International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 A Review on Application of Artificial Neural Networks for Injection Moulding and Casting processes Manjunath Patel G C Research Scholar, Department of Mechanical Engineering, NITK, Surathkal, INDIA Email: manju09mpm05@gmail.com; Abstract: Artificial Neural networks (ANNs) have potential challenges in the eve of prediction, optimization, control, monitor, identification, classification, modelling and so on particularly in the field of manufacturing. This paper presents a selective review on use of ANNs in the application of casting and injection moulding processes. We discuss number of key issues which must be addressed when applying neural networks to practical problems and steps followed for the development of such models are also outlined. These includes data collection, division of data collected and pre-processing of available data, selection of appropriate model inputs, outputs, network architectures, network parameters, training algorithms, learning scheme, training modes, network topology defined, training termination, choice of performance criteria for training and model validations. The suitable options available for network developers were discussed and recommended suggestions to be considered are highlighted. Keywords: Metal casting processes, Injection moulding processes, Casting Materials/Alloys, Artificial Neural Networks INTRODUCTION Neural network is one among the simplified models of our biological nervous systems. Our biological nervous system consists of massively parallel distributed large number of interconnected processing elements known as neurons [22]. For example [24], in human brain the most basic element is neuron. The beauty of human brain is its ability to think, remember and apply past experiences to our future actions. Our human brain comprises about 100billion neurons, each neuron connects up to 200000 neurons and approximately 1000-10000 interconnections are available. The neurons were arranged to form a layer and the connection pattern formed within and between the respective layers through weights referred as network architecture. There are mainly five basic neuron connection exists namely [24], 1. Single-layer feed forward network 2. Multi-layer feed forward network 3. Single node with its own feedback 4. Single layer recurrent network 5. Multi-layer recurrent network From the above five network connections currently more than fifty different network architectures were developed under supervised and unsupervised learning. A. Introduction to Metal castings Casting is one among the widely used near net shape manufacturing process started production during 4000-3000 B.C., Find major applications in both ferrous and I. Print-ISSN: 2231-2013 e-ISSN: 2231-0347 © RG Education Society (INDIA) Prasad Krishna Professor, Department of Mechanical Engineering, NITK, Surathkal, INDIA Email: krishnprasad@gmail.com non ferrous casting materials such as casting engines blocks, valves, pumps, machine tools, power transmission equipments, aerospace, home appliances and so on. Because of its wide applications, functional advantage, economical benefits, near net shape manufacturing capability more than 90% of the manufacturing goods produced through casting processes [91]. Moulding, melting followed by pouring, fettling, inspection and elimination/dispatch were the major steps followed in casting process. Control during each stage in casting process plays a vital role else it may lead to casting defects like rat-tails, scabs, cracks, blow holes, oxide films, misrun, shrinkage, porosity, cold shut, scar, blister, hot tears, segregations, cracks and so on. In past few decades neural networks were used to tackle production related problems, controlling the process by reducing casting defects, lead times, scrap rate, production cost and to avoid trial and error method, expert dependent advices in optimizing the process. B. Cast alloys and composite materials introduction Cast aluminium alloys widely used in various engineering applications due to its light weight characteristics, good heat, cast-ability and electrical conductivity, low density, melting point, coefficient of thermal expansion, and casting shrinkage respectively [86]. The mechanical properties of cast alloys mainly depend on weight fractions of alloying elements, applied heat treatments, microstructures, and morphologies of the various phases [79]. The use of composite materials in recent years is due to ever increasing fuel prices, need of weight reduction in aerospace and automotive sectors. Stir casting, diffusion bonding, squeeze casting, ultrasonic method, powder metallurgy, compo casting, thixo-forging and were the major techniques to fabricate composites. C. Introduction to Injection moulding process The complete injection moulding (IM) cycle follows three stages namely filling, post-filling and mouldopening for processing plastics. Plastic materials have following properties such as light weight, corrosion resistance, transparency, ability to make complex shapes, allow to integrate with other materials, functions, excellent thermal and electrical insulation properties. Quality related problems observed from the reviewed publications during injection moulding process were sink mark, flash, jetting, flow marks, weld lines, volumetric shrinkage, warpage, dimensional shrinkage variations and others. In recent years there is a rapid growth in ANNs to address and tackle the International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 problems that might occur in casting as well as IM processes either to enhance mechanical, micro-structure or surface characteristics as shown based on years of publications in table 1(a). Neural networks proved to be an excellent tool from the reviewed publications starting from preparation of mould to final inspection stage refer table 1(b). 48 out of 145 network architectures (NA) reviewed used for optimum process parameter prediction, 13 NA applied during moulding, 4 NA used to study the flow behaviour or metal fill during process cycle [28], [51] & [1]. 