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Tribological assessment of the interface injection mold/plastic part N. Crisan a, S. Descartesa,n, Y. Berthiera, J. Cavoreta, D. Baudb, F. Montalbanoc aUniv Lyon, CNRS, INSA Lyon, LaMCoS, UMR5259, F69621 Villeurbanne, France bCentre Technique de la Plasturgie et des Composites, 2 Rue Pierre corrosion phenomena that can greatly limit the lifetime of the mold cavity as a function of the type of injected polymer; problems of sticking and releasing in function of the injected materials and surface quality; problems in keeping the polishing quality; scratches or shocks during use or storage. Theavailableliteratureapproachesexperimentaland/or numerical, various aspects regarding the plastic injection molding process. One of them is the fi lling and fl ow behavior of molten polymers. Bociaga and Jaruga 3 studied the formation of fl ow, weld and meld lines by developing a new method of fl ow visua- lization, which can prove helpful in the identifi cation of weak Contents lists available at ScienceDirect journal homepage: Tribology International /10.1016/j.triboint.2016.04.015 0301-679X/ b) the matrix. Optical polished mold (simple geometry): c) the stamp; and d) the matrix. Table 1 Confi gurations mold-materials. Injected materialMirror polished mold Optical polished mold Why? SANxxReference material SurlynsxCorrosive PA66GF25xAbrasive PCxOptical applications Table 2 Injection Parameters for the mirror polished molds. Injection parametersInjected polymers SANSurlynsPA66GF25 Metal temperature which heats the polymer (C)245215300 Temperature inside mold (C)403080 Injection debit (cm3/s)303020 Quantity of injected material (cm3)26.64223 Injection time (s) Pressure to avoid shrinkage (bar)0.763.31.61 Time to maintain the pressure to avoid shrinkage (s) 5204 Time to cool down (s)73210 Clamping fore (T)507070 Length of ejection pin (mm)25.232.530 Ejection force (kN)3520 Ejection speed (mm/s)1305040 Table 3 Injection Parameters for the optical polished molds. Injection parametersInjected polymers SANPC Metal temperature which heats the material (C)280280 Injection speed (mm/s)3030 Quantity of injected material (cm3)3030 Pressure to avoid shrinkage (bar)55-3060-30 Time to maintain the pressure to avoid shrinkage (s)88 Time to cool down (s)1515 Clamping fore (kN)13001300 N. Crisan et al. / Tribology International 100 (2016) 388399390 The infrared technique could not be applied for the optical polished mold, because the possible present layers and deposit are too thin to be detected with this method. The interferometry measurements were made using a 3D non- contact optical profi ler (Sensofar) which combines the confocal and interferometry techniques. Two objectives were used, one ?10 with an acquisition area of 1200?800 mm and ?50 with an acquisition area of 250?150 mm. The acquired data was processed using Moun- tainsMap Universal software. The following height parameters for 3D areal surface (ISO 25178) were considered in this study: the arithme- tical mean height of the surface (Sa), the root mean square height of the surface (Sq), the maximum peak-to-valley distance (Sz), the height distribution (Ssk ) and the fl atness of the height distribution (Sku) 8. Bigerelle et al. 9 developed an original methodology applied to a grained mold surface, to analyze the infl uence of injection parameters on the roughness of injected plastic parts, by fi nding the evaluation length on which classical parameters can be esti- mated. This work raised the question of fi nding the right cut-off fi lter to ensure that the height parameters values are due to the roughness of the surface and not of the waviness. In this study a fi ve degree polynomial was applied to the acquired data to remove the form. By comparing the values of the height parameters: Sa, Sq, Szat different scales of the same area, it was observed that values are identical or very close. So if is taken into account that the level of polishing is very high (very smooth surface) and the variation of the values is insignifi cant, it be can concluded that the values of the height parameters presented in this paper are truly related to the roughness of the surface and not of the waviness. A study taking into account the methodology developed by Bigerelle 9 and Van Gorp 10 could be envisaged for the future in order to apply it on a very smooth surfaces. The interferometry measurements on the mold used for the Surlynsinjection were performed as shown in Fig. 3 (red color). 