Ci a ion: Piska, M.; U banco a, K.
Ad anced Machining o Join Implan
UHMWPE Inse s. Machines 2022,10,
1008. h ps://doi.o g/10.3390/
machines10111008
Academic Edi o : Yuwen Sun
Recei ed: 16 Sep embe 2022
Accep ed: 28 Oc obe 2022
Published: 1 No embe 2022
Publishe ’s No e: MDPI s ays neu al
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A ibu ion (CC BY) license (h ps://
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machines
A icle
Ad anced Machining o Join Implan UHMWPE Inse s
Mi osla Piska * and Ka e ina U banco a
Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, FME IMT, 2 Technicka, 616 69 B no, Czech Republic
*Co espondence: [email p o ec ed].cz; Tel.: +420-5-4114-2555
Abs ac :
The mode n o hopaedic implan s o applica ions in hips, knees, shoulde s, and spines a e
composed o ha d me al alloys o ce amics and a ibological sub-componen ha is made o so ma-
e ials, wi h good ic ional p ope ies—e.g., UHMWPE (Ul a High Molecule Weigh Polye hylene).
The UHMWPE implan s need o be machined in o hei inal shape a e he polyme iza ion and
consolida ion in o a blank p o ile o nea ne shaped implan . Thus, machining is a c ucial echnology
ha can gene a e an accu a e and p ecise shape o he implan ha should comply wi h he join s’
unc ion. Howe e , he machining echnology can a ec he opog aphy and in eg i y o he su ace,
ansmi ed s esses, and esis ance o wea . Technology, cu ing ools, and cu ing condi ions can
ha e an impac on he physical and mechanical p ope ies o he en i e implan and i s longe i y.
This pape shows an e ec i e and compe i i e echnology o acqui ing high-quali y inse shape,
dimensions, and su ace, needed especially o cus omized implan s.
Keywo ds: machining; UHMWPE; implan ; su ace in eg i y; ibology
1. In oduc ion
Cus omized pa ien -ma ched echnology join eplacemen p esen s a ision o be e
medical ca e ha e lec s oday’s knowledge o ma e ials and compu e aided echnolo-
gies [
1
]. The echnology can minimize he loss o human bone compa ed o he s anda d
su ge y, and he use o uni o med p oduc ion se ies p olongs he li e o implan s and im-
p o es he implan s’ sus ainabili y. The design o he join is based on a h ee-dimensional
image o a join made by magne ic esonance imaging, which allows he su geons o iden-
i y he ex en o a epai needed o he enginee s o p oduce a ue model o he join ,
all new pa s, and co ec sizes. This has been p o en success ul in he p oduc ion o he
me al knee componen s made o Ti-6Al-4V ELI wi h he elec on beam mel ing machine
A cam Q10+. The excellen ma e ial s uc u e was based on he use o globula Ti-6Al-4V
ELI pa s o size 50.65
±
22.46
µ
m (mean
±
s anda d de ia ion) in diame e , which we e
mel ed in a acuum chambe [
2
] in an op imized way o sin e ing. This ma e ial is di icul
o machine [
3
], bu exhibi s unique s uc u es and p ope ies needed o emo al and ibial
join p os hesis [
4
] in a wide ange o mo phologies. The e y dense, solid s uc u es can
be used o ibological, highly loaded applica ions and he spongious s uc u es o a e y
good ancho ing o he bone and bone ing ow h [
5
,
6
]. The ma e ial quali y, su ace in eg i y,
mechanical p ope ies, machinabili y o he alloy by g inding, polishing and umbling, and,
inally, a high esis ance o he implan o a igue and wea a e c ucial [
6
]. Such ad an ages
open no el echnologies o join eplacemen s [
7
]. Howe e , he p oblem a ose wi h he
p oduc ion o he “so ” pa ha makes he ibological con a pa . The pa o indi idual
knee design should be machined om a solid block o ma e ial. The basic machining
echnologies a e u ning and milling (each can be used as oughing o inishing). The e
a e many ways o machine hese su aces. Ne e heless, p oblems such as low igidi y o
he p oduc , poo he mal p ope ies o he ma e ial, high mel iscosi ies, and s icking
o he ma e ial o he cu ing edge (p oduc ion o buil -up edges) should be aced. The
UHMWPE ma e ial may be damaged by excessi e hea , eed a e, cu ing o ce, and ool
mic o-geome y. The shapes and dimensions o he cus omized implan s a y b oadly in
Machines 2022,10, 1008. h ps://doi.o g/10.3390/machines10111008 h ps://www.mdpi.com/jou nal/machines
Machines 2022,10, 1008 2 o 19
humans, which complica es he machining echnology. No s anda d p og ams like hose
used o uni ied implan shapes and dimensions can be used epea edly, so each join mus
be designed and p oduced indi idually. Howe e , his esul s in a longe implan li e
and be e com o o pa ien s. Many p oduce s do no publish he de ails abou p ecise
machining o he UHMWPE pa s, bu d y o c yogenic machining, acuum as ening o
he wo kpieces, e y sha p cu ing ools, and low cu ing condi ions a e gene ally known
and ecommended. The o a ional pa s a e u ned, bu he shaped pa s (as he emo al
join inse s) can be milled wi h ball milling cu e s only due o complex geome ies o he
unc ional su aces. This echnology p e ails o he s anda dized join pa s p oduced in
a ious sizes and se ies. The main ad an age o he echnology is o sa is y cu en needs
in si u a he join su ge y in he hospi al. Howe e , he machining should be s udied
mo e, because he quali y o he unc ional pa su aces a ec s he o al success o he
a h oplas y and longe i y o he implan s.
2. Ma e ials and Me hods
2.1. Ma e ial and Design o Join Inse s
The e a e mo e han one hund ed design solu ions o knee implan s all o e he
wo ld, bu he e is a s ong de elopmen in cus omized p oduc s, indi idual o each human
o animal. Basically, he solu ions a e p incipally simila , and mos o hem con ain an inse
o gua an ee a low ic ion in e ace be ween he emo al and ibia pa s made om me al
alloys o ce amics [
8
,
9
]. The use o UHMWPE o he inse seems o be p e ailing [
10
–
12
]
and longs anding [
13
] in su gical p ac ice. The ma e ial was polyme ized in he 1950s and
he i s UHMWPE ib es we e comme cialized in he la e 1970s. UHMWPE is a ype o
polyole in ha is composed o e y long chains o polye hylene wi h a e y high pe cen age
o pa allel o ien a ion and a high le el o c ys allini y. The ex emely long polyme chains
enable load ans e by s eng hening in e molecula in e ac ions. The ma e ials su e
om gamma adia ion, which a ec s hei molecula s abili y and mechanical p ope ies
like o he ma e ials o medical applica ions [
14
], bu so a hei ibological p ope ies
and esis ance o a igue a e unique [
15
] and he e a e many echnologies o p ocessing
i acco ding o he ad anced s anda ds [
16
–
19
]. This speci ica ion [
16
] co e s UHMWPE
powde and ab ica ed o ms o use in su gical implan s. UHMWPE powde shall be
o i gin polyme manu ac u ed om a homopolyme o e hylene. Tes s o iscosi y
numbe , elonga ion s ess, ash con en , ex aneous ma e , and ace elemen s shall be
pe o med o UHMWPE powde s, while es s o densi y, ash con en , ensile s eng h,
yield s eng h, elonga ion, and impac s eng h shall be pe o med o ab ica ed o ms. The
documen [
17
] speci ies he equi emen s and co esponding es me hods o moulding
ma e ials in powde o m made om UHMWPE o use in he manu ac u e o su gical
implan s. The s anda d [
18
] speci ies he equi emen s and co esponding es me hods o
moulded o ms, e.g., shee s and ods, made om UHMWPE o use in he manu ac u e
o su gical implan s. This documen is no applicable o di ec -moulded, i adia ed, o
inished p oduc s, o p oduc s manu ac u ed om polye hylene blended wi h addi i es o
by blending di e en o ms o polye hylene. The documen [
19
] speci ies a es me hod o
in es iga ing he oxida i e s abili y o UHMWPE ma e ials as a unc ion o he p ocessing
and s e iliza ion me hod.
The e a e plen y o o he applica ions in a wide ange o con ac ing su aces— o ming,
manipula ion o ma e ials, guiding pads, suppo ing uni s, e c. The e a e some ypical p oblems
wi h machining hem o he speci ied shapes, dimensions [
20
], and high-quali y su aces [
21
,
22
].
I has been equen ly con i med ha his inse pa su e s ei he om he high
speci ic loading and/o speci ic ma e ial p ope ies, so i s in ensi e wea in knee implan
applica ions is e y common–Figu es 1and 2. The o igins come gene ally om he ma e ial
composi ion [
10
], echnological p ocessing, applica ion, and use [
23
–
29
]. Obse a ions o
wea damage in [
23
] we e pe o med on e ie ed wo n po ous coa ed ana omic ibial
componen s. The noncon o ming a icula ing su aces on hin polye hylene componen s
would be a highe isk o damage han mo e con o ming su aces on hicke componen s.
Machines 2022,10, 1008 3 o 19
Mo eo e , he high cyclic loads a e mos esponsible o he deg ada ion in p ope ies o
he ma e ial nea he a icula ing su ace. Acco ding o [
24
], he wea o ma e ials causes
di e en consequences depending on he ype o he sys em in which i occu s. In biological
sys ems, pa icles p oduced by wea elici an en i onmen al esponse, e mina ing he li e
o he bea ing sys em as in hip and knee join p os heses. I is shown ha polye hylene wea
p oduces sub-mic ome e pa icles, which a e known o con ibu e o os eolysis and implan
loosening. The mechanisms esponsible o p oduc ion o polye hylene wea pa icles
a e mainly ploughing and ab asion by ha d aspe i ies and/o wea pa icles, as well as
delamina ion wea . The po en ial o os eolysis induced by wea deb is is highligh ed by he
wo k [
25
] also. These esul s sugges ha ea ly knees may ha e simila wea mechanisms
and indica e ha simila olumes o biologically ac i e mic ome e and sub-mic ome e
UHMWPE pa icles we e p oduced, which may ha e impo an implica ions in he longe -
e m ou come o o al knee a h oplas ies. Ano he wea mechanism—delamina ion, in
which a hin su ace shee o polye hylene sepa a es om he deepe laye s—was con i med
in [
26
] and some ea lie wo ks also. I has been con i med ha he delamina ion is no only
a p oblem o ca bon- ein o ced UHMWPE, bu he mechanism o damage can be ound in
no mal UHMWPE ma e ials. The same mechanisms and many o he s, such as ab asion,
e osion, adhesion, e c. a e also desc ibed ho oughly in [
27
]. The book [
28
] sugges s a new
app oach o explo ing he e ec o applied load and su ace de ec s on he a igue wea
beha iou o polyme s. I discusses e ec s o su ace c acks unde di e en s a ic and cyclic
loading pa ame e s on wea , and p esen s an in elligen algo i hm o map he ela ionship
be ween wea a e and ele an ac o s.
Machines2022,10,xFORPEERREVIEW3o 19
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
composi ion[10], echnologicalp ocessing,applica ion,anduse[23–29].Obse a ionso 
wea damagein[23]we epe o medon e ie edwo npo ouscoa edana omic ibial
componen s.Thenoncon o minga icula ingsu aceson hinpolye hylenecomponen s
wouldbea highe  isko damage hanmo econ o mingsu aceson hicke componen s.
Mo eo e , hehighcyclicloadsa emos  esponsible o  hedeg ada ioninp ope ieso 
hema e ialnea  hea icula ingsu ace.Acco ding o[24], hewea o ma e ialscauses
di e en consequencesdependingon he ypeo  hesys eminwhichi occu s.Inbiolog‐
icalsys ems,pa iclesp oducedbywea elici anen i onmen al esponse, e mina ing
heli eo  hebea ingsys emasinhipandkneejoin p os heses.I isshown ha polye h‐
ylenewea p oducessub‐mic ome e pa icles,whicha eknown ocon ibu e oos eol‐
ysisandimplan loosening.Themechanisms esponsible o p oduc iono polye hylene
wea pa iclesa emainlyploughingandab asionbyha daspe i iesand/o wea pa i‐
cles,aswellasdelamina ionwea .Thepo en ial o os eolysisinducedbywea deb isis
highligh edby hewo k[25]also.These esul ssugges  ha ea lykneesmayha esimila 
wea mechanismsandindica e ha simila  olumeso biologicallyac i emic ome e and
sub‐mic ome e UHMWPEpa icleswe ep oduced,whichmayha eimpo an implica‐
ionsin helonge ‐ e mou comeo  o alkneea h oplas ies.Ano he wea mechanism—
delamina ion,inwhicha hinsu aceshee o polye hylenesepa a es om hedeepe 
laye s—wascon i medin[26]andsomeea lie wo ksalso.I hasbeencon i med ha  he
delamina ionisno onlyap oblemo ca bon‐ ein o cedUHMWPE,bu  hemechanism
o damagecanbe oundinno malUHMWPEma e ials.Thesamemechanismsandmany
o he s,suchasab asion,e osion,adhesion,e c.a ealsodesc ibed ho oughlyin[27].The
book[28]sugges sanewapp oach oexplo ing hee ec o appliedloadandsu ace
de ec son he a iguewea beha iou o polyme s.I discussese ec so su acec acks
unde di e en s a icandcyclicloadingpa ame e sonwea ,andp esen sanin elligen 
algo i hm omap he ela ionshipbe weenwea  a eand ele an  ac o s.
Thepin‐on‐discwea  es s(pinso AISIF138s ainlesss eelo ASTMF136 i anium
alloyagains aUHMWPEdisc,wi hbo inese um)we es udiedin[29].Thelowe  alues
o  ic ioncoe icien we emeasu edwhena hin ilmo polyme was ans e ed o he
me allicsu aces,bu  he iscoelas icbeha iou o UHMWPEwas esponsible o  hein‐
c easein ic ioncoe icien a e a es ingpe iod.
Apa  ommanyo he  es s, hepa ame e ssuchas helocalizedHe ziancon ac 
s esses ha de elopas wocu edjoin su acescomeincon ac (andde o msligh ly)
unde  heimposedloadsandsliding eloci yp esen  hec ucialp oblem.Thead anced
designo acus omizedjoin implan should espec  heindi idualhumano animalana ‐
omyandallbodyp edisposi ions,andmakeab oad la con ac wi h egula ly ansmi ‐
edloadingind y ic ioncondi ions.

