外文.pdf

TOOLING Injection molding of products with functional surfaces by micro-structured, PVD coated injection molds Kirsten BobzinNazlim BagcivanArnold Gillner Claudia HartmannJens HoltkampWalter Michaeli Fritz KlaiberMaximilian Scho ngartSebastian Thei Received: 11 February 2011/Accepted: 29 April 2011/Published online: 15 May 2011 ? German Academic Society for Production Engineering (WGP) 2011 AbstractMoldingofmicrostructuresbyinjection molding leads to special requirements for the molds e.g. regarding wear resistance and low release forces of the molded components. At the same time it is not allowed to affect the replication precision. Physical vapor deposition (PVD) is one of the promising technologies for applying coatings with adapted properties like high hardness, low roughness, low Youngs modulus and less adhesion to the melt of polymers. Physical vapor deposition technology allows the deposition of thin fi lms on micro structures. Therefore, the infl uence of these PVD layers on the contour accuracy of the replicated micro structures has to be investigated. For this purpose injection mold inserts were laser structured with micro structures of different sizes and afterwards coated with two different coatings, which were deposited by a magnetron sputter ion plating PVD tech- nology. After deposition, the coatings were analyzed by techniques regarding hardness, Youngs modulus and morphology. The geometries of the micro structures were analyzed by scanning electron microscopy before and after coating. Afterwards, the coated mold inserts were used for injectionmoldingexperiments.Duringtheinjection molding process, a conventional and a variothermal temperature control of the molds were used. The molded parts were analyzed regarding roughness, structure height and structure width by means of laser microscopy. KeywordsMicro structures ? PVD ? Laser structuring ? Injection molding ? Variothermal ? CrAlN 1 Introduction Competition between manufacturing companies in high- wage and low-wage countries typically occurs within two dimensions: the production-oriented economy and the planning-oriented economy. Low-wage countries produc- tionsfocusoneconomies-of-scale,whilehigh-wage countries try to combine scale and scope. Within the sec- ond dimensionthe planning-oriented economycompa- nies in high-wage countries optimize processes with sophisticated, investment-intensive planning systems and production systems, while companies in low-wage coun- triesimplementsimple,robustvalue-stream-oriented process chains. These two dichotomies generate the poly- lemma of production. The polylemma of production can only be solved by an integrated research approach. One approach to solve the dichotomy between scale and scope is the hybrid produc- tion. This includes both hybrid production processes combining different physical mechanisms or production steps into one machine and hybrid products which combine e.g. a macro plastics product with a functional surface due to micro structures on the surface created in a one-step process by using microstructured molds. To generate these functional products in large numbers, the plastics industry requires structured, highly durable molds with low adhe- sion of the polymer melt and easy ejection for small sized K. Bobzin ? N. Bagcivan ? S. Thei ( Rapid, LumeraLaser) with an amplifi - cation stage at a wavelength of k = 355 nm. The laser operates at a repetition rate up to m = 500 kHz and at a pulse duration of s = 12 ps. For the micro structuring experiments the laser radiation was positioned on the sur- face of the samples by a galvanometer scanner system with a focus length f = 32 mm. The ablated geometries were lines. Different line geometries were ablated. To generate the lines the laser beam was either moved along the surface several times on the same line (for small groove widths) or the laser was moved along parallel lines several times to generate broader grooves. All experiments were performed at a repetition rate of m = 500 kHz. The ablated grooves were designed to have an aspect ratio of C1, which means that the width is equal to or smaller than the depth. The ablation parameters are shown in Table 1. The structures were analyzed regarding their width by using scanning electron microscope (SEM) micrographs (ZEISS DSM 982 Gemini) using SE (secondary emission electrons) mode and, additionally, analyzed regarding the width and the height of the structures using a color 3D laser scanning microscope (VK9710, Keyence). 