23 measurements were performed in case of the stamp and 17 measurements for the matrix. The location of the measured areas for the mold used for SAN and PA66GF25 injection is also shown in Fig. 3 (blue color). In this case, 31 measurements were considered for each mold component (Fig. 4). The different defaults and deposits present on the surfaces were not consider for the calculation, they were masked. The values lis- ted in Tables 4 and 5 represent the average of the measured values. 3. Results and discussions 3.1. Mirror polished mold 3.1.1. Injection Surlyns All along the stamp plane part, deposits different in texture and consistence can be observed (Fig. 5). Their location and morphology seem to indicate the fl ow direction of the molten polymer. Also it can be noticed, towards the end of the fl ow, the deposits grow in terms of thickness and occupied surface. The type of deposit observed in Fig. 5e and f is also observed after the fi rst injection campaign (3000 injected parts), and appeared that the cleaning operation has been able to remove it, but formed again during the second injection campaign (3700 injected parts). This particular deposit is located between the extremity of the oval bump and the hole where one of the ejection pins acts. Also in this location the fl ow changes direction, more precisely makes a left turn; fact also revealed by the deposit morphology. Its existence can be explained starting with the effect of the injection speed on the molten polymer viscosity, which is considered to be a heat transfer mechanism that occurs during the injection process. Due to the geometry factor, the viscous dis- sipation creates a temperature gradient which sensitizes this area. During the packing phase, as the mold continues to be fi lled, the location identifi ed is one of the last to be reached by the molten polymer. As the holding phase begins and with it the solidifi cation, the temperature gradient that appears in the injection phase continues to act and by doing so it delays the solidifi cation in this area. When the established time for the holding phase expires, the mechanism of ejection is set in motion. The ejection pin is close to the identifi ed location and as it was affected by the temperature gradient and has not yet been entirely solidifi es, it will also be the fi rst area to be separated from the mold surface. All these can explain the appearance of the adhesion phenomenon. In Fig. 5d, the deposit appears like a thin fi lm and is also located in an area where the fl ow changes direction. It could also be jus- tifi ed by the temperature gradient, but its aspect and composition suggest that may another phenomena can occur. The infrared analysis performed on this area (Fig. 6c) suggest that only some of the wavenumbers match with the ones from the spectrum regis- tered for the injected part (Fig. 6a). It is possible that the gases released from the contact of the molten polymer with the mold surface reacted with the additives from the raw material compo- sition and facilitated the separation of the thin layer that stick on Fig. 3. Interferometry measurements localization for the mirror polished mold. (For interpretation of the references to color in this fi gure, the reader is referred to the web version of this article.) Fig. 4. Interferometry measurements localization for the optical polished mold. N. Crisan et al. / Tribology International 100 (2016) 388399391 the mold surface. Also the “scraped” aspect of this deposit indi- cates that is more likely that this type of deposit has formed during the injection phase. The mold matrix surface presents small “islands” deposits that follow the fl ow direction (Fig. 7a). This aspect is in concordance with what it can be observed on the plastic part surface in the Table 4 The values of roughness parameters for the mold used for the Surlynsinjection. Parameters (Objective?10)Mold before injectionSurlynsinjection StampMatrixStampMatrix Sa(nm)5.4470.865.5271.398.6471.236.8871.15 Sz(nm)63.96713.7366.6971.79113.8875.7480.66721.5 Sq(nm)6.8671.156.9971.7910.8871.568.6671.44 Ssk?0.05470.18?0.15670.12?0.01770.14?0.04170.14 Sku3.1470.363,2970.383.2170.553.2370.38 Table 5 The values of roughness parameters for the mold used for the SAN and PA66GF25 injections. Paramtres (Objective?10)Mold before injectionSAN and PA66GF25 injection StampMatrixStampMatrix Sa(nm)5.4470.865.5271.395.9371,711.0273.18 