Dinky Collector Restoration Series

Section 6 - Casting

 

 

Although the majority of parts are available from suppliers - not all parts are available.
Sometimes, you may come across a model that has a broken part or a part that is missing, you check your suppliers listings and find that the part is not stocked.
So what do you do? Do you just throw the model away as being useless? No - not just yet as there is always a last resort.
You could try casting your own part.

The original Dinky models were what is known as die cast using a mixture of metals know as Mazak, Zamak or white metal. This is a zinc based alloy that has a fairly low melting point similar to lead. Some of the very first die cast models were actually made of lead.

This process can be done on a small scale at home by making a mould, melting metal and pouring it into the mould.

How successful this will be depends mainly on the shape of the part to be cast. Simple, flattish parts are the easiest to cast - parts with bits sticking out or holes are more difficult, if not impossible.

Before you start you will need a copy of the part to be cast. This can be an original part temporarily taken off another vehicle or a 'mock up' made of whatever substance suits you (clay, playdough, wood, metal or a mixture of all if need-be). You will need to have an exact replica of the part to start with.

Once you have your 'part' you will need to make a mould. In the process that follows I cast a new arm for the 561/963 Blaw Know Bulldozer.

Broken link arm on a Blaw Knox Bulldozer.

To start you will need some modelling clay (plasticene), some Plaster of Paris, an old model (to melt down), something to melt it in (I use an old stainless steel eggcup with strong wire formed round it it to form a handle), a gas hob, safety equipment (strong leather gloves and eye protection),some small bits of timber doweling, a wooden board (as a work surface) and of course - your replica of the part you are going to mould.

Start by taking some modelling clay. Work it in your hands until it is soft and pliable. Once it is soft, use a piece large enough to flatten out - use a bottle or rolling pin. Press your part into the clay to half its depth. If you are using an original part you may be able to use the original casting marks as a guide.

Using small bits of timber or dowel, matchsticks or wooden skewers (or even modelling clay if necessary) build up the pouring tube at one end. It is a good idea to add a couple of smaller vents at the outermost ends of the part to act as vents to allow any air out of the mould when the metal is poured in.

When this is done, use timber or modelling clay to build up a wall around the outside. Using a piece of dowel or a pencil, make a series of indentations into the modelling clay to act as locators for the second half of the mould. Finally, coat the whole of the inside including the side walls with a thin coating of petroleum jelly (Vaseline). Some modellers use French chalk instead of Petroleum jelly. Either of these will act as a release agent.
It should look something like this when finished.

Mix up enough Plaster of Paris (follow mixing instructions on the packet and mix to a consistency of double cream) to fill this mould.
Pour the mixture into the mould fairly quickly but avoid pouring directly on the original part. This is to prevent getting air bubbles in the mixture.
Once the mould is full, tap the baseboard underneath sharply and rapidly a number of times. This is to release any trapped air in the mixture. The trapped air will be seen as bubbles rising to the surface. Keep tapping until no more bubbles come to the surface.
Leave in a warm place )on top of a radiator is ideal) for 48 hours to ensure that the plaster is fully dried.

Leaving the part and the pouring hole timbers in place, remove the layer of modelling clay.
Rebuild the walls around the outside and apply a thin layer of petroleum jelly or French chalk. At this stage your mould should look similar to this -

It is now ready for the second half of the mould to be poured. Mix up Plaster of Paris as before and follow the same method of pouring and tapping.
Leave for at least 48 hours to dry.

You now have both parts of the mould - the thing is - you still have the part embedded in the centre.
Gently pull the 2 halves of the mould apart. Remove the blocks use to make the vents and then very carefully remove the original part. Try not to damage any of the plaster mould around the part when removing it as these will show up on the finished product as defects. Once all the bits have been removed give both halves of the mould a brush out to remove any plaster debris caused while removing the parts.

At this stage it is imperative that you ensure the plaster is fully dry. If any moisture is present in the mould when the liquid metal is poured in can in the worst scenario, cause the mould to explode and at best will cause the liquid metal to spatter - both of which will be detrimental to both you and your casting.
Both halves of the mould should be pure white and, if tapped, should have a clear ringing sound. Any off-whiteness or dull sound indicates that moisture is still present in the plaster. If in any doubt - leave to dry completely before proceeding any further.

Dust both inner surfaces of the mould with French chalk to act as release agent and put both halves together. Hold together firmly using elastic bands, tape or a clamp (if using a clamp don't put too much pressure on or the mould will crack).

Place the mould on a flat stable surface ensuring that it cannot fall over. You are going to be using extremely hot liquid metal so for safety, I would suggest you use a wooden baseboard in case of spills. Place this as near to your gas hob as possible so that you don't have to carry hot metal across rooms etc.

Break up some bits of white metal - an old scrap model is ideal for this. Place these into your melting pot. Melting pots can be purchased from hobby supply shops (Prince August metal casting) but I use an old stainless steel eggcup with a handle made from wire.

A old stainless steel ladle (not aluminium - it will melt) could also be used.
Hold the melting pot in the flames to heat it up. As the metal starts to melt add more bits until you have sufficient quantity to fill the mould.
When fully molten the metal will form a skin of scum on the top. Scrape this off using a screwdriver or similar implement.

Pour the molten metal into the mould slowly to allow air to escape. Stop pouring when you can see the vent hole starting to fill.
Leave to cool down enough to be able to handle and remove the bands holding the 2 halves together. Carefully separate the 2 halves. The casting will be in one of the halves.

Carefully remove the casting from the mould. Try not to chip the plaster so that the mould can be used again if required.

Clean up the part by cutting off the moulding sprues (vents and pouring tunnel) and clean up with a file (this is called 'fettling').

You should now have a new cast part the same as the original part.

All that needs doing now is to drill the holes for the pins and give it a coat of paint.


The model with the new part in place.


 

Return to Restoration Main page

Home Page & Index