Influence of polymer flow on polypropylene morphology, micro-mechanical, and tribological properties of injected part

Ci a ion: O sik, M.; Fuciko a, K.; Manas, L.; S anek, M. In luence o Polyme Flow on Polyp opylene Mo phology, Mic o-Mechanical, and T ibological P ope ies o Injec ed Pa . Lub ican s 2024,12, 202. h ps:// doi.o g/10.3390/lub ican s12060202 Recei ed: 15 Ap il 2024 Re ised: 31 May 2024 Accep ed: 2 June 2024 Published: 4 June 2024 Copy igh : © 2024 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). lub ican s A icle In luence o Polyme Flow on Polyp opylene Mo phology, Mic o-Mechanical, and T ibological P ope ies o Injec ed Pa Ma in O sik * , Kla a Fuciko a, Lukas Manas and Michal S anek Facul y o Technology, Tomas Ba a Uni e si y in Zlin, Va ecko a 5669, 760 01 Zlín, Czech Republic; [email p o ec ed] (K.F.); [email p o ec ed] (L.M.); [email p o ec ed] (M.S.) *Co espondence: [email p o ec ed] Abs ac : This esea ch in es iga es he mic o-mechanical and ibological p ope ies o injec ion- molded pa s made om polyp opylene. The ibological p ope ies o polyme s a e a e y in e es ing a ea o esea ch. Unde s anding ibological p ocesses is e y c ucial. Conside ing ha he mechanical and ibological p ope ies o injec ed pa s a e no uni o m a a ious poin s o he pa , his esea ch was conduc ed o explain he non-homogenei y o p ope ies along he low pa h. Non-homogenei y can be in luenced by nume ous ac o s, including dis ance om he ga e, mold and mel empe a u e, injec ion p essu e, c ys alline s uc u e, cooling a e, he su ace o he mold, and o he s. The key ac o om he mic o-mechanical and ibological p ope ies poin o iew is he polyme mo phology (deg ee o c ys allini y and size o he skin and co e laye s). The mo phology is in luenced by polyme low and he injec ion molding p ocess condi ions. Gained esul s indica e ha he inden a ion me hod was su icien ly sensi i e o cap u e he changes in polyp opylene mo phology, which is a key pa ame e o he esul ing mic o-mechanical and ibological p ope ies o he pa . I was p o en ha he mechanical and ibological p ope ies a e no equal in a ying egions o he pa . Due o cooling and p ocess pa ame e s, he di e ence in he inden a ion modulus in indi idual measu emen poin s was up o 55%, and he ibological p ope ies, in pa icula he ic ion coe icien , showed a di e ence o up o 20%. The a o emen ioned esul s indica e he impac his inding signi ies o injec ion molding echnology in echnical p ac ice. T ibological p ope ies a e a key p ope y o he pa su ace and, oge he wi h mic o-mechanical p ope ies, cha ac e ize he esis ance o he su ace o mechanical ailu e o he plas ic pa when used in enginee ing applica ions. A sui able choice o ga e loca ion, inishing me hod o he ca i y su ace, and p ocess pa ame e s can ensu e he imp o emen o mechanical and ibological p ope ies in s essed egions o he pa . This will inc ease he s i ness and wea esis ance o he su ace. Keywo ds: polyp opylene; ga e dis ance; mic o-mechanical p ope ies; ibological p ope ies; s uc u e; c ys allini y; su ace quali y 1. In oduc ion Injec ion molding is one o he mos commonly used manu ac u ing me hods o p oduce polyme pa s. I is cha ac e ized by a high deg ee o au oma ion, high p oduc i i y, and good olume ic s abili y o injec ed pa s. The polyme is exposed o he mal and mechanical e ec s du ing he injec ion molding cycle. The polyme expe iences a ansi ion om a mol en s a e o a ubbe , glass, o c ys alline s a e. The inal physical, op ical, and mechanical p ope ies o he injec ed pa closely co ela e wi h he c ea ed mic o-s uc u e. Du ing he mel ing phase o he injec ion molding p ocess, he polyme expe iences high shea s ess, no mal p essu e, and a he mal g adien . The high p essu e a he wall leads o he c ea ion o a highly o ien ed lamella mic o-s uc u e, commonly called he skin laye . On he o he hand, he low p essu e in he co e leads o he c ea ion o a sphe uli ic mic o-s uc u e. This di e ence in mo phology be ween he su ace and co e has al eady been in es iga ed in nume ous wo ks [ 1 – 6 ]. This phenomenon is commonly Lub ican s 2024,12, 202. h ps://doi.o g/10.3390/lub ican s12060202 h ps://www.mdpi.com/jou nal/lub ican s Lub ican s 2024,12, 202 2 o 25 called skin–co e mo phology, which can be obse ed by a pola ized op ical mic oscope (Figu e 1). Injec ion-molded semi-c ys alline pa s gene ally con ain 2–5 laye s [7–9]. Lub ican s 2024, 12, x FOR PEER REVIEW 2 o 26 sphe uli ic mic o-s uc u e. This diffe ence in mo phology be ween he su ace and co e has al eady been in es iga ed in nume ous wo ks [1–6]. This phenomenon is commonly called skin–co e mo phology, which can be obse ed by a pola ized op ical mic oscope (Figu e 1). Injec ion-molded semi-c ys alline pa s gene ally con ain 2–5 laye s [7–9]. C ys al g ow h occu s a empe a u es below he mel ing poin (Tm) and abo e he glass ansi ion poin (Tg). Highe empe a u es in e e e wi h he molecula a angemen , while lowe empe a u es lead o he eezing o molecula chains’ mo emen . Seconda y c ys alliza ion can occu unde Tg, al hough in he ime scale o mon hs and yea s. This p ocess in luences he mechanical p ope ies o polyme s and dec eases hei olume due o he mo e compac a angemen o polyme chains [10,11]. The g ow h o c ys alline egions occu s in he di ec ion o he bigges empe a u e g adien . In he case o a s ong g adien , his g ow h is unidi ec ional and has a dend i ic cha ac e . I he empe a u e dis ibu ion is iso opic and s a ic, hen he lamellae g ow adially and c ea e bigge quasi-sphe ical agg ega es called sphe uli es. Sphe uli es’ size anges om 1 o 100 µm [10]. Du ing obse a ion by a pola ized op ical mic oscope, sphe uli es c ea e a g ea numbe o colo ed igu es, including he ypical Mal ese c oss [10–12]. The c ys alliza ion mechanism o he polyme mel is qui e impo an o he injec ion molding o plas ic pa s. Diffe en ypes o c ys alliza ion occu , o example, in ex usion du ing he p oduc ion o ibe s and ilms. The c ys alliza ion heo y indica es ha he hickness o lamellae is always less han he expec ed leng h o molecules. This means ha molecules in c ys als can be olded nume ous imes. Du ing c ys alliza ion, only a pa o he mel is deposi ed in he c ys alline phase, while he emainde eezes in he amo - phous phase, which en elops he c ys alline egions. A common polyole in is a semi-c ys- alline ma e ial wi h a ela i ely complex in e io s uc u e [10]. (a) (b) Figu e 1. Mo phological s uc u e: (a) scheme o skin–co e s uc u e; (b) op ical mic oscope—skin– co e s uc u e. The p oblem o c ys alliza ion du ing injec ion molding was in es iga ed by Le e al. [13], who ocused on he in luence o p essu e on c ys alliza ion kine ics. The de ec ed change in c ys alliza ion empe a u e enabled he iden i ica ion o he p essu e depend- ence o c ys alliza ion kine ic pa ame e s Tm and Tg, which a e used in he Hoffman–Lau- i zen equa ion. C ys alliza ion is he abili y o a ma e ial o c ea e igid s uc u es, which gi e he ma e ial i s speci ic p ope ies. Liu e al. [14] compa ed he mo phology o iPP samples p epa ed by con en ional injec ion molding and mic o-injec ion molding. The samples we e s udied using PLM, SEM, DSC, and WAXD. The esul s showed ha mic o-injec ed samples con ained a much highe pe cen age o o ien ed shea laye s. Va ying c ys alliza ion in diffe en poin s o he injec ed pa was in es iga ed by Sun e al. [15] and Pan ani e al. [16], who ocused on he c ea ion o skin–co e s uc u e in he injec ed pa s. The change in polyp opylene mo - phology is desc ibed by he a angemen and size o sphe uli es in dependence on dis- ance om he wall. The p oblem o c ys alliza ion was esea ched by mo e au ho s [17,18]. Figu e 1. Mo phological s uc u e: (a) scheme o skin–co e s uc u e; (b) op ical mic oscope—skin– co e s uc u e. C