Zeitschrift Kunststofftechnik Journal of Plastics TechnologyTable 1: Material properties of polycarbonate (Makrolon 2805) The higher the mould temperature, the lower the residual stress

© Carl Hanser Verlag Zeitschrift Kunststofftechnik / Journal of Plastics Technology 7 (2011) 3

handed in/eingereicht: 06.10.2010 accepted/angenommen: 29.03.2011

Dipl.-Ing. Andreas Nau, Prof. Dr.-Ing. habil. Berthold Scholtes, IfW Institute of Materials Engineering - Metallic Materials, University of Kassel

Dipl.-Ing. Martin Rohleder, IfW Institute of Materials Engineering – Plastics Technology, University of Kassel

PH.D João Nobre, CEMDRX, Department of Mechanical Engineering, University of Coimbra, Portugal

Application of the Hole Drilling Method for Residual Stress Analyses in Components made of Polycarbonate The incremental hole drilling method is widely used and a cost-effective procedure to analyse residual stress depth distribution states in metallic components. By disturbance of the mechanical equilibrium due to a stepwise introduced hole, it is possible to calculate residual stress depth distributions due to the relaxed strains recorded by special strain gages. The calculation is based on modified elasticity assumptions and implemented calibration functions. In this publication, using polycarbonate as model material, important process conditions are outlined which have to be fulfilled to achieve reliable results in case of residual stress analyses in components made of plastics.

Anwendung des Bohrlochverfahrens für Eigenspannungsanalysen in Proben aus Polycarbonat Die inkrementelle Bohrlochmethode ist ein weit verbreitetes und wirtschaftliches Verfahren zur Bestimmung von Eigenspannungstiefenverläufen. Durch Störung des vorhandenen mechanischen Gleichgewichtes in Form eines stufenweise erzeugten Loches können mittels der mit einer speziellen Dehnungsmessstreifenrosette registrierten relaxierten Dehnungen Eigenspannungstiefenverläufe berechnet werden. Hierfür kommen modifizierte elastizitätstheoretische Ansätze zur Anwendung, die üblicherweise auf Kalibrierfunktionen basieren. Die in dieser Arbeit beschriebenen Untersuchungen geben am Beispiel des Werkstoffs Polycarbonat den Rahmen für die wichtigsten Prozessparameter vor, die für die reproduzierbare Bestimmung von Eigenspannungen in Bauteilkomponenten aus Kunststoffen eingehalten werden sollten.

Zeitschrift Kunststofftechnik Journal of Plastics Technology archival, peer-reviewed online Journal of the Scientific Alliance of Polymer Technology archivierte, peer-rezensierte Internetzeitschrift des Wissenschaftlichen Arbeitskreises Kunststofftechnik (WAK) www.plasticseng.com, www.kunststofftech.com

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Nau, Scholtes et al. Hole Drilling Method for Stress Analyses in PC

Journal of Plastics Technology 7 (2011) 3 67

Application of the hole drilling method for residual stress analyses in components made of polycarbonate

A. Nau, B. Scholtes, M. Rohleder, J. Nobre

1 INTRODUCTION

It is generally accepted that the manufacturing and processing of components made of metallic as well as non-metallic materials e.g. thermoplastic polymers is accompanied by the formation of characteristic residual stress states [1, 2], which may have a decisive influence on strength, lifetime and applicability. Therefore, a great interest exists in reliable methods of their analysis.

In thermoplastic polymers based on the highly dynamic processes during the production, shear forces create characteristic inhomogeneous stresses in the skin layer of the products, which are especially highly influenced by the injection speed and the mold temperature [3]. The high thermal expansion coefficient causes shrinkage of the material during the cooling process and leads to warpage or rather residual stresses in immobile areas [4, 5].

Residual stresses can be reduced or even prevented by lower cooling rates, special processing technologies like variothermic processing (fast heating and cooling of the mould), injection compression moulding or foaming [6, 7] and also by thermal annealing of the products afterwards. However, all of these methods except foaming increase the cycle time as well as investment and production costs.

