100 Giant Bells - advice please

A

ArtvsScience

New Member
Hello all.

Now isn't exactly for a movie or game prop but it's very similar. I'm just figuring out how i'm going to do it and am looking for any advice / suggestions from anyone who has experience making largeish silicone skin moulds and urethane castings

So the job is to manufacture 100 giant bells, they are going to be 750mm high x not sure yet. for the plug I'm going to turn a bell on a vertical lathe that we have set up, glass and fill the outside, melt the foam on the inside, fill and sand.

Then i'm planning on making a two part silicone skin mould joined around the bottom of the bell with a fiberglass back up case.

As I have to make so many in such a short time I have been thinking about simply solid casting them using a filled urethane casting resin the bell will only have a 5-10mm wall thickness.

I would pour in the resin from the top of the bell.

So what do you think? I usually make most things out of fiberglass but in this job strength isn't a major factor and I need a good finish both inside and out aswell as turning out at least two a day in a finished color.

questions?

What do you think about making such a big part out of casting resin ( the most suitable barnes product)
do you think it will work?
do you think the finished bell would be overly fragile?
What % of filler would you recommend to get the balance between economy and strength?
Do you think the silicone skin mould would keep it's shape inside the fiberglass case?
Any other advice / recommendations / cautions?

-- If it doesn't seem viable i'm just going to have to bite the bullet and make a solid glass mould and get them turned out by hand which is a lot more work and time.

Really looking forward to hearing responses, all welcome. cheers
 
I would recomend checking out smooth-on: http://www.smooth-on.com/ They have several products that can help you get the job done quick. Resin takes a while to set, so you might have trouble making two a day. Maybe a solid mold and expanding foam for the cast would work best, since durability isn't critical? Good luck! That's a big job :)
 
Thanks Cadeus! I'll check out the smooth on website although i'm not sure if they distribute in Australia where I am based. Barnes moulding here in Australia is quite a big franchise and they stock a good variety of moulding and casting materials i'm sure they would have the equivilant of the smooth on products. Which products would you recommend from the smooth on range specifically?

Casting with a polyurethane foam.. interesting.. I suppose the foam would 'skin' up against the mould and give a reasonable finish. Do you know if you can pigment a colour in to 2 p
part foam?

When I say durability isn't critical they are going to be hung once a year from the ceiling, taken down and then hung again the following year and so on.

2 a day is what i'm hoping for because the deadline is November. :wacko Please keep suggestions rolling. Once decided I will start the job and keep a build log. cheers
 
I don't know a lot about materials or molding. I just know smooth-on. And the foam was just an idea. I have little experience, but smooth-on has representatives you can call that could recommend products based on your needs. And I hope you get some more feed-back from others. i wish i knew more. sorry
 
you might consider vacuumforming the bells rather than pouring them. make a half-bell buck, then glue (or rivet) two copies together to make a bell then spray them with a metallic finish. with the right set up you might be able to make 10 or 20 a day rather than 2. I haven't done any vacuum-forming myself but I would think that even after scraping together a DIY facuumforming machine, the cost (and the weight) would be much lower than a hundred 2 1/2 foot tall bells worth of resin. then you could reproduce them easily since you still have the buck AND a vacuumforming machine. plus they may be stackable for storage. you may need another buck for the inside wall if you need a finished look.

this guy's 3PO torso looks ballpark the size you need...
http://www.youtube.com/watch?v=maH5Ech0wK8
 
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Wow. 100? I've done these type projects in all of the materials you guys have discussed. If you had a big vacuumformer, that would be my first choice, but scrapping together a machine big enough for this job is not a good idea for time, expense and learning curve. My second choice (for me to do) would be the fiberglass, as you already are very familiar with that. The rigid urethane foam is worth considering, but you will have to have a sturdy two part mold (inside and out) to fill. It helps, for self-skinning foam to warm the mold and to fill it well with the expanding foam. You'll probably have to have the bell thicker for a slightly sturdier part, but the rim will still be fragile. I do not like your urethane resin idea for the size part you are making. Sorry. So, if you go with fiberglass, and insist upon a silicone, jacketed mold, make sure you have enough registration keys which will lock the silicone into the fiberglass jacket. That said, I would probably just make a clean fiberglass mold. 100 parts of ****ing with the large floppy silicone does not sound like something I would enjoy. I would also make two or three molds so I'm not sitting around whacking the pud all day waiting. Hope this helps, and good luck.
 
