MICRO COMPONENTS WITH LOCALLY DIFFERENT PROPERTIES REALIZED BY SEGMENTED TEMPERED INJECTION MOLDING

Transcription

1 MICRO COMPONENTS WITH LOCALLY DIFFERENT PROPERTIES REALIZED BY SEGMENTED TEMPERED INJECTION MOLDING Christopher Fischer and Dietmar Drummer, Institute of Polymer Technology (LKT), Friedrich-Alexander-University Erlangen-Nuremberg, Germany Abstract Influencing mold temperature during injection molding via dynamic tempering affects the melt s cooling conditions and, therefore, can lead to a change in component properties. The current research presents an innovative dynamically tempered mold technology with different temperature zones within the cavity, which enables the production of micro components with locally different component properties. Results with ipp show that due to influencing internal component properties such as morphology and degree of crystallization, significant differences in mechanical component properties can be achieved. 1. Introduction Product miniaturization has decisively been determined by the microelectromechanical system industry and, therefore, components in the field of micro technology have gained in importance in recent years [1]. Especially in the areas of biotechnology and medical technology plastic micro components are particularly important because of their comparatively low production costs [2]. During the production of micro components, the high cooling rate over the entire component s cross section can lead to limitations in the mold filling due to a fast melt solidification. In order to allow a complete mold filling, an increase in the injection speed, the mold temperature or the mass temperature has been established. An increase in mold temperature can, for example, be realized by means of a dynamic temperature control of the mold, Figure 1. temperature T [ C] T s T e injection t heating t cooling t cycle time t [s] ejection Figure 1. Mold temperature profile during dynamic temperature control according to [3]. In the case of conventional dynamic tempering systems, the mold is first heated within the materialspecific solidification temperature range. After filling the cavity during the injection step, the mold is cooled below the ejection temperature. The higher mold temperature during the injection process leads to a lower cooling rate of the polymer melt. Thus, the formation of internal properties can be influenced in addition to the better filling behavior [4]. In particular, an increase in mold temperature can lead to more distinct internal properties (for example, higher degree of crystallization, coarser spherulitic structure, formation of thermally preferred crystal modification, etc.) owing to the reduction of the cooling rate of the polymer melt and, for example, a corresponding lower nucleation rate during the crystallization process [5, 6]. By means of influencing the internal properties, resulting external component properties (for example, mechanical, optical, tribological, etc.) can be influenced. For semi-crystalline materials, an increase in the degree of crystallization is associated with an increase in stiffness and strength as well as a reduction in the elongation at break [7]. For different crystal modifications, influences on mechanical component behavior could have been shown [8]. Regarding isotactic polypropylene primary four different polymorphs are discussed in literature: α-, β-, γ- and a mesomorphic-phase [9, 1]. Here, β- and γ-crystals primary form out under certain conditions such as specific nucleating agents or shear-stress during manufacturing [11, 12] (for β-form) and elevated pressure (for γ-form) [13]. The most stable form is discussed to be the monoclinic α-form, which develops at cooling the melt with low to moderate cooling conditions in a temperature range from 5 C to T m [14]. With increasing cooling velocity, the amount of the mesomorphic-phase increases. Here, temperatures less or equal 5 C are estimated to produce primary the mesomorphic-phase. Recent researches have shown comparable effects also for semi-crystalline micro components [15]. In addition, oriented component regions can have higher modulus of elasticity, higher stretching stresses as well as lower elongation stresses. Research work at the Institute of Polymer Technology (LKT) shows that structure and component properties of small components differ from those of conventional macroscopic components [3, 16, 17]. As a SPE ANTEC Anaheim 217 / 951