ys al g ow h occu s a empe a u es below he mel ing poin (T m ) and abo e he glass ansi ion poin (T g ). Highe empe a u es in e e e wi h he molecula a angemen , while lowe empe a u es lead o he eezing o molecula chains’ mo emen . Seconda y c ys alliza ion can occu unde T g , al hough in he ime scale o mon hs and yea s. This p ocess in luences he mechanical p ope ies o polyme s and dec eases hei olume due o he mo e compac a angemen o polyme chains [10,11]. The g ow h o c ys alline egions occu s in he di ec ion o he bigges empe a u e g adien . In he case o a s ong g adien , his g ow h is unidi ec ional and has a dend i ic cha ac e . I he empe a u e dis ibu ion is iso opic and s a ic, hen he lamellae g ow adially and c ea e bigge quasi-sphe ical agg ega es called sphe uli es. Sphe uli es’ size anges om 1 o 100 µ m [ 10 ]. Du ing obse a ion by a pola ized op ical mic oscope, sphe uli es c ea e a g ea numbe o colo ed igu es, including he ypical Mal ese c oss [10–12]. The c ys alliza ion mechanism o he polyme mel is qui e impo an o he injec ion molding o plas ic pa s. Di e en ypes o c ys alliza ion occu , o example, in ex usion du ing he p oduc ion o ibe s and ilms. The c ys alliza ion heo y indica es ha he hickness o lamellae is always less han he expec ed leng h o molecules. This means ha molecules in c ys als can be olded nume ous imes. Du ing c ys alliza ion, only a pa o he mel is deposi ed in he c ys alline phase, while he emainde eezes in he amo phous phase, which en elops he c ys alline egions. A common polyole in is a semi-c ys alline ma e ial wi h a ela i ely complex in e io s uc u e [10]. The p oblem o c ys alliza ion du ing injec ion molding was in es iga ed by Le e al. [ 13 ], who ocused on he in luence o p essu e on c ys alliza ion kine ics. The de ec ed change in c ys alliza ion empe a u e enabled he iden i ica ion o he p essu e dependence o c ys alliza ion kine ic pa ame e s Tm and T g , which a e used in he Ho man–Lau i zen equa ion. C ys alliza ion is he abili y o a ma e ial o c ea e igid s uc u es, which gi e he ma e ial i s speci ic p ope ies. Liu e al. [ 14 ] compa ed he mo phology o iPP samples p epa ed by con en ional injec ion molding and mic o-injec ion molding. The samples we e s udied using PLM, SEM, DSC, and WAXD. The esul s showed ha mic o-injec ed samples con ained a much highe pe cen age o o ien ed shea laye s. Va ying c ys alliza ion in di e en poin s o he injec ed pa was in es iga ed by Sun e al. [ 15 ] and Pan ani e al. [ 16 ], who ocused on he c ea ion o skin–co e s uc u e in he injec ed pa s. The change in polyp opylene mo phology is desc ibed by he a angemen and size o sphe uli es in dependence on dis ance om he wall. The p oblem o c ys alliza ion was esea ched by mo e au ho s [17,18]. The co ec se ing o injec ion molding p ocess pa ame e s is a key ac o no only o he s able manu ac u ing p ocess bu also o he expec ed inal p ope ies o injec ed pa s. The mos impo an pa ame e s ha signi ican ly in luence he en i e injec ion molding p ocess a e injec ion speed, injec ion p essu e, holding p essu e, du a ion o holding Lub ican s 2024,12, 202 3 o 25 p essu e, mel empe a u e, and mold empe a u e. The indi idual p ocess pa ame e s ac simul aneously and in luence each o he . The e o e, a change o one pa ame e a ec s o he pa ame e s [1]. Fu he mo e, he in luence o p ocess pa ame e s on he mechanical p ope ies o he injec ed pa has been s udied in nume ous o he publica ions. Wang e al. [ 19 ] ocused on he e ec o p ocess pa ame e s (especially injec ion speed) on he mechanical p ope ies o mic o-injec ed PP samples. I was ound ha inc easing injec ion speed led o highe ha dness, which inc eased mo e in he pe pendicula di ec ion o low han in he low di ec ion. A simila s udy was conduc ed by Glogowska e al. [ 20 ], who injec ed samples wi h di e ing p ocess condi ions. The samples we e hen g inded, injec ed again, and hei mechanical p ope ies measu ed. Syku e a e al. [21] in es iga ed he in luence o p ocess condi ions on he polyme iscosi y, which was measu ed di ec ly in he ca i y. In gene al p ac ice, iscosi y is measu ed in heome e s, bu he goal o his s udy was o p o ide eal alues coming s aigh om he manu ac u ing p ocess. S udies [ 22 – 25 ] deal wi h he e ec o mul iple p ocess condi ions (p essu e and empe a u e) on inal low leng h. In conclusion, he submi ed s udy ocuses on he in luence o low leng h (dis ance om he ga e) and p ocess condi ions on he c ea ion o c ys alline mo phology (skin– co e s uc u e). The inal s uc u e a ec s he mic o-mechanical p ope ies o he su ace laye o injec ion-molded polyp opylene. The li e a y esea ch on he p oblem o he in luence o dis ance om he ga e on he mechanical p ope ies o injec ion-molded pa s was conduc ed, and no exis ing publica ion conce ning his opic was ound. In mos cases, he mechanical p ope ies and some imes ha dness we e measu ed only locally and subsequen ly aken as esul s o he o e all pa . Howe e , he e was no s udy ha in es iga ed he p oblem o a ying p ope ies along he polyme low pa h o injec ion- molded p oduc s. The a o emen ioned s udies we e conce ned wi h pa ial esea ch, mainly wi h he in luence o p ocess pa ame e s in injec ion molding on mechanical p ope ies and he e ec o ool quali y on low leng h and su ace eplica ion. Fu he mo e, o he s udies ocused on changes in c ys allini y, bu once again, mos ly locally. Some in es iga ed p ope ies we e esea ched only o mic o-injec ion molding, which is qui e di e en om egula molding. In he ield o mic o-injec ion molding, se e al s udies we e conce ned wi h he in luence o su ace oughness on he low leng h o he polyme . The esul s o some s udies we e simila o mac o-injec ion molding, al hough he dimensions o he inal pa we e in he ange o mic ome e s; hus, he e ec o su ace oughness on polyme low was much g ea e . These esul s canno be di ec ly applied o hose o mac o-injec ion molding due o he signi ican di e ence in size. As he mechanical p ope ies o injec ion- molded p oduc s a e no uni o m along he low leng h, his esea ch was designed o speci ically a ge he non-homogenei y o he p ope ies o injec ion-molded pa s. This non-homogenei y can be in luenced by nume ous ac o s, o example, he dis ance om he ga e, he empe a u e o he mold and mel , injec ion p essu e, c ys alline s uc u e, deg ee o cooling, he su ace o he mold, and o he s. This p oblem has a signi ican e ec on injec ion molding in gene al p ac ice. 2. Ma e ials and Me hods The p epa a ion o he expe imen was inspi ed by he p ac ical equi emen s o manu ac u ing injec ion-molded echnical pa s. Indi idual designs we e checked by injec ion molding simula ion. In o ma ion gained om he simula ion was used o choose he mos sui able echnological pa ame e s. 2.1. Injec ion-Molded Ma e ial (Polyp opylene) This esea ch, as well as he selec ion o ma e ial (polyp opylene), is inspi ed by gene al p ac ice equi emen s, as polyp opylene is commonly used o injec ion molding o echnical pa s. The es ing was conduc ed on polyp opylene, which is a semi-c ys alline he moplas ic wi h he ade name Bo ealis BJ380MO p o ided by Bo ealis (Linz, Aus ia). The selec ed ma e ial is commonly used in he au omo i e indus y, om which he eques Lub ican s 2024,12, 202 4 o 25 o es ing p ope ies along he low leng h o igina ed. Polyp opylene is nowadays being pushed ou by mo e expensi e cons uc ion ma e ials. On he o he hand, polyp opylene is s ill qui e use ul, especially due o i s wide ange o applica ions and p ocessing pa ame e s. The ma e ial p ope ies we e aken om he p o ided ma e ial shee , as can be seen in Table 1. Table 1. Basic p ope ies o injec ion-molded ma e ial. P ope ies Uni Value MFI g/10 min 80 Densi y kg/m3905 Elas ic modulus GPa 1.3 Mel empe a u e ◦C 210–260 Mold