We’re back from the holiday break with Chris Rogers from ExploreComposites.com & the third installment of his Creating with Composite Materials posts. This week’s topic is Infusion methods including Vacuum Infusion, VARTM & Closed-Cavity Bag Molding.
So you place a stack of dry fiber reinforcement in the mold followed by a vacuum bag with some fancy plumbing over the top – you then allow the pressure differential between the inside of the bag and some resin in a pot outside to suck (push) the resin through the dry laminate – wetting it out. That’s vacuum infusion! It’s not intuitive and you really have to see it to get what’s happening.
Infusion can be done with disposable vacuum bags, re-usable vacuum bags (CCBM) and even hard two-sided tooling – which is sometimes called “vacuum assisted transfer molding” or “VARTM” if you’re an engineer. Because of the potential for careful fiber and core loading, repeatable resin content and re-usable vacuum bags, infusion is a great choice for production. Once the process is dialed in, variability can be low and part quality high – especially because resin fills in any potential voids.
Resin transfer molding (RTM) is similar, but with RTM the resin is forced through the laminate stack using much higher pressure. More on RTM below.
Vacuum Infusion (aka: Resin Infusion, VIP)
This is the standard disposable-bag vacuum infusion process. When people say “Infusion” this is what they mean! It is a very useful and efficient process for building large, complex parts with excellent laminate performance – exceeded only by pre-pregs.
Infusion usually requires some type of “flow media” to allow resin to travel through the part to displace air and wet out the reinforcement and core. This can be a disposable surface flow media, a interlaminar flow media, or a core with flow features. The process requires careful setup and training. Mistakes are often big – on part-scrapping scale! It is still probably the best overall manufacturing method for larger high-ish performance composite parts.
VARTM (aka: LRTM, Vacuum Molding, Light-RTM)
Vacuum Assisted Resin Transfer Molding (VARTM) can cover a lot of bases, but typically is describes a process like vacuum infusion, but where instead of a bag, there is a second hard or semi-rigid tool. Dry fiber is laminated in one (or both) halves of the tooling and then resin is infused into the cavity using atmospheric pressure only. VARTM is different from RTM in that RTM uses much more pressure – and requires tools to be very heavy duty and well clamped.
Closed-Cavity Bag Molding (CCBM)
CCBM is one name for reusable bag or “soft” B-side tool. Like VARTM or VIP, the resin in infused using vacuum only through a part cavity. In this case the bag-side is usually a silicone or rubber re-usable shaped bag with built-in seal features and often a perimeter vacuum clamp. It is a good process for larger or lower volume parts and produces parts with a smooth but not finished B-side surface. Reusable bags are faster and easier to use, as well as reducing the waste process materials that come with disposable-bag infusion. Generally this is only done with polyester or vinyl-ester resins as epoxy is hard on silicone bags!
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