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Tribological assessment of the interface injection mold/plastic partN. Crisana, S. Descartesa,n, Y. Berthiera, J. Cavoreta, D. Baudb, F. MontalbanocaUniv Lyon, CNRS, INSA Lyon, LaMCoS, UMR5259, F69621 Villeurbanne, FrancebCentre Technique de la Plasturgie et des Composites, 2 Rue Pierre & Marie Curie, Bellignat 01100, FrancecChristian Dalloz Sunoptics (CDS), Route de Genve BP 155, Saint-Claude 39206, Francea r t i c l e i n f oArticle history:Received 3 October 2015Received in revised form13 April 2016Accepted 13 April 2016Available online 25 April 2016Keywords:Injection processMold surfaceThermoplastic polymerRoughnessa b s t r a c tOne of the current challenges of the plastic injection process is linked to the importance given to productdesign that enables a strong differentiation. The key in developing optimal surface molds that canovercome the present disadvantages lies in the comprehension of the interactions that occur at the mold/injected piece interface. This paper focus on identification and evaluation of the contact conditions atthat interface, taking into account the effect of the polishing, of the mold geometry and of the injectedmaterial. A critical characterization of the surface topography was performed to study the corrosion-mechanical attack and the mechanical-physico-chemical one on two molds.& 2016 Elsevier Ltd. All rights reserved.1. IntroductionThe sector of plastics processing is relatively young comparedto cast iron, steel or glass industry. So it still has a very strongdevelopment potential. One of the current challenges of the plasticinjection process is linked to the importance given to productdesign that enables a strong differentiation 1. Plastic parts withan increased technical level of surface accuracy are required in theareas of luxury, packaging, automotive, including the medical andoptics. Their development involves the improvement of the fab-rication process, and one of the keys lies in the mastery of thesurface conditions of the molds.Injection molding is a cyclic process, characterized by 5 phases:dosing, injection, packing, cooling and ejection (Fig. 1). The rawmaterial that is dosed in the machine must be pure and conservedbefore use at an adequate temperature in order to be as dried aspossible. This is necessary to avoid condensation inside the mold.The injection phase is characterized by high flow rates and hencehigh shear rate (tangential effect). As the molten material entersthe mold, two heat transfer mechanisms occur: convection(between the melted material and mold surface) and viscousdissipation (due to the effect of injection speed on the injectedmaterial viscosity). As the filling is complete, the mold is uphold ata predetermined pressure and so the packing phase is initiated.During this phase, the molten polymer continues to be introducedinto the mold to compensate for the shrinkage of the alreadyinjected material as it cools down. After a specific time, the coolingphase (contrary to the cooling state which begins during injectionand packing phase) of the entire assembly starts and so also thesolidification process of the plastic part. As the material solidifiesand shrinks in the mold, the dominant heat transfer mechanism isconduction. When the part is sufficient solidified, it is ejected fromthe mold. During this last phase, a normal effect can be attributedto the ejection force and adhesion phenomena can occur betweenthe mold surface and the plastic part 2.Despite the undeniable diversity of configurations available (interms of combination: mold material, surface finish and processedmaterials), the producers are faced with similar difficulties. Thus,the key shortcomings that stand out, more or less combined, canbe summarize as follows: thefoulingphenomena whichrequirefrequentstopsforcleaning; corrosion phenomena that can greatly limit the lifetime of themold cavity as a function of the type of injected polymer; problems of sticking and releasing in function of the injectedmaterials and surface quality; problems in keeping the polishing quality; scratches or shocks during use or storage.Theavailableliteratureapproachesexperimentaland/ornumerical, various aspects regarding the plastic injection moldingprocess. One of them is the filling and flow behavior of moltenpolymers. Bociaga and Jaruga 3 studied the formation of flow,weld and meld lines by developing a new method of flow visua-lization, which can prove helpful in the identification of weakContents lists