Tag: casting

Hi, a new project and I’ll be adding to the documentation coming time. I hope it gives you useful information or inspiration.

last update: okt 5th.

Introduction:

Usually for casting concrete a wooden or steel form is made which is filled. For simple shapes that’s easy, but some shapes are hard or time consuming to make of wood. I wanted to try this already for some time, so I decided to make a 3D printed mold. Interesting study project but for such a simple shape way over the top 🙂

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We thought of a simple wooden cross with a weight (three tiles or a bag filled with sand) placed on it, a bucket filled with sand (or concrete). It has to be transportable, not too heavy, and it will be decorated (painted).

Rather simple, and it could well be made quickly without any problem. But for learning I’ll use 3D printing. The added value might be: nice shape, own logo, durable.

Questions (and answers)

-Q: How thick should the mold be. A: 9mm makes a strong mold, but even vase print works)

-Q: Will the concrete release well? Release agent or grease necessary? A: yes, very well with PLA. For proper release of bubbles that cling to the walls a releasing agent is recommended.

-Q: How many times can it be used. A: as long as your mold is strong enough to withstand pouncing with a hammer to release the concrete, you can use it over and over again.

-Aluminium or Portland cement ? A: Alu is much faster and fine for details. Portland is cheap. Both work well but Alucement may set too quickly if temperatures are high (summer).

An observation:

Normally the concrete mix is rather stiff, you need a vibrating needle to fill the form properly. I used plasticizer and a steel rod and some bouncing on the floor with the form, but that is not optimal. You could (but it’s not recommended!) add a (very!) little more water to make it more fluid, but for forms that have to be strong you should not do that.

A block with a center hole for a post, 2 handles to pick it up easily and two holes to put a rope through.

These parts would form the sides of a wooden mold. However, I abandoned this idea of separate parts and adjusted the plan for 3D printing the mold.

The 3D print design. The walls are still 9 mm thick, remnant of designing for wood 9mm.

The resulting shape. Looks nice to me. Print it

27 hours printing.

Settings: 0,20mm Standard @BBL A1M (Bambuslicer):

The next print ( the shape to form the opening for the hand) I printed at 160% (ludicrous). It turned out well.

Next: sanding before appling a coating.

K120 sanding paper, primer spray paint, 2 coats applied

Observation: I later noticed that this primer is not necessary. Save yourself some work and skip it.

I bought this material:

Gietsteen (casting stone). You can also use aluminiumcement mortar with sand. Alu cement is not sold everywhere like portland cement. I found it at sculptors supply store Beeldhouwwinkel. Others that have it: Keramikos and Hazelaar. 20 kg costs about 40 euro, transport 12,50 euro. That is much more than 25 kg portland ( around 8 euro / 25 kg mixed with sand (3x 25 kg, less than 10 euro). It better looks awfully nice for such a price. Bought it anyway. If this works well there will be follow up projects.

Observation: It casts well, but may show some shrinking cracks. Make your corners well rounded or add some sand to the mix.

https://www.beeldhouwwinkel.nl/nl/caltra-gietsteen-standaard-snelwerkend-801

For the through holes I use a PE tube around a metal rod, so I can remove it easily: just pull it out, it is flexible and does not stick to concrete. Metal tubes or wooden rods might get stuck. Then you have to do a lot of work to get them out and keep the mold intact.

For the big hole I cut a 2-piece foam core with the hotwire cutter (smoother than with the belt saw), made a 2-piece foam core (might be easier to remove than a 1-piece core). Taped around it to keep it together. I can, I suppose, remove this core quite easily with a piece of metal, a screwdriver or other sharp tool. We’ll see 🙂

Observation: just sawing it works fine as well.

The mold parts. Now: applying release agent ( bekistingsolie (NL), Schalol (GE) ). Oil or vaseline can also be used as well but is not recommended as it may stain the concrete. I found a jerrycan with still 3L in it at Marktplaats for a very reasonable price. That will do for many molds to come.

A thin coat of oil applied, 1 liter is good for 20 -30 square meter, that means a layer three times as thin as a paint layer, so apply sparingly. I applied it with a dry-ish brush and removed excess with a paper towe

Cement and water. Ready to go

Mixed. Now tapping against the bucket with a stick, to get the air out

I waited 5 minutes before casting, following the instructions. But it set very fast, I could hardly poor 80% into the mold and it got to stiff already. Probably too hot today (over 25°C). So I quickly prepared another batch, mixed, did not wait 5 minutes this time and still it set very quickly. But now I succeded to fill the mold to the rim and hit it gently with the plastic hammer to get the air out and level the surface. Even filled a couple of small test molds. Covered it with wet cloth and a plastic bag to prevent that the concrete dries out. Tomorrow we’ll see how it ended up. Will it release properly, will there be a visible separation between the first and the second cast? I must go home now and not have a look (and rip the fresh concrete apart)

Ok, some positive and some negative points:

but:

4 shrink cracks,

5 characters were damaged ( no draft)

6 casting in two times resulted in an ugly joint

Next cast:

Working at lower temperature or using a slower setting cement so I have enough time to fill the mold in one time

adding sand against fracture

Adding draft to the letters (15 degree?)

