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.
In this work, bubble instabilities of metallocene and Ziegler-Natta catalyzed polyethylenes (PEs) were studied by using an in-line scanning camera. Levels of long chain branching (LCB) and breadth of molecular weight distribution (MWD) of PEs were systematically varied to study their effects on bubble instabilities. Gel permeation chromatograph traces and small amplitude oscillatory shear data were used to verify the molecular structure of PEs. In addition, tensile stress growth coefficients and apparent uniaxial extensional viscosities were also determined by using Meissner type rheometer and converging die techniques. It was found that LDPE shows the most stable working windows confirming the previous arguments that the polymer showing strong strain hardening has the broader working window. It was also found that two branched metallocene catalyzed PEs show better bubble stability than PEs with broad MWDs implying that the presence of LCB plays a much more important role than the broadening of the MWD. The order of bubble instabilities can be properly evaluated based on a plot of normalized tensile stress growth coefficient vs Hencky strain or normalized extensional viscosities vs extensional stress.
In this research, a parallel finite volume method was developed to simulate non-isothermal non-Newtonian steady flow in a coat-hanger extrusion die on PC clusters. We implemented the algorithm by domain decomposition methods that distribute the computational parts equally among the PCs and balance the loading of each PC to the utmost. Each PC exchanged data and information according to MPI (message passing interface) standard, and the governing equations were solved by using the three-dimensional collocated cell-centered finite volume method. In this approach, the extrusion flow can be predicted efficiently and accurately. Moreover, the effects of interconnect network were also discussed in this paper. The present numerical approach proved to be a promising solution for complicated extrusion problems.
The use of injection molding (IM) in high precision manufacturing relies upon the capability of the process to deliver parts consistently conforming to specifications. Characterizing such capability is a matter of understanding the most important sources of variation in IM and to find ways to provide robustness to the process. In this study, a statistical analysis of several sources of variability in IM is presented to precede a future optimization task in which the aim will be to find variable settings that provide the balance between high performance in selected measures as well as low variability around these indicators. The results presented here are meant to be an evaluation of IM molds for use in high precision manufacturing. A method that capitalizes on the strengths of statistical analysis is demonstrated here through two case studies where the variability in parts produced by different molds is assessed. The adequacy of these molds for high-precision manufacturing is determined.
Injection molding (IM) is considered to be the most important mass production process for plastic products. A substantial amount of research has been directed towards finding settings for the IM process variables as well as the optimal location of the injection gates. These objectives have been mostly approached through the optimization of performance measures (PMs) as functions of the process’ variables. The use of computer-aided engineering (CAE) has played a pivotal role in trying to achieve these objectives. The aim of this work is to demonstrate a method based on CAE, artificial neural networks (ANNs), and data envelopment analysis (DEA) to find the optimal compromises between multiple PMs to prescribe the settings of IM process variables and the location of the injection gate. Two case studies are presented for this purpose. The first case refers to the production of a cylindrical canister where part shrinkage plays an important role for an effective mold release. The second case analyzes the production of a generic part with cut-outs such as a window frame where the location of weld-lines is critical. Also in this second case flatness is considered an important measure.
The control of plastic freeze-off and melt flow through a cylindrical nozzle is studied. Analysis of the temperature distribution of a nozzle contacting the mold shows a significant temperature distribution as a function of the axial and radial position in the metal and plastic. The temperature of the plastic melt determines the viscosity and subsequent flow through the nozzle. Experimental investigation validates the analysis by characterizing the pressure needed to induce flow as a function of nozzle and mold temperature. Control of the polymer freeze-off and melt flow is necessary for fully automatic production, as well as development of advanced molding processes.
This paper proposes a new virtual sensing approach for on-line monitoring process variables of injection molding process. In particular, a nozzle pressure virtual sensor has been developed. Exploiting the dynamic interaction between the machine and process variables, the virtual sensor utilizes the screw velocity data (a machine variable) to predict behavior of the nozzle pressure (a process variable). The virtual sensor was designed based on nonlinear observer theory. Experiment evaluation on a commercial injection molding machine was carried out, confirming the effectiveness of the virtual sensor.