2 result of the relatively fine structures and lower degrees of crystallization, compared to macroscopic components, microstructures exhibit less favorable mechanical properties under comparable processing conditions. A targeted influencing of the internal properties by a reduction in the cooling rate of the polymer melt (for example by an increase in the mold temperature or by the use of low-temperature conductive mold materials) can contribute to a significant improvement in the mechanical properties of the micro components. Currently, existing dynamic tempering concepts aim at a primary homogeneous temperature distribution over the entire cavity length to produce parts with homogenous properties. The aim of the present research is to investigate the influence of segmented temperature areas within the cavity to produce a component with locally different and defined properties. the entire mold. Thus, a short cooling phase can be ensured. Furthermore, the water flow rate can be set separately to synchronize the resulting cooling rates of the nozzle and ejector side. After the end of the cooling phase, the mold opens and the next cycle starts. 2. Experimental 2.1 Mold concept with heating ceramics The mold concept presented in this article was designed and built by the companies Ypsomed GmbH and gwk Gesellschaft Wärme Kältetechnik mbh in cooperation with the Institute of Polymer Technology. The heating of the dynamically tempered mold is realized by means of segmented electrical heating ceramics. For this purpose, four heating ceramics are arranged directly below the cavity region for each (ejector and nozzle) side. The temperature can be set separately in a temperature range from 25 to 3 C. Figure 2 shows the mold concept as well as an infrared image of the cavity side, with the two upper and the two lower ceramics being tempered to the same temperature and, therefore, two different temperature zones can be measured. The temperature control is realized by means of an integrat evolution by gwk, Figure 3. The integrated high performance mold inserts with ceramic power heaters (CPH) and close-to-cavity cooling channels are responsible for the dynamic temperature control. Independent from the temperature of the main mold they control the temperature profile of each individual ceramic. In addition to the specification of the respective heating ceramic temperature, the dynamic temperature system allows a setting of the heating power to achieve a defined heating rate. Process control is handled by the central controller of the heating unit which simultaneously ensures a uniform temperature of the mold. The ceramic high performance heater is located behind the cavities. Therefore, a rapid heating at comparatively low energy expenditure can be realized. The start of the cooling phase is initiated by a machine signal. The cooling via the closeto-cavity cooling channels is realized by means of water and simultaneously serves to insulate the mold from the heater and, therefore, can ensure temperature stability of Figure 2. Mold concept of the segmented dynamic temperature control; CAD-sectional view (A: cooling channel, B: heating ceramic, C: thermo sensor, D: cavity); Infrared image of the tempered cavity area. Figure 3. Dynamic tempering system integrat evolution by gwk. SPE ANTEC Anaheim 217 / 952

3 2.2 Material, specimen and processing Within this research, industrially used ipp (Sabic 55P), supplied by SABIC, was used. According to the datasheet, this PP-type is a homopolymer with medium isotacticity and a melt flow rate of 2 dg/min. As test specimens, rectangular plates with a dimension (length x width x thickness) of 38 x 35 x,55 mm 3 were injection molded by means of an Allrounder 37 U from Arburg (screw diameter of 15 mm). Relevant injection molding parameters are listed in Table 1. Table 1. Relevant injection molding parameters. mass temperature T m, C 25 mold temperature zone 1 T m,zone1, C 11 mold temperature zone 2 T m,zone2, C 6 injection speed v in, mm/s 1 holding pressure p h, bar 4 holding pressure time t ph, s.5 ejection temperature T e, C 25 For the present investigations, the two lower heating ceramics (close to the gate) were heated to 11 C, also compare Figure 2. The two upper heating ceramics were heated to 6 C. The maximum mold temperature of 11 C was chosen based on internal DSC measurements, after which the material crystallizes in the range of 12 C (cooling velocity 1 K/min). To avoid mold damage and to achieve a primarily homogenous temperature distribution within each cavity area, the temperature difference was set to 5 C. 2.3 Component characterization In order to assess the optical properties, a frontal component photography was taken. Furthermore, the transmission was measured by means of the spectrometer UV / VIS Lambda 18 from Perkin Elmer at a wavelength of 583 nm. To evaluate residual stresses and orientations in the different component regions, a photoelasticity test with polarized light was performed with an GS2 by Schneider Messtechnik using circular polarized light. In order to evaluate the morphology, a 1 μm thin section was taken along the flow direction in the middle of the injection molded component and analyzed at under 45 linearly polarized transmitted light by means of the microscope Axio Imager.M2 from Zeiss. The component positions for evaluating the transmission, the morphology and the degree of crystallization can be taken from Figure 4. For calculating the degree of crystallization, the melting enthalpy H m has been determined by DSC measurements using a Q 1 TMDSC by TA Instruments according to DIN EN ISO and set into correlation with H m (melting enthalpy of 1 % crystalline material) which is described to be 25 J/g [18]. For the determination of the mechanical parameters tensile modulus and yield strain, tensile bars were prepared from the different temperature areas transversely to the flow direction, Figure 4. The dimensions were scaled with 1 to 8 based on the campus tensile bar (DIN EN ISO 527). The tensile tests were carried out according to DIN EN ISO 527. Therefore, 5 tensile bars were tested using an Instron 5948 MicroTester by Instron at standard climate. 