Investigation of Interactions of Polymer Material, Mould Design and Process Condition in the Micro Moulding Process

Transcription

1 STR/3/6/FT Investigation of Interactions of Polymer Material, Mould Design and Process Condition in the Micro Moulding Process J. Zhao, G. Chen, P. S. Chan and M. Debowski Abstract Micro moulding process studies have been carried out using a Battenfeld Microsystem moulding machine with a maximum melt injection capacity of 1 cm 3. Micro moulds with different designs were used to study melt shear effect during the process. Both amorphous and crystalline polymers, including PMMA, PC, POM, PBT, PP and a PC/LCP blend were evaluated in the study. It has been found that, while the metering size is the dominating factor that affects the process, the mould temperature, melt temperature and injection speed can also affect the mould filling behaviour. Shrinkage and the speed of melt solidification are the two important material factors that need to be considered in the process condition setting. Since micro runner and cavity system solidifies very quickly in the micro moulding process, high melt shrinkage and quick melt solidification may result in short shot or high micro part distortion. Keywords: Micro moulding, Mould design, Micro parts, Viscosity, Polymer blend 1 BACKGROUND The rapid development of microsystem technology in recent years has placed increasing demands for production of miniaturized components for various micro mechanical electronic system (MEMS) device applications. Due to the advantages of polymer materials such as easy mass processing and tailorable properties, the micro moulding process is increasingly becoming a key enabling technology for microsystems. In order to produce micro parts that weigh no more than a few milligrams in a reliable and economical way, micro moulding machines that use a small diameter plunger injection system for melt injection and control have been developed and used in the moulding industry. Analysts predict that microsystem technology will have a far-reaching influence on device manufacture and will become one of the main technologies of the 21st century [1-2]. However, past research activities in microsystem technology were mainly focused on silicon material and micro devices were typically fabricated with batchprocessing techniques similar to those used for integrated circuits. In recent years polymer materials as a class of important materials for microsystem technology have been widely recognized since there are vast varieties of polymer materials that can be tailored to provide the desired application properties [3]. Polymer micro processing technologies, especially the micro moulding process, have become the desired processes for mass production of disposable polymer micro components at a low cost [4-5]. In order to meet the needs of component miniaturization, a lot of efforts have been devoted in the polymer industry to the development of micro process technologies and micro processing machinery. Several types of micro moulding machines have developed by major polymer machinery manufacturing companies. To control metering accuracy and homogeneity of the very small quantities of melt in the micro moulding process, a new generation of machines that use a separate screw plastication unit and a plunger injection system have been developed. Injection plungers as small as 5 mm in diameter have been used in micro moulding machines to produce polymer melt down to sub-gram levels. Micro moulding is a new branch of injection moulding process with a different set of challenges compared with the conventional injection moulding process [6]. The interactions of moulding machine, mould design, material properties and process conditions need to be specially addressed in order to produce micro components of high qualities. In the present paper micro moulding process studies have been carried out in a plunger type injection moulding machine to examine the interactions among polymer material, mould design and process condition in the moulding process. 2 OBJECTIVE The objective of the study is to understand the interactions among polymer material, mould design and process condition in the micro moulding process so that guidelines can be established for polymer material selection, mould design and process condition setup. 3 METHODOLOGY 3.1 Micro moulding machine The micro moulding machine used in the pre- 1