7 NA [15], [36] & [87] used during solidification stage to enhance properties by reducing defects, 2 NA [73] & [58] used to predict the micro-structure characteristics, 1 NA [31] applied during design stage for minimum porosity, 12 NA [8], [12], [13] & [53] used during inspection stage to predict the presence/absence of defect, 3 NA [66] utilized for heat treatment stage to enhance the mechanical properties, 15 NA [25] used for process planning, other 40 NA utilised for various purpose such as 4 NA [6] & [52] cost estimation before manufacturing the component, 33 NA used for mechanical properties prediction at different stages during the process, remaining 3 NA applied for data extraction [4], state of molten metal level [46] and metal temperature [21]. Neural networks have good learning and generalization capability; they learn from examples during training and generate outputs for new inputs which were not used during the training period. The above feature of ANNs helps to use as an alternative tool to simulate complex and ill-defined problems. As shown in table 1(c). 1 NA applied for identification of heat transfer co-efficient between metal mould interfaces [36], 9 NA were used to control the process [14], [37], [48-50] & [61], 9 NA [8], [12-13], [61] & [86] utilized for classification of defects, 95 NA utilized for prediction, 11 NA [9], [11], [18], [23], [26], [35], [59] & [66] adopted to optimize the process, 2 NA [28] & [51] used for to monitor the fluid flow and metal fill towards cavity and remaining 18 NA [2], [10], [38], [47], [55], [81] & [82] were used to map the relationship between input-outputs via modelling. ANNs applied by various researchers to achieve various objectives for different cast alloys, injection moulding and casting processes. 31 NA utilized for injection moulding (IM) processes to enhance product quality, 23 NA [1], [3], [21], [32], [57], [72-73], [78-79] & [87] used to improve mechanical, micro-structure and surface characteristics of cast material/alloys (CM/A), 20 NA [8-9], [12-13], [20], [30], [38], [40], [64-65] & [71] utilized for pressure die casting (PDC) processes namely low and high pressure die casting processes, 15 NA [25] adopted for investment casting (IC) process, 15 NA [48-49], [53-54], [58], [66], [68-69] & [76] used for continuous casting process, 14 NA [16], [17], [27], [33], [44], [47] & [60] presented for stir casting process (STIRC), 6 NA [41], [55], [74] & [83] used for sand casting (SC) process, 4 NA [81] & [89] used for centrifugal casting (CFC) process, 3 NA [61], [85] & [86] make used for cossworth (CP) process, 2 NA utilized for graphite sand mold casting (GSMC) [42], semisolid metal casting (SSMC) [70], lost foam casting (LFC) [28] & [51], squeeze casting (SQC) [2] & [81] and phosphate graphite mould casting (PGMC) [43] process respectively and remaining 4 NA adopted for each process such as green sand casting (GSC) [63], permanent mould casting (PMC) [77], strip casting (STRIPC) [46] and gravity die casting (GDC) [36] processes. II. CLASSIFICATION OF REVIEWED WORKS The search for papers was done utilizing available scientific and technology resource data bases namely journals, conferences, symposium and others. Total 145 NA found from 83 publications were chosen for review if they meet the following requirements, Focused on the casting processes, cast alloys and injection moulding processes, Neural network tools used as a modelling technique. From the reviewed publications following key issues were focused in applying ANNs to practical problems, 1. Generation of training samples for ANN 2. Selection of neural network type 3. Selection of input and output for network 4. Design of suitable network architecture 5. Selection of suitable learning algorithms for training 6. Selection of hidden layers, hidden neurons, activation functions, bias and other network parameters 7. Selection for evaluation of training performance, network termination, model validation 8. Applications of ANN model in casting and injection moulding processes III. PROBLEM DEFINITION Statistical design of experiments proven to be an cost effective tool for mapping the complex relationship between numbers of independent and dependent variables and optimize throughput in various manufacturing process with minimum experiments. Full factorial design (FFD), central composite design (CCD), box-behnken design (BBD) and taguchi parametric design were some of the tools available for experiment conduction and optimizing the process. Statistical tools help the designer in identifying the best set of parameter combination level to yield quality results for discrete values [4] & [18], however for continuous variables to conduct experiments at predetermined levels within the narrow range finds difficulties in some fields like foundry, welding, metal forming [55] and so on and may not help engineers to optimize the process. Modification of statistical models by incorporating new random data is not feasible. ANNs addressed these limitations effectively and refines the model by incorporating new random data at any stage during training. Form the reviewed literatures statistical methods were used to reduce the experiments/simulation runs and to fulfil the need of enough training data for ANNs. ANNs yield best prediction when compared to other statistical models in most of the case studies was observed. A. Data collection ANNs prediction accuracy depend upon the quality and quantity of the data used for training observed from the reviewed publications, various means of data collected for 2 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 TABLE 1. REVIEW OF LITERATURES IN CASTING AND INJECTION MOLDING APPLICATIONS VIA ARTIFICIAL NEURAL NETWORKS SL. No Number of network architectures among reviewed publications a Year of publications b Process applications c ANN applications Identification-1 Control-9 Classification-9 Prediction-95 Optimizaton-11 d Model validation CRC-10 Max. PE-7 RMSE-7 PE-25 Graph-35 MAE-5 SDQ-5 SDE-2 e Network training CRC-13 MPE-20 RMSE-26 MSE-44 NRMSE-1 MAE-9 SDQ-3 MAPE-2 f Training termination TE/TT-2 g Hidden layers h Input neurons One-3 Two-10 i Output neurons One-94 Two-12 j Topology definition k Training mode l Learning scheme Supervised-135 Unsupervised-4 NR-6 m Validation set Used-21 Not used-87 NR-37 n Simulation software o Data collection p Process and cast alloys q ANN Simulation software \ Network architecture Mat.NNTB-40 LVQ-2 