Figu e1.TheUHMWPEinse  o  hehumankneejoin :(a)newone,(b) hedamagedsu acea e 
se e alyea so use.
Figu e 1.
The UHMWPE inse o he human knee join : (
a
) new one, (
b
) he damaged su ace a e
se e al yea s o use.
Machines2022,10,xFORPEERREVIEW4o 19
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Figu e2.Ade ailo  hewo nsu aceo  heUHMWPEinse —Adeepca i ya ec ing hequali y
o  he ibologicalcon ac .
Fo  hesegoals, hedesigns a swi h3Dcompu e  omog aphy(CT)o magne ic
esonance(MR),andcompu e digi al econs uc iono  eno a iono  hede ec edjoin 
su aces ollows.A e  hep oduc iono  he emo aland ibiame allicpa s ia,e.g.,elec‐
onbeam echnology(EBM),apiece om henon‐me allicma e ialshouldbeins alled
a  hein e acebe ween heme allicimplan s.
Thispieceo ma e ialshouldgua an eepe ec con ac o  hesu aces o eachape ‐
ec sliding i  ha p oducesminimalcon ac p essu esandalowcoe icien o  ic ion o 
along imewi hou anylub ica ion.Thep ecisep o ilescanbeob ained,e.g.,byscanning
he inishedme alliccomponen sandanalyseso  heimplan c oss‐sec ionsinpe pendic‐
ula planes—seeFigu e3.
UHMWPEisasemic ys allinepolyme consis ingo anamo phousma ix ein‐
o cedbyas i e ,c ys allinelamellaedomain(hal o i ss uc u e)[8–10].UHMWPE
composi escanbeenginee edinawide angea amic o/nanome e ‐leng hscalebyblend‐
ingpolyme powde  esinwi hmic o‐o nanopa iclesand ib esbe o econsolida ion.
UHMWPE ib escanbewo en,kni ed,o manu ac u edin oshee s o mingcomplex
s uc u esandaniso opicp ope ies.UHMWPEcanplay he oleo ma ixand ib ein
ab oad angeo composi ema e ials,dependingupon heapplica ion.Polyme s,which
includeallplas ics,consis o chainso buildingblockscalledmonome s.Thesechains
g owbyaddingnewmoleculeson o hei ends—Figu e4.

