Feasibility Study of an Extrusion-based Direct Metal Additive Manufacturing Technique
Authors: Massimiliano Annoni, Hermes Giberti, Matteo Strano
Keywords: Direct metal deposition, Fused deposition modeling, Metal injection molding, Parallel kinematics
Purpose: This paper describes the feasibility study of a new additive manufacturing (AM) technique based on extrusion of a feedstock made of metallic (or ceramic) particles and a polymeric binder.
Design, benefits and limitations: The main innovations introduced with the extrusion-based AM system presented in this paper are:
-Parallel kinematics work table:
An extrusion-based system equipped with a parallel kinematics 5-axis work table is proposed in this paper, which is unprecedented, at least to the authors’ knowledge. The motivations for this innovative design are: x The extrusion head is stationary, which is an advantage in terms of positioning accuracy since the head is heavy (about 25 kg) and powerful (power = 3 kW; maximum injection pressure = 134 MPa); x Parallel kinematics allows high positioning accuracy of the TCP (tool center point); x The 5-axis work table orientation allows a better surface quality and limits the need for workpiece supports during the deposition process.
-Extrusion Head:
In this Extrusion Head a plasticizing screw, positioned at 45° with respect to the vertical direction, continuously rotates and loads the injection cylinder with heated feedstock. A vertical piston moves and pressurizes the heated feedstock through the injection nozzle. The diameter of the piston is 14 mm. This configuration allows for continuity and stability of the extrusion process and large extrusion force, i.e. the possibility of extruding highly viscous fluids through small nozzle exit diameters. The injection pressure on the heated material upstream the nozzle can be set at a very high value, up to 140 MPa. The maximum amount of material that can be deposited during one single piston stroke is 9000 mm3 . If a 0.6 mm diameter wire is extruded, an approximate wire length of 32000 mm can be deposited before recharging the injection cylinder.
-Extrusion nozzles:
The nozzle design is even more important in the proposed system than in conventional FDM due to the high pressure required by the extrusion (from 10 to 140 MPa). Three different nozzle designs have been prepared and tested (Figure 5): x Nozzle A: convergent-divergent profile, used in injection molding, suited for relatively large wire diameters and aimed at minimizing the pressure drops inside the nozzle; x Nozzle B: convergent-parallel profile, suited for medium wire diameters, aimed at stabilizing the direction of the outgoing viscous flow. The calibrated part of the nozzle makes it suitable for controlling the wire diameter; x Nozzle C: convergent-stepped profile, suited for small wire diameters, aimed at reducing the wire diameter while maintaining low pressure drops. Since the calibrated part of the nozzle is shorter that the nozzle B one, swelling is more likely to occur.
-Extruded materials:
Two types of commercial MIM feedstock based on AISI 630 stainless steel powders (nominal median particle size = 13 Pm) and zirconia (nominal median particle size = 0.6 Pm) were preliminary tested.
-Hybrid manufacturing :
The configuration of the presented AM machine is suitable for hosting also a milling head on board of the top plate.
Findings: The test described in this paper allowed to screen out some preliminary designs for extrusion nozzles and to verify the role of the binder percentage on the process capability. The possibility to bond the deposited wires after sintering was demonstrated on AISI 630, while zirconia seems to need a higher binder percentage to adhere during the deposition. Obtained material density is good, even if porosity exists, mainly for AISI 630 when sintered in argon instead of hydrogen. Future studies will be devoted to fully develop the proposed process/system.
Link: https://www.sciencedirect.com/science/article/pii/S2351978916300919
Full Reference: ANNONI, Massimiliano; GIBERTI, Hermes; STRANO, Matteo. Feasibility study of an extrusion-based direct metal additive manufacturing technique. Procedia Manufacturing, 2016, 5: 916-927.
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