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This paper describes the development of innovative temperature control concepts for use in additively manufactured inserts based on CO2. These have been successfully investigated for their suitability in small batch production. The additive manufacturing processes have been evaluated in terms of their suitability for the production of mold inserts. It has been possible to reduce the time required to prepare the inserts. In the investigation of suitable plastics, POM has proven to be suitable. Of the generative manufacturing processes investigated, stereolithography was found to be suitable. Robust manufacturing in the injection molding process with the other additive manufacturing processes was not possible. The manufactured components were examined with regard to their properties and compared with conventionally injection-molded components. It was found that a clear dependence on the manufacturing process of the insert used for production can be observed, especially in the crystalline microstructure of the manufactured components. This makes it difficult to use additively manufactured tool inserts in small-batch production, since the resulting properties of the components in terms of crystallinity and thus distortion are not comparable with injection-molded components. In further investigations, the minimum necessary thermal properties of the printing materials must be determined in order to ensure robust small series production with component crystallinity comparable to the injection molding process.
Aqueous polyurethane dispersions based on castor oil and lignin sulphonate (LS) were successfully synthesized in homogenous solution with no organic volatile compounds and excellent dispersion stability. Transparent thin films of PU-LS with different LS contents were obtained via solution (dispersion) cast technique. The glass transition temperatures (Tgs) of the PU-LS films were evaluated from the dynamic mechanical analysis (DMA) at 1 Hz and 2 oC/min heating rate. The Tg was found to be strongly influenced by the incorporation of the small LS content. The Tg (temperature of tand peak maximum) for PU-LS film with LS content lower than or equal 3 wt.% increases considerable with increasing the concentration of LS. For higher concentrations, no significant additional increase in the Tg was observed. The crosslink density was also calculated from the elastic modulus at a temperature of 40 oC higher than the Tg based on the rubber elasticity theory. The crosslink density increases with increasing the LS content of the thin films. The thermal-induced shape-memory effect was investigated using DMA according to cyclic thermomechanical tensile tests. The PU-LS thin film was found to have an excellent shape-memory effect and the recovery was strongly dependent on the LS content. Fast recovery (17 sec) to the permeant shape was observed once the temporary shape sample was immersed in water bath at the programming temperature.
Sean C. Kim, Esteban B. Marin, Christophe Desard, June 2022
A seamless modeling framework from injection molding simulation to anisotropic structural analysis is presented. Key features of the framework are anisotropic material modeling and fiber orientation data mapping, aspects that are facilitated by coupling Moldex3D, Digimat, and ANSYS software. The approach is exercised by modeling the mechanical response of injection molded tensile specimens with single and dual gates made of a thermoplastic resin with 20% glass fiber weight fraction. It is reassured that local fiber orientation is crucial for an accurate prediction of the mechanical strength of dual-gated tensile specimens with a weld line. Unlike the isotropic modeling approach, typical features of stress and strain concentrations along the weld line are clearly demonstrated. The capability of the approach is further highlighted by accurately predicting the break-off torque of a screw head used to adjust the seal compression in cable entry ports of optical closures.
Shardul Panwar, Royan J. D’Mello, and Umesh Gandhi, June 2022
The effects of the processing parameters on the curing of continuous carbon fiber composite made from Hexcel AS4/8552 prepreg tape are studied. A commercial process simulation finite element method, that takes in account the residual stresses due to chemical, thermal, and mechanical shrinkages, is utilized. This method solves the curing process sequentially. In the first step, the distribution of temperature and degree of cure in the composite is computed. In the second step, the information from the previous step is used to calculate the stress evolution during cure. At the end of the second step, the composite deformation due to tool removal is also calculated. The impact of three different process parameters on the final degree of cure and the residual stresses are studied in detail.
Nanocellular foam has attracted significant attention because of its superior physical and mechanical properties than microcellular foams. In this study, nanocellular foams were produced using the hot-bath and hot-press foaming methods. By lowering the saturation temperature (Tsat) to -30 ºC, the CO2 solubility was increased to 45.6%, and the cell size was reduced to less than 40 nm. Samples prepared by hot-bath exhibited smaller cell size, thinner solid skin, and transitional layer.