Although water-assist injection molding is still in its infancy, this enabling technology promises productivity improvements for applications that may otherwise be cost prohibitive with gas-assist injection molding. Cycle times are reduced through cooling time reductions and the utilization of water as a cost-effective cooling medium when compared to nitrogen. For example, automotive suppliers have the potential for a broad range of cost savings with the production of conduits like cooling pipes or oil pipes. Other parts with large cross sections may also be produced in a cost-effective manner with water-assist injection molding. In fact, production parts in Europe are now beginning.Compared to the gas-assist injection molding details of the process, equipment, and the special material developments will be examined in this paper.
Extensive experiments were conducted to study the effect of injection speed on gas penetration length, residual wall thickness, the melt front position and short-shot weight of gas-assisted injection molded part. Experiments were performed on polystyrene melts filling a spiral tube cavity at three different melt temperatures. Simultaneous measurements of the screw position and the evolution of gas pressure and melt pressure in the cavity were performed. At a constant shot size, the length of melt propagation and the weight of moldings were found to increase with an increase of injection speed. An implication of these finding for gas penetration in gas-assisted injection molding was discussed.
Theoretical and experimental studies have been carried out on the transient gas-liquid interface development and gas penetration behavior during the cavity filling and gas packing stage in the gas-assisted injection molding of a spiral tube cavity. The evolution of the gas/melt interface and as well as the distribution of the residual wall thickness of skin melt along with the advancement of gas/melt front have been investigated. The physical model for both the primary and secondary gas penetrations was developed based on the Hele-Shaw approximation combined with interface kinematics and dynamics. Numerical simulations were implemented on a fixed mesh covering the entire cavity. The residual thickness of a polymer layer and the length of gas penetration in moldings were calculated using a commercial software (C-Mold) and both the simulation and model developed in this study. Extensive molding experiments were performed on polystyrene at different processing conditions. The obtained results on the gas bubble dynamics and penetration behavior were compared with those predicted by the present simulation and C-Mold.
An expert system has been developed here to assist the machine setter eliminate common defects from a gas assisted injection moulded product. The system breaks down the range of possible mould tool configurations into four main modes of operation. A process starting point is determined by an initialisation routine. The process is changed according to specified product defects to provide acceptable products. A further routine optimises the process by ensuring the process envelope, defined by natural random variation, does not move outside the moulding window. A multicavity mould tool has been used to validate the routines.
In this study, the application of a finite volume discretization and volume-of-fluid method has been demonstrated to simulate three-dimensional gas-assisted injection molding processes. An effective fluid concept is employed to compute segregated multi-fluid flows. The modified Cross model and Arrhenius temperature equation are implemented in the numerical scheme in order to calculate the rheological properties of polymer flows. The numerical results successfully depict some important three-dimensional phenomena, such as the jetting effect, race-tracking effect, corner effect, and the flow asymmetry after the gas is injected, which could not be described by any two-and-half dimensional model commonly used in the current commercial CAE applications.
As a result of heat transfer properties superior to that of standard tool steel, copper-beryllium alloy inserts are expected to provide cycle time improvements in some injection molds. However, issues such as manufacturing cost can limit the benefits of using such inserts. A thin-wall single cavity container mold with inter-changeable H13 and Copper Beryllium core caps was used to quantify the possible differences between these materials. Under optimized single cavity process conditions (cycle time below 3 seconds), the cycle time could be significantly reduced by using the copper beryllium core cap. However, the impact properties of the resulting cups were reduced for some of the materials under investigation. No such disparity either in the process or in the mechanical properties could be observed when a stack mold process (cycle time around 5 seconds) was simulated. There is evidence that the container’ impact behavior is mostly determined by the local internal stresses near the injection gate.
The design of injection molds can be accomplished by the state-of-the-art software available on the market. However, in daily practice where quick estimates of the parameters involved are needed, the application of sophisticated software can be time consuming and costly. This paper deals with straightforward solution procedures for optimizing the mold design by taking thermal, mechanical and rheological design criteria into account. Easily applicable analytical methods are given for calculating the heat transfer between the melt and the coolant. It is shown in these calculations how the geometrical layout of the cooling channels is related to the mechanical strength of the mold material. Furthermore, explicit relationships based on resin rheology are presented for balancing the melt flow in runner systems. These proven equations are illustrated by numerous worked-out examples.
