RTM, RTML, and infusion all have different requirements.
RTM requires very heavy/strong molds due to the high cavity pressures involved. You do get section accuracy because of the mold structure. RTM typically takes a lot of parts to recoup the tooling costs.
RTML doesn’t require the heavy mold of RTM but it does require accuracy in mold construction. The flange details are more complex (at least the way I learned to do it) with two seal areas, vacuum clamp, mold stops, resin channel, and resin pinch. Laminate installation needs to be accurate as bridging and/or thin areas can cause racetracking and overlaps can result in insufficient wet-out of the fiber. Part section accuracy is pretty good but it can vary depending on mold geometry and construction. The counter mold, not heavily built, can flex a little so the section thickness can vary depending on how the mold is filled, both with fiber and resin. Resin fill pressures and vacuum can also affect section thickness.
By infusion I’m assuming you mean with a vacuum bag so, as you no doubt already know, infusion doesn’t require a counter mold so the initial tool cost is lower. Overlaps aren’t really a problem but bridging can be. The biggest problem with vacuum bags is getting a good seal. Get the leaks closed and the part should come out well. Another issue is the time it takes to make the bag. Complex molds can require a lot of time to build a bag that doesn’t have bridging problems.
If you are making single, or at least low volume parts (one or two parts every so often), I don’t think it pays to make counter molds like those used in RTM or RTML. If you are working towards production (2-5 turns a day), then the matching mold sets are the way to go. We use both RTML and vacuum bagged infusion, along with a couple of other processes, depending on the part. If we had to vacuum bag every part, we wouldn’t be in business. By the same token, if we had counter molds for every mold we have, we’d need a bigger building just to store them.
We mold in polyester and our parts don’t need to have “Class A” surfaces (infrastructure/industrial parts) so, for us, I think the cosmetics are pretty much the same regardless of the method used. You might find better cosmetics in one process over the others depending on the materials and methods you use.
I’ve attached a couple of screen shots that illustrate the mold details for RTML. These are models from a recent project that we did. The green part is the A mold and the the purple is the B mold. I’ve notated the various features so you can see what each one is for. The wing seal and fluid seal are not shown but they are glued in place in the locations indicated. Also, the fluid seal, when installed, forms the outer wall of the resin channel.
