Closed mold RTM design tips

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#1

I have a new project and it involves using composites and i was after some help.

The project is to develop a system for efficient production of the part using a closed composite mold and 2-3bar RTM, I’m making some test(scaled down) molds out of aluminium to investigate the process and i was wondering what sort of gap i should leave between the part and the mold, as in how oversized the mold cavity should be vs the preform thickness? The whole point of the exercise is research so im not after someone answering accurately just a ball park figure to get me off and running. Re machining the molds is easy but only as long as there is material on them to remove, so i didnt want to start out with no idea and find ive got to make 3-4 molds just to get in the ballpark.

Part thickness for these 1/4 scale tests is starting at about .6mm which is based on some FEA but part of the test is to validate the FEA findings with a mechanical test of the part so the thickness could increase, im assuming at pressure and that thickness i shouldn’t have too much trouble infusing the part, am i wrong?

Thanks very much for anyone who takes the time to reply.

#2

A rule of thumb for polyester and vinylester resin is 0.0018" shrinkage for every inch of dimension. I don’t have much experience with epoxies in RTM or VARTM so I don’t have a good number for you but epoxies have much less shrinkage than PE or VE.

#3

Wouldn’t the mold cavity be sized to final part dimensions?

As for resin flow and wet out… I would think it would depend on the mold setup for inputs and vacuum. It probably would depend on how thick the laminate is and what sorts of materials it’s comprised of. I know with RTM that an issue is migration of fibers form the high pressure of the resin injection. This is where sewn or stitched materials are useful.

I’m interested in finding out more as well. Is it common to use inter laminate flow media in RTM? And How and where to decide on puting the intakes vacuum ports?

#4

Thanks for the replies, I think i’ll start with a mold to fit the reinforcement thickness and see how the process goes, the plan is to use biaxial preforms, although that may change to quadaxials if the FEA turns out to have been off, hopefully this will help to minimize fibre migration, perhaps a woven outer layer to help things along, but im very new to all of this so its a case of learning from the tests.

Regards the injection of the resin i imagined i would inject the majority of the resin at a low pressure till its almost completely consolidated then ramp up the pressure to fill any voids, is this the right way to go about things?

cheers