Vitasta Jain, James Sternberg, Mark Johnson, Srikanth Pilla, June 2022
An alternative to bisphenol A was used to synthesize polysulfones (PSs) that are chemically recyclable. Vanillin was reacted with 4-aminophenol to generate a diphenol with an imine. The synthesis of PSs is done by means of polycondensation of dibasic phenols with sulfur-containing aryl halides by the mechanism of nucleophilic substitution. The lignin based diphenol replaces traditionally used bisphenols (a xenoestrogen) and is the site for recycling the polymer. The polymerization is studied under various conditions (temperature, time, monomer ratio) for best properties and product purity. The polymer structure was confirmed via NMR and its thermal properties studied using DSC and TGA (Tg~122°C, Td5~270°C, Td10~400°C, Tprocess~180). The stability of the imine bond was studied under the reaction conditions for reactant stability.
Ultrasonic joining is a novel friction-based joining technique to produce through-the-thickness reinforced hybrid joints between surface-structured metals and unreinforced or fiber-reinforced thermoplastics. The reinforcements’ presence is responsible for improving the out-of-plane strength of the parts, enhancing their damage tolerance. The process feasibility has been successfully demonstrated to join additively manufactured (AM) metal and polymer parts. However, further investigation of its main advantages and the joining process of subcomponents to support the technique’s further development is still missing. This paper aims to demonstrate the application of U-Joining to fabricate AM 316L and PEEK hybrid structures produced via laser powder bed fusion and fused filament fabrication, respectively. Firstly, the quasi-static single lap shear performance of coupon specimens produced with optimized joining parameters was assessed. The results indicate an improvement of 2.7 times in the ultimate lap shear force and 5.9 times in the displacement – when compared to non-reinforced flat samples. Fracture surface analyses of tested samples exhibited a mixture of cohesive and adhesive failure. Further microstructural analyses at the metal-polymer interface showed micromechanical interlocking between the parts. As observed, the PEEK was able to flow and penetrate the cavities at the metallic specimen’s rough surface due to the joining friction heat input. Finally, a selected skin-stringerbracket case study was analyzed, showing the potential of AM and U-Joining to drastically reduce the structure’s weight by about 64%. To validate this idea, a scaled-down skin-stringer-bracket technology demonstrator was successfully fabricated.
The use of in-mold melt-front detecting switches were used to control the velocity-to-pressure (v/p) transfer during injection and/or to monitor the injection in a 2-cavity, hot runner valve-gated mold. The switches were connected to a data acquisition/control system either independently, in series or in parallel. When the switches were not used for v/p transfer, screw position was used. It was found that using the in-mold switches for monitoring was more effective than either peak injection pressure or cushion monitoring to sort suspect parts and alert of changes in cavity balance. When the switches were either hooked up in parallel or independently, using the first switch closed for v/p transfer, overpacking of the mold was prevented when the heater in the drop/gate of one cavity was turned off.
Functionally gradient 3d printing is of great importance for polymer composites to be applied in soft robotics or smart electronic devices. Imparting mechanical gradients within the design of new materials would help to prevent premature failure of devices and could reduce strain mismatches. In this work, we first focus on investigating the mechanical gradients and water responsive behavior of cellulose nanocrystal (CNC) / thermoplastic polyurethane (TPU) films by changing the concentration of CNCs. After generating masterbatched feedstocks, CNC/TPU films were extruded with a single screw extruder to obtain 3D printable filaments. The thermal and rheological behavior of the nanocomposite system is characterized to evaluate the mechanical property gradient of CNC/TPU filaments as a function of CNC concentration within a 3D printed geometry.
A machine learning approach based on artificial neural network is presented and applied to injection molding process. Fill time, maximum fill pressure and transient cavity pressure profiles are predicted with the input process conditions of injection speed, melt temperature and mold temperature. The physics based model using Autodesk Moldflow is evaluated by comparing it with experimental fill pressure profiles for various process conditions, and it is used to generate enough data to train and validate the machine learning model. With the present machine learning model using 400 data samples, not only the fill time but also the transient pressure profiles are accurately predicted with less than 4.7% error. Further, a new machine learning model is trained with 200 data samples, instead of 400 samples, to check the dependence of the model accuracy on the sample size, and the error in prediction of transient pressure profiles increases only to 6.7%.