The mechanical properties of thermoplastic polymers are highly influenced by their residual stress state, especially in near surface volumes [4, 5]. A simple method to characterize the residual stress state is the environmental stress crack resistance (ESCR) test [8]. A disadvantage of this method is that the determination of residual stress can only be done in areas close to the surface and results are only qualitative and not quantitative. A further influencing factor is the corrosion medium used, which necessitates a comparison with a reference residual stress free material to relate the stress crack resistance to the existing residual stress state.

The hole drilling method, unlike diffraction methods, which are restricted to crystalline materials, is a widely used procedure to quantitatively analyse residual stress states in metallic and plastic components. It is based on the determination of strains relaxed by the disturbance of the mechanical equilibrium in the component as a consequence of a stepwise introduced small hole, Fig. 1. In general 6-blade face cutters are used to introduce an

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approximately ideal cylindrical hole with a flat bottom. This is essential because calibration functions based on numerical simulations assume an ideal drilling process excluding geometrical deviations from an ideal cylindrical hole. For the calculation of the initial residual stresses the theory of elasticity is used. The most important influencing factors with respect to strain sensitivity of the applied strain gage can be summarized as follows, see Fig.1:

The lower the stress state in the component, the lower are the measured strains on the components surface.

The deeper the position of the removed material increment, the lower are the measured strains on the components surface.

The larger the distance between the drilled hole and the applied strain gage, the lower are the measured strains on the components surface.

Because of the marginal mechanical disturbance the hole drilling method is classified by ASTM as semi-destructive [9].

In general, relaxed strains are measured by special strain gage rosettes. Other contactless techniques based on optical analyses, e.g. speckle interferometry [10], are available but more expensive as well as not fully developed for general practical application.

Fig. 1: Principle of the hole drilling method

left: introduction of a hole by incremental drilling with an end mill right: strain sensitivity dependent on stress state and geometrical

parameters

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If strain relaxation is analysed by strain gages, a careful surface preparation and the use of appropriate adhesives is necessary to ensure an excellent bonding between the strain gages and the components under investigation. Different solvents e.g. acetone, propanol as well as acid are used to obtain a high quality surface finish to apply strain gages in case of metallic materials [11]. In order to ensure the direct transmission of the released strains to the strain gages the layer thickness of the adhesive should be as thin as possible.

To get high-resolution output signals of the measurement amplifier, feeding voltages between 3 V up to 5 V are used. This results in a high current density due to the small diameter of the conducting paths of the strain gages [11], which may generate a non-negligible temperature rise in materials with low thermal conductivity.

Due to the notch effect of the drilled hole [12 - 15] measured residual stresses are only reliable if they are below 60 % of the materials’ yield stress [14]. A further important issue is the introduction of a hole into the specimen with a minimum amount of plastic deformation. For metallic materials the recommen-ded and most practical technique is the application of high speed drilling (HSD) [16] by air turbines with a speed of rotation up to 400000 rpm. In general, end mills with 6-blades are preferred in order to obtain a cylindrical hole with a flat bottom. In this way, only negligible microstructural alterations are generated in the vicinity of the drilled hole and elastic effects are by far prevailing.

However, plastics have quite different and strongly temperature dependent materials properties. As a consequence, it has to be expected that different tools and process parameters have to be applied in order to produce holes of the required quality. Otherwise, detrimental effects on the residual stress measurements cannot be excluded. It is well known that reliable stress analy-ses by the hole drilling method are based on special requirements concerning the drilling as well as the strain measuring procedure. Basically, it is of importance that only elastic deformations are relaxed by the drilling process. In practice, that means that plasticity effects due to the chip forming drilling operation have to be minimized as far as possible. In addition, the hole geometry should be as close as possible cylindrical with a flat bottom and the position of the strain gages relative to the hole should be exactly known. Finally, any additional fictitious strains not due to the drilling procedure and time dependent effects should be strictly avoided. There are several publications in literature using hole drilling experiments for residual stress analyses in plastics (see e. g. [2, 17 - 19]). However, clear recommendations about the necessary boundary conditions for the application of the hole drilling method in such materials are still lacking.

As a consequence, in a systematic experimental study important process conditions were identified which have to be fulfilled to achieve reliable results in case of residual stress analyses in plastic components and results are presented in this publication.