available at ScienceDirectjournal homepage: /locate/tribointTribology International/10.1016/j.triboint.2016.04.0150301-679X/& 2016 Elsevier Ltd. All rights reserved.nCorresponding author.E-mail address: sylvie.descartesinsa-lyon.fr (S. Descartes).Tribology International 100 (2016) 388399areas on injected parts. Also the effect of pressure and cavitythickness were assessed. Same topic was treated by Ozdemir et al.4, comparing the behavior of molten HDPE (high density poly-ethylene) and PE experimentally and numerically.During molding, friction forces act first between the moldsurface and the molten polymer and second when the plastic partis ejected from the mold. Bull et al. 5 adapted the ASTM rubberwheel abrasion test to simulate the conditions of wear producedby the glass filled polymers on the barrel surface of an injectionmolding machine. Various coatings were tested, but unfortunatelythey tended to have a weak performance on account of the testconditions.Pouzada et al. 6 developed a prototype apparatus to study thefriction properties of molding thermoplastics during ejectionphase. The measured friction coefficient had a tendency toincrease with the roughness. But when the roughness was reducedthe friction coefficient increase due to the rising adhesion forceseffect. The scanning electron microscopy images of the mold sur-face and the ones for the polycarbonate (PC) and polypropylene(PP) plastic parts, revealed a clear replication of the mold surfaceon the parts.Transient in nature, injection molding process involves not onlyseveral heat transfer mechanisms, phase change and time varyingboundary conditions, but goes further in adding the effect ofmaterial properties and geometry of the injected part. Bendadaet al. 7 performed a study to evaluate the nature of thermalcontact between polymer and mold through the different phasesof a typical injection cycle. Their findings concluded that thethermal contact resistance was not negligible, not constant withtime and was strongly linked with the process conditions.The existing number of studies concerning the phenomenapresent at the interface mold surface/polymer is relatively low toother related topics. Besides, they dont focus on studying thecurrent limitations of the plastic injection process at a microscopicscale, taking into account various macroscopic influences. Toovercome plastic injection molding shortcomings, the contactconditions at the interface between mold surface and plastic parthave to be identified. This work focus on the effect of the polishingquality, the mold geometry and the injected material on thatinterface, by studying the corrosion-mechanical attack and themechanical-physico-chemical one.2. Method and materials2.1. MaterialsFour polymers were chosen to be injected: ionomer resin (E-MMA SurlynsPC 2 000), styrene-acrylonitrile resin (SAN Tyril790), polyamide with 25% glass fibers (PA66GF25) and poly-carbonate (PC Makrolon LQ 2647). Surlynsis a copolymer ofethylene and methacrylic acid where some of the acid groups areneutralized to form the sodium salt. The acid in the polymer givespolarity and reduces crystallinity. The ionic bonding between thepolymer chains gives outstanding melt strength, toughness andclarity. The reason of choosing Surlynswas based on the experi-ence of our industrial partners, which find it particularly corrosivedespite its good properties. SAN is also a copolymer, opticallytransparent and brittle in mechanical behavior. Its considered inthis study a reference material, usually used in cosmetics, luxuryand automobile domains. PA66GF25 is an aliphatic polyamide,reinforced with 25% glass fibers. PA66 has an excellent balance ofstrength, ductility and heat resistance. The glass fibers exert anabrasive effect and thus affect the mechanical protection of thepolishing. PC is composed by carbonate groups. It has a highimpact-resistance, low scratch-resistance and is highly transparentto visible light. It is usually used for the production of eyewearlenses and exterior automotive components.2.2. MoldsTwo molds, made of hardened steel (52 HRC) containing 13% to15% of Chromium, with different geometries were used, one with amirror polished surface (complex geometry) (Fig. 2a, b) andanother with an optical polished surface (simple geometry)(Fig. 2c, d) (Table 1). The mold has two parts: the stamp and thematrix. For the mold with complex geometry the stamp is of149?119?80 mm in size and the matrix of 149?119?50 mm. Incase of the one with a simple geometry, the stamp is of50?70 mm in