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Printing hollow to use as a mold questions

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Posted · Printing hollow to use as a mold questions

Hey all, a few questions.


Printing hollow (0% infill) with 2 or 3 outline perimeters for the shell works well enough for me, but I'd like to use the inside of this hollow print as a mold to cast another material in (eg. silicone). Couple questions.


First, is there any option or setting I can change so that the inside of the print is printed with a clean surface and the outside gets all the visual deformities instead? For example, in the attached photo, the steep slope of the chin combined with having 3 or 4 bottom layers causes the inside of the print to be slightly extruded inwards and look a bit messy. I honestly don't care how mangled or terrible the outside of the print looks, so long as the inside looks great. Any way to do this? Printing from inside to outside instead doesn't seem to solve the issue.


Second, is there a setting so that the outline walls are built OUTWARDS instead of INWARDS? I essentially end up with a hollow print where the inside is actually smaller than it's supposed to be, since the slicer set up the walls to go towards the inside. I'd like to have the walls built outwards such that innermost wall should be the correct same size as my model, if any of that makes sense.


Just a heads up also, I'm aware of other methods for making molds, such as using boolean operations in other software or even using Cura's experimental mold setting, but these methods, the way they're designed, also blow the print times through the roof. Printing hollow with 2 shells works perfectly and keeps the print time down to 2 hours or so, instead of 16+, but I just can't seem to make the inside of the print have a clean surface and ensure that the walls are built outwards. Any advice would be greatly appreciated, thanks all!


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Posted · Printing hollow to use as a mold questions

I think you would best use the "shelling" commands in a CAD program. Or delete the top- or bottom surface in CAD, so you end up with a non-solid surface-only model, which has zero wall-thickness and is unprintable at that moment. And then thicken that surface *outwards* until it is 2x nozzle width, so it becomes a printable solid again.


And then design some supports, so that the print can stand upwards or upside down, whatever is required for casting, without toppling over. And if required: add pouring canals, venting canals, cut the mould in two halves, make flanges at the seams, provide alignment features, clamping features (e.g. holes for screws or clamps), etc... It all depends on the model, how many undercuts it has, and how flexible the casts are going to be.


I would suggest you watch Youtube videos on "mould making and casting". These mostly apply to old-style silicone and gypsum mould making, and casting resins. But a lot of the basic concepts and techniques can also be used in 3D-CAD-designed moulds. I learned a lot from these.


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