"Any tips or advice for casting large pieces?" Topic

Ive been doing this for about 3 years now, but this week is the first time Ive ever had to do a big 54mm figure thats been sent to me as one piece. I can see literally nowhere on it that I can slice it up (that was my first thought).

Problems are:
-never fully forms
-lots of pitting/comes out crumbly

Currently using 9" Bobweight Machine, and GWN Pewter 92 (lead free, and I use it for everything)

Now 6mm tanks, strips of 6mm infantry, 25-28mm infantry – fine! Im totally confident with casting and moulding those now, even things with tricky undercuts and shapes. But this has me stumped.

Perhaps I should be using a different alloy for really chunky models?
I drop the speed for large pieces, but should I drop the working temp too? Pewter 92 (lead free) has a high working temp anyway by comparison to leaded alloys so maybe I should order something different for casting this run in – what's your experiences/favourites?

Or am I just screwed, because counter to what my gut is telling me, you need to run large pieces fast and hot, and my bobweight machine cant apply the pressure needed to stop flash/spilling?

Pitting and crumbly sounds like too hot temperature, likewise slower spin rate is right. I suggest that you also dust the mould with talc every spin.

Yes to slow spin rate and a lower temperature might help a little. What are you running pewter 92 at? I am casting 54mm figures in it without any problems.

Also, run it for a long time – a LOT longer than you are used to, 2 minutes or more – the pitting is caused by the metal not being held out to edges while it cools.

Mike

290'c for my 6mm vehicles, I occasionally go to <300'c for some 6mm infantry that have spindly weapons. 280-286'c for 25-28mms.

As for spinning it longer, thanks I didnt consider that.

This is a very chunky piece too, pretty much ovoid in shape but detailed top and bottom- Ive asked them to send me the .stl file because it could very easily have been done in two 'clamshell' style halves but for the time being going to see if I can cast as it was sent to me.

(its the size of one of those old 4.5v batteries i.e bigger than a 9.v, and as its so solid theres no 'empty space' due to limbs etc.)

Pewter and big pieces are a for porosity. That sort of mass going to build in a lot of gas/porosity problems.
Probably reverse gate best and low temp, low speed and long spin time.
Dp you have the Principles of Centrifugal rubber mold casting book? Used the detail in there a lot when casting 90's was very useful.
L

Leon – not sure what you mean by reverse gate? Theres a lot of lingo I dont know as Im 90% self taught.

If thats the book I think it is, I couldnt find it anywhere when I first started, the one or two copies I could find were not only stateside, but north of £80.00 GBP – as with any new venture you're broke when you start! You've jogged my memory though so I might try and get a copy now.

I'm going to take a guess here that 'reverse gating' is the same as 'back gating' here in the US whereby the main feed sprue travels toward the outer edge of the mold before turning back toward the mold cavity and the gate enters into the back side of the cavity. It's a better tactic for large pieces and allows a more controlled and steady flow of alloy into the cavity. Venting is generally cut into the top of the piece near the main center hole and wherever else fill needs a help.
Your main problem of pitting and porosity most likely has been covered already. When something fails to cast the natural tendency is to apply more heat and rotation speed. Not So! Generally the reverse is better thinking the larger the cavity to fill is. Take your casting machine speed down as low as it will go and still rotate. Also lowest temperature your pot is able to go and still be able to pour a ladle of metal before it starts to freeze in the ladle. Work UPWARD incrementally – in units of 20 rpm and ten degree increments – in both speed and temperature until you get good results. You also don't need a lot of pressure clamping the mold halves unless the rpms are way high. If you have an air pressure machine I would start at 16 psi – but at low rpms you may be able to go as low as 10 or 12 psi. Flash is more a result of speed.
I think your main enemy on this piece is the nature of your alloy, too much heat and perhaps turbulence of metal entering the cavity. All of these contribute more-or-less to ease of metal flow and heat distribution. Antimony, cadmium and lead in very small percentages help the melted alloy flow better and remain pasty longer until consistent hardening is attained throughout the piece.
If your pot has no temp control, fill your ladle and let sit off to the side to cool a bit before pouring. Slow pouring may be the key, side pouring into the machines RTV cup may help too. Play with all these variables. And yes, be patient and allow the metal to cool fully and consistently in the mold. This could take minutes of spinning the mold and more minutes with the mold out of the machine and off to the side.
Once you find the right combination of speed, temp setting, pressure and pouring style/speed write it down with markers on the mold for next time.
The book Principles of Centrifical casting was sold by Conley Casting Supply in Warwick, Rhode Island USA and is shown in their catalog which can be seen on their web site:
conleycasting.com
They do have a price listed or it may be out of print. Go to their 'Contact Us' section and you will see a UK provider listed for Conley. Maybe they can get it for you. It is still the Bible for mold design etc. – which is in need of some updating considering some of today's new materials – but it will help. It's troubleshooting section is something I find myself going to even after 40 plus years of casting. Bite the bullet and get a copy if you want to become better at this.
There's always some piece that will throw you a curve. When faced with casting problems it's always best to go back to the basics and try to identify and diagnose the trouble. Designing sculptures, prototyping, laying out molds and casting are all separate arts with casting more of a science to me. Pouring hot metal into a mold while allowing the air to escape is, indeed, a very simple process. But a hundred or more exasperating problems can pop up and you will develope the experience to solve each and every type of problem.
But then that one bugaboo piece comes along…..
Toby Barrett
Thoroughbred Figures

Rob,
Well if you cant find one let me know ( check public library listings I knew it was in the Oxford library years ago) you can always borrow my copy for a while if we attend same trade shows, I wouldn't trust it to postal system!
Yes back and reverse gating same thing, totally counter intuitive but they work.
L

Othe thing I forgot to suggest is using a "Chiller" – put a piece of scrap metal in the cavity before you spin up the mould. This will cool the metal more rapidly and stop the sinkage problem. I use it with one piece 54mm horses..

Mike

Hmmmmm. 'Chiller'. Never heard of that. Thanks. Another arrow in my quiver of casting solutions…..
Toby Barrett