Small tables: foam sandwich or solid plywood

A couple of weeks ago I used some leftover pieces of foam to make two small tables. I put a thin glass laminate on both sides of the foam and the resulting panels were stiff enough to be used as tables.

I am not sure yet what I will do with these tables (garden, boat?). I just wanted to use the leftover foam.

Ofcourse, this isn’t a very efficient way to make a table. I could have used a piece of 18 mm plywood instead. That would have saved me a lot of time. But, so the sandwich theory goes, at least I saved a lot of weight. But did I really?

The tables weighed 3,3 and 3,8 kg respectively. The edge of the heavier table was filled with glass/epoxy. The edge of the lighter table was left untreated (at least for now).

How does this compare to a solid plywood table of similar thickness (18 mm)? The tables were 82 x 57 cm. Assuming a density of 500 kg/m3 (average Google result for okoume plywood) a plywood table would weigh around: 0,82 x 0,57 x 0,018 x 500 = 4,2 kg, The plywood table would still need to be faired and painted. Let’s say the finished product would weigh around 4,5 kg. So the difference is 1,2 and 0,7 kg.

Personally I think that’s quite a big difference (in relative terms). For weight critical applications (e.g. boat interior) I would certainly consider swapping plywood for foam sandwich. I would, however, reconsider the edge treatment. Filling the edge with glass/epoxy added an enormous amount of weight. That was a bit stupid.






(Mis)using finishing filler for fillets

A big can of unused epoxy finishing filler, a leftover from painting the Jeanneau, had been staring at me for a year or so.

Light weight finishing filler is not intended to be used for structural (gluing) applications but I decided to do some experiments anyway. Without any surface preparation and just using a single fillet on one side I glued together scrap pieces of MDF, marine plywood and foam.

The pictures show that even a light weight finishing filler is stronger than these materials. The materials fail before the fillet.




Vacuum bagging and resin infusion – first trial and weight comparison

Ik stap hierna over op Engels omdat (relatief) veel bezoekers van deze site niet uit Nederland blijken te komen. Eens kijken of ik dat volhoud.

All laminating work on the Jeanneau project was done by hand.

I did not take the time to explore vacuum bagging or vacuum resin infusion. But I would like to learn those techniques. Maybe for another project, who knows …

In my living room I made some small foam/glass/epoxy samples with both methods and I learned a lot.

The most important lesson was that I really need a big table to cut all the materials (glass fibre, vacuum bag, flow media, release film, tacky tape, tubes, etc.) to size. Shelves to store the materials (rolls) at working height are also essential. Even during this little trial I didn’t like the preparations. If I’m going to use these methods for real I would like to make my life as comfortable as possible.

Finding (inevitable) leaks on the other hand wasn’t that hard. I could often hear them. When I couldn’t hear them (I don’t have special equipment at this moment), I just went around the vacuum bag one more time compressing the tacky tape. Focussing on obvious problem areas: corners, pleats and around tubes. That took care of the leaks every time. During my trial the bag itself was never damaged or punctured (in a real life boat building environment that would be quite different I’m afraid!).

A big difference between “vacuum bagging” (compressing an already wetted out laminate with atmosferic pressure) and “vacuum resin infusion” (using atmosferic pressure to both push resin into dry glass fibre and compress the laminate) is the amount of time you have to solve problems.

With resin infusion you can solve all problems (leaks) without any time constraints.  You only start mixing the resin after these problems are solved.

With vacuum bagging you first mix the resin and wet out the laminate. Then the clock starts ticking. You don’t want the epoxy to gel before pressure can be applied. On small scale projects (e.g. panels) I don’t expect problems when using slow curing epoxy. On big and complicated projects (e.g. boat hull) things could get exciting. Maybe too exciting for me personally, especially if I build alone. That’s an argument in favour of resin infusion.

I measured and weighed my samples to calculate the weight per square meter.

In this, admittedly not very scientific, setting I found that resin infusion creates panels (foam with glass/epoxy on both sides) that are more than 20 percent heavier. An argument in favour of wet bagging.

This is not surprising: resin infusion fills every little irregularity on the foam surface with epoxy. The extra holes I drilled myself were also filled. The  percentage mentioned above only applies to my specific combination of foam and glass. If you use heavier and/or thicker foam and/or more glass the relative weight difference should drop: heavier/thicker foam or more glass increase the total weight of the panel but they don’t increase the surface area of the foam that can be filled with epoxy. Higher density foams are also less porous and should therefore absorb a little less resin.

The very old and noisy vacuum pump I used for this experiment probably wasn’t very good. I don’t know what vacuum level it reached. Maybe resin infusion suffered more from this old pump than wet bagging?

Overall I was pleasantly surprised by the quality of the wet bagged laminate. The pressure from the bag removed a considerable amount of excess resin from the laminate (last picture) and it looked nice and compact.