Lost Wax Casting

[SteveW]

Guys,

This is really aimed at Abby. I've just seen your intro (in "Say Hello") and that you're into lost wax casting. This is something else on my long list of thing to know more about.

The only book I've seen advertised is in the Camden catalogue and that is claimed to be aimed at third world craftsmen. I think I understand the technique but I'm not so sure of the materials used. Yes wax but what is it coated with? I've seen that things like cow dung is traditional because of the fibre content which provides some porosity but mostly strength but that's about it. There's also the question of where to get the stuff from.

[modeng2000]

You could try a farmer. ;D

[greasemonkey]

i thought cow dung mixed with straw was only used for castings bells? Investment casting is noramlly done is a plaster type material.

Andy

[baggo]

Hi Steve,

Abby will no doubt be able to answer your questions better but there was a series of articles in ME by J M Pace in vol 173 describing his methods and equipment used. There was another series later that described the commercial process used but I can't find that at the moment.
The actual moulds are made from a ceramic material like plaster. Materials used to be available from Alec Tiranti amongst others.

John

[Tony K]

I've seen that things like cow dung is traditional because of the fibre content which provides some porosity but mostly strength but that's about it. There's also the question of where to get the stuff from.

Steve - why not try number 10 Downing Street.

I think he has only the male variety though.

Regards, Tony.

[Deleted]

I've always made my own patterns for lost wax castings; wheels and cylinders.
I make a master pattern from aluminium then make the mould for the wax from cold cure silicon rubber, waxes are then sent to the foundry who attach all the runners and risers etc. I believe the clay is sprayed on though I could be wrong, it's certainly a thin layer. The whole sprue is then inverted and the wax vapourised out in an oven before pouring of the metal.

My moulds are usually 2 part, but the beauty of silicon rubber is that the mould can be cut up further with a Stanley knife to aid removal of the wax if problems are encountered. For cylinders I use stainless steel cores wiped with Vaseline for the bores, these need to be pulled out while the wax is still hot.
I found Tiranti's a little expensive and use South West Industrial Plasters in Wiltshire: 01380 850 616.
The rubber used is not suitable for high temperatures but is much cheaper than the stuff used for whitemetal casting.
The main problem with the rubber is getting rid of air bubbles created whist mixing in the catalyst, my own solution to this is to place the container on my jig saw and let the vibes do the rest!

One important point to remember is that you need to make a double allowance for shrinkage.

[SteveW]

Guys,

Thanks for the quick info. I found an article in amongst a load of old ME mags on lost wax. That guy used dental supplies. Like most problems that need solving it's mostly a question of familiarly with the items and materials in use. It's also handy to be able to pass by them in normal life.

From what I remember of bell founding I think horse hair was also required and the trouble with No 10 is his neighbour. If he ever finds out what I'm up to he'll be taxing cow SH1T along with everything else.

(SH1T gets converted to "nuts" for some reason. Huh?)

[Tony K]

If he ever finds out what I'm up to he'll be taxing cow SH1T along with everything else

Mmm! A tax on bullsh1t as well. Unlikely I think since it is likely to affect politicians more than anyone else by at least one order. Suppose they would have to have a bullsh1t allowance to offset the cost. Then there's the measuring of bullsh1t and the inevitable bullsh1t league tables. ;D
OK - I'll shut up.
Regards, Tony

[abby]

Hi guys , I reckon all your remarks are valid - especially the no.10 quip , lost wax casting is not difficult but does involve the use of specialised equipment and like everything small quantities of anything are expensive. I use the "block moulding" method using a plaster based refractory cement. This requires a heat treatment cycle of 24 hours to remove the wax and fire the mould and a kiln is necessary. Pouring of the molten metal is done with vacuum assistance to draw trapped air from awkward crevices. I could not in all honesty recommend anyone to set up their own facility unless they had a wish to learn rather than make models ,but I would be pleased to cast anyones wax patterns if required .

If anyone is interested this is from my G1 black 5 project , I can supply most details for anything you might require if you do decide to have a go

[Noddy]

Hi, somewhere I have some old photocopies from a booklet collated from old british engineering mag articles (from about the '50s) about investment casting, the booklet was in the library of camborne college about 8 years ago.

