Yet another anode casting fail - with photos this time :(

RichardS

RichardS

N/A
Joined
5 Nov 2009
Messages
29,212
Location
Home UK Midlands / Boat Croatia
Visit site
This is second time that my attempts to produce a plaster mold of a saildrive anode, to enable me to cast new anodes out of used ones, have failed miserably!

This time I used special hard plaster of Paris rather than the normal fine stuff. The hard stuff is supposedly designed for this purpose.

I attempted to drive the anode from behind the casting using two carefully drilled holes and a couple of rods with firm tapping on both rods in sequence. As you can see, the problem is that the centre part of the molding, which is not tapered like the outer surface, will never slip out from inside the the anode and always seems to rip out half of the molding with it.

Am I missing something obvious here or is this shape just too complex to create a mold without some specialised equipment or something.

Stage 1 looks great:

IMG_4032.JPG


Stage 2, after much tapping, looks like a bomb site:

IMG_4034.JPG


:(

Richard
 
It will never work like that Richard, the mold and the metal will key together too well. You need a split mould. How about : at the end of stage 1, bold the whole thing to a plate with four bolts. Cut the whole lot in half with a hacksaw from top to bottom, as viewed in the picture above. Undo the bolts and it should come apart. Bolt the mold back onto the plate and make a pair of spacers to fit into the hacksaw cuts. Next time you make a casting, remove the spacers, unbolt from the plate and it should give you enough clearance to get it apart.

The other, and probably easier solution might be to sand cast it.
 
Last edited:
Not too familiar with the casting process but would it be necessary to cover the old anode with Vasaline or similar to assist release?
 
There are plenty of foundries in your neck of the woods, Richard. Why not try a factory tour, and while you're at it, ask about your problem. I suspect you'll love the experience. It's a fascinating, inside-out world.

...and wot Paul wrote.
 
Your pattern looks too rough. You need to file and emery it to the best finish you can, using filler for any bad patches as required. Then ensure that the draught angles are good in every plane. I might paint it, it does have to be rather smooth

Sand is easier for this type of work. You can get Petrobond from these people:

http://anvils.co.uk/products/view/402?cat=33

Unfortunately, rather a lot, if you go on that foundry visit you may be able to scrounge some. It is an oil based product so no messing about with water.

Knock up a wooden box a couple of inches deep and fill with sand, level off with a straight edge, ram to a firmish consistency and push in your pattern. If you have a couple of bolts sticking out of the back of the pattern you can use them to wiggle and finesse the thing out. Tapping lightly also helps. If you collapse the mould all you do is start again. Talc is the traditional lubricant for easy removal from the mould so cheap talcum powder will do the job.

The sand can be re-used provided you remove the dried stuff which has been in direct contact with the melt.
 
doug748 said:
Your pattern looks too rough. You need to file and emery it to the best finish you can, using filler for any bad patches as required. Then ensure that the draught angles are good in every plane. I might paint it, it does have to be rather smooth

Sand is easier for this type of work. You can get Petrobond from these people:

http://anvils.co.uk/products/view/402?cat=33

Unfortunately, rather a lot, if you go on that foundry visit you may be able to scrounge some. It is an oil based product so no messing about with water.

Knock up a wooden box a couple of inches deep and fill with sand, level off with a straight edge, ram to a firmish consistency and push in your pattern. If you have a couple of bolts sticking out of the back of the pattern you can use them to wiggle and finesse the thing out. Tapping lightly also helps. If you collapse the mould all you do is start again. Talc is the traditional lubricant for easy removal from the mould so cheap talcum powder will do the job.

The sand can be re-used provided you remove the dried stuff which has been in direct contact with the melt.
Click to expand...

It's clearly a lot more difficult than I thought it would be. :(

I think that the roughness in general is not too bad as the surfaces which are tapered have separated cleanly. However, that central boss is both deep and parallel-sided so is always going to be a pig.

Do you think it's worth smoothing it off and possibly painting it and trying again or is a deep parallel-sided pattern (as you called it) always going to be an insurmountable challenge, even with the sand, which I suppose might collapse as it will be trying to sustain a high vertical wall.

Thanks to PaulRainbow as well as I think he is saying something very similar to you.

Has anyone on the Forum succeeded in casting a saildrive anode?

Richard
 
Last edited:
Sorry, I have not seen any other posts on the topic. I see now, I think.

That centre section will not come out of any simple mould regardless of material, as you say it is parallel. It would have been produced as a die casting and these are often very complex indeed.

However If you could file an angle a minimum of 2 degrees on that centre section ( without compromising it's function) you stand a chance with sand. This is because you do have some wiggle room with that medium and with care, good luck and a following wind you could get it out.
 
Back in the dark mists of time I worked for a short time in a casting shop. If I remember correctly, that centre section of the mould would have been a 'core', ie a separate piece of the mold. I can't remember if it would have been sacrificial or re-useable.
Then again I may be over-remembering it.
 