Hybrid injection molds with non-metallic components in the molding zone are being considered for short runs or pre-series. Cast resin tooling is one of the techniques for making the molding inserts. The nonmetallic materials being used have poor thermal properties that tend to increase substantially the molding cycle. In this study, the dependence of the thermal performance of hybrid molds with respect to the cooling layout was studied using epoxy inserts. Experimental data was gathered in terms of temperature at the polymer/mold interface and compared with simulation using the software C-MOLD. The thermal performance is discussed for different cooling layouts.
Due to the complication in operation mechanisms, commercial valve gate usually delays for about 0.3 to 0.5 seconds once the valve-opening command is given. This signal to operation delay limits its application to 3C thin-wall injection molded parts. In this study, a fast-response gas-driven unit developed for thin-wall gas-assisted injection molding was adopted to perform valve gate control. Verifications of valve-gate opening were monitored using CCD camera, cavity pressure transducers and accelerometer, respectively. All design parameters including gas-valve response characteristics, tolerance between inner piston and cylinder, gas pressure, melt temperature, etc., that would affect valve-gate opening were investigated. The delay time for vale-gate shaft movement in a non-melt environment can be reduced to about 50 milliseconds whereas it increases to about 80 milliseconds in a melt-filled environment. The improved system results in injection molded parts without weld line and good cosmetic quality.
An innovative injection mold system with a specific function has been developed for improving surface defects of molded articles. The system mold comprises an insulated thin metal cavity surface and a release-functioning core surface. Immediately after mold-filling under a low pressure such as one third of that in conventional molding, the cavity surface rapidly increases in temperature to develop wettability and adhering, while the resin on the core side is released and migrates toward cavity side to compensate the surface shrinkage. This will bring the merit to produce paint-free articles having accurate surface patterns in molding with downsized machines.
The main aim of the research was to study the influence of adjustable process parameters on the breaking load of electrical insulating parts. Those insulating parts have been made by injection molding of low-viscous epoxy resins. Based on planned experiments and statistical analysis it has been concluded that the most influence on breaking load had two parameters: cavity wall temperature and reaction time. Thus, the additional aim of investigation was defined, which is to study the temperature field in molds and to establish the heat balance of molds for this procedure. Based on our own experience in this field, starting with heat exchange in molds for injection molding of thermoplastics, powder thermosets like phenolic and rubber compounds, one difference has been established. Due to long cycle time, some additional factors influence the heat exchange and temperature field in these molds.
In injection molding process, gates design is of great importance to part quality and productivity. For a certain application, gate design includes selection of the gate number, location, type, and dimension, and it is dictated by the part and mold design. Numerical simulation of the molding process is an effective means that can be used to compare different effects of various gate designs. In this paper gate location design is studied, an industrially practical example illustrate how to use numerical simulation to optimize gate location.
Different grades of PC and PC/ABS blends were molded at a wall thickness from 1.5mm to 0.5mm. A comparison was made between high-pressure, high-speed injection molding and injection-compression molding to evaluate flow length, fill pressure, and molded-in stress. In addition several factors specific to injection compression molding, such as mold gap and screw position at the start of compression were examined for their effect on flow length. It was seen that through the use of injection-compression molding longer flow lengths can be achieved and parts as thin as 0.5mm are possible.
A special tool was designed to allow the injection molding of flat disc geometry under a wide window of thermo-mechanical conditions. This can be achieved by controlled mechanical actions of one of the cavity molding walls, which is able to rotate and move axially (in steady or oscillating modes) during the filling and holding stages. The movements are assured by two servo-actuated electric motors allowing for an accurate control of a previously defined moving sequence.This injection tool was used to mold polypropylene under different thermo-mechanical set-ups, including a filling stage under cavity wall movements (in rotation, compression or expansion modes).Polarized light transmission microscopy and small angle light scattering (SALS) were used to observe and assess the moldings microstructure, including the quantification of the core spherulite size.The results evidence a very large range of microstructural patterns, whose features can be associated to the specifically imposed thermo-mechanical conditions.Furthermore, the evolutions of the microstructure along the disc radius was also assessed.
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