Enclosures

So you are making the mold out of 'glass, too?

Yes :>

I had thought of trying something like plaster for the mold. I.e., something that could either be *dissolved* or *fractured*.

Ah!

Clever! What about just leaving a screw *in* the insert? (and backing it out later)

How did you later get the inserts "unscrewed" from the "locater screws" (since the locaters were part of the wooden block)?

Thanks! Lots to think about, here...

Reply to
D Yuniskis
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You can make the molds out of anything that will stand up, but if you're going to be doing fiberglass work you may as well use that for the mold.

Plaster is probably easier for onsie-twosie, but it doesn't last well.

Any cheap resin will work for a while, but the quality of the mold determines the quality of the part that comes out of it. If you just want a few, get whatever is on sale. If you're going to pull hundreds of parts out of the mold then use the best resin you can get.

(If you're going to pull tens of thousands of parts out of the mold then you should reconsider your process, and maybe plan on metal molds).

Only if you want a one-use mold. Even for your plug I'd suggest that you plan on something that'll stand up to making half a dozen molds -- this will give you the opportunity to re-do a screwed up mold, or modify the design without building a plug from scratch.

And if you _do_ want a one-use mold, then the lowest-effort way to get that is to make a male plug that's undersized by the thickness of the glass, and lay up on the outside of it. Then pull the part.

If the part has negative draft, or just if you want to, there's a "lost foam" method where you use a foam plug and destroy it to get it out. Use urethane foam and chip it out, or styrofoam (with epoxy resins, which are compatible) and dissolve it out with acetone or other solvent. This leaves you with some effort to finish the outside of the part, but it's likely less than the effort to make a really good mold.

Curse those glued-in screws!

At the risk of starting a flame war -- unless your part's exterior is textured, or you want it obvious that your part is fiberglass, _don't_ use woven cloth. The weave will "print through" the final finish any time your part is at a different temperature than it was when it acquired it's final finish. Clearly this isn't an issue if the part is textured (and try to keep a textured mold looking nice!), but if it's supposed to be smooth and shiny it does detract from the part's appearance.

Wax the screws, probably.

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott
[much elided]

I was thinking that plaster had the advantage of being easily destroyed (e.g., negative draft angles, etc.)

As I said, "small quantity (20)" :> I don't plan on going into the box making business!

Doesn't this affect the surface finish of the part? (it also imposes constraints on the shape of the part since you have to slide one out/off of the other -- regardless of M/F)

Ah!

I had also thought of making ceramic molds and shattering them.

Interesting! This is because the weave expands differently than the resin? (tempco)

Noted.

Set them *in* wax? (i.e., so they can be PULLED out instead of "unscrewed")

Reply to
D Yuniskis

But then you have to make a new mold each time.

Most certainly -- that's one of the downsides, but it's equivalent to your idea of draping the fiberglass over a mandrel.

That's the 'draft' that I keep babbling about. It's a casting term, meaning the degree to which all the sides slope 'the right way' to pull the part out of the mold. You can make a multiple-part mold for a part with negative draft that would otherwise get locked into the mold, but then you have parting lines on the part itself that have to be finished as a second operation.

In what are you going to mold the molds?

As far as I know, yes. Even sharp changes in the thickness of the fiberglass will do it. Look at a Chevrolet Corvette with a trained eye and you can see where the factory put all their bonds -- and whether it's been through a collision repair, and how good of a job the shop did with it.

No, wax them, then screw them into the inserts.

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

Yes. From a rubber mold -- pour in plaster, wait, lather/rinse/repeat. (I actually started thinking of this at a friend's ceramic business... but initially thought ceramic would be wasteful -- firing, etc.)

Ah, OK.

Yes, but I am refering to shapes that would otherwise require a two (or more) piece mold. E.g., imagine molding light bulb! (crappy example for this type of application but I was trying to illustrate how you couldn't do this in a one piece female mold)

Unless you make a mold that can be destroyed cheaply.

Make the positive out of something that can also be "shattered" (again, I keep thinking of the ceramic shop) or "flexible" (e.g., rubber). Use that to make lots of negatives (which you discard).

Remember, I'm not looking for a "production" solution -- just something I can use to crank out some reasonably identical pieces (I even thought of doing the pieces in ceramic but they are just way too brittle)

Hmmm... I'll have to keep that in the back of my mind.

I think I need to go play with some of these ideas and see what the *real* "costs" are -- and the quality of the results, etc.

Thanks!

Reply to
D Yuniskis

Sorry for the late reply. I realized Thunderbird decided to not show any thread I had posted to. Back to Pan.

I make the molds out of glass also. One example I have is an airplane instrument panel. I cut out the basic shape from plywood. I did a couple of 3/4" pieces to give me a 1.5" flange around the perimeter. I then used foam and bondo to form other features. I sanded and filled any imperfections in the plywood with bondo or resin / microballon filler. Basically trying to get a very smooth surface. Then I coated this with several coats of resin, letting it cure and lightly sanding between each coat. The final product was a male plug, very smooth.