2.2 Coating deposition The used mold inserts (X43Cr13, 1.2083, AISI 420, cold working steel) were all polished with a 6 lm diamond suspension to a roughness of 0.02 lm Ra. For deposition the pulsed magnetron sputtering ion plating (MSIP) PVD technology was used. The coating unit (CC800/9 SinOx, CemeCon AG) is equipped with an asymmetrical bipolar dual cathode arrangement. The unit is equipped with four rectangular cathodes (500 9 88 mm2). For deposition of high-Al-(Cr,Al)N two Al targets with twenty 15 mm Cr inserts (AlCr20) and two Cr targets with twenty 15 mm Al inserts (CrAl20) were used. For deposition of low-Al- (Cr,Al)N four CrAl20 targets were used to decrease the amount of aluminum in the coating. The deposition parameters are shown in Table 2. 2.3 Mechanical and chemical properties of the coatings In order to evaluate the morphology and the thickness, SEM micrographs of fractured cross sections of the coat- ings were taken also using SE mode. For this micrographs substrates made of cemented carbide (SNUN433, Kenna- metal Widia GmbH & Co.KG) were used due to their brittle fracture behavior. Within this SEM an energy dis- persive X-ray spectroscopy (EDS) was used to determine the chemical composition of the coatings surfaces. The hardness and Youngs modulus were, moreover, deter- mined using the method of nano indentation. A Nanoind- enter XP (MTS Nano Instruments) was applied for this purpose. The indentation depth did not exceed 1/10 of the coating thickness. The evaluation of the measured results was based on the equations according to Oliver and Pharr 13. A constant Poissons ratio of m = 0.25 was assumed. 2.4 Injection molding The structured and coated mold inserts were subsequently mounted into a high-precision injection mold, which was designed and built particularly for the replication of micro- structured plastics parts. All molding experiments were performed using a hydraulic CX 160-1000 injection molding machine (KraussMaffei Technologies GmbH) with a 40 mm diameter plasticizing unit and a heated extended nozzle. The injection molding process starts with the closing of the mold, followed by the injection of the molten polymer into the cavity. Once the cavity is fi lled, a holding pressure is applied to compensate for the shrinkage of the material until the sealing point is reached. After a cooling time the mold opens and the part is ejected. Table 1 Ablation parameters for the generation of the grooves Structure size (lm) Power (mW)Repetition rate (kHz) Scan speed (mm s-1) Number of parallel lines Distance between laser lines (lm) Repetition of laser lines 106005008004240 406005005002 9 33123 Table 2 Deposition parameters of deposited thin fi lms ParametersLow-Al-(Cr,Al)N/high-Al-(Cr,Al)N Deposition time2,300 s Temperature160?C Cathode power4 9 2,000 W Bias-voltage35 V Ar-fl ow300 sccm N2 -fl ow42.5/37.0 sccm Pressure650 mPa Prod. Eng. Res. Devel. (2011) 5:415422417 123 Ithadbeenshownthatelevatedmoldtemperaturesduring the injection are necessary in order to ensure a precise moldingofsurfacestructuresinthelm-range14.Whenthe hot melt contacts the relatively cold mold surface, a rapid decrease of temperature causes an increase of the viscosity. Thus, the melt pressure is not able to push the material into the fi ne surface structures. With a mold temperature close to thetemperatureofthepolymermelt,prematuresolidifi cation canbeprevented,leadingtoaprecisefi llingofthestructures. Atthesametime,atemperaturelowenoughforademolding without damaging the part is required. For this purpose, variothermal process control can be used to combine high temperatures during the injection and temperatures low enough for a demolding at reasonable cycle times. To achieve a well defi ned and fast changing temperature profi le, an inductive mold heating system in combination with a conventional mold cooling was used 15. This system for variothermal process control was composed of an inductor heating unit and a robot. A W721 robot (Wittmann Robot Systeme GmbH) automatically placed the inductor in front of the micro-structured cavity while the mold was open. Using a pyrometer-based temperature control, a defi ned heat-up of the cavity wall (up to 60 K s-1) was achieved without destroying the fragile micro-structures by uncontrolled high temperatures. After the mold was heated up, the inductor was moved out, the mold closed and the injection process started. A