Research in Dr. Mackay’s group concerns materials processing and the structures developed from processing effects. Most of his research centers on processing polymers using fused filament fabrication (FFF), a type of additive manufacturing or 3D printing. Our goal is to develop new materials and processing technologies to make strong products with this new polymer processing technology.
We use sophisticated characterization tools such as small angle neutron scattering (SANS), a technique we have many years of experience using, to understand the structure-property relationship. This technique requires us to use deuterated polymers to measure the radius of gyration. Deuterated polystyrene is blended with protonated polystyrene using a twin screw extruder and we make the filament for FFF. After FFF, we determine the radius of gyration using SANS with the aim of determining how oriented the molecules are in the product and then relate this to its strength. We are interested in generating highly oriented polymers using FFF to improve the product strength. To do this we make new polymers and polymer blends for FFF and optimize the “hot end” of our Taz 3D printers.
Our lab also utilizes rheometry to characterize materials used in FFF. This tool helps us understand why current materials are particularly good in FFF and to design new materials that will have good printing performance. Other tools that we use regularly include dynamic scanning calorimetry (DSC) and thermogravimentric analysis (TGA).
The weld strength between filaments is another aspect of FFF that dictates printed product strength. We have developed heat transfer models to predict the temperature of deposited filaments and relate this to molecular diffusion at the interface and subsequently to the weld strength. More recent work involves the use of new chemistries to form strong covalent bonds between deposited layers, resulting in increased product strength. Other work includes designing new printer heads that can print multiple materials simultaneously to take advantage of both materials’ properties.
In summary, we are using sophisticated techniques to optimize a new processing technique to make the strongest products possible in the shortest amount of processing time.
Computational fluid dynamics simulation of the melting process in the fused filament fabrication additive manufacturing technique
Numerical simulation is used to understand the melting and pressurization mechanism in fused filament fabrication (FFF). The results show the incoming fiber melts axisymmetrically, forming a cone of unmelted material in the center surrounded by melted polymer. Details of the simulation reveal that a recirculating vortex of melted polymer is formed at the fiber entrance to the hot end. The large viscosity within this vortex acts to effectively seal the system against back-pressures of order 1000 psi (10 MPa), which are typical under standard printing conditions. The Generalized Newtonian Fluid (GNF) model was appropriate for simulation within the region that melts the fiber, however, a viscoelastic model, the Phan-Thien-Tanner (PTT) model, was required to capture flow within the nozzle. This is due to the presence of an elongational flow as molten material transitions from the melting region (diameter of 3 mm) to the nozzle at the exit (diameter of 0.5 mm). Remarkably, almost half the pressure drop occurs over the short capillary (0.5 mm in length) attached to the end of the converging flow region. Increased manufacturing rates are limited by high pressures, necessitating more consideration in the nozzle design of future FFF printers.
Nonisothermal Welding in Fused Filament Fabrication
Fused filament fabrication (FFF), sometimes called material extrusion (ME) offers an alternative option to traditional polymer manufacturing techniques to allow the fabrication of objects without the need of a mold or template. However, these parts are limited in the degree to which the welding interface is eliminated post deposition, resulting in a decrease in the interlaminar fracture toughness relative to the bulk material. Here reptation theory under nonisothermal conditions is utilized to predict the development of healing over time, from the rheological and thermal properties of Acrylonitrile-Butadiene-Styrene (ABS). ABS is rheologically complex and acts as a gel and as such considerations had to be made for the relaxation time of the matrix which is important in predicting the degree of interfacial healing. The nonsiothermal healing model developed is then successfully compared to experimental interlaminar fracture experiments at variable printing temperatures, allowing future optimization of the process to make stronger parts.
Maximal 3D printing extrusion rates
Many applications of 3D printing are enhanced by increased printing speed. In the hot end of a 3D printer, the polymer feed stock flows in a heated cylinder at a set temperature. Since the polymer must be hot enough to reach a pliant state before extrusion, this establishes a maximum velocity beyond which the polymer is too rigid to be extruded. A mathematical model is presented for this system, and both amorphous and crystalline polymer systems are examined. The former is a heat transfer problem; the latter is a Stefan problem. Several different conditions for establishing the maximum velocity are considered; using the average polymer temperature in the hot end matches well with experimental data.