Olumide Aladesiun, Kyehwan Lee, Yooseob Song, Younggil Park, June 2022
Injection molding is the process of injection molten plastic into a mold to form desired shape of part and it’s widely used process for mass production of plastics over the world. This process is not complete without the mold as it is the most critical part of the process. The cost of producing mold is huge due to manufacturing process and technique, tool material and cost of labor. The more effective the mold, the more efficient the process and the more profitable to the business. A critical factor is the cooling time, and a well-designed mold can achieve even cooling in the shortest period, which leads to increased productivity and higher quality of molded parts. In this research, an alternative core design was employed, to achieve these goals during the molding process. The core has 2 parts: the core and core insert. The core insert was produced using SLA technology to achieve the conformal cooling while the core was machined, and the deflection was studied using finite element analysis.
This paper presents a process for fitting corrected viscosity data to constituent and temperature dependent data to a range of two-equation models. The process tests different models to determine the best fit model for each. Rheometer data for polymer melts, after corrections for shear rate and entrance pressure losses, may fit one model better than another, and as such the following constituent models are reviewed in the form as they are commonly applied in commercial software today: 1) Cross Model, 2) Modified Cross Model, and 3) Carreau-Yasuda. Once the constituent model is fit, the following temperature dependent models are compared: 1) WLF, Exponential, Arrhenius, and Masuko-Magill. The differences between the models are presented in order to highlight the need to compare different models to obtain a best fit. Lastly, a solution is presented to the problem of convergent viscosities with respect to shear rate as compared across a range of temperatures as no existing model in common use today can capture this specific behavior.
After nearly 80 years of research in constitutive modeling of polymeric fluids, simple yet capable models are still sought after today. In this work, we provide an explicit constitutive equation where the extra stress tensor is an explicit function of the objective velocity gradient while finite stretch of polymer chains are considered. With this model, the basic rheological functions in uniaxial extensional, planar extension and simple shear can all be obtained as closed-form analytical solutions with only elementary mathematical functions involved. The new model demonstrates excellent fitting to some sear and extensional data in the literature, and is able to simultaneously predict the major rheological functions in steady-state shear and extension.
Tracking the cure progress of slow reacting, uncatalyzed polyurethane systems is a tedious, time consuming process that has been largely neglected due to the availability of catalysts. The use of catalysts has enabled quick, nonisothermal studies to dominate the field of research, but when catalysis is not an option, these methods become impractical. In this context, we can use chemorheology to correlate viscoelastic data to several previously developed cure models. The models presented here examine viscosity buildup, reaction rate progress, and thermodynamic behavior, while emphasizing the importance of interpretation during data analysis. These chemorheological techniques focus on the development of thermally curing networks during subjection to flow fields, and apply to a vast array of thermosetting polymeric materials.
S.J. Coombs, M.A. Kanso, K.E. Haddad, A.J. Giacomin, June 2022
The complex viscosity of planar star-branched polymers has been derived from general rigid bead-rod theory, but only for singly-beaded arms. Here, we explore the respective roles of branch functionality, arm length of non-planar arrangements, analytically from general rigid bead-rod theory. For non-planar, we include polyhedral, both regular and irregular. We fit the theory to complex viscosity measurements on polybutadiene solutions, one quadrafunctional star-branched, the other unbranched, of the same molecular weight. We learn that when general rigid bead-rod theory is applied to quadrafunctional polybutadiene, a slightly irregular center-beaded tetrahedron of interior angle 134º is required (with 1,360,000 g/gmol per bead) to describe its complex viscosity behaviour.