There were also a series of articles in the industrial minerals mags, more from a refactories point of view...

apparently the comercial wax oused has a known thermal contraction (pretty essential).

depending on what you'r casting, the wax is dipped in a suitable binder and sprinkled with a mineral flour (typically fused silica (next to zero thermal expansion) or something more exotic like alumina or zircon for higher temperatures...), the flour gives a fine lining to give a good surface reproduction.
Following this the mould is then dipped an sprinkled or sprayed with gradually coarser refactory, to give physical strength and gas permeability.

Apparently at one time moulding boxes were used, but these introduced more labour requirement than simple spraying dipping and dusting.

from a model point of view, (am I diving in where angels fear to tread?) I don't see why a wax pattern should not be dusted with finely sifted conventional sand - coke mix then treated like a wooden pattern in a normal moulding box, with the bonus that you don't need to remove the pattern, just vapourise the wax by pre heating the mould.

Anyone any thoughts ?

From a gun point of view, Ruger and CZ / Brno use investment casting for rifle parts, these require only minimal machining and very little finishing. Apparently the Ruger parts are straightened before machineing with a big sledge hammer, and are annealed in a carbonising atmosphere to replace any carbon burned out during the casting. apparently the stressed parts are dimensioned slightly larger to allow for the random grain and any internal flaws compared to a forging, although with aerospace, hot isostatic pressing (compressing in an inert or hydrogen atmosphere (i heard of one company that adapted an old naval gun barrel as its pressure vessel for this) to close up any internal pores and voids) is used to improve the physical properties of high performance castings such as gas turbine blades. (things don't get much higher spec).

Returning to guns, surface porosity is the main reason for failure, and this can be addressed by grinding out, welding and re finishing.

gone on far too long... happy research

[abby]

Hi Kieth the process you describe is "ceramic shell" and is ok. for solid parts ,but hollow items are not easily cast this way. the shell consists of a binder and refractory powder. It may take 4 or more coats to build up a suitable thickness for the shell and each coat has to dry before adding the next. The waxes used for pattern making are actually low melt plastics rather than wax , they are normally injected under pressure into a die to create a pattern , often certain chemicals are added to control contraction but there will always be some, It is possible to use a polystyrene pattern in traditional sand moulds , which literally evaporates when the molten metal is poured in. Wax patterns are destroyed during the lost wax process and it is normal to dewax moulds before the firing cycle , this is accomplished by heating the moulds at 180 deg C for 3 or 4 hours with the moulds upside down to allow the wax to melt and run out , ceramic shells are dewaxed using steam as the slight expansion in the wax on heating can cause cracking of the shell if done "dry". As for using a wax pattern in a conventional sand mould -what would be the point ? the whole reason for investment casting is that shapes with undercuts and fine detail can be reproduced with high accuracy . How would you ram up a wax pattern ? If you melted the wax it would diffuse into the sand, and go off like a bomb if you poured molten metal in !Moulds are typically held at 750 degrees c for 5 hours or more to remove every last trace of wax before the metal is poured. You may find some details in old books but today investment casting is carried out at the leading edge of technology just like CNC machining or laser cutting and you won't find much thats relevant to modern working.
Abby

[SteveW]

Abby, All,

Brilliant stuff! Thanks, I'm glad I asked.

[Noddy]

Hi Abby,
I aggree, industrial investment casting is far removed from what we are capable of as model engineers, but (and it is a big one...) most of what we do is far removed from current large scale manufacturing practice.

Somewhere there has got to be a model engineer level of technology, that is part way between the BS and mud mix used to cast the Benin bronzes, and the fully automated investing and vacum casting used in aerospace...

The high spec refactories and binders are not readily available to amateurs, but surely by using finer grained and suitably bound linings to our moulds, better finishes and closer tolerances than with traditional sand casting should be possible, shouldn't they? or am I really diving in with both feet?

[SteveW]

Guys,

Having just read Keith's item above it occurs to add that in my researches I found the suggestion of a weak sand/cement mix as a suitable moulding material (not forgetting moisture control etc) but for general moulding and not lost wax. I know I'm not using the correct terms but I did do proper sand casting of alloy years ago at school. Apart from being potentially lethal in the hands of the novice it has got to be THE way of doing some things.

As an asside I recycled 30 years of smokeless coal dust back into usable bricketts using a 7 dust/1 cement and it kept me going a whole winter.