The importance of rheological behavior in the additive manufacturing technique material extrusion
Material extrusion (ME), sometimes called Fused Deposition Modeling® or Fused Filament Fabrication, is an additive manufacturing technique that places order 300 μm diameter molten polymer filaments sequentially onto a moving substrate to build an object. The feed material is a solid fiber that acts like a continuous piston in a heated barrel, which plasticates itself to push molten material through a nozzle. The barrel pressure is substantial, of order 30 MPa (4000 psi), and similar to that developed in contemporary polymer processing. The similarity does not end here with all the non-Newtonian and viscoelastic effects and heat transfer limitations that challenge extrusion operations coming to bear in the ME. These will be discussed in this review with suggestions of areas of study.
Rheological and heat transfer effects in fused filament fabrication
The fused filament fabrication (FFF) process is similar to classic extrusion operations; solid polymer is melted, pressurized, and extruded to produce an object. At this level of investigation, it appears no new science or engineering is required. However, FFF has heat transfer limitations that are unique to it, due to its small throughput, not encountered in contemporary polymer processing, negating the use of present-day correlations or heuristics. Here, we quantify heat transfer by rheological modeling of the pressure drop data in the process to generate a general Nusselt number–Graetz number correlation. This is the first time the pressure has been measured in the die (nozzle) during normal printing that we accomplished by monitoring the power used to drive the hot end. Ultimately, we find that fouling within the region used to melt/soften the polymer significantly reduces the heat transfer rate.
Increased fracture toughness of additively manufactured amorphous thermoplastics via thermal annealing
Polymeric structures fabricated using Fused Filament Fabrication (FFF) suffer from poor inter-laminar fracture toughness. As a result, these materials exhibit only a fraction of the mechanical performanceof those manufactured through more traditional means. Here we show that thermal annealing ofconfined structures manufactured using the FFF technique dramatically increases their inter-laminartoughness. Single Edge Notch Bend (SENB) fracture specimens made from acrylonitrile-butadiene-styrene (ABS) feedstock were isothermally heated in a supportingfixture, post-manufacture, across arange of times and temperatures. Fracture testing was then used to quantify the changes in inter-laminartoughness offered by annealing through measurements of the Mode I critical elastic-plastic strain energyrelease rate. Under the most aggressive annealing conditions, the inter-laminar toughness increasedby more than 2700% over the non-annealed baseline material. Void migration and aggregation duringthe annealing process was analyzed using X-ray tomography and provides insight into the tougheningmechanisms. Time-scales of reptation and polymer mobility at the interface during annealing are alsomodeled and agree with fracture experiments.
The performance of the hot end in a plasticating 3D printer
The failure (maximum) feed velocity in a LulzBot Taz 4 3D printer at various temperatures is determined for three polymers: Acrylonitrile butadiene styrene, poly(lactic acid) (PLA), and a PLA polyhydroxybutyrate copolymer. Through an approximate solution of the energy balance, we develop a model to correlate the dimensionless fiber feed velocity (represented by a Peclet number) with a dimensionless temperature. Using these dimensionless parameters, all polymers fall onto the same curve. However, when molten polymer is forced through a small nozzle to enable 3D printing, this curve also depends on another parameter: Nozzle diameter. Our model does not account for this parameter because it does not consider hydrodynamics due to the complexity of the coupled energy and momentum balances. Thus, we modify the Peclet number to account for hydrodynamics and produce a satisfactory master curve for all diameters and polymers. Our dimensionless numbers require determining the polymer thermal and rheological properties as well as the minimum possible temperature that can be used for 3D printing of any given polymer. We discuss a way to predict this temperature based on the entry pressure drop into the nozzle. Our results will enable designers and engineers to modify the extrusion die and polymer in order to obtain better 3D printed items, and these findings can be generalized to other 3D printers.