size and the matrix has a cylinder form with adiameter of 70 mm. The surface finish of the mirror and opticalpolished molds involved a polishing cloth and diamond paste.Further details on the polishing process are confidential.The mirror polished mold was specially designed for this studyby Technimold (a mold maker) to highlight the role of angles andobstacles in the formation of defaults. Also the mold design did notinclude a special feature that can evacuate the air. This was doneintentionally in order to submit the polished surfaces to aggressiveconditions. The molding process was performed at “Center Tech-nique de la Plasturgie et des Composites” (IPC, France) on a 50 TEngel machine. The injection parameters, listed in Table 2, werechosen in accordance with standard specifications for the injectedpolymers. Based on a numerical simulation they were adapted torespond in conformity with the mold design. Two injection cam-paigns were conducted on this type of mold. After the first cam-paign, on the plane part of the mold stamp, an insert with a dia-meter of 12 mm and a height of 8 mm, was mounted to facilitatethe morphology assessment.For the Surlynsinjection, 3000 parts were injected in the firstcampaign. After surface analysis, the mold was submitted to theindustrial cleaning operation. The second campaign consisted inthe injection of 3700 more parts. SAN and PA66GF25 were injectedon the same mold. During the first campaign, only 8000 SAN partswere injected. Before starting a second campaign, the mold waspolished entirely. The second campaign consisted in the injectionFig. 1. Plastic injection process phases 1.N. Crisan et al. / Tribology International 100 (2016) 388399389of 300 parts of SAN. The insert was changed before starting theinjection of 12 200 PA66GF25 parts.The optical polished mold was provided by Christian DallozSunoptics (CDS) and was used to evaluate the polishing mechan-ical protection and fouling phenomenon considered as composingelements of the mechanical-physico-chemical attack. The injectionprocess was also performed by CDS, on a 145 T Engel machine. Theinjection parameters are listed in Table 3.2.3. MethodThe surface expertize consisted in two main steps: the micro-scopy analysis and the interferometry measurements before andafter injection process. Due to their large dimensions and elevatedmass, the surface analysis of the mirror polished molds was per-formed using a classic optical microscope. For the optical polishedone, thanks to smaller dimensions, the microscope analysis couldbe carried out using a numerical optical microscope (Keyence) anda high resolution environmental scanning electron microscope(FEI XL30 ESEM). Although two injection campaigns have beenperformed, the results presented in this paper, refer only to thesurface expertize performed at the end of the second campaign.For the injected plastic parts, only the interface between moldmatrix plane part and plastic part is discussed in this paper.In order to identify the chemical composition of differentdeposits found on the mirror polished mold surfaces, a FourierTransform Infrared (FTIR) spectrometer was used for the analysis.Fig. 2. Mirror polished mold (complex geometry): a) the stamp; b) the matrix. Optical polished mold (simple geometry): c) the stamp; and d) the matrix.Table 1Configurations mold-materials.Injected materialMirror polishedmoldOptical polishedmoldWhy?SANxxReference materialSurlynsxCorrosivePA66GF25xAbrasivePCxOptical applicationsTable 2Injection Parameters for the mirror polished molds.Injection parametersInjected polymersSANSurlynsPA66GF25Metal temperature which heats the polymer (C)245215300Temperature inside mold (C)403080Injection debit (cm3/s)303020Quantity of injected material (cm3)26.64223Injection time (s)Pressure to avoid shrinkage (bar)0.763.31.61Time to maintain the pressure to avoid shrinkage(s)5204Time to cool down (s)73210Clamping fore (T)507070Length of ejection pin (mm)25.232.530Ejection force (kN)3520Ejection speed (mm/s)1305040Table 3Injection Parameters for the optical polished molds.Injection parametersInjected polymersSANPCMetal temperature which heats the material (C)280280Injection speed (mm/s)3030Quantity of injected material (cm3)3030Pressure to avoid shrinkage (bar)55-3060-30Time to maintain the pressure to avoid shrinkage (s)88Time to cool down (s)1515Clamping fore (kN)13001300N. Crisan et al. / Tribology International 100 (2016) 388399390The infrared technique could not be applied for the opticalpolished mold, because the possible