the dependency of material

8/3/2019 The Dependency of Material

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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: [email protected]

C. Kempmann

e-mail: [email protected]

123

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

382 Prod. Eng. Res. Devel. (2007) 1:381387

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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

Prod. Eng. Res. Devel. (2007) 1:381387 383

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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


123


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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


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


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


zone width : 38 mPEEKGF3b=63kJ/m2h0.5K

Tg

=143C

PETb=36kJ/m2h0.5K

Tg

=69C

Beginning of the bore:l = 2 mm


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


zone width WB: 20 m


zone width : 38 mPEEKGF3b=63kJ/m2h0.5K

Tg

PETb=36kJ/m2h0.5K

Tg

=69

Beginning of the bore:l = 2 mm


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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