Changing the Design Paradigm – 3D Printing on Molecular Texture in Plastics

MIT PIs:
Gregory C. Rutledge – Lammot du Pont Professor Department of Chemical Engineering

PT PIs:
Geoffrey Mitchell – Polytechnic Institute of Leiria, Centre for Rapid and Sustainable Product Development

Abstract: 3d printing has evolved from a prototyping technology to a viable manufacturing methodology. It offers a rapid pathway from concept to product and its particularly suited to applications where massive personalisation is required. In some areas it has enabled new shapes to be produced not possible with conventional manufacturing technology. Moreover the approach is waste free in that only the materials required to produce a part is used. Despite this freedom of geometry the approach is rather overfocused on replicating shapes rather than exploring new approaches.

This is the focus of this project, in which will explore the scope for depositing materials with different properties by controlling the structure and morphology of the polymer. It is well established that a high molecular weight polymer when extruded through a die with appropriate length to bore ratio can lead to the formation of a preferred orientation in the melt which when the temperature is lowered leads to the formation of a highly anisotropic crystalline morphology which exhibits quite different mechanical properties to the morphology which forms from a quiescent melt. Typically the variation in modulus will be in the range 2x to 3x. Now that enhancement in properties lies along the line of depositing the material on the build surface. We wish to exploit the opportunities for depositing in 3d so as to enable new designs and new products. 3d printing is often referred to as a layer by layer process and in the vast majority of cases the initial design is sliced in to a series of co planar layers. This is not a limitation of 3d printing, indeed the only limitation for the nature of the slicing in arbitrary shaped non planar layers is that they do not intersect. Several papers have highlighted the use of non-planar layers to lead to an enhanced surface finish, however we have identified another major advantage of the use of non-planar layers. We will give a specific example – let us consider a cylindrical object. The major stresses in such an object may lie along the length of the cylinder as for a supporting pillar, or around the edge of a cylinder as in the case of barrel. In traditional manufacturing of wooden barrels, metal hoops are placed around the circumference to prevent the barrel bursting. If we slice the cylindrical object in to a series of cylindrical surfaces we have the option of depositing lines of stiffer anisotropic material along the length or around cylinder. This methodology immediately allows the stiffer or stronger material to be deposited or printed where it is required and aligned with the principal stress directions. Such an approach will lead to reduced volume of material to deliver specific properties and will released 3d printing from the constraints of coplanar slices and thereby enable new design as well as new products.

The focus of this project is to explore these concepts and to generate a roadmap for the development of the new technology which emerges to include both hardware and software for control of printers and the optimisation of design. We will build on previous work by utilising an instrumented industrial scale pellet fed extruder which will facilitate the use of a wide/range of polymers including liquid crystal polymers, which exhibit a high level of flow induced anisotropy. By using this instrumented extruder we will identify the range of conditions which lead to the formation of solid material with an enhanced stiffness and anisotropy performing in-situ experiments with the extruder at the ALBA Synchrotron Light Source in Barcelona. We will use the output of multiscale modelling of the process at MIT to provide an understanding of the phenomena and to identify other adaptions such as localized temperature control of the extruded plastic before deposition. . Equally we will identify the conditions required to deposit softer isotropic material. We will explore the length to bore diameter of the extruder die and its use to optimise both the conditions to switch between isotropic and anisotropic material deposition and the build resolution and build speed. We will construct a prototype printer using this industrial scale pellet fed extruder to be able to print objects sliced with arbitrary shaped non-planar layers and adapt existing software to drive this printer. We will adapt existing slicing software to enable cad designs to be sliced with non-planar arbitrary shaped functions. We will identify some case studies of objects and print these using standard planar layers and compare the properties to parts prepared using the controlled deposition of anisotropic stiffer material in particular directions using non-planar slices. If time permits and the initial extruder results are promising we will explore the printing of what will be effectively molecular composites with the high performance struts printed using a liquid crystal polymer and in fill with a conventional thermoplastic.