Numerical And Experimental Investigation On The Powder Spreading And Binder Penetration Process In Binder Jetting

Binder Jetting Additive Manufacturing(BJAM)offers the advantages of high production rate,no need for support,a wide selection of materials,and occurring at room temperature and atmosphere.However,the widely used metal salt binder or organic binder needs to be heated before the powder removal,which will reduce the production rate and dimensional accuracy.These problems can be solved by photocurable binders,which will be cured under UV light.Powder spreading process and binder penetration process have great influence on the dimensional accuracy of printed parts in photocurable BJAM.In this paper,the influence of powder spreading and binder penetration process on printing accuracy in UV curing BJAM was studied by simulation and experiment.The main research contents and achievements are as follows:1.Numerical simulations of powder spreading.The physical and mathematical models of powder spreading in photocurable BJAM were established by using the discrete element method.The effects of powder spreading method,powder surface energy,roller speed and roller traverse speed on the forming quality and accuracy were studied.It was found that roller spreading is better than blade spreading;excessive powder flowability enlarges green part displacement;the printing quality is better only when roller speed and roller transverse speed are moderate.2.Experimental verification of powder spreading simulation.The influence of powder surface energy,roller speed and roller traverse speed on the quality of powder spreading and the displacement of printed part were verified by experimental method,and the experimental results were in consistent with the simulation results.Then two types of powder with different flowability were used to print small cubes,which further verified that too high flowability of powder would lead to displacement,and it was found that displacement was mainly concentrated in the first 10 layers.3.Numerical simulation of binder droplet impact and penetration into powder bed.Based on the assumption that the powder particles are fixed and the VOF model,the three-dimensional physical model and mathematical model of binder droplet penetration into the powder bed were established.The effects of particle size,droplet falling velocity and contact angle on the spreading and penetration process were studied.It was found that smaller particle size is not always better for higher dimensional accuracy;droplet falling velocity has large influence on the lateral spreading but little influence on the vertical penetration;too good wettability may lead to the entrapped gas bubbles,reducing the accuracy and mechanical properties of green parts.At the same time,the mechanism of micron droplets impact and penetration into the powder bed was modified,and the whole process can be divided into impact-driving stage(first0.5ms)and capillary-driving stage(after 0.5ms).4.Experimental verification of the influence of particle size on binder penetration.Two types of 316 L stainless steel powder with different particle size were used to print the same samples,finding that samples manufactured by smaller particles have a larger width and smaller layer thickness,which verifies the numerical simulation results of particle size and demonstrates that smaller particle size is not always better for higher dimensional accuracy and mechanical properties.