Injection molding and related techniques for fabrication of microstructures

 Injection molding technology and its different modifications represent established processes for manufacturing polymer products with high accuracy in large scale production. Enhanced machine and tool technologies like evacuation units and special temperization systems have already been adapted to the molding of microstructures with high aspect ratios. Cycle times are actually in the range of minutes and depend on the geometry of the microstructures and the materials used. Based on injection molding of lost plastic microforms new processes for the manufacturing of ceramic or metal microstructures are being developed. Received: 14 November 1996/Accepted: 25 November 1996     

Injection molding for microstructures controlling mold-core extrusion and cavity heat-flux

 In this work we constructed an injection press molding system with a mold-core extrusion mechanism and a small sensor assembly for effectively duplicating microstructures to the mold products. The mold-core extrusion mechanism is driven by a piezo element to apply force on important area with microstructures. For example, after injection it increases the cavity pressure from 20 to 34 MPa. Small sensors consist of the pressure, displacement, and heat flux sensor assemblies, arranged around the small cavity. The signals showed us the physical phenomena inside the mold and may be further used as control signal. In order to evaluate this injection press molding system, we formed micro triangular grooves of pitch 1 μm and angle 140°. The mold-core extrusion gave better diffraction intensity by several percents. Received: 16 May 2001/Accepted: 24 July 2001 The authors would like to thank FANUC Ltd. for cutting the triangular grooves on the mold-core. This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.     

Injection molding of polymeric LIGA HARMs

The primary goal of an ongoing research effort at LSU is to develop the three-step LIGA process to inexpensively manufacture high aspect ratio microstructures (HARMs). The first two steps of the process (lithography and electroplating) produce a metallic mold insert that can be used as a template for molding microstructures. The final step of LIGA is molding. This paper focuses on injection molding of thermoplastics to produce surfaces covered with HARMs. The resulting microstructures are hundreds of micrometers in height, tens of micrometers in width, and separated by gaps on the order of tens of micrometers. Injection molding experiments using high density polyethylene were performed using a commercially available injection molding machine. Experimental variables included injection speed, the tool temperature, and air pressure in the mold cavity. Elevating the tool temperature above the melting point ensured that the polymer completely filled the mold, producing microstructures with the desired geometry. As the temperature of the mold was reduced, higher injection speeds did not necessarily ensure filling of the mold cavity. The cycle time is shorter than the values previously reported in the literature [Madou (1996)]. Received: 30 March 1999 / Accepted: 12 April 1999     

Hot embossing - The molding technique for plastic microstructures

 Hot embossing is the technique to fabricate high precision and high quality plastic microstructures. Industrial fabrication of plastics components is normally achieved by injection molding. Hot embossing is actually used only for a few optical applications where high precision and high quality are important. The advantages of hot embossing are low material flow, avoiding internal stress which induces e.g. scattering centers infavorable for optical applications, and low flow rates, so more delicate structures can be fabricated, such as free standing thin columns or narrow oblong walls. The development of modular molding equipment, orientated on industrial standards has opened the door to the fabrication of plastic microcomponents in great numbers (for example LIGA-UV/VIS-spectrometers). Hot embossing has the potential of increasing production rates and therefore decreasing production costs by the enlargement of the molding surface and automatization of the molding process. Received: 25 August 1997/Accepted: 22 September 1997     

Injection molding and injection compression molding of three-beam grating of DVD pickup lens

The objective of this paper is to investigate the application of injection molding and injection compression molding processes to produce diffraction gratings. A mold was designed to produce a diffraction rating connected with the fixed bushing. The combined part was verified to have a good diffraction performance. Integrated grating eliminates the assembly cost and error. Photolithography was applied to make the mold insert. The Taguchi method and parametric analysis were applied to study the effects of molding parameters on grating quality. The design, fabrication of structured mold surfaces and the results of the replication by injection molding (IM) and injection compression molding (ICM) are presented and compared. The diffraction angle of ICM grating is more accurate than that of IM grating. Grating made by ICM has a much smaller warpage than that made by IM. The diffraction pattern shows that ICM is a better process than IM to replicate a diffraction grating.     