empe a u e ◦C 20–60 Holding p essu e MPa Min. 20 2.2. Injec ion Molding The es samples we e p epa ed using he injec ion molding machine All ounde 470 E 1000-290 Golden Edi ion, manu ac u ed by A bu g (Losbu g, Ge many). The mold empe ing was conduc ed by he oil empe ing uni Regloplas 150 Sma , manu ac u ed by Regloplas (S . Gallen, Swi ze land). The p ocess condi ions we e se based on alues gained om he injec ion molding simula ion and ma e ial shee (Table 2). The es samples we e manu ac u ed as ec angula blocks wi h dimensions o 6 × 1 × 240 mm. The selec ion o injec ion molding pa ame e s o his esea ch is once again based on he equi emen s o indus ial p ac ice. This esea ch is only a pa o ex ensi e wo k ega ding he p oblem o he polyme low pa h and i s inal p ope ies. The chosen injec ion molding pa ame e s o his esea ch we e aken close o he lowe bounda y o ecommended pa ame e s gi en by he manu ac u e wi h ega ds o he economic pe spec i e o he leng h o he injec ion molding cycle (mel empe a u e 215 ◦ C, mold empe a u e 30 ◦ C). The ecommended mel empe a u e ange is 215–255 ◦C, and he mold empe a u e is 30–50 ◦C. Table 2. Technological pa ame e s o he injec ion molding p ocess. Technological Pa ame e s Uni Value Injec ion p essu e ba 800 Holding p essu e ba 640 Holding p essu e du a ion s 1 Cooling ime s 20 Clamping o ce kN 1000 Mold empe a u e ◦C 30 Mel empe a u e ◦C 215 Sc ew zone 1 ◦C 215 Sc ew zone 2 ◦C 210 Sc ew zone 3 ◦C 205 Sc ew zone 4 ◦C 200 Sc ew zone 5 ◦C 200 The ca i y o he es ing injec ion mold was in he shape o ou g oo es wi h a ying leng hs, which we e de e mined acco ding o p e iously conduc ed injec ion molding analysis ha ocused on he es ima ion o p ocess pa ame e s ha could lead o he comple e illing o he mold. The leng h o indi idual ca i ies was 208 mm, 158 mm, 68 mm, and 38 mm (Figu e 2a). The ca i y had a ec angula c oss-sec ion wi h a dimension o 6×1 mm and a leng h o 208 mm (Figu e 2b). The connec i i y o he ca i y wi h he unne was ensu ed by a cold slug, which was o a ed by 90 ◦ o each leng h. The shape pla es we e changeable and i in o a uni e sal ame. These pla es we e empe ed h ough d illed channels. Lub ican s 2024,12, 202 5 o 25 Lub ican s 2024, 12, x FOR PEER REVIEW 5 o 26 The ca i y o he es ing injec ion mold was in he shape o ou g oo es wi h a ying leng hs, which we e de e mined acco ding o p e iously conduc ed injec ion molding analysis ha ocused on he es ima ion o p ocess pa ame e s ha could lead o he com- ple e illing o he mold. The leng h o indi idual ca i ies was 208 mm, 158 mm, 68 mm, and 38 mm (Figu e 2a). The ca i y had a ec angula c oss-sec ion wi h a dimension o 6 × 1 mm and a leng h o 208 mm (Figu e 2b). The connec i i y o he ca i y wi h he unne was ensu ed by a cold slug, which was o a ed by 90° o each leng h. The shape pla es we e changeable and i in o a uni e sal ame. These pla es we e empe ed h ough d illed channels. The ca i y o he injec ion mold was manu ac u ed by ine milling, as he equi ed quali y o he su ace was in he ange o Ra 1.6 µm o Ra 2.2 µm. The condi ions o he manu ac u ing we e se acco ding o he equi emen s. (a) (b) Figu e 2. Injec ion mold ca i y: (a) Tes mold, 1—sp ue holde , 2—mold pla e, 3—ejec ion pla es, 4— empe ing sys em igh , 5—mold ame, 6— es mold pla e, 7— empe ing sys em le , 8—sp ue inse ; (b) c oss-sec ion o mold ca i y. 2.3. Injec ion Molding Simula ion The injec ion molding simula ion was pe o med in o de o p o ide da a ha could be compa ed wi h eal esul s and o help wi h he se ing o he p ocess pa ame e s. The simula ion was pe o med in MoldFlow Syne gy, which was made by Au odesk (San Ra- ael, CA, USA). The condi ions we e se acco ding o he ma e ial shee in a way ha closely esembled condi ions du ing injec ion molding. The ma e ial o simula ion was selec ed om he so wa e da abase, which offe s he same ype o polyp opylene (Bo ealis BJ380MO) ha was used o p oduce he injec ion- molded specimens. The pa ame e s o he selec ed ma e ial can be seen in Table 3. Table 3. Basic p ope ies o polyp opylene (Bo ealis BJ380MO). P ope ies Uni Value MFI g/10 min 80 Densi y kg/m3 900 Elas ic modulus GPa 1.3 Mel empe a u e °C 210–260 Mold empe a u e °C 20–60 Ejec ion empe a u e °C 117 The impo ed model was ha ched by a 3D ne wo k made o 4-sided bodies (Figu e 3), which p o ided an adequa e ep esen a ion o he pa in i s en i e olume and Figu e 2. Injec ion mold ca i y: (a) Tes mold, 1—sp ue holde , 2—mold pla e, 3—ejec ion pla es, 4— empe ing sys em igh , 5—mold ame, 6— es mold pla e, 7— empe ing sys em le , 8—sp ue inse ; (b) c oss-sec ion o mold ca i y. The ca i y o he injec ion mold was manu ac u ed by ine milling, as he equi ed quali y o he su ace was in he ange o Ra 1.6 µ m o Ra 2.2 µ m. The condi ions o he manu ac u ing we e se acco ding o he equi emen s. 2.3. Injec ion Molding Simula ion The injec ion molding simula ion was pe o med in o de o p o ide da a ha could be compa ed wi h eal esul s and o help wi h he se ing o he p ocess pa ame e s. The simula ion was pe o med in MoldFlow Syne gy, which was made by Au odesk (San Ra ael, CA, USA). The condi ions we e se acco ding o he ma e ial shee in a way ha closely esembled condi ions du ing injec ion molding. The ma e ial o simula ion was selec ed om he so wa e da abase, which o e s he same ype o polyp opylene (Bo ealis BJ380MO) ha was used o p oduce he injec ion- molded specimens. The pa ame e s o he selec ed ma e ial can be seen in Table 3. Table 3. Basic p ope ies o polyp opylene (Bo ealis BJ380MO). P ope ies Uni Value MFI g/10 min 80 Densi y kg/m3900 Elas ic modulus GPa 1.3 Mel empe a u e ◦C 210–260 Mold empe a u e ◦C 20–60 Ejec ion empe a u e ◦C 117 The impo ed model was ha ched by a 3D ne wo k made o 4-sided bodies (Figu e 3), which p o ided an adequa e ep esen a ion o he pa in i s en i e olume and subsequen display o esul s h oughou he en i e hickness o he pa . The ga e was made in he o m o a conical sp ue. The gene a ed 3D ne wo k was subsequen ly checked o all equi emen s, and hen he simula ion was conduc ed (Table 4). Lub ican s 2024,12, 202 6 o 25 Lub ican s 2024, 12, x FOR PEER REVIEW 6 o 26 subsequen display o esul s h oughou he en i e hickness o he pa . The ga e was made in he o m o a conical sp ue. The gene a ed 3D ne wo k was subsequen ly checked o all equi emen s, and hen he simula ion was conduc ed (Table 4). (a) (b) Figu e 3. (a) Impo ed 3D model; (b) meshed model includes ga e sys em. Table 4. P ope ies o mesh. P ope ies Uni Value Mesh ype - Te ahed al Global edge leng h on su ace mm 1 En i y coun s - 111,686 Aspec a io - 4.61 Figu e 4 displays he empe ing sys em and he mold block. The ajec o y o em- pe ing channels, including he inle and ou le o he empe ing medium, is de i ed om he eal mold concep . The co e, ca i y, and o he mold pla es we e simpli ied as blocks o he sake o simula ion. Geome y de ined in his way was subsequen ly also ha ched by a 3D ne wo k and hen checked. A e his s ep, i was necessa y o se all o he condi- ions o he simula ion, which can be seen in Table 5. (a) (b) Figu e 4. (a) Cooling sys em; (b) mold block. Figu e 3. (a) Impo ed 3D model; (b) meshed model includes ga e sys em. Table 4. P ope ies o mesh. P ope ies Uni Value Mesh ype - Te ahed al Global edge leng h on su ace mm 1 En i y coun s - 111,686 Aspec a io - 4.61 Figu e 4displays he empe ing sys em and he mold block. The ajec o y o empe - ing channels, including he inle and ou le o he empe ing medium, is de i ed om he eal mold concep . The co e, ca i y, and o he mold pla es we e simpli ied as blocks o he sake o simula ion. Geome y de ined in his way was subsequen ly also ha ched by a 3D ne wo k and hen checked. A e his s ep, i was necessa y o se all o he condi ions o he simula ion, which can be seen