A Gap In Understanding of Cored Construction

[Chotu]

How do we close out the core on a panel when we need a square edge?

Throughout my project I've took a router to the edge of the core around the edges of a panel and used lightweight fiberglass cloth to glass the edges, closing off the core. It's worked well of course, but leaves an undesired round edge to the panel.

A cored door, for example, still needs to be square on the edges.

A cored area with a hole for a panel of glass needs a square "lip" all around the inside to make a flush, good looking window.

How can I achieve these edges using fiberglass, epoxy (polyester really in this case) and Airex foam core?

I'm stumped on how to make things like a small lip on a cutout of a foam cored panel for a window or even a square edge for a door.

How is it done?

 

[JamesG123]

Often a U channel of aluminum or plastic is used to close/protect/strengthen the edge of a sandwich panel. Likewise a frame of some shape of extrusion is used to both cover the exposed core and act as an interior window molding/frame.

When not doing that and using nothing but composite, usually the core is beveled that leads to a thickened layer of glass composite at the edge. This can create a square edge, but often the slight bevel is left in it because otherwise this uses a lot of epoxy and gets heavy.

 

[Chotu]

Often a U channel of aluminum or plastic is used to close/protect/strengthen the edge of a sandwich panel. Likewise a frame of some shape of extrusion is used to both cover the exposed core and act as an interior window molding/frame.

When not doing that and using nothing but composite, usually the core is beveled that leads to a thickened layer of glass composite at the edge. This can create a square edge, but often the slight bevel is left in it because otherwise this uses a lot of epoxy and gets heavy.

Brilliant, James.

Thank you for the thoughtful post. This is extremely helpful.

Window wise, I was looking to leave a simple rebate around the entire edge to make a frameless, flush window using 3M VHB tape, Dow silicone around the edges.

So probably the technique in the diagram will be best suited for that.

 

[Chotu]

Making the doors however, there is no way at all to do that with just composite, correct?

I'd have to put something square along the edge when glassing, such as a piece of wood or plastic or something or use a frame of U channel around the outside edge with 5200 to hold it on?

 

[JamesG123]

Yes generally. You have to protect the edge from knocks etc.

 

[SamSam]

If you don't wrap the glass around the corners it looses structural integrity. If you don't need much of that, you can laminate the faces of a panel, let it cure and then trim to the flat edge. Then laminate the edges and trim to the flat face. That will give more or less weak corners depending on how many laminations are used. It's easy(er) to rip the edge off with use or to have the corner crack and look bad, let in moisture etc.

You can wrap the edges and then apply filled resin putty and then sand for square corners. The sharper the corners, the more prone to chipping etc. and the harder it is to adequately cover with paint or gelcoat.

For the putty you put a filler in some resin such as Cabosil (fumed silica) or milled fibers (a powder like substance of milled fiberglass) or for an easier sanding , less durable filler use glass microspheres.
Wood Flour https://www.systemthree.com/products/wood-flour

You want to avoid pure gelcoats or paints on corners as much as possible as it's brittle and easily chips.

If using a mold and you have an inside corner (which would be the outside corner of the finished product), you can tease a cut edge of fiberglass into the corner from one way, say on the face, and then tease another cut edge into place on the edge, so you end up with FG filaments right into the corner. You can also use a filled putty filet and then solid pieces of fabric that will lay into the cove. In sharp corners in a mold, instead of filling it those two ways, I would sometimes take some strands of roving from a piece of woven roven (or straight from a chopper gun) and lay that in the corner lengthwise, running along the length of the corner. When there was enough in there to fill in and prevent air bubbles or resin rich corners, then fabric was applied for the rest of the lamination. All these corner solutions using a mold were done at one time with no waiting for anything to cure.

 

[fallguy]

Not sure if my answer will be too amateurish for you, but here goes.

Use a slotted saw on a router to remove core or remove by hand.

Backfill with thickened resins or insert higher density core rips in thickened resins.

For open edges; you need to get creative. This jig is backed with ship tape to sharpen back transom edges. Just like SamSam advised; I prefer milled glass in the mix; although it is more intuition than wisdom I admit.

I would like to see edge testing of edges with and without milled fiber.

I don't know how any of this pertains to rules. Just what I am doing..

 

[Chotu]

Ok. Thanks for all the help.

Finish work is a lot harder than building the hull. Wow

I am not working with molds because I'm making a few doors and odd pieces here and there. Molds sound much nicer for getting the glass around sharp corners but these are one off situations.

So I guess I'm down to laminating the faces of the doors and digging out core around the edges, except there are a couple spots where wood inserts need to go for door hanging hardware to grab hold.

For windows in these doors, maybe I can use a mold to make the lip around the window area take sharply turned fiberglass to make a rebate for the window pane to be stuck to?

Carry the flat face fiberglass on the front of the door around a really tight corner and into the window frame area, meeting the flat face fiberglass from the back side of the door. With the right mold piece to shove into this window area during layup, this should hold the tight curve, yes?

 

[JamesG123]

Depends on the weight and weave of the glass you are using. A fine twill will be more tolerant of sharp compound curves than course biaxial.

 

[ondarvr]

We make flat panels all day long.

Lay out the door on your table with a frame of the desired material, dense foam, wood, PVC, etc, then use the lighter weight foam inside that frame. We make the frame slightly oversized and trim to the correct dimensions afterwords.

 

[Chotu]

We make flat panels all day long.

Lay out the door on you table with a frame of the desired material, dense foam, wood, PVC, etc, then use the lighter weight foam inside that frame. We make the frame slightly oversized and trim to the correct dimensions afterwords.

So I don't need to worry about wrapping glass all the way around the panel edges?

I guess I've always misunderstood this. I thought I had to close the core off on the edge of the door with glass.

I thought the glass around the edges was to hold the front and back face glass together at the edge.

Am I over thinking this?

 

[Chotu]

Depends on the weight and weave of the glass you are using. A fine twill will be more tolerant of sharp compound curves than course biaxial.

I have some 6oz cloth I was thinking of using.

 

[ondarvr]

It depends on the core, more durable cores just need to be coated.

PVC boards will hold a screw fairly well, but if through bolting they work very well.

Or there are ways to strengthen the attachment points.

 

[Chotu]

It depends on the core, more durable cores just need to be coated.

PVC boards will hold a screw fairly well, but if through bolting they work very well.

Or there are ways to strengthen the attachment points.

Core is 6lb Airex foam. 1" thick. The doors will be sliders so I was going to put wood blocks in where appropriate to hang them.

It would be fantastic if I didn't have to do this edging. I hesitate to use wood all the way around the frame border but I guess anything really dense and waterproof (like pvc) could do the trick as a frame.

I mean the Airex is waterproof, but it could certainly be dented if hit wrong.


Maybe a really thin wood strip all the way around, thicker where the mounting hardware goes.

Helps a lot to talk this out.

Thanks everyone for the help.

 

[SamSam]

We make flat panels all day long.

Lay out the door on you table with a frame of the desired material, dense foam, wood, PVC, etc, then use the lighter weight foam inside that frame. We make the frame slightly oversized and trim to the correct dimensions afterwords.

If you can get a piece of window glass (or similar smooth stuff) big enough for what you want to make that will give a flat, gloss surface to at least one side of your projects. Old, single pane sliding glass doors work well. The frames come off easily leaving a 3' x' 6' piece of glass, then lay it on a workbench or table, 2x4s and sawhorses, driveway etc.