present layers and deposit aretoo thin to be detected with this method.The interferometry measurements were made using a 3D non-contact optical profiler (Sensofar) which combines the confocal andinterferometry techniques. Two objectives were used, one ?10 withan acquisition area of 1200?800 mm and ?50 with an acquisitionarea of 250?150 mm. The acquired data was processed using Moun-tainsMap Universal software. The following height parameters for 3Dareal surface (ISO 25178) were considered in this study: the arithme-tical mean height of the surface (Sa), the root mean square height of thesurface (Sq), the maximum peak-to-valley distance (Sz), the heightdistribution (Ssk) and the flatness of the height distribution (Sku) 8.Bigerelle et al. 9 developed an original methodology appliedto a grained mold surface, to analyze the influence of injectionparameters on the roughness of injected plastic parts, by findingthe evaluation length on which classical parameters can be esti-mated. This work raised the question of finding the right cut-offfilter to ensure that the height parameters values are due to theroughness of the surface and not of the waviness.In this study a five degree polynomial was applied to theacquired data to remove the form. By comparing the values of theheight parameters: Sa, Sq, Szat different scales of the same area, itwas observed that values are identical or very close. So if is takeninto account that the level of polishing is very high (very smoothsurface) and the variation of the values is insignificant, it be canconcluded that the values of the height parameters presented inthis paper are truly related to the roughness of the surface and notof the waviness. A study taking into account the methodologydeveloped by Bigerelle 9 and Van Gorp 10 could be envisagedfor the future in order to apply it on a very smooth surfaces.The interferometry measurements on the mold used for theSurlynsinjection were performed as shown in Fig. 3 (red color).23 measurements were performed in case of the stamp and 17measurements for the matrix. The location of the measured areasfor the mold used for SAN and PA66GF25 injection is also shown inFig. 3 (blue color). In this case, 31 measurements were consideredfor each mold component (Fig. 4).The different defaults and deposits present on the surfaces werenot 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 discussions3.1. Mirror polished mold3.1.1. Injection SurlynsAll along the stamp plane part, deposits different in texture andconsistence can be observed (Fig. 5). Their location and morphologyseem to indicate the flow direction of the molten polymer. Also it canbe noticed, towards the end of the flow, the deposits grow in terms ofthickness and occupied surface.The type of deposit observed in Fig. 5e and f is also observedafter the first injection campaign (3000 injected parts), andappeared that the cleaning operation has been able to remove it,but formed again during the second injection campaign (3700injected parts). This particular deposit is located between theextremity of the oval bump and the hole where one of the ejectionpins acts. Also in this location the flow changes direction, moreprecisely makes a left turn; fact also revealed by the depositmorphology. Its existence can be explained starting with the effectof the injection speed on the molten polymer viscosity, which isconsidered to be a heat transfer mechanism that occurs during theinjection 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 filled, thelocation identified is one of the last to be reached by the moltenpolymer. As the holding phase begins and with it the solidification,the temperature gradient that appears in the injection phasecontinues to act and by doing so it delays the solidification in thisarea. When the established time for the holding phase expires, themechanism of ejection is set in motion. The ejection pin is close tothe identified location and as it was affected by the temperaturegradient and has not yet been entirely solidifies, it will also be thefirst area to be separated from the mold surface. All these canexplain the appearance of the adhesion phenomenon.In Fig. 5d, the deposit appears like a thin film and is also locatedin an area where the flow changes direction. It could also be jus-tified by the temperature gradient, but its aspect and compositionsuggest that may another phenomena can occur. The infraredanalysis performed on this area (Fig. 6c) suggest that only some ofthe wavenumbers match with the ones from the spectrum regis-tered for the injected part (Fig. 6a). It is possible that the gasesreleased from the contact of the molten polymer with the moldsurface reacted with the additives from the raw material compo-sition and facilitated the separation of the thin layer that stick onFig. 