in Table 5. Lub ican s 2024, 12, x FOR PEER REVIEW 6 o 26 subsequen display o esul s h oughou he en i e hickness o he pa . The ga e was made in he o m o a conical sp ue. The gene a ed 3D ne wo k was subsequen ly checked o all equi emen s, and hen he simula ion was conduc ed (Table 4). (a) (b) Figu e 3. (a) Impo ed 3D model; (b) meshed model includes ga e sys em. Table 4. P ope ies o mesh. P ope ies Uni Value Mesh ype - Te ahed al Global edge leng h on su ace mm 1 En i y coun s - 111,686 Aspec a io - 4.61 Figu e 4 displays he empe ing sys em and he mold block. The ajec o y o em- pe ing channels, including he inle and ou le o he empe ing medium, is de i ed om he eal mold concep . The co e, ca i y, and o he mold pla es we e simpli ied as blocks o he sake o simula ion. Geome y de ined in his way was subsequen ly also ha ched by a 3D ne wo k and hen checked. A e his s ep, i was necessa y o se all o he condi- ions o he simula ion, which can be seen in Table 5. (a) (b) Figu e 4. (a) Cooling sys em; (b) mold block. Figu e 4. (a) Cooling sys em; (b) mold block. Lub ican s 2024,12, 202 7 o 25 Table 5. P ocess se ings o simula ion. P ope ies Uni Value Molding ma e ial - Bo ealis BJ380MO Mold ma e ial - Tool s eel P-20 (1.2311) Injec ion molding machine - All ounde 470e 143 ons 16.4 oz Mel empe a u e ◦C 215 Ejec ion empe a u e ◦C 117 Cycle ime s 30 Mold empe a u e ◦C 30 Coolan - oil Flow a e li /min 50 A e he simula ion was s a ed, all necessa y calcula ions an acco ding o p e- se condi ions. The complex Mold low analyses p o ided nume ous impo an esul s ha we e signi ican o he isualiza ion o e en s aking place in indi idual phases o he manu ac u ing cycle and allowed he e alua ion o quali a i e pa ame e s o he p oduc . The goal o he simula ions was o analyze he ime o illing, injec ion p essu e, and o ien a ion in he skin–co e laye . The esul s o hese analyses p o ide impo an in o ma ion o he s abiliza ion o he injec ion molding p ocess and he p edic ion o polyme beha io in he ca i y. 2.4. T ibological P ope ies T ibological p ope ies we e measu ed using he Mic oCombi es e MCT 3 om An on Paa (G az, Aus ia). The measu emen s we e ca ied ou using he mic o-inden a ion es , which allows he coe icien o ic ion and ab asion esis ance o he es ed su aces o be de e mined. The p inciple o his measu emen is based on he s aigh o wa d mo emen o he inden o (Rockwell cone) wi h a ip angle o 120 ◦ and a ip adius o 100 µ m along he su ace o he es specimen. The p ocess pa ame e s can be seen in Table 6. T ibological p ope ies we e measu ed on a Mic oCombi es e , which was also used in he wo ks o o he au ho s [26,27]. Table 6. Measu emen pa ame e s o ibological p ope ies. Measu emen Pa ame e s Uni Value Applied load N 1 Speed mm/min 10 Leng h mm 5 Acquisi ion Ra e Hz 30 Ahead o he p ocess, he inden o mo es along he su ace wi h a de ined o ce o ini ia e he measu emen de ice. Following he ini ia ion, he inden o pene a es he es sample wi h no mal o ce F n, which leads o ma e ial de o ma ion and he c ea ion o an imp in . The senso s eco d he ic ion o ce F , which is p opo ional o he no mal o ce, and a so-called p e-scan, which is conce ned wi h he su ace p o ile o he es sample be o e he pene a ion dep h P d (pene a ion dep h) and pos -scan o he imp in R d ( esidual dep h), which a e impo an o polyme elaxa ion e alua ion. Valuable in o ma ion abou he ibological p ope ies o he ma e ial can be ob ained om he di e ence in p o ile dep hs (P d− R d ). Finally, he c i ical loads can be accu a ely measu ed using he acous ic emission AE and ic ion coe icien µ . Figu e 5shows a schema ic diag am o he mic o- ibome e . Lub ican s 2024,12, 202 8 o 25 Lub ican s 2024, 12, x FOR PEER REVIEW 8 o 26 Figu e 5. Schema ic diag am o he mic o- ibome e . 2.5. Mic o-Mechanical P ope ies The mic o-mechanical p ope ies measu emen was conduc ed by dep h sensing in- den a ion (DSI) on a Mic o-Combi es e (MCT 3 ) manu ac u ed by An on Paa (G az, Aus- ia). A ou -sided diamond py amid wi h a op angle o 136° (Vicke s inden o ) was used as he inden ing body. The measu emen was conduc ed by he DSI me hod (dep h sens- ing inden a ion), and he da a gained we e e alua ed by he Oli e and Pha me hod. The measu emen was conduc ed acco ding o he ČSN EN ISO 14577 s anda d [28] . Two loading o ces we e used o obse e he changes in he s uc u e (skin–co e laye ). The use o loading o ce 1 N led o a 20 µm dep h o inden a ion, while he applica ion o 5 N led o a 100 µm dep h o pene a ion. The measu emen pa ame e s can be seen in Table 7. Since i was a mic o-ha dness measu emen , which wo ks wi h low inden ing o ce wi h dep h in he ange o µm, he only e alua ed pa ame e s we e inden a ion ha dness, in- den a ion modulus, and inden a ion c eep. Table 7. Pa ame e s o mechanical p ope y measu emen . Measu emen Pa ame e s Uni Measu emen o Skin Laye Measu emen o Skin + Co e Laye Applied load N 1 5 Maximum load du a ion s 90 90 Loading and de-loading speed N/min 2 10 Poisson’s a io - 0.4 0.4 In o de o de e mine mechanical p ope ies and s uc u e changes a a ying dis- ances om he su ace, he es samples we e cu , ixed in esin, and polished. The es samples p epa ed in his way can be measu ed in a di ec ion pe pendicula o he su ace. Indi idual cu s we e conduc ed in i e sec ions e enly dis ibu ed along he low di ec ion (ga e loca ion 0 mm, and hen 79 mm, 158 mm, 198 mm, and 220 mm om he ga e). Figu e 5. Schema ic diag am o he mic o- ibome e . 2.5. Mic o-Mechanical P ope ies The mic o-mechanical p ope ies measu emen was conduc ed by dep h sensing inden- a ion (DSI) on a Mic o-Combi es e (MCT 3 ) manu ac u ed by An on Paa (G az, Aus ia). A ou -sided diamond py amid wi h a op angle o 136 ◦ (Vicke s inden o ) was used as he inden ing body. The measu emen was conduc ed by he DSI me hod (dep h sensing inden a ion), and he da a gained we e e alua ed by he Oli e and Pha me hod. The measu emen was conduc ed acco ding o he ˇ CSN EN ISO 14577 s anda d [ 28 ]. Two loading o ces we e used o obse e he changes in he s uc u e (skin–co e laye ). The use o loading o ce 1 N led o a 20 µ m dep h o inden a ion, while he applica ion o 5 N led o a 100 µ m dep h o pene a ion. The measu emen pa ame e s can be seen in Table 7. Since i was a mic o-ha dness measu emen , which wo ks wi h low inden ing o ce wi h dep h in he ange o µ m, he only e alua ed pa ame e s we e inden a ion ha dness, inden a ion modulus, and inden a ion c eep. Table 7. Pa ame e s o mechanical p ope y measu emen . Measu emen Pa ame e s Uni Measu emen o Skin Laye Measu emen o Skin + Co e Laye Applied load N 1 5 Maximum load du a ion s 90 90 Loading and de-loading speed N/min 2 10 Poisson’s a io - 0.4 0.4 In o de o de e mine mechanical p ope ies and s uc u e changes a a ying dis ances om he su ace, he es samples we e cu , ixed in esin, and polished. The es samples p epa ed in his way can be measu ed in a di ec ion pe pendicula o he su ace. Indi idual cu s we e conduc ed in i e sec ions e enly dis ibu ed along he low di ec ion (ga e loca ion 0 mm, and hen 79 mm, 158 mm, 198 mm, and 220 mm om he ga e). As desc ibed in he ISO 14577 s anda d [ 28 ], he ollowing pa ame e s we e e alua ed: inden a ion ha dness, modulus, and c eep. The calcula ion o he indi idual alues has been ca ied ou using he Oli e and Pha me hod (Figu e 6). Lub ican s 2024,12, 202 9 o 25 Lub ican s 2024, 12, x FOR PEER REVIEW 9 o 26 As desc ibed in he ISO 14577 s anda d [28], he ollowing pa ame e s we e e alu- a ed: inden a ion ha dness, modulus, and c eep. The calcula ion o he indi idual alues has been ca ied ou using he Oli e and Pha me hod (Figu e 6). Figu e 6. Schema ic ep esen a ion o he inden a ion p ocesses shows he dec ease in inden a ion dep h du ing loading (acco ding o Oli e and Pha ). Inden a ion ha dness (HIT) can be de ined as he abili y o a ma e ial o esis plas ic de o ma ion. Inden a ion ha dness HIT (Equa ion (1)) is exp essed by he a io be ween he applied load Fmax and he con ac a ea (Ap) be ween he inden e and he specimen a he maximum dep h and load. The con ac a ea Ap (Equa ion (2)) is speci ied by he shape cons an o he inden o (24.50 o he Vicke s inden e ) and he dep h o con ac o he inden o wi h he es specimen hc [29,30]. 