Large-scale reactive extrusion deposition of sparse infill structures with solid perimeters

Large-scale additive manufacturing of reactive polymer systems offers significant improvements over thermoplastics through improved mechanical properties, faster deposition rates, and deposition at ambient temperatures. At ambient temperatures, the resin exists as a viscoelastic liquid which require additional additives for improved rigidity and bead stability. These filler-reinforced resins, or yield stress fluids, are soft solid-like materials able to withstand compressive forces from successive layer deposition until curing leads to bead solidification. When bead deposition is unsupported by previous layers, as is the case when bridging a gap, the creep due to gravitational loading can lead to bead instabilities which result in excessive sagging or bead breakage. These bead instabilities are material dependent and susceptible to changes in filler composition and temperature rises due to exothermic reactions. This work implements a design solution for bridging sparse infill patterns in additively manufactured parts which are essential in the reduction of weight. A series of span tests over a range of distances, as multiples of nozzle diameter, were conducted to determine the critical distance where a bead would break. Additional layer depositions determined the number of layers needed to recover from sagging effects. This recovery was essential in the transition of sparse infill to solid infill printing, where solid infill is milled for a smooth surface finish. Without the capability of sparse-solid transition, additively manufactured parts would require further surface treatments such as spray coating, adding to cost and increasing production time. A standard domed-mold geometry is presented to show the success of this printing process.