3. Interferometry measurements localization for the mirror polished mold. (For interpretation of the references to color in this figure, the reader is referred to the webversion of this article.)Fig. 4. Interferometry measurements localization for the optical polished mold.N. Crisan et al. / Tribology International 100 (2016) 388399391the mold surface. Also the “scraped” aspect of this deposit indi-cates that is more likely that this type of deposit has formedduring the injection phase.The mold matrix surface presents small “islands” deposits thatfollow the flow direction (Fig. 7a). This aspect is in concordancewith what it can be observed on the plastic part surface in theTable 4The values of roughness parameters for the mold used for the Surlynsinjection.Parameters (Objective?10)Mold before injectionSurlynsinjectionStampMatrixStampMatrixSa(nm)5.4470.865.5271.398.6471.236.8871.15Sz(nm)63.96713.7366.6971.79113.8875.7480.66721.5Sq(nm)6.8671.156.9971.7910.8871.568.6671.44Ssk?0.05470.18?0.15670.12?0.01770.14?0.04170.14Sku3.1470.363,2970.383.2170.553.2370.38Table 5The values of roughness parameters for the mold used for the SAN and PA66GF25 injections.Paramtres (Objective?10)Mold before injectionSAN and PA66GF25 injectionStampMatrixStampMatrixSa(nm)5.4470.865.5271.395.9371,711.0273.18Sz(nm)63.96713.7366.6971.7965.52718.7124.99742.77Sq(nm)6.8671.156.9971.797.4772.1513.8373.99Ssk?0.05470.18?0.15670.120.000770.14?0.0570.24Sku3.1470.363,2970.383.1370.323.2470.44Fig. 5. Mold stamp used for the Surlynsinjection various type of deposits found on the plane part.N. Crisan et al. / Tribology International 100 (2016) 388399392same area, shown at a magnified scale in Fig. 8a. This type ofdeposit due to the molten polymer viscosity has led to the for-mation of burrs (excess material in thin layer) possible during theejection phase.Holes (from 14.6 nm to 404 nm deep) are observed beforeinjection probably due to polishing. Their morphology evolvesduring injection process: the holes expand in occupation area anddepth (39.7 nm to 877 nm). In Fig. 7b and c, the pointing redarrows indicate the presence of the evolved holes. They exhibittwo types of morphology. The first type illustrated in Fig. 7b showsvery small holes focused altogether in smaller or larger spots andthe second type illustrated in Fig. 7c presents a hole surrounded bya “cloud” of small holes.Due to the inclusions in the bulk material, grains dislocationcould occur causing the formation of holes during polishing pro-cess. Those holes are modified in term of depth and area duringinjection process. As reported in 11, stress corrosion cracking canaffect the molds, starting at a microscopic level and revealing itselfas crack. The primary causal elements are the metallurgy of steel,the presence of chlorine in the water used in the cooling lines ofthe mold and the stresses on the tool during molding. It is knownthat Chromium gives the steel corrosion resistance, by providing aprotective oxide layer. Thus it is possible that due to the polishingdefects (holes), the thickness of this layer is compromised andthus when a high viscous corrosive polymer, like Surlyns, isinjected, the areas affected by holes, are submitted to corrosionattack. The fact that the feature to evacuate the air was excludedfrom the mold design in conjunction with the corrosion nature ofSurlyns(based on the experience of industrial project partners),can create an aggressive environment at the mold/molten polymerinterface due to the gases release. The high viscosity of Surlynsand its capability to stick onto the mold surface also plays a role interms of exerting a mechanical-physico-chemical attack on thearea where the defaults are located. All these statements allow tocatalog 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 influence is manifested by the formation of accentuatedburrs (Fig. 8b) or accumulation of debris or fragments presented insame form that corrosion pits exhibit on the mold surface.Although the damage mechanism exhibited by the stamp isdifferent than the one for the matrix, some deposits can be foundon the surface. Near the end of injection, where the flow changedirection, small deposits can also be observed (Fig. 7d). Theirlaying out indicate the flow