Dog’s Head: Part 2


The end goal of this exercise is to cast, from metal, a bust of my dog’s head in metal: perhaps to use as a coat hook. The first step, modelling the dog’s head’ went well. This is just an ‘interim’ post with a few photos of the 3D printing of scaled test attempt.

The video above is just my evidence that my 3D printer is working again! Next time I’ll frame the print a little better eh?

PLA print of dog’s head. It’s at half scale with a 0.4mm layer height so resolution not great. Wanted a quick print for testing

 

I feel this was a good idea. I got a cheapo kids paint brush and went over the print with melted candle wax: (1) To smooth out layers, and (2) Give impermeable skin for when I pour the (liquid) casting mould around it.

In readiness for the pouring plaster of Paris mould. I made to funnels: one where the molten metal goes in and the other is a vent. I bonded them to the back of the 3D print with more wax.

 

Plaster of Paris poured. I would add ‘precisely’ to that, for alliteration….but it was a bit of a FAIL. Weights on top to counteract the buoyancy of the largely hollow 3D print in the liquid plaster of Paris.

A Word (to the Wise – not Me)

As nice as the photos are (taken with my DSLR, not my phone), this post is a bit of a hoax:

I bought the Plaster of Paris a long while ago from the dollar store. I do not know if age and source are the reasons but when I made the first batch, following the quasi-English instructions to a ‘T’ the stuff pretty much set as I was stirring it. I never even got to pour it. I was then left with a third of what I needed to make a second batch. So what you see in the last photo is a very dilute mix….because I was impatient and could not just wait to go buy some more at the weekend.  D-AAAAAHHHH! Batch II did set, but it was really friable, delicate and spongey almost. The best word for it: useless.

Despite the above, I went ahead and put the set mould in the oven, as per a million youtube videos. I expected the PLA was going to miraculously melt and smoothly pour out. It did not. I think the mass of plaster holding so much water prevented the centre getting up to heat. It was in the oven a fair while – 3 hours? Perhaps if I had been a little more patient? After 3 hours the plastic was not even soft.

A technicality: By coincidence, I have been doing some research at work on gypsum board: they use it in North America in cladding build-ups. So, the process: You mine gypsum. You grind it up. You heat it (to about 150°C) which drives out some of the water:

(from Wikipedia) CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).

The Calcium Sulphate hydrated with the 0.5 water of crystallisation (the hemihydrate) is Plaster of Paris. You then add some water and it rehydrates back to the dihydrate, gypsum, and sets. And one can form gypsum board, dental moulds, kids trinkets etc etc.. Here lies the problem: PLA’s softening temp is around 170°C and I guess it is really free-flowing at 210°C? I would have to brush up my school chemistry and do a little more research but there are a couple of things that seem wry here:

  1. To get the plastic to a liquid state I am going to cause the reaction above. I am not sure ‘Plaster of Paris’ is as good a solid material (and so mould material?) as Gypsum.
  2. The reaction above is endothermic (which apparently is why Gypsum board is a pretty good in fires). So this might explain why getting the oven’s heat into it was SOOO SLOOOOW.
  3. I have also read that up at 180°C you get another version of Calcium Sulphate called γ-anhydrite….then at 250°C+ you get the completely anhydrous form called β-anhydrite or dead burned plaster.  Again, I am not sure how these stand up as a solid material capable of holding 660°C+ molten aluminium.

The upshot of the above is that I am not sure that I am ever going to get a good result trying to melt PLA out of a gypsum plaster mould. I am sure with additives etc (which I gather the Big Boys do) it might work. Of course, as an engineer, there is a small voice whispering “concrete” at the back of my head. But, by all accounts that is a BAD IDEA – ignore the voices.

Back to the drawing board.

 

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