The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
While gains are achieved via cost reduction and increased portability, thinner and smaller parts encounter more difficulty in molding because of the frozen layer problem. Due to coupled filling and cooling involved in standard injection molding, the relative contribution of the frozen layer in the total part thickness drastically increases as the part thickness decreases, thus resulting in increased difficulty of flow. Resin providers recommend using high speed and high pressure to alleviate the increased molding difficulty. However, the high-speed and high-pressure strategy appears inadequate when molding ultra thin wall parts and microstructures and cannot be efficiently used for delicate structures due to localized high stresses. A thin-wall molding process, wherein the mold surface is rapidly heated during the filling stage, was investigated in this paper. By rapidly raising the mold temperature to above the polymer melting temperature, thin-wall cavities can be easily filled without frozen layer induced flow resistance. Characteristics of this molding process were studied and compared with those of standard injection molding with the aid of molding simulation. A thin-wall molding process for an electrical connector is used to demonstrate the advantages of the new molding strategy over the high-speed and high-pressure molding strategy.
It is well known that reorientation of interfaces is key to efficient distributive mixing. However, how to achieve reorientation is not well known. This paper describes how interfaces can be reoriented in screw extruders and which method leads to the most effective reorientation. A new mixing device was developed to achieve highly efficient reorientation. This mixer can produce excellent mixing quality over a short axial length, as short as one diameter.This makes it possible to incorporate this new mixer into the non-return valve of an injection molding screw. Results will be presented from injection molding studies that compare the mixing action of the new mixing non-return valve to other mixing devices.
The challenge of replication of microstructures by injection molding depends very much on the size and the aspect ratio of the features as well as the size of the area covered with such structures. The filling of the structure has to compete with the filling of the underlying thicker substrate. The degree of filling of structures with high aspect ratios is investigated in a mold with a single continuous thin wall section attached to a flow leader for a large number of thermoplastic polymers. The degree of filling of the thin wall part is found to depend very much on the distance from the gate. Materials with the smallest pressure drop in the flow leader tend to give the best filling. Processing conditions do not have a strong influence except for the injection speed which can change the filling picture even qualitatively. It is found that the degree of achieved filling depends mainly on the local ‘time-to-pressure’, which is inversely related to the slope of the pressure rise curve. Numerical simulation overpredicts the achieved filling of the thin section for all investigated polymers.
A laser printer frame, which is one of the tightest tolerance injection-molded parts in a printer, was simulated using injection molding simulation software fromMoldflow® Corporation. The part was simulated with different types of gating, including multi-gate cold and hot runner systems. The simulations were used to predict the part deformation using the different gating options. The optimized gating was determined by minimizing the part’s shrinkage and warpage. The optimized gating was used in the production mold for the frame. The predicted part shrinkage and warparge was in good agreement with the actual frame deformation.
During the extrusion process, whether it is film, sheet or injection molding, the need to obtain consistent quality and output requires an extensive quality control program. Unfortunately, these operations are time consuming and wasteful, often requiring many pounds of extrudate be expended before the desired result is achieved.Also, as extrusion equipment becomes worn, output rates decline and product quality gradually falls, often going unnoticed, until significant problems occur.Thirdly, when a new screw purchase is required, time-consuming laboratory trials are normally set up to evaluate screws from different manufacturers before deciding the best one for that particular operation.To quickly assess changes in operation conditions, whether from day-to-day running, equipment performance or assessing new equipment, a concept has been developed for an extrusion product, which utilizes the principal of mixing two colors to achieve a homogeneous third color. Observing the homogeneity of the third color mix and the flow pattern it generates will indicate the screw performance and the quality of the product. This is a quick, efficient way to test the process without sacrificing product or running time.A series of experiments was performed to evaluate two different screw designs in an injection molding process. In addition, molded parts from the same mold but the different screw designs were evaluated for quality consistency. In a separate trial, the amount of wear on production screws and barrels in a color compounding process were evaluated.This paper is based on these experiments and prospective new products.
