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.
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Effect of Injection Speed on Gas Penetration Length, Residual Wall Thickness and the Melt Front Position during Gas-Assisted Injection Molding
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.
Transient Gas/Melt Interface and Gas Penetration during Gas-Assisted Injection Molding: Simulation and Experiment
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.
On the Breakup of a Non-Newtonian Drop in an Extensional Flow
The condition for the breakup, of a power-law non-Newtonian slender drop in a Newtonian liquid in an axisymmetric extensional flow, has been theoretically studied. The problem is governed by four dimensionless numbers: The capillary number, the Reynolds number, the viscosity ratio and the power-law index. The results suggest that the critical capillary number for drop breakup increases as the Reynolds number, the viscosity ratio and the power-law index decrease.
Molecular Dynamics Simulation of Nano-Scale Polymeric Rheological Properties and Extrusion Flows
In this work the rheological properties of polymer have been studied by molecular dynamics simulation. Couette flow with various shear rates are used to investigate the degree of slip, shear viscosity and normal stress difference. The fluid consists of chains of n-hexadecane and is confined between two structured gold atomic walls. Isothermal simulations (350K) of 4 to 1 unsteady extrusion flow with various extrusion rates are conducted.
Optimization of a Flat Die Geometry
Geometry of a flat die for polymer sheet extrusion is optimized to obtain a uniform velocity distribution across the exit of the die. While optimizing the exit velocity distribution, the constraint optimization algorithm used in this work enforced a limit on the maximum allowable pressure drop in the die. Effect of the shear as well as elongational viscosity of the polymer on the flow in the flat die is taken into account.
The Effect of Stabiliser Type and TiO2 Concentration on the Rheology of uPVC Profile Formulations
A range of unplasticised polyvinylchloride (uPVC) profile extrusion grade formulations, containing calcium/zinc, organotin and lead based stabiliser systems were blended with different concentrations of TiO2. Rheological analysis showed that the concentration of TiO2 (2phr - 8phr) had little effect on viscosity over the shear rate (200-1000s-1) and temperature range (170 – 190°C) studied. Mechanical analysis showed higher tensile and flexural modulii for the organotin stabilised formulations.
Setup and Optimisation of the Gas Assisted Injection Moulding Process Using an Expert System
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.
Three-Dimensional Simulation of Gas-Assisted Injection Molding Process
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.
Comparative Performance of a H13 and Beryllium-Copper Core Caps in a Thin-Wall Injection Molding Application
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.
A General Method of Designing Injection Molds by Straightforward Solution Procedures
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.
Thermal Performance of Hybrid Injection Moulds with Epoxy Inserts
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.
Medical Grade Copolyesters for Profile Extrusion
Resins used in profile extrusion require high viscosity at low shear rates to improve melt strength and low viscosity at high shear rates to prevent melt fracture. This paper discusses the development of copolyester resins with the desirable rheological properties, as well as good optical and physical properties. The processing, biocompatibility, and sterilization of these resins will also be discussed.
Modeling Polymer Balloons for Angioplasty: From Fabrication to Deployment
Postulating that arterial injury resulting from the angioplasty intervention is a possible predictor of restenosis, a three-dimensional finite element model is proposed to predict stresses during balloon angioplasty. The model simulates balloon folding, insertion and deployment into a diseased artery. This work focuses on the balloon material model and properties, using experimental characterization and inverse modelling. A numerical example, including balloon folding and deployment inside a stenosed artery is also presented.
The Design of the Small Punch Test and its Application to Testing Medical Polymers
The small punch test is a useful technique in the mechanical testing of polymers where limited material is available. This investigation focuses on the latest developments in the small punch test design, including integrated temperature control and environmental conditioning and its use in analysis of polymethyl methacrylate (PMMA) bone cement.
Plastic Medical Enclosures Made without Molds
It has never been easy, in the Medical Products Industry to design and build a custom plastic enclosure when the initial or lifetime quantities do not justify molds or tooling. In the last few years, a toolless technology has been commercialized to allow the manufacture of such enclosures, with minimal up front costs and broad design flexibility. This paper describes the technology, its application, strengths and limitations and provides an economic comparison to the other enclosure technologies used in the industry.
Sequential Injection Molding Using Fast-Response Valve Gate System
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 Advanced Cavity/Core System Mold for Ultra-Low Pressure Injection Molding-“ULPAC Mold”
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.
Heat Exchange in Molds for Injection Molding of Low-Viscous Epoxy Resins
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.
The Optimized Design for Gates Location of Injection Molds Based on Filling Simulation and Industry Application
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.
Thin Wall Molding: Achieving Longer Flow Lengths and Decreased Internal Stress with Injection-Compression Molding
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.
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