eXPRESS Polymer Letters Vol.13, No.

4 (2019) 379–389
Available online at www.expresspolymlett.com
https://doi.org/10.3144/expresspolymlett.2019.31

Laser markability of PVC coated automotive electric cables

E. Bitay*

Sapientia Hungarian University of Transylvania, Faculty of Technical and Human Sciences, 540485 Târgu-Mureş, Op. 9.,
Cp. 4., Romania

Received 26 June 2018; accepted in revised form 29 November 2018

Abstract. This article describes the test results for laser markability of automotive electrical cables. The insulation is PVC,
but the colour and construction of the insulations are different. Two types of laser workstations were used, one with a wave-
length of 1064 nm and another with 532 nm. The penetration depth of the laser beam was determined by optical microscopy
on cross sections. The 1064 nm laser beam can mark all investigated materials with good contrast, except the yellow insu-
lation. The 532 nm laser beam with fast speed can hardly produce contrast with any of the materials. The laser markability
of the yellow insulation was found to be the most problematic. On the two-layer insulation, despite the whitening of the
inner material, dark marking is produced because the heat developing on the interface of the two layers will heat up and car-
bonize the transparent layer.

Keywords: materials testing, industrial application, laser markability, PVC coated cable

1. Introduction Laser surface treatment can serve purposes other than

Laser marking of plastics is the most widespread laser marking, such as increasing the wettability [30, 31],
surface modification procedure. Marking is part of surface texturing [32], structuration [33–35], pattern-
mass production, and the speed of the product often ing [36], hydrofobization [37–39] or stereo-lithogra-
cannot be followed with the naked eye, and also, phy [40, 41]. Automotive and aerospace cables can
sometimes all the products have to be marked with only be laser marked by thermal bleaching since all
an individual marker. other marking methods would change the structure or
Polymers can be marked with a laser with one of the amount of the material to such an extent that it would
following mechanisms: ablation, bleaching (thermal compromise the basic function of the cable.
bleaching) [1], carbonisation [2], colour change Markings must satisfy certain technological criteria,
(colour-formation, colouring, colour marking [3, 4]), such as visibility, legibility, durability or laser mark-
darkening/whitening [5, 6], dehydration [7], doping ability [42–45], but marking speed (cycle time) is
[8, 9], engraving [10], foaming [8, 11–13], melting, also a basic criterion in industrial applications.
optical breakdown [14], oxidation/reduction on met- During laser marking of thermoplastics, various in-
allized surfaces [15], transfer and unzipping [16, 17]. teractions take place in the polymer, some of which
Laser beam can make pits and rims [18], craters [19], are hardly known as the pulse time of the laser can
but there are papers in which the same interaction be- be measured in µs, ns, ps or fs [46–51], but the tem-
tween laser beam and material is called once ablation perature during marking can reach up to 800 °C [8].
[20], sometimes etching [21]. Laser ablation is a cur- Thermoplastics without fillers or pigments can be di-
rent application for thin layer machining [22–29], vided into three main groups according to their suit-
but is not applied for marking of electric cables. ability for laser marking:

*
Corresponding author, e-mail: ebitay@ms.sapientia.ro
© BME-PT

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– Members of group 1 absorb laser rays well, there- wavelength. There are lasers from UV to the far in-
fore they are carbonized and the marking will be frared, and they can be continuous or high-peak power,
dark. Such materials are polyesters (PES) and poly- short-duration pulsed laser beams [11]. The most
sulfones (PSU). common lasers for automotive cable marking in-
– In the materials of group 2, absorption and car- clude the following:
bonization are irregular. This group includes poly- – CO2 gas laser with 10 640 nm wavelength (far-in-
styrenes (PS), some foamable copolymers, such frared); it is used also for thermosetting polymers
as ABS (acrylonitrile-butadiene-styrene), and PET [60],
(polyethylene terephthalate) and PBT (polybuty- – Xenon chloride (XeCl) excimer gas laser with a
lene terephthalate). With the addition of the proper wavelength of 308 nm (UV),
pigment or additive, these polymers can also be – Nd:YAG solid-state laser with a wavelength of
made well markable. 1064 nm (near-infrared),
– Group 3 includes polymers, such as POM (poly- – Ytterbium fibre laser with a wavelength of 1060 nm
oxymethylene), PP (polypropylene), PE (polyeth- (near-infrared),
ylene) and PPS (polyphenylene sulfide), which – Frequency-doubled Nd:YVO4, 532 nm wavelength
cannot be marked with laser in their original un- (visible, green),
coloured state; the laser does not cause a change – Frequency-tripled Nd:YVO4 or Nd:YAG solid-
in colour or reflection. For laser marking of poly- state laser with a wavelength of 355 nm (UV).
urethane with a Nd:YAG laser (YAG means yttri- UV lasers have been spreading fast recently. The rea-
um-aluminium-garnet), bismuth oxide was applied son for this is that the absorption coefficient of poly-
successfully [52]. mers can increase up to 20 times higher if the wave-
The basic criterion for laser markability is that the ma- length of the laser decreases from 1064 to 355 nm.
terial should absorb the energy of the laser. The poly- In the case of the hard PVC (poly(vinyl chloride)), the
mer can absorb laser radiation itself but if it does not, increase is ‘only’ five times (from 0.06 to 0.30 1/mm)
colorants (pigments) or other additives need to be [16]. However, the newest trend in the laser marking
added [53, 54]. Therefore, additives play a very im- of polymer materials shows the substitution of UV
portant role in laser marking; they can make the mark- lasers by ps green lasers (532 nm), because of the in-
ing process quasi-independent of the polymer [55, creased process speed, the sufficient contrast, and
56]. The first additives to improve the laser marka- the reduced cost (less maintenance and higher life-
bility were mica-based pigments. Carbon black and time).
rutile were first used for this purpose in 1994. Be- This paper focuses on the investigation of the laser
sides TiO2, that is used in thermoplastic elastomers markability of cables of various colours, and one of
[5], other white pigments such as Ba2SO4, Al2O3, and the important issues was how much the cycle time
ZnO are also used [16]. One of the newest additives of laser marking can be reduced in special ‘marking
to improve the laser markability of PA (polyamide) on the fly’ process.
is antimony-trioxide [57], combined with Ti, Fe, Cr,
Ni oxide, and 10–50 nm, organic pigments (copper 2. Materials and equipment
phthalocyanines, dioxazines, anthraquinones, dike- We examined thermoplastic automotive electric ca-
topyrrolopyrrole) to make laser transparent polymers bles with PVC insulation, but the diameter of the ca-
(ABS, PC (polycarbonate)) markable [58]. bles, and the colour and structural layout of the in-
Additives that improve laser markability can be dis- sulation were different. The insulations were laser-
persed or aggregated in the material or in a coating. marked with an experimental program of four series,
In multi-layered coating, there is a first pigment (ru- on TRUMPF VectorMarc VMc5 and VMc4 work-
tile or anatase, antimony trioxide, PEEK (polyether- stations, by using the ‘marking on the fly’ process. The
etherketone), PES) markable with ultraviolet laser, main technological variables (not all details) can be
and a second pigment (silica, magnesia alumina or found in Table 1.
diamond), which does not absorb ultra-violet (UV) After the experiments, the samples were first exam-
radiation and looks white [59]. ined with the naked eye, and then some representa-
There are practical criteria of choosing the appropri- tive samples were examined with a microscope to
ate laser, but the most important characteristic is determine the penetration depth of the laser, which

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causes meso-scale structural transformations in the

zone under the surface of the insulation.
Cables with the following insulation colours were
examined: yellow, red, red + black, green + yellow,
black and dark brown. The identification of the insu-
lation’s polymer and its additives (e.g. plasticizers,
pigments) was not a part of this work, but differential
scanning calorimetry (DSC) – using a Perkin-Elmer
Pyris type calorimeter – and energy dispersive spec-
trometry (EDS) were carried out for the identifica-
tion of the polymer and the inorganic additives.
The plasticizers mainly used in PVC are phthalates,
e.g. dibutyl phthalate (DBP), dioctyl phthalate (DOP)
[61], diisodecyl phthalate (DIDP) and di(2-ethyl-
hexyl) phthalate (DEHP) [62, 63]. There are also
many bio-based plasticizers [64–67], in which the
soybean oil and the epoxidizes soybean oil (ESBO)
and the acetic acid ester (AAE) are the most com-
mon [68, 69]. Cardanol is a natural resource with low
cost and low toxicity [70]. Cardanol is also used as a
perfect raw material for the high grade insulating coat-
ings [71, 72], and this is valid for the cardanol acetate
(CA) [73], the epoxidized cardanol acetate (ECA)
[74] and the cardanol derivatives as well [63].
A Nikon SMZ-2 type stereo-microscope, Jeol JSM-
6380 and Zeiss EVO 10 type scanning electron mi-
croscopes and Olympus PMG-3 metallurgical mi-
croscope were used for the examination of the cable
surfaces and the cross-sectional cut samples. The pen-
etration depth of the laser beam was determined in
the microstructural images with the ImagePro-Plus
image analyser software.