RBF-11 MLP-algorithms BP-58 LM-40 r s 1997-1 1999-2 2000-3 Molding-13 2001-4 Design-1 2003-1 Process parameter-48 EL-10 One-82 Three-30 Three-13 NEB-20 2002-3 2005-6 2006-11 Heat treatmet-3 NR-73 Two-29 Three-1 Five-23 Four-8 Five-4 OBT-25 Six-6 Six-2 2008-8 Process planning-15 EL/TE-37 Four-29 2007-10 DOE-6 IM-31 REPL-4 IC-15 C-Mold-3 Meltflow-6 DOE-T/SS-11 2011-7 Inspection-12 Monitor-2 Mean.PE-21 PQI-2 2012-5 Others-46 Modelling-18 RE-1 MAPE-19 None-22 NR-4 NR-29 NMSE-1 SSE-15 TQ-6 NTEP-18 EL/TT-5 NR-9 Between seven to nine-15 Between ten to thirty-24 NR-5 Above ten-4 Nr-5 Between seven to nine-3 OAT-4 2010-13 Solidification-7 OMO-29 PPN-23 On-line mode-17 FEA/FEM-9 2009-13 T&E-44 Off-line-128 Moldflow-4 Procast-22 Virtual casting-1 ENRDOE/SS-38 TB/HB/HD-6 ENRDOE-65 GSMC-2 STIRC-14 CC-15 SC-6 Matlab-21 Qnet-2 DPL-3 VC/C++/JPL-6 SLP-4 MLP-120 SCG-5 PDC-20 CG-2 3 CFC-4 SSMC-2 NPV4.5-2 SOM-2 V-shrink-1 DOE-T-12 CM/A-23 CP-3 NNPII/Plus-3 PNN-1 MAL-2 Others-5 DOE-SS-1 SQC-2 LFC-2 Others-6 SNNS-14 NR-50 Others-4 NR-4 NR-6 Others-2 Other-1 Others-7 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 SS has the following advantages observed from the literatures [65], SS replace the real experiments need to be conducted. It helps in decision making-where human experts are not available and process calibration procedure takes longer than the manufacturing lead time, Where large number of process variables need to be examined with a minimum number of process range values, when die preparation cost and time were high. SS have some limitations such as simulation model are not suitable for large number of repetitive analysis required for optimization process because of high computational cost [11]. Simulation programs require need of human experts to interpret the output data [20]. Large number of simulation trials, coupled with lengthy execution or computation time/run might make investigator impractical [65]. The analyses and interpretations of simulation results were still empirical and enough computation time cannot meet the requirements of online control [71]. So to eliminate the trial and error method, reducing computational cost, repetitive analysis, need for experts and implementation of online control, advance methods are in high demand to model and optimize the process. Artificial intelligence (AI) based ANN model addresses these limitations and predict large number of outputs within few seconds once the training has completed. C. Training data quantity Collection of adequate quantity of training data is essential to enhance neural network performance. During the training process network architecture and there network parameters such as connected weights, learning rate, activation functions, constants, bias etc., need to be optimized for the desired problem under experimentation. When neural network is trained (weights are fixed) it then becomes possible to generate satisfactory results when presented with any new input data it has never experienced before. Total number of weights (TW) can be calculated for the given network [34] using TW = (Number of input neurons (IN) * Number of hidden neurons (HN)) + (Number of hidden neurons (HN) * Number of output neurons (ON))). HN increases network connections and weights, this number cannot be increased without any limit because one may reach the situation that, number of the connections to be fitted is larger than the number of the data patterns available for training. Though the neural network can still be trained, but it is not possible to mathematically determine more fitting parameters than the available data patterns for training [80]. Table 2. Represents various researchers used extremely limited data patterns wherein network connections and weights to be fitted were more than the data patterns used for training. To study and compare the effectiveness of neural network training techniques and learning algorithms must be done with large data patterns to improve generalization and avoid over-fitting but not with small data points [80]. Approximately 1000 set of data used for training yielded better results by various researchers [6], [20], [23], [29], [30] and [42]. It was observed from the literatures collected data base was divided into 3 stages namely training set, validation set and test sets. The main purpose of training was to decide the weight and bias of the training is as shown in table 1(o). 6 NA [3], [50], [57] & [75] used data collected from design of experiments (DOE) like response surface methodologies (RSM) namely CCD & BBD, 12 NA [4], [18], [43], [45], [56], [59], [63], [70] & [84] used DOE utilizing taguchi parametric design (DOET), 65 NA utilized either trial & error method of experiment conduction, experts advice/ experience from industry refers to experiments not resembling DOE (ENRDOE), 11 NA [11], [26], [35], [40], [65], [67], [73] & [82] adopted combination of DOE-T and simulation software (SS), SS was used to avoid the need of real experiments to be conducted because it adds material, processing, labour, inspection costs and so on and the purpose of DOE-T is to reduce the number of simulation trials because of high computational time for each trial may lead to costly simulation. 1 NA [19] used integration of DOE and SS (DOE-SS), 38 NA utilized combination of ENRDOE and SS (ENRDOE/SS), 4 NA [23] & [42] used regression equations from the design of experiments in published literatures (REFPL) to satisfy need of huge data for training, Text books (TB), hand books (HB) [79], historical data (HD) [52], [68] & [76] were the other means of collected database. Some of the limitations observed from the reviewed literatures such as, DOE-T reduce the experiments need to be conducted but fails to provide enough training data for ANN simulation in reference to example 13 shown in table 2. SS fulfils need of huge training data for ANNs, however simulation model is not suitable for large number of repetitive analysis required to optimize the process leads to high computational cost [11] and need of experts to interpret the results. ENRDOE follows large number of experiments to yield best results leads to high manufacturing cost and may not provide required quantity training data. So alternative method to overcome this problem is use of DOE for