications have been developed using sub-micron ripples to modify the tribological, wetting and adhesion properties of surfaces [10]. LIPSS have also been used to treat the surface of injection moulds in order to produce hydrophobic plastic parts [11]. However, to guarantee high replication of the sub-micron features the mould surface temperature has to be higher than the polymer glass transition temperature (for amorphous polymers) or melting temperature (for semicrystalline ones) [12]. In this work, the effect of differently oriented LIPSS on the melt flow resistance of polyethylene terephthalate (PET) was experimentally investigated. The slippage of polymer melt over the cold, nano-structured cavity surface was modelled to understand the effect of the laser treatment on the filling pressure. 2. Experimental 2.1. Mould design The cavity considered for this analysis is an open flow channel characterized by a length of 47.5 mm, a width of 6 mm and three alternative thickness values of 1, 1.45 and 1.9 mm. The modular mould assembly was designed to mount inserts with different surface nano-structures, on both its moving and fixed half (Fig. 1). The thickness of the channel was varied using different front plates on the moving side insert.
https://doi.org/10.1016/j.cirp.2018.04.102 0007-8506/© 2018 CIRP. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
G Model
CIRP-1835; No. of Pages 4 M. Sorgato et al. / CIRP Annals - Manufacturing Technology xxx (2018) xxx–xxx
2
Fig. 1. Schematic representation of the moving half of the mould.
The use of flat surfaces will not limit the applicability of the method to free-form mould cavities. Algorithms and procedures for laser texturing of free-form surfaces was already demonstrated in literature and 5 axes laser texturing systems are nowadays commercially available. 2.2. Mould surface treatments and characterization Mould inserts were treated using ultrashort laser nanopatterning, allowing the generation of LIPSS on the steel surface. Before the laser treatment, mould inserts were ground to obtain surface roughness (Sa) smaller than 0.3 mm. This preparation of the surface is compatible with the manufacturing process usually adopted for injection moulds of preforms. A femtosecond laser source (Coherent HyperRapid NX), operating at the fundamental harmonic of 1064 nm with pulse duration of about 8 ps, was used to create ripples on the cavity surfaces. The generated beam was deflected by a scanner (Raylase Focusshifter CS) equipped with a 160 mm focal length lens. The focused spot, measured with a beam analyser, presented a diameter of about 30 mm. The setup allows control of the polarization plane direction, thus control of LIPSS orientation with respect to the direction of the polymer melt flow. The periodic nano-structures were created on both sides of the mould cavity using a repetition rate of 1 MHz, scanning speed of 2500 mm/s and a step of 10 mm between scan lines. The laser mean power was set to 16 W, resulting in pulse energy of 16 mJ. The surfaces of different mould inserts were treated in two orthogonal directions to obtain topographies with ripples aligned in the flow direction (parallel LIPSS — P) and transversal to the flow direction (transversal LIPSS — T), respectively. The morphology of the nano-structures was evaluated using Scanning Electron Microscopy (SEM — FEI, Quanta 400). Their regularity was characterized considering their local orientation, which was evaluated using the Dispersion of the LIPSS Orientation Angle (DLOA) method [13]. This procedure is implemented in the open-source software ImageJ and is based on the Riesz Filters structure tensor analysis from the Orientation Distribution plugin. For each analysis, the half width at half maximum of the angle distribution was extracted to obtain the DLOA. The topography was characterized using Atomic Force Microscopy (AFM — Veeco Digital Instruments, CP II). The instrument was equipped with a silicon sharp tip, which had curvature radius of 6 nm, cantilever length of 125 mm, width of 30 mm and thickness of 2 mm. For each LIPSS treatment, both mould inserts were characterized in three 10 mm 10 mm areas along the flow direction.
