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Mechanism-based wear models for plastic injection moulds
Wear 440–441 (2019) 203105
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Mechanism-based wear models for plastic injection moulds �pez-Ortega a, E. Fuentes a, R. Bayo �n a, B. Zabala a, *, X. Fernandez a, J.C. Rodriguez a, A. Lo a b A. Igartua , F. Girot a
IK4-TEKNIKER, Calle I~ naki Goenaga 5, 20600, Eibar, Spain University of the Basque Country, UPV-EHU, Faculty of Engineering, Department of Mechanical Engineering, IKERBASQUE, Basque Foundation for Science, Alameda Urquijo S/n, 48013, Bilbao, Spain
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Plastic injection moulding Moulds Wear Friction Erosion Abrasion Corrosion
Injection moulding is a single-step manufacturing process for plastic parts that require very precise dimensions, geometries and low part roughness (Ra 0.2–0.025 μm). The high cost of moulds (typ. > 100,000 €) and unac ceptable wear of their cavities can limit the cost competitiveness of the process. Mould cavity surfaces are deteriorated by different wear mechanisms which may drastically reduce the quality of injection moulded parts, interrupt their production, raise maintenance and repair costs, or delay delivery. Therefore, a better under standing of the wear mechanisms and the main parameters affecting mould wear is needed. In this research, three major failure mechanisms were studied: abrasion and erosion (due to fibre-reinforcements in the plastics), and corrosion (due to gas release from the polymers). Testing protocols were developed for each failure mechanism. Abrasion was simulated with a block-on-plate tribological test, erosion was evaluated in an air-jet erosion tester combined with a gravel-o-meter test, and corrosion was simulated using electrochemical techniques. Three different thermoplastic materials and several mould surface solutions (i.e., mould steel and four alternative surface treatments) were tested. Furthermore, this paper presents an innovative approach to simulate the behaviour of mould and plastic materials at the laboratory scale, in order to minimise the cost associated to mould failures, through mould lifetime prediction. The prediction, with some limitations has been made based on the model generated through laboratory tests, by using a multiparameter design of experiments, where different working conditions (such as pressure or speed) and plastic and mould materials were evaluated. A single best solution was not found to resist all failure mechanisms, but useful recommendations for potential solutions for each failure mechanism were proposed. The TiN coating applied by Physical Vapour Deposition represents a good compromise between wear (abrasive and erosion) and corrosion resistance, especially for fibre reinforced plastics. Ni-PTFE coating is proposed to reduce the friction and improve the corrosion resistance of the mould materials when high wear resistance is not needed, as in the case of non-reinforced plastics.
1. Introduction Injection moulding is a single-step manufacturing process for plastic parts that require very precise dimensions, geometries and low part roughness (Ra 0.2–0.025 μm). The high cost of moulds (typically above 100,000 €) and unacceptable wear of their cavities can limit the competitiveness of the process. The occurrence of failures involves the production interruption, reparation of the mould, and even the change of the injected material and reestablishment of the injection process parameters. This problem supposes an over cost and a waste of resources (e.g., raw material and energy). The costs associated with the injection machine shutdown can reach 3000 €/day.
A current challenge in injection moulds materials is the necessity to resist high injection pressures (>100 bar) and temperatures (150� C300 � C) of the polymers and additives. This is particularly important when the polymer contains glass fibre reinforcements, which are very hard and abrasive leading to noticeable wear of the mould. Other ad ditives like titanium oxide (TiO2), used as a white pigment, can generate a similar problem [1,2]. According to literature [3,4], this kind of hard additives can lead to two different wear mechanisms, depending on the flux conditions: (i) erosion in mould regions where there is a frontal impact of the plastic flux, or (ii) abrasion mechanism when the flux is parallel to the mould surface. From these studies, it can be extracted that the phenomena
* Corresponding author. E-mail address: [email protected] (B. Zabala). https://doi.org/10.1016/j.wear.2019.203105 Received 9 October 2018; Received in revised form 9 October 2019; Accepted 26 October 2019 Available online 31 October 2019 0043-1648/© 2019 Elsevier B.V. All rights reserved.
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depend principally on: (i) the location in the mould due to variable flux motion in a complex geometry, (ii) the injection conditions and (iii) the properties of the mould material. The overheating of plastic materials can lead to gas release resulting in a local corrosion of the mould. Regarding the chemical composition of polymers, Polyamide (PA) can release NH3, and Polyvinyl chloride (PVC) releases Hydrochloric acid (HCl). Additives like flame retardants usually contain halogens (F, Cl, Br) that can release the corresponding acids being highly corrosive for steel moulds [5,6]. Other polymers could release other types of acids [7]. The high aesthetic requirements of the parts are one of the main reasons that make necessary the avoidance of any kind of wear scar in the cavity surface, since it can be copied in the surface of the injected part. In the mould, the polymer molecules and fibres are typically oriented in the flow direction, but during cavity filling they are perpendicular to the flow. The wall friction makes the speed at the extreme of the fibre and molecules in the centre to be higher than those near the walls. In turn, the flow speed difference results in a shear that rotates the mole cules and fibres until they are properly oriented in the flux direction. However, speed in the centre is more uniform, and the orientation of the reinforcements is not modified. The shear rate generated in the mould is caused by the high speed applied to the injected polymer, and the reduction of the speed due to the friction with the mould surface. This friction depends on the surface of the mould and the type of plastic and makes more difficult, the filling process. To avoid incomplete filling, it is necessary to increase the speed and pressure of injection, generating high shear rate. Therefore, these measures can lead to several problems in plastic parts such as flow marks, burns, or flashes [8]. The friction between mould and polymer can also generate problems during the part ejection. Higher friction will require larger ejection force, resulting in ejection marks in the parts. In this context, testing procedures need to be developed to reproduce the friction and wear phenomena adequately at laboratory scale. Ideally, a test could be designed on an actual test mould in a real injection environment. Nevertheless, the risk of damaging an expensive and complex mould, and the cost of interrupting the production make this approach onerous. Consequently, it is necessary to find simple labora tory scale tests, that can simulate as close as possible the actual pro duction failure mechanisms and provide results in a reasonable period of time, in order to evaluate different material candidates and gain a better understanding about their behaviour. The abrasive mechanisms generated in PIM (Plastic Injection Moulding) mould surfaces due to glass fibres have been simulated in literature with, at least, three different testing protocols based on the modification of standard tests. The “ball cratering test” [1] is considered relatively far from the actual mould conditions, since even if the abra sive slurry of TiO2 was carefully selected, there is some influence of the metal-metal contact between the ball and the flat sample. The “rubber wheel test” [9] is considered to be closer to the actual mould conditions, since the reinforced plastic is used as abrasive in a condition near to fluid, and the sample temperatures are quite high, close to the injection temperature. In this case, some type of wear due to the metal-metal contact is expected as well. In the “block on ring test” [10,11], the metal-metal contact is avoided, but the plastic is in a solid state. The simulation of mould erosion in the laboratory has been mainly performed using solid particles with devices similar to those described in the ASTM G76-18 standard [12,13]. Rotated slurry erosion rig has also been used [7] with the aim of simulating the combination of erosion and corrosion phenomena. For the simulation of corrosion of mould surfaces, different tech niques have been reported in literature, such as “salt spray methods” [14] or “potentiodynamic polarisation tests” [9]. Another key aspect in these evaluations is the selection of an appropriate electrolyte. Different solutions have been used in literature for PIM application (5% HNO3 þ 1% HCl, 10% CH3COOH [15] or 3.5% NaCl [9]).
The mathematical modelling of PIM moulds wear is not as mature as other wear processes simulation. Two references identified tried to model the wear generated by glass fibres in the mould surface. One of them [3] built a phenomenological model, based on the indentation of fibres in a plastic flow. Even though, it is still a long way to be applicable on simulation software’s, since the validation of the values predicted would need a lot of injection tests and simulations of their conditions. The second contribution [16] is an empirical model, constructed directly from results of tests performed in full-scale injection machines. This model was built for a specific mould with specific injection conditions, correlating the number of cycles performed with the roughness gener ated due to wear scars. This approach would need to perform a set of experiments on different mould types, in order to predict their failure. Regarding corrosion, even if some tests have been performed, there is no contribution in the literature about models regarding corrosive-wear degradation mechanism. This paper presents an approach to simulate the behaviour of mould and plastic materials in PIM process at the laboratory scale, in order to minimise the cost associated to mould failures, through mould lifetime prediction. The prediction, with some limitations, can be made based on the model that has been obtained through laboratory tests, by using a multiparameter design of experiments, where different working condi tions (such as pressure or speed) and plastic and mould materials were evaluated. 2. Materials and methods 2.1. Plastics and mould materials studied Different thermoplastic materials have been used to study their behaviour against different mould materials: a non-reinforced acrylic resin (AR), a non-reinforced polyester family thermoplastic (PT), and a reinforced polyamide (r-PA). For the case of (r-PA) tests, the reinforce ment orientation percentage was checked to be the same in all the samples. The procedure for reinforcement inspection was based on organic material removal in an oven at 600 � C for 2 h. A reference mould steel material was selected, which is supplied at two different states, as base material (BM) or at pre-hardened condition (HH). Different currently available commercial surface treatment al ternatives not commonly used in PIM were considered: Salt Nitriding, Ionic Nitriding, PVD-TiN and Ni-PTFE. The hardness of the different mould surfaces was measured by Vickers durometer, and by Fischerscope H-100 for thinner films. The order from higher to lower hardness was: TiN, Ionic Nitriding, Salt Nitriding, Ni-PTFE, HH steel, and BM steel. The roughness of the samples surfaces was measured with a Perth ometer M2 (Mahr GmbH) profilometer. Table 1 shows the hardness and roughness values obtained for each treatment. The roughness mea surements showed that all the surface treatments increased the rough ness of the substrate. The microstructures of the two steels HH and BM were also investi gated. Both steels presented a Tempered Martensitic structure, as shown in Fig. 1. BM steel showed a grain size number of 4–5, while that of the Table 1 Hardness and Roughness measurements of the different surface solutions.