2 sent work was an electrical driven Battenfeld Microsystem moulding machine [7]. The injection system of the machine is composed of a screw plastication barrel, a plunger injection system and a melt dosage control barrel as shown in Fig. 1. The screw diameter of the plastication unit is 14 mm and a 5 mm diameter injection plunger is employed for melt injection. The maximum injection capacity of the machine is 1 mm 3. the chip, as shown in Fig. 3. These types of chips are widely used in bio-analysis and medical diagnostic applications by utilizing the micro channels for separating various components of liquid samples. The chip used in this work had an overall nominal dimension of 2 x 2 x 2 (W x L x T) mm and the length of the channels was 15 mm. Compared with the other two components the volume of the chip, about 8 mm 3, is very large, which is good from comparison point of view. Fig. 1. The Battenfeld injection system and a threeplate micro mould. Fig. 2. Moulded gears of 1 mm diameter. 3.2 Micro components Moulding experiments have been carried out on a micro gear, a micro lens array, and a polymer biochip component to study interactions of polymer material, mould design and process condition in the micro moulding process. The micro gear studied was a micro spur gear with 8 gear teeth and a tip diameter of 1 mm. Shown in Fig. 2 is a photograph of the moulded gears. The dimension of the gear teeth is in the range of a few tens of microns, which makes it a good component to study filling behaviour of polymer materials. The total volume of the gear is about.45 mm 3, which is only a very small fraction of the maximum machine capability of 1 mm 3. The micro lens array studied is shown in Fig. 3. The lens array has nineteen micro lens surfaces designed on the top and bottom sides of the lens array. The overall dimension of the part is 12 x 3 x 2 mm, which gives a total volume of around 72 mm 3. Compared with the micro gear, this component is relatively large, but its total volume is still less than 1% of the capacity of the moulding machine. The third part studies was a so called biochip component which was a flat component with micro channels in dimensions from 1 micrometers to 4 micrometers on the surface of 12 mm Lens array 2 mm Moulded biochip Fig. 3. Moulded micro lens array and biochip. 3.3 Micro mould design 2 mm A three-plate two-cavity micro mould was used for the micro gear. Due to the small volume of the component, a relatively large runner system, about 142 mm 3, is selected for the mould to increase the total melt amount involved in each moulding shot to avoid the difficulties involved in handling very tine amount of melt [8-9]. The micro lens array mould was a single cavity, threeplate mould to facilitate automatic de-gating. A two-plate mould was fabricated for the biochip component as shown in Fig. 4. A side gate system was selected for the component so that polymer melt from the injection barrel can flow 2

3 through the runner system designed on the parting plane into the mould cavity. shrinkage factors behaviour during the process. These polymers included: the Mitsubishi S-3 PC, a Mitsubishi Rayon s Shinkolite-P VH PMMA resin, a DuPont s Crastin polybutylene terephthalate (PBT) resin, a Mitsubishi F2-3 polyoxymethylene (POM) resin, and a Tokuyama MJ17 polypropylene (PP) resin. 4 RESULTS & DISCUSSION 4.1 Effects of viscosity on mould filling Fig. 4. The two-plate mould used for the biochip moulding. 3.4 Polymer materials A Mitsubishi S-3 polycarbonate (PC) resin was used in the lens array moulding studies. A GE Plastics Lexan PC and its blend modified with a liquid crystalline polymer (LCP) were used in the biochip and the micro gear moulding to study viscosity effects on the moulding process. Compared with the original PC, the blend showed a 6% viscosity reduction [9]. The LCP used in this work was synthesized from 4- hydroxybenzoic acid (HBA), hydroquinone (HQ), sebacic acid (SEA) and suberic acid (SUA) [9, 1]. A weight percentage of 2% LCP was applied for the PC-LCP blend used in this work. The viscosity of the PC material and the PC- LCP blend are presented in Fig. 5 as a function of shear rate and process temperature. Apparent Viscosity (Pa s) 1 1 PC at 26 C PC at 28 C 1 PC/LCP at 26 C PC/LCP at 28 C Apparent Shear Rate (1/s) In order to study the effects of rheological properties of polymer materials on micro moulding process the mould filling behaviour of the GE Plastics Lexan PC and the PC-LCP blend were studied using the micro gear and the biochip moulds. In Fig. 6 the gear diameter is presented as a function of metering size for both the gear moulding processes of the PC resin and the PC- LCP blend. It can be observed from the figure that the diameter of both the PC and the PC- LCP gears increased with metering size until a steady state was reached, which indicates full filling of the mould cavities. However, for the PC- LCP gears the metering size for the steady state was much lower than that for the PC gears. At a metering size of 23 mm 3, the PC-LCP gears were fully filled, while a metering size of 24 mm 3 is required for the PC resin to fill the cavities properly. This clearly indicates that micro cavities are much easier to be filled using polymers or blends with lower viscosity. Gear Diameter (mm) Blend PC Metering Size (mm 3 ) Fig. 6. Gear diameter as a function of metering size for the Lexan PC and the PC-LCP blend. Fig. 5. Viscosity of the Lexan PC and its PC-LCP blends as a function as shear rate and temperature. Several other polymer materials were used in the micro moulding process of the biochip to study how different materials with different In Fig. 7 the part weight of the moulded biochips are presented as a function of mould temperature used in the moulding process of the Lexan PC resin and the PC-LCP blend. It was noted from the figure that while the blend filled the mould cavity well at a low mould temperature of 8 C, the PC resin had difficulties to fill the 3