direction. Fine scratches can beobserved in the area where the flow changes direction, but at acloser look, in fact they are traces of deposits.A significant deposit can be observed all along the area wherethe two melting polymer flows encounter (Fig. 7e). The infraredanalysis reveal that this deposit has a different composition thanthe 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 flowdirection area (Fig. 7f). Due to the limitations caused by the dimen-sions of the matrix, the optimal conditions for an appropriate infraredanalysis were not possible. But optically, the texture of these depositsare very similar with the one found on the stamp (Fig. 5e).The initial roughness parameters for the two parts of the moldare listed in first two columns of Table 4. Taking into account theinitial values of Skewness parameter (Ssk) and Kurtosis (Sku), it canbe concluded that the mold surface has a slight tendency to havemore valleys than peaks (Ssko0) and the flatness of the high dis-tribution is wide (Sku43), so the surface is rather plane. It can beobserved also from Table 4, that the values of Sa, Szand Sqare higherafter the mold has been submitted to the injection process.Although initially, there is no significant difference between thevalues of Sa(?0.08 nm) and Sq(?0.13 nm) for the stamp and thematrix, a slight more significant one is observed after injection(?1.76 nm for Saand ?2.2 nm). This difference consists in highervalues of the roughness parameters after injection for the stamp.The peak-to-valley height represented by Szhas a higher value afterinjection in case of the stamp. This supports the statement that thestamp and the matrix present different damage mechanism.Fig. 6. Mold stamp injected with Surlyns registered infrared spectrum for: a) the injected part final product; b) deposit identified on the mold surface (Fig. 5e); c) depositidentified on the mold surface (Fig. 5d).N. Crisan et al. / Tribology International 100 (2016) 388399393Fig. 7. Mold matrix used for the Surlynsinjection identified damaged areas on the plane part.Fig. 8. Injected Surlynspart identified damaged areas on the plane part.N. Crisan et al. / Tribology International 100 (2016) 388399394Although during data processing the defaults and deposits aremasked, it appears that the sticking-releasing phenomena (foundon the stamp) affects more the integrity of the surface than thecorrosion pits (found on the matrix).All the roughness parameters evolved along the flow direction.The value of Saparameter was chosen to highlight this surfaceevolution along the flow direction. The Savalue considered torepresent this variation is the average of the measured valueslocated on a line perpendicular on the flow direction. For bothstamp and matrix, the lowest Savalue can be found at thebeginning of flow on the plane part (C1 on the graphs in Fig. 9). Forthe matrix, this value is not much higher than the one measuredbefore injection. If the Savariation is considered on the lines ofobservations, it can be observed for the stamp that the roughnesshas the tendency to increase on the flow direction (Fig. 9a). Also inthe case of the stamp, the Savalue also varies in function ofobservation column. It has the tendency to increase and decreasefrom one measured area to the next. If the microscopy analysisperformed on the stamp is correlated with the roughness varia-tions, the presence of a lot of deposits with various thicknessjustifies the increase of roughness towards the end of flow.In case of the matrix, the Savalue increases along the centeredobservation lines (L3, L4, L5, L6 Fig. 9b) towards the end of flow.But it decrease on the observation lines situated on the edge. Thevariation of Savalue on the observation columns C3 and C4(Fig. 9b) is very pronounced, as Savalue decreases or increasesdramatically from a measured area to the next. Apparently thematrix surface flatness is severely compromised, but it is not aninjection consequence, it is due to the polishing process that hasproven to be challenging when it comes to high surface finishingon complex geometries.3.1.2. Injection SAN and PA66GF25SAN and PA66GF25 polymers were injected successively on thesame mold. The first polymer to be injected was SAN. When theinjection finished, the insert mounted on the stamp was changedwith a new one and the injection of PA66GF25 began. No sup-plementary cleaning operations were performed. The microscopyanalysis presented below was performed after the PA66GF25injection, and so only the plastic part made of this polymer