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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 F 69621 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 http dx doi org 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 388 399390 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 388 399391 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 Param tres 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 388 399392 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 388 399393 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 388 399394 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 exhibits also elongated holes produced by the glass fi bers Fig 11a Only a small polymer deposit located on a hole was found on matrix surface Fig 11b The type of default pre sented in Fig 11c is also observed before injections and so their presence can only be cataloged as a polishing default Unfortu nately this default can affect the quality of the injected part in terms of esthetical aspect For the example the default in Fig 11c can be observed on the plastic part in Fig 12b The effect of mold g