The effects of different LIPSS orientation and cavity thickness were characterized by monitoring the cavity pressure drop in injection moulding experiments, conducted using a micro injection moulding machine (Wittmann-Battenfeld, MicroPower 15). The surface temperature of the cavity was set and controlled using a chiller directly connected to the mould cooling circuits and two thermocouples, one for each mould half. Two piezoelectric pressure transducers (Kistler, 6182C) were used to measure the pressure during cavity filling at 5.5 and 40 mm from the injection location, respectively. The piezoelectric charge signals were acquired, processed and logged using a charge amplifier (Kistler, 5039A), a 16-bit analog input module (National Instruments, NI 9205) and a software specifically coded in Labview. The acquisitions were performed with a number of samples of 10 at a rate of 80,000 s1 (i.e. with a time step of about 0.1 ms). The acquired signals are characterized by a linear growth of the pressure during cavity filling. When the flow front reaches the open end of the cavity, the two pressure values stabilize to a steady-state value, due to the equilibrium between heat convected by the injected polymer melt and that remove by conduction through the mould. The effect of the laser treatment on the flow resistance was analysed considering the pressure drop in the channel, which was evaluated as the difference between the steady-state values of the pressure signals acquired in the two locations. 2.4. Experimental approach The shear rate of the polymer melt was experimentally varied by modifying the cavity thickness (t) and the injection speed (Vinj). The latter was varied from 200 to 600 mm/s. The other process parameters were fixed for all the experiments (Table 1). To ensure the stability of the injection phase, 20 cycles were performed before the acquisition of the cavity pressure. For each moulding condition 10 acquisitions were collected, one every 5 cycles. Table 1 Processing conditions for the experiments. Parameter
Unit
Value
Mould temperature Melt temperature Injection speed
15 300 200, 300, 400, 500, 600
C C mm/s
3. Modelling For a straight, rectangular channel having length L, width w, and thickness h, with the assumptions of a fully developed steady state laminar flow with no-slip on the wall, the apparent shear rate and real shear stress in the slit model are given by [3]:
g_ wðappÞ ¼
6Q 2
wh
ð1Þ
and
t wðrealÞ ¼
wh DP 2ðw þ hÞ L
ð2Þ
where Q is the volumetric flow rate and DP the pressure drop. The Graetz number (Gz) was calculated for each experimental run to evaluate the ratio between the heat convection along the flow direction and the heat conduction in the transverse one: 2
uh aL
2.3. Injection moulding setup
Gz ¼
The polymer used is a semi-crystalline polyethylene terephthalate (PET, Cepsa PET SR08), which has a Melt Flow Index (200 C — 5 kg) of 6. The material was dried for 8 h at 180 C before moulding, to achieve residual humidity smaller than 50 ppm.
where u and a are the average velocity and the thermal diffusivity of the polymer melt, respectively. Gz can be used as a measure of the anticipated thickness of the skin layer. Higher values of Gz indicate the formation of thinner skin layers and no skin develops
ð3Þ
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
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if Gz > 100 [14]. This latter condition is satisfied for all experimental runs except the one conducted at Vinj = 200 mm/s and t = 1 mm. Therefore, in the calculation of the shear stress at wall, which neglects the thickness of the skin layer, this specific data point was not considered. For a steady-state, laminar, incompressible non-Newtonian flow under wall slip, the slip velocity, us, which is assumed to be solely a function of the wall shear stress, can be calculated from the following expression [15]:
g_ wðappÞ ¼ g_ wðappÞ;s þ
6us h
ð4Þ
where g_ wðappÞ;s is the apparent shear rate corrected for the effect of slip, which was determined by calculating the deviation of the flow curve affected by wall slip from the no-slip flow curve [6]. 4. Results and discussion 4.1. LIPSS characterization The topography of the nano-structured mould inserts was initially evaluated using SEM, showing good overall regularity of the ripples (Fig. 2). Average values of dispersion of the orientation angle of 48 for insert T and of 26 for insert P were calculated using the DLOA method, as shown in Fig. 3. This indicates overall better regularity for the ripples in insert P compared to insert T. In fact, the grinding texture in insert T is perpendicular to the LIPSS direction, leading to higher presence of bifurcation points.
Fig. 2. SEM micrographs of the mould surfaces with the differently oriented nanostructures.
Fig. 4. Experimental measurements of cavity pressure drop for the different LIPSS orientation as a function of injection speed and cavity thickness.
Fig. 4. Conversely, when they are transverse to the polymer melt flow, LIPSS have negligible effect on drag reduction. However, the effect of the treatment varies with cavity thickness. In particular, significant average reductions of 23% and 7% were observed with cavity thickness of 1.45 and 1 mm, respectively. A negligible effect was observed for cavity thickness of 1.9 mm. These results indicate that the polymer flow in the thinner cavities is affected by wall slip. Indeed, the decrease of pressure drop with increasing flow rate observed with insert P, shown in Fig. 4, clearly indicates that the rise of slip velocity with flow rate prevails over the frictional forces in the flow. The smaller effect of the parallel ripples on drag reduction observed, despite the higher shear stresses, with a cavity thickness of 1 mm, indicates that the interactions at the polymer/tool are stronger compared to the 1.45 mm thick geometry. In fact, the higher cavity pressure leads to higher adsorption of the macromolecules at the mould surface, which results in lower slip velocity. The negligible effect of LIPSS on the melt flow resistance for the thicker cavity, suggests that with a thickness of 1.9 mm the shear stresses are too low to promote the onset of wall slip. Moreover, this indicates that the laser treatment and the generated nanostructures do not modify the thermal boundary condition at the polymer/mould interface. 4.3. Wall slip
Fig. 3. Normalized distribution of the orientation angle for inserts T and P.
