PVD—Coatings in Injection Molding Machines for Processing Optical Polymers

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  • PVDCoatings in Injen

    B

    In the last few years transparent polymers are gaining

    importance. In comparison to glass, they offer more flexi-

    bility in design, reduce weight, and offer the possibility of

    integrating functional elements.[13] By processing poly-

    mers with injection molding machines (IMMs) the high

    degree of automation leads to high reproducibility and low

    product prices.[1,3,4] On the other hand, the processing of

    polymers with IMMs produces faults that cannot be

    tolerated in transparent products. When the adhesion of

    the polymer melt to the surrounding metal surfaces is too

    high, the melt sticks to the metal for a long time. Due to

    extended exposure to heat and shear stress the polymer

    degrades and changes to dark colors. While plasticizing,

    parts of this dark material are torn off and can be traced in

    the parts as dark cords or black spots.

    To reduce the adhesion between optical polymers and the

    were developed. For round substrates (diameter 20 mm, height

    10 mm) the hardened steel A422 (X35CrMo17, 1.4122, 300 HV0.1)

    and the plasma-nitrided steel A355 (X34CrAlNi7, 1.8550, 1 100 HV0.1)

    were chosen. These two steels are used for conventional plasti-

    cizing components and represent chromium steels with a good

    corrosion resistance and nitriding steels with a high abrasion

    resistance.[5] All the substrates were ground and finally polished

    with a 6 mm diamond suspension, until a surface roughness of

    Ra 0.05 mm was achieved. Before the coating, all the sampleswere cleaned in a six bath industrial cleaning station (Meiner

    Technik Mllenbach, Marienheide-Mullenbach, Germany) with

    ultrasonic baths of an alkaline cleaning fluid and water.

    All the coatings of this work were deposited by the arc ion

    plating PVD processes on a PVD 20 machine (METAPLAS IONON

    GmbH, Bergisch Gladbach, Germany). TiN and CrN were deposited

    from single targets. The other coatings were all deposited with a

    separate aluminum target (Table 1). By using separate targets for

    the two metal components of the coatings the ratio could be

    changed by switching between different currents. Because of the

    good antiadhesive properties of AlN[68] and Al2O3[9,10] the para-

    by varying the reactive gas composition. After 5 min of coating

    with pure nitrogen, oxygen was added with an increased share,

    Full Paper

    ntoepdasangsbefo

    S144plasticizing units new plasma vapor deposition (PVD)-coatings until the maximum oxygen share of 20% was reached.

    For the characterization of PVD-coatings, basic tests were first

    carried out. Beginning with Rockwell hardness tests[11,12] and a

    subsequent optical analysis of the indents, the adhesion class

    could be derived.[13] After cup grinding tests, the thickness of the

    coating was calculated.

    K. Bobzin, R. Nickel, N. Bagcivan, F. D. ManzSurface Engineering Institute, RWTH Aachen University, Augus-tinerbach 4-22, Aachen 52062, GermanyFax: 49 (241) 8092264; E-mail: manz@iot.rwth-aachen.deExperimental Partmeters were chosen to have a high percentage of aluminum. In the

    coatings (Ti0.4Al0.6)ON and (Cr0.6Al0.4)ON oxygen was integratedIntroduction

    Transparent parts with faults down to a teprocessing transparent polymers, injection mused. To reduce these faults, plasma vapor dused in this paper to reduce adhesion, wear, anout on the two steel types ASTM A422 and pl(Ti0.4Al0.6)N, (Ti0.4Al0.6)ON, CrN, (Cr0.6Al0.4)N,adhesion to the substrate. With the PVD coatinadhesion of the polymers to the surface couldscrew tips and used. After all the tests were pereasily from the polymers.Machines for Processi

    Kirsten Bobzin, Reimo Nickel, NazlimPlasma Process. Polym. 2007, 4, S144S149

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimction Moldingg Optical Polymers

    agcivan, Florian D. Manz*

    h of a micrometer lead to reject parts. Forlding machines (IMMs) are most commonlyosition (PVD) coatings for injection tools arecorrosion. All the investigations were carriedma-nitrided ASTM A355. The coatings of TiN,d (Cr0.6Al0.4)ON were developed with goodof (Ti0.4Al0.6)ON and (Cr0.6Al0.4)ON the lowestrealized. These two coatings were applied onrmed, the coated parts could be cleanedmuchDOI: 10.1002/ppap.200730507

  • They were mounted in an IMM. Each polymer was tested in a

    separate test run on an IMM type TM 1600/1000 of Battenfeld

    PVDCoatings in Injection Molding Machines for . . .

