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T O O L I N G
The dependency of material properties and process conditionson the cutting temperatures when drilling polymers
Klaus Weinert Florian Brinkel Christoph Kempmann Klaus Pantke
Received: 13 February 2007 / Accepted: 9 March 2007 / Published online: 6 October 2007
German Academic Society for Production Engineering (WGP) 2007
Abstract Construction parts consisting of modern poly-
mer materials still need to be machined. Thereby specialattention has to be paid to the machining quality. The
machining quality implies dimensional accuracy as well as
a defect-free peripheral zone. Machining defects often
occur as a consequence of excessive mechanical loads,
which are often caused by unfavorable process conditions.
Besides mechanical loads, the thermal influence on the
composite material, which is induced by the cutting process
itself, has to be considered as crucial. According to the
thermo physical material properties of polymer materials
the boundary conditions differ from the machining of
metals. Especially the drilling of polymer composites is
introduced in this article and moreover the influences of the
material properties and the process conditions on the pro-
cess temperatures are presented.
Keywords Production process Polymers
Cutting temperatures
1 Introduction
The placement of reinforcing fibers into a polymer matrix in
1930 opened up great potential for lightweight
constructions. Since that time high performance fiber
composites gain more and more importance for lightweightstructures. This fact is founded in their immense flexibility
and innovative diversity for construction. Usually machin-
ing by drilling and milling follows after forming of fiber-
reinforced plastic parts [1, 2]. Therefore, the production
quality and tool wear play a role especially when machining
reinforced polymers [3]. Numerous studies in the past have
shown that tool wear and the development of measurement
and form errors and also the surface quality are caused by
mechanical stress during machining in combination with
reinforcement-fibers acting strongly abrasive. Even though
thermal load seems to have significant impact on the quality
of the work piece, temperatures resulting while drilling
polymers and their influence on plastics are still not ana-
lyzed in detail [47]. Because of their unfavorable thermo
physical characteristics, knowledge about reducing thermal
load while drilling plastics from this material group is
important. Within this article measurements of tool tem-
peratures while drilling reinforced and non reinforced
thermoplastic polymers are introduced and effects of tem-
perature on the production quality are analyzed.
2 Experimental set-up
In the studies presented here tool temperatures while dry
drilling four different thermo plastic polymers are studied.
The tested materials are polyamide 6.6 (PA), polyethere-
therketone (PEEK), polyoxymethylene (POM) and
polyethyleneterephthalat (PET). Furthermore the tempera-
ture development while drilling glass- and carbon
reinforced PEEK (PEEK GF 30 und PEEK CF 30) has been
investigated. The fiber volume percentage of these mate-
rials is 30 vol.%. Table 1 lists the most important
The investigations presented in this paper are funded by the
Deutsche Forschungsgemeinschaft (DFG)
K. Weinert F. Brinkel C. Kempmann K. Pantke (&)
Department of Machining Technology,
University of Dortmund, Dortmund, Germany
e-mail: pantke@isf.de
C. Kempmann
e-mail: kempmann@isf.de
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Prod. Eng. Res. Devel. (2007) 1:381387
DOI 10.1007/s11740-007-0015-y
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mechanical and thermal characteristics of the materials
used. Micrographs of the studied materials are shown in
Fig. 1. The drills used for measuring tool temperature are
straight fluted tools made of carbide with K10/20 classifi-
cation and a straight fluted tool with a polycrystalline
diamond (PCD) cutting edge. The tools have a diameter of
d= 8 mm. Tool temperatures are measured with thermo
elements, which are embedded into the clearance face of
the drilling tools. The measurement of temperature directly
at the area of impact is almost impossible because of
mechanical load and difficult accessibility. Due to this fact
the thermo elements are positioned with a defined distance
away from the cutting edge.
Thermo elements need a cable joint for data transfer
therefore, it is not possible to perform the experiments with
a rotating tool. For this reason an experimental set up with
a rotating work piece have been used [8]. During a second
test series temperature is measured inside the work piece
with a distance of 0.1 mm from the bore edge and 10 mm
from the beginning of the borehole. Figure 2 shows the
experimental set-up for measuring tool temperatures and
the position of the thermo elements in the clearance faces
of the drilling tools.
