Using PLA as molds for casting

[Toby]

I've been experimenting printing molds in PLA for casting.  The stuff I'm casting with is called Permastone, which is just plaster mixed with something to make it stronger.  I got it at Hobby Lobby where I'm sure it's vastly overpriced and is probably the same stuff you can get online for about 15% of the cost.

I did a few simple tests to verify it works and then tried something more involved- an 8 sided box.  The problem in designing the mold is to get something that you can not only pour the liquid in and get the shape you want, but also get the cast out of when it dries.  I've only read about how to do it, but based on that my first tests didn't work out.  I tried printing two-part molds in PLA but they weren't flexible enough to get the model out without breaking it.

So I went the other direction and broke the mold down into a lot of pieces that would be easy to take off one by one without putting any stress on the cast.  In order to assemble the mold I used the CAD drawing of the box to design and print custom "clips" that fit the geometry accurately and held all the pieces to together.  

This worked out better than I expected.  Attached are four pics illustrating the process.

1. The first pic shows all the parts of the mold laid out.

2. The second pic is the assembled mold.

3. The third pic is the mold filled with plaster.

4. The last pic shows two completed casts.  The one on the left is as it came out of the mold.  The one on the right has been lightly sanded but nothing else.

The mold is reusable- both these casts were made from the same mold.  Cleaning it between casts is just dipping the pieces in water and wiping with a paper towel.

I mean to make a top for the boxes the same way, which is an easy mold.  But I'm not sure how to finish the boxes.  I tried acrylic paint on an earlier model.  It sealed well but the appearance was kind of ho-hum.  The plaster has a certain softness to it I want to try to preserve in the finish.

[Jin Choi]

Nice. The outside can come off easily, but how about removing the inside pieces that are surrounded by the plaster, how hard is that?

How do your clips work?

[Tony Shulthise]

I love this stuff.  Keep posting.  Its great learning as a group.

I haven't made plaster molds but I have made silicone molds from PLA prints then cast plastic and chocolate parts in those molds.  I also made a very small two piece PLA mold to make a flexible silicone rubber plug which worked out pretty well.  

Silicone molds are expensive to make but they can last a long time and are really easy to use.  I use Autodesk Inventor so its easy to make the negative of any model or assembly to print then make a silicone part from the 3D printed negative.  Or, make a 2 part silicone mold around the "positive" and pour a plastic or plaster duplicate using the silicone mold.

I tried making a couple metal parts by pouring Bismuth (around 170F melting temp metal) into silicone molds made from 3D printed parts but they were FULL of bubbles.  That turned out to be fine for the part I used it for but I'm not sure how you keep the bubbles from forming.

Thanks for sharing.  I really believe that making molds from 3D printed parts is something that will grow exponentially over the next couple of years.

[Toby]

Tony- I think you're right about 3D mold-making becoming bigger.  It solves the problem of how long it takes to print parts, especially large ones, while still allowing people to share designs and make things locally.

Jin,  The inside walls aren't hard to remove.  Here's a picture of the underside of the mold where most of the clips are.  For reference, next to it is the plaster cast in the same orientation as it would be if it were in the mold.

The clips hold three layers together: The outer wall, the inner wall, and the 8-sided flat piece that corresponds to the top surface of the plaster cast.  That last is just one piece and printed at 4mm thick to give the outer and inner walls something to grip.  The outer and inner walls are each in four pieces, which you can puzzle out by the fact that they are held together inside each clip.

When the cast is done I remove the clips.  Then the outer layer peels away.  Then the single piece around the top is taken off.  Then the two middle sections of the inner walls come out- the joints there are angled in such a way that the end walls don't interfere with removing the middle walls.  Then the two end walls come out without interference.

The hardest part came next, but it was still easy enough.  This is the flat surface which corresponds to the inside bottom of the cast.  That's printed in three pieces, each with a raised handle to grab with a pair of pliers (which doubles as what holds the piece in place relative to the inner walls, which you can sort of see in the picture).  But because the surface area in contact with the plaster is larger and there was some ooze of plaster through the cracks, there's relatively more resistance to lifting it out.  But just cutting away the ooze with a knife and gently working on the pieces with the pliers got them out without doing any damage.

[jimc]

Madesolid now has a resin specifically for investment casting. It burns off leaving little to no ash or residue.

[Ketil Froyn]

I've read that PLA can also be used directly. From http://diamondage.co.nz/?page_id=21

"Clear PLA is suitable as the core for investment castings, as the plastic vapourises leaving no char."

Cheers, Ketil

On 24 March 2014 12:21, jimc <xtremekr...@gmail.com> wrote:

Madesolid now has a resin specifically for investment casting. It burns off leaving little to no ash or residue.

--
You received this message because you are subscribed to the Google Groups "MakerGear - Make Today, Change Tomorrow" group.
To unsubscribe from this group and stop receiving emails from it, send an email to makergear+...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

--
-Ketil