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Why is cupping bad? (I don't mean hollow parts)

Example:

Lets say that I want to print a simple cup without a handle.

There are two obvious orientations:

  • In one orientation it won't require any support, which I quite like, but it will then form a 'cup' which my 3D program tells me is bad.
  • In the opposite orientation it needs to be filled with support, but the resin can freely run out.

I don't want to add a drainage hole to my cup for obvious reasons :) Also, I am using a formlabs form 3, if it makes a difference.

A cup:

enter image description here

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  • $\begingroup$ It seems you already answered: it's bad because resin cannot flow away easily (a sort of vacuum is created) when the already solidified object is slightly lifted to let further resin flow in. $\endgroup$ – FarO Jun 30 at 19:00
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The printer prints, then moves up, then down again. The print surface stays inside the resin vat at all times. As a result, we have this experiment:

enter image description here

The "bottle" is resting in a vat of liquid. As we raise it more and more, it does not drain until the lower lid is free of the liquid surface, which happens in a rather violent way - which in turn could deform the print in the making.

Even if printing the mouth against the plate you'll have cupping if you have a solid plate to print against. This can be mitigated with a little angle but trap liquid in the print at the end or including a couple of small gaps close to the surface to allow air to get into the print - yet unless the resin can flow out at the bottom some will be trapped in any case.

To prevent cupping, I would turn the cup to print sideways, that way resin and air can be exchanged.

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