Experimental and theoretical studies of internal cavity pressure during injection molding of a spiral tube cavity were carried out. The frozen layer thickness and the evolution of internal cavity pressure were calculated using a commercial software (C-MOLD). The evolution of the internal cavity pressure was recorded during injection molding of polystyrene into a spiral tube mold. To explain the differences observed between the calculated and measured internal cavity pressure, a pressure correction factor (PCF) was introduced based on the plane stress theory. This factor was determined by analyzing the stress state in the melt and calculating the frozen layer thickness near the mold wall. The corrected and experimental pressures have been compared to validate the applicability of the pressure correction factor.
Cavity pressure is an important injection molding parameter. It is regarded as a good indicator of molded part quality and injection machine control performance. It not only indicates the material condition in the mold but also affects the microstructure and part quality. On the other hand, almost all users prefer better accuracy of CAE simulation. The discrepancy results from neglecting some important factors, such as the pressure-dependent viscosity, variable heat transfer coefficient, and variable material properties. The goal of the study is to understand how pressure-dependent viscosity, heat capacity, heat transfer coefficient, juncture pressure loss and pvT-data affect pressure prediction, and the importance of each parameter. Then the method to improve the prediction accuracy will also be discussed.
The combination of increased quality standards and just in time (JIT) production has yielded a series of issues for injection molders. While striving to maintain high quality and a JIT posture they are often faced with the potential for scrap molded during start up procedures. An often-unseen contributor to start up scrap is the stabilization period required by the mold. Until the mold has reached temperature equilibrium, consistent production is questionable. Building process tolerance to this instability and accurately predicting the amount of time required for equilibrium are the two logical solutions to solve this problem. This research focused on the use of various injection mold core materials to determine their effect on process equilibrium.
Strain gauges are one of the best measures of clamping force on the toggle clamp units. By means of FEA-calculations, it will be demonstrated that a problem with this kind of clamping force measurement lies with the superposition of strain and bending in the tie bars. In addition, FEA-results will show that only parts of the toggle are elongated, and therefore measuring at these parts will lead to higher precision. In the following paper, measurements of clamping forces with the new Kistler piezoelectric strain transducers will be presented and compared to measurements of a strain gauge.
A mold has been constructed with a continuous wave ultrasound transducer installed, along with post gate and end of fill cavity pressure transducers. Signals from these transducers and the position signal from the injection ram were used in turn to control the switch/over from injection to packing phase on a standard industrial injection molding machine. The accuracy with which this point was identified was measured for each sensor. The results show that cavity pressure and ultrasound are significantly more repeatable as switch/over sensors than the ram position sensor.
This paper describes the techniques employed to measure the dynamics of the micromoulding process and assess the influence of the processing parameters on the properties of the product. A number of sensors were fitted to a commercial micromoulding machine and a custom data acquisition system was assembled to record process data. Nano-indenting and Atomic Force Microscopy techniques were used to assess the mechanical properties and morphology of the micromoulded products. Results indicate that process conditions influence the morphology and mechanical properties of the product. Mould surface features of the order of ?ms were shown to be replicated on the surface of the product.
Injection molding has been used for mass production of polymer products with microstructures. Conventional 2.5D midplane simulation based on Hele- Shaw approximation was unable to describe the local flow pattern around the microstructures. This simulation tends to over predict the effects of microstructures on global flow pattern. A x-z 2D planar simulation was developed in our lab to achieve better accuracy and to retrieve more detailed flow and heat transfer information around the microstructures. For the unidirectional flow, it is able to obtain a good resolution, similar to that of Moldflow 3D simulation. The mold-melt heat transfer coefficient and injection speed are very important factors to determine the filling depth in microstructures. Since the velocity and stress fields vary significantly in the main flow and microstructure regions, the heat transfer coefficient and wall slip as a function of location need to be considered in the simulation.
The idea of raising the mold temperature to enhance part quality is not new. However, its application is limited because of prolonged cycle time. The rapid thermal response (RTR) molding process can facilitate extremely rapid changes in the mold surface temperature, thus reducing the prolonged cycle time due to heating. While cycle time reduction via RTR molding is apparent for parts that need an elevated mold temperature, such as micro parts, ultra-thin parts and stress-free parts, it is not clear whether the process could also be used to reduce cycle time for standard parts. In this paper a RTR molding process for polycarbonate samples with varied thicknesses was simulated and the cycle times were compared with those in standard molding with the recommended mold temperature from the resin supplier. The simulation result indicated that, by application of RTR molding to standard parts, both quality improvement and cycle time reduction can be achieved especially for thick parts.
