Idea wanted for Silicon mold using Dow Corning Silicon

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#1

The question(s) I have are:

  1. The 3120 is sort of hard but can I add a sheet of punched hole stainless steel mesh into the curing silicon to make the mold stiff(er) (molds only going to be about 10-15mm thick). My other option is to lay vinyl-ester and CSM glass after the silicon has set to give a stiff base to prevent flexing of the mold.

  2. I am using the 3120 so I can bake the prepreg at 150degrees. Which gives me about a calculated 9 min cure? The 3120 is hi heat tolerant to 300degrees. So my other question is the intensifier is 1:1 on the tool face, will this still allow 200gm carbon cloth to be sandwiched and vacuumed down into the tool face, without air pocket voids?

The mold has 2 radius corners but 2 sets of 90 degree corners. the part it self is about 250mm long 150 wide and 10mm thick. I am trying not to use a clear gell coat if necessary.

Your guys help would be appreciated.
Hetal

#2

huh?

Welcome to the forums. Please give us some idea of what you are trying to accomplish.

#3

Yes hello, i have been reading this forum for the last 6 months, just didnt have anything to contribute!

jeepers. The top part of my message was cut off :?

I had a tool made from perspec/acrylic of a spark plug cover for a 4 cylinder engine. I tried to get a mold made but i destroyed 3 tools (each tool has 5 parts embeded). Due to the draft angles a ridged part is very very difficult to remove and i ended up throwing a mold away bec i couldn’t get the part out. So i have been told about using Silicon, both from a performance aspec, no need to wax etc…basically you can’t stuff anything up.

Also i can also oven the silicon mold and pregreg which has a accelerative cure from 5 hours at 60 to 10min at 120degrees.

I have a pic that i can email if it helps. Wanting to pull 50-80 parts off this mold. This is just for fun, nothing big time commercial type stuff.

#4

You would want to check the thermal expansion if you are planning to bake the parts. Silicone does expand. I would highly suggest building a box to contain the silicone. The box would help stabilize the silicone rubber. You would want to make the box so that it screws together. It’s also a good idea to insert some dowel pins into the box so that it will located the silicone and keep it from moving. At 10-15mm it is still quite flexible. Key is, you want good bit of wall thickness and the box to keep it rigid when you do the lay up. But you want it to be flexible when you demold.

If you can’t get the part out of the mold due to the draft angles, you might want to consider building the mold in multiple pieces.

#5

So your saying lay a frame around the tool to act as a fence. Pour in silicon. Wait to set. Then put a few layers of glass to act as a bath enclosure? I have heard that the 3481 silicon is alot softer thus will expand more. This is desirable as it will push the carbon into the harder mold, my assumption is that this will allow greater consolidation of the carbon?

Any other hints? Does anyone think the stainless steel or some form of embedded steel will help instead of the enclosure or is necessary with silicon of shore 60?

Thanks

#6

Shore 60 is pretty damn soft. Building a box to drop your silicon mold in is actually a very good suggestion. That’s how it is done normally. Other than that is there any particular reason why you are using silicon for your tool? Silion, polyurethane are normally used in cast molding and not really for laying up FRP.

#7

The tool (the part i am making a female mold for) is made from perspec/acrylic, the mold will be made from Silicon. I had a really hard time trying to get the cured part out of a glass mold with tooling gell coat. It would end up breaking/chipping the edge of the tooling before it would pop out, and thats with PVA. The tollerances are very tight and if anything may even have a slight negative draft.

#8

You will have difficulty in producing a part with high tolerance from a silicone mould. In every practical case (except for casting a part), you will need to apply pressure (vacuum bag, two-sided mould, etc) to the layup to make it conform- and this will cause stress on the fiber. This will in turn deform the mould. If you can live with a few % deformation, a silicone mould can work. A good supplier is perma-flex at http://www.perma-flex.com/. I have used them several times- and the owner loves to talk shop.

Rob