3 Influence of matrix material on thermal development
The following chapter deals with the tool temperatures
measured during the first test series. With 400C the tool
temperature measured when drilling 20 mm deep blind
holes into thermo plastic polymers is unexpectedly high
and almost always reaches the melting temperatures of the
tested polymer materials. It is remarkable that the tool
temperatures while using a carbide drill are up to 50C
higher than while using a PCD-tool. The tests show that
rising cutting speeds cause a higher tool temperature and
on the other hand increasing the feed causes a smaller
thermal load.
The thermal load, which is induced into the tool, is a
result of the conversion of mechanical energy into heat.
Here, the mechanical material properties contribute to the
generated heat quantity, but moreover the main influence is
given by the thermal material characteristics. The tensile
strength and hardness of the polymer materials as well as
the friction conditions determine the height of the occur-
ring feed forces as well as drilling torques. The cutting
power is in turn direct proportional to the feed force and
drilling torque. The mechanical cutting power which is
Table 1 Properties of the
deployed polymer materialsMaterial PA6.6 PEEK PEEK CF 30 PEEK GF 30 PET POM
Tensile strength Rm in MPa 90 50100 224 156 81 65
Therm. cond. coeff. ak in 106 J/K m 70 47 4 22 20 110
Heat capacity c in kJ/kg K 1.7 0.32 1.85 1.71 1.1 1.5
Heat conductivity k in W/m K 0.23 0.25 0.92 0.43 0.24 0.31
Glass trans. temp. Tg in C 78 143 143 143 69 38
Melting temp. Ts in C 255 334 334 334 255 175
Therm. intr. coeff. b in kJ m2 h0.5 K 40 19.5 94 63 36 49
Fig. 1 Microstructure of
analyzed thermo plastics
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necessary for the drilling process is converted into heat by
internal friction and friction of the process counterparts.
Thereby, the separating work as well as the kinetic energy
of the chips is considered to be neglectable. Thus, heat is
distributed to the tool, the work piece and the chips as well
as the environment. The quantity of heat distributed to the
active components is controlled by the thermal properties
of the tool and the polymer material. The heat capacity c
describes the quantitative amount of energy per tempera-
ture and mass, which can be absorbed by a material.
Temporary effects are not considered by this physicalvalue. In contrast, the heat conductivity k specifies which
time is needed for a certain amount of energy to spread into
the inside of the material. Comparing the measured tool
temperatures with the heat capacity and the heat conduc-
tivity no clear trend can be recognized. However, when
taking into account the thermal intrusion coefficient b, a
clear trend can be observed.
Thus, the thermal intrusion coefficient b is characteristic
for the magnitude of tool temperatures, which describes a
correlation between the magnitude of temperatures inside
the tool and the physical attributes of the matrix polymer.
This value relates heat conductivity k, heat capacity c anddensity q and is defined as
b ffiffiffiffiffiffiffiffikqc
pin kJ=m2 s0:5 K:
Accordingly, the thermal intrusion coefficient is a
measure of how much heat Q enters the work piece per
time and area. In previous tests the shear zone was
identified as the location for heat development. Because of
the higher thermal intrusion coefficient b of the cutting
material compared to the polymers, most of heat quantity
developed during the machining process flows directly into
the tool. After 20 mm drilling distance the PEEK material
shows the highest tool temperature with 360C compared
to all other non-reinforced polymers. This effect results
from the lowest thermal intrusion coefficient
b = 19.5 kJ m2 h0.5 K for PEEK compared to the other
tested materials. During the machining process, this thermo
plastic conducts more heat into the tool compared to allothers.
As a result maximum tool temperatures when drilling the
other materials develop in an opposite way compared to their
thermal intrusion coefficient b. Consequently the lowest
temperature of 190C occurs when drilling POM, which has
a thermal intrusion coefficient of b = 49 kJ m2 h0.5 K.
Figure 3 demonstrates the characteristics of tool and work
piece temperatures when drilling non-reinforced thermo
plastics with a cutting speed ofvc = 120 m min1 and a feed
off = 0.1 mm.