    Table

    1.De

    posit

    ionpa

    ram

    eter

    sfor

    the

    inve

    stig

    ated

    coat

    ings

    .

    Co

    ati

    ng

    TiN

    (Ti 0

    .4A

    l 0.6

    )N(T

    i 0.4

    Al 0

    .6)O

    NC

    rN(C

    r 0.6

    Al 0

    .4)N

    (Cr 0

    .6A

    l 0.4

    )ON

    Ta

    rget

    ma

    teri

    al

    an

    d

    pu

    rity

    1T

    Ti

    99

    .61T

    Ti

    99

    .6R

    1T

    Al

    99

    .51T

    Ti

    99

    .6R

    1T

    Al

    99

    .51T

    Cr

    99

    .51T

    Cr

    99

    .5R

    1T

    Al

    99

    .51T

    Cr

    99

    .5R

    1T

    Al

    99

    .5

    Rea

    ctiv

    eg

    as

    Nit

    rog

    enN

    itro

    gen

    Nit

    rog

    enR

    0

    21

    %o

    xy

    gen

    Nit

    rog

    enN

    itro

    gen

    Nit

    rog

    enR

    0

    21

    %o

    xy

    gen

    Ga

    sp

    ress

    ure

    (Pa

    )2

    1.2

    1.2

    21

    .21

    .2

    Ta

    rget

    curr

    ent

    (A)

    60

    45

    (Ti)

    45

    (Ti)

    60

    45

    (Cr)

    45

    (Cr)

    50

    (Al)

    50

    (Al)

    50

    (Al)

    50

    (Al)

    Bia

    sv

    olt

    ag

    e(V

    )5

    05

    05

    05

    05

    05

    0

    Co

    ati

    ng

    tim

    e(m

    in)

    40

    30

    30

    40

    30

    30

    Ro

    ug

    hn

    ess

    aft

    er

    coa

    tin

    gR

    a(m

    m)

    0.1

    10

    .11

    0.1

    20

    .15

    0.1

    60

    .18

    Plasma Process. Polym. 2007, 4, S144S149

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimGmbH, Meinertzhagen, Germany. After a stable run was achieved,

    250 pieces were produced. Plates with the dimensions of

    200150 4 mm3 were produced to estimate the influence ofthe coatings on black spots and dark cords. After each polymer the

    complete screw was pulled out of the plasticizing unit to clean and

    analyze the coated parts.

    Results and Discussion

    The coatings, produced with the parameters given in

    Table 1, are all optimized to have a good adhesion on the

    two substrates of the project. Testing with Rockwell HRC,

    all coatings have a very good adhesion to the substrate

    (adhesion class 1). This is especially important for

    processing optical polymers. Every piece of coating, which

    is torn off the substrate, produces a black spot in the optical

    product. Furthermore, the coating-free spot on the

    substrate has the conventional high adhesion to the

    polymer melt. This produces an increase in the black spots

    and dark cords. The goal of minimizing these faults can

    only be achieved with coatings that have a very good

    adhesion. For a better comparison of the coatings, they

    were all adjusted to have a thickness of about 2 mm.

    The compositions of the developed coatings were

    determined by energy dispersive X-ray analysis. The ratio

    between titanium and aluminum was about 2:3. So the

    coating was labeled (Ti0.4Al0.6)N. By integrating oxygen the

    ratio remained the same, so the label for this coating was

    (Ti0.4Al0.6)ON. The chromium-based coatings had a ratio ofFor morphology studies one sample of each coating was

    fractured. The sample broke at the phase boundaries. In this way

    the morphology of the coating could be examined by scanning

    electron microscopy (SEM).