Figu e3.Digi alc oss‐sec ionso a emo alkneeimplan , hede ini iono  adiiinsagi al(a,b)and
ans e seplanes(c,d) o  igh kinema icso  headjacen pa o  hejoin – heUHMWPEinse .
Figu e 2.
A de ail o he wo n su ace o he UHMWPE inse —A deep ca i y a ec ing he quali y o
he ibological con ac .
Machines 2022,10, 1008 4 o 19
The pin-on-disc wea es s (pins o AISI F138 s ainless s eel o ASTM F136 i anium
alloy agains a UHMWPE disc, wi h bo ine se um) we e s udied in [
29
]. The lowe alues
o ic ion coe icien we e measu ed when a hin ilm o polyme was ans e ed o he
me allic su aces, bu he iscoelas ic beha iou o UHMWPE was esponsible o he
inc ease in ic ion coe icien a e a es ing pe iod.
Apa om many o he es s, he pa ame e s such as he localized He zian con ac
s esses ha de elop as wo cu ed join su aces come in con ac (and de o m sligh ly)
unde he imposed loads and sliding eloci y p esen he c ucial p oblem. The ad anced de-
sign o a cus omized join implan should espec he indi idual human o animal ana omy
and all body p edisposi ions, and make a b oad la con ac wi h egula ly ansmi ed
loading in d y ic ion condi ions.
Fo hese goals, he design s a s wi h 3D compu e omog aphy (CT) o magne ic
esonance (MR), and compu e digi al econs uc ion o eno a ion o he de ec ed join
su aces ollows. A e he p oduc ion o he emo al and ibia me allic pa s ia, e.g.,
elec on beam echnology (EBM), a piece om he non-me allic ma e ial should be ins alled
a he in e ace be ween he me allic implan s.
This piece o ma e ial should gua an ee pe ec con ac o he su aces o each a
pe ec sliding i ha p oduces minimal con ac p essu es and a low coe icien o ic ion
o a long ime wi hou any lub ica ion. The p ecise p o iles can be ob ained, e.g., by
scanning he inished me allic componen s and analyses o he implan c oss-sec ions in
pe pendicula planes—see Figu e 3.
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Figu e2.Ade ailo  hewo nsu aceo  heUHMWPEinse —Adeepca i ya ec ing hequali y
o  he ibologicalcon ac .
Fo  hesegoals, hedesigns a swi h3Dcompu e  omog aphy(CT)o magne ic
esonance(MR),andcompu e digi al econs uc iono  eno a iono  hede ec edjoin 
su aces ollows.A e  hep oduc iono  he emo aland ibiame allicpa s ia,e.g.,elec‐
onbeam echnology(EBM),apiece om henon‐me allicma e ialshouldbeins alled
a  hein e acebe ween heme allicimplan s.
Thispieceo ma e ialshouldgua an eepe ec con ac o  hesu aces o eachape ‐
ec sliding i  ha p oducesminimalcon ac p essu esandalowcoe icien o  ic ion o 
along imewi hou anylub ica ion.Thep ecisep o ilescanbeob ained,e.g.,byscanning
he inishedme alliccomponen sandanalyseso  heimplan c oss‐sec ionsinpe pendic‐
ula planes—seeFigu e3.
UHMWPEisasemic ys allinepolyme consis ingo anamo phousma ix ein‐
o cedbyas i e ,c ys allinelamellaedomain(hal o i ss uc u e)[8–10].UHMWPE
composi escanbeenginee edinawide angea amic o/nanome e ‐leng hscalebyblend‐
ingpolyme powde  esinwi hmic o‐o nanopa iclesand ib esbe o econsolida ion.
UHMWPE ib escanbewo en,kni ed,o manu ac u edin oshee s o mingcomplex
s uc u esandaniso opicp ope ies.UHMWPEcanplay he oleo ma ixand ib ein
ab oad angeo composi ema e ials,dependingupon heapplica ion.Polyme s,which
includeallplas ics,consis o chainso buildingblockscalledmonome s.Thesechains
g owbyaddingnewmoleculeson o hei ends—Figu e4.

Figu e3.Digi alc oss‐sec ionso a emo alkneeimplan , hede ini iono  adiiinsagi al(a,b)and
ans e seplanes(c,d) o  igh kinema icso  headjacen pa o  hejoin – heUHMWPEinse .
Figu e 3.
Digi al c oss-sec ions o a emo al knee implan , he de ini ion o adii in sagi al (
a,b
) and
ans e se planes (c,d) o igh kinema ics o he adjacen pa o he join – he UHMWPE inse .
UHMWPE is a semic ys alline polyme consis ing o an amo phous ma ix ein o ced
by a s i e , c ys alline lamellae domain (hal o i s s uc u e) [
8
–
10
]. UHMWPE composi es
can be enginee ed in a wide ange a a mic o/nanome e -leng h scale by blending polyme
powde esin wi h mic o- o nanopa icles and ib es be o e consolida ion. UHMWPE
ib es can be wo en, kni ed, o manu ac u ed in o shee s o ming complex s uc u es and
aniso opic p ope ies. UHMWPE can play he ole o ma ix and ib e in a b oad ange o
composi e ma e ials, depending upon he applica ion. Polyme s, which include all plas ics,
consis o chains o building blocks called monome s. These chains g ow by adding new
molecules on o hei ends—Figu e 4.
Machines 2022,10, 1008 5 o 19
Machines2022,10,xFORPEERREVIEW5o 19
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Once o med,polyme scanbeshapedin o3Dobjec susinginjec ionmouldingand
machinedins anda dwaysby u ning,milling,d illing,e c.Howe e ,polyme sa edi ‐
icul  ocu mainlydue o hei  iscoelas icp ope ies,low he malconduc i i y,high
elas ici y,ando he ma e ialcha ac e is ics—Table1.All hesep ope iesa ec  hechip
o ma ionandwo kpiecede o ma ion,and esul inde e io a iono su ace inish ia he
o ma iono buil ‐upedges,chips icking,andpoo su acep oduc ion.Aldwelle al.[22]
ecognizesix ypeso chips o medinpolyme machining,including hedecisi e oleo 
shea ing,c acking,and oolmic o‐geome y.I hasbeen ound he e ha wo kpiecep e‐
cooling oc yogenic empe a u es(100–150K)inc easeswo kpieces i nessandimp o es
su acequali yin hemachiningo UHMWPE.Tomee  unc ional equi emen sand o
becompe i i einaneconomiccon ex ,i isnecessa y o educe hecos so implan p o‐
duc ion.Machinings a egyshould espec  hesu ace obemachined,soballendmilling
cu e swe e he i s choice.Machining imedependson hepa geome y,i sp ecision,
andcu ingcondi ions o  hecalcula ed oolpa hs.Theop imiza ionp oblemcanbe
sol ed iaop imiza ionwi h heobjec i eso  educingcu ing ime,cu ingcos s,su ace
quali y,o maximal empe a u e eachedwhenmachining.Themanu ac u edsu ace
quali ys ems om hecu ingcondi ions.The oolpa hsa ecompu edacco ding o he
su acecu es, ool adius, eedpe  ee h,and adialandaxialdep hso cu s.