wasconsidered for the results presented in this paper.Fine scratches can be observed on the stamp surface atbeginning of the plane part (Fig. 10 a). These scratches becomedenser and pronounced in the areas towards the end of injection,where the flow change direction. Oval holes like the one in Fig. 10c are scattered all over the mold surface. Initially formed duringpolishing, it seems that they were enlarged on a perpendiculardirection to the injection flow, by the abrasive action of the glassfibers.The insert and the rest of the stamp surface present differentmorphology in terms that on the insert the scratches are morehighlighted (Fig. 10 b). The interferometry measurements confirma slight difference for the Savalue (?0.76 nm). It is possible that avery thin layer of polymer remained after the SAN injection, and sothe surface was somehow protected against the glass fibers action.The area, where the two injection flows encounter, presentsburn marks and a lot of small deposits like drops (Fig. 10d). Theinfrared analysis reveal that these deposits are not similar incomposition of the injected polymers.The matrix exhibits also elongated holes produced by the glassfibers (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 theirpresence can only be cataloged as a polishing default. Unfortu-nately, this default can affect the quality of the injected part interms of esthetical aspect. For the example, the default in Fig. 11ccan be observed on the plastic part in Fig. 12b.The effect of mold geometry combined with the injectedmaterial effect manifests through the damage effect of glass fibers,and is more visible in the area near the oval bump.Burn marks and deposits like drops are also found in thesame area than in the case of the stamp (Fig. 11d). Also the plasticpart presents burns marks in the same area (Fig. 12c). This can bedue to the lack in the mold design of the feature that would havepermitted air evacuation. But it can also be caused by the abrasiveeffect exerted by the glass fibers in conjunction with physico-chemical reactions at the mold surface asperities level.The PA66GF25 part surface presents black areas, that seem tobe burned (Fig. 12a). Small void pockets can also be identified onthe plastic part surface (Fig. 12d), probably due to different soli-dification times and/or the water vapors that have managed toinfiltrate the mold.In Table 5, the average values of the considered roughnessparameters are listed and the same initial roughness parametersare considered. The values of roughness parameters (Sa, Szand Sq)measured after injection are higher in the case of the matrix. Thedifference between is ?5.5 nm for Sa, ?6.36 nm for Sqand?5947 nm for Sz. For the stamp, the values of Sa, Sqand Szare veryclose to the ones before injection. This can confirm the suspicion ofa thin layer remaining on the surface after the injection of SAN.In this case also the Saparameter was chosen to analyze surfacemodification along flow direction. For the stamp, the Savalueincreases along the centered observation lines (L4,L5,L7, L8 Fig. 13a) towards the end of flow. But along the observation linesFig. 9. Surlynsinjection arithmetic mean roughness (Sa) variation along the flow direction for the: a) stamp; and b) matrix.N. Crisan et al. / Tribology International 100 (2016) 388399395located at the edge, the Savalue decreases in the flow direction.The variation of Savalue along the observation columns C2 and C3,is constant for the majority of measured areas. But towards the topborder (the top edge as viewed in Fig. 3), the Savalue increasedconsiderably (for example, for C3 increases from a value of3.96 nm to 11.1 nm).The variation of Savalue for the matrix, has the same behavioras the one for the stamp. But along the observation columns C4and C2 (Fig. 13b), the value of Saincreases and decreases sig-nificantly from one measured location to the next. The highest Savalue (about 14 nm) can be found in the area around the pinejection holes.Like in the case of the mold used for Surlynsinjection, thesurface flatness of the two mold components is also affected.3.1.3. Comparison between Surlynsinjection and SAN/PA66GF25injectionThe mold surface injected with Surlynsis governed by adhe-sion and corrosion pits. Instead for the mold injected with SAN andPA66GF25, although scarce deposits may be found, polishingdefaults stand out the most. The type of polymer deposits foundon the plane part of the stamp in case of the Surlynsinjection isobserved neither in the case of the SAN injection, nor PA66GF25injection. An explanation is that the change of flow directioninduces an effect on the injection speed. But the injection speedcan be linked with the injected material effect, more precisely theviscosity of the molten polymer exerts a thermic transfer betweenthe molten polymer and the mold surface.The holes found on the mold surface, differ in terms of mor-phology as a function of injected material. For the Surlyns, theFig. 