2
SURFACES
HARDNESS (HV)
ROUGHNESS, Ra (μm)
HH Steel HH Steel þ Ionic Nitriding HH Steel þ Salt Nitriding HH Steel þ Ni-PTFE HH Steel þ TiN BM Steel BM Steel þ Ionic Nitriding BM Steel þ Salt Nitriding BM Steel þ Ni-PTFE
275 � 4 753 � 39 586 � 12 382 � 7 1592 266 � 2 861 � 37 513 � 27 321 � 14
0.55 0.75 0.80 0.60 0.70 0.55 0.75 0.80 0.60
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HH steel was of 9–10 (0.016–0.011 mm).
maximum depth of the wear scar at the three locations along the track, as shown in Fig. 3. The volume of material loss was quantified by approaching the shape of the track to a semi-ellipsoid.
2.2. Abrasion test
2.3.2. Gravel-o-meter The gravel-o-meter device was designed and manufactured by IK4TEKNIKER, in order to evaluate the resistance of the mould material and coatings to chipping caused by the impact of gravel, screws or other flying objects. Air pressure was used to project the gravel. The test samples were weighted before and after each test, to compare the amount of material lost. The testing protocol followed was similar to ASTM D3170-14 standard [18], but using pellets (plastic raw material) as gravel. The plastic gravel was charged in a tank and discharged from there into the supply hopper thanks to a vibratory feeder. The pipe of the gun has a nozzle with a diameter of 5 mm. The target was positioned at 350 mm (See Fig. 4). The gravel was recollected after each test series to be re-used. The metal samples had a square geometry of 50 � 50mm2. Five discharges of 6 kg of raw material each time were performed. The wear of the mould material sample was quantified by mass loss mea surements in the high precision balance. r-PA, RA, PT were used as plastic materials, whereas the two reference mould steels, i.e., base and treated with the four surface treatments of the study, were tested as mould materials.
The abrasion test was performed by a general-purpose reciprocating movement tribometer (UMT3-CETR), using “Block-on-Plate” configu ration. A flat surface of mould material described a reciprocating movement sliding against a static 4 � 4 � 4mm3 block manufactured by plastic injection. The testing conditions were selected in order to simulate as close as possible the PIM moulds abrasion mechanism. The steel sample was heated to usual mould working temperature of 100 � C, while the plastic was expected to reach a temperature near the plastic melting point based on the values measured on literature [10,11], simulating similar boundary conditions as in PIM. The contact pressure between the two surfaces was selected in the range of pressures (8–18 MPa) and sliding speeds (20–50 mm/s) employed in real injection operations. Considering this value range, four tests were performed: 8 MPa at 20 and 50 mm/s, and 18 MPa at 20 and 50 mm/s. In order to ensure these speed ranges, the movement stroke was kept constant at 8 mm, while the frequency was modified. Abrasive wear tests were performed mainly on r-PA, since the wear generated on AR or PT samples was negligible. The duration of the wear tests was of 4 h. Friction tests were carried out for a duration of 5 min. In some cases where the pin wear was very high, the test was stopped slightly before the 5 min time. After the tests, the wear was evaluated by mass loss measurements in a Mettler Toledo XP205 balance of high precision (accuracy down to 0.00001 g). The wear depth was analysed in a Nikon ECLIPSE ME600 Confocal Microscope, as shown in Fig. 2. In some cases, the surface of the sample was also analysed by Field Emission Scanning Electron Micro scope (FE-SEM) in a ZEISS ULTRA plus device, equipped with Energy Dispersive X-ray Spectrometer (EDS).
2.4. Electrochemical corrosion test Electrochemical corrosion tests were performed through Electro chemical Impedance Spectroscopy (EIS) measurements at different im mersion times for several materials immersed in NH4Cl 0.5%. The selection of the electrolyte was made according to the composition of the common plastics used in injection processes. It is likely that the NHþ 3 and the Cl presented in the electrolyte appear in real application, from the gases released by the Polyamide when overheated, or for example the PVC that contains Cl-R in its chain. Fluorides and halogens (Cl, Br, F) could also appear in the plastics due to additives like flame retardants, and they are somehow represented through the presence of Cl ion in the electrolyte. The tested materials were the two reference mould steels, base and treated with the four surface treatments considered in this study. The exposed surface was 625 mm2 for all the test samples. The electrochemical setup consisted of a three-electrode electro chemical cell, connected to an Autolab-Metrohm PGSTAT30 potentio stat shown in Fig. 5. A commercial Ag/AgCl (KCl 3 M) electrode and a platinum wire of 1 mm in diameter were used as reference (RE) and counter electrodes (CE), respectively. The test sample corresponding to each material tested was connected as the working electrode (WE). The potential of the reference electrode was of 0.207 V vs Standard
2.3. Erosion tests 2.3.1. Air Jet erosion tester An Air Jet Erosion Tester TR-471 from Ducom Instruments was used for the erosion study. The device was designed to characterise erosion resistance of different materials and coatings based on ASTM G76-18 standard [17]. In these tests, a sample of 25 � 25 � 5 mm3 received the impact of a 50 μm diameter alumina erodent on compressed air flow. The studied parameters were the particle speed (32 and 64 m/s), the sample tem perature (25 and 100 � C), the impact angle (15� , 45� , and 90� ), and the mould material (BM and HH steels). The erodent discharge rate (1.8 mg/ min) and sample exposition time (10 min) were kept fixed. The wear was evaluated by confocal microscopy, measuring the
Fig. 1. Microstructure of BM (a) and HH (b) steels. 3
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Fig. 2. Sample aspect after wear test (a) and cross-section profile of the wear scar (b).
Fig. 3. Confocal measurement of the wear scar.
Fig. 5. Experimental Setup, the three electrodes, the electrochemical cell and the potentiostat.
3. Results and discussion 3.1. Abrasion tests 3.1.1. Effect of injected plastic on the friction coefficient The non-reinforced polymers (AR and PT) were found to generate very high friction against the mould surfaces, but the wear generated was negligible and could not be measured. Instead, the reinforced Polyamide exhibited low friction but generated measurable wear in most of the mould surfaces. Regarding the friction, AR and PT presented very similar friction against all the different surfaces, with a friction coefficient around 0.8, while that of r-PA was around 0.5, as it can be seen in Fig. 6. AR and PT are not considered as tribological polymers, unlike PA, which offers a lower friction coefficient. The friction-generated heat at high pressures and high speeds can increase the sliding interface temperature of a polymer to values much greater than the metal surface temperature [20], and thus the polymer can start to plasticate at metal surface temperatures appreciably below its thermodynamic glass transition temperature of all these three glassy polymers. This material condition leads to high friction coefficient corresponds to an adhesive mechanism [21], as high as the 0.8 friction coefficient of AR and PT. It has been proven in literature [22–24] that reinforcements affect tribological properties of the polymer composites to a great extent. Their friction is determined by the adhesion of the polymer matrix, abrasion by glass fibres, the temperature at the sliding interface, and friction films produced on the counter surface.
Fig. 4. Gravel-o-meter test layout.
Hydrogen Electrode (SHE), and all the potentials registered in this document are referred to this electrode. The Electrochemical Impedance Spectroscopy (EIS) measurements were performed under a sinusoidal perturbation of �10 mV amplitude and at a frequency range from 100 kHz to 1 mHz. Four EIS measurements were carried out at different immersion times: 24 h, 72 h, 168 h, and 336 h. The electrochemical behaviour of the studied surface can be described by means of Electrical Equivalent Circuits (EEC). For this aim, the experimental data was fitted by using the Randles equivalent circuit [19], where Rs represents the electrolyte resistance between the refer ence electrode and the working electrode, and Rp correspond to the polarisation resistance. The Constant Phase Element (CPE) represents the double layer capacitance at the electrolyte/steel interface. For the comparison of the corrosion resistance of the different materials, the Rp polarisation resistance has been selected.
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Fig. 6. Mean friction coefficients of the different surfaces against AR, PT and r-PA.
The unreinforced PA, without glass fibre, has a higher friction co efficient attributed to the adhesion of the PA to the steel surface. The lower friction level with glass fibres is due to the reduced adhesion by smaller polymeric contacts on the sliding surface [22]. The inorganic fibres reinforced composites show good performance decreasing friction coefficient with increasing glass fibre content until it increased slightly beyond a glass fibre content of 30% [25]. For the case of AR and PT, the Ni-PTFE was the most effective surface treatment reducing the friction coefficient compared to the bare sub strate, while the rest of the surface treatments showed higher co efficients. In Fig. 7, it can be seen that at the beginning of the test, NiPTFE started with a coefficient of friction of 0.4 for the PT and 0.6 for the AR instead of the constant 0.8 afterwards. These results are in coherence with that reported in literature, since PTFE is currently used as a lubricity agent in several applications [26]. As it can be seen in the friction curves, after rubbing certain time, the increased adhesion of the polymer, reduces progressively the effect of Ni-PTFE until equalling the friction coefficient of the other surfaces, that is led by the friction of the transfer layer of the polymer. In fact, the AR, which has a lower glass transition temperature and then enhanced adhesion tendency, has more limited operation time of the Ni-PTFE low friction, compared to the PT. The range of variability of this test was found to be below the 10%, confirming it to be considerably reproducible. In the case of r-PA, the Ni-PTFE treatment was found to be less effective. For this case, the best surface treatment was TiN, reducing the mean friction coefficient to 0.4, being 0.53 the reference BM substrate. This behaviour is related to the abrasive wear mechanism that takes place in the surface rubbing against the harder reinforced plastic. The reduction of the friction coefficient due to the fibres, is partially compensated due to the abrasive wearing of the metallic material. In fact, the hardest the metallic surface, the lower will be the indentation of the fibres (Fig. 8), limiting the abrasive mechanism, as was demon strated by the ranking of the friction coefficient from lower to higher friction of the hardest to softest of the surfaces.
inspection performed by EDS after the tests. Salt Nitriding presented more abrasive wear than the Ionic Nitriding. The former showed a wear track depth of around 4 μm and 0.35 mg of total wear, whereas the wear track depth of the latter was below 1 μm with no measurable mass loss. Both TiN and Ionic Nitriding were the only surfaces presenting negligible wear after the 4-h tests. This behaviour was ascribed to the higher hardness provided by these surface treatments. The dependence between mould material hardness’s and resulting abrasive wear is presented in Fig. 8. There is logarithmic trend showing a decrease in the material loss with increasing hardness. Above a hardness of 800 HV1, it is hardly possible to make any scratch on the mould surface, and the material loss is negligible. Regarding the different substrate materials, the result showed little influence of the substrate on the response of the system, which depended mainly on the hardness of the surface treatment employed.
