Warp In 3D Printed Parts
One of the most common phenomena experienced in 3D Printing is that of warp. Warpage leads to undesired part deformations rendering the part inoperable. As a result, most of the time is spent on either re-fabricating the warped parts, or in post-processing to mitigate the same. In this article, we will discuss some of the probable reasons of part warpage and how one could mitigate the same with proper part design and considerations.
Part warpage is a generalized phenomena existent across all the manufacturing processes, and 3D Printing is no different. Plastics and almost all the materials for that matter, have a tendency to expand when you heat them, and a tendency to contract when you cool them. Warpage is a result of thermal deformations due to uneven temperature gradients and non-uniform cooling of the part geometry.
Let’s address these concerns one by one. Polymers, in general, are poor conductors of heat, and thus they do not dissipate the heat away quickly and as effectively as desired. In case of laser sintering technologies, when the laser impinges onto the bed of powder material, it selectively sinters the powder material. This raises the temperature of the powder locally where the sintering takes place. When bigger parts having large cross-sectional area per layer are exposed to the laser, it results in too much heat being accumulated per layer which is one of the prime causes for part warpage. More the surface area, more will be the heat generated, and dissipating this heat effectively and efficiently becomes a challenge. We recommend that the parts be oriented such that there is minimum cross-sectional area exposed per layer to the laser. Less exposed surface area, less would be the heat generated, and thus lower the chances of warpage.
Fig. 1: Parts should be oriented such that there is less cross-sectional area exposed per layer to the laser (as shown in Fig. 1(b)).
Part warpage isn’t much of a concern in smaller parts as it is in larger part geometries. In case of large, flat parts as shown in Fig. 1, the edges will dissipate the heat to the neighboring unsintered(raw) powder, and cool down faster than the center of the part which is unable to dissipate the heat quickly since it is surrounded by hot sintered powder material from all sides. Thus, the center of the part will stay warmer for a longer period of time as compared to the outer part edges. This temperature gradient gives rise to thermal stresses in the part causing it to warp and curl.
It has been observed that when a large, flat part cools, the edges tend to shrink/contract lengthwise. These pulling forces along the length give rise to a resultant force which, if plotted on a force vector, would act in a direction that tries to curl the part upwards and inwards (as shown in the figure below).
Another reason for part warpage is faster cooling time. Cooling rates can be better controlled by heating the build chamber. When the build chamber is heated, the part stays warm for a prolonged period of time and it cools down slowly and uniformly in a controlled environment. Thus, the part can retain its shape. When the cooling times are fast, there are chances of uneven cooling giving rise to residual stresses in the part geometry leading to part deformations. That is why it is suggested that when manufacturing the part in laser sintering, the part must be cooled for the same time duration as it takes to manufacture it. That is, if it takes, say, 7 hours to manufacture a part, it should be cooled down in the machine for the next 7 hours. Thus a total part build, in such a scenario, would last for 14 hours. In order to cut down on the idle time, if one were to take the part out of the build prematurely, then there are high chances that the part would warp owing to sudden exposure to the ambient temperature.
Warpage can also be mitigated by ensuring a proper part design. Flat parts should be redesigned with ribs. In case of FFF technology, part warpage due to rapid cooling is more predominant. A heated build platform, if not a heated build chamber should be incorporated within which the part can be isolated from the ambient air and cooled uniformly. Another approach to mitigate warpage in FFF technology would be to improve the bed adhesion by putting an adhesive on the build platform. This is effective in stopping the part edges from curling up. Warping can also be mitigated by reducing the stress concentration areas in the part design by incorporating fillets and radii wherever necessary.
Image Source: Fab lab