Figu e4.Selec edbasics uc u alcha ac e is icso polyme sa e syn hesis—(a)linea polyme 
chain,(b)b anchpolyme ne wo ks uc u es.
Table1.Mechanicalp ope ieso selec edma e ialsusedinmedicalapplica ions(adap ed om
[8–12]).
PMMA PEEK PUR PP UHMWPE
Mola mass[g/mol]1.3×104
−2.2×105
1.4×104
−1.0×105
7×103
−1.2×104
1×105
−6×105
3×106
−6×106
Densi y[kg/m3]1180 1320 15–600 900–910 941–965
C ys allini y[%]0 16–47 0–13 60–75 39–75
Flow empe a u e
T [°C]1.6 334 141–150 176 135
Tensilemodulus[MPa]1800–3100 3000–4000 6 1325 655–1077
Commen :PMMA—polyme hylme hac yla e,PEEK—polye he e he ke one,PUR—polyu e hane,
PP—polyp opylene,UHMWPE—ul ahighmolecula weigh polye hylene.
TheUHMWPEma e ialwaspu chased om heB i ishcompanyO hoplas ics.I 
wasex uded om ineg anulesbyGUR®2024,manu ac u edby hecompanyCelanese.
Themoduluso elas ici yo  hema e ialwasapp oxima ely830MPaandi sdensi ywas
930kg/m3.Thesemi‐ inishedp oduc had heshapeo ap ismwi hdimensionso 80×
100–500mm.Sixblocksampleso 20mmin hicknesswe ecu o usingaBOMARB no
bandsaw,and hebandsawPILANA1640×13×0.65/10zC125WSTANDARDa a
cu ingspeedo 20m/minanda eedspeedo 50mm/minwi hou cooling.
Figu e 4.
Selec ed basic s uc u al cha ac e is ics o polyme s a e syn hesis—(
a
) linea polyme
chain, (b) b anch polyme ne wo k s uc u es.
Once o med, polyme s can be shaped in o 3D objec s using injec ion moulding and
machined in s anda d ways by u ning, milling, d illing, e c. Howe e , polyme s a e
di icul o cu mainly due o hei iscoelas ic p ope ies, low he mal conduc i i y, high
elas ici y, and o he ma e ial cha ac e is ics—Table 1. All hese p ope ies a ec he chip
o ma ion and wo kpiece de o ma ion, and esul in de e io a ion o su ace inish ia he
o ma ion o buil -up edges, chip s icking, and poo su ace p oduc ion. Aldwell e al. [
22
]
ecognize six ypes o chips o med in polyme machining, including he decisi e ole o
shea ing, c acking, and ool mic o-geome y. I has been ound he e ha wo kpiece p e-
cooling o c yogenic empe a u es (100–150 K) inc eases wo kpiece s i ness and imp o es
su ace quali y in he machining o UHMWPE. To mee unc ional equi emen s and o be
compe i i e in an economic con ex , i is necessa y o educe he cos s o implan p oduc ion.
Machining s a egy should espec he su ace o be machined, so ball end milling cu e s
we e he i s choice. Machining ime depends on he pa geome y, i s p ecision, and
cu ing condi ions o he calcula ed ool pa hs. The op imiza ion p oblem can be sol ed ia
op imiza ion wi h he objec i es o educing cu ing ime, cu ing cos s, su ace quali y, o
maximal empe a u e eached when machining. The manu ac u ed su ace quali y s ems
om he cu ing condi ions. The ool pa hs a e compu ed acco ding o he su ace cu es,
ool adius, eed pe ee h, and adial and axial dep hs o cu s.
Table 1.
Mechanical p ope ies o selec ed ma e ials used in medical applica ions (adap ed om [
8
–
12
]).
PMMA PEEK PUR PP UHMWPE
Mola mass [g/mol] 1.3 ×104
−2.2 ×105
1.4 ×104
−1.0 ×105
7×103
−1.2 ×104
1×105
−6×105
3×106
−6×106
Densi y [kg/m3]1180 1320 15–600 900–910 941–965
C ys allini y [%] 0 16–47 0–13 60–75 39–75
Flow empe a u e
T [◦C] 1.6 334 141–150 176 135
Tensile modulus
[MPa] 1800–3100 3000–4000 6 1325 655–1077
Commen : PMMA—polyme hylme hac yla e, PEEK—polye he e he ke one, PUR—polyu e hane, PP—
polyp opylene, UHMWPE—ul a high molecula weigh polye hylene.
The UHMWPE ma e ial was pu chased om he B i ish company O hoplas ics. I
was ex uded om ine g anules by GUR
®
2024, manu ac u ed by he company Celanese.
The modulus o elas ici y o he ma e ial was app oxima ely 830 MPa and i s densi y
was 930 kg/m
3
. The semi- inished p oduc had he shape o a p ism wi h dimensions
o
80 ×100–500 mm
. Six block samples o 20 mm in hickness we e cu o using a BOMAR
B no band saw, and he band saw PILANA 1640
×
13
×
0.65 /10z C125 W STANDARD a
a cu ing speed o 20 m/min and a eed speed o 50 mm/min wi hou cooling.

Machines 2022,10, 1008 6 o 19
2.2. Machining Tes s
The samples we e subsequen ly machined wi h ace milling cu e Oc omil and six
indexable cu ing inse s OFEX05T305FN-M05 PCD20 D125 mm om SECO Tools. The
cu ing speed was 300 m/min and eed a e 315 mm/min a an axial dep h o cu 1 mm,
wi hou any cooling, espec ing he cu ing condi ions men ioned in [
22
,
23
] and ou p elim-
ina y es s. The ec angula pla es we e di ided in o ou a eas by g oo es
10 mm×5 mm
along he samples, so 24 combina ions o cu ing speeds and eed a es could be es ed by
milling in he p epa ed a eas. The spindle was inclined a 45
◦
in all machining es s. Each
a ea was es ed wice, so eal cu ing speeds 70.3, 113.0, 175.8, and 282.6 m/min, adial
s epo e 0.35 mm, and eed a es 315, 450, and 630 mm/min we e se , gi ing he eed
0.012–0.094 mm/ oo h a he milling machine FNK 25, TOS Ku im—Figu e 5a. Due o he
use o a ounded cu ing edge, he mean chip hickness co esponding o he angle
φmax
was ecalcula ed om he maximal chip c oss sec ion and adius o he milling cu e R
acco ding o he Equa ion (1) and Figu e 5b–d:
hm=
ADmax
R.ϕmax , (1)
when he non-de o med chip c oss sec ions a e equalled by he ela ion (2)
ADmax =Am(2)
In addi ion, a special a angemen o clamping ( la con ac o a acuum as ening)
should be a anged, o he wise he UHMWPE la samples end o buckle.
The speci ic cu ing o ce was calcula ed as a a io o he mean cu ing o ce and he
mean chip c oss sec ion—see Equa ion (3):
kc=
Fc
Am(3)
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2.2.MachiningTes s
Thesampleswe esubsequen lymachinedwi h acemillingcu e Oc omilandsix
indexablecu inginse sOFEX05T305FN‐M05PCD20D125mm omSECOTools.The
cu ingspeedwas300m/minand eed a e315mm/mina anaxialdep ho cu 1mm,
wi hou anycooling, espec ing hecu ingcondi ionsmen ionedin[22,23]andou p e‐
limina y es s.The ec angula pla eswe edi idedin o ou a easbyg oo es10mm×5
mmalong hesamples,so24combina ionso cu ingspeedsand eed a escouldbe es ed
bymillingin hep epa eda eas.Thespindlewasinclineda 45°inallmachining es s.
Eacha eawas es ed wice,so ealcu ingspeeds70.3,113.0,175.8,and282.6m/min, a‐
dials epo e 0.35mm,and eed a es315,450,and630mm/minwe ese ,gi ing he eed
0.012–0.094mm/ oo ha  hemillingmachineFNK25,TOSKu im–Figu e5a.Due o he
useo a oundedcu ingedge, hemeanchip hicknessco esponding o heangleϕmax
was ecalcula ed om hemaximalchipc osssec ionand adiuso  hemillingcu e R
acco ding o heEqua ion(1)andFigu e5b–d:
ℎ𝑚𝐴𝐷𝑚𝑎𝑥
𝑅.𝜑𝑚𝑎𝑥
,(1)
when henon‐de o medchipc osssec ionsa eequalledby he ela ion(2)
𝐴
 
𝐴
(2)
Inaddi ion,aspeciala angemen o clamping( la con ac o a acuum as ening)
shouldbea anged,o he wise heUHMWPE la samples end obuckle.
Thespeci iccu ing o cewascalcula edasa a ioo  hemeancu ing o ceand hemean
chipc osssec ion—seeEqua ion(3):
𝑘𝑐𝐹𝑐
𝐴
𝑚
(3)