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2 MATERIAL INVESTIGATED AND EXPERIMENTAL DETAILS

The material investigated was polycarbonate (Makrolon 2805 - Bayer MaterialScience) with medium viscosity and an MVR of 9.5cm³/10 min (300 °C; 1.2 kg). It is a transparent general purpose grade with release additive produced intended for a mould. Important material parameters are given in Table 1. Most remarkable is the thermal conductivity of 0.2 W/(m K), which is considerably smaller in comparison to metallic materials (>15 W/(m K)). As a consequence, all process steps producing heat, e.g. manufacturing of the soldering connections of the strain gages, have to be carried out very carefully in order to avoid detrimental temperature effects.

Residual stresses are highly influenced by the production process of the specimens. Main influencing parameters are the mould, the melt temperature and injection velocity.

tensilemodulus

yieldstress

poissonratio

glas transition t.

thermalconductivity

in MPa in MPa - in °C in W/(mK)

2373 66 0.35* 148** 0.20***assumption, **Bayer AG

Table 1: Material properties of polycarbonate (Makrolon 2805)

The higher the mould temperature, the lower the residual stress of the produced sample, based on the lower cooling rate and the possibility to relax emerging stresses by a creep process. Furthermore a low mould temperature generates high shear forces by freezing the melt on the surface of the mould which leads to residual stress in these areas of the sample. This can be intensified by an increase of the injection velocity.

The mould used in this study had a plate geometry with a size of (160 x 60 x 4) mm³. The melt was injected through a film gate which should lead to a homogeneous residual stress value in flow direction over most of the specimen. Specimens were manufactured according to DIN EN ISO 294-1 and one batch of the specimens was additionally tempered at 128°C for 72 hours in order to obtain a residual stress free starting condition.

Further specimens were manufactured with two different manufacturing conditions. These specimens were produced by an injection moulding process (Engel E-Motion 100 – clamp force 100 to, screw diameter 35 mm) with different processing parameters. Condition “l” was produced according to DIN EN ISO 294-1 [20] and DIN EN ISO 7391-2 [21]. To achieve higher residual stress amounts condition “h” was produced with a higher injection velocity and lower melt and mould temperatures, Table 2.

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processtemperature

mouldtemperature

injectionvelocity

basic processconditions

in °C in °C in mm/s

condition l 300 80 200DIN EN ISO 294-1

DIN EN ISO 7391-2

condition h 280 40 300 DIN EN ISO 294-1

Table 2: Injection moulding processing parameters

The test equipment used for hole drilling measurements, Fig. 2, consists of the milling guide RS-200 (Vishay Measurements Group), the measurement amplifier Picas (Peekel Instruments GmbH), strain gage rosettes of the type CAE-13-062UM-120 (Vishay Micro-Measurements) and the evaluation software BOP (MPA Stuttgart).

Fig. 2: Experimental equipment and investigated drilling techniques

RS-200 is a high-precision device for analysing residual stresses by the hole drilling method, but without any automatic functions for in depth feed rate or circular direction. The parameters have to be adjusted manually by the operator. It is possible to assemble the RS-200 with an air turbine or with a milling rod. The latter can be connected to an electrical drilling machine or be

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operated manually. In this study the air turbine and manual drilling were applied. The drilling process can be carried out in a conventional manner by advancing the rapidly rotating end mill into the specimen as well as in an orbital manner with an additional rotary motion around a radial offset relative to the axial center line of the turbine assembly. Orbital drilling unlike conventional drilling has advantages in chip removal, reduction of overheating and results in increasing tool lifetime [22].

To connect the strain gage rosette with the amplifier, a 3-wire technique is the best choice to compensate cable resistance of the back wire. A quarter Wheatstone bridge circuit without an additional strain gage for temperature compensation was selected due to laboratory conditions as well as stable temperature conditions of the specimen.

To assess the influence of the different drilling techniques on the materials damage in the vicinity of the hole, specimens were investigated in a first step by means of photoelasticity with linear polarized light in the dark field to get a qualitative information about the drilling induced plastic deformation.

