Elephant's Foot

Elephant's foot on 3d resin printers is characterized as an increase in size where a printed part is attached to the build plate. Two primary causes of elephant's foot are:

• insufficient wait times prior to exposing the resin
• longer duration exposures for bottom layers

Time is needed for the build plate to settle into its equilibrium position due to large viscous forces associated with squeezing resin from between the build plate and the resin vat film. If the wait time is not long enough, resin will cure as it continues to move. This bulk movement of curing material is usually the largest contributor to elephant's foot. This contributor is easily fixed with longer wait times.

Longer exposure times for bottom and transition layers also leads to elephants foot. Light traveling through resin can scatter to areas outside of the intended target. The longer the exposure, the more likely resin outside of the targeted area will receive a dose of light high enough to solidify.

The "ScriptElephantsFootDimming" script is intended to address elephant's foot caused by longer exposure times of bottom and transition layers.

Matching Light Bleed of Bottom Layers to that in Normal Layers

Resin solidifies when it absorbs a certain minimum dose of UV light. The Beer-Lambert law suggests that light will attenuate exponentially through the material. As a result, doubling the exposure time of UV-light directed into resin will not double the thickness of the cured material, just as halving the exposure time will not reduce the thickness of the cured material in half. The best chance of reducing elephant's foot caused by light bleed is to match the effective exposure at the boundary of objects of bottom layers to the exposure of normal layers.

Effective exposure can be reduced at model walls by dimming pixels. The thickness of elephant's foot on a printed part can be used to estimate how many pixels should be dimmed. Take the added thickness caused by elephant's foot and divide it by the printers pixel size to get the minimum number of pixels which should be dimmed. Pixels should be dimmed in an amount proportional to the normal layer exposure time divided by the exposure time of the layer being modified.

Dimming Model Walls

Open a sliced file in UVTools and open the Scripting window under Tools. After loading the "ScriptElephantsFootDimming" script, model wall thickness must be chosen. On a Saturn 2 8K printer with Siraya Tech Fast Navy Gray resin, a wall thickness of 10 pixels works well. The user selected wall exposure defaults to the exposure of normal layers. The script will never dim a pixel more than 90% . A toggle switch is automatically activated to apply changes to bottom and transition layers. If the toggle switch is left active, the "layer range selector" range will be ignored. To improve adhesion between pixels of varying cure states, the selection to use a dimming gradient is automatically activated. The gradient size can be modified, but is limited in code to never be more than half the total wall thickness. Once all selections have been made, run the script.

Benefits

By dimming model wall pixels to compensate for elephant's foot caused by the mismatched exposure between bottom layers and normal layers, a more dimensionally accurate print is made. Other methods commonly erode walls to the point where details are lost, and the outcome is sensitive to the amount of erosion chosen. Dimming model walls instead of eroding walls is more fault tolerant. The user can often make model walls thicker than necessary will little adverse consequence.