Analysis of morphology and performance of PP microstructures manufactured by micro injection molding

In this paper, PP (polypropylene) microstructures were manufactured by micro injection molding (MIM). The surface topography and internal defect under different process conditions were studied. An internal defect named “hollow” was observed in microstructures made without vacuum. To investigate the morphology (crystal and phase), the microstructures samples were cut to slices with 10 μm thickness along the filling direction. Results of polarized light microscopic observation reveal that these microstructures also represent “skin-core” morphology, i.e. a highly oriented non-crystalline skin layer, a shear zone with column crystal essentially parallel to the injection direction and a spherulites core. However the morphology distribution of microstructures is different from the macroscopic structure: the non-crystalline layer is much thinner, the ratio of skin layer (non-crystalline and column crystal layer) to core thickness is very big, there is no change of spherulites dimension from skin to center. So the microstructures must have a special mechanical performance differ from the macroscopic parts.     

Fabrication of high-aspect-ratio ceramic microstructures byinjection molding with the altered lost mold technique

A fabrication process for complex ceramic microstructures was proposed that combines a lost mold technique and ceramic injection molding. Two key points in this process were studied. First, the solubility of several engineering plastics in various organic solvents was tested to find appropriate combinations of mold material and solvent for dissolving molds. Secondly, the binder extraction rate and the strength of a green body during debinding were investigated. Experimental results indicate that acrylonitrile-butadiene styrene and acetone are the best combinations selected for this lost mold technique. We also propose that using gasoline as the debinding solvent and performing the debinding at room temperature will give a good time-saving effect and avoid toppling the microstructure if paraffin wax, stearic acid, and polyethylene were selected to compound the binder system. This process has been successfully applied to fabricate several ceramic microstructures, such as an integrated punch     

Post-buckling behavior of cold-formed thin-walled stainless steel beams

A numerical approach for predicting the post-buckling responses of cold-formed thin-walled stainless steel beams is presented. In the analysis, the nonlinear and unsymmetrical stress-strain relations in tension and compression for both flat and corner materials are considered. The effective width concept is used to account for the post-buckling strength of the thin compression flange of the beam. Using this concept, the value of the effective width varies with the stress borne by the flange edge. In view of the nonlinearities introduced by considering the material characteristics and the post-buckling behavior of the individual elements, an iterative procedure is employed to establish the nonlinear moment-curvature relation of a section. In a beam subjected to bending, the effective moment of inertia of the section varies from point to point along the length of the beam depending upon the magnitude of the moment at the point considered. In this analysis, the continuously flexible beam is replaced with a finite number of discrete rigid elements connected by flexible joints at which the continuously varied curvature is lumped through numerical integration. In the case of a continuous beam, it is further complicated by the fact that the bending moment distribution along the beam is not known a priori and its determination is part of the solution to the problem. An iterative algorithm is developed to obtain the approximate true moment distribution considering the compatibility conditions at the support. A computer program following th e above solution scheine has been developed and is capable of predicting the nonlinear responses of most of the common thin-walled sections encountered in light-gage steel design. Comparisons with some available experimental data are provided.     

汽车仪表板注射成形浇注系统方案及翘曲分析

针对汽车设计中的轻量化需求,分析不同浇口模型下汽车仪表板的注射成形过程.分析3种浇注模型下模具型腔的充填情况,发现熔接痕的数量不仅与浇注系统的布局有关,还受产品结构影响;分析3种浇注模型下的充填压力,发现浇口数量越多,充填压力损失越小,但浇口数量的增加会导致熔接痕数量增多;4浇口模型与5浇口模型对应的制品收缩率分布基本一致,3浇口模型的收缩率略高;采用高性能有限体积法（High Performance Finite Volume Method,HPFVM）分析3种浇注模型下制品的翘曲情况,发现3浇口模型的总翘曲变形量分布最宽,5浇口模型的总翘曲变形量分布最窄.     

