When printing high-end parts that have to conform to tight tolerances, compensation for shrink is required. This is, however, not just a straightforward overdimensioning of the original design; to compensate for warpage, for example, the shape has to be corrected in the opposite direction. The compensation has to be included in the standard printing procedure of designing, slicing, determining toolpaths, and printing the part, as illustrated in Figure 4. Given the freeform nature of 3D printing, this compensation can best be determined using finite-element model (FEM) simulations including mechanical and thermal effects. These simulations can provide the input for modifying the 3D shape (geometry) of the part to compensate for its shrink after printing.
Figure 4: Standard printing procedure:
a) The 3D model (design) of the product is created.
b) The slicer partitions the model into layers, or slices, that are to be printed on top of one another; each layer is a set of 2D shapes.
c) The slicer determines the toolpaths that need to be printed (similar to the toolpaths for other CNC machines) and the amount of material to be extruded.
d) The printer executes the print run using the toolpath data (gCode).
e) The 3D part is produced.
In the traditional approach of compensation (Figure 5), the complete part is subjected to a thermomechanical FEM simulation in each iteration step. The resulting new geometry is passed through the slicer again. With curved surfaces due to warpage, the slicing may lead to unexpected results, such as the emergence of the well-known staircase effect in designs that should not suffer from this phenomenon (Scheme 1).
Figure 5: Traditional shrink-compensation approach in the printing procedure.
Scheme 1. Staircase effect emerging from the traditional shrink-compensation approach.
b) Sliced model
d) Design compensated for shrink
e) Sliced model of shrink-compensated design.
f) Printed part
Bond3D has developed, in collaboration with simulation specialists AniForm Engineering, a new, advanced shrink- compensation approach (Figure 6). This incorporates the dynamics of the printing process in the simulations, while at the same time minimising the computational effort.
Figure 6: Bond3D’s shrink-compensation approach incorporated into the standard printing procedure.
The compensation procedure is iterative, because the first modification usually will not provide full compensation. Therefore, a new simulation is performed until the feedback loop converges to a stable input geometry that produces the desired output geometry after shrink.
In Bond3D’s approach, the slicing is no longer included in the optimisation loop. The toolpaths as initially defined by the slicer are preserved, but they do move along with the shape modifications in the procedure: the strict printing of horizontal layers is abandoned and the height of each line can also be varied. As a result, no staircase effect will be created (Scheme 2).
Schema 2: Smooth part, without staircase effect, emerging from Bond3d’s shrink-compensation approach.
b) Sliced model
d) Shrink-compensated sliced model (e) Printed part