experiment conduction, develop regression equation from the real experiments through analysis of variance, statistically rearranging the input variables between their respective ranges and can predict the output through regression equation obtained from DOE which satisfy the need of huge training data. B. Simulation software In recent years, numerical simulation software’s were rapidly developed and applied successfully in many casting and injection moulding industry to enhance product quality, improve properties and reduce manufacturing costs. The list of commonly used and commercially available SS for IM process observed form the reviewed literatures shown in table 1(n) namely 3 NA [5] used c-mold, 4 NA [11], [19], [26] & [35] utilized mold flow, 1 NA [67] adopted Timon-3DTM and 1 NA [29] used Partadviser CAE. Similarly in casting applications, 22 NA [25], [36], 22 NA [25], [36], [64], [65], [71] & [82] used Procast software, V-shrink [1], Virtual casting [73], Any cast [21], Maxwell [28] and finite element methods (FEM) [33], finite element analysis (FEA) [51], fluent, calcosoft, magmasoft were the advanced commercial codes were available for simulation. 4 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 network, validation set verifies whether the estimated outputs from the network were accurate and testing determines whether network was over-trained or not. It is quite interesting to note that only 21 NA used validation sets, 37 NA not used and 87 NA not reported information about validation sets as shown in table 1(m). Qnet for windows version 2.0, NeuralLab, havBPNet++, IBM Neural network utility, NeuroGenetic Optimise (NGO) version 2.0, Aree 3.0 Adaptive logic network development system for windows, TDL version 1.1 Neuroon-line, NeuFrame and OWL Neural network library. From the reviewed publications as shown in table 1 (q), following ANN simulation software has adopted by various researchers namely, 21 and 40 NA adopted MATLAB and MATLAB neural network tool box (Mat. NNTB), 14 NA employed using statistica neural network software (SNNS) [8], [13], [81], [78] & [85-86], 2 NA [14] & [37] uses Qnet, Neuro-planner version 4.5 (NPV) [32], 6 NA used either visual basic c (VC)/c++/ java programming language (JPL), 3 NA [16], [44] & [88] employed delphiprogramming language (DPL), 3 NA [66] professional II/plus (NNPII/plus), 1 NA [15] utilized Neuro solutions (NS)], PC neuron (PCN) [75], Neuro-computer (NC) [52], Fast artificial neural network library (FANNL) [63] and remaining networks not reported (NR) the kind of ANN simulation software adopted. From the literature survey, it has been observed that selection of ANN simulation software based on the type of neural architecture used for prediction. Because all ANN simulation software cannot predict all type of network architectures. E. Network type and architectures Currently more than 50 different neural network architectures were available from the available literatures. In casting and IM applications five different type of NA were observed from the reviewed literatures as shown in table 1(r). 120 NA make use of multi-layer perceptron network (MLP) because of its good generalization capability, 11 NA utilized radial basis function network because of its training speed and easy to construct, 2 NA [65] & [71] used learning vector quantization (LVQ) network, 2 NA [4] & [15] used self organize mapping (SOM) as an un-supervised learning network, 4 NA [13], [74] & [78] adopted single layered perceptron (SLP) network, 1 NA [13] employed probabilistic neural network (PNN), 1 NA [4] adopted the combination of SOM and MLP network, 4 NA [3], [28], [63] & [84] NR the kind of network architecture they employed for their application. MLP with different training algorithms adopted by various researchers as shown in table 1(s), namely 58 NA used back-propagation (BP), 40 NA adopted levenberg-marquardt (LM), 2 NA employed conjugate gradient (CG), 2 NA momentum adaptive learning (MAL), 5 NA employed scaled conjugate gradient (SCG), newton method, Bayesian regularization (BR), rprop [87], variable metric chaos to improve BP algorithm [69], combination of LM and BP [78] and LM and BR [57] were the other few algorithms used to tackle and improve the prediction accuracy. F. Learning (or) Training The process of modifying the neural network parameters by making proper adjustments lead to result in production of desired response. During training the network parameters were optimized, as a result of which it undergoes internal process of fitting. Table 1(l), clearly shows 135 NA TABLE 2. LIMITED DATA SET USED FOR TRAINING 1 Network architecture IN-HN-ON 4-7-4 Total weights (TW) 56 9 [2] 2 1-12-8 108 10 [16] 3 5-10-3 80 13 [79] 4 5-8-1 48 19 [82] 5 1-8-2 26 10 [88] 6 5-15-1 90 80 [50] Examples Ref. 7 3-4-1 16 8 [9] 8 11-23-5 4-4-1 4-10-1 1-8-3 368 20 50 32 65 10 10 12 [41] 11 3-7-1 28 20 [71] 12 3-15-1 60 6 [58] 13 5-6-6 66 18 [59] 14 2-10-3 50 46 [27] 9 10 D. Training Data used [65] [44] ANN simulation software Simulation uses a model to develop conclusions providing insight on the behaviour of real-world problems being studied. Computer simulation is used to reduce the risk associated with creating new systems or upgrading the existing systems. Over the past few decades, computer simulation software together with statistical analysis techniques was used as decision making tool for the given task and is growing more technical, precision by reducing the error percentage in the field of business, industry and so on [62]. Computer simulation is very important because it helps in prediction of future events and occurrence. For example, simulation gives for the particular operating conditions; it predicts outputs or responses based on the past data without conducting any experiments. There are various freeware and shareware software available for ANN simulation for measurement fields [90] namely Rochester connectionist simulator, NeurDS, PlaNet 5.7, GENESIS 2.0, SNNS 4.1, Aspirin/MIGRAINES 6.0, Atree 3.0 educational kit for windows, PDP++, Uts, Multi-module neural computing environment, NevProp, PYGMALION, Matrix Back Propagation (MBP), Win NN, BIOSIM, AINET, DenoGNG, nn/xvv, NNDT 1.4, Trajan 2.1, Neural network at your finger tips etc., and some of the commercial software packages for