Injection molding of microstructured components from plastics, metals and ceramics

 Micro injection molding is presently on its way to become an established manufacturing process in commercialising Microsystem Technologies. Enhanced products from plastics for micro optical or medical applications are entering the market. New developments like the different kinds of injection molding for microstructured components from plastics, metals or ceramics will increase the material range available in microdimensions. This will open up opportunities for increasing economic efficiency, for new fields of applications as well as for innovative products in the future. Received: 30 April 2001/Accepted: 30 August 2001     

Simulating the ferrite-to-austenite transformation in stainless steel welds

Under some conditions, the transformation of as-solidified ferrite to austenite in stainless steels has characteristics of a massive transformation. Calculations were carried out to determine if experimental evidence from Koseki of such a transformation could be modeled by a diffusion-controlled transformation. Two different conditions at the transformation interface were considered: ortho-equilibrium and para-equilibrium. Simulations solved the multi-component diffusion problem for various thermal conditions, including isothermal and continuous cooling conditions. It was found that neither interface condition could explain the experimental results of Koseki. The ortho-equilibrium interface condition resulted in transformation rates that were too slow. In addition, continuous cooling conditions led to non-uniform compositions, contradicting the experimental evidence. The para-equilibrium condition resulted in sufficiently fast transformation rates but the temperature of the transformation was considerably higher than that found experimentally. It is suggested that a condition of limited partitioning of substitutional solute elements at the interface could better explain the experimental results, but implementation of this condition at the interface is not possible with currently available simulation capabilities.     

Protective and non-protective oxide formation on 304 stainless steel

Parameters controlling the formation of protective and non-protective oxides on 304 stainless steel were examined by using Auger electron spectroscopy to monitor oxides formed in the vacuum chamber. Variables found to influence the oxide formation include: oxygen partial pressure, the location of the oxide over a grain or grain boundary, sample grain size and anneal treatments. Low pressure oxidation (<10⁻⁵ Torr oxygen pressure) at 800°C produced at thin Cr-oxide on annealed samples with 40 μm grain size; this thin (≈ 2000 Å) protective oxide did not grow with 15 min additional oxidation in air at 800°C. In contrast, initial oxidation of the same annealed material at oxygen partial pressures greater than 10^-3 Torr caused the formation of thicker Fe-rich oxides. The oxides formed at these higher pressures were often non-uniform having thin Cr-rich oxides near grain boundaries and thicker Fe-rich oxides over the grains. Annealing treatments influenced oxidation by varying the amount of chromium available at the surface for oxide formation. At temperatures above 800°C Cr evaporates from the sample decreasing the amount of Cr on the surface and altering the chromium profile into the metal. If carbon is present on the sample surface near 500°C a chemically induced Cr segregation occurs (similar to grain boundary sensitization) which also severely alters the Cr distribution in the near surface region of the material.     

不锈钢衬里背侧缺陷的超声波检测技术研究

针对目前采用的超声检测法对衬里层背侧裂纹检测时,检测效率不高且易漏检的现状,采用友联PXUT-350C型便携式超卢探伤仪、2.5P13×13K1的纵波探头,对自行设计制作的8mm厚的316 Mod尿素级不锈钢板的人工模拟缺陷试块进行了超声波探伤的实验研究.结果表明,超声探伤对深度为0.5 mm以上的缺陷比较敏感,缺陷回波的高度与缺陷深度成正比.该实验的成功为衬里层背侧裂纹的快速检测技术的形成奠定了实验基础.     