9 mm 21 mm Figure 4. Position for the evaluation of transmission, morphology, degree of crystallization, tensile modulus and yield strain (blue: cold temperature zone, red: warm temperature zone). 3. Results 3.1 Morphology and degree of crystallization The morphology of the different mold temperature zones is depicted in Figure 5. At 11 C, in comparison to 6 C, a significantly coarser-spherulitic structure can be seen over the entire cross-section of the core area. Regarding the skin layer, except of a more distinct layer thickness at 6 C, no significant effects can be detected. The differences in the morphology can be explained by the different cooling rates of the polymer melt during the injection process. Accordingly, due to the slow cooling and the resulting lower nucleation rate, larger spherulitic structures are formed at 11 C. At 6 C, the higher cooling rate over the component s cross section leads to a higher nucleation rate. As a consequence, smaller spherulites are formed. Regarding the oriented skin layer thickness, with in sum 245 µm at 6 C the oriented layer thickness is slightly greater than for 11 C (21 µm), although, the area near the gate (here 11 C) normally shows the greatest oriented skin layer. This could be SPE ANTEC Anaheim 217 / 953

4 explained with the higher shear effects at lower temperatures. Here, the melt solidifies faster, which favors the building of an oriented skin layer. manufacturing. The faster cooling of the melt leads to a faster solidification, which favors the formation of orientations. Figure 7 shows the results for the tensile modulus and yield strain. The results show that the 11 C component regions achieve higher tensile modulus and lower yield strain characteristics. Comparing the mean values, an increase in tensile modulus in the range of 2 % and a decrease in yield strain of 55 % is measured. The differences in the resulting component properties are attributed to the different internal component properties. Therefore, it is assumed that the more distinct crystalline properties of the 11 C component area lead to an increase in stiffness as well as a decrease in the strain properties. Figure 5. Morphology of the produced component at a mold temperature zone of 6 C (left) and 11 C (right). Regarding the degree of crystallization, with 44 % the component area manufactured at 6 C reaches a similar value as the component region manufactured at higher mold temperature (43 %). Therefore, no clear differences can be measured. The similar values can be attributed to the integral measurement. Here, the melting enthalpy results from melting the complete sample (skin and core area). As a consequence, different degrees of crystallization in skin and core area can t be distinguished. It is assumed that in the core area, the 11 C sample has a higher degree of crystallization while for the oriented skin layer a higher degree of crystallization is assumed for 6 C. Nevertheless, these effects are balanced by the integral measurement and further measurements on thin sections need to be carried out. 3.2 Optical and mechanical properties Figure 6 shows the frontal component photography as well as the results of the photoelasticity test with polarized light. The component photograph shows similar behavior of the component regions produced at different temperature. Regarding the results of the transmission measurements, the transmission in the 6 C temperature zone is 43 %, whereas at 11 C 37 % is measured. The differences can be correlated to the different morphologies. Therefore, the more distinct spherulitic structure absorbs more of the visible light. Regarding orientation effects as well as residual stresses, the component area manufactured at lower temperature shows more distinct properties. Here, the differences can be explained by the more distinct shearing effects during Figure 6. Component photography (left) and photoelasticity test with polarized light (right). tensile modulus E t [N/mm 2 ] tensile modulus yield strain 6 11 mold temperature zone T m,zone [ C] Figure 7. Mechanical characteristics tensile modulus and yield strain of injection-molded plates produced at a mold temperature zone of 6 C and 11 C. 4. Conclusion In this paper, the influence of a locally different cooling behavior during the injection molding was investigated. Therefore, a novel, dynamically tempered mold temperature concept with segmented heating yield strain ε y [%] SPE ANTEC Anaheim 217 / 954