Micrographs of the cross sectional area of the drilled hole were prepared to obtain information about the hole geometry. For this purpose, the polycarbonate plates were cut close to he holes by a low speed saw using diamond coated saw blades and water cooling. Subsequently, the prepared pieces were cold embedded with epoxid resin and grinded as well as polished to the center of the cross sectional area of the hole. For polishing a special polishing plate appropriate for plastic materials was used.

To investigate the thermal influence on the material induced only by a single strain gage, measurements were carried out with a thermal sensor applied directly on the strain gage grid as well as 1 mm below the strain gage in the material, as a function of different feeding voltages. For the latter thermal sensor, a small hole was drilled into the specimen filled with heat transfer paste. Additional measurements were carried out by a thermography camera in order to obtain a temperature distribution field. The required emission coefficient for the applied strain gage on polycarbonate was identified using the two thermal sensors, mentioned above and set to 1 for a black body.

For all experimental steps, a relaxation time after drilling of about 5 min was taken into account before strain values were recorded.

To assess the selected experimental parameters based on previous investi-gations, residual stresses were simulated by applied external uniaxial loading stresses using a tensile testing machine, Fig. 3. To avoid any effects that might arise due to unknown residual stress states, two different stresses max and min were applied and according to the equation below, the calibration stress cal was determined (Eq. 1). The applied loads were controlled by an additional calibration strain gage. An appropriate specimen for the tensile machine was milled out of a polycarbonate plate and was subsequently tempered.

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RSRScal minmax (1)

Fig. 3: Working with calibration stresses by means of a tensile machine

To calculate residual stresses from the measured strain distributions, software BOP with the evaluation algorithm MPA II developed by Kockelmann and Schwarz [23] was used which has calibration functions implemented for a component geometry of a thick wide plate. However the calibration functions are also appropriate for other geometries within well defined geometrical boundary conditions, if the restrictions listed in Table 2 are fulfilled [24, 25].

Table 2: Geometrical boundary conditions for the MPA II standard [24]

3 EXPERIMENTAL RESULTS AND DISCUSSION

3.1. Drilling operation

For metallic materials, HSD using appropriate end mills, e.g. 6-blade face cutters, is recommended. However, polymer materials have a lower thermal conductivity and a low glass transition temperature. Thus, thermal effects due to

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frictional heating cannot be neglected. In order to demonstrate qualitatively the influence of different drilling techniques, only photoelasticity was used in a first step. Fig. 4 shows the tempered polycarbonate plate before as well as after drilling, grouped for HSD (Pos. 1-10) and for manual drilling (Pos. 13-15). It is quite obvious that in all cases HSD introduces coupled thermal / mechanical effects into the specimen. In this way, unintentional strains are produced. On the other hand, manual drilling results in the smallest effects. Additionally, Fig. 4 highlights the reproducibility of the manual technique, which was carried out three times under the same testing conditions.

Fig. 4: Specimen before and after drilling with different conditions

A detailed investigation of the effects of the different drilling techniques is illustrated in Fig. 5. It is quite obvious that, the higher the turbine air pressure and, hence, the rotational speed, the more pronounced are deformations due to the drilling procedure on the material. Additionally, the hole diameter increases with higher air pressure of the turbine due to higher radial vibrations of the end mill which results in an expanding of the holes. However, there is a difference among the three HSD-techniques. The orbital unlike the conventional one seems to have less detrimental effects on the materials state near the hole. In [17] this technique is preferred because air can circulate in the hole and is able to cool the drill and to prevent chips from being trapped between the drill bit and the edges of the hole. Because the theory of the hole drilling method and existing evaluation standards require an ideal geometry of a cylindrical hole with a flat bottom, the resulting hole geometries dependent on the different drilling processes have to be investigated, too.

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Fig. 5: Photoelastic analysis of different drilling techniques for PC

For this purpose, cross sections of the drilled holes were prepared and analysed which is shown in Fig. 6. The differences of the geometries are marginal. The best cylindrical geometry can be achieved with the conventional HSD and a high turbine air pressure. But manually drilling also results in appropriate cylindrical geometries with even sharper vertices at the bottom of the hole.