standard Polypropylene (Sabic?PP513MNK40) was used for the injection molding experiments. This material exhibits an excellent fl owability, which is important for the precise replication of microstructured surfaces. Each mold insert was tested both in the conventional and the vario- thermal process, with the injection molding parameters held constant. The maximum injection speed was 40 cm3s-1 and the holding pressure was set to 900 bar. A melt tem- perature of 200?C was chosen, with a mold base tempera- ture of 30?C. In the variothermal injection molding process, the cavity was heated up to a temperature of 160?C by the inductive mold heating system before the injection of the polymer. After molding the parts were analyzed regarding the width and the height of the structures using a color 3D laser scanning microscope (VK9710, Keyence). 3 Results and discussion 3.1 Analyses of mechanical and chemical properties of the coatings After deposition the specimens were subjected to basic characterization in accordance with the above mentioned methods. The results of the characterization are shown in Table 3. By EDS the chemical composition of the coatings was measured to 80 at-% chromium and 20 at-% aluminum for the low-Al(Cr,Al)N coating and 57 at-% chromium and 43 at-% aluminum for the high-Al-(Cr,Al)N coating. As expected the hardness increases by increasing the alumi- num content. Furthermore, the morphology changes from a columnar to a fi ne grained structure (Fig. 2). It is possible that the higher hardness and the smoother structure of the (Cr0.57Al0.43)N coating lead to better results during replication in the injection molding process. 3.2 Analyses of structured and coated mold inserts At fi rst the 40 lm structure was analyzed. Figure 3 shows the SEM micrographs of the surfaces of the coated and uncoated molds. Clearly, a rough wall and ground can be seen in the structured parts. There are three different types of structures on the ground, which create the rough surface. A linear nano structure in the dimension of several hundred nm, a linear micro structure, and a hole structure (Fig. 4). The nano structure can be related to a laser induced peri- odic surface structure (LIPSS). The linear micro structure and the hole structure can be related to the recently described periodic substructure and the cone-like protru- sions (CLP). These occur on the surface and spread on the surface for increasing number of ablated layers 1619. This roughness leads to a slightly varying width of the structures with mean values of 43.2 lm for the uncoated, 42.0 lm for the (Cr0.80Al0.20)N coated and 40.6 lm for the (Cr0.57Al0.43)N coated mold. By coating the structures, the LIPSS and the periodic substructure can be leveled out completely. The holes of the CLP can only be moderated a little bit. This leads to the conclusion that additionally to the function of the coating small faults on the mold surface can be corrected by the coating. The roughness at the edges, the walls and the holes in the ground may lead to an increase of the demolding force, which then causes a stretching of the polymer during the ejection process in the injection molding experiments. However, a contraction of the polymer during the cooling process may lead to an easier ejection. Furthermore,the10 lmstructureswereanalyzed (Fig. 5). Here, actually the same structuring effects can be Table 3 Results of the basic characterization of the coating systems, deposited on 1.2083 (Cr0.80Al0.20)N(Cr0.57Al0.43)N Coating thickness (lm)1.61.3 Hardness (GPa)17.0 1.319.8 2.5 Youngs modulus (GPa)343.6 22.7342.3 36.6 Mean roughness Ra (lm)0.080.07 418Prod. Eng. Res. Devel. (2011) 5:415422 123 seen. The reached mean width of the structures is 12.3 lm for the uncoated, 13.4 lm for the (Cr0.80Al0.20)N coated and 11.7 lm for the (Cr0.57Al0.43)N coated mold. Due to the fact that the structures will be fi lled by a smaller amount of the molten polymer, the contraction of the polymer may not lead to an easier ejection. Additionally, 3D laser scanning micrographs of the structures on the uncoated mold insert were taken to obtain the depthofthestructures (Fig. 6).Themeasurements ofthe widthandthedepthofthestructuresresultin39.7 1.5 lm and 37.4 4.1 lm, respectively, for the larger structures. For the 10 lm structures width and depth of 11.6 1.1 lm and22.5 1.0 lmweremeasured,respectively.Theresults showthatthelaserablationhasreachedanaspectratioof&1 for the 40 lm structures. An aspect ratio of &2 was reached for the 10 lm structures. Due to the roughness of the laser structures the standard deviation of the depth of the struc- tures is very high. This method was used in the following investigations to determine also the width and height of the different structures in the molded parts. 