𝐻 = 𝐹  𝐴  (1) 𝐴 =24.50∙ℎ   (2) Ano he ma e ial p ope y ha can be ob ained om inden a ion es ing using he DSI me hod is he inden a ion modulus EIT (Equa ion (3)). The inden a ion modulus EIT is compa able wi h Young’s modulus o he ma e ial. In gene al, he inden a ion modulus can be de e mined om he slope o he angen line used o calcula e he inden a ion ha dness HIT. As desc ibed in he ISO 14577 s anda d, he educed modulus, E , is used o accoun o he ac ha he elas ic displacemen s occu in bo h he inden e and he sam- ple. The ins umen ed elas ic modulus in he es ma e ial, EIT, can be calcula ed om E (Equa ion (5)). The calcula ions in ol e Poisson’s a io ( s), which is usually be ween 0.2 and 0.4 o me allic ma e ials and 0.3 and 0.4 o polyme ic ma e ials [29,30]. The plane s ain modulus E* is calcula ed om he ollowing equa ion 4, whe e Ei is he elas ic modulus o he inden e (diamond 1141 GPa), E is he educed modulus o he inden a ion con ac , and i is Poisson’s a io o he inden e (0.07) [29,30]. Reduced modulus E is calcula ed om he ollowing equa ion 5, whe e C is con ac pliabili y and Ap is con ac a ea, 𝐴= 4.950 ∗ ℎ o he Vicke s inden o [29,30]. 𝐸 =𝐸 ∗∙(1−  ) (3) i i E E E2 * 1 1 1 − − = (4) Figu e 6. Schema ic ep esen a ion o he inden a ion p ocesses shows he dec ease in inden a ion dep h du ing loading (acco ding o Oli e and Pha ). Inden a ion ha dness (H IT ) can be de ined as he abili y o a ma e ial o esis plas ic de o ma ion. Inden a ion ha dness H IT (Equa ion (1)) is exp essed by he a io be ween he applied load F max and he con ac a ea (A p ) be ween he inden e and he specimen a he maximum dep h and load. The con ac a ea A p (Equa ion (2)) is speci ied by he shape cons an o he inden o (24.50 o he Vicke s inden e ) and he dep h o con ac o he inden o wi h he es specimen hc[29,30]. HIT = Fmax Ap(1) Ap=24.50·h2 c(2) Ano he ma e ial p ope y ha can be ob ained om inden a ion es ing using he DSI me hod is he inden a ion modulus E IT (Equa ion (3)). The inden a ion modulus E IT is compa able wi h Young’s modulus o he ma e ial. In gene al, he inden a ion modulus can be de e mined om he slope o he angen line used o calcula e he inden a ion ha dness H IT . As desc ibed in he ISO 14577 s anda d, he educed modulus, E , is used o accoun o he ac ha he elas ic displacemen s occu in bo h he inden e and he sample. The ins umen ed elas ic modulus in he es ma e ial, E IT , can be calcula ed om E (Equa ion (5)). The calcula ions in ol e Poisson’s a io ( s ), which is usually be ween 0.2 and 0.4 o me allic ma e ials and 0.3 and 0.4 o polyme ic ma e ials [29,30]. The plane s ain modulus E* is calcula ed om he ollowing Equa ion (4), whe e E i is he elas ic modulus o he inden e (diamond 1141 GPa), E is he educed modulus o he inden a ion con ac , and iis Poisson’s a io o he inden e (0.07) [29,30]. Reduced modulus E is calcula ed om he ollowing Equa ion (5), whe e C is con ac pliabili y and Apis con ac a ea, pAp=4.950 ∗hc o he Vicke s inden o [29,30]. EIT =E∗·1− 2 s(3) E∗= 1 1 E −1− 2 i Ei (4) E = √π 2·CpAp (5) Inden a ion c eep is de ined as he ela i e change in he inden a ion dep h while he applied load is kep cons an . I is de ined in he ISO 14577 ins umen ed inden a ion Lub ican s 2024,12, 202 16 o 25 owa ds he cen e (Figu e 10b). The di e ence be ween he su ace a he ga e and in he middle o he sample was 24%, while i was 53% a 0.5 mm dep h. Fo inden a ion c eep, a simila imp o emen be ween su ace and cen e was measu ed, up o a 39% inc ease (Figu e 10c). Table 10. S a is ical pa ame e s o inden a ion c eep (%). Leng h o Flow S a is ical Pa ame e s Dis ance om Su ace (mm) mm % 0 0.25 0.5 0.75 1 0 x 12.56 11.92 11.54 12.05 12.56 s 0.31 0.32 0.34 0.36 0.31 76 x 14.32 11.37 11.21 12.13 14.32 s 0.32 0.13 0.09 0.24 0.32 154 x 14.94 10.87 10.69 11.70 14.94 s 0.34 0.19 0.11 0.08 0.34 192 x 14.05 11.64 11.46 12.05 14.05 s 0.36 0.07 0.15 0.31 0.36 208 x 11.00 12.32 11.94 12.05 11.00 s 0.37 0.32 0.34 0.36 0.37 Lub ican s 2024, 12, x FOR PEER REVIEW 17 o 26 10a). A simila end was measu ed o he inden a ion modulus, which ose owa ds he cen e (Figu e 10b). The diffe ence be ween he su ace a he ga e and in he middle o he sample was 24%, while i was 53% a 0.5 mm dep h. Fo inden a ion c eep, a simila imp o emen be ween su ace and cen e was measu ed, up o a 39% inc ease (Figu e 10c). These esul s con i m he indings discussed in Sec ions 3.2 and 3.3 while also show- ing ha he polyme s uc u e is no he same ac oss he en i e c oss-sec ion o he es ed sample. (a) (b) (c) Figu e 10. In luence o dis ance om he su ace o he es sample and ga e dis ance on mechanical p ope ies: (a) inden a ion ha dness; (b) inden a ion modulus; (c) inden a ion c eep. 3.5. Su ace Quali y Resul s o su ace eplica ion (Figu es 11 and 12) indica e ha he su ace quali y o injec ion mold eplica es on es samples o a limi ed deg ee and diffe en ly a indi idual poin s o he pa . Milled su aces displayed de iances in su ace quali ies ha could be caused by he di ec ion o milling. Acco ding o he esul s, he su ace quali y changes o e he cou se o he low leng h. The su ace quali y o he es sample nea he ga e was Ra 1.1 µm, while he mold had 2.1 µm. The su ace quali y a he end o he es sample (225 mm om he ga e) was simila o he ga e. Su ace quali ies a ga e dis ances o 77 mm and 154 mm inc eased o 1.4 µm in compa ison wi h mold, which displayed Ra 1.8 µm. A e his measu emen poin , su ace quali y dec eased all he way owa ds he end. The su ace quali y a he ga e was Rz 7.5 µm o he es sample and Rz 11 µm o he Figu e 10. In luence o dis ance om he su ace o he es sample and ga e dis ance on mechanical p ope ies: (a) inden a ion ha dness; (b) inden a ion modulus; (c) inden a ion c eep. These esul s con i m he indings discussed in Sec ions 3.2 and 3.3 while also showing ha he polyme s uc u e is no he same ac oss he en i e c oss-sec ion o he es ed sample. Lub ican s 2024,12, 202 17 o 25 3.5. Su ace Quali y Resul s o su ace eplica ion (Figu es 11 and 12) indica e ha he su ace quali y o injec ion mold eplica es on es samples o a limi ed deg ee and di e en ly a indi idual poin s o he pa . Milled su aces displayed de iances in su ace quali ies ha could be caused by he di ec ion o milling. Acco ding o he esul s, he su ace quali y changes o e he cou se o he low leng h. The su ace quali y o he es sample nea he ga e was Ra 1.1 µ m, while he mold had 2.1 µ m. The su ace quali y a he end o he es sample (225 mm om he ga e) was simila o he ga e. Su ace quali ies a ga e dis ances o 77 mm and 154 mm inc eased o 1.4 µ m in compa ison wi h mold, which displayed Ra 1.8 µ m. A e his measu emen poin , su ace quali y dec eased all he way owa ds he end. The su ace quali y a he ga e was Rz 7.5 µ m o he es sample and Rz 11 µ m o he injec ion mold, while a he end, he su ace quali y was Rz 4.2 µ m a he ga e and Rz 10.5 µ m o he mold. Lub ican s 2024, 12, x FOR PEER REVIEW 18 o 26 injec ion mold, while a he end, he su ace quali y was Rz 4.2 µm a he ga e and Rz 10.5 µm o he mold. The su ace quali y o he mold eplica ed wi h be e su ace quali y on he es sam- ple. The eplica ion can be in luenced by nume ous ac o s, such as injec ion p essu e, mold empe a u e, enclosed ai , e c. The main pa ame e ha in luences su ace eplica- ion is p essu e d op, which mani es s in a dec ease in su ace