5 ceramics was developed to produce parts with locally different properties to allow for new areas of application. Therefore, microplates made of ipp were injectionmolded at locally different mold temperature zones, whereby the mold temperature zones were chosen with 11 C and 6 C according to the material s crystalline behavior. The components were analyzed in terms of their inner properties as well as the resulting optical and mechanical properties to show the influence of locally different temperature zones on resulting inner and, as a consequence, global component properties. To determine inner component properties, the morphology as well as the degree of crystallization were measured. For mechanical component properties, tensile tests in the respective component area were performed. First results show that for ipp, as a material with medium crystallization kinetics, a locally different temperature can influence the inner component properties. Therefore, regarding the higher mold temperature zone, the crystalline superstructures have developed more coarsely. Furthermore, the oriented skin layer increases with decreasing mold temperature. Regarding the degree of crystallization, no clear differences between the two temperature areas were measured. Here, although a higher degree of crystallization is assumed in the core region of the high temperature area, the more distinct oriented skin layer of the lower temperature area could increase the degree of crystallization due to shear induced crystallization. As a result of the different inner component properties, components with locally different mechanical properties could be produced. Here, the tensile bars produced at higher mold temperature show an increase in the measured tensile modulus in the range of 2 % in comparison to the lower temperature. In summary, the component area manufactured at higher mold temperature reaches higher stiffness and lower strain than the component areas produced at lower mold temperature what can be explained by the more distinct spherulitic structure. In further investigations, in addition to ipp, other semi-crystalline thermoplastics should be investigated with regard to their crystallization behavior. Moreover, the acquired knowledge should be transferred to different test body geometries. In particular, a study of thin-walled tip geometries in the field of medical technology, which is brittle in the injection area and comparatively ductile in the shaft area, can enable new product solutions by means of the segmented process strategy. Acknowledgments The authors would like to thank the Deutsche Forschungsgemeinschaft for the financial support of the work carried out as part of the DFG DR 421 / 25-1 project, Ypsomed GmbH for the general mold construction and manufacturing and gwk Gesellschaft Wärme Kältetechnik mbh for the dynamic tempering system. References 1. M. T. Martyn, B. Whiteside, P. D. Coates, P. S. Allan, and P. Hornsby, SPE-ANTEC Tech. Papers, 48, (22). 2. A. K. Angelov and J. P. Coulter, SPE-ANTEC Tech. Papers, 5, 748 (24). 3. A. Jungmeier, Struktur und Eigenschaften spritzgegossener, thermoplastischer Mikroformteile, Ph.D. thesis, Friedrich-Alexander-University Erlangen-Nuernberg, (21). 4. R. Künkel, Auswahl und Optimierung von Kunststoffen für tribologisch beanspruchte Systeme, Ph.D. thesis, Friedrich-Alexander-University Erlangen-Nuernberg, (25). 5. G. W. Ehrenstein, Polymer-Werkstoffe: Struktur - Eigenschaften Anwendungen, Carl Hanser, Munich (211). 6. B. G. Millar, P. Douglas, W. R. Murphy, and G. M. Mc Nally, SPE-ANTEC Tech. Papers, 51, 2258 (25). 7. F. R. Schwarzl, Polymermechanik: Struktur und mechanisches Verhalten von Polymeren, Springer, Berlin Heidelberg (199). 8. I. Kolesov, D. Mileva, and R. Androsch, Polymer Bulletin, 71, pp (214). 9. S. Brückner, S. V. Meille, V. Petraccone, and B. Pirozzi, Progress in Polymer Science, 16, pp (1991). 1. R. Androsch, M. L. Di Lorenzo, C. Schick, and B. Wunderlich, Polymer, 51, pp (21). 11. R. H. Somani, B. S. Hsiao, A. Nogales, H. Fruitwala, S. Srinivas, and A. H. Tsou, Macromolecules, 34, pp (21). 12. H. Huo, S. Jiang, L. An, and J. Feng, Macromolecules, 37, pp (24). 13. J. A. Sauer and K. D. Pae, Journal of Applied Physics, 39, pp (1968). 14. C. Silvestre, S. Cimmino, D. Duraccio, and C. Schick, Macromolecular Rapid Communications, 28, pp (27). 15. C. Fischer and D. Drummer, Advances in Mechanical Engineering, 214, 1 pages (214). 16. D. Schmiederer and E. Schmachtenberg, Journal of Plastics Technology, 2, pp (26). 17. S. Meister and D. Drummer, International Polymer Processing, 28, pp (213). 18. G. W. Ehrenstein, G. Riedel, and P. Trawiel, Thermal Analysis of Plastics, Carl Hanser, Munich (23). SPE ANTEC Anaheim 217 / 955