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Accurate printing of mold shell - reverse inner and outer walls


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Posted · Accurate printing of mold shell - reverse inner and outer walls

I'm printing the shell for plaster of paris part mold  The final mold will be made up of five parts that need to fit together accurately. The .STL model is exactly the size that the plaster of paris part needs to be. If I print the model as is and then fill it with plaster, it will be inaccurate by the thickness of the printed wall. Is there a way to reverse the inner and outer wall selection so that Cura maps the wall thickness on the outside of the model rather than the inside?

Screenshot 2021-03-09 at 11.24.11.png

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    Posted · Accurate printing of mold shell - reverse inner and outer walls

    Hi - thanks but the uniformity of the mold mode doesn't allow for the more complex shape of interlocking pieces.

    I have no trouble designing the mold parts - I'm really just looking for a way to specify which faces of the model to treat as outside. 

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    Posted · Accurate printing of mold shell - reverse inner and outer walls

    If line width is 0.4mm and wall thickness is 1.2mm then you can set horizontal expansion to 1.2mm and you will get what you want on the walls.  This won't affect the top and bottom however.  The height of the final part will be the same as the model which is probably not what you want.


    To clarify - any walls that are not horizontal will be the way you want.  Only walls that are 100% horizontal will be off.


    I guess it's more complicated as there is a setting called top layers and associated settings that can put extra printing underneath a sloped wall.  And bottom layers which can put layers above a sloped wall.


    Hmm.  It's complicated.


    Another solution of course is to create the mold in CAD.  Most CAD programs allow you to do "binary" operators which include "subtract" where you can create the exterior of the mold and then subtract another model from that to get the mold.

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    Posted · Accurate printing of mold shell - reverse inner and outer walls

    If I had to do this, I would probably do the mould-making in CAD: subtract the model from a solid block. And then cut the block to pieces along natural seam lines, so the cast can be demoulded. Next add flanges to clamp mould parts together, add alignment features ("keys"), add a stable baseplate so it does not fall over or slide away when pouring heavy plaster in it, add air venting holes if required, add pouring holes if required, add features to lift the whole mould, add features to insert a screw-driver in-between mould-parts to wiggle them apart later on so you can open the mould, etc... Requires lots of work, and lots of thinking, but it gives you way more control and understanding.


    Making a mould is not that difficult. But afterwards opening it, and getting the cast out undamaged is...  :-)


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    Posted · Accurate printing of mold shell - reverse inner and outer walls

    I did some work designing tool cutter paths for molds.  I would scale the part by the shrink factor and then as Geert says, subtract if from a solid block.  At the determined split line, cut the mold into two pieces and they become (usually) the top and the bottom.  Alignment pins, pry slots, vent holes, fill holes, pockets for inserts, etc., all get added and there it is.  A mold.  The part might require the addition of draft angles to some areas so it could be removed from the mold.


    It's worse than useless to make a real nice mold that you can't get the part out of.  The locating features you have there may require removeable inserts.  After the part hardens the inserts get pulled out and the part can be removed.

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    Posted (edited) · Accurate printing of mold shell - reverse inner and outer walls
    On 3/9/2021 at 11:27 PM, gr5 said:

    Maybe he can melt away the mold?


    Yes, but then it's a single use mould. Not the best option if you need a lot of casts. It sort of defeats the idea of moulding and casting.


    A good alternative to a fully 3D-printed mould, could be a silicone mould in a hard 3D-printed shell. It goes a bit like this:


    - Print the real model in PLA or whatever.

    - Design in CAD a shell that is sitting at some distance from this real model, maybe 5mm to 10mm (depending on the size of the model: bigger models need a bigger distance). This can be a very simple shell: two parts, with clamping flanges, alignment keys, stable baseplate, and big opening on top. Make sure to minimise details and undercuts in the shell.

    - Mount the real model in the shell, and verify there is some distance around the model. Bolt or glue the model to the shell. Removable glue like "rubber cement" might be a good choice. If not fixed, the model will go swimming around.

    - Carefully seal all seams, they must be absolutely watertight, or the silicone will leak away.

    - Pour silicone into the shell, around the model. Preferably transparent silicone, so you can see the model through it. Let cure (often overnight, depending on the silicone).

    - Remove the shell. This should be very easy if well designed, due to the simplicity of the mould and the flexibility of the silicone.

    - Now using a scalpel or very sharp knife, cut the silicone in half, or in multiple parts as required by the model, so the model comes out. While cutting, make zig-zag movements, so you get natural alignment keys in the silicone. It looks ugly, but it works very well. Cut along natural seam lines of the model.

    - Treat the mould with plenty of silicone release spray before casting, so it really soaks in.


    In this way, you get a reusable silicone mould, easy to clean, easy to demould, suitable for many many casts.


    There are lots of excellent tutorials on Youtube, search for: moulding and casting




    3D-printed shell. Model (teeth) was mounted with plasticine. Alignment of both mould-halves is by M4-screws, also used for clamping the mould. Note the lots of draft in the mould, required for easy demoulding from the shell. Also note the zig-zag pattern in the silicone halves.



    Old-school mould, with hard shell of two-component epoxies. Note the zig-zag in the silicone, for alignment. Also note the bottom plate for positioning it upright stably, and the big pouring opening on top, and the alignment keys.



    Side-view. The flanges of the shell are for clamping the mould and for the alignment keys. This was made manually, long before 3D-printing. But you can use the same concept.


    This is fast-curing silicone: advantage is that no sealing of the mould is required: it cures before it leaks away. Disadvantage is that bubbles can not be evacuated: it cures too fast...


    For casting solvent-like stuff like polymethylmetacrylates (PMMA) or PU, lots of silicone release spray are required. Otherwise the solvent penetrates the mould and makes its life much shorter. Silicone is watertight, but not oil, parafine and solvent-tight: these seep into it, and would cure in there, thus hardening and destroying the mould. So, saturating the silicone with silicone oil prior to casting helps a lot.


    Edited by geert_2
    Corrected typo and unclear wording
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