3.1.2. Effect of mould material and surface treatment on abrasive wear Wear tests were performed on r-PA plastic against the mould mate rial treatment alternatives. The HH steel showed a higher wear resistance than the BM steel. The Ni-PTFE treatment on BM substrate did not improve the wear resistance of the base material. The treated sample was found to be less wear resistant than the substrate itself, losing the coating and reaching the substrate in some areas along the wear track, as confirmed by the
3.2.1. Effect of mould material, surface treatment and plastic type on erosive wear of PIM Three different plastic materials were tested against BM steel to evaluate their erosive power in the Gravel-o-meter test rig (Fig. 10). As expected, the reinforced plastic (r-PA) was found to be much more erosive than the rest of the plastic materials. The other two plastics (AR and PT), had a combination of mild erosion and adhesion. The different surface solutions were tested against the reinforced plastic (r-PA). The response of the surface treatments in the erosion tests
3.1.3. Influence of different injection parameters on the abrasive wear Wear tests were performed at all the four extreme testing conditions of pressures and speeds, for BM substrate against r-PA, and the results are presented in Fig. 9. As it was expected, exist inter-relationships among the temperature, pressure and speed dependences of the fric tional coefficients and resulting wear [20]. For the lower pressure (8 MPa), the wear was independent of the speed within the tested range. Similarly, in the tests performed at the lower speed, the wear was in dependent of the applied pressure within the tested range. Nevertheless, the increase of both pressure and speed led to higher material losses. The results showed that both speed and pressure were critical for modelling the wear of PIM moulds. There is a PV (pressure x velocity) limit that is critical for modelling the wear of PIM moulds probably due to a change in the failure mechanism. 3.2. Erosion tests
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Fig. 7. Friction coefficient evolution during the tests of the different surfaces against PT, AR and r-PA.
Fig. 8. Correlation between hardness and abrasive wear during the tests for the studied surfaces.
Fig. 9. Surface response of abrasive wear tests dependence on pressure and speed.
were in coherence with that obtained in the abrasion tests, being the ranking exactly the same, from the best to the worst-behaving solution. Abrasion and erosion can have some differential behaviour related to
hardness [27], but for the case of PIM was expected to have a parallel behaviour of these mechanisms [3]. TiN and Ionic Nitriding were the most adequate surface treatments. It is interesting to remark that the Salt 6
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Fig. 12. Correlation between hardness and erosive wear during the tests for the studied surfaces. Fig. 10. Erosive wear subjected by different plastics on the reference BM steel.
Nitriding underwent most of the wear in the first discharge, showing progressively less material loss in the following discharges, as shown in Fig. 11. This can be attributed to the two different layers that compose the nitrided surface, e.g., compound and diffusion layers, which are known to have different properties [28]. The compound layer present at the topmost surface is usually very thin and brittle, and it can be easily removed during the first batch discharge. The correlation between the hardness of the mould material and the resulting erosive wear (Fig. 12) was in coherence with that of the abrasive wear (Fig. 8). Above a hardness of 800HV1, the erosion per formed in the moulds was negligible. From the results obtained in these experiments, a model was built, which is presented at the end of the manuscript. Fig. 13. Surface response of wear depending on impact angle (X2) and impact speed (X3).
3.2.2. Influence of different injection parameters on erosive wear for PIM The Design of Experiments (DOE) for this test was performed, varying the parameters: particle speed (32, 64 m/s), sample temperature (25, 100 � C), and impact angle (15� , 45� , 90� ). The test set was fully completed for both HH and BM steels. The erosion test results in the Air Jet erosion tester showed a little influence of the different microstructure and grain size of steels and mould temperature on the wear generated. This can be explained with the little influence on steel hardness of the temperature range of PIM moulds (up to 100 � C). Fig. 13 shows the influence of the impact angle (coded as X2) and speed (coded as X3) on wear. The impact angle and speed were the most significative wear factors, being the impact speed, the parameter with bigger influence in the maximum wear depth. 3.3. Electrochemical corrosion tests
Fig. 14. Time evolution of the corrosion resistance of the different materials immersed in NH4Cl.
The corrosion resistance obtained from the EIS measurements for the different materials and treatments in NH4Cl is presented in Fig. 14, in function of the immersed time. After 24 h of immersion in the NH4Cl solution, the highest resistance was that of the salt nitriding HH, followed by the Ni-PTFE (HH and BM)
and by TiN (HH and BM). In general, the corrosion resistance of the HH substrate was slightly higher than that of the BM one, possibly favoured by their smaller grain size. The resistance of the salt nitriding HH remained similar after 72 h of immersion and further up to the 336 h, whereas the values registered for the Ni-PTFE samples significantly decreased at 72 h, reaching very similar values for both HH and BM substrate materials. Ni-PTFE main tains the corrosion resistance after this initial decrease up to the final 336 h of immersion. The resistance of the TiN on HH substrate was maintained very similar during the whole exposition time. After 168 and 336 h, the resistance of these samples was close to those of the Ni-PTFE samples. The values registered for the rest of the materials were considerably smaller. The ionic nitriding BM and HH, presented a corrosion resistance values lower than 130 Ω after every exposition time registered, from 24 up to 336 h of immersion, which were lower than the resistance regis tered for the steel substrates. Salt nitriding BM samples, did not improve
Fig. 11. Wear generated on mould material samples by r-PA plastic in different discharges. 7
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the corrosion resistance of the bare substrate neither. Regarding the corrosion mechanism, general corrosion was observed on substrate materials, while the coatings suffered more localized pitting-like corrosion. The great difference between the salt nitriding applied to BM and HH was related to the nitriding process, which presented an inhomogeneous appearance, independently of the sample to be BM or HH. In fact, the compound layer (white layer composed of hard nitrides and outer postoxidation layer) was inhomogeneous and very brittle. Probably the HH sample has few local areas that exposed directly the substrate to the electrolyte. In the regions where the treatment seems to be properly applied, the compound layer presented an adequate corrosion inhibi tion. However, in the surface where the layer was not adequately applied and the diffusion region was exposed to the electrolyte, the corrosion resistance of the treated samples was like the bare substrate. This treatment was not reliable in terms of corrosion protection. After salt nitriding HH, the second highest resistance corresponded to the Ni-PTFE for all the exposure times (HH followed by BM) and the third one to the TiN (HH followed BM). However, the values registered for these treatments at high immersion times were at least 3 times smaller in comparison with the salt nitriding HH sample. Different studies have shown that the corrosion protection of mould steels by nitriding processes can be maximized through the nitride vol ume fraction at the surface [29], and especially excels when is more composed of ε-nitride [30]. Furthermore, the post-oxidation that natu rally appears in the salt nitriding process will improve the corrosion protection significantly. The generation of these nitrides in the surface are very dependent of nitriding time and temperature [29,30], but the steel composition as well [31]. Depending on these conditions the compound layer can be very thin, while in the diffusion layer, the precipitation of chromium nitride during the nitriding treatment, generates segregation of chro mium via a precipitation hardening mechanism which leads to compromised corrosion performance (intergranular corrosion, pitting corrosion …) [31], which has most probably occurred on ionic nitriding processes used in this study. TiN has been demonstrated as an adequate solution for corrosion protection due to barrier effect, and as shown in Fig. 14, its corrosion protection is nearly independent on the exposition time and it is less dependent on the steel type than nitriding as shown in the literature [31]. The performance of this coating can be further improved in some
WHmax ðμm = hÞ ¼ 607:68 þ 10:27α þ 12:09v þ 0:1αv
� 0:13α2 � ð33:24
injection of reinforced r-PA samples. The inputs needed in this forecast would be the surface hardness of the mould material, the flow speed, and the pressure. In order to calculate the cycles to failure, an assumption of the sliding time of the polymer during the filling operation should be revised in each case (a filling time of 5 s can be considered as a reference value). Another parameter that should be defined by the end user is the maximum accepted wear depth on the mould prior to its replacement. The resulting model built from the experimental results can be written as follows: Wðμm = hÞ ¼ 5:0918
0:0597P
0:0262v þ 0:0031Pv
0:818 ln H
where: W: Abrasive wear rate in the contact area, in μm per hour P: Pressure in MPa (applicable from 8 to 18 MPa. Below the 8 MPa value no wear is expected. The pressure value is necessary to cali brate the model, and convert the pressure range on the PIM to the ones measured on the tests). v: Speed in mm/s (applicable from 20 to 50 mm/s. Below the 20 mm/ s value no wear is expected) H: Hardness in Vickers, HV (applicable up to 900 HV. Above 900 HV no wear is expected) The regression coefficient of the Analysis of Variance of the model is: R2 ¼ 0.9811. 4.2. Modelling of erosion From the results obtained in the erosion tests, a model was built to quantify the wear on moulds. The inputs needed in this model would be the surface hardness of the mould material, the flow speed, and the pressure. In order to calculate the cycles to failure, an assumption of the duration in seconds of flux impact against the mould surface should be made. Furthermore, the maximum accepted wear depth on the mould prior to its replacement must be also defined by the end user. For the specific application of an injection mould, the wear depth is probably the critical parameter, since these defects will be copied in the injected part. The dependence of the wear extent on the speed, the impact angle, and hardness showed before can be written in an equation as follows:
4:92 ln HÞ
where:
cases through a duplex treatment (combining plasma nitriding and TiN) [31]. Nickel coatings containing PTFE particles showed good protection properties because of the double nature of the coating: the outer layer includes PTFE particles (self-lubricating properties), the inner one in creases the barrier-effect [32].