quali y om 154 mm om he ga e all he way o he end o he es sample. The su ace p o ile, as shown by he 3D su ace image (Figu e 12), shows ha he diffe ence in eplica ion is signi ican . Du ing low, he polyme ailed o ill he bigges i egula i ies in he mold due o he empe a u e p o ile. Fo all measu ed poin s, a posi- i e end in su ace eplica ion was measu ed. The esul s o he ool’s and es sample’s 2D su ace quali y p o iles indica e ha he highes i egula i ies o he ool su ace we e no eplica ed on op o he es sample, which mani es ed in diffe ing su ace quali y a indi idual measu emen poin s. These endencies we e mos likely in luenced by he p es- su e d op in he ca i y, enclosed ai , and mel empe a u e. (a) (b) Figu e 11. In luence o su ace eplica ion a a ying dis ances om he ga e: (a) su ace quali y Ra, (b) su ace quali y Rz. Figu e 11. In luence o su ace eplica ion a a ying dis ances om he ga e: (a) su ace quali y Ra, (b) su ace quali y Rz. The su ace quali y o he mold eplica ed wi h be e su ace quali y on he es sample. The eplica ion can be in luenced by nume ous ac o s, such as injec ion p essu e, mold empe a u e, enclosed ai , e c. The main pa ame e ha in luences su ace eplica ion is p essu e d op, which mani es s in a dec ease in su ace quali y om 154 mm om he ga e all he way o he end o he es sample. The su ace p o ile, as shown by he 3D su ace image (Figu e 12), shows ha he di e ence in eplica ion is signi ican . Du ing low, he polyme ailed o ill he bigges i egula i ies in he mold due o he empe a u e p o ile. Fo all measu ed poin s, a posi i e end in su ace eplica ion was measu ed. The esul s o he ool’s and es sample’s 2D su ace quali y p o iles indica e ha he highes i egula i ies o he ool su ace we e no eplica ed on op o he es sample, which mani es ed in di e ing su ace quali y a indi idual measu emen poin s. These endencies we e mos likely in luenced by he p essu e d op in he ca i y, enclosed ai , and mel empe a u e. Lub ican s 2024,12, 202 18 o 25 Lub ican s 2024, 12, x FOR PEER REVIEW 19 o 26 Figu e 12. Replica ion o he ool’s su ace on he es sample—Ra 1.6 µm. Figu e 12. Replica ion o he ool’s su ace on he es sample—Ra 1.6 µm. Lub ican s 2024,12, 202 19 o 25 3.6. In luence o Ga e Dis ance on Polyp opylene S uc u e This pa se ed o he obse a ion o mo phology changes ha occu ed du ing he illing and cooling o es samples in he ca i y. The indi idual s uc u al changes we e measu ed a he same dis ances om he ga e as he mechanical p ope ies. 3.6.1. Pola ized Op ical Mic oscope The changes in su ace laye (skin) hickness we e obse ed along he leng h o he pa by a pola ized op ical mic oscope. Mic o ome cu s wi h 20 µ m hickness we e made a indi idual dis ances om he ga e. Du ing injec ion molding, he polyme is o ced o low owa ds he cold su ace o walls, whe e i cools and solidi ies; his is called he oun ain low. This ype o laye displays a high deg ee o o ien a ion (Figu e 13), which di ec ly ansla es o speci ic polyme p ope ies. Lub ican s 2024, 12, x FOR PEER REVIEW 20 o 26 3.6. In luence o Ga e Dis ance on Polyp opylene S uc u e This pa se ed o he obse a ion o mo phology changes ha occu ed du ing he illing and cooling o es samples in he ca i y. The indi idual s uc u al changes we e measu ed a he same dis ances om he ga e as he mechanical p ope ies. 3.6.1. Pola ized Op ical Mic oscope The changes in su ace laye (skin) hickness we e obse ed along he leng h o he pa by a pola ized op ical mic oscope. Mic o ome cu s wi h 20 µm hickness we e made a indi idual dis ances om he ga e. Du ing injec ion molding, he polyme is o ced o low owa ds he cold su ace o walls, whe e i cools and solidi ies; his is called he oun- ain low. This ype o laye displays a high deg ee o o ien a ion (Figu e 13), which di- ec ly ansla es o speci ic polyme p ope ies. Figu e 14 illus a es c oss-sec ional iews o diffe en mo phological s uc u es on he su ace and in he middle o an injec ion-molded ensile sample. As can be seen, he mo phological s uc u e changes wi h he hickness o he injec ed samples. The s uc u e wi h high o ien a ion appea s in he skin laye , and he sphe uli ic s uc u e wi h essen- ially no p e e ed o ien a ion appea s in he co e laye . I has been epo ed ha in he skin laye , because o he high shea s ess and shea s ain, he ex ended polyme chains lead o ex ended chain c ys als. In he co e laye , because o he absence o shea , he an- dom polyme chains lead o lamella , chain- olded c ys als, and, inally, sphe uli es. Hence, he s uc u e is ela ed o low-induced c ys alliza ion, and he sphe uli ic s uc- u e is ela ed o quiescen c ys alliza ion. As can be seen in Figu e 14, he skin laye is no he same along he injec ion-molded pa . The hickes skin laye can be ound a ga e dis ances o 158 mm in he sample (up o 20 µm), while he hinnes skin laye can be ound a he end o he pa . These diffe - ences in he hickness o he highly o ien ed laye a e due o diffe ences in he in ensi y and eloci y o cooling o he polyme du ing mold illing. This is e lec ed in he esul ing skin laye . The high o ien a ion o mac omolecules in he su ace laye p e en s he PLM ligh om passing, as seen in Figu e 14. This su ace laye has a signi ican in luence on he a ying mechanical p ope ies o he pa . Figu e 13. Mechanism o s uc u e c ea ion du ing injec ion molding. Figu e 13. Mechanism o s uc u e c ea ion du ing injec ion molding. Figu e 14 illus a es c oss-sec ional iews o di e en mo phological s uc u es on he su ace and in he middle o an injec ion-molded ensile sample. As can be seen, he mo phological s uc u e changes wi h he hickness o he injec ed samples. The s uc u e wi h high o ien a ion appea s in he skin laye , and he sphe uli ic s uc u e wi h essen ially no p e e ed o ien a ion appea s in he co e laye . I has been epo ed ha in he skin laye , because o he high shea s ess and shea s ain, he ex ended polyme chains lead o ex ended chain c ys als. In he co e laye , because o he absence o shea , he andom polyme chains lead o lamella , chain- olded c ys als, and, inally, sphe uli es. Hence, he s uc u e is ela ed o low-induced c ys alliza ion, and he sphe uli ic s uc u e is ela ed o quiescen c ys alliza ion. Lub ican s 2024, 12, x FOR PEER REVIEW 21 o 26 Figu e 14. Changes in su ace (skin) laye hickness a a ious dis ances om he ga e. 3.6.2. Diffe en ial Scanning Calo ime y This sub-pa ag aph deals wi h obse ed changes in c ys allini y (hea low, Table 11) due o diffe ing dis ances om he ga e (Figu e 15). Resul s o su ace laye c ys allini y indica e ha he highes con en o he c ys alline phase is nea he ga e and a he end o he pa . Towa ds he middle o he sample, he con en o c ys allini y dec eases (Figu e 15b). C ys allini y in he co e laye (Figu e 15d) poin s owa ds he opposi e end as ob- se ed in he skin laye . The highes c ys allini y was measu ed a he cen e o he sample (158 mm). These esul s ag ee wi h he mechanical p ope y measu emen s, which ol- lowed a simila end. I is ob ious om he DSC measu emen s ha he c ys alline phase con en , and hus mic o-mechanical p ope ies, change along he low leng h (Figu e 15). These changes co - espond wi h changes in mic o-mechanical p ope ies. The c ys alliza ion a e is no uni- o m du ing polyme cooling, and so diffe en s uc u es a e c ea ed, including shea -o i- en ed lamellae and sphe uli es. These s uc u es p o ide diffe en mechanical p ope ies. The injec ion molding p ocess is sensi i e o polyme empe a u e, especially du ing cooling, du ing which he molecula chains o ien in he di ec ion o low. In he co e laye , he longe chains can emain in a s e ched-ou s a e, while he sho e chains a e o ien ed andomly du ing illing. In he inal s uc u e, he p e alence o sphe uli es is signi ican . Table 11. Hea low 𝛥𝐻m (J/g). Leng h o Flow Hea Flow 𝛥𝐻m (J/g) mm 100% C ys alline Polyp opylene Skin Co e 0 88.78 84.45 76 87.48 88.69 154 207 84.19 92.01 192 88.28 86.42 208 91.45 86.51 Figu e 14. Changes in su ace (skin) laye hickness a a ious dis ances om he ga e. Lub ican s 2024,12, 202 20 o 25 As can be seen in Figu e 14, he skin laye is no he same along he injec ion-molded pa . The hickes skin laye can be ound a ga e dis ances o 158 mm in he sample (up o 20 µ m), while he hinnes skin laye can be ound a he end o he pa . These di e ences in he hickness o he highly o ien ed laye a e due o di e ences in he in ensi y and eloci y o cooling o he polyme du ing mold illing. This is e lec ed in he esul ing skin laye . The high o ien a ion o mac omolecules in he su ace laye p e en s he PLM ligh om passing, as seen in Figu e 14. This su ace laye has a signi ican in luence on he a ying mechanical p ope ies o he pa . 