    Contact angle measurements were realized to analyze the

    adhesion of the polymer melt to the sample surface.[14,15] After

    cleaning the samples in ultrasonic baths of acetone and

    isopropanol, they were heated to 100 8C in a closed heat chamberplaced on a heating plate. The temperature in the chamber was

    kept within a tolerance of about 2 8C. The heat chamber has glasswindows, so the digital camera can take pictures of the sample in

    the chamber continuously. To ensure a similar starting point, a

    single granulated pellet of one optical polymer was placed on top

    of the sample and the temperature was held at 100 8C for 10 min.Because the temperature in the heat chamber rises very fast, a

    break temperature (Table 2) was set to ensure a complete melting

    of the polymer, before the start of the test. After another 10 min,

    the temperature was set to the low cylinder temperature at the

    screw nozzle (Table 2). After the temperature was reached, the

    melt appears on the surface and the contact angle could be

    measured. Within the first minutes the contact angle changes

    rapidly until it asymptotically reaches a final value. Here the

    contact angles are measured after 120 min.

    Screw tips and back flow valves were coated for practical tests.3:2, so they were labeled (Cr0.6Al0.4)N and (Cr0.6Al0.4)ON.

    www.plasma-polymers.org S145

  • To study the morphology of the coatings, SEM micro-

    graphs of fractures through the PVD-coatings were taken.

    In Figure 1 the micrographs for the titanium-based

    coatings are shown. The TiN coating has a columnar

    structure like shown in Figure 1(a). Between the columns

    are phase interfaces that can have flaws like voids.

    Especially when a tangential force is applied at the

    surface, these phase interfaces of columnar coatings result

    K. Bobzin, R. Nickel, N. Bagcivan, F. D. Manz

    Table 2. Optical polymers and temperatures for the contact angle measurements.

    Polymer Polymethyl

    methacrylate (PMMA)

    Polycarbonate (PC) Cyclic olefin

    copolymer (COC)

    Polyether

    sulfone (PES)

    Type Plexiglas 7N OQ Makrolon LQ2647 Topas 5013 Ultrason E 2010

    Producer Rohm GmbH & Co. KG,

    Darmstadt, Germany

    Bayer AG, Leverkusen,

    Germany

    Topas Advanced

    Polymers, Frankfurt,

    Germany

    BASF AG,

    Mannheim,

    Germany

    Glass transition temperature (-C) 110 148 135 225

    Low cylinder temperature at

    the screw nozzle (-C)

    220 280 240 350

    Starting temperature (-C) 100 100 100 100

    Heat up break temperature (-C) 200 200 190 250

    Test temperature (-C) 220 280 240 300

    Test time (min) 120 120 120 120

    S146Figure 1. SEM micrographs of fractured titanium-based PVD coating(Ti0.4Al0.6)N coating has a finer structure. The oxygen in the (Ti0.4Al0.6The nanolaminate structure can be seen especially in the SEM micro

    Plasma Process. Polym. 2007, 4, S144S149

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheims. TiN has a columnar morphology. Because of the aluminum, the)ON coating reduces the grain size even more and is a nanolaminate.graph with a higher magnification.

    DOI: 10.1002/ppap.200730507

  • PVDCoatings in Injection Molding Machines for . . .

    atinFigure 2. SEM micrographs of fractured chromium-based PVD coin breaking off of the particles. By integrating aluminum

    and oxygen into the coating system, the structure of the

    coating is changed. The aluminum is integrated by adding

    an aluminum target to the coating process. By varying the

    current of the targets the ratio between titanium and

    aluminum can be changed. With this setup a coating with

    60 at.-% aluminum and 40 at.-% titanium was realized. The

    SEM micrograph of (Ti0.4Al0.6)N [Figure 1(b)] shows the

    very smooth structure of this coating. By adding oxygen to

    the reactive gas the structure gets even finer [Figure 1(c)].

    By magnifying a section of the coating the nanolaminate

    structure can easily be recovered [Figure 1(d)].

