What is Cupping?

Cupping is a phenomenon inherent to all bottom-up SLA type systems, including our systems. It occurs when printing a convex shape relative to the build platform. During membrane peel, a vacuum force traps resin in the void, then during return to print position a pressure of resin builds up. The pressure will cause cause surface defects or a blowout failure.

Symptoms

• Thin walls bowed inward or surface defects
• Blowout holes within the wall
• Delamination of the part
• Part coming off the build plate midprint

How to Fix it

• Reorient the part
• Add a vent hole
• Adjust your design to have thicker walls
• Reduce the retraction speed

Reorient the Part

For most geometries, there is usually at least one orientation that can avoid cupping. This orientation will likely require more support. Looking at the figure below, we can see that flipping it 180 degrees will eliminating the cupping but will require significant support built in an inner cavity. The third option shows the part rotated at 135 degrees in order to both eliminate cupping and the amount of support needed. 

Add a Vent Hole

In some cases, you may not be able to find a proper orientation to avoid cupping or you may just want to avoid the extra support another orientation may need. You can use the perforation tool to create a tapered hole in your model to prevent a vacuum from being created. You will want to place this hole as close to the base of that concave as possible.  Keep the plug and attach it with resin prior to post-cure. Use as large as a plug as possible.

Adjust your Design

While cupping does exert force on the part, having thick enough walls that can resist this force may prevent blowouts. It is likely that there will still be small surface quality issues since this method does not eliminate cupping. It just mitigates the effects of it. 

Reduce Retraction Speed

Avoid a blowout by reducing the speed at which the model approaches the membrane, Reduced speed will reduce pressure build up during retraction, but will not completely eliminate it except for low viscosity materials. You can expect minor surface defects depending on the amount of pressure build and green strength of the material chosen. If you can only have a small hole, or if supports are dense and causing fluid restriction, you may also need to reduce retraction speed.

Updated January 23, 2024 16:40