Figu e5.Machiningexpe imen alse ‐up,(a)ano e iewo  he es a angemen ,(b)unde o med
chipc oss‐sec ionADi,(c)s epo e whenmilling o  eedpe  oo h z,(d)a ecalcula iono  he a ‐
iablechip hicknesshialong hecu ingedge o henominalmean alueo chip hicknesshm.
Thedownd ymillingwaspe o medwi haspecialballendmonoli hiccu e SECO
JS730200D3B.3Z6‐HXT,ISOK10,co e edwi hPVDAlTi(N)coa ing( hickness2m),jus 
de eloped o  hemachiningo aluminaalloyswi hex asha pcu ingedges o pa s
usedinae onau icalenginee ing—Figu es6and7.AliconaIFG‐5(B uke Alicona,G az,
Aus ia)wasused o  heanalysiso  hecu ing oolgeome yandsu ace opog aphyo 
machinedsamples,aswellas hescanningelec onmic oscopeMIRA‐3(TESCANB no,
CzechRepublic).Thepiezo‐elec icdynamome e Kis le 9575B, heKis le cha geampli‐
ie sType2825Aand heDynoWa eso wa e o uni e salda aacquisi ion(Kis le ,Win‐
e hu ,Swi ze land)we eusedwo kinga  hesampling equencyo 1000Hz, helow‐
pass il e sa 100–450Hz,and helong imecons an pe each o cecomponen induced
incu ingo  he ibological es ing.D ymachiningisalways hes ingen  ule o im‐
plan p oduc ion oa oidsu acecon amina ionwi hanemulsion,soakingo  heliquid
Figu e 5.
Machining expe imen al se -up, (
a
) an o e iew o he es a angemen , (
b
) unde o med
chip c oss-sec ion A
D
i, (
c
) s epo e when milling o eed pe oo h
z
, (
d
) a ecalcula ion o he
a iable chip hickness hialong he cu ing edge o he nominal mean alue o chip hickness hm.
The down d y milling was pe o med wi h a special ball end monoli hic cu e SECO
JS730200D3B.3Z6-HXT, ISO K10, co e ed wi h PVD AlTi(N) coa ing ( hickness 2
µ
m), jus
de eloped o he machining o alumina alloys wi h ex a sha p cu ing edges o pa s
used in ae onau ical enginee ing—Figu es 6and 7. Alicona IF G-5 (B uke Alicona, G az,
Aus ia) was used o he analysis o he cu ing ool geome y and su ace opog aphy
o machined samples, as well as he scanning elec on mic oscope MIRA-3 (TESCAN
B no, Czech Republic). The piezo-elec ic dynamome e Kis le 9575B, he Kis le cha ge
ampli ie s Type 2825A and he DynoWa e so wa e o uni e sal da a acquisi ion (Kis le ,
Win e hu , Swi ze land) we e used wo king a he sampling equency o 1000 Hz, he low-
pass il e s a 100–450 Hz, and he long ime cons an pe each o ce componen induced in
cu ing o he ibological es ing. D y machining is always he s ingen ule o implan
Machines 2022,10, 1008 7 o 19
p oduc ion o a oid su ace con amina ion wi h an emulsion, soaking o he liquid in o he
plas ics, e c. Each combina ion o cu ing condi ions was es ed a minimum o six imes o
ob ain s a is ically signi ican esul s (p< 0.05) o measu emen o o ces, su ace quali y
and ibological es ing. The da a unde wen Gaussian dis ibu ion, p esen ed by a e ages
and s anda d de ia ions.
Machines2022,10,xFORPEERREVIEW7o 19


in o heplas ics,e c.Eachcombina iono cu ingcondi ionswas es edaminimumo six
imes oob ains a is icallysigni ican  esul s(p<0.05) o measu emen o  o ces,su ace
quali yand ibological es ing.Theda aunde wen Gaussiandis ibu ion,p esen edby
a e agesands anda dde ia ions.

Figu e6.Ballendmillingcu e SECOJS730200D3B.3Z6‐HXT.(a)ano e iew,(b)a on al iew,
(c)cu ingedgegeome yin heo hogonalplane.

Figu e7.Theballendmillingcu e andi sdimensions.DMM=20mm,OAL=121mm,APMXS=
62mm,DC=20mm,Z=6,(a)ske ch,(b) he akeplaneA, he lankplaneA, heo hogonalplane
Po,(c) hesu ace oughnessa  heo hogonal akeplanePo.
NewCu ingTool
Thenex  esea chwasbasedon he e e ences[20,22],ou expe ience,andexcellen 
esul so singlepoin diamond u ning[30]used o  hemachiningo polyme s o op ics.
Apa  omi suniqueha dness,diamondhasse e al imes(2–10×)highe  he malcon‐
duc i i y hano he cu ingma e ials(cemen edca bides,ce amics, ools eels),so he
de o ma ionhea  omcu ingcanbe ansmi ede ec i ely h ough hecu ingedge,
supp essingas ickye ec andbu  o ma ionon hechip– oolin e ace.Un o una ely,
he u ningope a ionsp esen acon inuouspolyme chip lowa highcu ingspeedsand
i isno applicable o asymme icalo mo ecomplica edshapesusedinimplan designs.
Thus,dec easing hecu ingspeedwassuppo edinou  es s,bu a ec ed he imepe 
machining,sowedecided omodi y hecu ingedgeand e lec  heapp oxima eEqua‐
ion(4) o  hea e age oughness[31]:
𝑅𝑎  32.
𝑓

𝑟

(4)
Thegene alo e iewo  henewcu ingblade(aso o inne sca ing)canbeseen
inFigu e8.Thediame e o  he oolwas30mmand he ip adiuswas0.4–0.6m.The
Figu e 6.
Ball end milling cu e SECO JS730200D3B.3Z6-HXT. (
a
) an o e iew, (
b
) a on al iew,
(c) cu ing edge geome y in he o hogonal plane.
Machines2022,10,xFORPEERREVIEW7o 19


in o heplas ics,e c.Eachcombina iono cu ingcondi ionswas es edaminimumo six
imes oob ains a is icallysigni ican  esul s(p<0.05) o measu emen o  o ces,su ace
quali yand ibological es ing.Theda aunde wen Gaussiandis ibu ion,p esen edby
a e agesands anda dde ia ions.

Figu e6.Ballendmillingcu e SECOJS730200D3B.3Z6‐HXT.(a)ano e iew,(b)a on al iew,
(c)cu ingedgegeome yin heo hogonalplane.

Figu e7.Theballendmillingcu e andi sdimensions.DMM=20mm,OAL=121mm,APMXS=
62mm,DC=20mm,Z=6,(a)ske ch,(b) he akeplaneA, he lankplaneA, heo hogonalplane
Po,(c) hesu ace oughnessa  heo hogonal akeplanePo.
NewCu ingTool
Thenex  esea chwasbasedon he e e ences[20,22],ou expe ience,andexcellen 
esul so singlepoin diamond u ning[30]used o  hemachiningo polyme s o op ics.
Apa  omi suniqueha dness,diamondhasse e al imes(2–10×)highe  he malcon‐
duc i i y hano he cu ingma e ials(cemen edca bides,ce amics, ools eels),so he
de o ma ionhea  omcu ingcanbe ansmi ede ec i ely h ough hecu ingedge,
supp essingas ickye ec andbu  o ma ionon hechip– oolin e ace.Un o una ely,
he u ningope a ionsp esen acon inuouspolyme chip lowa highcu ingspeedsand
i isno applicable o asymme icalo mo ecomplica edshapesusedinimplan designs.
Thus,dec easing hecu ingspeedwassuppo edinou  es s,bu a ec ed he imepe 
machining,sowedecided omodi y hecu ingedgeand e lec  heapp oxima eEqua‐
ion(4) o  hea e age oughness[31]:
𝑅𝑎  32.
𝑓

