making moulds

I've used two-part silicone without a release agent, and it worked really well in casting some resin parts. Using a release would make the molds last longer, but I just needed to cast a couple of pieces.

Are you talking about the molds, per se, or the pattern?  I assume you mean the mold.  What material is the pattern?

hi don i want to make moulds of wind0ws and windshield. there moulded into the fuslage in same plastic . i was going to make them by pouring mould agent into back side of said than vacuum for clear plastic to replace them. am i on the right track? thx for your reply

Ah, okay, yeah, that should work.  Use RTV mold material for the mold, then cast in either urethane or polyester resin.  There are tricks to designing the mold to minimize air bubbles in the resulting mold.

I use the two part stuff with success.

One thing I do to minimize bubbles.  I secure the mold on a plywood stand on top of a small block of 2x4.  Pour the resin slowly using a stick to direct the flow, holding it as high as you can and still fill the mold (avoiding spills).  Then I use one of those battery powered vibrating massage things, running it around the mold on the plywood for a minute or two. If there are bubble when I demold, they are few and small.  I also make an entrance and exit on each mold.  The entrance is made such that it fills the mold from the bottom letting the air rise out the top.  Sometimes I use a release agent, sometimes not.  For large pieces I haveno problem. But for smaller, thinner ones I use release.

Have fun !

The mold should have some vertical extent to increase hydrostatic pressure, and, most importantly, do not let the pour sprue be the only way air gets out of the cavity.  The incoming resin will tend to block air leaving, when the resin is poured in.  You need vent tubes, seperate from the pour tube to allow air to freely leave the mold cavity.  The ideal design is a J-shaped pour sprue, so resin enters cavity from the bottom, and pushes out the air which then leaves by vent tubes going from top of cavity to top of mold.