3.6.2. Di e en ial Scanning Calo ime y This sub-pa ag aph deals wi h obse ed changes in c ys allini y (hea low, Table 11) due o di e ing dis ances om he ga e (Figu e 15). Resul s o su ace laye c ys allini y indica e ha he highes con en o he c ys alline phase is nea he ga e and a he end o he pa . Towa ds he middle o he sample, he con en o c ys allini y dec eases (Figu e 15b). C ys allini y in he co e laye (Figu e 15d) poin s owa ds he opposi e end as obse ed in he skin laye . The highes c ys allini y was measu ed a he cen e o he sample ( 158 mm ). These esul s ag ee wi h he mechanical p ope y measu emen s, which ollowed a simila end. Table 11. Hea low ∆Hm (J/g). Leng h o Flow Hea Flow ∆Hm (J/g) mm 100% C ys alline Polyp opylene Skin Co e 0 88.78 84.45 76 87.48 88.69 154 207 84.19 92.01 192 88.28 86.42 208 91.45 86.51 I is ob ious om he DSC measu emen s ha he c ys alline phase con en , and hus mic o-mechanical p ope ies, change along he low leng h (Figu e 15). These changes co espond wi h changes in mic o-mechanical p ope ies. The c ys alliza ion a e is no uni o m du ing polyme cooling, and so di e en s uc u es a e c ea ed, including shea - o ien ed lamellae and sphe uli es. These s uc u es p o ide di e en mechanical p ope ies. The injec ion molding p ocess is sensi i e o polyme empe a u e, especially du ing cooling, du ing which he molecula chains o ien in he di ec ion o low. In he co e laye , he longe chains can emain in a s e ched-ou s a e, while he sho e chains a e o ien ed andomly du ing illing. In he inal s uc u e, he p e alence o sphe uli es is signi ican . The a o emen ioned esul s co espond wi h polyme beha io in he ca i y, whe e he polyme lows by oun ain low om he middle owa ds he cold su ace o he walls. The polyme mel s, cools apidly a he wall, and c ea es a solid laye . This signi ican cooling imposes a high deg ee o elonga ion o ien a ion in he skin laye , while in o he laye s, he molecules ha e mo e ime o elax. The combined e ec o solidi ica ion and elaxa ion c ea es se e al egions wi h a ying deg ees o o ien a ion (su ace laye , shea laye , and co e). The su ace laye solidi ies quickly wi h nex o no elaxa ion and con ains highly o ien ed molecules. This is caused by elonga ion de o ma ion b ough on by oun ain low. The deg ee o o ien a ion co esponds wi h he low leng h a he momen he mold is illed. The inal o ien a ion along he low leng h is s ongly a ec ed by he holding p essu e phase, as men ioned abo e. The deg ee o o ien a ion and especially di e ences in c ys alline mo phology in indi idual laye s o a pa ha e a signi ican e ec on esea ch p ope ies. In echnical p ac ice, a pa is gene ally unde s ood o ha e uni o m p ope ies along i s en i e leng h. Al hough his does no co espond wi h eali y, he p ope ies can a y a di e en poin s. The indings o his wo k show ha i is no possible o iew one pa as homogenous ( om he mechanical and mo phological poin o iew), bu i is necessa y o ocus on speci ic poin s o he injec ed pa . A sui able choice o ga e loca ion and p ocess Lub ican s 2024,12, 202 21 o 25 condi ions, such as holding p essu e o mold empe a u e, can esul in imp o ed local p ope ies. This can be especially bene icial in pa s wi h local s aining, as hey can be modi ied wi hou equi ing mo e expensi e ma e ial. Lub ican s 2024, 12, x FOR PEER REVIEW 22 o 26 (a) (b) (c) (d) Figu e 15. Measu emen o c ys allini y: (a) DSC cha ac e is ic—skin laye ; (b) c ys allini y o su - ace (skin) laye ; (c) DSC cha ac e is ic—co e laye ; (d) c ys allini y o co e laye . The a o emen ioned esul s co espond wi h polyme beha io in he ca i y, whe e he polyme lows by oun ain low om he middle owa ds he cold su ace o he walls. The polyme mel s, cools apidly a he wall, and c ea es a solid laye . This signi ican cooling imposes a high deg ee o elonga ion o ien a ion in he skin laye , while in o he laye s, he molecules ha e mo e ime o elax. The combined effec o solidi ica ion and elaxa ion c ea es se e al egions wi h a ying deg ees o o ien a ion (su ace laye , shea laye , and co e). The su ace laye solidi ies quickly wi h nex o no elaxa ion and con- ains highly o ien ed molecules. This is caused by elonga ion de o ma ion b ough on by oun ain low. The deg ee o o ien a ion co esponds wi h he low leng h a he momen he mold is illed. The inal o ien a ion along he low leng h is s ongly affec ed by he holding p essu e phase, as men ioned abo e. The deg ee o o ien a ion and especially di - e ences in c ys alline mo phology in indi idual laye s o a pa ha e a signi ican effec on esea ch p ope ies. In echnical p ac ice, a pa is gene ally unde s ood o ha e uni o m p ope ies along i s en i e leng h. Al hough his does no co espond wi h eali y, he p ope ies can a y a diffe en poin s. The indings o his wo k show ha i is no possible o iew one pa as homogenous ( om he mechanical and mo phological poin o iew), bu i is necessa y o ocus on speci ic poin s o he injec ed pa . A sui able choice o ga e loca ion and p o- cess condi ions, such as holding p essu e o mold empe a u e, can esul in imp o ed local p ope ies. This can be especially bene icial in pa s wi h local s aining, as hey can be modi ied wi hou equi ing mo e expensi e ma e ial. Figu e 15. Measu emen o c ys allini y: (a) DSC cha ac e is ic—skin laye ; (b) c ys allini y o su ace (skin) laye ; (c) DSC cha ac e is ic—co e laye ; (d) c ys allini y o co e laye . 4. Discussion This esea ch aims o explain he ibological and mechanical beha io o polyp opy- lene o suppo he bene icial in oduc ion o hose ma e ials in ac ual applica ions. Gen- e ally, he designe s ha e o ake in o conside a ion a se o ibological and mechanical pa ame e s, no only one, including ic ion coe icien , wea , con ac du abili y ela ed o applica ion, and he ha dness o he su ace. Polyme s a e e y p omising ma e ials o be used o ubbing componen s in machines and de ices. Howe e , he selec ion o ma e ials wi h app op ia e ibological and mechanical p ope ies is c i ical. Unde s anding he ic ional and wea mechanisms con olled, in pa icula by he in ensi e and decisi e ans e o ma e ial du ing he ope a ion o polyme ic ibosys ems, is a e y impo an ask o ibologis s. Low cos , co osion esis ance, damping o ib a ions, abili y o adap o wo k in he p esence o con amina ion, and many o he ad an ages o he use o polyme s in sliding (as well as olling) sys ems open up a e y in e es ing esea ch a ea o ibology. This esea ch is pa o la ge-scale esea ch based on sub-pa s om p ac ice, whe e i has been ound ha he mechanical and ibological p ope ies a e no he same along he leng h bu a y a di e en poin s in he injec ion-molded p oduc s. Based on his inding, es molds wi h di e en leng hs, shapes, and c oss-sec ions (s aigh ca i y (Figu e 2), spi al ca i y, and ca i y in he o m o a ensile es body) we e designed, in which he su ace o he ca i y was manu ac u ed by di e en echnologies wi h manual su ace oughness (milled ca i y Ra 1.6 µ m, g inded ca i y Ra 0.8 µ m and Ra 0.45 µ m, polished Lub ican s 2024,12, 202 22 o 25 ca i y Ra 0.1 µ m, and coa ed ca i y TiB 2 ). Subsequen ly, he es ed ma e ials (PP, PA6, e c.) and condi ions o injec ion molding (mel empe a u e in he ange o 215 o 255 ◦ C, mold empe a u e 30–50 ◦ C, and injec ion p essu e 20–80 MPa) we e changed. Based on hese a ia ions, i is possible o decla e ha he esul s demons a ed in his wo k can be used e en in he case o di e en mold ca i ies and su aces, ma e ials, and p ocessing pa ame e s. The main condi ion is ha he p oduc has a shape wi h a longe low pa h. Then, i can be said ha he p oduc ’s p ope ies a e no he same along he leng h o he pa and a e in luenced by nume ous ac o s ha can be a ec ed du ing injec ion molding. These changes ha e a signi ican e ec on he dis ibu ion o he skin–co e laye and, hus, he mechanical and ibological p ope ies. These indings a e c ucial o echnical p ac ice and can be used in he injec ion molding o polyme ma e ials in he indus y. Cu en ly, in echnical p ac ice, he iew o an injec ion-molded p oduc ’s p ope ies is qui e simpli ied, wi h no ega d o he non-homogenei y along he low leng h. Based on he indings o his s udy, mold o p ocess pa ame e s can be modi ied o imp o e he p ope ies o injec ion-molded p oduc s. The a eas o he p oduc ha a e mo e mechanically s ained can be locally ein o ced by hese modi ica ions. The bigges in luence on mechanical and ibological p ope ies along he low is exe ed by ga e placemen , which is closely ollowed by polyme cooling in he ca i y. A local change in mold empe a u e in conc e e a eas could lead o signi ican changes in mechanical p ope ies. Due o he a o emen ioned e ec s, he e could be a change in he mo phological s uc u e o he polyme in he speci ied a ea, which could hen mani es as changed mechanical and ibological p ope ies. This publica ion opens new oppo uni ies o modi ica ion o he injec ion molding p ocess ha could be used o complex applica ions wi h speci ic equi emen s. This knowledge was applied o a p ac ical pa , which was a headlamp bezel, whe e he clamping poin s we e subjec o c acking. By changing he loca ion o he injec ion ga e, changing he mel and mold empe a u e, and modi ying he empe ing ci cui , he mechanical p ope ies we e imp o ed (up o 38%). This was achie ed by mo e in ensi e cooling in he p oblem a ea and also by changing he injec ion ga e loca ion so ha he polyme pa h was no oo long, hus mo ing he s onge spo in o he clamping poin a ea. I is also possible o imp o e hese p ope ies by applying a TiB 2 coa ing o he mold ca i y, which will imp o e he lowabili y o he polyme and inc ease he mechanical p ope ies. As he ollowing esea ch shows, he applica ion o he coa ing inc eased he mechanical p ope ies by 33%. The ibological, mechanical, su ace, and mo phological p ope ies o he injec ion- molded polyp opylene samples we e in es iga ed. I was ound ha he injec ion molding p ocess and he ga e loca ion can be used o inc ease he ab asion esis ance o polyp opy- lene. We ha e also ound ha he ela ionship be ween ibological cha ac e is ics and mo phological cha ac e is ics (skin–co e laye , c ys allini y, e c.) has a majo in luence on he inal p oduc . This can be a ibu ed o he injec ion molding p ocess, he loca ion o he injec ion ga e, and he beha io o he polyme du ing cooling, esul ing in a highe su ace esis ance o he polyp opylene. Thus, i can be concluded ha in o de o achie e op imum ibological and mechanical p ope ies o injec ion-molded polyp opylene, i is necessa y o moni o he mo phological p ope ies o he ma e ial wi h a ocus on c ys allini y, which has a majo in luence on he a o emen ioned p ope ies. Gained esul s o a ying p ope ies along he low pa h o injec ion-molded p ope ies change he iew o polyme beha io du ing injec ion molding and can ha e a signi ican e ec on echnical p ac ice. Fo a complex desc ip ion o his beha io , u he in es iga ion is necessa y, especially wi h a ocus on o he polyme ma e ials o p ocess pa ame e s. 5. Conclusions This wo k deals wi h he in luence o ga e dis ance on he p ope ies o injec ion- molded polyp opylene pa s. The impo ance and cu en esea ch in he a ea o ibological and mechanical p ope ies o injec ion-molded p oduc su aces ha e been ex ensi ely Lub ican s 2024,12, 202 23 o 25 e iewed o p o ide an unde s anding o hei impo ance and bene i s ela ing o hei use in indus y. The e ec o he echnology i sel , manu ac u ing p ocesses, and ela ed p ocess pa ame e s on he ibological p ope ies o injec ion-molded polyp opylene has been discussed in e ms o he complex beha io o he polyme su ace. Unde s anding he mechanisms o ic ion and wea on an injec ion-molded pa is e y impo an o designe s and, when co ela ed wi h mechanical p ope ies and especially mo phological p ope ies, opens up a e y in e es ing a ea o esea ch in he ield o ibology. P epa ed es samples we e measu ed o hei mic o-mechanical p ope ies (inden a ion ha dness, inden a ion modulus, inden a ion c eep), ibological p ope ies ( ic ion o ce, acous ic emission, ic ion coe icien ), su ace quali y, and s uc u al changes. The es samples showed he e ogenic beha io along he low leng h as well as wi hin indi idual dep hs o he pa . In he su ace laye (dep h o measu emen 20 µ m), he mechanical p ope ies dec eased om he ga e o he middle o he sample (158 mm). This dec ease was in o al 15% o inden a ion ha dness and 55% o inden a ion modulus. The ibological p ope ies also showed simila beha io o he mic o-mechanical p ope ies. The coe icien o ic ion has i s maximum alue a he beginning and end o illing and dec eases owa ds he middle dis ance o illing. The coe icien o ic ion inc eases signi ican ly wi h c ys allini y. The di e ence in he ibological p ope ies (coe icien o ic ion) be ween he indi idual poin s o he pa was up o 20%. Towa ds he end o he sample, an opposi e end was obse ed. Deepe in he su ace laye (100 µ m), an opposi e end was ound o mechanical p ope ies, wi h i s maximum a he cen e o he sample. The inc ease owa ds he cen e o he sample was 43% o inden a ion ha dness and 120% o inden a ion modulus. Also, he eplica ion o he ool su ace on he es sample su ace showed signi ican changes along he polyme low. The measu ed esul s we e p obably in luenced by he illing p ocess as well as he p ocess pa ame e s o injec ion molding. These pa ame e s a ec ed he c ea ion o he inal skin–co e s uc u e and c ys allini y, which a ied along he low leng h bu also h ough he c oss-sec ion. The esul s indica e ha he p ope inden a ion me hod can ca ch changes in ibological and mechanical p ope ies ha we e in luenced by polyp opylene mo phology. Thus, mo phology changes can be co ela ed wi h ibological and mechanical changes. In conclusion, his wo k demons a es ha he p ope ies o injec ion-molded pa s a e no uni o m along he en i e sample bu change locally acco ding o condi ions wi hin he mold. This signi ican ly al e s how ibological and mic o-mechanical p ope ies a e looked upon in injec ion-molded pa s. An impo an pa is played by he way a mold is illed and how his, oge he wi h low beha io , in luences he inal p ope ies a speci ic poin s o a pa . Au ho Con ibu ions: Concep ualiza ion, M.O.; me hodology, M.O. and M.S.; o mal analysis, M.S. and K.F.; da a cu a ion, M.O. and M.S.; w i ing—o iginal d a p epa a ion, M.O.; isualiza ion, M.O. and L.M.; p ojec adminis a ion, M.O.; unding acquisi ion, M.S. All au ho s ha e ead and ag eed o he published e sion o he manusc ip . Funding: This a icle was w i en wi h he suppo o he p ojec TBU a Zlin In e nal G an Agency (No. IGA/FT/2024/003). Ins i u ional Re iew Boa d S a emen : No applicable. In o med Consen S a emen : No applicable. 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