measurement field [90] are also available for ANN simulation like BrainMaker, SAS, MATLAB Neural Network Tool, Propagator, Neuroforecaster and visual data, NESTOR, Neuroshell2, NeuroWindows, NeuFuz4, PARTEK, NeuroSolutions v2.0, 5 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 used supervised learning scheme, 4 NA makes use of unsupervised learning and 6 NA not reported the kind of learning procedure adopted. Generally there are three kinds of learning process namely supervised learning, unsupervised learning & reinforcement learning. Supervised learning: Learning with the help of a teacher refers to supervised learning. Here input-output data were available for training. Error determination is possible by comparing neural network prediction and target output data. This error is feedback to the network for updating the weights until predicted network results close to desired target values [24]. Un-Supervised Learning: Learning without the help of a teacher referred as un-supervised learning. In un-supervised learning the target output data were not available; hence error prediction is not possible. Because no feed-back information available from the environment to inform what the output should be or whether the predicted output is right or wrong. In such case neural network must discover, patterns, regularities, features (or) categories from input data patterns and relates the input data over output, such kind of process is called as self-organizing process [24]. Reinforcement Learning: Reinforcement learning process is similar to supervised learning. Here in supervised learning correct target output values are known for each input data. But in some cases, very less information might be available. For example the network output might tell that its actual output is only 50% correct. Thus only the critic information is available not the exact information. The reinforcement signal acts as feedback information to the network during learning process [24]. G. Training modes The process of modifying the neural network parameters by updating connection weights, bias and other network related parameters if any refers to training [22]. There are normally two types of training, Incremental mode of training (on-line training)- refers to an approach in which training sample/scenarios are collected on-line and pass to the network to be trained one after the other in sequence. Batch mode of training (off-line training)- The necessary data is collected before the commencement of the training and entire training data (say more than one) is passed to the network at a time and average error in predictions is determined and is back propagated to the network to update weights and bias values to enhance prediction accuracy. From reviewed publications shown in table 1(k), off-line approach has been adopted in 128 NA, 17 NA make use of on-line prediction applications namely, to monitor the metal fill in LFC [28], prediction of aluminium silicon modification level in AlSiCu alloys [57], to monitor product quality through strip casting process [46], to predict optimal flow behaviour of molten metal inside die cavity [64], to predict crack in continuous casting process [48-49], The authors [68] analyzed characteristics of existing mould breakout prediction systems, influence factors on breakout, feasibility that mould friction is used to forecast breakout and investigated mould friction through online training method for mould breakout prediction, researchers [75] developed in-process mixed-material caused flash monitoring system for IM process. Incremental mode of training has the following advantages, easier to implement, computationally faster than batch mode of training [22], occurrence of defects/problems can be tackled at any stage during the process leads to increase in productivity, decreasing human labour, reduces manufacturing lead time and costs [41] H. Network Topology During development stage of network, understanding of the following factors are more important such as initialization of weights, selection of activation function, learning rate, bias value, momentum constant, activation constants, hidden layers/neurons and so on. These factors not only affect network convergence but also affect prediction accuracy. Table 1(j), shows the way researchers adopted to define their network topology. Not even the best topology (NEB) used in 20 NA [38], authors dint mentioned either how many hidden layers/neurons, transfer function, training termination, learning algorithms, bias, learning rate, momentum constant, model validation, network performance criteria they adopted. 25 NA presents only best topology (OBT) which includes network parameters, hidden layer/neurons with no justification regarding selection, One at a time strategy (OAT) adopted by 4 NA [23] & [42], where in topology selection done through network parametric study, 29 NA only mentioned optimization, but actual optimization procedures followed were not reported, 23 NA [4], [6], [25], [52] & [89] proposed new methodology to define their network, authors [25] presented optimum topology selection by brute-force exhaustive enumeration of alternatives. When relatively few alternatives and/or network training time is low, in such situations genetic algorithm tool may find best possible solution to optimize networks. 44 NA adopted the most common method trial and error method (T&E). Though some new method proposed in selection of hidden layer/neurons and network topology, but they were not popular because they might fail for other applications. So till date there is no perfect methodology for defining network topology and still it is under intensive study. I. Neural network architecture parameters Normalization and De-normalization: Scaling of inputoutput values plays crucial role because improper scaled values cause incompatibility, leads to inaccurate results [5]. Normalization process helps in speed up the training process, avoids numerical overflows due to very large or small values [21]. Statistical techniques such as zero score, median, sigmoid, mini-max and statistical column normalization were available for data normalization, developed under different rules namely max rule, sum rule, product rule and so on [39]. Data normalization range selection is