The use of a pulsed supply of a cooling medium to a mould tool has been shown to have benefits on the cycle time and energy consumption in the injection moulding process. Three papers at the ANTEC meeting last year reported on this technology. A definitive explanation for the effects reported was not submitted at that time. Since that meeting further experimental work has been carried out to compare direct cooling with the pulsed cooling technology. The results to be presented will also show the effects of thermally conductive additives on the injection moulding cycle time. Results from the first stage of a study to model the functions of pulsed cooling in injection moulding will also be discussed.
The objective of this paper was to study the effect of different process parameters on the conductivity of injection molded graphite samples and stainless steel samples. Different percentages of graphite powder and binder were mixed using a premixer. The mixture then was grinded into small granules; these granules were then injection molded to flexural bars. The injection molded flexural bars were then debinded at different times and temperatures. Conductivity testing was conducted on the bars and volume resistance was calculated for all samples. The effect of powder/binder concentration, debinding time and debinding temperature was studied on the conductivity of the samples. As the powder concentration was increased in the sample the volume resistance of the sample decreased. Also, with the increase in debinding time and temperature there was a considerable decrease in volume resistance.
A hybrid finite element/finite difference method is employed to solve the temperature and pressure fields of an injection-compression molding process using a non-isothermal compressible flow model. The process simulation is coupled with a thermal viscoelastic material model to predict residual stress, warpage, and birefringence. A finite element analysis is formulated using axisymmetric plate elements to simulate the thermal stress and warpage. Flow and thermally induced birefringence is calculated by applying the stress-optical rule to the predicted residual stress. Experimental validation of injection-compression molded CD-R substrates shows that the simulation well predicts the process and part qualities under various processing conditions.
In this work, a three-dimensional finite element flow analysis code is used to solve sequential co-injection molding problems. Non-Newtonian, non-isothermal flow solutions are obtained by solving the momentum, mass and energy equations. Two additional transport equations are solved for tracking polymer/air and skin/core polymers interfaces. Solutions are shown for a rectangular plate filled with polypropylene. The numerical solutions are compared with experimental results.
In-mold coating (IMC) is carried out by injecting a liquid low viscosity thermoset material onto the surface of the thermoplastic substrate while it is still in the mold. The coating will then solidify and adhere to the substrate. IMC process is being integrated with conventional thermoplastic injection molding to improve the part surface quality and to protect it from outdoor exposure. This paper presents a Hele-Shaw based mathematical model to simulate the coating flow during the IMC process. Power-law viscosity model is employed to describe the rheological behavior of the coating material. The continuous deformation of the thermoplastic substrate caused by the coating injection is analyzed by means of the PVT relationship of the substrate. The corresponding computer code based on the Control Volume based Finite Element Method (CV/FEM) has been developed to predict the fill pattern and pressure distribution during the coating flow. The predicted results have been verified by experiments.
This paper develops a true three-dimensional numerical approach to simulating the melt filling and fiber orientation in injection-molded part of complex geometry. An efficient finite volume method is combined with VOF method to solve the melt flow during molding. The fiber orientation distribution is described by the second-order orientation tensors. The three-dimensional orientation of fibers is determined through the Folgar-Tucker equation, in which an interaction coefficient is introduced to account for the fiber-fiber interaction. Several example cases are simulated to verify the predicted melt front advancement and the corresponding fiber orientation. An industrial case with complex geometry is also studied to illustrate the capability of the proposed methodology.
Mold cooling system design in injection molding is of great importance because it is crucial not only to reduce molding cycle time but also to improve part quality. Traditional mold cooling analysis is based on the hybrid finite-difference/ boundary element (FD/BEM) approach. This approach was developed to accommodate the conventional 2.5D Hele-Shaw flow-based shell element model. In this paper, a true three-dimensional mold cooling analysis approach is developed. A fully tree-dimensional numerical analysis faithfully simulates the effects of part geometry, cooling system design, and ambient temperature on the solidification of the part. Finite volume method (FVM) is adopted as the numerical engine of the new approach. This developed approach is proved from numerical experiments to be a cost-effective method for true 3D simulation in injection mold cooling analysis.
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