Sz(nm)63.96713.7366.6971.7965.52718.7124.99742.77 Sq(nm)6.8671.156.9971.797.4772.1513.8373.99 Ssk?0.05470.18?0.15670.120.000770.14?0.0570.24 Sku3.1470.363,2970.383.1370.323.2470.44 Fig. 5. Mold stamp used for the Surlynsinjection various type of deposits found on the plane part. N. Crisan et al. / Tribology International 100 (2016) 388399392 same area, shown at a magnifi ed scale in Fig. 8a. This type of deposit due to the molten polymer viscosity has led to the for- mation of burrs (excess material in thin layer) possible during the ejection phase. Holes (from 14.6 nm to 404 nm deep) are observed before injection probably due to polishing. Their morphology evolves during injection process: the holes expand in occupation area and depth (39.7 nm to 877 nm). In Fig. 7b and c, the pointing red arrows indicate the presence of the evolved holes. They exhibit two types of morphology. The fi rst type illustrated in Fig. 7b shows very small holes focused altogether in smaller or larger spots and the second type illustrated in Fig. 7c presents a hole surrounded by a “cloud” of small holes. Due to the inclusions in the bulk material, grains dislocation could occur causing the formation of holes during polishing pro- cess. Those holes are modifi ed in term of depth and area during injection process. As reported in 11, stress corrosion cracking can affect the molds, starting at a microscopic level and revealing itself as crack. The primary causal elements are the metallurgy of steel, the presence of chlorine in the water used in the cooling lines of the mold and the stresses on the tool during molding. It is known that Chromium gives the steel corrosion resistance, by providing a protective oxide layer. Thus it is possible that due to the polishing defects (holes), the thickness of this layer is compromised and thus when a high viscous corrosive polymer, like Surlyns, is injected, the areas affected by holes, are submitted to corrosion attack. The fact that the feature to evacuate the air was excluded from the mold design in conjunction with the corrosion nature of Surlyns(based on the experience of industrial project partners), can create an aggressive environment at the mold/molten polymer interface due to the gases release. The high viscosity of Surlyns and its capability to stick onto the mold surface also plays a role in terms of exerting a mechanical-physico-chemical attack on the area where the defaults are located. All these statements allow to catalog this default as corrosion pit. As can be seen in Fig. 8b and c, these corrosion pits have also an effect on the plastic part surface. Their infl uence is manifested by the formation of accentuated burrs (Fig. 8b) or accumulation of debris or fragments presented in same form that corrosion pits exhibit on the mold surface. Although the damage mechanism exhibited by the stamp is different than the one for the matrix, some deposits can be found on the surface. Near the end of injection, where the fl ow change direction, small deposits can also be observed (Fig. 7d). Their laying out indicate the fl ow direction. Fine scratches can be observed in the area where the fl ow changes direction, but at a closer look, in fact they are traces of deposits. A signifi cant deposit can be observed all along the area where the two melting polymer fl ows encounter (Fig. 7e). The infrared analysis reveal that this deposit has a different composition than the injected polymer. The plastic part surface presents a weld line (Fig. 7e), on the corresponding area. Deposits are also found on top of corrosion pits, in a changing fl ow direction area (Fig. 7f). Due to the limitations caused by the dimen- sions of the matrix, the optimal conditions for an appropriate infrared analysis were not possible. But optically, the texture of these deposits are very similar with the one found on the stamp (Fig. 5e). The initial roughness parameters for the two parts of the mold are listed in fi rst two columns of Table 4. Taking into account the initial values of Skewness parameter (Ssk) and Kurtosis (Sku), it can be concluded that the mold surface has a slight tendency to have more valleys than peaks (Ssk o0) and the fl atness of the high dis- tribution is wide (Sku43), so the surface is rather plane. It can be observed also from Table 4, that the values of Sa, Szand Sqare higher after the mold has been submitted to the injection process. Although initially, there is no signifi cant difference between the values of Sa(?0.08 nm) and