Artificial neural network modeling to evaluate and predict the deformation behavior of stainless steel type AISI 304L during hot torsion

The deformation behavior of type 304L stainless steel during hot torsion is investigated using artificial neural network (ANN). Torsion tests in the temperature range of 600–1200 °C and in the (maximum surface) strain rate range of 0.1–100 s^?1 were carried out. These experiments provided the required data for training the neural network and for subsequent testing. The input parameters of the model are strain, log strain rate and temperature while torsional flow stress is the output. A three layer feed-forward network was trained with standard back propagation (BP) and Resilient propagation (Rprop) algorithm. The paper makes a robust comparison of the performances of the above two algorithms. The network trained with Rprop algorithm is found to perform better and also needs less number of iterations for convergence. The developed ANN model employing this algorithm could efficiently track the work hardening, dynamic softening and flow localization regions of the deforming material. Sensitivity analysis showed that temperature and strain rate are the most significant parameters while strain affects the flow stress only moderately. The ANN model, described in this paper, is an efficient quantitative tool to evaluate and predict the deformation behavior of type 304L stainless steel during hot torsion.     

Multi-objective optimization and bio-inspired methods applied to machinability of stainless steel

Nowadays, due to the growing needs of market, the simultaneous optimization of various responses is configured as a necessary strategy in real process. Machinability of stainless steel has always been considered a difficult task and any movement toward optimization of this process are really worthy. Traditionally, the treatment of this problem is done through the application of the desirability function that consists in transforming the original multi-response problem in a similar with one objective. In spite of various applications involving this methodology, the quality of the solution obtained is dependent on the choice of the inferior and superior limits and on goals for each one of the responses. To overcome this disadvantage, the present work proposes a methodology to solve the original multi-objective problem by using the Bio-inspired Optimization Methods (BiOM). The strategy proposed consists in the extension of the BiOM to problems with multiple objectives, through the incorporation of two operators into the original algorithm: (i) the rank ordering, and (ii) the crowding distance. The proposed algorithm is applied to the machinability of stainless steel AISI (ABNT) 420 using a model that considers the tool life and cutting forces responses in terms of cutting speed, feed per tooth and axial depth of cut, in end milling process. The effects of these variables in the responses were investigated crossing information contained in response surfaces of material removal rate and cutting forces. The results obtained showed that the methodology used represents an interesting approach to the treatment of the optimization problem formulated.     

Micro-punching process of stainless steel foil with micro-die fabricated by micro-EDM

Micro-punching has become a potential micro-manufacturing process due to the simplicity of the process, low cost, high production capability and accuracy. But the size of a punched micro-hole is limited due to the difficulties of tool fabrication. In the paper, we propose a design procedure according to buckling theory and a kind of micro-die fabrication method for micro-punching process. Micro-punch and female die with high dimensional accuracy were fabricated by micro-EDM with block electrode. Micro-punching processes of stainless steel foil were conducted with the micro-punch and female die equipped in the precision micro-punching tool. Micro-holes of 300, 150 and 100 μm in diameter were pierced with a good profile and high dimensional accuracy, which indicate that the micro-die fabricated by micro-EDM is suitable for micro-punching processes.     

Mathematical modelling and parameter identification of a stainless steel annealing furnace

A new, comprehensive mathematical model of continuous annealing furnaces is developed, under consideration of both the radiative and convective heat transfer of the furnace components. Based on measured normal operating data from an industrial stainless steel plant, parameter identification is basically carried out using a nonlinear least-squares optimization algorithm for the whole annealing furnace, to estimate optimal values of uncertain parameters, such as emissivities. Due to the complexity of the model, a sequential approach for parameter identification is proposed and implemented, i.e. the parameter set is divided into different subsets, and the parameter estimation is carried out sequentially in several steps and iterations. The performance of the model with the estimated parameters is then evaluated on a different test data set. It is shown that the obtained model can predict temperature evolutions along the furnace in good agreement to measured data, under both steady-state and transient conditions. The presented model is suitable for controller design and process optimization.