WHmax: maximum wear depth rate in the centre of the eroded area, in μm/h v: flux speed in m/s (valid from 32 to 64 m/s. The speed value is necessary to calibrate the model, and convert the pressure range on the PIM to the ones measured on the tests) α: impact angle in � (valid from 15 to 90� ) H: Hardness in HV (valid from 200 to 900 HV. Above 900 HV, no wear is expected)
4. Wear modelling implementation 4.1. Modelling of abrasion Due to the small differences found in the abrasion tests regarding friction, no friction model was built. However, the results of this work can be useful as friction database to reduce the friction in moulds, to apply for example, in the cases, were friction-related problems can be expected, e.g., part ejection problems or high shear stresses that lead to burns in the final part. Regarding wear, a model was built considering the influence of the injection parameters and the hardness of the mould material for the
The regression coefficient of the Analysis of Variance of the model is: R2 ¼ 0.8053. 4.3. Modelling of corrosion Considering the results obtained in this work, the corrosion of mould materials has been found to depend on the surface characteristics. In this 8
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Table 2 Safety factors referred to the basic BM steel for PIM solutions. Mould material BM steel HH steel Ni-PTFE on BM Ni-PTFE on HH Salt Nitriding on BM Salt Nitriding on HH Ionic Nitriding on BM Ionic Nitriding on HH TiN on BM TiN on HH
Table 3 Inputs necessary for the PIM moulds wear prediction.
Safety factor 1.00 0.69 5.48 10.47 0.75 31.05 0.38 0.62 2.89 6.20
INPUT Pressure Impact speed Impact angle Mould material Hardness Basic maintenance Unacceptable wear
18 50 30 BM steel 260 600 100
MPa mm/s �
HV cycles μm
Table 4 Outputs of the model of PIM moulds wear prediction.
case, a table representing the risk of corrosion occurrence on mould surfaces has been built (Table 2). The table uses as reference the number of cycles needed to corrode the mould by the reference steel. The safety factor from this data can be calculated as follows:
OUTPUT Abrasion Erosion Corrosion
73659 16671 600
Cycles Cycles Cycles
Safety factor ¼ measured Rp of the surface (ohm) / measured Rp of the reference material (ohm) Table 5 Inputs for the mould wear prediction of the improved case.
Where Rp is the corrosion resistance of the samples obtained by electro chemical corrosion tests, in ohms.
INPUT
4.3.1. Application example The user of the mould-lifetime model can introduce the input data, as collected in Table 3 for the critical areas of the mould. The selection of the process values (impact angle, velocity, and applied pressures) can be done using injection process simulation tools or user experience. Once the cycle parameters are defined, the output of the model is the forecast of the expected wear, depending on the failure mechanism suffered by the mould as shown in Table 4. This information is relevant to schedule maintenance work of the mould and take counter measures if the wear is unacceptable. The model allows the modification of pro cess parameters or surface material characteristics. To improve this reference cycle, different steps can be made. For instance (see Table 5), the application of Ni-PTFE coating is proposed to reduce the friction and improve the corrosion resistance of the mould materials. It is scheduled that this change might allow injecting almost 5 times more cycles: > 3200 cycles instead of the 600, without surface cleaning or anti-oxidant application. Regarding abrasion and erosion wear, this surface treatment has slightly higher hardness and combined with a slight reduction of the process pressure and speed would allow to increase by a factor of 2 the time necessary for a mould repair due to abrasion or erosion wear scars (see Table 6). Notwithstanding the models developed in this work, further cali brations with the experience of end users (e.g., adding a weighting factor) and experimental trials when possible, will be needed to improve the accuracy of the proposed models.
Pressure Impact speed Impact angle Mould material Coating Basic maintenance Unaccepable wear
16 40 30 BM steel 321 600 100
MPa mm/s �
Ni-PTFE HV cycles μm
Table 6 Outputs for the mould wear prediction of the improved case. OUTPUT Abrasion Erosion Corrosion
188693 35483 3288
Cycles Cycles Cycles
� Ionic nitriding and TiN were the most promising alternatives for wear protection, in both the abrasion and erosion tests. � The behaviour of the different surface treatments was very similar for both substrates, HH and BM steels. � Friction reduction of AR and PT were mainly achieved by using an Ni-PTFE coating. TiN was the most effective in the case of r-PA. � Regarding the corrosion tests, the salt nitriding treatment provided good corrosion protection on HH steel, but the process itself was not reliable showing a high scattering in the results. The best solution was the Ni-PTFE coating, followed by the TiN that showed the most stable corrosion resistance.
5. Conclusions
Finally, from all the results obtained from laboratory tests simulating PIM conditions, different models were built. These models could be useful to evaluate the wear resistance by abrasion, erosion, and corro sion of candidate materials and coatings, and sensibility to working conditions as relative speed or pressure. The TiN coating represent a good compromise between wear and corrosion resistance, especially for fibre reinforced plastics.
In this work, the performance of two mould materials with different surface treatments has been evaluated by means of abrasion, erosion, and corrosion simulated tests. The following conclusions can be drawn: � The reinforced plastic material (r-PA) wore the moulds significantly, while the expected wear for non-reinforced plastics (PT and AR) is minimum both for abrasion and erosion mechanisms, despite the high friction they generate. � The processing parameters increasing the moulds erosive wear, were the particle speed and the impact angle increase. The parameters accelerating the abrasive wear were the increase of the relative speed and pressure. Both wear mechanisms were very sensible to the mould material hardness. The higher the hardness of the surface treatment, the lower was the wear.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
[15] C. Kerschenbauer, M.O. Speidel, G. Lichtenegger, J. Sammer, K. Sammt, Plastic Mould steels- wear resistant and corrosion resistant martensitic chromium steels, in: 6th International Tooling Conference ISBN 9189422813, 2002, pp. 203–216. ISBN 9189422813. [16] A. Pereira, P. Hern� andez, J. Martínez, J.A. P�erez, T. Mathia, Study of morphology wear model of moulds from alloys of aluminium EN AW-6082 in injection process, Key Eng. Mater. 554–557 (2013) 844–849. [17] Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets, ASTM G76–18. [18] Standard Test Method for Chipping Resistance of Coatings, ASTM D3170/D3170M14. [19] A. L� opez-Ortega, R. Bay� on, J.L. Arana, Evaluation of protective coatings for offshore applications. Corrosion and tribocorrosion behavior in synthetic seawater, Surf. Coat. Technol. 349 (2018) 1083–1097. [20] C.I. Chung, W.J. Hennessey, M.H. Tusim, Frictional behavior of solid polymers on a metal surface at processing conditions, Polym. Eng. Sci. 17 (1977) 9–21. [21] N.K. Myshkin, M.I. Petrokovets, A.V. Kovalev, Tribology of polymers: adhesion, friction, wear, and mass-transfer, Tribol. Int. 38 (11–12) (2006) 910–921. [22] Sung Soo Kim, Shin, Ho Jang, Tribological properties of short glass fiber reinforced polyamide 12 sliding on medium carbon steel, Wear 274– 275 (2012) 34–42. [23] D.X. Li, X. Deng, J. Wang, J. Yang, X. Li, Mechanical properties of polyamide6polyurethane block copolymer reinforced with short glass fibers, Wear 269 (2010) 262–268. [24] S.N. Kukureka, C.J. Hooke, M. Rao, P. Liao, Y.K. Chen, The effect of fiber reinforcement on the friction and wear of polyamide 66 under dry rolling-sliding contact, Tribol. Int. 32 (1999) 107–116. [25] Sung Soo Kim, Min Wook Shin, Ho Jang, Tribological properties of short glass fiber reinforced polyamide 12 sliding on medium carbon steel, Wear 274– 275 (2012) 34–42. [26] E. Dhanumalayan, Girish M. Joshi, Performance properties and applications of polytetrafluoroethylene (PTFE)—a review, Adv. Compos. Hybrid. Mater. 1 (2018) 247–268. [27] G. Sundararajan, The differential effect of the hardness of metallic materials on their erosion and abrasion resistance, Wear 162–164 (1993) 773–781. [28] D. Caliari, G. Timelli, An investigation in to the effects of different oxynitrocarburizing conditions on hardness profiles and corrosion behavior of 16MnCr5 steels, Int. J. Metall. Mater. Eng. 110 (2015) 1–9. [29] A. Basu, J. Dutta Majumdar, J. Alphonsa, S. Mukherjee, I. Manna, Corrosion resistance improvement of high carbon low alloy steel by plasma nitriding, Mater. Lett. 62 (2008) 3117–3120. [30] Dong Cherng Wen, Microstructure and corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding, Appl. Surf. Sci. 256 (2009) 797–804. [31] N. Dingremont, E. Bergmann, M. Hans, P. Collignon, Comparison of the corrosion resistance of different steel grades nitrided, coated and duplex treated, Surf. Coat. Technol. 76–77 (1995) 218–224. [32] S. Rossi, F. Chini, G. Straffelini, P.L. Bonora, R. Moschini, A. Stampali, Corrosion protection properties of electroless Nickel/PTFE, Phosphate/MoS2 and Bronze/ PTFE coatings applied to improve the wear resistance of carbon steel, Surf. Coat. Technol. 173 (2003) 235–242.