based on intended transfer function adopted, which helps in modifying summing weights into an output. Normalization range between (0, 1) for logistic sigmoid transfer function and (-1, +1) for hyperbolic tangent transfer 6 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 function [66]. The purpose of de-normalization is to convert predicted scaled values into actual output. J. Initialization of weights In neural NA neurons of one layer connected to other neighbouring layer by means of connection weights. Weights contain information about input signals to solve complex problems [24]. From the review literatures it has been observed that network performance relies partly on initialization of weights. Smaller weights tend to increase the training time for network convergence, whereas larger weight matrix may cause instability in the network performance. Network weights have to initialize at small random values because if all weights start with equal weights and if the solution requires an unequal weights to be developed the network won’t converge and may fail to learn from training examples with error stabilizing or even increasing as learning continues during training period[19] & [30]. K. Transfer or Activation functions Activation function is used to convert the weighted sum of input values of a neuron to a value that represents the output of the neuron. The output of a neuron is largely depends on the intended transfer function used. Different types of transfer function are used in MLP networks are hard-limit-it generates output either 1.0 or 0.0 depending on the input, log-sigmoid- output lies in the range between 0 and 1, linear-output of transfer function is made equal to its input and ranges varies between -1.0 to 1.0 and tan-sigmoid transfer function-generates output lies in the range between -1.0 to 1.0 [22]. Radial basis function is the most commonly used transfer function in RBF networks. The RBF network trains faster than multilayer networks, but requires many neurons for high-dimensional input spaces [57]. The nature of the sigmoid transfer function used in BP algorithm does not reaches its extreme values of 0 and 1 without infinitely large weights, therefore input-output patterns are to be normalized between the ranges from 0.1 to 0.9 [71], [36]. From the literature review linear, log and tan sigmoid transfer function finds in maximum applications. Linear transfer function is used in almost many NA in input and output layers. Log and tan sigmoid transfer function used in MLP network and radial basis function in case of RBF networks finds maximum applications in case of hidden layers. Linear transfer function in RBF networks and MLP networks was normally used which helps to speed up the training process. However in case of output layer log or tan sigmoid transfer functions also used by many researchers [59], [23], [42], [67], [58], [74] to improve predictions while sacrificing the higher computational cost. Authors [42] and [23] used activation constants in the transfer function which helps in reducing the mean squared error to small values, selection of constants based on network parametric study. L. Learning rate (η) and Momentum (α) constant The purpose of using network parameter-learning rate (η) is to prevent over-learning and error vibration. The learning rate parameter range varies between 0-1. Lower value of η ensures a true gradient descent but increases total number of learning steps to obtain the optimal solution. Higher value of η indicates rapid convergence but results in local minima by over-shoot and may never converge to minimum [30]. The momentum (α) constant parameter helps in weight updating in order to speed up the search in local minima region leads to faster convergence, increases prediction accuracy and shortening the computational time. To prevent over learning rate, error vibration and to avoid local optimum value α + η = 1.0 is adopted by authors [6]. Hence to conclude optimal vale of η and α depends on the problem to be solved and chosen experimentally via trial and error method, network parametric study by varying one at a time between the range 0-1 [42]. M. Bias Bias neuron does not receive any input and emits constant output across the weighted connections to neurons in the next layers [21], [29], and [33] The main function of using bias neuron is to prevent the weight matrix from stagnation, controlling activation function during learning process and finally helps neurons to be flexible and adaptable [77]. The range of bias value is varies between (00.15) [36]. N. Number of Input and Output neurons Table 1(h) & (i) represent the number of inputs and outputs used among reviewed publications. A single input neuron used in 3 NA [44], [76] & [88], two neurons in 10 NA, three neurons in 30 NA, four neurons in 29 NA, five neurons in 23 NA, six neurons in 6NA, seven to nine neurons employed in 15 NA, ten to thirty neurons in 24 NA [12], [54], [78] & [86] and 5 NA not reported the number of input & output neurons used. For one input variable conventional, simple regression, moving average and smoothed line function in micro-soft excel perform better than neural network. ANNs were best suited to model complicated interactions between several numbers of input variables [80]. A single output neuron used in 94 NA, two neurons in 12 NA, three neurons in 13 NA, four neurons in 8 NA, five neurons in 4 NA, six in 2 NA, seven to nine neurons employed in 3 NA, ten to hundred neurons is used in 2 NA [8] & [15], 108 output neurons used in 2 NA [85] & [86]. It is more effective to develop separate models for individual output, because training time/computational time increases dramatically when the number of outputs increases [80]. Neural networks proved to simulate with high accuracy for higher input-output neurons only when we have huge training data and ready to sacrifice computational time. O. Selection of Hidden neurons and Hidden layers Hidden layer(s) & hidden neurons play significant role in terms of convergence rate during training, learning time and prediction accuracy of the network [3]. The possible means of size choice observed from the reviewed literatures such as selection based on experience and experimentation with the problem or application in hand [3], neural network prediction accuracy can further be improved by increasing hidden layers and neurons [11] & [19], alkaikes information criterion, network information 7 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 criterion, neural network information criterion methods [4] available based on statistical probability and an error energy function. 