4 mould at this mould temperature. This shows the advantages of the low viscosity blend over the PC resin. In the micro moulding process since the volume of the moulded parts is very small, the amount of heat that transfers from the melt to the mould steel is small and therefore the mould generally needs to be heated up for better melt flow and cavity filling. However, higher mould temperature will inevitably result in longer cycle time and higher energy consumption since the melt in the mould runner and cavities needs to be cooled down to the desired ejection temperature before the parts can be ejected out. For polymer resins with better flowability, the mould temperature setting can be lower and therefore shorter cycle time and lower energy consumption can be expected. It can also be observed from Fig. 7 that at the same process conditions the part weight of the components moulded from the blend is higher than those of the PC components because of the lower viscosity of the blend. Part Weight (g) Blend PC Mold Temperature ( o C) Fig. 7. Mould temperature effects on the moulding process of the biochip using the Lexan PC and the PC-LCP blend. In Fig. 8 the weight of the runners produced in the chip moulding process is presented as a function of the metering settings. It can be seen that while the runner weight of PC remained fairly constant over the metering size range, the runner weight of the blend increased with the increase of the metering size setting. This indicates that at the low metering size setting of 82 mm 3, there was already enough polymer melt to fill the runner and cavity spaces. For the blend, due to its good flowability the melt could fill the cavity at this low metering setting. With the increasing of the metering size, the runner weight increased because very little extra melt was forced into the mould cavity. However, for the PC resin, due to the high force required to push the melt into the cavity, the melt was not fully injected into the cavity at low metering settings, instead, the plunger movement was stopped and resulted in large runners. As the metering size increased, the injection pressure built up would also increase, resulting in more melt being pushed into the mould cavity and the runner weight remaining unchanged. This observation indicates that for materials with good flowability, a smaller melt metering size can be used to fully fill a mould cavity and a smaller runner is generated compared with materials with high melt viscosity. Runner Weight (mg) Metering Size (mm 3 ) Fig. 8. Variation of runner weight during biochip moulding of the Lexan PC and the PC-LCP blend. 4.2 Material shrinkage effects In moulding processes, application of packing pressure is essential to overcome material shrinkage problems and obtain good part without severe sink mark and warpage. In the micro moulding process, holding pressure is applied through a small further forward movement of the plunger [7]. For the Battenfeld moulding machine, the packing capability of the system is limited since the maximum post-injection forward movement of the plunger is limited up to 1 mm. This will only pack a volume of 2 mm 3 melt into runner system and mould cavity during the pressure holding stage [11]. There are therefore limitations for the process to overcome melt shrinkage problems in conditions where the part volume is large, or the material shrinkage is high. For the biochip component studied in this work, 2 mm 3 is only 2.5% of the total volume of the component. If the volume shrinkage of the polymer processed is larger than 2.5%, then severe sink mark can be expected. Fig. 9 shows moulding shrinkage data for the materials used in the biochip moulding and the sink mark depth measured from the surface of the moulded biochips for each of the materials. A Wyko profile meter was used in the measurement of the sink mark depth. It can be seen from the figure that the depth of the sink mark is closely related to the moulding shrinkage value. When the shrinkage value is low the process is capable to control the sink mark to certain degree. However, for materials with high moulding shrinkage values such as PBT and POM, the PC Blend 4