Michael C. Coco, Michael J. Bortner Ph.D., June 2022
Rheological testing of new material formulations can require significant quantities, specifically when considering development of new chemistries at the laboratory scale. In order to minimize the quantity of material required for evaluation, we are developing approaches suitable for characterization of high solids content formulations using micro-capillary rheometry. The goal of this investigation is to design and produce a micro-capillary rheometer capable of characterizing basic rheological properties, such as viscosity and shear-thinning behavior, while requiring the least amount of sample possible. In our current design, we implement a micro-dispensing approach combined with calibrated force transducers. With this approach we can further elucidate an understanding of the differences between typical capillary rheometry and behavior at reduced dimension flow fields. Issues such as pressure relaxation and free volume compaction can therefore be studied through readily modified geometries and testing rates. This design will lead to a better understanding of micro-capillary rheometer design and enable a unique approach for rheology measurements for new chemistries and formulations, including high solids content formulations (up to 60+ vol%). Additionally, this framework will facilitate the study of a variety of flow geometries applicable to a wide range of applications including precision dispensing of adhesives and sealants, and direct ink write additive manufacturing.
Karun Kalia, Benjamin Francoeur, Alireza Amirkhizi, Amir Ameli, June 2022
The purpose of this study was to investigate the feasibility of in-situ foaming in fused filament fabrication (FFF) process. Development of unexpanded filaments loaded with thermally expandable microspheres, TEM is reported as a feedstock for in-situ foam printing. Four different material compositions, i.e., two grades of polylactic acid, PLA, and two plasticizers (polyethylene glycol, PEG, and triethyl citrate, TEC) were examined. PLA, TEM and plasticizer were dry blended and fed into the extruder. The filaments were then extruded at the lowest possible barrel temperatures, collected by a filament winder, and used for FFF printing process. The results showed that PLA Ingeo 4043D (MFR=6 g/10min) provides a more favorable temperature window for the suppression of TEM expansion during extrusion process, compared to PLA Ingeo 3052D (MFR=14 g/10min). TEC plasticizer was also found to effectively lower the process temperatures without adversely interacting with the TEM particles. Consequently, unexpanded filaments of PLA4043D/TEM5%/TEC2% was successfully fabricated with a density value of 1.16 g/cm3, which is only ~4.5% lower than the theoretical density value. The in-situ foaming in FFF process was then successfully demonstrated. The printed foams revealed a uniform cellular structure, reproducible dimensions, as well as less print marks on the surface, compared to the solid counterparts.
For several decades, the Tait model has been used in simulation software to describe the volumetric mechanical behavior of thermoplastic polymers as they cool. It is used to compute the residual strains and stresses of the polymer as it solidifies, but there is a problem. Many data sets have coefficients where there exists a discontinuity at the transition between the molten and solid domains. This paper outlines some basic checks that can be done to detect this problem and a procedure to fit the coefficients to data so that this problem does not arise.
Aliya J. Kaplan, Bradley P. Sutliff, Michael J. Bortner, June 2022
Nanofibrillated cellulose (NFC) has properties ideal for applications in the packaging and medical industries. To understand if cellulose-based polymers could become a replacement for synthetic polymers in these fields, NFC suspensions were repeatedly exposed to elevated shear stresses to simulate industrial processing procedures and allow for observation of changes in material properties. A capillary rheometer was used to run aqueous NFC suspensions of 10 wt% at room temperature at shear rates beyond 30,000 s-1. Due to repeated shear rate exposure, a decrease in volume resulting from unavoidable water loss informed the observable increase in apparent viscosity and suggested that this increasing trend was not caused by a change in material morphology. Noisy data as a result of flocs was detrimental to the analysis of material behavior during rheological testing. Once preprocessing procedures are successfully designed to reduce noise in the data, material behavior at high shear rates will be further defined.
Myung-Ho Kim, JaeSik Hyun, InSu Seol , Sunwoong Choi, June 2022
The shear rate-dependent viscosity of natural rubber and three types of synthetic rubber was measured using the Rubber Screw Rheometer. Viscosity values with Mooney viscometer, which has traditionally measured rubber viscosity, have a high correlation with the values of RSR shear rate 10 [1/s]. Thus the Mooney Viscosity value can be estimated using the RSR shear viscosity measurement. Also, in the case of virgin rubber, the accuracy of the measured value increases when it has a pre-shear history. It was confirmed that the viscosity measurement value was a measurement value having a deviation within +3% when comparing the three times repeated measurements. The measured value was correlated to Mooney Viscosity successfully with a first- order equation.
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