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.
Finite element analysis plays a crucial role in modern engineering problems, enabling engineers to predict the response of designed parts at any point in the design process. Specifying a constitutive model that accurately captures the mechanical response of a polymer material is paramount to obtaining useful results. In order to understand the capabilities of commercial FE packages used to simulate problems involving polymers, we have tested the uniaxial response of polyamide in tension and compression over six decades of strain rate. We then calibrated four constitutive models to the experimental data: an Abaqus Parallel Rheological Framework model, the LS-DYNA SAMP-1 model, the ANSYS Bergström-Boyce model, and the PolyUMod Three Network model. We compared the performance of the four models in predicting the experimental data; the Three Network model had the lowest error. Additionally, we compared the runtime of a simple test case for each model; the ANSYS Bergström-Boyce model being the fastest.
The deformation of the material during injection molding of fiber filled composites causes a process-induced change in the fiber configuration. The local fiber orientation, fiber concentration, and fiber length within the molded part varies in thickness direction and along the flow path. This heterogeneous fiber microstructure inevitably results in anisotropic and locally varying mechanical properties. This paper presents a detailed experimental analysis of the microstructure of long glass fiber reinforced polypropylene (PP) plates and its influence on the mechanical properties. Large and thin center-gated plates are injection molded with three different nominal fiber concentrations (20, 40, and 60 wt%) and an initial fiber length of 15 mm. The analysis comprises local fiber orientation, fiber concentration, and fiber length measurements conducted by means of advanced measurement techniques, including micro-computed tomography (µCT) and digital image processing. Tensile test results reveal the correlation between the process-induced fiber configuration and the mechanical properties. The results of this experimental study verify a distinct seven-layered fiber orientation pattern for industry relevant nominal fiber concentrations. Besides a nominal fiber concentration and flow length dependent reduction of the average fiber length, the measurements suggest a non-uniform fiber orientation and fiber concentration distribution through the part thickness and along the flow path. Tensile test results show that tensile modulus increases with nominal fiber concentration, whereas tensile strength does not increase above 40 wt%. The process-induced fiber configuration causes a larger degree of anisotropy of the mechanical performance in high fiber-filled components (40 wt% and 60 wt%).
Lifetime evaluation methodologies are gaining more focus and importance in the area of fiber reinforced engineering thermoplastics. Plastic applications are potentially subjected to harsh environments, where the lifetime of components can be significantly reduced. Failure due to fatigue and its consequent lifetime evaluation is particularly based on empirical studies, due to the interaction of multiple factors. The methodology proposed here based on 3D-optical techniques together with digital image correlations (DIC), establishes a generalized energy based fatigue model. The methodology is illustrated on a short fiber reinforced plastic (SFRP) specimen, typically used as a representative part in pressurized fluid applications. This fatigue model will include different influential parameters like ????-ratio, pressure, temperature, and the presence of a weldline, which are seen as critical parameters for failure. The fatigue model is developed using algorithms with surface strain energy density (SSED) that acts as a damage parameter of the component lifetime.
Plastic sheet is prestretched in a plane strain fashion as predicted by the Mooney-Rivlin constitutive equation of state. Once the plastic sheet is heated to its desired thermoforming temperature range, it is ready to be stretched into on onto the mold. The sheet is stretched differential pressure across its surface. To achieve a more uniform part wall thickness the sheet is often prestretched. Prestretching is achieved either by differential air pressure or by mechanic means. In the latter case, a shaped solid is pressed into the sheet prior to applying differential pressure. The solid, usually referred to as a plug or pusher, stretches the sheet into or over the mold, prior to the sheet touching the mold surface. After I briefly discuss the general characteristics of the stretching of thermoplastic sheet during thermoforming, I focus on the plastic sheet response during the mechanical prestretching phase of the thermoforming process.
Low-density thermoplastic foams primarily heat by volumetric absorption of incident infrared energy and are primarily formed into functional parts by shear compression in matched tooling. Thermoforming is a secondary process that follows sheet extrusion. The way in which low-density foam sheet is extruded is key to understanding the complex technical issues involved in heating and stretching the sheet into its desired product. I begin by summarizing the general desired methodology needed to produce quality low-density foam sheet. I follow this with discussion of a heating protocol. And conclude with the rationale behind forming the sheet into useful products.
I lay the groundwork for a thorough comparison of radiopaque and volumetric absorbing heat transfer models. I define the technical models for thin-gauge thermoplastic sheet through what I have called the Lumped Parameter Model (LPM) where conduction through the plastic plays no role. And I define the radiopaque and volumetric absorbing models for thick-gauge thermoplastic sheet. I call these models the Distributed Parameter Models (DPM) where conduction plays an important role in energy transfer from sheet surface to core. This is preparatory to my solving the arithmetic for these models.
This is the third of a three-part series examining the role of volumetric absorption in heating of thermoplastic sheet. In this paper I compare the traditional radiopaque distributed parameter transient one-dimensional heat conduction model (DPM) with a transient heat conduction model with internal heat generation, viz, volumetric absorption of inbound radiant energy. It is apparent from calculations for two sheet thicknesses and two plastics that the two models produce distinctly different temperature profiles. I conclude that more attention needs to be paid to the role volumetric absorption of inbound radiant energy plays in the heating of both thin and thick plastic sheet.