3. Results and discussion

3.1. Analysis of the insulation and the burning
residue
Figure 1a shows the DSC heat flow curves of two
cable insulations; this measurement denotes that the
Figure 1. a) DCS heat flow curves of two (red and yellow)
materials of the insulations are a generally used,
PVC insulations. b) The surface of the red colour
medium plasticizer containing PVC. A detail of the PVC insulation. c) The residue of the red cable
surface of the red insulation is seen in Figure 1b, and after burning.
the burning residue in Figure 1c respectively. The
small white particles in Figure 1b were analysed as this analysis cannot determine the nature of organic
particularly Ca as well as low quantity of Zn, Mg, pigments and plasticizers.
Al, Ti and Fe compounds. These are also the con-
stituents of the burning residue as the EDS analysis 3.2. Visual examinations and evaluation
determined. The performed EDS analyses prove that Since this investigation was connected to the manu-
the inorganic additives are particularly Ca as well as facturing development program of a car manufacturer,
low quantity of Zn, Mg, Al, Ti and Fe compounds; first, with the visual inspection, the most important

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Figure 2. The 2.0×0.6 mm laser markings on the insulation Figure 3. Laser marking (a) and its certain details on the
of cables of various colours. dark brown insulation, in the middle of the mark-
ing (b) and at the edge of the marking (c).
visibility characteristics had to be determined: colour
of marking, contrast, homogeneity, and scratch re- Laser marking formed a special surface texture on
sistance. Scratch resistance also has to be evaluated the dark brown insulation. The scanning electron mi-
visually, after a special rubbing test (but this article croscopic examination of the texture (Figure 3)
does not discuss this in detail). showed that the 30 micron diameter laser pulse series
Figure 2 shows the representative, selected markings produced individual craters in the surface, positioned
whose microscopic test characteristics this article like a raster.
presents.

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Table 1. The area of marking is 2.0×0.6 mm on the insula- 3.3. Microscopic examination of the
tion of each cable. penetration depth of laser marking
Series 1 Series 2 Series 3 Series 4 Cross-sectional samples of the cables were embed-
Laser type VMc5 VMc5 VMc5 VMc4
ded in Duracryl resin, and then wet grinded on SiC
Power [%] 95 95 95 95
grinding papers of gradually finer grades. Figure 4a
Wavelength [nm] 1064 1064 1064 532
shows the effect of laser marking on the cross-sec-
Optical focus [mm] 163 163 163 160
tion of the yellow insulation: the 532 nm laser pen-
Spot size [µm] 40 40 40 20
etrates relatively deep through narrow channels, but
Filling distance [µm] 50 50 50 50
Frequency [kHz] 65 55 55 20
the visibility of the marking is weak; this is clearly
Speed [mm/s] 5000 500 500 500 visible in Figure 2. Figure 4a also depicts the method
Pulse width [µs] 5 5 5 5 of determining penetration depth: penetration depth
Marking time [s] 0.12 0.26 0.60 0.23 is the distance between the two outer dashed lines,
as the arrows show. The penetration depth of the
1064 nm laser is considerably smaller in both the
The inner part of the craters and the zones between yellow and the green insulators, as can be seen in
them were melted for a short time, then quickly Figure 4b, but this is good because laser energy is ab-
cooled and solidified. In addition to the variables
given in Table 1, surface morphology is also strongly
affected by the defocus and the number of pulses per
point. The optimum of these is usually sensitive and
protected data.
Table 2 contains the general evaluation of the visibil-
ity of laser markings during visual inspection; these
evaluations are built into the manufacturing process
as grading categories. In the whole DOE (design of
experiment) the following can be said about the suit-
ability of the two kinds of laser marking systems
(VMc4 and VMc5), taking into account that there are
two technological variables provided, one with fast
speed (5000 m/s) and the other one with slow speed
(500 m/s).
• The VMc5 system with slow speed can mark all
the different materials with good contrast, except
the yellow insulation.
• The VMc5 system with fast speed can only mark
black, brown and green/yellow insulations with
good contrast.
• The VMc4 system with slow speed can mark with
good contrast all the materials, except yellow, but
with fast speed it can hardly produce contrast with
any of the materials.