82 NA employed for general problems with single hidden layer, 29 NA adopted for complicated using 2 hidden layers, 1 NA adopted three hidden layers [64], 9 NA not reported the kind of network architecture adopted as shown in table 1(g). Too many hidden neurons may have risk of over fitting in training data leads to poor generalization and increases training time [30]. Too few neurons (say one) are not sufficient to capture non-linear mapping from the collected data base. Mathematically proved by Hyken, single hidden layer with enough neurons yields successful results for feed forward neural network [17]. Kolmogorov theorem [40] & [89] H = 2p+1 (H is hidden neurons, p is input neurons) adopted to fix hidden neurons, l=Sqrt(n+m) + a, (n= input neurons, m= output neurons, a= constant (1, 10) and l is hidden neurons) employed for size choice [58], network purning technique [68], H= (input neurons + output neurons)/2 used by authors [6] & [79], network parametric study using one at a time [23] & [42] strategy was employed to fix hidden neurons. There are some few examples [13], [19], [51], [61] and [78] observed, where neural networks predict best results even when hidden neuron numbers are more than 50. According to authors [34], when the number of hidden neurons is equal to the number of training data used, the learning could be fastest but loses its generalization capability; hence for good generalization the ratio 10:1 must be used means for every 10 training patterns 1 hidden neurons yields better results. The exact size choice is still under intensive study with no conclusive solutions available because of network mapping complexity and nondeterministic nature of many successfully completed training procedures; however choice was done based on network prediction accuracy [30]. P. NA adopted mean square error (MSE), 15 NA used sum of squared error (SSE), 9 NA employed mean absolute error (MAE), 6 NA used training quality (TQ), 20 NA adopted mean percent error (MPE), 3 NA used standard deviation quotient (SDQ), 2 NA utilized mean absolute percentage error (MAPE), 1 NA used normalized mean squared error (NMSE), 26 NA make use of root mean squared error (RMSE), 2 NA used performance quality index (PQI), 1 NA used normalized root mean squared error (NRMSE), 13 NA used correlation coefficient (CRC) and remaining . 29 NA not reported. Some literatures adopted more than one statistical index for evaluation of network training performance few examples in [6], [10], [78] and [61]. The goal of any training algorithm is to reduce the error between the predicted and the actual value. A lower error & higher CRC indicates the estimated value of network is closer to the true value [6]. Among various statistical quality indexes MSE followed by RMSE, MPE and SSE were widely used. R. Model validation The ultimate goal at the end of training is to evaluate the network prediction accuracy (Model validation) with new set of data which are not used in training; the predicted model has to provide accurate and reliable forecasts. There are various approaches used by researchers as shown in Table 1(d), [4] to evaluate the ANN model namely CRC [7], RMSE, Maximum percent error (Max. PE), Percent error (PE) [83], MAE, SDQ, Standard deviation error (SDE), Mean percent error (Mean PE), Relative error (RE), MAPE, authors [4], [27], [43], [67], [78], [79], [81] and [89] used more than one approach for model validation and attempt made by the authors [3], [4], [8], [20], [21], [27], [30-32], [36], [45], [50], [57], [66-67], [70], [72], [76], [77], [87] and [88] Expressed their prediction accuracy through graph by comparing with experimental and predicted neural network results. S. Sensitivity analysis & reverse process model Sensitivity analysis (SA) is of paramount importance to identify the most significant process parameters during manufacturing process. SA can be applied during pre-processing stage to predict which factors contributing more and less, less contributed will be discarded and more will be considered and the difference in prediction with and without can also be checked. SA on post-processing stage determines the most critical factors from the selected process parameters. 1 NA [72] presented study made on the effects of different alloying elements, chemical composition, grain refiner, modifier and cooling rate on porosity formation in Al-Si casting alloys via SA. Development of reverse process model is always in great demand because industrialist are more interested to know the optimal setting process parameters for achieving desired castings quality. Reverse process model configures the response as the inputs and the process parameters as the output during training and learns the correlation between the responses and process parameters. The statistical methods [42] such as DOE might fail to carry out reverse mapping because the transformation matrix might not be invertible at all. This problem limitation could be handled effectively by Neural network training termination Table 1(f) represents researchers adopted various criteria to terminate the network training will fall among the following categories such as when the error (MSE, RMSE, etc.,) between neural network predicted value and the target value is reduced or meets the pre-set value (Error goal), when the preset training epochs completed and when crossvalidation takes place between training and test data. In most of the reviewed neural network architectures the first two approaches were used. 73 NA not reported, 37 NA adopted (EL/TE) during training if the preset error limit (EL) reaches, network terminates else it continues until the preset number of training epochs (TE). 18 NA employed fixed number of training epochs (NTEP) alone, 10 NA used only EL, 5 NA utilized (EL/TT) – if EL reaches within the preset timing network terminates training else it continues until the preset TT to reach. 