Using the deviations from the no-slip flow curve (Eq. (4)), the slip velocity values associated to the mould surface treated with parallel LIPSS were calculated for cavity thicknesses of 1.45 and 1 mm. Fig. 5 shows the calculated slip velocities as a function of the shear stress at wall for the mould insert with parallel LIPSS at
The AFM measurements showed that ripples on Insert T and P are characterized by similar dimensions. In particular, an average periodicity of 780 nm for the nano-structures was measured, with a standard deviation of 60 nm. The average depth of the troughs was of 220 nm with a standard deviation of 50 nm. Thus, the grooves on mould topography are characterized by an aspect ratio of about 0.3. 4.2. Drag reduction The pressure drop results at different injection speed show that LIPSS effectively reduce the melt flow resistance when the nanostructures are aligned along the flow direction, as shown in
Fig. 5. Slip velocity of PET as a function of wall shear stress and mould cavity thickness for the LIPSS parallel to flow.
Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102
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different values of cavity thickness. The two sets of data are shifted and differently inclined due to the different conditions of pressure and shear rate. The linearity and the high values of both slip velocity and shear stress at wall indicate that, for both values of cavity thickness, the melt flow is in a strong-slip regime [16]. In order to understand the peculiar influence of LIPSS parallel to flow on the onset of strong wall slip, it is important to consider the physical mechanisms at the origin of the slip at the wall, as presented in the scientific literature [6]. For high molecular weight polymer melts (as PET), the wall slip phenomenon occurs within the first monolayer of macromolecules adsorbed at the wall. In fact, these chains form loops and attach to the wall at several sites along their backbone. These chains are also entangled, at several sites depending on the entanglement density, with those in the bulk. For increasing shear stress, the macromolecules at the wall deform and stretch in the flow direction. Above a critical shear stress value, which depends on the density of adsorbed chains and on their capability to deform under flow, the macromolecules in the bulk suddenly disentangle from those absorbed at the wall. This leads to the onset of strong slip at this newly formed polymer/ polymer interface [8]. In injection moulding, the surface of the cavity is maintained at temperature values much lower than the polymer melting point and the macromolecules that contact the mould during filling are not able to enter the LIPSS troughs [12]. Hence, in the case of a wall surface nano-structured with ripples, the macromolecules are adsorbed near the crests (Fig. 6(a)). According to this mechanism, the untreated surface is characterized by higher density of adsorbed chains (Fig. 6(b)), and thus by higher polymer/wall interface interactions. Therefore, the untreated mould surface is characterized by higher value of the critical shear stress for the onset of strong wall slip. However, the different orientation of LIPSS on mould inserts does not modify the density of adsorbed chains. Both nano-structured surfaces, compared to the untreated mould, are characterized by less interaction with cavity walls due to the presence of absorption points on ripples peaks only. Under strong flow conditions, transversal ripples obstruct the deformation of the macromolecules adsorbed on ripples crests by counteracting their bending (Fig. 6(c)). Conversely, parallel LIPSS allow higher deformation of the adsorbed chain loops due to the lack of support given by the near empty troughs, as schematized in Fig. 6(d). Hence, drag reduction
Fig. 6. Schematics of macromolecules adsorption on (a) ripples crests and on (b) the untreated surface. Points of adsorption at cavity wall and entanglement with bulk chains are represented with blue and red dots, respectively. (c) Support effect of the transversal LIPSS and (d) bending mechanism of chain loops in troughs for the parallel LIPSS.
is higher with the parallel ripples due to the lower shear stress required to disentangle the macromolecules in the bulk from those adsorbed at the wall. 5. Conclusions This work focused on the effects of laser-induced periodic surface structures on the filling flow resistance in injection moulding of PET. The results from this study showed, for the first time in the literature, that periodic nano-structures aligned along the flow direction can significantly reduce the critical shear stress for the onset of strong wall slip in injection moulding. Low mould temperature and sub-micron periodicity are essential conditions to restrain macromolecules, which contact the mould during filling, from entering the LIPSS troughs and, therefore, to decrease the density of adsorbed chains. Moreover, LIPSS orientation plays a fundamental role in triggering strong wall slip: LIPSS parallel to flow allow higher deformation of the adsorbed chain loops, due to the lack of support given by the empty troughs, promoting the disentanglement of the macromolecules in the bulk. The obtained maximum pressure reduction of 23%, due to the high values of slip velocity, could allow the design of thinner plastic parts. The consequently lower resource consumption and shorter cycle times would bring significant cost saving and reduced environmental impact.
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Please cite this article in press as: Sorgato M, et al. Effect of different laser-induced periodic surface structures on polymer slip in PET injection moulding. CIRP Annals - Manufacturing Technology (2018), https://doi.org/10.1016/j.cirp.2018.04.102