𝑟

(4)
Thegene alo e iewo  henewcu ingblade(aso o inne sca ing)canbeseen
inFigu e8.Thediame e o  he oolwas30mmand he ip adiuswas0.4–0.6m.The
Figu e 7.
The ball end milling cu e and i s dimensions. DMM = 20 mm, OAL = 121 mm,
APMXS = 62 mm
, DC = 20 mm, Z = 6, (
a
) ske ch, (
b
) he ake plane A
γ
, he lank plane A
α
, he
o hogonal plane Po, (c) he su ace oughness a he o hogonal ake plane Po.
New Cu ing Tool
The nex esea ch was based on he e e ences [
20
,
22
], ou expe ience, and excellen
esul s o single poin diamond u ning [
30
] used o he machining o polyme s o
op ics. Apa om i s unique ha dness, diamond has se e al imes (2–10
×
) highe he mal
conduc i i y han o he cu ing ma e ials (cemen ed ca bides, ce amics, ool s eels), so he
de o ma ion hea om cu ing can be ansmi ed e ec i ely h ough he cu ing edge,
supp essing a s icky e ec and bu o ma ion on he chip– ool in e ace. Un o una ely,
he u ning ope a ions p esen a con inuous polyme chip low a high cu ing speeds and
i is no applicable o asymme ical o mo e complica ed shapes used in implan designs.
Thus, dec easing he cu ing speed was suppo ed in ou es s, bu a ec ed he ime pe
machining, so we decided o modi y he cu ing edge and e lec he app oxima e Equa ion
(4) o he a e age oughness [31]:
Ra =32. 2
ε
(4)
The gene al o e iew o he new cu ing blade (a so o inne sca ing) can be seen
in Figu e 8. The diame e o he ool was 30 mm and he ip adius was 0.4–0.6
µ
m. The
Machines 2022,10, 1008 8 o 19
p o o ype was made o he ha dened s eel DINX155C VMo121 (62
±
1 HRC). The cu ing
speed was 50 m/min, axial dep h o cu 0.005 mm, and adial wid h o cu ( he s epo e )
0.010 mm.
Machines2022,10,xFORPEERREVIEW8o 19


p o o ypewasmadeo  heha deneds eelDINX155C VMo121(62±1HRC).Thecu ing
speedwas50m/min,axialdep ho cu 0.005mm,and adialwid ho cu ( hes epo e )
0.010mm.

Figu e8.Thecylind icalcu ingblade o  inishing hemachinedsu acebylongi udinalsca ing.
(a)cu ingend,(b)de ailo  hecu ingedge.
2.3.T ibologicalTes s
Se e al esea chwo ksha ebeendealingwi h hepin‐on‐disc echniquesande ec 
o sliding eloci yon he ic ionandwea o UHMWPE o usein o ala i icialjoin s
[32]and ibologicalbeha iou o ul a‐highmolecula weigh polye hylene(UHMWPE)
unde d y ecip oca ingsliding[33].
The ibological es sin hiswo kuseda ealcus omizedimplan  omTi6Al4VELI
(g oundand umbled oRa=0.03m)whichwasused o  heanalysiso  ic ion—Figu e
9.Theimplan  as ened o heholde wasp essedby o ceso 20,25,100,and300N,and
henwasd i enalong heUHMWPEsu acea speedso 200,800,1400,and2000
mm/min.Eachcombina iono  he h us loadingand eedspeedwasmeasu edsix imes,
and hea e agecoe icien o  ic ionwascalcula eda  i s app oxima ionas he a ioo 
angen ialandno mal o ce(5):
𝜇






.(5)

Figu e9.Theexpe imen alse up o  es ing he ic ion.(a) hemachineFV25CNC/Heidenhain
iTNC530, hedynamome e Kis le 9575B,and hesampleclampedin o ice,(b)cus omisedjoin 
implan in hemechanicalholde .
Figu e 8.
The cylind ical cu ing blade o inishing he machined su ace by longi udinal sca ing.
(a) cu ing end, (b) de ail o he cu ing edge.
2.3. T ibological Tes s
Se e al esea ch wo ks ha e been dealing wi h he pin-on-disc echniques and e ec
o sliding eloci y on he ic ion and wea o UHMWPE o use in o al a i icial join s [
32
]
and ibological beha iou o ul a-high molecula weigh polye hylene (UHMWPE) unde
d y ecip oca ing sliding [33].
The ibological es s in his wo k used a eal cus omized implan om Ti6Al4V ELI
(g ound and umbled o Ra = 0.03
µ
m) which was used o he analysis o ic ion—Figu e 9.
The implan as ened o he holde was p essed by o ces o 20, 25, 100, and 300 N, and hen
was d i en along he UHMWPE su ace a speeds o 200, 800, 1400, and 2000 mm/min.
Each combina ion o he h us loading and eed speed was measu ed six imes, and he
a e age coe icien o ic ion was calcula ed a i s app oxima ion as he a io o angen ial
and no mal o ce (5):
µ=
F
Fn
=
Fx
Fz. (5)
Machines2022,10,xFORPEERREVIEW8o 19
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
p o o ypewasmadeo  heha deneds eelDINX155C VMo121(62±1HRC).Thecu ing
speedwas50m/min,axialdep ho cu 0.005mm,and adialwid ho cu ( hes epo e )
0.010mm.

Figu e8.Thecylind icalcu ingblade o  inishing hemachinedsu acebylongi udinalsca ing.
(a)cu ingend,(b)de ailo  hecu ingedge.
2.3.T ibologicalTes s
Se e al esea chwo ksha ebeendealingwi h hepin‐on‐disc echniquesande ec 
o sliding eloci yon he ic ionandwea o UHMWPE o usein o ala i icialjoin s
[32]and ibologicalbeha iou o ul a‐highmolecula weigh polye hylene(UHMWPE)
unde d y ecip oca ingsliding[33].
The ibological es sin hiswo kuseda ealcus omizedimplan  omTi6Al4VELI
(g oundand umbled oRa=0.03m)whichwasused o  heanalysiso  ic ion—Figu e
9.Theimplan  as ened o heholde wasp essedby o ceso 20,25,100,and300N,and
henwasd i enalong heUHMWPEsu acea speedso 200,800,1400,and2000
mm/min.Eachcombina iono  he h us loadingand eedspeedwasmeasu edsix imes,
and hea e agecoe icien o  ic ionwascalcula eda  i s app oxima ionas he a ioo 
angen ialandno mal o ce(5):
𝜇






.(5)

Figu e9.Theexpe imen alse up o  es ing he ic ion.(a) hemachineFV25CNC/Heidenhain
iTNC530, hedynamome e Kis le 9575B,and hesampleclampedin o ice,(b)cus omisedjoin 
implan in hemechanicalholde .
Figu e 9.
The expe imen al se up o es ing he ic ion. (
a
) he machine FV 25CNC/Heidenhain
iTNC 530, he dynamome e Kis le 9575B, and he sample clamped in o ice, (
b
) cus omised join
implan in he mechanical holde .
Machines 2022,10, 1008 9 o 19
3. Resul s
3.1. Resul s o Machining Tes s
The o ce decomposi ion, o ce a ios, and ime se ies we e e y simila in all cu ing
condi ions—see Figu e 10a. Fo he e alua ions, he s abilized pa o he ime se ies was
selec ed o de ine he a e age s a is ical pa ame e s. The dominan componen was he
passi e pe pendicula o ce Fcn, which was nea ly wice as high compa ed o he cu ing
o ce Fc. On he con a y, he speci ic milling o ce exhibi ed a e y ypical cou se, known
o mos known ma e ials and echnologies—see Figu e 10b.
Machines2022,10,xFORPEERREVIEW9o 19
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3.Resul s
3.1.Resul so MachiningTes s
The o cedecomposi ion, o ce a ios,and imese ieswe e e ysimila inallcu ing
condi ions—seeFigu e10a.Fo  hee alua ions, hes abilizedpa o  he imese ieswas
selec ed ode ine hea e ages a is icalpa ame e s.Thedominan componen was he
passi epe pendicula  o ceFcn,whichwasnea ly wiceashighcompa ed o hecu ing
o ceFc.On hecon a y, hespeci icmilling o ceexhibi eda e y ypicalcou se,known
o mos knownma e ialsand echnologies—seeFigu e10b.

b)

Figu e10.A ypical imese ieso cu ing o ceswhenmillingUHMWPE.(a)Fc‐cu ing o ce,Fcn‐
pe pendicula cu ing o ce,Fp‐passi e o ce,(b)speci iccu ing o ceasa unc iono chip hick‐
ness.
Su ace opog aphyin heballendmillingp ocesswas es ed i s wi h ans e se
andlongi udinals eps—Figu e11a,bu  hencon inuedinonlylongi udinals eps—Fig‐
u e11b.Thecu ingspeedand imeo millingexhibi edasigni ican e ec on hemo ‐
phologyo chipswhenmillingUHMWPE,and equen lysome aceso chipmel ingand
highadhesioncanbeseen—Figu e12.The ypicalphenomenaa chipp oduc ionwe e
e ysimila .