Camelina (Camelina sativa (L.) Crantz, family Brassicaceae) is an emerging oilseed crop which produces high oil content but has a press cake that contains glycosinolates which are potential health risks if employed as an animal feed. As an alternative to a dietaric use Camelina press cake (CAM) was employed as a filler material to fabricate lignocellulosic plastic composites (LPC). LPCs were generated by blending polypropylene (PP) with 25% or 40% CAM with 0% or 5% by weight of maleated PP (MAPP) via a twin screw compounding and injection molding. Injection molded test specimens had mechanical and flexural properties comparable to neat PP.
Bulk molding compound (BMC) compositions are characterized by a comprehensive property profile, which makes this thermoset material attractive for a wide range of high-performance applications. BMC processing by injection molding allows high production rates and the fabrication of parts with a considerable shape complexity. Although the injection molding machine offers a high reproducibility and process reliability, several effects such as material induced disturbances or changing ambient conditions may cause fluctuations in BMC injection molding. The result is a varying part quality for example in the form of volumetric filling differences which causes rejects. The manual adaption of certain process setting parameters presents a possibility to react on disturbances in order to achieve constant part properties. However, the outcome of these adjustments is dependent on the experience of the operator, since an accurate knowledge of the influence of certain setting parameters on individual part quality features is required. In this paper a correlation between process setting parameters and the part filling volume for BMC injection molding is introduced and discussed. The main aim is the development of an adaptive machine function which autonomously compensates occurring disturbances and ensures a constant part filling in BMC injection molding.
Uniform cooling plays a large role in the production of high quality plastic parts. However, simulations to help with this task are often too expensive and complex to perform in the mold’s early design phases. The literature on this topic provides various equations dealing with this issue. The so-called ‘cooling error’ presents an example of the issues surrounding uniform cooling. Some strange experiences with the equation describing the cooling error revealed the need to validate it. This paper examines the cooling error through thermal FEM simulations. The simulated results are then compared to those of the equation given in the literature. It turned out that it is necessary to re-evaluate the equation. Thereafter, the usefulness of the equation is analyzed with further thermal analysis.
A test system was designed to evaluate the failure behavior of a thin-wall small-diameter polyethylene tube under internal pressure. The test setup was capable of delivering constant (static) and cyclic (dynamic) pressure patterns as well as maintaining an elevated testing temperature to accelerate the failure. Desired pressure patterns were obtained by controlling the opening/closing duration of the solenoid valves accordingly. A water-sensing system was used to detect the failure time, particularly for small brittle failure. A data acquisition system based on LabView™ was used to control and record the applied pressures and the failure times. The constant pressure tests were performed at 65 and 75°C and the cyclic pressure tests were performed at 75°C. The test data obtained from the constant pressure tests exhibited two distinguishable linear regions in a log-log plot of hoop stress versus failure time. Slope values of -0.034 and - 0.113 were obtained for ductile and brittle regions, respectively. A brittle failure curve with slope of -0.039 was obtained under the cyclic pressure testing condition. The slow crack growth (SCG) failure was considerably accelerated by the cyclic loading.
Package integrity assurance for liquid packaging is of paramount importance to safety of packaged products. A transportation related failure mode is described as the development of pinholes in the package. ASTM F392 (Gelbo test) is often used as a screening tool for selecting materials for liquid packages. The objective of the current study is to develop an understanding of the mechanics of the film deformation during this test to help identify mechanical properties of interest. The model predicts that ‘bending rigidity’ and stiffness/toughness ratio of the film play an important role in the performance of the material in a Gelbo test based on simplified assumptions for material behavior. These predictions were verified with experimental data.
A variety of applications use flexible film for shipping solid granular materials. The film/packages must have sufficient toughness and/or strength to endure impact during shipping and handling. Package performance is commonly evaluated using a drop test typically from a certain height specified by ASTM and/or ISTA standards. One limitation of the drop test is that it can only qualitatively determine whether a package will survive. In order to better understand impact type loading which can lead to package failure, more quantitative information such as stresses/strains developed during the drop tests is required. We have developed a model to simulate drop tests of a flexible package containing solid particles. The model utilized some of the capabilities available in Abaqus software package, such as Abaqus/Explicit, Finite Element Method (FEM), and Discrete Element Method (DEM) to capture the interaction between the flexible package and a large amount of solid particles inside. Preliminary results compare well with drop tests carried out in our laboratory.
A chilled twin-screw extruder-based processing technique called solid-state shear pulverization (SSSP) explores an opportunity to create a new set of composites made from temperature-sensitive natural fibers and high-temperature melting thermoplastics. Model polyamide 6/ flax fiber composites produced with SSSP are compared to those made by conventional compounding methods. Mechanical property tests indicate that SSSP can defibrillate the flax into elementary fibers, which have superior specific mechanical properties, while retaining the fiber lengths above the critical values. SSSP can produce PA6/flax composites on an industrial scale without excessively degrading or damaging fibers.
