Whitemetal Casting for Beginners Part 1 -The TheoryReturn to main vee tipper page
Nigel Lawton: 3,074 Words Version 2
This is not a condescending ‘for beginners’ but a ‘by a beginner’ article – my prior experience in this area is restricted to lead smelting as a child the prospect of which, now as a parent, I find horrifying! I used to collect lead which had fallen of the roof of mainly my parents’ house – surprisingly plentiful as its an old and much repaired slate roof (no really, honest). I would melt it down in an old pan over a campfire in my parents’ garden creating many ingots of lead by pouring the resulting melt into the lids of Humbrol Enamel tinlets. I had no real use for the ingots – I just did it for the fun of doing it (no change there then). More recently I have made use of resin casting both professionally as a way of generating models of plastic mouldings during the product design process and at home to reproduce the occasional item for toys and even models. The former makes use of rubber moulds then vacuum-casting specialist resins, the latter involves making a plasticine or even Blu-Tack mould by pressing in a master and pouring in ordinary rapid-setting epoxy glue. I have successfully reproduced an OO Thomas the Tank type ‘face’ (used in converting a GWR pannier tank to ‘Duck’) and even OO9 works plates. Of course this method is only useful for items which have one flat and featureless face.
The main reasons I decided to have a go at whitemetal casting were firstly that I had for some time bemoaned the non-availability of a certain type of rolling stock which have now become the subject of my casting efforts. Secondly I came across a ready source for the materials and some basic directions on ‘how to do it’ in ‘Hobby’s Annual’ which is the catalogue of Hobby’s of London from whom I have since purchased silicone mould-making and whitemetal casting materials normally sold for ‘toy soldier’ and chess-piece making purposes. Other materials needed include plasticine and tools to be found in most modellers toolkits, plus the use of a hotplate. The set-up costs at £30-£40 are not prohibitive in my view although I recognise that some are on restricted budgets for whom this may cause difficulties. The complete process of whitemetal casting is described below with reference to the materials used shown in the associated picture.
The process can be described in three simple steps:-
1. Make a master.
2. Make a silicone rubber mould around the master.
3. Reassemble the mould and fill with whitemetal to replicate the master.
Firstly a master is needed from which to generate the mould. This is presumably going to be an item which you can scratch build but will want to produce in quantity – the list of possibilities for OO9ers and other modellers is almost endless. In a later article I will describe and show photographs of a casting project based on a OO9 V-tipper truck or ‘skip’. Although its possible to make a one-piece mould as for most practical purposes a two-part mould will be needed and the remainder of this article assumes this approach.
An item’s suitability for moulding is mainly judged on two factors:- wall thickness and ‘can it be removed from the mould’. Wall thickness is simply the thickness of metal cast in any part of the mould - the minimum wall thickness I have successfully cast over larger areas is 0.7mm and this is close enough to the limit for minor variations in the casting process to result in holes in the castings. Localised areas can be significantly thinner than this down to perhaps 0.25mm depending on the size of the feature and its position in the mould, smaller features cast better in a pre-heated mould. These wall thicknesses are ambitious but can be achieved using the simple gravity approach described in this article. Note that this does not include very small details like rivets which will be reproduced well so long as the wall thickness on which they are positioned is adequate. Very thick walls or sections can in theory lead to uneven shrinkage problems as the whitemetal cools but as I have not tried anything more than about 3mm this remains only theory.
‘Can it be removed from the mould’ is best judged by how many holes the item has right through it – I mean holes you need to reproduce not small holes which can be drilled out later. This is where you have to consider the ‘split line’ of the two moulds - one hole right through is no problem so long as the split line is through the hole so the two parts of the mould can be removed from either side of the hole. If there are several holes all the way through the item should be sub-divided into two or more castings to avoid this situation and the castings assembled to form the complete item.
