Update on the foam 'worms' question raised several weeks ago. Many helpful hints were posted and most have been run to ground. The problem is definitely in retreat so it seemed appropriate to share what's been learned.
One source of the problem appears to be defects in the foam - either inherent or introduced by me in handling before bagging. When coupled with vacuum above ~ 15in Hg, the surface of the foam appears to collapse slightly along sinuous tracks, primarily in the spanwise direction. Keep in mind that this observation is limited to Foamular 150, which has a rated compression strength of ~ 15psi. Thanks to Phil Barnes for noting that baking the wax probably wasn't the root cause. I've gone back to a single wax application with no deliberate delay before bagging and the release is just fine and the 'worms' appear to be relatively unaffected. Subsequent evaluations indicate the initial favorable outcome was fortuitous. After following the method in Phil's video in detail, the problem persisted. The one exception I'll note is I mix the epoxy while standing but I doubt that's it. I'll also suggest that using anything less than 4 mil for the release behind the breather is not a good idea - too easy to wrinkle. Those wrinkles don't get through the mylar but it's easier to work with 4 mil or thicker anyway. I'll also note that working with anything but 3M77 is a pain - especially when setting up the breather. Photo-Mount adhesive doesn't have the tack and Elmer's spray contact doesn't do much of anything at all. Go with the good stuff. Suggestions about epoxy mixing and keeping the layup dry were evaluated. No problems there. The layup was so dry that you couldn't really sop anything out of it anyway. The breather arrangement was worked over some more. As noted, the method in Phil's video is a lot easier than laying down the layers separately. But all breather arrangements seemed to give about the same result. Layups were tried with the beds inside and outside the bag. No difference. I took a sample to the recent KCSE 'Turkey Fly' (pctures posted on their web site, by the way: http://www.kcse.us/). Jim Frickey suggested the tracks looked a lot like problems he had with defective foam when he was bagging a few years ago with Mark LeVoe. Pat McCleave took a look and thought that might be it as well. So we tried a third batch of foam - and had the same problem. Following the thought that this was mechanical damage, older samples were hit with a heat gun and all the 'worms' popped back out. Now I've got all these great looking samples but still lacking an appropriate solution! Meanwhile, lurking in the corner of the basement is the old CST Mighty Mini-Vac which pulls ~ 6in Hg. A vague memory stirs about using this level of vacuum for white beaded foam and obechi and anything higher would cause damage. Hmmm. Bottom line - after two cycles of DLG wings with the Mighty Mini, I've got two of the most gorgeous sets wings I've ever made. No worms, no dents, no errors. I'll allow as how I may have gotten lucky but one of these wings has the old foam in the root and the new foam in the tip and both sections are in great shape. So a potential solution is excess pressure for this foam. It is possible - maybe even likely - that some of this may arise from handling, especially when de-fuzzing the surface while prepping the cores. However, a very close inspection of the cores after sanding does not show any obvious dents on the surface. If it's handling damage then it's incipient until enough pressure is applied to stress concentrate the weakened foam in that area. A trial was run (at 20in Hg) in which no attempt was made to clean the core surfaces before layup. It had 'worms' also. Another was run with typical handling and sanding but 6in Hg - no worms. As soon as I can get to Tulsa and get some 25psi Dow or Owens product, we'll try it again with full vacuum. However, from others' experience, and the comments in Phil's video, this should be successful. Regarding Foamular 150, full vacuum is right at the compressive strength of this material. Partial vacuum may be within the variability of the strength of this material in localized pockets. But one result here is that using 6in Hg or thereabouts seems to take care of this problem with this type of foam. Since the core weight is not a large part of the final wing weight, the only reason to use 150 is that it's the only thing readily available at hardware outlets like Home Depot (at least in this part of Oklahoma). Thanks for many suggestions from many folks. If this observation is significantly at odds with other experience, I'd appreciate comments. - Dave R PS - One solid conclusion - the vacuum bagging process as taught in Phil's video is incredibly robust. RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.
