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[tonyH]

I seem to remember reading that some commercial hulls were being produced from a silicone mould to get more definition and easier release. Does anyone know whether these are full silicone moulds, as used for fittings, or a silicone secondary layer in a more stable 'rough' mould? If the latter, how does the silicone stick to the outer mould?
I've got a small project I want to try on and, if it works, a much bigger one so any ideas would be appreciated.
 
Thanks
 
Tony

[Circlip]

Ideally, the Silicone shouldn't stick to the backing form but should be supported by it when making a cast in the Silicone. When the item has been molded and cured, the molding and the Silicone is removed from the backing and then the Silicone can be peeled from the molding. The Silicone form can then be replaced into the backing form and start again.
 
  Check on the forum how Neil made some master moulds for Lifeboat crewmen.
 
  Regards  Ian.

[tonyH]

Thanks Ian,
 
I want to practice with a 12" hull for a small jet-boat but the next one will be a complex hull, about 1m long, with lots of tumblehome, plate details, square portholes etc.etc. and I was wondering how the whole thing worked. It would appear to save a lot of effort in mould preparation, or am I deluding myself?
 
Tony  

[Circlip]

You still have to make a "Finished" model to mould to and don't forget, when you get to the point of creating your final hull from the mould, you still have to fill the mould so getting into all the undercuts etc. may/will be a problem.
 
   RTV ain't cheap so even working on a thickness of 50mm (2") over all the master mould and a shaped backing to achieve this may increase the time and expense bit for how many?
 
  Not trying to wet blanket it but all points to be worked out before committing loads a money.
 
  Regards  Ian.

[tonyH]

No, I appreciate that the master has to be as good as you can get, and that the planning stage is most important, it's just that I'm always worried about the release bit from the normal mould.
It's always seemed to be the 'do or die' point and having experienced moulding fittings in silicone, it seems logical to use the same benefits for a hull.
 
I've always found that the hardest bit of hull building for me is lining the inside of a plank on frame hull between the bulkheads in a suitably tidy manner, especially when all the ones I build have tumblehome, so I hope that laying up the hull in the mould might be a tad easier.
I'll probably be wrong but.............
 
Is there a 'best' silicone to use, or is a 'standard' pouring type the sort to go for?
 
Thanks again
 
Tony

[Subculture]

A few things to note with silicone tooling (plenty of good tutorials online BTW).

1. You cannot use conventional polyester gelcoat resin with silicone tooling. So you need to use normal lay-up resin thickned to form the gelcoat. Better still, use epoxy, as this has much lower shrinkage rate, and doesn't compromise the rubber so much.

2. It is a much (MUCH) more expensive way to make a tool.

3. The tool will burn out after a certain amount of pulls, typically 50-100 pulls, but it does depend on how well the tool is treated.

4. Silicone rubbers tend to come in two main types- paltinum cured and tin cured. The former are better, but unsurprisngly more expensive. There are also different hardness rubbers. Tiranti are a good place to get rubber. Smooth-on are another.

Personally I would only use rubber for a mould of very steep undercuts. Most boat hulls only require a split mould, so a hard case tool isn't that difficult. Getting a release is easy, but you do need to be patient and very methodical in your approach to application of the release agent.