Hemisphere Press

[schroedinger]

Hi

 

As I`m reading a little bit around last time, i really got interested in the way companies are building fireworks.

It`s quite amazing how professional companies buil'em and how amateurs are doing it.

 

But one thing i could'nt find much about, is the hemispheres press, just a couple youtube movies how to make the punch, but nothink more.

Is there maybe somewhere a good tutorial how it's made?

[Col]

I figured i`d post to this thread to save making a new one as its on the same topic.

Here`s the tooling i knocked up today for a 1T arbour press to churn out 3 inch hemi`s. Its made from polyurethane fast cast resin, i guess it took around 500g but its hard to be sure as i was casting other stuff at the same time.

The male side has a 40mm neodymium magnet embedded below a thick steel washer to spread the load. The magnet option seemed a quick, clean method of securing the tool to the ram without drilling holes. It works a treat.

 

Edited August 14, 2015 by Col

[schroedinger]

Thats a nice idea. Did you just cast out a normal hemi, or how did you do it?

[Col]

Aye, used a standard paper hemi, it automatically creates the correct spacing for the wall thickness. The rest of the mold materials were just odds and ends that most pyro`s would have. Mdf, large diameter spiral wound cardboard tube, a chunk of pvc drainpipe, pva woodglue, hotglue and vaseline for release agent.

[schroedinger]

Thank you for the anser, sounds like a much better way then using plaster.

[Dean411]

Col,

I had the same idea but I used delrin for the inside of the dome, I haven't made the outside part of the die yet though

 

Dean

[Col]

I dont think plaster would last long in the press, the 1 tonne arbor puts around 200psi on a 3" hemi. To be honest, the PU resin is just as easy to use as plaster, maybe easier. Weigh out equal parts of A and B, mix for 1 minute and pour it into the mold, You cant hang around with the fast cast stuff as it sets in 2 minute, you dont get any warning its the consistancy of milk one second and a hardhat the next

 

Hi Dean

That has a nice finish, was it cast or turned on a lathe?

[Dean411]

I machined it on my CNC lathe,

I wanted to do it for the hell of it as I now get all my Pyro stuff free , once I finish it if you want it I will ship it out.

Dean

[Col]

It`ll be a top quality item when its done. Thanks for the offer but the postage would be steep to the uk. Its prolly better to donate it to a stateside pyro. I suspect you`ll have a big queue of people offering it a new home

[schroedinger]

Yes plaster brakes up after a few pressings.

Well PU is a problem since they got banned, but polyester and epoxy is still free.

[Col]

polyurethane casting resin is banned where you are?

[schroedinger]

Yes, because they contain isocyantes which are quite toxic. They even banned the stuff used to fill the holes when building doors, stupid things.

[mikeee]

Isocyanates will kick your ass hard if you are exposed to enough of the fumes while working with them.

Really bad for your lungs and takes a long time to recover from the effects. Nasty Stuff, I know........

[Arthur]

For the few hemis an amateur builder might want, there seems little value in making an expensive tool, especially if you only need a few hemis (say 5"+). As long as you can find a sphere of the right size then hand laminating it with pasted newspaper should form a good hemi. Go through all the sports balls in your culture, see what fits your pyro habit.

 

I see more value in selecting food cans by diameter to use as cylinder shell formers.

[Col]

Less than a tenner`s worth of resin for an unlimited supply of perfectly formed 3" hemi`s is a reasonable investment in my book, Breakeven point is around 20 sets

[Col]

Here`s a 3" pressed hemi with a commercial 3" hemi for comparison. Outside diameter for both is 66.7mm (+/- 0.1), wall thickness 2.4mm. Cost to make 6.5p /10 cents per set

 

Edited September 16, 2015 by Col

[calebkessinger]

Shipping to the uk should be under 30 bucks if it's under 4 lbs and fits in a flat rate envelope.

 

Those are nice hemi's.

 

I have sprayed hundreds of barrels of iso foam..... they do start to bother you after a while.. and you can become or start out allergic to it. They have changed a lot of it to waterborn. Trying to get rid of the iso. I don't think they have completely figured out how to get it to work without it though. Just make it "less toxic"

[mikeee]

I have turned about 50 different sized wood hemi balls over that last year used on the Wasp

machine to make paper hemis rolled onto the wood balls, and then cut off with a razor.

By turning a wood ball on a lathe, you could also turn a hollow bowl to fit the ball and use them

as a mold to press a paper slurry in the bowl with the ball. Or you could use either form to lay up

strips of pasted paper to form a paper hemi and trim them with a razor knife after the paper and

paste is dry.

[Col]

I`d imagine spraying iso foam would pose more of a risk than a one-off job involving half a litre of casting resin. You could always do it outside wearing a respirator, tbh, it didnt have any real smell to it perhaps a faint whiff similar to furniture polish.

If you have a wasp, wooden balls are the way to go. Over here, it would work out cheaper to buy the hemi`s than the gummed tape to make them

[Arthur]

There were some hemis made from newspaper with paste.

[Col]

I found newspaper hemi`s good and you cant moan about the price. Only downside i found was they are much less rigid than a standard hemi even with the same wall thickness, 2.4mm wall takes 34 layers of newspaper

[calebkessinger]

My buddy uses the two part molding foam for making gun molds... It's not the least bit smelly really. Dang hard when it cures. He makes kydex holsters off of the molds..

[schroedinger]

Col if you use newspaper hemis, you want them to be less rigid than commercial hemis. It helps with getting a better pattern. If you use newspaper 10-15 layers are enough, better paste more on the outside and use a thinner hemi.

[Col]

My pasting machine has a mean spring loaded clamp, anything weaker than a commercial hemi wont survive it Newspaper hemi`s are ok for handpasting or wasp which doesnt use a clamp. I only put shells on rockets so the pasting thickness isnt restricted.

 

Modified the female mold so the hemi`s can be pushed out with compressed air..works a treat

Edited September 18, 2015 by Col

[Col]

Added a die to cut out the hemi pattern discs