Having selected the split line the master is embedded in Plasticine (5) up to the split line to allow the first half mould to be produced. Its worthwhile carefully considering which half of the mould to do first as some masters may ‘float’ in the liquid silicone rubber perhaps only slightly but a small movement can spoil the mould. As for the first half of the mould the master is secured by plasticine (or even some Blu-tack in the critical area if plasticine isn’t sticky enough) this is the best time to do the more problematic half. When making the second half of the mould the master is only secured by the first half mould and if this has no significant side projections problems with floating could happen, particularly with plastic rather than metal masters. Particularly for lightweight masters create the half of the mould on which the master as the largest side projection first. With the master embedded in plasticine create a plasticine slab around the master level and in line with the split and cut to a rectangular shape. Leave about ½"/1cm border outside the master on three sides with about 2"/5cm on the side you think best for a metal entry point. This is where you will later cut a channel into the cavity left by the master to allow the molten whitemetal to be poured in. When selecting the best place for this choose either a hidden area or one with no detailed features which can easily be shaped back to form with files etc. Build up a wall around the slab to ½"/1cm higher than the highest point of the master using card, and/or plasticard and/or cellotape. At this point you need to identify some alignment ‘pins’ which will keep the two halves of the mould in the correct relation ship to each other during use. You can get away with two of these in theory positioned diagonally in two corners of the mould in the ½"/1cm border. In fact I think you get better alignment with four pins, one in each corner of the mould. Use some metal rod or tube about 3-4mm in diameter – I use brass tube, don’t be tempted to use very thin rod as this gets loose quickly in use I think because of the smaller bearing area with the mould material. Embed the alignment pins into the plasticine the desire positions.
The first half mould can now be poured. Note that there is no need for mould release suspension at this stage. This is only required to prevent rubber-to-rubber adhesion. Calculate the amount of rubber needed by using the length x width x depth of the mould to be made – this gives the volume in ml if all dimensions are in cm – you can make an allowance for the displacement of the master if you think it significant. You now need to make up this volume of rubber. Mixing the silicone rubber (1) and hardener (2) must be done quite precisely – I strongly recommend the use of a measuring beaker (4) which I already had to hand. If you don’t get enough hardener in there the rubber just stays liquid – its not like epoxy glues where they will still set ‘eventually’ by migration of the hardener. The silicone I use specifies 1-2% hardener and gives helpful ‘so many drops per 100ml’ guidelines which I have successfully used. The correct amounts of rubber and hardener having been put into the beaker (see suppliers instructions) these must be mixed very thoroughly as I found that despite stirring well small volumes of some of my earlier moulds had un-set rubber which I then hardened off locally by cutting and dropping in hardener. The rubber is now ready to pour but before just chucking it in use a suitably sized paintbrush to paint a thin layer of rubber onto all exposed parts of the master. This reduces the risk of bubbles at the surface, the brush can be completely cleaned with white spirit. Now fill the walled in area around the master by pouring in the rubber to completely cover the master by about ¼"/6mm. Pour the rubber into the border area, not over the master – again this reduces the likelihood of surface bubbles.
Leave the rubber to cure for the prescribed time – 3 hours minimum is to be expected. Then carefully remove the half mould, plasticine and master from the walls (you’ll need these for the second half mould) and carefully separate the plasticine, master and half mould. In fact you can leave the master in the mould until the second half-mould is complete unless the master includes styrene parts which will be affected by the mould release suspension (3), the type which I used being based on a solvent which would dissolve styrene. Re-fit the walls around the half mould ensuring that there are a snug fit without cracks etc and paint the exposed parts of the half mould with mould release suspension. This is simply very fine Teflon (like on non-stick pans) powder in suspension in a solvent. Once this has ‘dried’ replace the master if it has been removed. Also place the alignment pins into the first half mould.
The second half-mould can now be poured in the same way as the first half mould and left to cure as before. Again remove the walls and carefully slightly separate the two half moulds all the way around along the join line using a very sharp craft knife to cut through any joining sections of rubber caused by less than perfect fitting walls. Again very carefully fully remove one half mould from the other half mould an master then remove the master from the second half mould cutting through any ‘trapped’ holes with your very sharp knife. These cuts need to be really clean to ensure the mould goes back together with minimum distortion.
The next task is to cut an entry channel for the whitemetal. Much of the decision as to where this should already have been taken when deciding on which side of the master to position the wider boarder. A decent length of metal feed channel provides some hydraulic pressure into the mould during pouring and improves the filling of fine detail and thin wall sections. Cut the channel with a sharp craft knife in one or both halves of the mould narrowing the channel sharply at the point it meets the cavity formed by the master to minimise the amount of filing. If you get it right and your mouldings are reasonable robust the castings will also conveniently ‘break off’ at this point. Don’t choose a very small thin wall section to introduce the metal – start small and increase the opening if the mould does not fill well in practice. The main part of the channel should be at least ¼"/6mm x ¼"/6mm with a slight funnel at the point where it exits the mould to assist with pouring. You now have a complete2-part mould (6).
I must admit to impatience over the next bit which is to leave the mould to fully harden (vulcanise) for a minimum of 4 days before use! Patience is rewarded by a mould which will last.