Kubla Kraus - yep vacuum forming would be the way to go I agree. unfortunately I have had mixed results with vac forming and have never tried to form something quite so tall but I will look in to it.

Wesman - I understand your aversion to messing around with a floppy mould x 100. If I cant cast them I will have to make them out of solid fiberglass I suppose. it's just that's an awful lot of glassing and I wont get that two sided finish i'm after. (unless I make a mould insert)
Just out of curiosity why not the urethane resin? Weight? Strength? Cost? I come a boatbuilding composites background so have only really just discovered urethane resins and the strength in them seems amazing to me.

Skinned foam may be a good one but I agree edges will be fragile.

Thanks so much for the input it really is a big help
 
I cast in urethane resin every day but admittedly in a chamber. Urethane can be expensive and 100 bells, wow. 40 kg usually comes in around £400 and you will need a lot. Depending on what you use it can be brittle or sort of flexible and de mould can be from minutes to a few hours. Smooth on is expensive. The urethane I use has about 7 minutes pot life but you can get quicker.

if pouring in from the top you need one that will allow you the time to get that much resin in without trapping air. Vac chamber is out the question at this size usually and the pour hole needs to be big enough and you will have to remove that. Any air is likely to get trapped at the top curvature.
As mentioned above, you want at least 4 moulds if you are doing 2 a day. One for back up and 2 in order to maintain casting when taking curing into account.

If you have to start in 8 days I would be trying out now to see what works :) but I wouldn't take a job on that big without some idea as to how I was going to do it.
Good luck
 
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Thanks for the reply TK7713Gra30. You have helped me make up my mind and i'm steering away from Urethane now due to high cost and too many unknowns. I can get two + fiberglass bells out of two moulds per day. cost of tooling for fiberglass moulds is far cheaper than silicone but there will most likely be a slight join line to deal with.

I'm confident I can pull the job off just the ideal of poring rather than laminating and a two sided finish appealed to me.. there are other ways to achieve a two sided finish with fiberglass.. for example an insert mould that you vacuum in to the bell at the end of the job. I don't really get a choice on what jobs I have to take on I just get told what to do by the boss. not complaining though.. I love it.

I will post a few progress updates on this thread just incase anyone's interested hope to make a start mid next week. Cheers.
 
Vacuum forming and fiberglassing are both decent options.
I also do urethane casting on a daily basis, so that would be an obvious choice for me. It can be pricing for good size parts like yours, but I'm really unsure of your budget. There are a ton of material choices to get the strength you need of course the design and wall thickness will play a big part. A thinner wall will be extremely difficult to cast unless you have extensive experience and equipment. A large vacuum or pressure chamber will help the process, but also dry the silicone out more quickly.
Good luck with what you choose.
 
Hi Pork and Beans thanks for your help. After talking to reps at smooth on Im currently leaning towards a hardish expanding urethane foam cast in a fibreglass mould material cost will be around $30 a bell so that's in our budget. I still have a bit of time to decide but just started on the plug yesterday. Got my old wood lathe turned it on it's side made a big plate to attach the polystyrene too and attacked it with a grinder. It was like it was snowing in the workshop....
 
uploadfromtaptalk1406665679906.jpguploadfromtaptalk1406665692539.jpg
 
Obviously plug is nowhere near shaped yet . Still open to suggestions for casting / construction methods? I'm currently leaning towards casting a flexible expanding urethane foam in a fibreglass mould.
 
OK done. Going to fill it with a single part filler, epoxy glass it, fill again gel coat the outside melt out the inside and sand smooth [emoji12]
 

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Hi Just a quick update. Got some self skinning soft urethane foam today and cast it in a small test bell mould with some pigment and managed to get a good part out pre colored and reasonably strong so things are looking good.
 
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