After dealing with tool temperatures, the second test
series focused on the work piece temperatures also shows arelation between the thermal intrusion coefficient b and the
temperature of the work piece. In polyamide PA and
polyoxymethylene POM, both having a high thermal
intrusion coefficient, the lowest work piece temperatures
(35 and 29C) are measured at a distance of 0.2 mm from
the drill edge and at a drill depth of 10 mm. On the one
hand, the tool heats up more when drilling PEEK and PET
while passing the measurement point, since a higher
amount of energy is converted. On the other hand, the heat
conducted from the cutting edge of the tool is more slowly
absorbed by the work piece, according to the higher ther-
mal intrusion coefficient b. Thus, heat distributes slower to
the inside of the work piece material and consequently the
measured work piece temperature is higher. In the case of
PEEK and PET more heat is accumulated at the drill edge
and at the bore hole wall because their thermal material
properties limit the transportation of heat to the inside of
the work piece. Accordingly, the quantity of heat is con-
centrated on a smaller amount of material at the tool tip and
the borehole wall. Thus, higher temperatures are generated
at the cutting edge as well as in the borehole wall.
Regarding the lower thermal intrusion coefficient b of
PEEK it is expected to have higher work piece tempera-
tures than PET. Accordingly, the work piece temperatures
in PEEK should be higher than in PET. The measured
temperatures display the opposite. The reason for the lower
work piece temperature of PEEK than PET seen in Fig. 3
can be found by temporal effects. At 10 mm drill depth the
tool temperature by drilling PEEK is clearly lower than it is
by drilling PET. By reaching 13 mm drill depth the tool
temperature in PEEK exceeds that of PET. Thus, the rate of
heating up is different. Consequently the work piece tem-
perature of PEEK would be higher as the work piece
Positions of embedded thermocouples
Collet with
sample
Boring tool with
thermo couple
Load cell
0.9
0.
5
1.6
0.
6
Top view: Cemented-Carbide-Tool
0.8
0.
4
0.
4
Top view: PCD-Tool
Tool fitting
Spindle
1.7
(Dimensions in mm)
Positions of embedded thermocouples
Collet with
sample
Boring tool with
thermo couple
Load cell
0.9
0.
5
1.6
0.
6
Top view: Cemented-Carbide-Tool
0.8
0.
4
0.
4
Top view: PCD-Tool
Tool fitting
Spindle
1.7
(Dimensions in mm)
Fig. 2 Experimental set-up and position of thermo elements in the
tool
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temperature of PET if the temperature was measured in a
depth of more than 13 mm.
4 Effect of reinforcement-fibers on temperature
development
Tool and work piece temperatures also show a dependency
on the thermal intrusion coefficient when drilling fiber-
reinforced thermo plastics and thereby on thermo physical
characteristics of the material. Figure 4 visualizes tem-
peratures measured in the tool and the work piece.
After a drilling distance of 20 mm non-reinforced PEEK
causes the highest tool temperatures. This is a result of
PEEK possessing a lower thermal intrusion coefficient b
(19.5 kJ m2 h0.5 K) than fiber-reinforced PEEK-materials.
Consequently, maximum tool temperatures are measured in
an opposite relation to the height of their thermal intrusion
coefficient when drilling fiber reinforced PEEK CF and
PEEK GF materials. Tool temperature when drilling
PEEK CF with a thermal intrusion coefficient of b = 94
kJ m2 h0.5 K is the lowest at 290C. Also the height of
work piece temperatures directly results from the depen-
dency on the thermal intrusion coefficient of the material.
Due to these fact materials with high thermal intrusion
coefficients results in lowest temperatures measured in the
borehole wall.
5 Surface development and tool temperatures
Besides the analysis of the occurring temperatures in the
cutting tool as well as in the work piece material the
influence of the temperatures on the materials integrity was
studied. In order to analyze the effect of the tool temper-atures on the drilled surfaces Fig. 5 shows pictures at
different drilling depths of the bore hole wall, made by a
scanning electron microscope of characteristic areas of the
drill surface in non-reinforced PEEK.