This work was developed within the European Project MUSIC (MUlti-layers control & cognitive System to drive metal and plastic production line for Injected Components, FP7-FoF-ICT-2011.7.1, under Contract nº 314145). The authors would like to thank the EU Commis sion and all the consortium partners, and especially the close collabo ration of industrial partners by supplying materials and for their feedback and kind suggestions. References [1] K. Van Acker, K. Vercammen, Abrasive wear by TiO2 particles on hard and on low friction coatings, Wear 256 (2004) 353–361. [2] E.J. Bienk, N.J. Mikkelsen, Application of advanced surface treatment technologies in the modern plastics moulding industry, Wear 207 (1997) 6–9. [3] J. Bergstrom, F. Thuvander, P. Devos, C. Boher, Wear of die materials in full scale plastic injection moulding of glass fibre reinforced polycarbonate, Wear 251 (2001) 1511–1521. [4] I. Martínez-Mateo, F.J. Carri� on-Vilches, J. Sanes, M.D. Bermúdez, Surface damage of mold steel and its influence on surface roughness of injection molded plastic parts, Wear 271 (2011) 2512–2516. [5] M. Heinze, G. Mennig, G. Ballet, Wear resistance of PVD coatings in plastics processing, Surf. Coat. Technol. 74–75 (1995) 658–663. [6] Bohai He, Research on the failure and material selection of Plastic mould, Procedia Eng. 23 (2011) 46–52. [7] Dong-Cherng Wen, Erosion–corrosion behavior of plastic mould steel in solid/ aqueous slurry, J. Mater. Sci. 44 (2009) 6363–6371. [8] Dominick V. Rosato, Donald V. Rosato, Marlene G. Rosato, Injection Moulding Handbook Vol. 1, ISBN 978-1-4613-7077-1. [9] P. Boey, W. Ho, S.J. Bull, The effect of temperature on the abrasive wear of coatings and hybrid surface treatments for injection-moulding machines, Wear 258 (2005) 149–156. [10] A. Guzanov� a, J. Brezinov� a, Influence of reinforced plastomers on injection moulds wear, J. Met. Mater. Miner. 18 (2008) 7–13. [11] A. Guzanov� a, J. Brezinov� a, The wear of injection mould functional parts in contact with polymer composites, Mater. Eng. 16 (2009) 26–33. [12] J.R. Laguna-Camacho, L.A. Cruz-Mendoza, J.C. Anzelmetti-Zaragoza, A. MarquinaCh� avez, M. Vite-Torres, J. Martínez-Trinidad, Solid particle erosion on coatings employed to protect die casting molds, Prog. Org. Coat. 74 (2012) 750–757. [13] A. Mohammed, M.B. Marshall, R. Lewis, Development of a method for assessing erosive wear damage on dies used in aluminium casting, Wear 332–333 (2015) 1215–1224. € [14] Orhan Oztürk, Ortaç Onmus, D.L. Williamson, Microstructural, mechanical, and corrosion characterization of plasma-nitrided plastic injection mould steel, Surf. Coat. Technol. 196 (2005) 341–348.
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Wear journal homepage: http://www.elsevier.com/locate/wear
Mechanism-based wear models for plastic injection moulds �pez-Ortega a, E. Fuentes a, R. Bayo �n a, B. Zabala a, *, X. Fernandez a, J.C. Rodriguez a, A. Lo a b A. Igartua , F. Girot a
IK4-TEKNIKER, Calle I~ naki Goenaga 5, 20600, Eibar, Spain University of the Basque Country, UPV-EHU, Faculty of Engineering, Department of Mechanical Engineering, IKERBASQUE, Basque Foundation for Science, Alameda Urquijo S/n, 48013, Bilbao, Spain
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Plastic injection moulding Moulds Wear Friction Erosion Abrasion Corrosion
Injection moulding is a single-step manufacturing process for plastic parts that require very precise dimensions, geometries and low part roughness (Ra 0.2–0.025 μm). The high cost of moulds (typ. > 100,000 €) and unac ceptable wear of their cavities can limit the cost competitiveness of the process. Mould cavity surfaces are deteriorated by different wear mechanisms which may drastically reduce the quality of injection moulded parts, interrupt their production, raise maintenance and repair costs, or delay delivery. Therefore, a better under standing of the wear mechanisms and the main parameters affecting mould wear is needed. In this research, three major failure mechanisms were studied: abrasion and erosion (due to fibre-reinforcements in the plastics), and corrosion (due to gas release from the polymers). Testing protocols were developed for each failure mechanism. Abrasion was simulated with a block-on-plate tribological test, erosion was evaluated in an air-jet erosion tester combined with a gravel-o-meter test, and corrosion was simulated using electrochemical techniques. Three different thermoplastic materials and several mould surface solutions (i.e., mould steel and four alternative surface treatments) were tested. Furthermore, this paper presents an innovative approach to simulate the behaviour of mould and plastic materials at the laboratory scale, in order to minimise the cost associated to mould failures, through mould lifetime prediction. The prediction, with some limitations has been made based on the model generated through laboratory tests, by using a multiparameter design of experiments, where different working conditions (such as pressure or speed) and plastic and mould materials were evaluated. A single best solution was not found to resist all failure mechanisms, but useful recommendations for potential solutions for each failure mechanism were proposed. The TiN coating applied by Physical Vapour Deposition represents a good compromise between wear (abrasive and erosion) and corrosion resistance, especially for fibre reinforced plastics. Ni-PTFE coating is proposed to reduce the friction and improve the corrosion resistance of the mould materials when high wear resistance is not needed, as in the case of non-reinforced plastics.
1. Introduction Injection moulding is a single-step manufacturing process for plastic parts that require very precise dimensions, geometries and low part roughness (Ra 0.2–0.025 μm). The high cost of moulds (typically above 100,000 €) and unacceptable wear of their cavities can limit the competitiveness of the process. The occurrence of failures involves the production interruption, reparation of the mould, and even the change of the injected material and reestablishment of the injection process parameters. This problem supposes an over cost and a waste of resources (e.g., raw material and energy). The costs associated with the injection machine shutdown can reach 3000 €/day.
A current challenge in injection moulds materials is the necessity to resist high injection pressures (>100 bar) and temperatures (150� C300 � C) of the polymers and additives. This is particularly important when the polymer contains glass fibre reinforcements, which are very hard and abrasive leading to noticeable wear of the mould. Other ad ditives like titanium oxide (TiO2), used as a white pigment, can generate a similar problem [1,2]. According to literature [3,4], this kind of hard additives can lead to two different wear mechanisms, depending on the flux conditions: (i) erosion in mould regions where there is a frontal impact of the plastic flux, or (ii) abrasion mechanism when the flux is parallel to the mould surface. From these studies, it can be extracted that the phenomena
* Corresponding author. E-mail address: [email protected] (B. Zabala). https://doi.org/10.1016/j.wear.2019.203105 Received 9 October 2018; Received in revised form 9 October 2019; Accepted 26 October 2019 Available online 31 October 2019 0043-1648/© 2019 Elsevier B.V. All rights reserved.
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depend principally on: (i) the location in the mould due to variable flux motion in a complex geometry, (ii) the injection conditions and (iii) the properties of the mould material. The overheating of plastic materials can lead to gas release resulting in a local corrosion of the mould. Regarding the chemical composition of polymers, Polyamide (PA) can release NH3, and Polyvinyl chloride (PVC) releases Hydrochloric acid (HCl). Additives like flame retardants usually contain halogens (F, Cl, Br) that can release the corresponding acids being highly corrosive for steel moulds [5,6]. Other polymers could release other types of acids [7]. The high aesthetic requirements of the parts are one of the main reasons that make necessary the avoidance of any kind of wear scar in the cavity surface, since it can be copied in the surface of the injected part. In the mould, the polymer molecules and fibres are typically oriented in the flow direction, but during cavity filling they are perpendicular to the flow. The wall friction makes the speed at the extreme of the fibre and molecules in the centre to be higher than those near the walls. In turn, the flow speed difference results in a shear that rotates the mole cules and fibres until they are properly oriented in the flux direction. However, speed in the centre is more uniform, and the orientation of the reinforcements is not modified. The shear rate generated in the mould is caused by the high speed applied to the injected polymer, and the reduction of the speed due to the friction with the mould surface. This friction depends on the surface of the mould and the type of plastic and makes more difficult, the filling process. To avoid incomplete filling, it is necessary to increase the speed and pressure of injection, generating high shear rate. Therefore, these measures can lead to several problems in plastic parts such as flow marks, burns, or flashes [8]. The friction between mould and polymer can also generate problems during the part ejection. Higher friction will require larger ejection force, resulting in ejection marks in the parts. In this context, testing procedures need to be developed to reproduce the friction and wear phenomena adequately at laboratory scale. Ideally, a test could be designed on an actual test mould in a real injection environment. Nevertheless, the risk of damaging an expensive and complex mould, and the cost of interrupting the production make this approach onerous. Consequently, it is necessary to find simple labora tory scale tests, that can simulate as close as possible the actual pro duction failure mechanisms and provide results in a reasonable period of time, in order to evaluate different material candidates and gain a better understanding about their behaviour. The abrasive mechanisms generated in PIM (Plastic Injection Moulding) mould surfaces due to glass fibres have been simulated in literature with, at least, three different testing protocols based on the modification of standard tests. The “ball cratering test” [1] is considered relatively far from the actual mould conditions, since even if the abra sive slurry of TiO2 was carefully selected, there is some influence of the metal-metal contact between the ball and the flat sample. The “rubber wheel test” [9] is considered to be closer to the actual mould conditions, since the reinforced plastic is used as abrasive in a condition near to fluid, and the sample temperatures are quite high, close to the injection temperature. In this case, some type of wear due to the metal-metal contact is expected as well. In the “block on ring test” [10,11], the metal-metal contact is avoided, but the plastic is in a solid state. The simulation of mould erosion in the laboratory has been mainly performed using solid particles with devices similar to those described in the ASTM G76-18 standard [12,13]. Rotated slurry erosion rig has also been used [7] with the aim of simulating the combination of erosion and corrosion phenomena. For the simulation of corrosion of mould surfaces, different tech niques have been reported in literature, such as “salt spray methods” [14] or “potentiodynamic polarisation tests” [9]. Another key aspect in these evaluations is the selection of an appropriate electrolyte. Different solutions have been used in literature for PIM application (5% HNO3 þ 1% HCl, 10% CH3COOH [15] or 3.5% NaCl [9]).