A little later: august 18th.

I used a new casting recipe:

3 vol parts sand, 1 vol part cement powder, 0.5 part water ( standard mixture)

Slower, but / and much cheaper as well.

This turned out much more relaxed, enough time to work with it. However, it did not flow, I had to scoop it into the form. It worked though and with some rocking of the form it also became dense enough and it filled the bubbles welll enough.

The letters now have a draft ( 15 degrees , roughly 1 : 4)

I did apply release agent again, but more to avoid that bubbles aggregate than for releasing the cast ( the agent, apart from forming a non adhesive interface, also changes the surface tension so bubbles release better)

Releasing the next cast two days later. Here are the results. Much better. Still not good enough I think. Too many bubbles, mix too stiff to cast easily ( not liquid enough)

I let the cast harden for two days, covered with wet towels. It thyen felt strong enough, but I still had to be careful, because it was not very hard yet and easy to scratch. The form released easily, just a few gentle knocks with the (wooden) hammer were enough.

The cast was still rather soft, edges break easily. But can also be rounded easily with a rough file. This makes handling more hand-friendly. Not all fillets could be included in the mold, so it is good this could be done easily (with the file) after releasing.

Observation: In later casts i skipped the rounding.

No cracks in the corners this time, due to the added sand I suppose.

The form is in a bucket with water now to prevent drying out too fast before the concrete is completely hard. One week keeping it wet should be enough. I’m curious how strong it will be, I expect some rough handling so it better be tough and strong.

Next test will follow, with gravel in the mix as well, so it resembles more like concrete. And i’ll add some plasticizer to make it more liquid without adding extra water, for easier casting. Maybe I should also use a vibrating needle.

For my next mold:

I would choose a splitting one. It is not much more work for the printer to make two (or more) parts or much more drafting and makes work later easier. You can also consider a release pin to push your form out of the mold, like it is used in dye casting. But for a (still) fragile cast i would be carefull not to push it to much on one place like a pin would.

Next step: where can I get plasticizer for concrete. It used to be sold in cans, but….not anymore, not in Enschede anyway. Should I commit to the ready made products? No thanks, I’m a maker. I like to make my own recipes: cheaper, adjustable. Even though i will make mistakes first. Online? Ok, but which one?

Ask the expert:

Sika. And they will send me a sample for my specific sand – cement mix 🙂 That’s is very nice of them. So now I have a sponsor 🙂

Next test, the Sika plastiziser had not arrived yet, i tried premixed concrete, straight from the bag. Good to knowchow that works as well. Sacrete constructiebeton C20/25, bag of 25 kg, i used 12.5 kg with 1.15 l water ( recommended quantity). Cadting was ok, i could scoop it in and vibrated it with a metal rod, did so in three times, vibrating every batch. I used a trowel for the surface which turned out ok this way

Releasing from the mold after one day was easy. I did test a sample though by hammering on it with a plastic hammer, just to know how strong it already was. Good enough for releasing

Next time: does the plasticizer affect the amount of bubbles on the surface?

Sep 24th. Casting with plasticizer.

That stuff works like magic. I mixed the recipe that I got from Sika:

for 1000 l concrete:

cement 500kg,

sand 800 kg

grit 800 kg 4/8 mm

water 180 – 190 l

plasticizer 2.5 a 5 kg

mixed for 13.7 kg in a 65 l tray ( mengkuip) en added just a little bit of plasticizer, just enough to get it (almost) liquid.

It mixed really well and casting was easy. I scooped it in with a trowel. Compacting went well just bouncing a couple of time with the mold on the floor (not too often to avoid separating the mix) and flattened the top with a trowel. Now it has to remain in the mold under a wet towel for at least a day. I’m very curious how the cast will turn out

releasing

the concrete dried for two days, while being kept wet with moist textile on top.

The tray that holds the core in place could be removed very easily, as well as the tubes and rods

So, satisfactory results. The mix turned out very well and most bubbles from sides and bottom are gone. The bubbles that formed under the tray were disappointing though 🙁

Next casting will have some cosmetic improvements such as avoiding the indent caused by of the towel (with weights on top to avoid flotation of the top tray) and avoiding the trapped bubbles under the tray.

The casting will stay in a bucket submerged under water for a few more days to harden properly.

a few days later, next casting with grit, sand and cement. Same recipe to confirm if this works well.

It’s becoming a routine: sifting, mixing, adding water, casting. I prepared a bit more than necessary, last time it was just enough. Always leave some margin.

note the Indian safety shoes🙂

The results though had more bubbles at the walls. The mix turned a bit stiffer, less plastic than last time . I probably added a bit more water than recommended then. This immediately affects viscosity. It also affects the strength though. Practice will show how the two casts will withstand use and time.

Recommendations: best casts will were made with:

• Premixed Sacrete constructiebeton C20/25, use plasticizer
• Cement, sand and grit (4-8mm) , use some plasticizer but also use a vibration needle or (if strength is not the main concern) use a tiny (!) extra amount of water

• No need to buy expensive casting stone
• Don’t use primer on PLA, but do use releasing agent
• Even though this single-piece mold released well try to make a mold that has several pieces. This will release easier, without needing a hammer that could damage the mold