The corresponding tool temperatures can be read from
the line chart. At a drill depth of 2 mm and a tool tem-
perature of 120C a relatively flat surface is formed. Here,
the temperature is lower than the materials glass transition
temperature (Tg = 143C) and the polymer condition is
defined by high internal bond strength. An indication for
this effect is given by the characteristic feed marks. At a
drilling depth of 10 mm the tool temperature of 280C is
situated between glass transition temperature (Tg = 143C)
and melting temperature (Ts = 335C). In this temperature
range polymers become more ductile and their static fric-
tion coefficient rises. Local adhering and tearing of the tool
causes a stick slip effect. This effect generates an uneven
surface. The ductile material is smeared over the surface of
the borehole wall by the minor cutting edges of the rotating
drilling tool. This causes detectable unevenness at the
surface. At 18 mm drilling depth the tool exceeds the
Matrix:
PET, PEEK
l: PCD
:
Cutting velocity.: vc = 120 m/minFeed rate: f = 0.1 mm
Therm.
intrusion
coeff.
b
kJ(mhK)-1
0
Tool
temperatureTWZ
PA 6.6
POM
PETPEEK
19,5
36
40
49
10
40
30
20
60
T
5153
35
29
CTWS 0.2 mmmeasured by thebore edge
:
PET, PEEK
l:
:
.:F : f =
Material:
PA 6.6, POM,
PET, PEEK
Tool:
Diameter: d = 8.0 mmCutting Material:
Cutting Parameter
=
Lubrication-concept: dry
Therm.
intrusion
coeff.
b
kJ(m
hK)
0
PA 6.6
POM
PETPEEK
19,5
36
40
49
10
40
30
20
60
T
5153
35
29
TWS 0.2 mmmeasured by thebore edge
0
10
40
30
20
60
PEEK PET PA POM0
PEEK PET PA POM
Drilling depth l
0
50
100
150
200
250
300
C
0 2 4 mm206 8 10 12
PA 6.6
POM
PETPEEK
PA 6.6
POM
PETPEEK
19,5
36
40
49
10
40
30
20
60
Componentte
mperatureTWS 51
53
35
29
TWS 0.2 mmmeasured by thebore edge
14 16
50
Fig. 3 Impact of polymer
material on thermal
development in tool and work
piece
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melting temperature of the polymer. In this temperature
range the material partly adheres at the minor cutting
edges. Subsequently, the material is pressed out between
the lead chamfer and borehole surface.
6 Peripheral zone and tool temperatures
An important criterion to prove the thermal influence on
the work piece is an analysis of the created peripheral zone.
hK)-1
400
19,5
63
94
0
50
100
150
200
250
300
C
0 2 4 mm 206 8 10 12 14 16
PEEK
PEEK GF
PEEK CF
ComponenttemperatureTWS
29
51
34
Material:: PEEK
,
Tool:Diameter: d = 8.0 mmCutting Material: PCD
Cutting Parameter:Cutting velocity.: vc = 120 m/min
Lubrication-concept: dry
100
Thermalintarusio
ncoefficientbkJ(m
0
20
60
40
19,5
63
PEEK PEEK GF
94
PEEK CF
Drilling depth l0 2 4 mm 20
TooltemperatureTWZ
6 8 10 12 14 16
PEEK
PEEK GF
PEEK CF
0
10
40
30
20
60
29
51
34
PEEK PEEK GF
C
PEEK CF
TWS0.2 mmmeasureded by the
bore edge
:Matrix: PEEKFibre: not reinforced
30 Vol.-% GF,30 Vol.-% CF
Tool:Diameter: d = 8.0 mmCutting Material: PCD
Cutting Parameter:Cutti ng velocity.: v = 120 m/min
Lubrication-concept: dry
80
Feed rate: f = 0.1 mmFeed rate: f = 0.1 mm
Fig. 4 Impact of fiber material
on thermal development in tool
and work piece
0
50
100
150
200
250
300
C
400
2 10 200 4 6 8 12 14 16 mm
Thermocouple 2Thermocouple 1
Melting temperature PEEK: 334 C
20 kV 75x 200 m
l = 2 mmEnd of the bore:l = 18 mm
Material:Matrix: PEEK
Tool:
Diameter: d = 8.0 mmCutting material: HM K10/20
Cutting parameters:
Cutting velocity: vc = 120 m/minFeed rate: f = 0.1 mmLubrication: dry
T
ooltemperatureTWZ
20 kV 75x 200 m 20 kV 75x 200 m
0
50
100
150
200
250
300
400
2 10 200 4 6 8 12 14 16 mm
Thermocouple 2Thermocouple 1
Melting temperature PEEK: 334
Drilling depth l
20 kV 75x 200 m
Beginning of the bore: Middleof the b ore:l = 10 mm
Material:Matrix: PEEK
Tool:
Diameter: d = 8.0 mmCutting material: HM K10/20
Cutting parameters:
Cutting velocity: vc = 120 m/minFeed rate: f = 0.1 mmLubrication: dry
T
ooltemperatureTWZ
20 kV 75x 200 m 20 kV 75x 200 m
Fig. 5 Effect of tool
temperature on drill surface
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The generated microstructure exhibits the influence of hightemperatures on the polymer material. Light optical
microscope pictures show that plastic at the peripheral zone
has changed in color (Fig. 6). Tests on PEEK [9] in the past
have shown that these discolorations are amorphous areas,
which were created from crystalline areas through high
temperatures. Tool temperature modifies the microstructure
of the material in such a manner, that the material becomes
thermally weak and the crystalline areas in the micro-
structure begin to melt. Since the mechanical
characteristics of polymers are defined by their level of
crystallinity, the heat affected zone, where crystalline areas
are damaged by high temperatures, is a significant damageof the material. Destruction of the crystalline structures
causes reduction of mechanical stability and lower tem-
perature resistance during later application. In a few cases
it is possible, that the machined work piece is rendered
defective. Furthermore high process temperatures can lead
to chemical disruption of the polymer in the peripheral
zone. Because of missing verification methods this thesis
could not be proved yet.
