Quality Of Injection Molded Parts Research Articles

Mechanism-based wear models for plastic injection moulds

Injection moulding is a single-step manufacturing process for plastic parts that require very precise dimensions, geometries and low part roughness (Ra 0.2–0.025 μm). The high cost of moulds (typ. > 100,000 €) and unacceptable wear of their cavities can limit the cost competitiveness of the process. Mould cavity surfaces are deteriorated by different wear mechanisms which may drastically reduce the quality of injection moulded parts, interrupt their production, raise maintenance and repair costs, or delay delivery. Therefore, a better understanding of the wear mechanisms and the main parameters affecting mould wear is needed. In this research, three major failure mechanisms were studied: abrasion and erosion (due to fibre-reinforcements in the plastics), and corrosion (due to gas release from the polymers). Testing protocols were developed for each failure mechanism. Abrasion was simulated with a block-on-plate tribological test, erosion was evaluated in an air-jet erosion tester combined with a gravel-o-meter test, and corrosion was simulated using electrochemical techniques. Three different thermoplastic materials and several mould surface solutions (i.e., mould steel and four alternative surface treatments) were tested. Furthermore, this paper presents an innovative approach to simulate the behaviour of mould and plastic materials at the laboratory scale, in order to minimise the cost associated to mould failures, through mould lifetime prediction. The prediction, with some limitations has been made based on the model generated through laboratory tests, by using a multiparameter design of experiments, where different working conditions (such as pressure or speed) and plastic and mould materials were evaluated. A single best solution was not found to resist all failure mechanisms, but useful recommendations for potential solutions for each failure mechanism were proposed. The TiN coating applied by Physical Vapour Deposition represents a good compromise between wear (abrasive and erosion) and corrosion resistance, especially for fibre reinforced plastics. Ni-PTFE coating is proposed to reduce the friction and improve the corrosion resistance of the mould materials when high wear resistance is not needed, as in the case of non-reinforced plastics.

Effect of Mould and Melt Temperature on the Properties of Styrene Acrylonitrile Moulded Parts

The quality of injection moulded parts can be influenced by processing parameters. The effect of mould temperature and melt temperature on the weight and shrinkage of Styrene Acrylonitrile moulded part was observed in this research. It was found that by increasing mold temperatures at constant melt temperature and injection speed, the weight of the part decreased and the shrinkage value slightly decreased. On the other hand, by increasing melt temperature at constant mold temperature and injection speed, the part weight increased. Both mold temperature and melt temperature gave only a small effect because the temperature difference was not too large.

Influence of clamping force on tie-bar elongation, mold separation, and part dimensions in injection molding

In injection molding, the magnitude of clamping force may affect the quality of plastic parts. A small clamping force may produce defects such as flashes and poor geometrical accuracy, whereas a large force could result in insufficient air venting during mold filling/packing, leading to the generation of short shot. Traditionally, clamping force is set at the highest machine specification, which may lead to additional energy consumption. Moreover, heavy loading at the tie bars is detrimental to the durability of processed molds and the machine itself. In this study, we developed a measuring system for recording tie-bar elongation and mold separation during injection molding at each corner of the mold. To determine the consequences of proper and improper clamping force settings on the quality of injection-molded parts, we systematically evaluated the effects of relevant parameters such as cavity layout and gate location on cavity pressure distribution during packing and the reactive force from cavity pressure acting on mold walls. Correlation analysis on experimental results depicted that the maximum increase in tie bar elongation and the maximum mold separation during injection molding are highly correlated with the thickness of plastic parts, and we suggested that the two features are good indicators of part quality. Moreover, cavity layout of generating diverse filling patterns and the distribution of cavity pressure is influential to mold separation as well as tie-bar elongation during mold filling. This phenomenon is further change the degree of mold separation at each corner of the injection mold and should be concerned in practice.

Influence of Technological Parameters on Surface Quality of Injection-Molded Parts

In the most cases manufacturing of injection mold and its cavities comprise more than half of the total production time. Very fine finishing operations can increase production time and cost of injection mold. Appropriate choice of finishing operations and injection technological parameters can reduce price of injection mold production, shorten production time and thus lower price of small and thin injection molded parts. Tool material X38CrMoV5-1, used for cavity in injection mold, was finished by fine turning, grinding, lapping and polishing with different technological conditions to achieve various surface roughness. The main aim of this research is to find an influence of surface quality of cavity and injection technological parameters on the surface quality of injection molded parts.

Study of the Structural Characteristics of Mold for Precise Injection Molding

With the improvement of accuracy requirements for industrial products, the precise injection molding is replacing the traditional injection molding quickly and widely. Many factors influence the quality of injection-molded parts greatly, such as the property of the plastics, mold structure and the manufacturing accuracy, injecting machine and the injecting process parameters. In this paper, the work is emphasized for the influence of mold structure on the quality of injection-molded parts. Eight portions of injection mold are analyzed, including the cavities and cores, the guide components, the runner system, the ejection system, the side-core pulling mechanism, the temperature balance system, the venting system and the supporting parts. The structural characteristics of the above eight portions are presented.

