3D Printed Molds and Cast Concrete

Hobby 3d printers get a bad rep for being only used for making trinkets and toys.

While that’s not  completely unjustified

Today I wanted to show you my process, for making 3d printed mo(u)lds, to create cost effective prototype products, with cast concrete.

This video is a tutorial of process I use when designing and creating my 3d printed moulds. Below I'll also do a write up of the process for those of you that enjoy reading more than watching.

If you would like to DOWNLOAD the project files, you can find them here.

 

CONTENTS:

In the first half of this tutorial we’re going to deal with the design and creation of the molds using the computer and 3d printing. 
And in the second half I’ll show you how I go about casting the products,  with some tips and tricks thrown in there as well of course.

 

DESIGNING THE PRODUCT AND MOULD:

We’re going to use these faceted candle holders for an example today because I was already making them for a client, but you’ll see that this process can be applied to pretty much anything you can think of.

I’m using a free software called fusion 360 by autodesk. This is most definitely easier to follow/explain with the video, so let me add this .gif for you instead.

cading gif1.gif

The first piece and really only piece of information we need is the dimensions of the candle. My candle has a diameter of 20mm and is about 300mm long. So I’ll model that in fusion by creating a sketch, choosing the circle tool, adding the dimensions and extruding out it’s length. I’m going to move through this stuff quite quickly, because it’s the process as a whole that’s important not the exact clicks that I’m making.

To create the product, I’m going start by moving into the sculpt environment, creating a cube and throwing in some rough dimensions.
I’m hitting alt & 1 to toggle on box mode and now, I can manipulate the cube by using the ‘modify’ tool to change its shape and position. I’m just eyeballing the basic size and shape, trying to get the proportions right and general form as a foundation to work from. 
Once I’m happy with that, I’ll start messing with the details. Again I’m using the modify tool to adjust and manipulate the control points. 
I want to create a triangle faceted look to my candle holder. To do that I need to insert some more edges into the form and move the points around until they protrude a little bit.
Fusion's sculpt environment isn’t big on creating triangles as it based on t-spline quads, which are better suited for creating more organic forms. Which you see in a sec how I deal with that little nugget.
During that little rigamarole I also deleted the bottom face of the form, technically leaving me with a surface instead of a watertight solid. Which will be important in a moment when I turn it into a mould.
Now that I don’t hate this form, I can exit the sculpt environment. And.. It’ll snap you back to a lovely organic form, which in this case isn’t what I’m after. So I’ll hop back into sculpt, select all, and crease the edges, so that they hold their faceted form.  
Now is a great time to double check how everything’s looking by bouncing over to the render workspace to get a more realistic feel of how light and shadow are hitting the form. I thought some of the facets could ‘pop’ a bit more, So I jump back and adjust the shape slightly.


To make the mould from this form I need to create the negative.
So back in sculpt mode I’m going to use the thicken tool to offset the surface - I’ve found a distance of 8mm to be about right.
And boom. Instant mould. - almost. You can see we’ve got ourselves an upside down cup. Which would work as a basic mould. But we’re going to do a bit better.

Before we go further we need to have a little talk about undercuts and draft angles. In its most basic sense undercuts and draft angles are all about making sure your form gets wider towards the opening, so that it can slide on out. 
A good way to get around undercuts and making your casting release easier in general is to create a two part mould.
Which conveniently is what I’m going to show you how to do right now.

Before I forget though, it’s helpful to make this bottom edge horizontal because it’s going to become our top edge when we pour a casting.

To create a two part mould I duplicate the body and then hide one.
Then I shift into the patch workspace so that I can modify the surface of the form.
I select the place I want the moulds to split and delete the faces in that area, leaving me with half a mould.
I can see that's left me with open edges so I need to come in and close those up so that I end with a solid form for 3d printing. To do that I’ll just use the loft tool and select opposing edges to close across.
If you end up with gold faces instead of grey, don’t sweat it, we can just reverse the normals to flip them back round.
Then I can join the new faces to the old ones with the stitch tool.
And that’s the hardest part done. To get the other half of the mould, we repeat the exact same process with the duplicate body we created before but deleting all the ‘other’ faces.

Before you pat yourself on the back there’s still a couple more steps.
First
First, we need to create a flat spot for the mould so sit when it’s being cast.
To make that. I create an offset construction plane. Project the edges here up to it. Then create an extrusion back down to the surface. And boolean join them back together.
second.
we need to get the candle shape into the form. To do that I simply boolean join the two bodies and then slice the exposed top off.

If everything’s worked out you should now be left with two solid bodies that fit nice and snugly together.
Finally we can export out as .stl and drop them into your 3d print software of choice. I’m using just regular old slicer.

 

PRINT THEM OUT:

Make sure your orientation is all good, with the flat side down. and preview the print layers to make sure you don’t have any dangerously thin areas or other problems.
Then we can go ahead and hit print.
I’m printing in PLA with a 0.3mm layer height and 10% infill. These parts need to be sturdy, but they don’t have to be like. extreme.


So that I get a tight fit between the two mould halves I'm just going to smooth down some of the areas that have 3d printing blob artifacts. I’m using a sharp chisel instead of sandpaper to do this, because I find PLA doesn’t sand that well on its own and will leave you with a fuzzy finish as it likes to scuff and melt rather nicely sand like ABS filament does. If you want to get a super nice finish, spray a few coats of filler primer on and sand that back instead. But it’s not really necessary here.

 

PREP THE MOULDS:

You need to lubricate the mould, otherwise the concrete will just stick to the mould and you’ll have made yourself a very complicated lump instead. I’m using petroleum jelly because it’s easily available and I like its viscosity - that’s a great word - which helps create a more substantial barrier between the 3d print and the concrete going in.
Don’t half ass this either, make sure you cover all the surfaces and get right into the nooks and crannies.
To hold the two halves of the mould together I just snap a couple of rubber bands around it.

 

CASTING THE CONCRETE:

You’ll need a bag of your favourite concrete product. I’m using literally the cheapest stuff I could find - i think it’s used for making fence posts.
If its got stones in it you can just steal a sieve from the kitchen and deal with those. If they end up in the mould it’s pretty likely they’ll ruin any sharp corners that you have.
I like to mix my concrete to a wet pancake batter consistency so it flows into the mould nicely - so just keep adding water and concrete until it seems about right.
And then quickly pour it into the mould but stop about half an inch from the top so we can bang the mould a few times to loosen any air bubbles that might have formed. I found plunging around inside the mould also works really well to create a bubble free cast. Top off the mould so it’s just humping over the top edge.

Leave it to firm up a bit and then trim the top flat with a straight edge. Then leave it alone.

 

DEMOULDING AND FINISHING:

To find out whether the cast is ready to demould I like to do the fingernail test. If you jam your nail into the surface and it doesn’t leave a mark, you’re good to go.

Remove the cast by lifting away the one side and then tapping gently to get it to slide out.
Now it’s not fully dry yet, so I recommend leaving it for a few days. 
Then if you feel like it you can use some sandpaper to make the surface a little nicer and use some concrete sealer to protect it.

And that’s it. let's see the final product!