Figu e11.Su ace opog aphyinballendmillingp ocess:(a)millingwi h ans e seandlongi u‐
dinals epswi h adials epo e sae×ae,(b)millingsolelyinlongi udinals epsae.
Figu e 10.
A ypical ime se ies o cu ing o ces when milling UHMWPE. (
a
) Fc-cu ing o ce, Fcn-
pe pendicula cu ing o ce, Fp-passi e o ce, (
b
) speci ic cu ing o ce as a unc ion o chip hickness.
Su ace opog aphy in he ball end milling p ocess was es ed i s wi h ans e se and
longi udinal s eps—Figu e 11a, bu hen con inued in only longi udinal s eps—Figu e 11b.
The cu ing speed and ime o milling exhibi ed a signi ican e ec on he mo phology o
chips when milling UHMWPE, and equen ly some aces o chip mel ing and high adhe-
sion can be seen—Figu e 12. The ypical phenomena a chip p oduc ion we e e y simila .
Su ace opog aphy con i med he signi ican e ec o eed speed and cu ing speed
on su ace quali y: o he bes quali y, only he slowes cu ing and lowes eeds gi e he
bes esul s—Figu e 13. Howe e , he in eg i y o he machined su aces wasn
´
op imal.
Some semi-de ached pa icles (Figu es 14 and 15) o p o ube an ib es (Figu e 16) a ec ed
he solid su ace and possibly could con ibu e o he p oduc ion o deb is on he su aces
in sliding con ac . The p oduc ion o he aspe i ies s ems om he kinema ics o he milling
echnology (Figu e 17) in gene al, bu is enhanced by he plas ic low o he polyme
ib es also.
Machines2022,10,xFORPEERREVIEW9o 19
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3.Resul s
3.1.Resul so MachiningTes s
The o cedecomposi ion, o ce a ios,and imese ieswe e e ysimila inallcu ing
condi ions—seeFigu e10a.Fo  hee alua ions, hes abilizedpa o  he imese ieswas
selec ed ode ine hea e ages a is icalpa ame e s.Thedominan componen was he
passi epe pendicula  o ceFcn,whichwasnea ly wiceashighcompa ed o hecu ing
o ceFc.On hecon a y, hespeci icmilling o ceexhibi eda e y ypicalcou se,known
o mos knownma e ialsand echnologies—seeFigu e10b.

b)

Figu e10.A ypical imese ieso cu ing o ceswhenmillingUHMWPE.(a)Fc‐cu ing o ce,Fcn‐
pe pendicula cu ing o ce,Fp‐passi e o ce,(b)speci iccu ing o ceasa unc iono chip hick‐
ness.
Su ace opog aphyin heballendmillingp ocesswas es ed i s wi h ans e se
andlongi udinals eps—Figu e11a,bu  hencon inuedinonlylongi udinals eps—Fig‐
u e11b.Thecu ingspeedand imeo millingexhibi edasigni ican e ec on hemo ‐
phologyo chipswhenmillingUHMWPE,and equen lysome aceso chipmel ingand
highadhesioncanbeseen—Figu e12.The ypicalphenomenaa chipp oduc ionwe e
e ysimila .

Figu e11.Su ace opog aphyinballendmillingp ocess:(a)millingwi h ans e seandlongi u‐
dinals epswi h adials epo e sae×ae,(b)millingsolelyinlongi udinals epsae.
Figu e 11.
Su ace opog aphy in ball end milling p ocess: (
a
) milling wi h ans e se and longi udi-
nal s eps wi h adial s epo e s ae×ae, (b) milling solely in longi udinal s eps ae.
Machines 2022,10, 1008 16 o 19
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Figu e23.Acompa isono  hesu aces.(a)be o ei oning(b)a e linea i oningwi h hep ehea ed
i oncylinde .
Table2.A e agesu ace oughnessa e machiningand he mali oning.
Ra[μm]Rz[μm]Sa[μm]
Su acea e milling2.49–2.6118.68–19.843.63–3.68
I oning( la con ac )1.29–1.369.21–9.721.37–1.42
I oning(cylind icalcon ac )1.70–1.8612.29–13.453.27–3.41
I oning(ballcon ac )1.93–2.0214.17–14.842.58–2.72
Thenewcu ing oolpe o meda lowcu ingspeedswi hexcellen su ace ough‐
ness, eeo  hebu s—Figu e24,exhibi ingRa<0.1m.The eisoneimpo an condi ion
o  eachingsuchagoodsu ace— omake he oughingand inishingcondi iona one
clamping,wi hou any eleasingo  hemachinedpa  om heclampingandCNCma‐
chine.The oolli eexp essedin he o almachineda eawasabou 12,000–14,000mm2pe 
ool.


Figu e24.Scanningelec onmic oscopyo  he esul s:(a) inishedUHMWPEsu ace‐ eeo bu s,
smoo h( c=5m/min,ap=0.01mm,ae=0.2mm),(b)su ace oughnesscha ac e is ics,(c)apiece
o a e y inechip.
Figu e 24.
Scanning elec on mic oscopy o he esul s: (
a
) inished UHMWPE su ace - ee o bu s,
smoo h (
c
= 5 m/min, a
p
= 0.01 mm, a
e
= 0.2 mm), (
b
) su ace oughness cha ac e is ics, (
c
) a piece
o a e y ine chip.
3.2. Resul s o T ibological Tes s
The esul s—see Table 3and Figu e 25—con i med excellen ibological beha iou
o he pai o su aces and ex emely low ic ion was ound: 0.07–0.11. A sligh e ec o
no mal o ces on he coe icien o ic ion was p o ed, bu mo e signi ican eed speed
was con i med. Then, he mean alue and s anda d de ia ions (SD) we e calcula ed om
all o ce da a a a cons an eed speed. A sligh ise o he coe icien o ic ion wi h he
inc easing eed speed was ound also. Possibly, he highe speeds gene a ed mo e hea
and he su ace s a ed o be plas ically de o med, which esul ed in a s icking e ec o he
con ac egions.
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3.2.Resul so T ibologicalTes s
The esul s—seeTable3andFigu e25—con i medexcellen  ibologicalbeha iou 
o  hepai o su acesandex emelylow ic ionwas ound:0.07–0.11.Asligh e ec o 
no mal o ceson hecoe icien o  ic ionwasp o ed,bu mo esigni ican  eedspeed
wascon i med.Then, hemean alueands anda dde ia ions(SD)we ecalcula ed om
all o ceda aa acons an  eedspeed.Asligh  iseo  hecoe icien o  ic ionwi h he
inc easing eedspeedwas oundalso.Possibly, hehighe speedsgene a edmo ehea 
and hesu aces a ed obeplas icallyde o med,which esul edinas ickinge ec o 
hecon ac  egions.
Table3.Coe icien so  ic ionas unc iono no mal o ceand eedspeed.
Th us Fo ce
[N]
FeedSpeed[mm/min]
20080014002000
200.0740.0800.0820.080
250.0850.0890.0930.094
1000.0800.0960.1030.094
3000.0880.0980.0970.099
Mean±SD0.082±0.0060.091±0.0080.093±0.0090.092±0.008


Figu e25.The ibological esul s o  hecoe icien o  ic ion.(a) hee ec o no mal o ceand
ansla i espeed,and(b) hee ec o  he ansla i espeedoni s alue o all es edno mal o ces
(a e age alues± s anda dde ia ions).
4.Discussion
Fi s , hema e ialexhibi eda e ycommoncou seo speci iccu ing o cea milling,
likemanyo he  echnicalma e ials.The hinne  helaye sepa a edbychip emo al, he
highe speci ic esis anceo  hede o medma e ialcanbeexpec ed.On hecon a y, es ‐
ingo  hecu ingspeedmagni udescon i med ha  hebes su acequali ywasachie ed,
su p isingly,a  helowes cu ingspeedand helowes  eedspeed.Theuseo highcu ing
speedsand eedwi h hecemen edca bide ool esul edin heaccumula iono plas ic
ene gya  heplaceo chip emo al,wi halimi edhea  lux.Thema e ialhea edupand
s a ed omel ,wi hallde e io a inge ec sonchip o ma ionandquali yo  hema‐
chinedsu ace,e ena mildcu ingcondi ions.Thec ucialp oblemis hep incipleo 
milling,whichin e up scu ingac ionandini ia es hep oduc iono  he“ ishscales”a 
allcu ingcondi ionsusedin hiswo k.Thedis ibu iono  helocali egula i iesdepends
oncu ingcondi ions,andcanbe educedpa iallywi hlow eedpe  oo h, oolo e s eps,
andlowcu ingspeed.Howe e ,i a ec s hep oduc i i yo  hemachiningope a ion.
Thesamep oblemso su acein eg i y,ma e ialpile‐ups,ando he  echnologicalphe‐
nomenao su acequali yha ebeen oundin[22]and[30]also.Shin okuandNa i a,in
hei s udyo inclinedplanemilling echnology[22],concluded ha  hebes ‐quali yma‐
chining esul sha ebeen ounda a eed a ein he angeo 120–240mm/minandacu ‐
Figu e 25.
The ibological esul s o he coe icien o ic ion. (
a
) he e ec o no mal o ce and
ansla i e speed, and (
b
) he e ec o he ansla i e speed on i s alue o all es ed no mal o ces
(a e age alues ±s anda d de ia ions).

