Differential Shrink

It is common to tilt/ angle geometry to reduce the surface area per-layer in order to limit in-print curl in acrylate-based materials.  Rotating the part about the X and/or Y axis is a common way to reduce the differential shrink effects.  Sometimes, the support structure allow for too much flex resulting in worse differential shrink effects. A best practice is to always consider uniform wall thickness as a goal similar to injection molding best practices.

In the images below, Part A was tilted at 45 degrees to reduce support attachment points on the bottom surfaces.  This orientation allowed for some movement early on as the part grew in Z.  The smaller layers were able to shrink in the Build Plane (X/Y plane) more than the larger cross-sections later in the part due to a base of material already present for the larger cross section to grow from.  The left and bottom edges are not straight or square to each other as designed. 

Part B was printed with the first flat surface drawn in one layer.  This orientation produced a better foundation for the rest of the layers to build on.  It measured much closer to the original geometry and produced straighter edges that are perpendicular to each other.


Figure # 6. Parts A and B orientation effects on differential shrink


The next example shows two halves of a drill housing rotated about the X and Y axis to place the curves in as many layers as possible (minimize stair-stepping) and reduce the number of attachment points needed to support the part. This leads to good part quality, less support scarring, and easier to finish.

The highlighted edges of both halves in image A below are designed to be in a straight line.  Differential shrink and maybe some in-print movement caused this surface to print at an angle.  The bottom of the two parts are not planer in image B.  More support attachment points are needed to stabilize the part or a different orientation is needed to change the cross section of the part as it grows.


Figure # 7. Examples of minimal support and orientation on organic part quality


The images below show three examples of parts that show the effect that differential shrink has in the transition from thin-wall to larger cross section.


Figure # 8. Examples of differential shrink due to minimal support and orientation on organic part quality


In addressing differential shrink, consider the tradeoffs per orientation. For visualization of surfaces with less finishing, the tilted orientation is often the better option.  When accuracy is critical, be careful tilting the part at an angle less than 60 degrees and properly add more than normal supports or anchor points.  Another compromise is to orient the part flat (will see more curl), tilt it at a higher angle (easier to support) or even stand it vertically (be careful about sufficient supports and sweep velocity) to minimize differential shrink without sacrificing part accuracy and quality.