The next stage is of course to actually make some castings! The process is simple in concept (melt the metal and pour it in) however observing a few instructions will shorten the time you take to produce the first useable casting. My rate started at several hours per casting and reduced to several minutes, certainly when making several I would say less than 5 minutes for a simple casting and nearer 10 for mor complex ones (more wastage and more time setting up the mould). Firstly don’t start with a cold mould, wrap it in aluminium foil and stick it in a preheated oven at 180C for 15 minutes or so (the foil is to protect the mould from all that gunge on the oven). If you start with a cold mould all you will do in the first few pourings is make partial castings and warm up the mould. Although you can re-melt whitemetal it does lose its properties and this should be minimised. A note here about the type of whitemetal to use – I strongly recommend a type intended for making small detailed models – I use ‘Model Metal’ by Prince August (10). Don’t be tempted by cheaper or lead-free alternatives unless you don’t mind your castings looking like they were made in a jelly mould.
Before you take your mould out of the oven set up your ladle and hotplate and melt a whitemetal ingot or part of one ready for use. The temperature is important at some level – put simply too cool and it won’t flow through the mould well and too hot and it will damage the mould. The rule of thumb is to use a struck matchstick and if it smokes slight when dipped in the melt that’s about right whereas if it chars quickly that’s too hot. Take the mould out of the oven and dismantle it. Lightly coat the inside of the mould including the joining surfaces with talcum powder (8). This allows air to escape from the mould and provides some degree of protection to the rubber. There should be no ‘talcum drifts’ just a light overall coating. Reassemble the mould making sure all alignment pins are correctly fitted. The two halves of the mould should be very gently clamped together using some pieces of plywood or hardboard and a mould clamp or rubber bands. If you use the type of clamp I did you will have to weaken it considerably by bending it so as to reduce the clamping pressure on the mould. Too much pressure results in poor flow and compression of the mould cavity making the cast the wrong shape! A gentle pressure is enough to close the mould join enough to prevent the metal escaping but still allowing the air in the mould to escape. I have found that gentle all-around pressure is preferable to cutting air channels as suggested in some instructions as this results in further loss of detail whereas even an over-light pressure only results in some flash.
Now for the final step – pouring the casting! The whole process of preparing the mould from the oven should have taken only a couple of minutes and the mould should still be quite hot. Sit the mould on newspaper on a flat surface with the inlet funnel to the top. Pour the molten whitemetal into the funnel at the top of the feed channel gently tapping the mould with something (not fingers!) with the other hand. This helps to release air bubbles in the mould. If molten metal pours out everywhere you have insufficient clamping pressure! Otherwise fill to the top of the feed channel. Now wait until the visible surface has set and then a few more seconds, now carefully remove the clamping arrangement and remove one half of the mould. Try not to disturb the casting at this stage. You will now be able to see if you have succeeded – frankly you should not expect to at the first try. You need to carefully examine your attempt and work out why it is not perfect. It may be that the entry channel is not big enough or that flow through the mould inadequate because of too much clamping force, too thin wall thickness or not enough talc. Make the appropriate adjustments and keep trying! Some advice on pouring castings contained in the ‘Hobby’s Annual’ recommends the cutting of additional small channels to allow air to escape if problems are experienced in filling the mould. I suggest that this should be a last resort as it inevitably leads to loss of detail and the need for much more cleaning up of the resulting castings.
The major satisfaction in this activity is the sense of achievement experienced, after trial and error and gradual improvement of your mould and technique, in opening the mould to find a perfect casting shining and new like a Horse Chestnut when you tear open its shell. Once I reach this stage I find it pretty addictive and just keep on making castings until I run out of metal – usually many more than I actually need! All that remains is to carefully remove the casting from the mould minimising any bending of the mould which can lead to splitting, and put the casting aside to cool whilst you apply more talc and reassemble the mould for the next pouring. Keeping the mould closed as much as possible keeps its temperature up and makes continued successful casting more likely. The inlet funnel casting needs to be removed from your casting using a razor saw or junior hacksaw. If you do this whilst the next casting is cooling you can immediately recycle the whitemetal in this which is likely to be most of the quantity used in the pouring.
A continuation of this article will contain an illustrated description of production of ‘master’ items and moulds for a 009 Vee tipper or ‘skip’. stock.
W. Hobby Ltd.
Knight’s Hill Square
London SE27 0HH
Phone 020-8761 4247
Fax 020-8761 8796