Casting your own models

A resin model of the Nebelkraftwagen (SdKfz 11/4) which will serve as an example in this article. (Master by Henry Klom)

Here I like to go one step further and share my experience in casting a complete 1/76 scale model.
1/76 scale models are very suitable for casting yourself, because of their small size the amount of r.t.v. rubber required for the mould is not so big and so the costs involved are low. Also because the small quantity of resin in the model there is no problem with heat build-up during curing, this makes it possible to cast the model hulls solid and so you can avoid the complications of making two piece moulds.

The materials: Right, silicon rubber with its harder; to the leftt, polyurethane resin component "A" & "B".

Safety:
First a word about safety, always remember you are working with chemicals.

This means:
-Read the instructions carefully.
-Work in a well ventilated area.
-Protect your eyes, hands and lungs.
-Do not smoke or eat while you are working.
-When finished lock everything up in a safe place out of reach of children.

In most cases the shop where you buy your resin and rubber will also sell disposable gloves, safety goggles and suitable filtermasks.

Even complicated parts, like this armoured car hull, can be cast out of a single piece mould.

The materials:
At the moment my favourite materials to work with are PS81020 RTV silicone rubber and PS115 Polyurethane both from Polyservice Amsterdam The rubber comes in 1/2, 1 or 5kg tins with harder in a separate bottle, The polyurethane in 1 ltr cans of component "A" and "B".
These materials are available in the Netherlands, you can find similar products under different names all over the world. I plan to add a list of products at the end of this article with brand names and suppliers from other countries.
If you have information about the various brands of resin and rubber in your country please e-mail this information to me so I can include it in the list.

The Vacuumcaster, the vacuumpump is fitted under the table.

One or two piece moulds:
After some experiments with two piece moulds I have returned to making my moulds out of one piece. Although with two piece moulds it is possible to make more complicated forms I found that, since I started to cast my models under vacuum, with a little bit care in design it is possible to cast all except the most complicated parts out of a single piece mould.

Casting under vacuum:
Some time ago I "invented" a vacuum casting machine which I will describe below. Since then I have used it for making most of my castings and moulds. If you want to cast your models under normal atmospheric pressure you have to take other precautions to eliminate our biggest enemy "Mr. Airbubble".

The Vacuumcaster:
This consists of a steel vessel (You can use an old pressure cooker )to which a vacuum pump is connected, as a vacuumpump I am using a wrongly assembled air compressor, by changing the valves I reversed the airflow .Now it is pumping the air out of the vessel creating a vacuum. The vessel is closed with a removable lid of 2cm perspex. I use two lids, one is just a piece of perspex, the other has a small hole drilled in it and a funnel made from silicone rubber fitted on top of it. The first I am using when making moulds, the second when casting resin in the mould. A small valve serves to release the vacuum.

The master model, it does not matter out of what material it is made but built it with care as every fault will show up in the castings.

The master model:
First start with a simple model, for a breakdown of the parts take a look at one of the commercial resin models. When building your master remember that you have to be able to remove the cast models from the mould. For instance notice how in this master the window and the openings in the front shaft are closed with thin plastic card.

If you plan to make the mould using vacuum make the model sturdy so it will not "explode" when you are creating a vacuum or be crushed by the atmospheric pressure when the vacuum is released. When completed I fill up the hollow space in the hulls of my masters with resin so they become solid models, but when you do this be sure that the hull is watertight!

The various parts of the master glued to the bottom.

The double bottom:
Make a ca 5 mm high bottom of plastic card and glue all the parts of the master on this. With this model I have fitted the smaller parts on a piece of sprue and glued this to the bottom. This makes the removal of small parts from the mould a lot easier. Take your time for this job, if you use a clever layout you can save a lot of money on R.T.V. rubber
Now make a box from plastic card like the one in the picture. Make the bottom of the box ca 5 mm bigger than the bottom on which you have glued the master, this way the finished mould will have a raised edge around its corners. Later this prevents spilling of the resin when it is poured in the mould.
If you are going to use vacuum when making the mould the box should be at least twice the height of the mould.

The box, made of 0.75mm plastic sheet stuck together with heavy duty adhesive tape.

Casting the R.T.V. rubber using vacuum:
Glue the bottom with the parts of the master to the bottom of the box. Check if all the parts are really fixed, if not they will start to float in the liquid R.T.V. rubber. Now mix the rubber and hardener and pour it in to the box. Place the box into the vacuumcaster and start the airpump. As the air is pumped out you will see the level of the R.T.V. rubber in the box going up because of the increase of the volume of the airbubbles. It looks like boiling milk, if you have not made the box high enough it will boil over. If all the air is drawn out of the R.T.V. rubber, usually after 10 minutes the level of the rubber will drop to its normal height, you can now release the vacuum and let the rubber cure for 24 hours.

The cured mould, now we can start casting the models.

Casting rubber without vacuum:
To prevent airbubbles on the surface of the model first mix the rubber and hardener and using a brush cover the whole master with a thin layer of rubber. Then fit the master to the bottom of the box and pour in the rest of the R.T.V. rubber. Let the rubber cure for 24 hours.

The moment of truth:
When the rubber is cured remove the box. Now carefully take the master out of the mould and check that no parts of the master are left behind in the rubber. Depending of the construction of the master model it sometimes will come out in one piece, but more often it will be destroyed. Remember this if you borrow a model from someone else.

3 Steps in Vacuumcasting
1)Pumping a vacuum.
2)Pouring in the resin under vacuum.
3)Releasing the vacuum.

Casting the model:
After all this casting the model is very simple, use pieces of wood or plastic to keep the mould open. If you are not using vacuum just pour in the mixed resin, with a stick you can check if the resin has entered every corner. Remove the pieces of wood holding the mould open.

With the vacuumcaster:
Place the mould in the vessel, use the lid with the funnel. Keep the small opening closed with a pencil. Start the vacuumpump, if it has pumped out all the air pour the mixed resin in the funnel on top of the lid. Partly remove the pencil so the resin can flow through the hole into the mould, when the resin is poured into the mould close the opening again. Now use the valve to release the vacuum, do not use the hole through which the resin was poured for this as the air entering the vessel will blow all resin out of the mould. Now you can take out the mould and remove the excess resin

Depending on the temperature the resin is cured within 10 minutes. I prefer to remove the castings from the mould when the resin is still a little bit flexible, then there is less chance on breaking parts.
After cleaning the mould it is ready again for the next casting.

The first cast parts, note that by closing the window of the master with thin plastic card we made it possible to remove the casting without breaking off the window frames

The assembled unpainted model.

On an average I manage to make about 25 castings out of one mould , then the mould gets damaged and the quality of the castings becomes worse. If you want to cast larger amounts you can keep one of the best castings of the first series and use this as a master for the next mould.

For others experience and usefull links goto:
Stephen Brezinski's article on Missing-Lynx