Fiberglassing a round-to-rectangular inlet using the "lost foam" method

[Latham]

I'll use this post to introduce myself.

I'm a retired, long-time sawdust producer with limited room in my shop. Sound familiar?

I've got the wall mounted Rockler DC with their plastic-lid separator sitting on a thirty gallon can. The separator is better than nothing but too much dust ends up in the bag. I use Rockler's (4") flexible hose to chase from tool to tool.

I've been aware of the Thien separator for a while and have finally decided to crank one out.

While lurking on this forum, I came across Retired2's build thread. The round to rectangular inlet makes a lot of sense. I've downloaded a couple of PDF patterns for a sheet metal R/R inlet. After cutting one pattern out, and attempting to fold the paper as a dry-run I ended up a bit cross-eyed.

I've done a fair amount of fiberglassing in my day and having epoxy, fiberglass, and a sheet of 1/2" pink foam taking up room in the shop, I decided to build one using the so-called "lost foam" method. Figured it'd be good to build the inlet first and then build the top-hat around it.

Here's how I did it:

Foam glued up with a 1" dowel running down its center:


Here's the block cleaned up and some waste trimmed from the to-be round end:


Originally, while planning this out, I decided to put the dowel through the center so I could build a jig to hold the block while I laid it up. However, having a 16" lathe, I decided to just use it to shape the 4" round end and lay it up while on the lathe. Obviously I covered te tools bed with black plastic to protect it from the epoxy.



I turned the round section with a rasp and sand paper. You've got to keep the paper moving or you'll start to melt the foam.



I don't recall the weight of the fiberglass cloth. I used four layers on each side, plus three layers of bias-cut cloth on the corners. Even with the bias cut layed over the edges, I had problems with the layup wanting to pull off the 90º corners. That light colored area is air between the foam and the layup.



Having anticipated this problem, I wrapped the part with cloth tape I'd picked up at a dry goods store. The cloth was held in place on each end with pins pushed into the foam.


That helped pull the laminate back on to the plug, but didn't solve the problem so I wrapped it with electrical tape. You can see that I now have a good tight layup on the foam plug:


After the epoxy cured overnight, I pulled the tape off the part and sanded the exterior out a bit. I screwed one end of the dowel to a piece of scrap plywood to hold the part upright and poured about a cup of acetone into one end of the part. (after I laid down that piece of black plastic!)  Most of the foam ended up on the plywood but it took me probably thirty minutes to clean up the interior of the inlet.

A coat of gray primer and here it is:


It's not a work of art but I think it'll do the trick. The rectangular outlet side is 3 x 5.25" and the inlet is 4".

I've read the thread on using a curved inlet and thought about creating one using this lost-foam method, but after thinking about it, it too started to make my eyes cross. Simple can be a good thing.

L

p.s. Don't try this with polyester resin, it'll puddle like the foam!

[jdon]

Interesting technique! My experience with fiberglass is pretty limited (patching holes), but it seems like you just laid the cloth and resin right over the styrofoam- that you don't need any mold release agent (e.g. wax). Is that so?

BTW, it never occurred to me to use eggs as weights for glue-ups!  :)

[Latham]

No mold release, the acetone simply dissolves the foam into a small (four or five cubic inches?) of grayish goo.

Those eggs are about about fifteen pounds of lead. I also had paint cans on both sides of the layup to keep the foam layers from wandering around.

L