3DPS Feature Generation in the YZ and XY Planes
Three-dimensional sand printing is not exempt from the same limitations as other additive manufacturing methods. The build orientation, properly exported geometry, and part placement is critical to the success of any additive project. KIP WOODS, UNIVERSITY OF NORTHERN IOWA (CEDAR FALLS, IOWA)
T
hree-dimensional sand printing (3DSP) is the process of selectively bond- ing sand aggregates together
utilizing the same resin systems as those present in modern foundries. One major difference from traditional sand production methods is that the sand and resin are applied at a defined layer thickness based upon the manufacturer specifications of the sand printer. Te beginning of any additive manufactur- ing project begins with the computer aided drafting (CAD) files. Before the CAD file can be printed it needs to be in a stereolithography (STL) file format. Te STL is imported into the manufac- turer’s software to produce the common layer interface (CLI) file that will enable the printer to produce the desired final shape. Te flow of this process is shown in Figure 1. Te process of converting a CAD
file to STL file will reduce the part accuracy related to the cusp height and layer thickness as depicted in Figure 2. Te cusp height is a value selected by the operator to control file size. Te process of creating the CLI file
will additionally create errors based upon the method of slicing being utilized by the software. Figure 3 shows the three most widely utilized methods of slicing geometry.
36 | MODERN CASTING June 2017
Fig. 3. Three popular slicing methods for CLI creation are used by the industry.
Several trials were conducted on
Fig. 1. This flow chart shows the intermediate steps from digital file to the sand printer for 3DSP.
Fig. 2. This image explains the cusp height of the tessellated STL surface compared to the original CAD model.
printed test geometry to better under- stand the feature generation capa- bilities of 3-D sand printers. Te test geometry consisted of rectangles with through holes. Te diameters of the holes tested were 3/8-, 1/2-, 3/4 and 1-in. Te geometry under investigation was printed in two different orienta- tions, the YZ and XY plane. Te circles were evaluated on the accuracy, preci- sion, and roundness. Te circles printed in the XY plane experienced a lower amount of deviations when compared to the circles printed in the YZ plane. Te improved feature generation in the XY plane was attributed to the greater print head resolution on the X and Y axis compared to the Z axis. Te YZ samples did experience a flat defect at the start and finish layer of the through holes. Tis defect averaged to be 28% smaller in the printed samples than within the CLI file.
Materials and Methodology Test trials were conducted using
the University of Northern Iowa sand printer. Te silica sand used for the trials was a round grain. Tis silica sand is a three-screen sand with over 10% reten- tion at the 70, 100 and 140 meshes and an overall grain fineness number of 82 (Figure 4). Te printed layer thickness
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