Minimization of Warpage and Sink Index in Injection

Minimization of warpage and sink index in injection-molded thermoplastic parts using Taguchi optimization method

The objective of this paper consist of minimization of the warpage and sink index in terms of process parameters of the plastic parts have different rib cross-section types, and rib layout angle using Taguchi optimization method. The polymeric materials were selected PC/ABS, POM, and PA66. Taguchi optimization method was used by exploiting mold analyses based on three level factorial design. Orthogonal arrays of Taguchi, the signal-to-noise (S/N) ratio, the analysis of variance (ANOVA) are utilized to find the optimal levels and the effect of process parameters on warpage and sink index.

In the Taguchi method, three stages such as system design, parameters design, and tolerance design are utilized. System design involves the application of scientific and engineering knowledge required in manufacturing a product. Parameter design is employed to find optimal process values for improving the quality characteristics. Tolerance design consists of the determining and analyzing of the tolerances in optimal settings recommended by parameter design. By applying Taguchi method based on orthogonal arrays, time and cost required to conduct the experiments can be reduced. Taguchi recommends the use of the S/N ratio for determination of the quality characteristics implemented in engineering design problems. The S/N ratio characteristics can be divided into three stages: the smaller the better, the nominal the best, and the larger the better. The effects of several process parameters can be determined effectively by carrying out matrix experiments based on the Taguchis orthogonal design.

In this study, the process parameters ranges recommended from MPI software. In order to reduce the time and cost a number of mold analyses consisting of 243 trails based on a full factorial, three level, and design were conducted to collect the warpage and sink index data obtained from mold analyses results using the plastic injection molding commercial program MoldFlow. Geometry of the injection-molded thermoplastic part utilized in this study. It has width, length and thickness of 60, 300 and 2 mm, respectively. The experimental data are created by MoldFlow which is a commercial software based on hybrid finite-element/finite difference method for solving pressure, flow and temperature fields. FE analysis of the plastic part includes 4020 tetrahedron elements.

The S/N ratio is defined as =-10log (MSD), where MSD is the mean-square deviation for the output characteristic and defined as MSD= 1/N)

Where Yi is the value of warpage and sink index for the ith test, n is the number of tests and N is the total number of data points. Since log is a monotone decreasing function; it implies that we should maximize the S/N ratio. Thus, the S/N ratio values are calculated by taking into consideration both equations.

The analysis of variance (ANOVA) carried out to examine the influence of process parameters on the quality characteristic in this study. If some parameters do not significantly affect warpage-sink index, they can be fixed.

The warpage and sink index values of 1.095 mm and 1.432 (%) are the smallest values involving in analyses results.

Similarly, the maximum S/N ratio is calculated to determine whether or not the minimum warpage and sink index are acceptable. Also, the maximum S/N ratio varies for polymeric material PC/ABS from the minwarpage = ((5.3 dB) < (3.9 dB) < +1dB) and from the minsink index = ((10.0 dB) < (3.6 dB) < +1dB)).

Confirmation analysis test with the optimal levels of process parameters are carried out in order to demonstrate the goodness of Taguchi method. From this, it can be concluded that Taguchi method is very suitable to solve the quality problem occurring the injection-molded thermoplastic parts.

An Optimization of Plastic Injection Molding Parameters Using Taguchi Optimization Method

During producing a product using injection molding process, various defects such as warpage, weld lines, shrinkage and sink mark can be occurred, optimal setting up of injection molding process is very important to reduce the defect and controlling the quality defect of the injection molded product. The purpose of this paper is to minimizing warpage defect on Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) thermoplastic and simulates the injection molding process using Moldflow Plastic Insight software (MPI). The approach was based on Taguchis method.

Analysis of Variance (ANOVA) has been implemented to analyze and optimize the processing parameters such as mold temperature, melt temperature, packing time, packing pressure, cooling time, cooling temperature, ambient temperature and runner size.

In order to produce the optimized setting for producing plastic, they are various tools and technique of optimization. In this project, we focused on Design of Experiment (DOE). DOE is the most powerful quality improvement techniques to reduce process variation, enhance process effectiveness and process capability. The choice of a DOE strategy (Taguchi or classical DOE) depends a great deal on the degree of optimization required, resolution required, time and cost constraints, nature of the problem and they are illustrates the power of Taguchi approach to DOE.

In this study, warpage for PC/ABS material shows the different values when several of parameter level are used for this computer simulation (mold flow plastic insight software). Based on this experiment, it may be concluded that Taguchi method was successfully help to solve the problem to optimize the parameters within each levels. Computer simulation and Taguchi method provides an efficient and economical way of replacing the traditional method of trial and error, for engineering design and analysis. The conclusions of the study are as follows:

Melt temperature (B) contributes the most significant which is 54.22% followed respectively by Runner size (G) 14.62%, ambient temperature (G) 11.17%, mold temperature (A) 8.54%, packing pressure (D) 4.61%, and packing time (C) 4.53% as the influence factor for warpage defect.

Cooling temperature (F) and cooling time (E) only contributed 1.85% and 0.46%, it is not the significant factor for the warpage defect in this study.