The mathematical modelling of PIM moulds wear is not as mature as other wear processes simulation. Two references identified tried to model the wear generated by glass fibres in the mould surface. One of them [3] built a phenomenological model, based on the indentation of fibres in a plastic flow. Even though, it is still a long way to be applicable on simulation software’s, since the validation of the values predicted would need a lot of injection tests and simulations of their conditions. The second contribution [16] is an empirical model, constructed directly from results of tests performed in full-scale injection machines. This model was built for a specific mould with specific injection conditions, correlating the number of cycles performed with the roughness gener ated due to wear scars. This approach would need to perform a set of experiments on different mould types, in order to predict their failure. Regarding corrosion, even if some tests have been performed, there is no contribution in the literature about models regarding corrosive-wear degradation mechanism. This paper presents an approach to simulate the behaviour of mould and plastic materials in PIM process at the laboratory scale, in order to minimise the cost associated to mould failures, through mould lifetime prediction. The prediction, with some limitations, can be made based on the model that has been obtained through laboratory tests, by using a multiparameter design of experiments, where different working condi tions (such as pressure or speed) and plastic and mould materials were evaluated. 2. Materials and methods 2.1. Plastics and mould materials studied Different thermoplastic materials have been used to study their behaviour against different mould materials: a non-reinforced acrylic resin (AR), a non-reinforced polyester family thermoplastic (PT), and a reinforced polyamide (r-PA). For the case of (r-PA) tests, the reinforce ment orientation percentage was checked to be the same in all the samples. The procedure for reinforcement inspection was based on organic material removal in an oven at 600 � C for 2 h. A reference mould steel material was selected, which is supplied at two different states, as base material (BM) or at pre-hardened condition (HH). Different currently available commercial surface treatment al ternatives not commonly used in PIM were considered: Salt Nitriding, Ionic Nitriding, PVD-TiN and Ni-PTFE. The hardness of the different mould surfaces was measured by Vickers durometer, and by Fischerscope H-100 for thinner films. The order from higher to lower hardness was: TiN, Ionic Nitriding, Salt Nitriding, Ni-PTFE, HH steel, and BM steel. The roughness of the samples surfaces was measured with a Perth ometer M2 (Mahr GmbH) profilometer. Table 1 shows the hardness and roughness values obtained for each treatment. The roughness mea surements showed that all the surface treatments increased the rough ness of the substrate. The microstructures of the two steels HH and BM were also investi gated. Both steels presented a Tempered Martensitic structure, as shown in Fig. 1. BM steel showed a grain size number of 4–5, while that of the Table 1 Hardness and Roughness measurements of the different surface solutions.
2
SURFACES
HARDNESS (HV)
ROUGHNESS, Ra (μm)
HH Steel HH Steel þ Ionic Nitriding HH Steel þ Salt Nitriding HH Steel þ Ni-PTFE HH Steel þ TiN BM Steel BM Steel þ Ionic Nitriding BM Steel þ Salt Nitriding BM Steel þ Ni-PTFE
275 � 4 753 � 39 586 � 12 382 � 7 1592 266 � 2 861 � 37 513 � 27 321 � 14
0.55 0.75 0.80 0.60 0.70 0.55 0.75 0.80 0.60
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HH steel was of 9–10 (0.016–0.011 mm).
maximum depth of the wear scar at the three locations along the track, as shown in Fig. 3. The volume of material loss was quantified by approaching the shape of the track to a semi-ellipsoid.
2.2. Abrasion test
2.3.2. Gravel-o-meter The gravel-o-meter device was designed and manufactured by IK4TEKNIKER, in order to evaluate the resistance of the mould material and coatings to chipping caused by the impact of gravel, screws or other flying objects. Air pressure was used to project the gravel. The test samples were weighted before and after each test, to compare the amount of material lost. The testing protocol followed was similar to ASTM D3170-14 standard [18], but using pellets (plastic raw material) as gravel. The plastic gravel was charged in a tank and discharged from there into the supply hopper thanks to a vibratory feeder. The pipe of the gun has a nozzle with a diameter of 5 mm. The target was positioned at 350 mm (See Fig. 4). The gravel was recollected after each test series to be re-used. The metal samples had a square geometry of 50 � 50mm2. Five discharges of 6 kg of raw material each time were performed. The wear of the mould material sample was quantified by mass loss mea surements in the high precision balance. r-PA, RA, PT were used as plastic materials, whereas the two reference mould steels, i.e., base and treated with the four surface treatments of the study, were tested as mould materials.
The abrasion test was performed by a general-purpose reciprocating movement tribometer (UMT3-CETR), using “Block-on-Plate” configu ration. A flat surface of mould material described a reciprocating movement sliding against a static 4 � 4 � 4mm3 block manufactured by plastic injection. The testing conditions were selected in order to simulate as close as possible the PIM moulds abrasion mechanism. The steel sample was heated to usual mould working temperature of 100 � C, while the plastic was expected to reach a temperature near the plastic melting point based on the values measured on literature [10,11], simulating similar boundary conditions as in PIM. The contact pressure between the two surfaces was selected in the range of pressures (8–18 MPa) and sliding speeds (20–50 mm/s) employed in real injection operations. Considering this value range, four tests were performed: 8 MPa at 20 and 50 mm/s, and 18 MPa at 20 and 50 mm/s. In order to ensure these speed ranges, the movement stroke was kept constant at 8 mm, while the frequency was modified. Abrasive wear tests were performed mainly on r-PA, since the wear generated on AR or PT samples was negligible. The duration of the wear tests was of 4 h. Friction tests were carried out for a duration of 5 min. In some cases where the pin wear was very high, the test was stopped slightly before the 5 min time. After the tests, the wear was evaluated by mass loss measurements in a Mettler Toledo XP205 balance of high precision (accuracy down to 0.00001 g). The wear depth was analysed in a Nikon ECLIPSE ME600 Confocal Microscope, as shown in Fig. 2. In some cases, the surface of the sample was also analysed by Field Emission Scanning Electron Micro scope (FE-SEM) in a ZEISS ULTRA plus device, equipped with Energy Dispersive X-ray Spectrometer (EDS).
2.4. Electrochemical corrosion test Electrochemical corrosion tests were performed through Electro chemical Impedance Spectroscopy (EIS) measurements at different im mersion times for several materials immersed in NH4Cl 0.5%. The selection of the electrolyte was made according to the composition of the common plastics used in injection processes. It is likely that the NHþ 3 and the Cl presented in the electrolyte appear in real application, from the gases released by the Polyamide when overheated, or for example the PVC that contains Cl-R in its chain. Fluorides and halogens (Cl, Br, F) could also appear in the plastics due to additives like flame retardants, and they are somehow represented through the presence of Cl ion in the electrolyte. The tested materials were the two reference mould steels, base and treated with the four surface treatments considered in this study. The exposed surface was 625 mm2 for all the test samples. The electrochemical setup consisted of a three-electrode electro chemical cell, connected to an Autolab-Metrohm PGSTAT30 potentio stat shown in Fig. 5. A commercial Ag/AgCl (KCl 3 M) electrode and a platinum wire of 1 mm in diameter were used as reference (RE) and counter electrodes (CE), respectively. The test sample corresponding to each material tested was connected as the working electrode (WE). The potential of the reference electrode was of 0.207 V vs Standard
2.3. Erosion tests 2.3.1. Air Jet erosion tester An Air Jet Erosion Tester TR-471 from Ducom Instruments was used for the erosion study. The device was designed to characterise erosion resistance of different materials and coatings based on ASTM G76-18 standard [17]. In these tests, a sample of 25 � 25 � 5 mm3 received the impact of a 50 μm diameter alumina erodent on compressed air flow. The studied parameters were the particle speed (32 and 64 m/s), the sample tem perature (25 and 100 � C), the impact angle (15� , 45� , and 90� ), and the mould material (BM and HH steels). The erodent discharge rate (1.8 mg/ min) and sample exposition time (10 min) were kept fixed. The wear was evaluated by confocal microscopy, measuring the
Fig. 1. Microstructure of BM (a) and HH (b) steels. 3
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Fig. 2. Sample aspect after wear test (a) and cross-section profile of the wear scar (b).
Fig. 3. Confocal measurement of the wear scar.
Fig. 5. Experimental Setup, the three electrodes, the electrochemical cell and the potentiostat.