Fig. 6: Hole geometries dependent on different drilling techniques

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3.2. Effect of the strain gage feeding voltage

Besides the drilling technique there is still another more important aspect as far as thermal effects are concerned. If the strain measurement is carried out with electrical resistance strain gages, the feeding voltage has a non negligible effect due to the low thermal conductivity of the material. Working with a feeding voltage of 5 V, which is commonly used in case of metallic materials, results in a current density of 46 A/mm² for a 120 strain gage in a symmetric bridge circuit. The generated Joule heat cannot be adequately dissipated in the material itself, see Fig. 7. The mean temperature of the two thermal sensors (dotted line), one directly on the grid (MP1) and the other one inside the material below the grid (MP2), increases up to 50 °C, working with a maximum feeding voltage of 5 V. The investigation with the thermography camera yields maximum values up to nearly 80°C (see upper part of Fig. 7).

Fig. 7: Influence of feeding voltage of a strain gage rosette applied on PC

Fig. 8 highlights the thermal strain output signal of the grid of the applied strain gage rosette when the feeding voltage is stepwise activated and switched off from 0.5 V up to 5 V The strain values were recorded, when the strain signal reached a constant state. A holding time of about 10 min was required. It is

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clearly shown that due to the heat produced, a remarkable thermal strain develops. Moreover, after voltage switch off, the strain signal does not disappear and, obviously, a permanent strain has been produced. Only for a feeding voltage of 0.5 V, the strain signal behaviour is acceptable. To ensure that no thermal influences affect the strain analyses by temperature induced thermal strains, it is recommended that a low feeding voltages of about 0.5 V is used in order to get reliable results.

Fig. 8: Influence of feeding voltage on the output signal of a strain gage rosette applied on PC, displayed for grid 2.

3.3. Specimen preparation

First measurements were carried out by manual drilling technique and a feeding voltage of 0.5 V on tempered, i. e. residual stress relieved PC. Hence, negligible strain relaxation was expected due to the tempered state of the specimen on the one hand and the selected sensitive drilling technique and the low feeding voltage on the other hand. But in contrast to that, considerable strain relaxation was observed. It turned out that the solvent used to provide an appropriate clean surface condition for applying strain gages has an enormous conse-quence on the measurement because of its influence on the near surface region of the specimen. Fig. 9 illustrates the measured strains for a tempered PC-specimen, prepared with different solvents for different drilling techniques. Again, it is confirmed by several measurements that the strain values measured if HSD is used are considerably distorted by the drilling process itself, introducing undesirable machining strains, because the results are absolutely unrealistic for a tempered PC-specimen. Additionally the investigation clearly points out the consequences of different solvents for surface preparation. In both cases the measured strains are clearly higher in the region close to the surface of the specimen, where the solvent can affect the material properties, if propanol is used compared to the use of ethanol. Cleaning with propanol leads to considerable strain relaxation during the following drilling process, which is not the case for ethanol. Propanol is known to damage the affected areas of plastics [26] and this might be the reason for the observed effects. Therefore it

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is recommended to clean the surface of PC with ethanol before applying strain gages, because it does not affect the material [26].

Fig. 9: Influence of the surface preparation on measured stress values

3.4. Examples of stress analyses

To validate the measuring procedure developed in the foregoing sections, two different kinds of stress analyses were carried out: Tests with calibration stress, simulating residual stress by appling loading stress, and tests with specially manufactured specimens where due to the individual manufacturing process characteristic residual stress states were expected.

Results of the calibration stress experiments are plotted in Fig. 10. On the left hand side, the applied strain gage on the specimen clamping in the tensile testing machine and the directions of the three grids as well as the relaxed strains under uniaxial tensile loads of cal, max = 15 MPa (solid lines) and cal, min = 10 MPa (dotted lines) resp. are shown. The diagram on the right hand side shows the calculated first principal stress (dashed line) compared with theoretical expectations (solid lines). A good agreement between measured and applied stress can be stated, which confirms the reliability of the measuring procedure. There are only devations close to the surface and for the maximum hole depth, which is commonly observed for the hole drilling method.

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Fig. 10: Residual stress analysis with applied calibration stress

Fig. 11 shows results of measurements of specimens with manufacturing induced residual stress compared with a tempered condition.

