RTV Silicone is a silicone compound used to make moulds in order to replicate parts. RTV comes in various hardnesses which is helpful when producing parts which are more complex. As a general rule you want a more flexible RTV silicone as the part you are replicating becomes more complex. At some point your complex part will need to be produced from a Split or Two Part Mould to ensure it can be released without damage or difficulty. Today we are producing a fairly simple RTV Mould as an introduction to Silicon Mould making.
RTV Silicone is used to castings using Polyester and Epoxy Resins, PU Fast Cast Resins and Jesmonite. Once you have made your Silicone Mould you can reuse it time and again and replicate your casting many times. It is a very cost effective way to re create items you would like to make.
RTV is able to capture fine detail from the part that you are using as a prototype and done well will create an exact copy of your original.
Materials:
When making a silicone mould we use polypropylene sheets as a perfect release to our silicone. As seen in the pictures we build up a frame into which we sit the original part we want to replicate. Silicone will release from melomine or board covered with packing tape so if you don't have the polypropylene there are plenty of release alternatives available. As you can see we back the Polypropylene sheet up with some 2 by 1 batton simply glued to one side which gives the structure rigidity. Obviously a glue gun is extremely useful.
Tip……For really small parts old plastic butter containers or tupperware can work really well. Cardboard boxes cut to size and covered in packing tape will also do a great job if you are not worried about the look of the outside of your finished silicone mould.
Once you have constructed your frame it is a good idea to seal the joins. This can be done with a bead of hot glue, silicone or plastacine, all of which will not stick to the finished silicone. This seals your frame making sure none of the RTV will seap out at any point.
Once construction of the frame is complete you are ready to place the part you want to replicate in position. Our part is a Cat Wall Hanging and we benefit from the base of the cat being completely flat. It can therefore be fixed in place with the hot glue gun. Modelling Plastacine also works well. Both materials release from the polypropylene and hold the part firmly when pouring the RTV. A small bead around the perimeter of your opiece is all that is needed.
Once you are happy that you have positioned your project correctly and will not move with the addition of your RTV you are nealy ready to pour the RTV Silicone. You will find that different RTV Silicones have variable cure and working times to be sure to double check how long you have between adding the RTC Catalyst, Mixing and Pouring into your finished Frame. Make sure that you mix the Catalyst well into the body of the RTV as unmixed material will cause the final mould to fail. Mix methodically and try not to mix too vigorously causing the build up of air bubbles.
When pouring RTV remaining on one position and allowing the material to self level helps eradicate any bubbles. Most RTV Silicones will release trapped air but RTV is fairly thick and this tecknique helps. For this pour we covered the cat shape and allowed 6mm of RTV above the highest point. This can vary depending on the shape and complexity of design and the anticipated release stage. It can ber very fiddly to get the cured part you have made out of the RTV Silicone mould! As you become more experienced you will play with the tolerances for your desired outcome.
Once poured allow 24hrs for the RTV to cure completely before demoulding your prototype. Bescuase we used the Polypropylene, Batton and Hot Glue construction we were able to gently tap the sides of our frame with a small crafting rubber mallet until it broke away. Once one side had been removed we were able to gently pull the sides of the frame away to reveal the finshed mould. The prototype Cat part released easily as the project was fairly simply with no complex shapes to free from the RTV.
Our RTV Silicone Mould is now ready to reproduce our Cat replicating perfectly all the detail of the original. You will be able to use your RTV Silicone mould over and over again making it a simply and cost effective way of making small and meduim size parts.
Bear in mind that the surface of the RTV will eventually break down and be less easy to release parts over time. This is effected by what you are making parts from. Polyester Resins are likely to decrease the amount of parts taking from your mould as the resin contains more damaging chemicals. Likewise, Epoxy Resins can eat away at the surface of your RTV Mould over time. Using Jesmonite or Plaster will extend the working life of RTV as these have less damaging chemicals in the part material.