3. Results and discussion 3.1. Abrasion tests 3.1.1. Effect of injected plastic on the friction coefficient The non-reinforced polymers (AR and PT) were found to generate very high friction against the mould surfaces, but the wear generated was negligible and could not be measured. Instead, the reinforced Polyamide exhibited low friction but generated measurable wear in most of the mould surfaces. Regarding the friction, AR and PT presented very similar friction against all the different surfaces, with a friction coefficient around 0.8, while that of r-PA was around 0.5, as it can be seen in Fig. 6. AR and PT are not considered as tribological polymers, unlike PA, which offers a lower friction coefficient. The friction-generated heat at high pressures and high speeds can increase the sliding interface temperature of a polymer to values much greater than the metal surface temperature [20], and thus the polymer can start to plasticate at metal surface temperatures appreciably below its thermodynamic glass transition temperature of all these three glassy polymers. This material condition leads to high friction coefficient corresponds to an adhesive mechanism [21], as high as the 0.8 friction coefficient of AR and PT. It has been proven in literature [22–24] that reinforcements affect tribological properties of the polymer composites to a great extent. Their friction is determined by the adhesion of the polymer matrix, abrasion by glass fibres, the temperature at the sliding interface, and friction films produced on the counter surface.
Fig. 4. Gravel-o-meter test layout.
Hydrogen Electrode (SHE), and all the potentials registered in this document are referred to this electrode. The Electrochemical Impedance Spectroscopy (EIS) measurements were performed under a sinusoidal perturbation of �10 mV amplitude and at a frequency range from 100 kHz to 1 mHz. Four EIS measurements were carried out at different immersion times: 24 h, 72 h, 168 h, and 336 h. The electrochemical behaviour of the studied surface can be described by means of Electrical Equivalent Circuits (EEC). For this aim, the experimental data was fitted by using the Randles equivalent circuit [19], where Rs represents the electrolyte resistance between the refer ence electrode and the working electrode, and Rp correspond to the polarisation resistance. The Constant Phase Element (CPE) represents the double layer capacitance at the electrolyte/steel interface. For the comparison of the corrosion resistance of the different materials, the Rp polarisation resistance has been selected.
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Fig. 6. Mean friction coefficients of the different surfaces against AR, PT and r-PA.
The unreinforced PA, without glass fibre, has a higher friction co efficient attributed to the adhesion of the PA to the steel surface. The lower friction level with glass fibres is due to the reduced adhesion by smaller polymeric contacts on the sliding surface [22]. The inorganic fibres reinforced composites show good performance decreasing friction coefficient with increasing glass fibre content until it increased slightly beyond a glass fibre content of 30% [25]. For the case of AR and PT, the Ni-PTFE was the most effective surface treatment reducing the friction coefficient compared to the bare sub strate, while the rest of the surface treatments showed higher co efficients. In Fig. 7, it can be seen that at the beginning of the test, NiPTFE started with a coefficient of friction of 0.4 for the PT and 0.6 for the AR instead of the constant 0.8 afterwards. These results are in coherence with that reported in literature, since PTFE is currently used as a lubricity agent in several applications [26]. As it can be seen in the friction curves, after rubbing certain time, the increased adhesion of the polymer, reduces progressively the effect of Ni-PTFE until equalling the friction coefficient of the other surfaces, that is led by the friction of the transfer layer of the polymer. In fact, the AR, which has a lower glass transition temperature and then enhanced adhesion tendency, has more limited operation time of the Ni-PTFE low friction, compared to the PT. The range of variability of this test was found to be below the 10%, confirming it to be considerably reproducible. In the case of r-PA, the Ni-PTFE treatment was found to be less effective. For this case, the best surface treatment was TiN, reducing the mean friction coefficient to 0.4, being 0.53 the reference BM substrate. This behaviour is related to the abrasive wear mechanism that takes place in the surface rubbing against the harder reinforced plastic. The reduction of the friction coefficient due to the fibres, is partially compensated due to the abrasive wearing of the metallic material. In fact, the hardest the metallic surface, the lower will be the indentation of the fibres (Fig. 8), limiting the abrasive mechanism, as was demon strated by the ranking of the friction coefficient from lower to higher friction of the hardest to softest of the surfaces.
inspection performed by EDS after the tests. Salt Nitriding presented more abrasive wear than the Ionic Nitriding. The former showed a wear track depth of around 4 μm and 0.35 mg of total wear, whereas the wear track depth of the latter was below 1 μm with no measurable mass loss. Both TiN and Ionic Nitriding were the only surfaces presenting negligible wear after the 4-h tests. This behaviour was ascribed to the higher hardness provided by these surface treatments. The dependence between mould material hardness’s and resulting abrasive wear is presented in Fig. 8. There is logarithmic trend showing a decrease in the material loss with increasing hardness. Above a hardness of 800 HV1, it is hardly possible to make any scratch on the mould surface, and the material loss is negligible. Regarding the different substrate materials, the result showed little influence of the substrate on the response of the system, which depended mainly on the hardness of the surface treatment employed.
3.1.2. Effect of mould material and surface treatment on abrasive wear Wear tests were performed on r-PA plastic against the mould mate rial treatment alternatives. The HH steel showed a higher wear resistance than the BM steel. The Ni-PTFE treatment on BM substrate did not improve the wear resistance of the base material. The treated sample was found to be less wear resistant than the substrate itself, losing the coating and reaching the substrate in some areas along the wear track, as confirmed by the
3.2.1. Effect of mould material, surface treatment and plastic type on erosive wear of PIM Three different plastic materials were tested against BM steel to evaluate their erosive power in the Gravel-o-meter test rig (Fig. 10). As expected, the reinforced plastic (r-PA) was found to be much more erosive than the rest of the plastic materials. The other two plastics (AR and PT), had a combination of mild erosion and adhesion. The different surface solutions were tested against the reinforced plastic (r-PA). The response of the surface treatments in the erosion tests
3.1.3. Influence of different injection parameters on the abrasive wear Wear tests were performed at all the four extreme testing conditions of pressures and speeds, for BM substrate against r-PA, and the results are presented in Fig. 9. As it was expected, exist inter-relationships among the temperature, pressure and speed dependences of the fric tional coefficients and resulting wear [20]. For the lower pressure (8 MPa), the wear was independent of the speed within the tested range. Similarly, in the tests performed at the lower speed, the wear was in dependent of the applied pressure within the tested range. Nevertheless, the increase of both pressure and speed led to higher material losses. The results showed that both speed and pressure were critical for modelling the wear of PIM moulds. There is a PV (pressure x velocity) limit that is critical for modelling the wear of PIM moulds probably due to a change in the failure mechanism. 3.2. Erosion tests
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Fig. 7. Friction coefficient evolution during the tests of the different surfaces against PT, AR and r-PA.
Fig. 8. Correlation between hardness and abrasive wear during the tests for the studied surfaces.
Fig. 9. Surface response of abrasive wear tests dependence on pressure and speed.
were in coherence with that obtained in the abrasion tests, being the ranking exactly the same, from the best to the worst-behaving solution. Abrasion and erosion can have some differential behaviour related to
hardness [27], but for the case of PIM was expected to have a parallel behaviour of these mechanisms [3]. TiN and Ionic Nitriding were the most adequate surface treatments. It is interesting to remark that the Salt 6
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Fig. 12. Correlation between hardness and erosive wear during the tests for the studied surfaces. Fig. 10. Erosive wear subjected by different plastics on the reference BM steel.
Nitriding underwent most of the wear in the first discharge, showing progressively less material loss in the following discharges, as shown in Fig. 11. This can be attributed to the two different layers that compose the nitrided surface, e.g., compound and diffusion layers, which are known to have different properties [28]. The compound layer present at the topmost surface is usually very thin and brittle, and it can be easily removed during the first batch discharge. The correlation between the hardness of the mould material and the resulting erosive wear (Fig. 12) was in coherence with that of the abrasive wear (Fig. 8). Above a hardness of 800HV1, the erosion per formed in the moulds was negligible. From the results obtained in these experiments, a model was built, which is presented at the end of the manuscript. Fig. 13. Surface response of wear depending on impact angle (X2) and impact speed (X3).
3.2.2. Influence of different injection parameters on erosive wear for PIM The Design of Experiments (DOE) for this test was performed, varying the parameters: particle speed (32, 64 m/s), sample temperature (25, 100 � C), and impact angle (15� , 45� , 90� ). The test set was fully completed for both HH and BM steels. The erosion test results in the Air Jet erosion tester showed a little influence of the different microstructure and grain size of steels and mould temperature on the wear generated. This can be explained with the little influence on steel hardness of the temperature range of PIM moulds (up to 100 � C). Fig. 13 shows the influence of the impact angle (coded as X2) and speed (coded as X3) on wear. The impact angle and speed were the most significative wear factors, being the impact speed, the parameter with bigger influence in the maximum wear depth. 3.3. Electrochemical corrosion tests
Fig. 14. Time evolution of the corrosion resistance of the different materials immersed in NH4Cl.