Sq(?0.13 nm) for the stamp and the matrix, a slight more signifi cant one is observed after injection (?1.76 nm for Saand ?2.2 nm). This difference consists in higher values of the roughness parameters after injection for the stamp. The peak-to-valley height represented by Szhas a higher value after injection in case of the stamp. This supports the statement that the stamp and the matrix present different damage mechanism. Fig. 6. Mold stamp injected with Surlyns registered infrared spectrum for: a) the injected part fi nal product; b) deposit identifi ed on the mold surface (Fig. 5e); c) deposit identifi ed on the mold surface (Fig. 5d). N. Crisan et al. / Tribology International 100 (2016) 388399393 Fig. 7. Mold matrix used for the Surlyns injection identifi ed damaged areas on the plane part. Fig. 8. Injected Surlyns part identifi ed damaged areas on the plane part. N. Crisan et al. / Tribology International 100 (2016) 388399394 Although during data processing the defaults and deposits are masked, it appears that the sticking-releasing phenomena (found on the stamp) affects more the integrity of the surface than the corrosion pits (found on the matrix). All the roughness parameters evolved along the fl ow direction. The value of Saparameter was chosen to highlight this surface evolution along the fl ow direction. The Savalue considered to represent this variation is the average of the measured values located on a line perpendicular on the fl ow direction. For both stamp and matrix, the lowest Savalue can be found at the beginning of fl ow on the plane part (C1 on the graphs in Fig. 9). For the matrix, this value is not much higher than the one measured before injection. If the Savariation is considered on the lines of observations, it can be observed for the stamp that the roughness has the tendency to increase on the fl ow direction (Fig. 9a). Also in the case of the stamp, the Savalue also varies in function of observation column. It has the tendency to increase and decrease from one measured area to the next. If the microscopy analysis performed on the stamp is correlated with the roughness varia- tions, the presence of a lot of deposits with various thickness justifi es the increase of roughness towards the end of fl ow. In case of the matrix, the Savalue increases along the centered observation lines (L3, L4, L5, L6 Fig. 9b) towards the end of fl ow. But it decrease on the observation lines situated on the edge. The variation of Savalue on the observation columns C3 and C4 (Fig. 9b) is very pronounced, as Savalue decreases or increases dramatically from a measured area to the next. Apparently the matrix surface fl atness is severely compromised, but it is not an injection consequence, it is due to the polishing process that has proven to be challenging when it comes to high surface fi nishing on complex geometries. 3.1.2. Injection SAN and PA66GF25 SAN and PA66GF25 polymers were injected successively on the same mold. The fi rst polymer to be injected was SAN. When the injection fi nished, the insert mounted on the stamp was changed with a new one and the injection of PA66GF25 began. No sup- plementary cleaning operations were performed. The microscopy analysis presented below was performed after the PA66GF25 injection, and so only the plastic part made of this polymer was considered for the results presented in this paper. Fine scratches can be observed on the stamp surface at beginning of the plane part (Fig. 10 a). These scratches become denser and pronounced in the areas towards the end of injection, where the fl ow change direction. Oval holes like the one in Fig. 10 c are scattered all over the mold surface. Initially formed during polishing, it seems that they were enlarged on a perpendicular direction to the injection fl ow, by the abrasive action of the glass fi bers. The insert and the rest of the stamp surface present different morphology in terms that on the insert the scratches are more highlighted (Fig. 10 b). The interferometry measurements confi rm a slight difference for the Savalue (?0.76 nm). It is possible that a very thin layer of polymer remained after the SAN injection, and so the surface was somehow protected against the glass fi bers action. The area, where the two injection fl ows encounter, presents burn marks and a lot of small deposits like “drops“ (Fig. 10d). The infrared analysis reveal that these deposits are not similar in composition of the injected polymers. The matrix ex