February 19, 2013
Mike Bergen (Custom Technologies LLC) and Mark Bishop (Waterfront Composite Solutions LLC) join us with a preview of their IBEX 2013 seminar on this process which offers and interesting alternative to VARTM.
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Bergen starts by giving his perspective from the Navy side, although he has recently separated from the service and has started his own company.
The Navy became interested in this technology as early as 2001 as a possibility for composites running gear to be used during dry docking. “We were interested in alternative fabrication methods,” says Bergen, “and by then Dr. Tom Juska had made some prototype fairwaters which we installed in 2002 and they are on those ships to this day.”
Soon after, the Navy began to work with a shop in Florida which was also using a prepreg system, and the Navy got behind it. “This was a solution to the roadblocks we’d been facing with VARTM,” says Bergen. The Navy was also working with Seeman Composites for very large projects such as hatch covers for submarines.
“The drawbacks with VARTM were the inability to guarantee wet-out for large thick parts, meaning these an inch and a half or two inches thick. We had spent a lot of money on modeling and in-situ flow sensors to ensure we’d get the quality we desired. Another issue we identified was the need for spray tack to hold laminate in place . . . epidemic to VARTM. We were also interested in transitioning to very large structures. That was all from the programmatic side.
“In more recent years, we’ve also seen a shift to prepreg because its become more readily available in the U.S. and we no longer have to wait three months for it to be shipped from England.”
Mark Bishop says people aren’t really sure why prepreg offers an advantage. “In high-end yachts, builders are looking for every possible advantage, but other builders are finding that prepreg gives them improved cosmetics, too. There are other very real spin-off. Other benefits are from a supply standpoint: You can use a wider range of fiber types. And keep in mind, you don’t have to do the entire boat in prepreg either. You may just want to go to prepreg for a series of unidirectional planks or a specific structural element. There are options for whatever you want to do.”
Bishops also talks about improved mechanical properties. “Absolutely true. It comes down to improved resin characteristics and improved sheer modulus and other details that define what you can and cannot do from an engineering standpoint.”
What you need to do first is understand what you are doing and how your project will be governed by how you plan to use the material. For example, if you are building a lightweight skiff, or your using a thin skin on a core structure, you may not ever achieve the ultimate strain that could be achieve by that laminate, but you may be looking for other features such as cosmetics and that could be what draws you to prepreg.
It used to be you needed more lead time, but that’s no longer a big issue these days. A lot of prepreg supplies are now stock materials. More than ever, there are a huge range of materials these days, including partially impregnated Prepregs. Its also well-suited to large structures since it shifts your infrastructure from creation of your material on the jobsite to the process.
Bergen adds, “One of the apprehensions that the Navy had was about the process of VARTM was the need for a post-cure. But with prepreg, that time has been reduced greatly. Once we got our heads – and money – into it, we realized how easy it was to set up an oven. We’d had so much apprehension. But all of that went away once we found people who understood the process.”
Bergen says prepreg is not only a stronger structure, but it is also a lighter structure. “The Navy likes balsa,” he says, “but prepreg still gave us a lighter structure because there was less resin ‘creeping’ in.”
Bishop makes a nice parallel about talking to his five-year-old son about seeing monsters in the night. “They are the same thing you see with the lights on. So maybe the boogie man is hiding in the oven?”
At the lower-temperature ovens, that temperature becomes a friend—as long as you know what you’re doing, says Bishop.
Bergen and Bishop end their conversation with a discussion of concerns about post-cure and how pre-preg fits in to those concerns.