2 NA used (TT/TE) – if preset number of training epochs reaches within preset time network stops training else continues until the TT should reach. Q. Neural network training performances To check performance of network training ability on estimated results, various statistical evaluation indexes observed in review of 145 NA is reported in table 1(e). 44 8 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 using neural networks, as it works like a black-box data processing unit. V. CONCLUSIONS Following are the major observations drawn from the literatures; 1. ANNs can be employed for casting and IM processes starting from moulding to final inspection stage. 2. Multiple regression equation obtained from DOE fulfils the need of huge training data for ANN 3. ANNs addresses the limitations of simulation software by reducing repetitive analysis, computational cost and time, replaces need of experts for results interpretation and helps in implementation for online process control. 4. An open field for studies aiming at on-line approach to address future problem or defects and correcting using adaptive and real time manner adopting ANNs 5. Supervised learning, MLP networks, one or two hidden layer(s), BP & LM algorithm, off-line training mode and Matlab simulations were more common in casting and IM process. 6. Very few authors attempted on use of activation constants in transfer function and bias values for MLP networks, even though it reduces the MSE to small values and improves the prediction accuracy. 7. ANN is best used to model complex interaction among several number of input & output parameter and predicts with good accuracy only limitation is need of enough training data and ready to sacrifice for computational time and cost. 8. Although author’s attempted and proposed some empirical equations for optimum hidden layer/neurons selection, however it is not generalized one and fails in other applications while using those methods. Hence optimum selection for hidden layer (s)/hidden neurons is still under intensive study. 9. Single hidden layer finds maximum applications in MLP networks from reviewed publications, reason might be increase the hidden layers increases the computation time or mathematically proven by Simon Hyken, single hidden layer with enough neurons yields better prediction accuracy. Only few NA used validation data set and recommended that use of validation sets prevents over-fitting. 10. Extremely limited amount data sets was used by few authors for training, where the network connections to be fitted are larger than the data available for training. Hence recommendation made by some author’s neural network has to be trained using large data base to avoid over-fitting and improve generalization. 11. It is important to note that very few authors reported the criteria adopted when to stop the neural network training, because prediction accuracy relies on neural network training termination. 12. In regards to model validation based on test sets or experimental results with network predicted IV. HYBRID MODELS Soft computing is a collection of computational techniques in computer science, artificial intelligence, machine learning and some engineering disciplines, which attempts to study, model and analyze complex relationship, which are difficult from conventional methods (hard computing) to yield better results at low cost, analytic and complete solutions [24]. Neural networks (NN), fuzzy logic (FL) and genetic algorithms (GA) are few examples of soft computing. If more than one soft computing technique employed to solve problems refers to hybrid computing or systems. Hybrid systems classification based on the following three types [34], 1. Sequential hybrids, 2. Auxiliary hybrids and 3. Embedded hybrids. Examples [34], GA obtains the optimal process parameters first and hands over pre-processed data to the NN for further processing in Sequential hybrids, Neuro-genetic system for Auxiliary hybrids, in which an NN employs GA to optimize the network architecture parameters. NN-FL is an example for embedded hybrid system, in which NN receives fuzzy inputs to process it and extract fuzzy outputs. Observations made from the reviewed publications that PSO-BPNN [6] for product and mold cost estimation of PIM process, Neural-Fuzzy model [14] to dimensionally control the molded parts in IM process, Neural-fuzzy model composed of NN for learning relationship between inputoutput data and FL for reasoning to generate more reliable suggestion for modifying induced output values from the trained neural network. Neuro-Fuzzy Model [15] developed to compare the results of modified and un-modified Al-Si alloys under vibration & non-vibration conditions which influences on mechanical properties. ANN & GA models [31] have been implemented to optimize the effective process parameters on porosity formation in Al-Si casting alloys. A hybrid of back propagation neural network and genetic algorithm for optimization of injection moulding process parameters [35], GA-NN based model is used to tackle problems related to quality of castings in green sand mould system under both forward as well as reverse mappings [42]. To determine the optimum process conditions for improving the IM plastic part quality through combination of ANN/GA model [45], To predict the optimal process conditions using NN by avoiding what if scenarios raised in casting simulation software, time consuming and GA is used to yield best output parameter values [65]. To optimize the thin walled component manufactured through the use of LPDC process for aluminium alloys by combination of NN and GA [71]. From these applications on hybrid models in casting and injection moulding processes, neural networks found to be an effective estimation and optimization tool. 9 International Journal of Advances in Engineering Sciences Vol.3, Issue 1, January, 2013 13. 14. 15. 16. [12] Paliniswamy S, C.R. Nagarajah, K. Graves and P. Loventti, A Hybrid Signal Pre-Processing approach in Processing Ultrasonic Signals with noise, Int J Adv Manuf Technol (2009) 42:766–771 [13] Dobrzanski L.A, Krupinski M and Sokolowski J. H, Computer aided classification of flaws occurred during casting of aluminum, Journal of Materials Processing Technology 167 (2005) 456–462. 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