Tips for first time using balsa core

I'm currently working on a race car roof that in order to get the rigidity I need I'm going to hafta use a balsa core. I can post pics if needed. The roof is like most cars it's has a 1/2 rib running from front to rear helping gain some rigidity however the customer wanted the middle to be flat the area of the flatness is roughly 3'x4' I have tried foam strips running front to rear in the middle and also an X out if foam strips but they do not seem to stiffen the laminate as much as needed. Another factor I have is weight. They want to keep them in the 6-7 lb range. My current layup is gel, 1.5 mat 1/4x 3/4 foam core strips in and x and 6oz cloth comes in at 7.6lbs and sitting on a table if I pick any corner up it flexes about 6 inches before lifting the rest of the roof. I have no experience with a balsa core. I have 1/4" 2x4 sheets I was planning to cut to size. Could anyone lend advice to how to use them in a layup. I plan to drop to 3/4 mat under the balsa. And 6oz on the other side. I prefer to not have to bag the part to keep cost down but I don't want any issues with the balsa not bonding. Thankyou I'm advance

 

Here's some general info http://www.corematerials.3acomposites.com/fileadmin/pdf/Proper_core_installation_Part_III.pdf

Important to resin prime surfaces, is the balsa plain sheet or contour cut, by the sheet size it sounds contour cut, if you have plain sheet you'll need to bag unless you laminate from each side, if contour cut a core bedding putty is more reliable than pushing into laminate-
Your X framing sounds as bad as it can get, down the CL or shortest distance across is better. with applied framing/stiffeners a uni cap helps a lot, the deeper = more effective.
If your really chasing weight & simplicity a core is good. You could drop the gelcoat & use post painting, epoxy resin may be a better choice or vinylester if it's gunna be a dark colour.
Maybe bagging or infusion would be cool for you.
Jeff.

 

Off the topic a bit - that excellent reference, with actual testing results, answers some claims that I have seen over the years. It is also a strong case for always infusing skins over balsa.

eg.


"areas are often diagnosed by the surveyor after the fact as a delamination, or a failure in service. However, further investigation almost always determines that the core never made contact with its bedding layer during construction ....the vast majority of cases of reported problems with sandwich laminates in the field can be traced to what can now be defined as poor core installation."

"if wet balsa is detected, ...... Stripping the outside skin and replacing the core, if the moisture is extensive, may cause more harm than good. This is especially true for foam cores, where the moisture can often be sucked out of the kerfs, and the presence of moisture has no adverse effects on the physical properties of the core."

"water in the core is not desirable, and should be avoided at all cost, but there is no evidence that the freezing and thawing of this water leads to any core delaminations."

"a line of thought that if water vapor can migrate through the gelcoat to form blisters, then that vapor would eventually find it’s way to the core, especially balsa core. A 14 year water immersion study conducted by Alcan Baltek Corp put that theory to rest when typical cored laminates were immersed in water, some samples exposed on both sides, and the moisture level monitored each year. At the end of 14 years the laminates were stripped of their skins and the cores weighed and then dried, and weighed again to determine the level of moisture. In every case, no significant increase in the moisture level of the cores was detected."

 

I'd use also biaxial instead of mat and cloth..

 

I am going to presume this race car needs a quality surface for slick paint jobs ?
Also, you don't want to/can't do vacuum bagging ?
So.....
Lay up the part with your skin laminate, cure. (To prevent/ameliorate block print through)
Lay the pre wetted balsa or foam into a foam bonding mix (pre mixed resin and filler). Use an orbital sander with some plastic on the pad to vibrate the balsa into it's adhesive.
After that has gone off tidy up any sharp edges and laminate inside laminate.
If you want to make it pretty use coloured waxed resin and some surface tissue or peel ply.
For what it's worth I would use foam rather than balsa.