3.3 Injection molding In the molding experiments without external inductive mold heating, each mold insert was used successfully to mold more than 200 parts without any demolding prob- lems. No sticking of the part to the mold was observed, so (Cr0.80Al0.20rC(N) 0.57Al0.43)N Cemented carbide Cemented carbide Fig. 2 SEM cross section fracture micrographs of (Cr0.80Al0.20)N (left) and (Cr0.57Al0.43)N (right) uncoatedrC( 0.57Al0.43)N (Cr0.80Al0.20)N 43.2 m 42.0 m 40.6 m Fig. 3 SEM surface micrographs of uncoated (left), (Cr0.80Al0.20)N coated (middle) and (Cr0.57Al0.43)N coated (right) structures with a width of approximately 40 lm LIPSS Holes of CLP Periodic substructure Fig. 4 SEM surface micrographs of uncoated structures with a width of approximately 40 lm. A high roughness can be seen because of structuring effects Prod. Eng. Res. Devel. (2011) 5:415422419 123 that the injection molding machine could be operated in fully automatic mode. This is essential for a high repro- ducibility from cycle to cycle, making sure that every part is molded under the same thermal conditions. Different results were expected from the molding trials with variothermal process control. As described above, due to the laser ablation process there are certain substructures on the microstructures in the range of less than 1 lm. These structures form many undercuts, which are fi lled with polymer when the mold is heated up close to the melting temperature of the polymer before the injection. In the subsequent demolding process, these undercuts lead to an increase of the demolding force which acts against the demolding of the part. Nevertheless, ductile polymer materials such as Polypropylene with a high breaking elongation can be demolded. The increase in demolding force leads to a stretching of the structure and a lateral shrinkage at the same time. Thus, the part can be demolded completely. In these experiments with Polypropylene, no clogging of the mold insert could be seen. However, the unique surface structure created in this process may not exactly refl ect the surface structure of the mold insert. The molded parts of both processes, conventional and variothermal, were analyzed regarding the width at the base and the height of the structures. Figure 7 shows the 40 lm structure molded in PP with the uncoated mold. It is obvious that the structures are not fi lled completely in the injection molding process by using a conventional tem- perature control. Otherwise, the structures formed with a variothermal temperature control are much more accurate. Figure 8 shows the results of the measurements carried out at the 40 lm structures of the three different types of molds. Additionally, the target value of 40 lm (aspect ratio & 1) is presented. Especially the (Cr0.80Al0.20)N coating leads to good results concerning the targeted struc- ture sizes. Furthermore, the use of the variothermal process shows a magnifi cent conformance with the targeted struc- turesizes.Withnoneoftheconventionaldrivenexperiments a respectable height of the structures can be reached. Moreover, variothermal process control leads to an increase of the mean height of the structures for each mold insert. The values of the mean roughness of the planar areas between the 40 lm structures are presented in Fig. 9. Due tothehigherroughnessoftheseareasonthe (Cr0.80Al0.20)N coated mold compared to (Cr0.57Al0.43)N the results measured on the molded plastics parts do not meet the expectations. However, the parts created with the (Cr0.80Al0.20 )N coated mold exhibit signifi cantly lower roughness values, both for the conventional and the variothermal injection molding process. In summary these results show an advantage of the columnar (Cr0.80Al0.20)N coating. In a next step the 10 lm uncoatedrC( 0.57Al0.43)N (Cr0.80Al0.20)N 12.3 m 13.4 m 11.7 m Fig. 5 SEM surface micrographs of uncoated (left), (Cr0.80Al0.20)N coated (middle) and (Cr0.57Al0.43)N coated (right) structures with a width of approximately 10 lm 100 m 20 m 40 m structures 10 m structures Fig. 6 3D laser scanning micrograph of the 40 lm structures (left) and the 10 lm structures (right) on t