5 sink mark is high because the part volume is high and the machine is not capable to supply enough melt to compensate the shrinkage. Molding Shrinkage (%) Shrinkage Sink Mark PMMA PC PP POM PBT Polymer Material Fig. 9. Moulding shrinkage factors and sink mark depth measured on the surface of moulded biochip components. 4.3 Parameter effects on process In the micro moulding process, due to the small amount of polymer melt involved in the process and the small dimensions of runner system and mould cavities, polymer melt cools much faster compared with the conventional moulding process. The actual cooling rate is affected by many factors such as material property, melt temperature, mould temperature, and injection speed. Fig. 1 shows the effects of injection speed on part weight and injection pressure during the moulding process of the lens array using the Mitsubishi PC. It can be seen that the part weight did not change very much in most of the injection speed regions used. However, at very low injection speed, the runner solidified before the mould cavity was fully filled, resulting in improperly filled components. It can also be observed from the figure that with the increase of the injection speed, the injection pressure decreased quickly at very low injection speed region. Once the injection speed reached a certain level there was only slight decreases in injection pressure over a wide injection speed range. In Fig. 11 the effects of the mould temperature on injection pressure is presented as a function of injection speeds for the lens array moulding. It is seen that at high injection speed mould temperature did not make much differences in injection pressure because the melt is quickly injected into the mould cavity before the mould temperature could play any significant roles. While in low injection speed region, if the mould temperature is low, polymer melt could be cooled down very quickly, and in severe cases solidified, before the mould cavity is fully filled Sink Mark Depth (µm) This will result in very high injection pressure and high stresses in moulded parts. To prevent polymer melt to cool down too quickly, micro moulds are usually heated up to prevent melt temperature from dropping too quickly during mould filling process. Part Weight (mg) Injection Speed (mm/s) Fig. 1. Variation of lens array part weight and injection pressure as a function of injection speed. Injection Pressure (bar) Fig. 11. Injection pressure as a function of injection speed at different mould temperatures. 5 CONCLUSION Part Weight Injection Pressure Mold Temperature 13C C Mold Temperature 14C C Mold Temperature 15C C Injection Speed (mm/s) The micro mould filling behaviour is very much affected by the rheological properties of the polymer materials used. Low viscosity polymers are desired in the moulding process for better mould filling. Among different grades of the same material the best results are achieved with low-viscosity grades. Sink mark and shrinkage are main quality concerns for moulded components with relatively large volume and using high shrinkage materials because the melt packing capability in plunger injection moulding machines is limited. Melt cooling takes place faster in the micro moulding process compared with the conventional moulding process owning to small amount of melt involved. To produce components with desired properties it is also necessary to select a good combination of process Injection Pressure (bar) 5

6 parameters especially the metering size, the injection speed and the mould temperature. 6 INDUSTRIAL SIGNIFICANCE With the global trend in plastics injection moulding industry slanting towards precision moulding and miniaturization, there are growing demands in the manufacturing of micro-precision components in plastics. Micro moulding technologies can therefore open completely new possibilities for many applications in different disciplines. The micro moulding activity encompasses the full spectrum of polymer moulding technologies from polymer material selection, product design to tool design and fabrication and the use of innovative injection techniques. Understanding of the interactions among polymer materials, micro mould design and process conditions is essential for wide industrial applications of the micro moulding process. The findings of this study can be used as guidelines for material selection, micro mould design and process setup in plastic micro component development. REFERENCES [1] N. Sparrow, The Microtechnology Revolution, European Medical Device Manufacturer, (1999). [2] K.J. Gabriel, Engineering Microscopic Machines, Scientific American, September, pp. 118, (1995). [3] C. Schneider and G. Maier, Small, but Potent Special Plastics for Injection Moulding Microparts, Kunststoffe, Vol. 91(3), pp , (21). [4] C. Kerkland, The Micromoulding Superexpress, Injection Molding, pp , (1999). [5] L. Weber, W. Ehrfeld, H. Freimuth, M. Lacher, H. Lehr and B. Pech, Micro Moulding A Powerful Tool for the Large Scale Production of Precise Microstructures, SPIE Proceedings, Austin, Texas, USA, October 1996, Vol [6] S. Hill, Micromoulding A Small Injection of Technology, Materials World, Vol. 9(6), pp , (21). [7] M. Ganz, Microsystem the innovative solution for microprecision parts, in Polymer Process Engineering 99, P.D. Coates, IOM Communications Ltd, London, pp. 8-17, (1999). [8] J. Zhao, R. Mayes, G. Chen, P.S. Chan and Z.J. Xiong, Polymer Micromould Design and Micromoulding Process, Plastics, Rubber and Composites, Vol. 32(6), pp. 1-8, (23). [9] W. Zhao, C.C. Yang and X. Lu, Effect of Interfacial Interaction on Rheological Behavior of Blends of a Semiflexible Liquidcrystalline Polyester and Polycarbonate, Journal of Applied Polymer Science, Vol. 9(11), pp , (23). [1] L. Liu and X. Lu, Polyesters Based on Hydroxybenzoic Acid, Hydroquinone, Sebacic Acid, and Suberic Acid Molecular Design towards Broad Nematic Range, Plastics, Rubber, and Composites, Vol. 31(7), pp , (22). [11] J. Zhao, R. Mayes, G. Chen, H. Xie and P.S. Chan, Effects of Process Parameters on the Micro Moulding Process, Polymer Engineering & Science, Vol. 43(9), pp , (23). 6