The corrosion resistance obtained from the EIS measurements for the different materials and treatments in NH4Cl is presented in Fig. 14, in function of the immersed time. After 24 h of immersion in the NH4Cl solution, the highest resistance was that of the salt nitriding HH, followed by the Ni-PTFE (HH and BM)
and by TiN (HH and BM). In general, the corrosion resistance of the HH substrate was slightly higher than that of the BM one, possibly favoured by their smaller grain size. The resistance of the salt nitriding HH remained similar after 72 h of immersion and further up to the 336 h, whereas the values registered for the Ni-PTFE samples significantly decreased at 72 h, reaching very similar values for both HH and BM substrate materials. Ni-PTFE main tains the corrosion resistance after this initial decrease up to the final 336 h of immersion. The resistance of the TiN on HH substrate was maintained very similar during the whole exposition time. After 168 and 336 h, the resistance of these samples was close to those of the Ni-PTFE samples. The values registered for the rest of the materials were considerably smaller. The ionic nitriding BM and HH, presented a corrosion resistance values lower than 130 Ω after every exposition time registered, from 24 up to 336 h of immersion, which were lower than the resistance regis tered for the steel substrates. Salt nitriding BM samples, did not improve
Fig. 11. Wear generated on mould material samples by r-PA plastic in different discharges. 7
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the corrosion resistance of the bare substrate neither. Regarding the corrosion mechanism, general corrosion was observed on substrate materials, while the coatings suffered more localized pitting-like corrosion. The great difference between the salt nitriding applied to BM and HH was related to the nitriding process, which presented an inhomogeneous appearance, independently of the sample to be BM or HH. In fact, the compound layer (white layer composed of hard nitrides and outer postoxidation layer) was inhomogeneous and very brittle. Probably the HH sample has few local areas that exposed directly the substrate to the electrolyte. In the regions where the treatment seems to be properly applied, the compound layer presented an adequate corrosion inhibi tion. However, in the surface where the layer was not adequately applied and the diffusion region was exposed to the electrolyte, the corrosion resistance of the treated samples was like the bare substrate. This treatment was not reliable in terms of corrosion protection. After salt nitriding HH, the second highest resistance corresponded to the Ni-PTFE for all the exposure times (HH followed by BM) and the third one to the TiN (HH followed BM). However, the values registered for these treatments at high immersion times were at least 3 times smaller in comparison with the salt nitriding HH sample. Different studies have shown that the corrosion protection of mould steels by nitriding processes can be maximized through the nitride vol ume fraction at the surface [29], and especially excels when is more composed of ε-nitride [30]. Furthermore, the post-oxidation that natu rally appears in the salt nitriding process will improve the corrosion protection significantly. The generation of these nitrides in the surface are very dependent of nitriding time and temperature [29,30], but the steel composition as well [31]. Depending on these conditions the compound layer can be very thin, while in the diffusion layer, the precipitation of chromium nitride during the nitriding treatment, generates segregation of chro mium via a precipitation hardening mechanism which leads to compromised corrosion performance (intergranular corrosion, pitting corrosion …) [31], which has most probably occurred on ionic nitriding processes used in this study. TiN has been demonstrated as an adequate solution for corrosion protection due to barrier effect, and as shown in Fig. 14, its corrosion protection is nearly independent on the exposition time and it is less dependent on the steel type than nitriding as shown in the literature [31]. The performance of this coating can be further improved in some
WHmax ðμm = hÞ ¼ 607:68 þ 10:27α þ 12:09v þ 0:1αv
� 0:13α2 � ð33:24
injection of reinforced r-PA samples. The inputs needed in this forecast would be the surface hardness of the mould material, the flow speed, and the pressure. In order to calculate the cycles to failure, an assumption of the sliding time of the polymer during the filling operation should be revised in each case (a filling time of 5 s can be considered as a reference value). Another parameter that should be defined by the end user is the maximum accepted wear depth on the mould prior to its replacement. The resulting model built from the experimental results can be written as follows: Wðμm = hÞ ¼ 5:0918
0:0597P
0:0262v þ 0:0031Pv
0:818 ln H
where: W: Abrasive wear rate in the contact area, in μm per hour P: Pressure in MPa (applicable from 8 to 18 MPa. Below the 8 MPa value no wear is expected. The pressure value is necessary to cali brate the model, and convert the pressure range on the PIM to the ones measured on the tests). v: Speed in mm/s (applicable from 20 to 50 mm/s. Below the 20 mm/ s value no wear is expected) H: Hardness in Vickers, HV (applicable up to 900 HV. Above 900 HV no wear is expected) The regression coefficient of the Analysis of Variance of the model is: R2 ¼ 0.9811. 4.2. Modelling of erosion From the results obtained in the erosion tests, a model was built to quantify the wear on moulds. The inputs needed in this model would be the surface hardness of the mould material, the flow speed, and the pressure. In order to calculate the cycles to failure, an assumption of the duration in seconds of flux impact against the mould surface should be made. Furthermore, the maximum accepted wear depth on the mould prior to its replacement must be also defined by the end user. For the specific application of an injection mould, the wear depth is probably the critical parameter, since these defects will be copied in the injected part. The dependence of the wear extent on the speed, the impact angle, and hardness showed before can be written in an equation as follows:
4:92 ln HÞ
where:
cases through a duplex treatment (combining plasma nitriding and TiN) [31]. Nickel coatings containing PTFE particles showed good protection properties because of the double nature of the coating: the outer layer includes PTFE particles (self-lubricating properties), the inner one in creases the barrier-effect [32].
WHmax: maximum wear depth rate in the centre of the eroded area, in μm/h v: flux speed in m/s (valid from 32 to 64 m/s. The speed value is necessary to calibrate the model, and convert the pressure range on the PIM to the ones measured on the tests) α: impact angle in � (valid from 15 to 90� ) H: Hardness in HV (valid from 200 to 900 HV. Above 900 HV, no wear is expected)
4. Wear modelling implementation 4.1. Modelling of abrasion Due to the small differences found in the abrasion tests regarding friction, no friction model was built. However, the results of this work can be useful as friction database to reduce the friction in moulds, to apply for example, in the cases, were friction-related problems can be expected, e.g., part ejection problems or high shear stresses that lead to burns in the final part. Regarding wear, a model was built considering the influence of the injection parameters and the hardness of the mould material for the
The regression coefficient of the Analysis of Variance of the model is: R2 ¼ 0.8053. 4.3. Modelling of corrosion Considering the results obtained in this work, the corrosion of mould materials has been found to depend on the surface characteristics. In this 8
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Table 2 Safety factors referred to the basic BM steel for PIM solutions. Mould material BM steel HH steel Ni-PTFE on BM Ni-PTFE on HH Salt Nitriding on BM Salt Nitriding on HH Ionic Nitriding on BM Ionic Nitriding on HH TiN on BM TiN on HH
Table 3 Inputs necessary for the PIM moulds wear prediction.
Safety factor 1.00 0.69 5.48 10.47 0.75 31.05 0.38 0.62 2.89 6.20
INPUT Pressure Impact speed Impact angle Mould material Hardness Basic maintenance Unacceptable wear
18 50 30 BM steel 260 600 100
MPa mm/s �
HV cycles μm
Table 4 Outputs of the model of PIM moulds wear prediction.
case, a table representing the risk of corrosion occurrence on mould surfaces has been built (Table 2). The table uses as reference the number of cycles needed to corrode the mould by the reference steel. The safety factor from this data can be calculated as follows:
OUTPUT Abrasion Erosion Corrosion
73659 16671 600
Cycles Cycles Cycles
Safety factor ¼ measured Rp of the surface (ohm) / measured Rp of the reference material (ohm) Table 5 Inputs for the mould wear prediction of the improved case.
Where Rp is the corrosion resistance of the samples obtained by electro chemical corrosion tests, in ohms.
INPUT
4.3.1. Application example The user of the mould-lifetime model can introduce the input data, as collected in Table 3 for the critical areas of the mould. The selection of the process values (impact angle, velocity, and applied pressures) can be done using injection process simulation tools or user experience. Once the cycle parameters are defined, the output of the model is the forecast of the expected wear, depending on the failure mechanism suffered by the mould as shown in Table 4. This information is relevant to schedule maintenance work of the mould and take counter measures if the wear is unacceptable. The model allows the modification of pro cess parameters or surface material characteristics. To improve this reference cycle, different steps can be made. For instance (see Table 5), the application of Ni-PTFE coating is proposed to reduce the friction and improve the corrosion resistance of the mould materials. It is scheduled that this change might allow injecting almost 5 times more cycles: > 3200 cycles instead of the 600, without surface cleaning or anti-oxidant application. Regarding abrasion and erosion wear, this surface treatment has slightly higher hardness and combined with a slight reduction of the process pressure and speed would allow to increase by a factor of 2 the time necessary for a mould repair due to abrasion or erosion wear scars (see Table 6). Notwithstanding the models developed in this work, further cali brations with the experience of end users (e.g., adding a weighting factor) and experimental trials when possible, will be needed to improve the accuracy of the proposed models.
Pressure Impact speed Impact angle Mould material Coating Basic maintenance Unaccepable wear
16 40 30 BM steel 321 600 100
MPa mm/s �
Ni-PTFE HV cycles μm
Table 6 Outputs for the mould wear prediction of the improved case. OUTPUT Abrasion Erosion Corrosion
188693 35483 3288
Cycles Cycles Cycles
� Ionic nitriding and TiN were the most promising alternatives for wear protection, in both the abrasion and erosion tests. � The behaviour of the different surface treatments was very similar for both substrates, HH and BM steels. � Friction reduction of AR and PT were mainly achieved by using an Ni-PTFE coating. TiN was the most effective in the case of r-PA. � Regarding the corrosion tests, the salt nitriding treatment provided good corrosion protection on HH steel, but the process itself was not reliable showing a high scattering in the results. The best solution was the Ni-PTFE coating, followed by the TiN that showed the most stable corrosion resistance.
5. Conclusions
Finally, from all the results obtained from laboratory tests simulating PIM conditions, different models were built. These models could be useful to evaluate the wear resistance by abrasion, erosion, and corro sion of candidate materials and coatings, and sensibility to working conditions as relative speed or pressure. The TiN coating represent a good compromise between wear and corrosion resistance, especially for fibre reinforced plastics.
In this work, the performance of two mould materials with different surface treatments has been evaluated by means of abrasion, erosion, and corrosion simulated tests. The following conclusions can be drawn: � The reinforced plastic material (r-PA) wore the moulds significantly, while the expected wear for non-reinforced plastics (PT and AR) is minimum both for abrasion and erosion mechanisms, despite the high friction they generate. � The processing parameters increasing the moulds erosive wear, were the particle speed and the impact angle increase. The parameters accelerating the abrasive wear were the increase of the relative speed and pressure. Both wear mechanisms were very sensible to the mould material hardness. The higher the hardness of the surface treatment, the lower was the wear.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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