10. Mold stamp used for the SAN and PA66GF25 injections identified damaged areas on the plane part.Fig. 11. Mold matrix used for the SAN and PA66GF25 injections identified damaged areas on the plane part.N. Crisan et al. / Tribology International 100 (2016) 388399396holes are more or less round, surrounded by other small ones.Meanwhile, the ones for the PA66GF25 injection, are elongated ona perpendicular direction to the flow one.The interferometry measurements has been useful to deter-mine a classification for the two mold parts in function of the Savalue. For the stamp, the average Savalue before injection issmaller the ones used for the SAN/PA66GF25 and Surlynsinjec-tions. But the mold stamp injected with Surlynshas a higheraverage Savalue than the one used for the injection of SAN andPA66GF25. Also in case of the mold matrix, the average measuredSavalue before injection remains smaller than the ones obtainedfor the injected molds. But the mold matrix injected with Surlynshas a higher Savalue than the one used for the SAN and PA66GF25injections.3.2. Optical polished moldThe optical polished mold surfaces and the injected plastic partwere analyzed by interferometry and optical microscopy. Theplastic parts were analyzed only on the surface that comes incontact with the mold stamp for both injected polymers (SAN andPC). Due to its smaller size, the stamp could be analyzed with thehigh resolution environmental scanning electron microscope(ESEM).As revealed by ESEM analysis, the mold stamp surface for theSAN injection present very narrow grooves (Fig. 14a). On the otherhand, friction tracks can be observed for the PC injection (Fig.14b).These can occur during injection and can be correlated with thepresence of the flow lines on the injected plastic part.SAN “followed” very well the morphology of the surface of themold. In consequence the micro indentations present on the moldstamp, can be identified as distinguished peaks on the plastic partsurface (Fig. 15). The SAN parts surface presents no visible flowlines. On the contrary the surface of the injected parts in PCexhibits highly visible flow lines (Fig.16a). But their amplitude andperiodicity decrease from the region near the injection sitetowards the area near the end of the injection (Fig. 16b).In Tables 6 and 7 the values for the chosen height parametersare listed. For the mold stamp, the values are higher in case of theone used for the PC injection. The values for the mold matrix aresimilar for the PC and SAN injections. In case of the plastic part itseems that the one injected with PC has higher values than theone injected with SAN, understandable considering the presenceof flow line on the PC part.Fig. 12. Injected PA66GF25 part identified damaged areas on the plane part.Fig. 13. SAN and PA66GF25 injections arithmetic mean roughness (Sa) variation along flow direction for the: a) stamp; and b) matrix.N. Crisan et al. / Tribology International 100 (2016) 388399397Fig. 14. ESEM analysis for the optical mold stamp injected with: a) SAN; and b) PC.Fig. 15. Interferometry measurements: a) the surface on the upper part of the mold; and b) surface of the injected piece.Fig. 16. Interferometry measurements same line of observation: a) surface near the injection point; and b) surface at the end of the injection flow.N. Crisan et al. / Tribology International 100 (2016) 3883993984. ConclusionsThis study has allowed the identification and evaluation ofdefaults that occur during plastic injection process, at microscopicscale.Theresultsobtainedhighlightthedifferentdamagemechanisms sustained by the mold surface, as a function of pol-ishing, geometry and injected material. It can be also observed thatfor each material injected there is a difference of level of wear anddamage mechanism between the stamp and the matrix.Surlynsinjection exhibited considerable amount of deposits onthe mold stamp. It seems that the physico-chemical conditions,created during the injection by this type polymer, favored theadhesion. Also in this case, the couplin
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