Machines 2022,10, 1008 17 o 19
Table 3. Coe icien s o ic ion as unc ion o no mal o ce and eed speed.
Th us Fo ce
[N]
Feed Speed [mm/min]
200 800 1400 2000
20 0.074 0.080 0.082 0.080
25 0.085 0.089 0.093 0.094
100 0.080 0.096 0.103 0.094
300 0.088 0.098 0.097 0.099
Mean ±SD 0.082 ±0.006 0.091 ±0.008 0.093 ±0.009 0.092 ±0.008
4. Discussion
Fi s , he ma e ial exhibi ed a e y common cou se o speci ic cu ing o ce a milling,
like many o he echnical ma e ials. The hinne he laye sepa a ed by chip emo al, he
highe speci ic esis ance o he de o med ma e ial can be expec ed. On he con a y, es ing
o he cu ing speed magni udes con i med ha he bes su ace quali y was achie ed,
su p isingly, a he lowes cu ing speed and he lowes eed speed. The use o high cu ing
speeds and eed wi h he cemen ed ca bide ool esul ed in he accumula ion o plas ic
ene gy a he place o chip emo al, wi h a limi ed hea lux. The ma e ial hea ed up and
s a ed o mel , wi h all de e io a ing e ec s on chip o ma ion and quali y o he machined
su ace, e en a mild cu ing condi ions. The c ucial p oblem is he p inciple o milling,
which in e up s cu ing ac ion and ini ia es he p oduc ion o he “ ish scales” a all cu ing
condi ions used in his wo k. The dis ibu ion o he local i egula i ies depends on cu ing
condi ions, and can be educed pa ially wi h low eed pe oo h, ool o e s eps, and low
cu ing speed. Howe e , i a ec s he p oduc i i y o he machining ope a ion. The same
p oblems o su ace in eg i y, ma e ial pile-ups, and o he echnological phenomena o
su ace quali y ha e been ound in [
22
] and [
30
] also. Shin oku and Na i a, in hei s udy
o inclined plane milling echnology [
22
], concluded ha he bes -quali y machining esul s
ha e been ound a a eed a e in he ange o 120–240 mm/min and a cu ing speed o
50 m/min. The ange o cu ing condi ions is compa able o he milling pa ame e s used in
ou esea ch (315 mm/min, 70 m/min). In ha wo k, i was also con i med ha he eed
a e has a highe impac on he esul ing quali y han he cu ing speed, bu his is logical
due o Equa ion (2). The high a io o passi e o ces ound in [
22
] cons i u es a new inding,
because he ac is known ypically o g inding echnology ( he use o a nega i e ake
geome y o g inding g ains). Howe e , in he milling o plas ics, e y sha p cu ing edges
a e used no mally, so he highe alues o Fcn han Fc can be explained by he isco-elas ic
beha iou o he ma e ial, he in ensi e plas ic low o he machined ma e ial, and an
ex eme ploughing e ec o he cu ing ool when he chip is o med. In o he wo ds, he
ene gy needed o he e ia y plas ic de o ma ion [
31
] was highe compa ed o he ene gy
consumed o he p ima y plas ic de o ma ion. The s uc u e o he UHMWPE, he low
modulus o elas ici y, he low he mal conduc i i y, and he low empe a u e o plas ic low
suppo he occu ence o his phenomenon.
The he mo-mechanical pos -p ocessing con i med ha i may be a way o smoo hing
o he su aces, bu he e-mel ing and in ensi e plas ic de o ma ion may ha e an e ec on
he polye hylene chains wi h unp edic ed a e e ec s. The ac ha mechanical milling can
cause a phase ans o ma ion o UHMWPE can be ound in [34] also.
P e-cooling o he ma e ial, o c yogenic machining, is a e y good echnology o
implan p oduc ion [
35
,
36
], e i ied especially on he Ti-6Al-4V alloy, bu e y ad anced
equipmen and echnology is needed. The u ning echnology using diamond ools and
high cu ing speeds [
22
] is di icul o apply o he gene al shapes and ad anced ool
ajec o ies needed o eal a i icial join s. Ou inno a ion, based on he use o ex emely
sha p ools and he echnology o sca ing, seems o be mo e ealis ic o se ial use. A
special design o a ounded ool simila o inne sca ing o welded ubes was designed
(pa en pending) and has been es ed on.
Machines 2022,10, 1008 18 o 19
The ibological es con i med he esul o he ex a-low ic ion coe icien agains he
p o o ype knee implan (made o Ti6Al4V ELI by he echnology EBM) and he ic ion de-
penden on he mo ion speed, no he no mal o ce, bu no e y d ama ically. I con i med
a e y small e ec o speed on ic ion and wea , as has been ound in he pin-on-disc
es [
32
,
33
]. The highe speeds can gene a e some ic ional hea , bu hey a e a enough in
hei magni udes compa ed o a ange used in no mal walking.
5. Conclusions
Fo he high longe i y o he implan , i is necessa y o achie e he bes possible su ace
in eg i y in o de o a oid any possible s ess ise s in he ma e ial. This esea ch wo k
con i ms ha UHMWPE is a e y good ma e ial o ibological applica ions in implan
inse ions, bu i is di icul o machine. To ge an accep able su ace quali y, a e y low
cu ing speed and a combina ion o milling ( o oughing wi h cemen ed ca bide ball
milling ool) and sca ing o he aspe i ies ( o inishing wi h a ha dened s eel cu ing ool)
o he unc ional su aces a e ecommended, wi h no cooling. This has a se ious impac in
p ac ice because no c yogenic appa a us is needed. Howe e , a igid machine wi h a e y
sha p cu ing edge and p ese a ion o echnological sequence ( oughing ollowed wi h
inishing, wi hou any elease o he inse om a clamping de ice and no in e up ion o
he cu ing ac ion a he su ace) should be kep . I he ma e ial is consolida ed well, he
inished su aces a e machining a e smoo h, glossy and wi hou any de ec o in eg i y.
All he esul s a e impo an o inalize he whole assembly o cus omized implan s and
ex end he li espan o he inse , hus maximizing he e-ope a ion pe iod o pa ien s o
elimina ing he necessi y o eplace he inse al oge he .
Au ho Con ibu ions:
Concep ualiza ion, M.P. and K.U.; me hodology, M.P.; alida ion, M.P. and
K.U.; o mal analysis, K.U.; in es iga ion, M.P.; esou ces, K.U.; da a cu a ion, M.P.; w i ing—o iginal
d a p epa a ion, M.P.; p ojec adminis a ion, M.P.; unding acquisi ion, M.P. All au ho s ha e ead
and ag eed o he published e sion o he manusc ip .
Funding:
This esea ch was unded by he PID g an FV40313, Applica ion o he New Su ace
T ea men Technologies in Me al Packaging Indus y, and by he B no Uni e si y o Technology,
Facul y o Mechanical Enginee ing, Speci ic esea ch “Mode n echnologies o p ocessing ad anced
ma e ials used o in e disciplina y applica ions”, FSI-S-22-7957.
Da a A ailabili y S a emen : No applicable.
Acknowledgmen s:
A special hanks go o he company As a Mo o s, s. .o., B no, Czech Republic,
ep esen ed by Jan Kep da, o his kind help in he echnology o d ag umbling, and o Phillip Jones
o his English co ec ions.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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