The research shows a direct connection between tooltemperature and depth of damaged areas. The width of the
heat-affected zone at the beginning of the drill hole is
smaller than at the end of the drill hole, where tool tem-
peratures reached their maximum. High tool temperatures
cause a great temperature difference between the material
and drilling tool. This immense gradient allows the
exchange of a greater heat quantity Q between the drilling
tool and work piece as the smaller gradient at the beginning
of the drill allows. In plastic large heat quantities create
critical temperatures deep within the work piece. This can
be proven by a wider heat affected zone. The thermal
intrusion coefficient b is the determining factor for thedepth of heat impact. Figure 6 shows light optical micro-
scope made pictures of the peripheral zone of drills in PET,
PA and PEEK GF 30 at the beginning and the end of the
drill hole. It becomes apparent, that materials with low
thermal intrusion coefficients like PET are thermally
damaged in greater areas because of a smaller heat con-
duction into the material. Consequently more heat is
formed in a small area.
medium thermal heat impactzone width WB: 43 m
medium thermal heat impactzone width : 54 m
PA6.6
b=40kJ/m2h0.5K
Tg
=78C
medium thermal heat impact
zone width WB: 20 m
medium thermal heat impact
zone width : 38 mPEEKGF3b=63kJ/m2h0.5K
Tg
=143C
PETb=36kJ/m2h0.5K
Tg
=69C
Beginning of the bore:l = 2 mm
medium thermal heat impact
zone width WB: 47 m
medium thermal heat impactzone width 60 m
End of the bore: l = 18 mm
medium thermal heat impactzone width WB: 43 m
medium thermal heat impactzone width : 54 m
PA6.6
b=40kJ/m2h0.5K
Tg
medium thermal heat impact
zone width WB: 20 m
medium thermal heat impact
zone width : 38 mPEEKGF3b=63kJ/m2h0.5K
Tg
PETb=36kJ/m2h0.5K
Tg
=69
Beginning of the bore:l = 2 mm
medium thermal heat impact
zone width WB: 47 m
medium thermal heat impactzone width 60 m
End of the bore: l = 18 mmFig. 6 Changes in the
peripheral zone of PET, PA and
PEEK caused by heat
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7 Summary
Drilling reinforced and non-reinforced thermo plastics
causes high tool temperatures. High tool temperatures lead
to melting and thermal damage of the material in the
peripheral zone of the drill hole. A direct connection
between the magnitude of the tool temperature, the thermal
intrusion coefficient b and the depth of damaged areas inthe borehole wall exists. The higher the tool temperature is,
the deeper is the thermal material damage and the weak-
ened areas at the drill hole wall. Furthermore, low thermal
intrusion coefficients of the studied materials cause a
concentration of heat within small areas. Consequences of
this effect are high temperatures at the drilling tool, which
results in damages of the microstructure.
In order to avoid thermally induced destruction of
polymers at the bore hole wall tool temperatures have to be
kept as low as possible during the process. Low cutting
speeds and high feeds are helpful to realize these favored
tool temperatures. Due to the large amount of existingpolymers with different material characteristics, no general
conditions for cutting polymers can be given. Each dif-
ferent case needs to be analyzed individually.
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