[abby]

Hi Keith - I appreciate your comments but in fact the modern materials necessary are readily available , they are typically sold for jewelery production and are supplied in quantities suitable for modelers . waxes can be purchased in pellet form for casting patterns or in sheet and section for fabrication of patterns , a metal melting furnace is quite easy to build , I built my first after reading an article in ME many years ago.
The other bits and pieces are available from various sources , but regular visits to your local junk yard will pay dividends.
I buy my investment cement ready made , but basically it is one third plaster of Paris and two thirds flour grade refractory such as crystobalite .
I have made my own which works well for castings that are finished all over as it sometimes gets hairline cracks which appear as fine lines or fins on the castings.
Fine castings can be produced by sand techniques , but the moulding of complex patterns with undercuts is a very skillful trade requiring techniques that take a great deal of practice. Lost wax removes the need for such skill and makes it possible to produce castings with walls as thin as 1 mm or less and multiple undercuts.
I have spent many years producing castings on a home workshop level and would be pleased to share the knowledge I have gleaned with any modelers who are interested.
I am sure most modelers would find it easy to produce their own wax patterns for something like an engine cylinder block and it would be then simple to send these to a lost wax foundry for turning into metal. Perhaps we could run an article on such a project ?

[alanstepney]

Abby, It seems that you have the knowledge and experience to write such an article.
If you have the time, and enthusiasm, to do so, I would send it to the model press and see if they are interested.

If not, I would willingly host it on my website (with full acknowledgement of course), so that the information is available to everyone.

[lampstart]

hi Thirty years ago I set up a number of investment cast parts in the course of my job,most were short batch runs ie several hundred in alumunium prior to ful production runs of die castings plus I was involved in castings in various metals.The advantage of investment is relatively high precision,good surface finish,minimal porosity,but quite expensive due to high labour content.The wax has to be produced from a mould usually made from aluminium,the wax is injected into the mould under low pressure,sometimes on complex castings more than one mould is used and the waxes are bonded together,the big advantage with investment is that parts can be any shape and do not have to be "drawn" from a mould .Cored holes can be made by using sliding cores in the aluminium mould but unless well fitting can form flash on the wax which has to be removed by hand.The waxes when complete are either sprayed with a number of layers of the plaster or placed in a can which has no bottom and plaster is poured around the wax,the wax is melted out.In the case of the can process the mould is set over a hole in a bench and vaccuum applied,the plaster being porous,as the metal is poured air in the mould is removed by the vaccuum.The whole process is not suitable for low quantities,it would be easier to machine say a cylinder set from solid than make the mould tooling,if a cast material was originally specified for its bearing qualities ie cylinder bore,then fit a separate liner.
In the replies to the query,I saw mentioned the use of animal dung,this has nothing to do with investment casting,up to about fifty years ago it was used as one of the ingredients in the cores used in sand castings,it made the cores porous and allowed trapped air to escape reducing the porosity in the casting,as one foundry man commented porosity is difficult to eliminate,the skill is putting it into an area of the casting that was non functional.

[abby]

On the subject of patterns for lost wax casting , these photos are of a oo/oh scale 1804 Trevithick locomotive boiler , obviously non -working, but cast in one piece and the boiler is hollow - see picture 4.




the boiler is about one and a half inches diameter and the hole into the boiler is about a quarter of an inch across , that makes the square nuts pretty small.
The surface quality is fairly pitted - if it were rusty it would look like the real thing lol!
But I cheated - I used a plastic model kit , just to see what would happen ! the pitting is caused by the fillers in the plastic as they are inert and don't burn away 100%.
I post this to show that anything can be used as a pattern as long as it will burn away during firing, in fact much old jewelry was made in this way using insects and small animals , there is a story that Michelangelo was accused of using a human body to produce his sculpture "David" and to prove this untrue he made an identical sculpture half as big again !
Abby

[mesuno]

Hi folks, I'm new here but about to go on a model engineering adult education course with my dad. Crazy christmas present idea.

Anyway, I don't know much about model engineering however I've spent a large part of today reading about casting using sand moulds, and recycled aluminium. Is there any reason why you would prefer lost wax over sand? There seem to be fewer stages involved, you keep the original and can do it all at home. I can see that it may take a little more cleaning up than lost wax would as the surface may be less clean.

www.abymc.com/forum/

These guys do a lot of their own casting and use essentially free scrap metal (aluminium cans etc..) and charcoal as fuel for melting it down.

[Myford Matt]

Most interesting Abby - hope you take up Alan's offer/suggestion