The Bond 3D printing technology is an extrusion process. Bulk material is fed to the print head where it is heated to around 400 °C, upon which the molten material is pushed through the print nozzle and deposited on the print table, then subsequently onto the previously deposited layers.
The conventional extrusion process proceeds in a flow-controlled manner: material is fed at a rate that is proportional to the distance the nozzle travels. Inevitably, there are errors. For example, variations in the filament diameter and fluctuations in the temperature of the compressible molten material will affect the flow rate of the highly viscous material through the nozzle. As a result, perfect extrusion cannot be achieved; either more or less material than required will be deposited and the distance between the print nozzle and the top layer of the print will vary. This may lead to an inhomogeneous distribution of the printed material, by either under- or overextrusion (Figure 3). In the latter case, the moving print head causes a kind of bow wave and ‘ploughs’ through the deposited, solidifying material (solidification only takes seconds, so interaction forces will be high).
Overextrusion results in the accumulation of residual material at the side of the nozzle, which will quickly degrade under the influence of the hot print head, as polymers can only withstand temperatures beyond their melting point for a short time. The degraded material might flake off in black specks and contaminate the printed product, deteriorating its aesthetic and mechanical properties. Moreover, the increasing forces between the ‘ploughing’ head and the product being printed may eventually lead to the print being ripped off the build plate. The properties of the build plate (for example steel) are chosen as a compromise regarding the adhesion force with the printing material; i.e., high enough to hold the print, but low enough to facilitate removal of the finished product from the build table.
As overextrusion can end disastrously, underextrusion is generally preferred and the process control is set accordingly. This, however, creates voids in the printed product (Figure 3a). Bond 3D’s solution is pressurecontrolled printing (Figure 4), where the molten material flows to completely fill the gap between previously printed lines; material flow continues until the pressure in the ‘melt pool’ below the print nozzle starts to exceed the setpoint, indicating that no more material can be added. Then the gap has been filled and adjoining lines have bonded completely – this is what Bond 3D calls ‘bondability’ from where the company derives its name.
The pressure can be derived from the upward force exerted on the print nozzle at the bottom of the print head by the printed material in the melt pool below. As a first implementation, this force was derived from the motor current required for driving the extruder that delivers print material to the nozzle. This motor current, however, provides a rather noisy signal, so filtering is required, which makes feedback control slow. As an accurate highbandwidth control alternative, Bond 3D designed a flexurebased, friction- and hysteresis-free mounting of the print head on the gantry to which sensitive force sensors are attached; Figure 5 shows a schematic design.