Wax the hell out of the male plug and lay on a very thick layer of resin and let it set for just a couple of minutes. Then start laying down the cloth. This gives you a smooth surface on the female mold. This was not for some production setup, I did 1/2 dozen panels this way. Any small imperfections in the mold you can fix with Bondo and a little sanding. Unless you have a part with very long sides you dont have to worry much about draft. The final part is going to shrink some anyway. Wax and mold release are the key. The instrument panel was about 40+ inches by about 18" width and popped out of the female mode without any real problems.

I did this for the counter weights on the elevators. I wanted to increase the hinge to weight distance so I needed to rework the skin. It works for small one off pieces but generally a PIA.

There is some polyurethane mold "rubber" you can get. It's a 2 part catalyzed process. You mix it up and pour it into a container and then you can put a part into it. It makes a copy of the part as a female mold. I dont remember the name or where I got it now. What I do remember is it makes one hell of a exothermic reaction!

That works also. I've done that for nut plates and such where some resin could get into the screws. Vaseline or some paraffin on the threads and you are good to go.

I drilled holes for the locater and had a insert. Basically the locater was screwed into a insert in the wood. Then I just unscrewed them from the wooden guide (and the insert now bonded to the part).

There are lots of ways to do tops and bottoms, inserts, etc. For small qty with minimal stress you can glue nuts on the back of a small flange to hold a panel in place. Fill the threads with hot glue and bond with some milled fiber. This is basically finely chopped fiberglass. You can get it from suppliers in bags. If you cannot find it locally try wicks aircraft or aircraft spruce. Look in their composite sections. You are better off with a small nut plate but finding the correct size can be difficult. Let's say you have a bottom plate of the box and you have made a small flange on the inside of the top. You want to screw the bottom on. You can use the nut plate on the back side of the flange. If the thickness is not enough, add a few small layers of glass, drill and be done with it. Again, small qty units, rivet the nut plate in with counter sunk rivets and a hand squeezer.

Another method is using simple "brackets". Think of a box, sides and top and you want to mount a bottom plate

1) take a 2x4, maybe 8" long and warp tightly with a oven baking bag. The resin wont stick to this. 2) Cut 4 strips of cloth, 4 inches by 7in long fold them in half length wise to give you a "guide" 3) Put a coat of resin on the 2x4 and then lay down the cloth. Wet the cloth out layer by layer. Let it cure and pop it off the 2x4 form. If viewed from the end of the 2x4 the glass will be about 2" on the top of the 2x4 and 2" down one the side of the 2x4

What you have is a L shaped piece about 7 inches long. From this cut brackets maybe 1" wide, so you would get about 6 or 7 brackets. Trim as needed. You can use a pair of tin snips or a cutoff wheel for this.

Now, mount the nut plate or bond a nut to the inside of the L. -------- | Nut plate | |

Now measure carefully and drill holes in the bottom plate located such that this bracket will be as close to the side of the box as possible. What you are going to do is bond the vertical side of the bracket to the box. Mount the brackets and box bottom together. Mix up a thin slurry of resin and milled fiber. Coat the vertical side of the bracket with this mix. It will be thick enough to hold in place for what you want to do. Push the brackets and box bottom in place in the top, tape and let cure. Presto, unscrew the box bottom and you have aligned brackets bonded to the box sides. What you have is:

bottom-------------------- outer|x============ outer|x| nutplate outer|x| outer|x|

Reply to
Joe Chisolm

OK. I.e., you could use that (suitably painted) for a dog-and-pony (to show client what the "real" part will basically look like)

Presumably you can flex the part enough to overcome "small problems" with the mold (?)

"Elevators" as in "control surfaces" (not "people conveyances")?

Presumably retaining some of its flexibility so you can "peel" it off/out?

*Always* Cool! :>

OK.

OK. I imagine strength is related to how much material is

*around* it (as well, perhaps, as the exterior shape of the insert? "rotation-deterring")

There's a place nearby (non profit) that restores vintage aircraft. I suspect a contact I have there can help me out with more of these details...

("warp" presumably is "wrap"?) What's an oven cooking bag? (any time I cook in a bag, I do so with just a paper "garbage/shopping" bag -- since usually you're only at ~350F). Is this similar to parchment paper?

(I will google)

So, like you are making "angle iron" out of glass?

OK.

To minimize stress on that connection?

OK. Though, instead of cutting the "raw" 7" "angle iron" into 1" strips, I could just as easily (?) have used a length of it that was almost as wide as the box's dimensions, right? I.e., so you end up with a "lip" all along the bottom...

It won't stick to the plastic (I assume you mean nylon?) hardware?

Ah, that's worth noting!

Thanks for all your time! This has been really informative!

--don

Reply to
D Yuniskis

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