Mold temperature 100 0C, melt temperature 280 0C, packing pressure 140 MPa, packing time 10 s, cooling time 17s, cooling temperature 30 0C, ambient temperature 35 0C and runner size 8mm is using as the best parameter for minimum warpage.12% decrease of warpage defect after run conformation using this parameter. By doing this study, it will ensure manufacturers to start production with a better starting data and furthermore could reduce time consume and material production

Modeling and Analysis of Injection Moulding Process Parameters for Plastic Gear Industry Application

Gears have been in use for more than three thousand years and commonly utilized in power and motion transmission under different loads and speeds. In comparing with metal gears, plastic gears have several advantages such as light weight, noiseless running, resistance to corrosion, lower coefficients of friction, and ability to run under none lubricated conditions The performance of plastic gears in wide variety of power and motion transmission applications is rather limited due to weak mechanical properties and divergent mechanism of failures. A methodical simulation is carried out to analyze the gear performance with various gating system types, gate locations, and processing parameters via grey-based Taguchi optimization method. With the obtained optimum results in simulation stage, the flow patterns of polymer melt inside the mould during filling, packing, and cooling processes are studied and the plastic gear failures mechanism related to processing parameters are predicted. The output results in the future can be used as guidance in selecting the appropriate materials, improving part and mould design, and predicting the performance of the plastic gear before the real process of the part manufacturing takes place. There are several different types of nylon (e.g., Nylon 6, Nylon 6/6, and Nylon 12) widely used in gear production that offer great toughness and wear well against other plastics and metals

Preliminary study of injection moulding flow analysis is undertaken by using MoldFlow plastic insight (MPI) version 6.1 software. For the gear three dimension (3D) geometrical drawing, it was initially done in SolidWorks.

After creating the initial 3D mesh for the gear model, a preliminary filling analysis is conducted. Filling pattern plays an important role in determining and identifying any potential aesthetic issues such as short shot, hesitation, air traps, and weld line due to wrongly gear types and locations selected. Location of gate have great effect on the filling pattern and anisotropy of the material. Three different types and locations of gate for the model gear. The selected gating system should produce a balanced flow front within the part, with no underflow or over packing effects as well as unidirectional. Therefore, in this case the model gear with diaphragm gate is selected as best location gate. Cool + Flow + Warp (CFW) Analysis is conducted after the optimized gating system for the studied model gear is completely determined in the preliminary filling analysis. In this study, volumetric shrinkage and deflection were selected as response variables to characterize and evaluate the gear simulation model related to injection moulding process parameters. The selection of the OA (orthogonal array) is concerned with the total degree of freedom (DOF) of the injection moulding process parameters. The DOF is defined as the number of comparisons among the process parameters required to optimize the parameters.

Data Preprocessing is required in view of the fact that the range and unit in one data may differ from the others. Moreover, it is necessary when the sequence scatter range is too large or the target sequence directions are different. The data pre-processing involves the transfer of the original sequence to a comparable sequence.

The Grey relational analysis (GRA) associated with the Taguchi method is applied to analyze the data obtained in CFW analysis as well as to determine the optimal processing parameters for the desired multiple quality characteristics of the moulded plastic gear. Apart from material selection, a proper part or mould design also plays a major role in getting the most out of plastic gears. Grey relational analysis is actually a measurement of absolute value of data difference between sequences and could be used to measure approximation correlation between sequences. Considering multiple quality characteristic in terms of volumetric shrinkage and deflection, two opposite trends are observed where the increment of melt temperature, mould temperature, packing pressure, and cooling time result in greater volumetric shrinkage and deflection of the moulded gear. On the contrary, the increment of packing time and injection time reduces the volumetric shrinkage and deflection. As in this case, the best combination of processing parameters and levels could easily be obtained from the main effect analysis by selecting the level of each parameter with the highest grey relational grade. Only two parameters, including melt temperature and packing time, are considered as significant on the examined quality of the moulded gear. The melt temperature showed the strongest comparability sequence with the percentage contribution of 67.579% followed by packing time of 25.664%. Injection time was found to have least importance on volumetric shrinkage and deflection concurrently with the lower percentage contribution of only 0.003%. Research on the reduction of sink mark and warpage of the molded part in rapid heat cycle molding process

Rapid heat cycle molding (RHCM) is a recently developed innovative injection molding technology to enhance the surface quality of the plastic parts without extending the molding cycle. Most of the common defects that occur in the plastic parts produced by conventional injection molding (CIM), such as flow mark, silver mark, jetting mark, weld mark, exposed fibers, short shot, etc., can be well solved by RHCM. However, RHCM is not a nostrum for all the defects in injection molding. Sink mark and warpage are two major defects occurring in RHCM. The purpose of this study is to investigate and further solve the sink mark and warpage of the molded parts in RHCM. To solve the problem of sink mark, a new bench form structure for the screw stud on the product coupling with a lifter structure for the injection mold was proposed. The external gas assisted packing was also proposed to reduce the sink mark in RHCM. To solve the problem of warpage, design of experiments via Taguchi methods were performed to systematically investigate the effect of processing parameters.

The plastic material used for the front shell is a type of bright and high-gloss ABS/PMMA, HG-0760TV, supplied by Samsung Cheil Industries.