Prediction of Quality Index of Injection-Molded Parts by Using Artificial Neural Networks

Based on the deep analysis of injection molding process and factors influenced on the quality of injection-molded parts, and considering of the non-linear, multi-variable coupling of process and the problem of data deficiencies, a feed-forward three-layer neural network was presented to predict the part weight based on the melt temperature, holding pressure and holding time, which is the most important quality index of injection-molded parts. Furthermore, an architecture optimization approach for neural networks was used and the performance of ANN was evaluated and tested by its application to verification tests with process parameters randomly selected which all of them were not used in the network training. Results showed that the ANN predictions yield mean absolute percentage error (MAPE) in the range of 0.1％ and the maximum relative error (MRE) in the range of 0.47％ for the test data set, and which can comparatively accurately reflect the influence relation of the injection process parameters on part’s quality indexs under the circumstance of data deficiencies.

Fast Optimization of Injection Velocity Profile Based on Graph Theory

Injection velocity profile is an important process parameter that affects the quality of injection-molded parts. In this paper, a fast optimization method for injection velocity profile is proposed, which aims at a constant melt-front-velocity throughout the filling stage. According to the relationship between cavity geometry and the melt-front-velocity, the ideal injection velocity is proportional to the melt-front-area. Through studying the filling behavior of molten polymer and the characteristic of 3D surface mesh model, a modified Dijkstra's algorithm is employed to predict the melt-front-area at different filling steps. Based on the predicted melt-front-area, a least square method is adopted to find out the optimum injection velocity profile by minimizing the difference of injection velocity and the ideal injection velocity. Finally, a short shot experiment and a Moldflow simulation experiment have been conducted to verify the proposed optimization method, which demonstrate that the proposed method is correct and effective.

Injection molding optimization with weld line design constraint using distributed multi-population genetic algorithm

Weld lines not only detract from an injection-molded part’s surface quality, but also significantly reduce its mechanical strength. It is not always easy to completely eliminate weld lines by simply adjusting the relevant injection mold design or the molding conditions. One solution is to prevent the weld lines from forming in regions that are structurally or aesthetically sensitive. The influence of weld lines on the quality of injection-molded parts cannot be overlooked and weld lines should be regarded as an important design constraint, especially for parts with aesthetic concerns. Since precisely predicting the number of weld lines and their positions and lengths is difficult without executing simulation routines, especially when part geometry is considered a design variable, this study adopts an enhanced genetic algorithm, referred to as distributed multi-population genetic algorithm (DMPGA), combining an optimization algorithm and commercial MoldFlow software with a dominance-based constraint-handling technique and a master–slave distributed architecture. MoldFlow obtains relevant data regarding warpage and weld lines and evaluates the corresponding designs. The dominance-based constraint-handling technique handles the weld line design constraint without needing additional penalty factors. Finally, the master–slave distributed architecture reduces the formidable computational time required for injection molding optimization. To illustrate the high viability of DMPGA, this study provides an outer frame of a digital photo frame as an optimization example. The results of this study show that DMPGA cannot only effectively decrease maximum part warpage without violating the weld line constraint, but also conquer hurdles attributed to constraint handling and computational demand.

A brief history of the filling simulation of injection moulding

Injection moulding is one of the most important manufacturing processes for mass production of complex plastic parts. The quality of injection moulded parts depends not only on the material, shape, and function of the part design, but also on how the material is processed during moulding. Traditional design approaches based on intuition, prior experience, and trial-and-error methodology have been becoming less efficient and effective. With advances in numerical modelling and computer simulation techniques, there have been tremendous efforts made to develop computer simulation tools to facilitate injection moulding design and process set-up. This paper reviews the history of research and development in the filling simulation of injection moulding. The existing models are classified into three categories: one-dimensional models, 2.5D models, and three-dimensional models. The basic features and relative key techniques about these models have been discussed. The techniques of tacking the moving flow front have also been presented. It is then followed by conclusions and discussions of these mentioned models.

A study on quality monitoring of injection-molded parts

Quality of injection-molded part is dependent on both mold design and processing conditions. From the design point of view, an optimal shrinkage should be used to compensate for the shrinkage of molded parts. From the processing point of view, it is important to analyze the priority of processing conditions because a number of processing conditions affect the quality of molded parts. Difference between designed and measured shrinkage at the specific position of a specimen box is minimized as a characteristic value to decide the quality of molded parts. Optimal processing conditions are 53.56 mm/s of injection velocity and 41.89 bar of holding pressure. The ranges of indirect control parameters, such as maximum cavity pressure or weight of molded part measured at the optimal processing condition, are used for monitoring the quality of molded parts in process. The judged results are displayed at the monitor by alarming a sound or lighting a lamp in case of bad-conditioned part.

A Dynamic Injection-Molding Process Model for Simulating Mold Cavity Pressure Patterns

The quality of injection-molded parts is typically controlled in the plant using statistical techniques that involve measuring parts as well as monitoring processing parameters. Part quality is also controlled by machine operators who adjust processing conditions in response to trends in process behavior. To achieve direct on-line monitoring and automatic control of part quality, a multivariable, nonlinear process model must be developed that relates process behavior to machine-controllable parameters. Presented in this work are the details of such a model derived from first principles and proven correlations. Because recent work has shown that complete mold cavity pressure patterns are good indicators of part quality, the focus of this lumped-parameter model is to simulate mold cavity pressure patterns observed during the filling, packing, and cooling stages of the process, given machine set points for barrel temperature and holding pressure. The model is validated experimentally using a production injection-molding machine and parameter-sensitivity case studies are presented.