looking for ideas on how to mold a mountain!

[DrHoneydew]

I am trying to come up with a good technique to mold a small, mound-like piece. Picture a small mountain. It will need to be about 2' x 4', and about 14" tall. The piece will need to be structurally sound enough to support the weight of a person sitting on it. Other necessary characteristics: UV resistant ( i.e. won't fade, degrade, etc. in prolonged direct sunlight at high temps [ to 120 deg. F ] ); translucent enough to have LEDs on it's interior visible from outside it...

...any recommendations on techniques would be much appreciated. I was thinking of vacuum forming, but can I achieve these properties? ...with what type of material? Can I use a polyester resin? Are polyester resins vacuum form-able or will I need to use another technique?

Thanks ahead of time for any responses.

Cheers,
DHD

 

[btrueblood]

Vacuum forming will give you the shape in a thin sheet of material, but you will probably need/want to fill the cavity formed on the backside with something to help give it enough strength to support a person's weight. Some possibles: fill with concrete for a permanent installation, or use a high-density urethane foam if this is something that needs to be transportable. The same technique could work with a female mold and hand-laid fiberglass/epoxy or glass/urethane resin.

As for what sheet material to make a vacuum-formed piece from - probably a polycarbonate or acrylic, with UV inhibitors. ABS would work, but I'm not sure how translucent it can be. Patprimmer will be along soon, I'm sure, with some more precise information.

 

[DrHoneydew]

Thanks for the reply.

...in the short time I've been gone plans have changed entirely on the project! ha.
...we are now looking to form a sort of hollow disc, roughly Frisbee shaped, but about 3' dia., maybe 6" tall
...we would love to use this product, EcoResin [ www.3-form.com ]. Would a polyester resin work for vacuum forming?

Thanks again to anyone who takes the time.

Cheers,
DHD

 

[Pud]

Cast Acrylic (PMMA).

Not cheap though, but transparent, excellent UV, etc, etc.

Cheers

Harry

 

[patprimmer]

I agree with Harry

Cast acrylic sheet. It will need to be quite thick to bear the load as it tends to be brittle.

It is used extensively in the outdoor sign industry and in aircraft windows.

It has extremely good UV resistance, and can be so clear it is used in very large lenses and outperforms glass.

It is probably the easiest material to vacuum or thermoform.

During th Disco era, it became popular for dance floors with flashing lights or even spectators below. I think dance floors were up to 1" thick. I don't know how far they spanned.

Engineering data is available as it was developed during WW11 as aircraft canopies and gun turret material and has been used extensively in building applications since then.

Polyester based FRP with UV stabilised resin will also work, but even with UV stabiliser it will eventually degrade, but so will PU and epoxy. I would rate polyester as the best of those 3 to UV.

I would think hand lay up would be the best production method for FRP.

Acrylic will look best and retain its looks longest, but will be expensive and it should have a big safety factor in the design as if it does fail, it will be a brittle break with sharp edges.

Impact modified acrylic is also available, but it loses other properties as it gains impact.

FRP Will not give a sharp edge as it breaks, so is inherently safer.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

Pat,

I had in mind casting the thing directly - I am assuming that being a "mountain", there would be a need for "rocky" detail on the outer surface. Envisioned a silicone(?) female part and a simple plug to keep the wall thickness down.

Pricey I guess.....

Cheers


Harry

 

[patprimmer]

Real pricey I would think

The frisby shape should be possible with blow or vacuum forming, but the mould would need to be 2 piece and opened for part removal past the undercut.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[DrHoneydew]

Thanks all for the advice.
Pat,
I'm not sure I understand. With vacuum forming the mold would need to be 2 pieces? Why not just one solid frisbee shaped mold?

Cheers,
Bill

oh, p.s., 3-form [www.3-form.com] tells me their EcoResin panels are vacuum formable. Can anyone foresee any issues with forming co-polyester resin panels? thanks.

 

[patprimmer]

If the frisby shape rolls under at the edges, the moulding would be trapped inside the mould. The mould would need to spilt horizontally at the point of maximum diameter.

Maybe I am imagining a different shape, but all Frisbees I saw had outside rim roll back under about 3/4" or so.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[DrHoneydew]

Oh, I see..

...no, all we need is the disc shape curving to contact the horizontal plane at a 90deg angle.

not difficult, no?

 

[patprimmer]

That makes it to easy. At an 85 deg angle makes it easier still as it releases from the mould with clearance.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.