Table 2. Evaluation of laser markings on the base of visual

and functional requirements.
Series 1 Series 2 Series 3 Series 4 Figure 4. Cross-sectional images of two cables showing the
Colour of marking Dark Dark/Clear Dark Dark penetration of the laser beam; a) Penetration of the
Contrast Middle High High High 532 nm laser beam into the yellow insulation; the
Homogeneity High High High High
arrows show the determination of the penetration
depth. b) Penetration of the 1064 nm laser beam
Scratch resistance Yes Yes Yes No
into the green-yellow insulation.

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sorbed in the material’s surface zone and a clearly form of whitening. This whitening is the result of the
visible black (dark) mark is produced. thermally induced change of the colorant. However,
In the red insulations, the 1064 nm laser is absorbed Figure 2 shows that a black mark formed on the red
relatively deeply (Figure 5a), and marking takes the surface of the red-black insulation. How is this pos-
sible? There can be such situations in which the
colours can be the results of the electron transfer from
photo-effected reaction; e.g. the PVA (poly(vinyl al-
cohol)) supplies the electrons to the self-bleached
processes [75], but the examined situation is much
simpler and clearer.
Figure 5b clearly shows that on the given cable, the
red insulation (which contains two black stripes) is
covered by an outer, completely transparent layer of
about 50 microns thickness. This is a so-called auto-
motive composite wire (ACW), with a radiation cross-
linked fluoropolymer outer insulation protecting the
PVC inner insulation; therefore the cable withstands
thermal and mechanical loads in the engine bay.
Transparency also holds for the 1064 nm laser, at least
the images inside Figure 5b and 5c show that the
outer surface of the transparent layer was not dam-
aged by the laser beam. Figure 5c also shows that
the material of the red insulation was whitened by
the laser.
The marking, however, still became black since the
heat generated on the outer surface of the red insula-
tion heated the transparent layer. This heating affected
less and less volume as it moved outwards, this is why
the cross section of the zones heated with the laser
looks like triangles pointing outwards. The transparent
layer can similarly heated and plasticized by means
of heat conduction [48], but at the boundary of the red
and the transparent layer so much heat was generated
that gas was produced due to thermal decomposition,
and its pressure separated the two layers; a similar ex-
planation is described by Zelenska et al. [76].
The cross section of the black cable is shown in Fig-
ure 6. This cable had the smallest diameter (1.6 mm),
therefore there is a greater difference between the
angles the laser hits the edge and the centre of the
marked area. In spite of the greater surface curvature
change, the laser had relatively uniform penetration
and penetration depth was small.
The microscopic examination of the black and dark
Figure 5. The cross-sectional images of the cables with red brown insulation with the Olympus PMG-3 inverted
insulation; a) the cross section of white marking microscope required special imaging modes. The
on the simple red insulation, b) the cross section cross-sectional examination of the dark brown insu-
of black marking on the red ACW, c) the separa- lation in Figure 7a required dark-field observation
tion of the whitened red layer and the transparent
(Figure 7b) in addition to bright-field observation
fluoropolymer layer above it.

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(Figure 7c), and Nomarski differential interference showed that the laser produced little pores inside the
contrast (N-DIC) imaging as well. N-DIC imaging material (Figure 7d), that is, during marking, foam-
ing also started in addition to whitening.
Table 3 shows the penetration depth of laser mark-
ings in Figure 2, as determined with the microscopic
Table 3. The penetration depth of markings in metric units
and as a percentage of the thickness of the insulation.
Wavelength of the Penetration depth
Colour of the
laser beam
insulation
[nm] [μm] [%]
Yellow 532 159 39.0
(Green) Yellow 1064 45 11.0
Green (Yellow) 1064 32 7.9
Red 1064 88 21.2
Red-black 1064 67 (+50) 23 (40.3)
Figure 6. The cross-sectional image of the cables with black Black 1064 72 18.4
insulation; marking with the 1064 nm laser Dark brown 1064 66 15.7
changed the surface into a light colour.

Figure 7. The cross-sectional image of the laser marking of cables with dark brown insulation; a) stereo-microscopic image,
b) dark-field image, c) bright-field image, d) N-DIC image.

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examination of the cross sections. Penetration depths with the 1064 nm laser in the case of PVC, but in the
are given in metric and relative units as well. Re- case of black and dark brown, the first signs of foam-
garding the red-black insulation the +50 μm means ing can appear, which is not always acceptable.
the thickness of the transparent external coating. The
data show that with the use of 1064 nm laser, pene- Acknowledgements
tration depth can be kept under 100 micrometers, Enikő Bitay was supported in her research project by the MTA
while the visibility characteristics of marking are Domus Hungarica Grant Program: 5634/5/2017/ HTMT
completely acceptable.
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