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Molds and castings 1, liquid latex rubber.

 

The mold is made from latex paste, and the casting is done with clear casting resin with an catalyst. This is a time consuming job, since it can take days to make an mold. Another disadvantage is the smell from the clear casting resin, which should be done outside, or in a well ventilated area.

After  a few weeks the smell will be less noticeable. The big benefit with this method is the low price.

The products used in this sample are from "Environmental Technology, Inc." I use "clear casting resin" and "mold maker". Click this link for there website.

 


 


  1. latex rubber paste.
  2. brush, to apply latex paste.
  3. liquid resin.
  4. catalyst, needed for the resin.
  5. mixing cup and stir.
  6. always plenty of VENTILATION.

This method is the cheapest and easiest to use. From the original part to copied, a mold is made with special latex rubber. A mold can be made from almost any kind of part, but not from the bottom. This method will not work to make a two piece mold. The latex copy each detail, like brick and such, perfectly clear. Before the mold is made from the part, the part needs to be finished and glued to a flat surface. The latex will flow everywhere during the process, so there can be no open gaps in places like under windows and the ends of the walls.

When the mold has dried some excess latex can be cut off with a scissor, when needed. My experience however is to avoid window panes at the bottom, which gives problems after the resin is cast. It is better to make the window panes with strip styrene.

With the latex I make the first layer of "skin" on top of the part. This will take about two hours to dry, after I add another layer. I repeat this until I have a strong latex "skin". After 5 layers I add some cloth to make a stronger mold. This can be gaas verband or borduur materiaal. The purpose is that this material will let the latex go trough to the previous layer, and it will be imbedded in the mold after a few more layers of latex. This will strengthen the mold, a weak (thin) mold will roll up and can not be used again. When the mold is finished it can be protected with some baby powder, to prevent it from sticking to itself.

Casting is done with liquid resin mixed with a catalyst. After the catalyst is mixed with the resin it needs to be inserted in the mold right away, since it starts hardening quickly. The time of hardening depends on the used resin and catalyst mixture. Mixing the resin with the catalyst starts a chemical reaction as it heats the resin, until it has hardened. The best place to do this is outside or in a well ventilated area. The fumes are not healthy and it smells real bad. The smell will remain for a few days, and then it will weaken. I usually wait a few hours before removing the casting out of the mold. A few days later I continue working with the casting.

The caste parts are going to be sanded and filed until the are acceptable. The resin feels sticky at times, but that might be from a wrong resin/catalyst mixture. Before painting the part apply a primer and then use any kind of paint. I like the water based paint from tamiya the best, but that's a personal taste. Gluing castings together or other materials is best done with superglue.

Here is a photo session of this method.


The original part to be copied is glued to a sheet of styrene.

The first latex layer is applied with a brush.

This repeats until I have at least 5 layers.

After at least 5 layers of latex I add pieces of cloth to the mold with the same latex and continue building up more layers of latex.


By overlaping the small pieces of cloth it will cover the entire mold as one piece.

When the mold is coverd with cloth I still add at least 3 more layers of latex.

The average drying time for 1 layer latex is about 2 hours.

When the latex has dried, it can be carefully and easily removed.

The latex has found it's way under the windows panes, which is why I recommend to leave the window planes out and use strip styrene.


Casting with the resin/catalyst mixture.

The mixture has to be precisely made according to the manufactor specification.

Usually just a few drips of catalyst.

An mold does not necessarily have to be made from a flat surface.

Here I make a mold from the roof top, the same way with multiple layer's.


And by adding some cloth among the layers of latex, to make it stronger.


 

The result is a strong mold.


This method worked fine, with exception of the smell from the resin. However when I learned about liquid silicone rubber, I won't be using this product any more.

 


This page is last updated on 15 July 2012

NLMS - Dutch ModelRailroad - www.nlms.info