Fig. 11: Residual stress analysis for different manufacturing conditions of PC-specimens in comparison to a tempered one

In condition “h” and “l”, small tensile residual stresses are measured at the surface and in both cases max. compression residual stresses between -3 and -

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4 MPa at a surface distance of approximately 0.4 mm. By contrast, tempered specimens are almost free of residual stresses in agreement with expectation.

4 SUMMARY

The presented experimental results clearly show that only a careful execution of the drilling procedure and measurement of the relaxed strains by strain gages results in reliable residual stress values. Established procedures and process parameters valid for stress analyses in metallic materials cannot be applied without appropriate adaptation. In the case of polycarbonate, the typical elastoplastic materials behaviour of plastics as well as their characteristic thermophysical properties are of importance. From the experimental results gained in this work, guidelines and instructions are deduced for the application of the hole drilling method, which can be transferred and generalized to other components made of plastics. A comprehensive summary of the recommendations is given in Fig. 12.

Fig. 12: Recommendations for residual stresses analysis in PC

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ACKNOWLEDGEMENT

The work presented in this paper was carried out within the scope of TRR 30 funded by German Research foundation DFG, which is gratefully acknowleged.

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[14] Nobre, J.P.; Kornmeier, M.; Dias, A.M.; Scholtes, B.

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Eigenspannungsanalysen nach der Bohrlochmethode unter Verwendung geometriespezifischer Kalibrierfunktionen, MP Materials Testing 6 (2010), pp. 356-362

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[26] Business Developement– Polycarbonate Bayer MaterialScience AG (Hrsg.)

Makrolon Chemische Beständigkeit, 2004

Keywords: residual stresses, hole drilling method, polycarbonate, plastics

Stichworte: Eigenspannungen, Bohrlochmethode, Polycarbonat, Kunststoffe

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Author/Autor: Dipl.-Ing. Andreas Nau University of Kassel IfW Institute of Materials Engineering - Metallic Materials Mönchebergstr. 3 34125 Kassel Prof. Dr.-Ing. habil. Berthold Scholtes University of Kassel IfW Institute of Materials Engineering - Metallic Materials Mönchebergstr. 3 34125 Kassel Dipl.-Ing. Martin Rohleder, University of Kassel IfW Institute of Materials Engineering – Plastics Technology Mönchebergstr. 3 34125 Kassel PH.D João Nobre University of Coimbra, Portugal CEMDRX, Department of Mechanical Engineering

E-Mail: [email protected] Website: uni-kassel.de Phone.: +49(0)561/804-3697 Fax: +49(0)561/804-3699 E-Mail: [email protected] Website: uni-kassel.de Phone.: +49(0)561/804-3660 Fax: +49(0)561/804-3699 E-Mail: [email protected] Website: kutech-kassel.de Phone.: +49(0)561/804-3688 Fax: +49(0)561/804-3692 E-Mail: [email protected]

Editor/Herausgeber: Europe/Europa Prof. Dr.-Ing. Dr. h.c. G. W. Ehrenstein, verantwortlich Lehrstuhl für Kunststofftechnik Universität Erlangen-Nürnberg Am Weichselgarten 9 91058 Erlangen Deutschland Phone: +49/(0)9131/85 - 29703 Fax.: +49/(0)9131/85 - 29709 E-Mail: [email protected]

The Americas/Amerikas Prof. Dr. Tim A. Osswald, responsible Polymer Engineering Center, Director University of Wisconsin-Madison 1513 University Avenue Madison, WI 53706 USA Phone: +1/608 263 9538 Fax.: +1/608 265 2316 E-Mail: [email protected]

Publisher/Verlag: Carl-Hanser-Verlag Jürgen Harth Ltg. Online-Services & E-Commerce, Fachbuchanzeigen und Elektronische Lizenzen Kolbergerstrasse 22 81679 Muenchen Phone.: 089/99 830 - 300 Fax: 089/99 830 - 156 E-mail: [email protected]

Editorial Board/Beirat: Professoren des Wissenschaftlichen Arbeitskreises Kunststofftechnik/ Professors of the Scientific Alliance of Polymer Technology

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Zeitschrift Kunststofftechnik Journal of Plastics TechnologyTable 1: Material properties of polycarbonate (Makrolon 2805) The higher the mould temperature, the lower the residual stress

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