Thermoplastic polyurethanes (TPUs) are a class of thermoplastic elastomers (TPEs) that are used in a variety of medical applications. Their characteristics including low temperature flexibility, excellent abrasion resistance, high tensile strength, good processing characteristics, and biocompatibility make them particularly attractive for medical applications by the device design engineers. In general current medical grade TPUs have the unique property of undergoing a reduction in flex modulus when placed in the body. This characteristic is referred to as “softening.” A new class of TPUs has been developed that retains many of the characteristics found attractive by designers but now offers the non-softening characteristic, broadening the design space for TPUs in medical devices. A non-softening TPU may be an attractive material for use in percutaneous interventional catheters where retention of stiffness is desired to enhance trackability and torqueability. Three grades of non-softening TPUs, covering low to mid durometers have been introduced into the market: Tecobax™ 25D, 40D, and 45D. The performance characteristics of these TPUs were measured and compared to other softening TPUs as well as non-softening TPEs. These materials maintain many of the typical TPU characteristics which make them widely used. For example, the 25D, 40D and 45D durometers have excellent tensile strengths and abrasion resistance. Softening characteristics were tested by comparing flexural modulus values at ambient conditions (22°C, 50% RH) to body conditions (100% RH, 37°C). The degree to which these new materials soften, especially in the harder grades, is significantly less than other TPUs but is comparable to non-softening TPEs. Torqueability at body conditions is also comparable to common non-softening TPEs. These materials also offer other unique characteristics which are of particular interest to the medical device community. For example, these materials have a high Vicat softening temperature; the 40D and 45D are above 100°C, resulting in compatibility with steam sterilization. In addition, these materials produce excellent quality extrusion components using standard extrusion equipment. Other unique characteristics and potential medical applications will be discussed.
Polymer Engineering Center at the University of Wisconsin-Madison has developed a particle level simulation model to predict fiber motion at industry relevant fiber concentrations. The model provides a solution to study fiber alignment for long and short fiber-reinforced thermoplastics at the particle level by accounting for all relevant effects, including fiber-fiber interactions. This simulation couples with the Reduced Strain Closure (RSC) Folgar-Tucker model to describe the orientation evolution and interaction coefficients for fibers placed in a simple shear flow. In this work, the process is outlined and the results are compared to existing models for predicting interaction coefficients. Results are then compared to those obtained through injection molding experiments of long glass fiber-reinforced polypropylene. A new relationship between fiber aspect ratio and volume fraction will then be proposed.
The shish-kebab structure has been investigated for many years and it has been widely applied in many field, while the formation mechanism is attracting researcher. In this study, different electrospun poly(e-caprolactone) (PCL) fibers were applied as shish material in the self-induced crystallization and two different self-induced crystal structure were obtained. By comparing with the surface crystalline structure, it seems that the self-induced nanohybrid shish-kebab (SINSK) structure follows a crystallographic matching mechanism in the crystallization process. The PCL fibers with different internal crystalline structure led to different induced crystal lamellae morphology. This study might helps people to screen the materials for formation of SINSK structure.
Residence time distribution (RTD) in an extruder has been studied extensively, but not many experiments have focused on the RTD in an extrusion based production process, such as a blown film line. Two inline methods on the film bubble, i.e., UV-Vis spectroscopy and optical imaging, were verified to measure the RTD in a lab scale blown film line using copper phthalocyanine tracer pulses. Both methods measured similar RTD results and can be used for research and troubleshooting of the blown film line. A full factorial design of experiments was also conducted to study the effects of rate, blow up ratio, tracer type, and tracer concentration on the measured RTD by UV-vis spectroscopy. The results showed that rate was the strongest factor for the RTD in the blown film line (as expected), blow up ratio had no effect, and tracer type and tracer concentration has some minor effects.
The relationship of shear history, morphology-microstructure and mechanical properties of the micro-scale parts was investigated based on the polypropylene parts with thickness 0.2mm and 0.5mm molded under varied injection speed. Shear rate was analyzed using Moldflow. 0.5 mm parts showed skin-core structure in the thickness direction with imperfect shish-kebab structure appeared in the transition layer between skin layer and core layer, however, the transition layer of 0.2 mm parts shows columnar crystal. The whole shear level in shear history increased with injection speed increasing for all the parts with two thicknesses. The ratio of skin layer of 0.5 mm parts decreased as the injection speed increased, which result in the decreasing of yield stress, modulus, breaking strength and elongation at break. The ratio of skin layer of 0.2 mm parts increased with injection speed increasing, and results in increasing of yield stress, modulus and breaking strength, and decreasing of elongation at break.
In this work, the prediction of final cell size of high-pressure foam injection molded parts has been attempted. An in-situ visualization technique was used to capture real-time cell growth data from high-pressure foam injection molding experiments conducted with PS and CO2. The simulated cell growth profile was compared with experimental measurements. For the PS/CO2 system, quantitative agreement (over 80%) between predicted and measured growth profile were achieved. With the validated simulation, the effect of cooling history on final cell size and cell size distribution was investigated. It is shown that in high-pressure foam injection during which all the gate nucleated cells are dissolved, final morphology is characterized by having large cells in the center and smaller cells near the skin.
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