Warpage is a distortion where the shape of the molded part deviates from the intended shape of the design. It is caused by the residual stresses within the molded plastic part after ejection. The residual stresses are in turn resulted from the non-uniform shrinkage of the plastic material in different positions of the molded part.

The geometrical structure of the front shell of a 46-in. LCD TV. The length, width and thickness of the front shell are 1125 mm, 697 mm and 3 mm, respectively. The front surface must have a very high appearance quality to meet the esthetic requirements. Therefore, any surface defects, such as flow mark, silver mark, sink mark, weld mark etc., should be completely avoided.

For the front shell of the LVD TV, the formation of sink marks on the front surface can be mainly attributed to the ribs and screw studs on the back surface. The imbalanced in-mold stresses will cause a distortion of the shape of the molded plastic part. Under the imbalanced in-mold stresses, the molded part will warp and deviate from the shape of design. In actual RHCM production of the LCD TV front shell, it was found that the molded plastic part tends to produce a very large concave warpage.

There are two methods for reducing sink mark i.e. Bench form structure for screw stud and external gas assisted packing.

Studs used for fixing and assembling are often designed on the back surface of the LCD TV front shell. In order to ensure high enough strength, the wall thickness of studs is usually higher than the thickness of the main substrate of the part. . With such bench form structure, the screw studs do not stand directly on the main substrate but on a shell bench which connects the main substrate and the screw studs. Although the bench form structure coupling with the lifter structure can effectively solve the problems of the sink marks resulted from the screw studs, a large number of lifters will increase the difficulty of mold manufacturing and assembling. For this reason, other methods, such as external gas assisted packing will be used. Sink marks resulted from the ribs and screws studs can be eliminated as long as the high-pressure gas penetrates into the positions of the ribs and screw studs. To achieve this goal, the gas channels should be carefully designed and the injections of the polymer melt and the high-pressure gas should also be carefully controlled. Filling, packing and warping analysis of RHCM process was performed

On commercial software, Autodesk Moldflow Insight 2010. The processing parameters including melt temperature, injection time, packing pressure, packing time and also cooling time on the warpage. Injection molding simulations based on Moldflow were conducted to acquire the warpage of the plastic parts produced under different processing conditions. A signal to noise analysis was conducted to analyze the effect of the factors, and the optimal processing parameters were also found out. ANOVA was also conducted to quantitatively analyze the percentage contributions of the processing parameters on the warpage. The verification results show that part warpage can be reduced effectively based on the optimal design results.

Optimization of plastic injection molding process parameters for manufacturing a brake booster valve body

The plastic injection molding (PIM) process parameters have been investigated for manufacturing a brake booster valve body. The optimal PIM process parameters is determined with the application of computer aided engineering integrating with the Taguchi method to improve the compressive property of the valve body. PIM is an important manufacturing technique to plastic products due to the advantages of product quality, competitive cost, high productivity, and good mechanical properties. The quality of PIM parts is deeply influenced by many factors, such as the material, mold design, and process parameters applied to manufacture them. Due to the high cost and time consuming, the trial-and error process is not suitable for complex manufacturing process in determining the optimal PIM process parameters [5]. Thus, the Taguchi method, artificial neural networks (ANNs), and genetic algorithm (GA) are applied to optimize the PIM process parameters to achieve the high quality product. The statistical software MINITAB 14 was used for the robust design methodology based on the Taguchi method. The Taguchi method consists of three stages which are system design, parameter design, and tolerance design. Phenolic molding compound was used to manufacture the brake booster valve body. Due to the complex manufacturing process, there are many parameters of the PIM process can affect the quality of a brake booster valve body. Seven PIM process parameters are considered in this study that they are number of gates, gate size, molding temperature, resin temperature, switch over by volume filled, switch over by pressure, and curing time. Finite element (FE) analysis of valve body is performed using MPI v5.0 software. Geometry of FE model used in this study corresponds to the real conditions such as, the real dimensions of the product and number of gates for production.

The Taguchi method uses the S/N ratio to qualify the quality characteristic deviating from the desired value. In this study, resin viscosity, curing percentage, and compression strength were chosen as three responses for the optimization. The parameter having higher rank corresponding to higher delta value means that this factor contributes to the PIM process more effective than other lower rank parameters. Analysis of variance (ANOVA) was also applied to find the influence of process parameters. The brake booster valve body produced using the optimal PIM process parameters has a compression strength of 36.36 MPa. When compared with the average compression strength out of the 18 design experiments, the compression strength of the valve body produced using the optimal PIM process parameter showed a nearly 12% improvement. It is clearly indicated that the Taguchi method optimizes efficiently the PIM process parameters and the efficient improvement can improve the safety performance of a vehicle. Finally, a verification test was carried out to validate the optimal parameters for improving compression strength. From these analyses, the following points are noted

The S/N ratio of the resin viscosity case was calculated by the objective function of smaller-is-better. The S/N ratios of both cases of curing percentage and compression strength were calculated by the objective function of larger-is-better. The results of the verification test at the optimal PIM process parameters show that the valve body has a 12% improvement.