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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, 2020 at 19:00

1 Answer 1

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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 or some point of the shell delaminates. The release of the resin can happen in a rather violent way - which in turn could deform the print in the making. Delamination rips through the part till air can enter the enclosed space, destroying the print in the process.

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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