I trained many years ago in fiberous plasterwork and use to do all sorts of mouldings the reason has already been hinted at by Alahol and Doug748, what your taking a cast off has some vertical surfaces, there doesnt appear to be an undercut to it which would allow the model to come out of the plaster cast easily. The only thing you could try is a mould release oil, that may help it slip out.
 
If you get a chance to visit a foundry, jump at it. I have never forgotten a tour of a valve manufacturer, many years ago. The skill and dexterity of the (women, mostly, IIRC) making and assembling the moulds was very impressive. The cores were made from finer sand than that for the outside of the casting and in answer to alahol2’s point, the core sand in that instance got mixed with the outer sand on being shaken out, so had to be replaced each time. The outer sand was re-used.

Readers of David Lodge’s novel Nice Work may recall that the top of a sand mold is called the cope, which is a metaphor, and the bottom is called the drag, which is a metonymy. I’ll get my coat …
 
I would also suggest sand casting, or a sacrificial core for the middle of your plaster mould.

With the method you are using, your next issue will be that the material you are trying to cast will shrink at a higher rate than the mould, so it will lock it's self on. A relativly large draft angle may eliminate this if your application allows.
 
I did some lead casting recently, and used sand (ordinary builders - mortar for use in) but made the mould with the sand pretty damp. That let me do the shaping that I wanted, and any mistakes were easy to fix with a spray of water and tamping the rogue bit. I then dried it in the oven for a goodly while and poured the lead while the mould was still hot.

My object didn't have parallel sides like yours, but I'm pretty certain that damp sand will let you get it out OK.
 
Might I suggest that you are trying to cast a machined shape?
While the anode might have started life as a casting it seems likely that the inner surface was machined afterwards. If that is the case then you need to add some material to your original (plasticine or the like) to make it taper and release from the mould, both the original and the new cast item. You then machine the centre to match.

AFAIK green sand for casting is bentonite clay (non clumping mineral cat litter) and silica sand plus a bit of horse poo for authenticity.
 
Firstly, thanks to everyone who has offered advice.

I'm starting to think that I would have difficulties in casting this anode in sand. I've never used sand and I know that this is special sand, but I suspect that whilst pouring the zinc I would catch the vertical central edge so that even if it could stand up on it's own, I would probably collapse it!

I would feel much happier pouring the zinc into a plaster mold, even if I had to smash the plaster each time to get the new anode out.

So, I've been thinking about this a little laterally and have come up with two suggestions which you might be able to help me with.

Firstly, I could make a perfect mold by heating the block in photo 1 until the zinc melts and runs out. I now how a perfect plaster mold which I could, in theory, cut cleanly in half and make a re-usable split mold. But, I can't figure out how to split it in a way that would allow removal of that centre core. I had to smash it with a hammer to get in out of the anode in photo 2.

But, once I have that perfect plaster mold, I could make a perfect anode copy in another material which I could use in future mold making.

I firstly thought of candle wax. I could make a duplicate anode which I could use to make more plaster molds and each time I could just melt out the wax and then cast the zinc version. I would have to smash the mold each time to release the finished anode but if I buy the plaster in bulk it's not expensive compared to the anodes. However, I think there's something wrong with my logic here as how would I get the first wax anode out without smashing the mold. OK, I could do that and use the wax anode to make a good mold but it seems like one step forward and one step back each time? :confused:


So my next thought was how about casting the duplicate anode out of some kind of slightly squishy rubber. It would need to be firm enough to allow me to make new plaster molds from it but soft enough so that I could then squish the rubber anode out of the plaster and leave the perfect plaster mold to pour the zinc into. I could use the rubber anode over and over again which is a lot less hassle than the wax idea, even if that could be made to work.

So, what does anyone think about split molds or candle wax (and feel free to say that even for PBO this is all just toooooooo much hassle!), and, if you think the squishy rubber idea has any mileage in it, does anyone know of any kind of DIY rubber molding material? :)

Richard
 
I would be surprised if you couldnt get away with a few degrees of taper on the central hole of the anode.

As for a plastic moulding material, the obvious answer is silicone rubber. Or what I would try is first to make a female mould using glass fiber filler and after coating the master anode with release wax - you might have to fine sand this anode to get it smooth enough. Then I would use that female mould to make a male mould / copy of the master anode in silicone. If that is coated with a thin grease or maybe PTFE before making the plaster female mould then it should be releaseable.

You might like to try re-inforcing the plaster mixture for the female mould using glass strands.

As an alternative strategy, how about making a central plug in hardwood with only the outside bit in plaster?

How did you melt the zinc or havent you got to that stage yet?
 
Last edited:
Don't imagine that the sand made using the link I posted is something like the stuff you or your children make sandcastles with! It is considerably stronger and holds up well. This is the stuff commercial foundries use for far more difficult casting metals than zinc and considerably greater complexity.

I don't know of any rubber that will take the melting temperature of zinc for any length of time. You might get away with silicone but I suspect it will remain at very close to its degradation temperature for some time unless quenched.
 
You must log in or register to reply here.

Share this page

Top