    The micrograph of the fraction through the CrN coating

    also shows a columnar structure [Figure 2(a)]. Again a

    second target, consisting of aluminum, is used to change

    the content of the coating. The (Cr0.6Al0.4)N coating is

    also columnar, but with interruptions in the columns

    Figure 3. Contact angles of PC melt on a steel reference and chromi

    coating has interrupted columns. Oxygen reduces the grain size inobvious in a higher magnification.

    Plasma Process. Polym. 2007, 4, S144S149

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimgs. The CrN coating is columnar. With aluminum the (Cr0.6Al0.4)N[Figure 2(b)]. With the same currents on the targets, but

    with a reactive gas, consisting of 80% of nitrogen and

    20% of oxygen, the structure of the coating changes to a

    superfine nanolaminate [Figure 2(c)]. The magnified struc-

    ture is shown in Figure 2(d).

    The contact angle of a liquid drop, lying on a solid

    surface, is influenced by the adhesion between the solid

    and the liquid and the cohesion of the fluid.[14] When

    testing a polymer on different surfaces, the cohesion of the

    fluid is constant, while the adhesion changes.[15] High

    contact angles can be found when the adhesion is low. In

    theory the lower adhesion results in a shorter maximum

    dwell time. Because most of the polymers have a high wall

    adhesion, they flow in a shear flow. In the plasticizing unit,

    the walls are heated so that the viscosity is almost the

    same or lower than at the center of the polymer material.

    So the shear velocity has its maximum, close to the walls.

    um-based coatings after 120 min at 260 8C.

    the (Cr0.6Al0.4)ON coating. The nanolaminate morphology becomes

    www.plasma-polymers.org S147

  • of aluminum and oxygen, the contact angle increases,

    K. Bobzin, R. Nickel, N. Bagcivan, F. D. Manz

    awthe

    S148When the adhesion to the wall is lower the material can be

    withdrawn from the wall easily. With lower adhesion of

    the polymer to the walls, the polymer melt is often with-

    drawn off the wall which reduces the maximum dwell

    time. Polymer melt that sticks to a wall degrades with the

    time and begins to change its color. When the degraded

    darkened polymer is pulled off the surface by the flowing

    polymer melt, it is transported into the product. Particles of

    this dark material result in dark cords or black spots. As an

    example Figure 3 shows the contact angles of PC on

    different surfaces. On the reference steel A355, the contact

    angle was 55.58. With the CrN coating the contact anglecould be increased to 66.18. The implementation of alu-minum into the coating results in an even higher contact

    angle. The highest contact angle (72.28) was found for(Cr0.6Al0.4)ON. The PC melt on (Ti0.4Al0.6)ON had a contact

    angle of 688, which means that the adhesion is higher thanthe adhesion for the (Cr0.6Al0.4)ON coating. By varying the

    contact angle of the polymers with the surface roughness

    Ra (Table 1), the implantation of aluminum and oxygen

    results in rougher structures and higher contact angles.

    This would fit into the lotus effect, where beneath the low

    adhesion of the water to the surface of micro tips force the

    water to stay away from the bottom surface. But for this

    effect the tips on the surface of the coatings are too few

    Figure 4. (Cr0.6Al0.4)ON-coated screw tip and back flow valve are flthat can hardly be cleaned from steel surfaces could be torn offand too irregular. In this case, it is even the other way

    round with the higher roughness resulting in a greater

    interface area, which increases the adhesion. Since the

    difference in roughness Ra is below 0.1mm, the effect of the

    surface topography can be neglected in this case. By testing

    all the surfaces with all polymers of the project it was

    found that the contact angles are not always consistent. By

    testing COC the surface of the polymer melt reacts with the

    oxygen of the air and builds a skin that influences the

    contact angle.

    For a final field test, screw tips and back flow valves

    were coated with (Ti0.4Al0.6)ON and (Cr0.6Al0.4)ON. After

    running all the four polymers for 250 cycles there were no

    faults on the coatings. Figure 4(a) shows the (Cr0.6Al0.4)ON

    screw tip and back flow valve after the production of all

    Plasma Process. Polym. 2007, 4, S144S149

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwhich means that the adhesion of the polymer melt to the

    coating decreases. The polymers can be easily removed

    from surfaces with high contact angles. This conf...

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