How to make a plastic torispherical dome?

[cklow01]

I need to do a plastic torispherical dome for my project. As I need only 1 dome so I plan to do plastic moulding, but I don't know where to start and what to prepare. Can anybody teach how to make it or any other alternatives to produce the plastic torispherical dome instead of plastic moulding? The diameter of the dome is about 40cm and its thickness is not more than 1cm. Thanks in advance.

ps. My budget is limited.

[One of Many]

Here is a link to the math in creating one.

Torispherical Dome equations

What equipment do you have available? How accurate does it need to be?

You could vacuum form the dome on a piece of hard wood, then trim the .04 thick material to suit.

How about metal spinning out of soft copper or aluminum sheet? For that matter, you may be able to do this by hand working the copper with a wood mallet on a form tool too.

DC

[cklow01]

I am an university student and I am not sure what facility is avalaible in my faculty. But I know my faculty has injection moulding facilty but according to the technician there, he said the mould is very expensive and he said I need to do my own mold if I want to do injection moulding. He also added that the mould preparation is time consuming and I don't have much time left. The dome is needed so that I can do some experiment on it to verify my theoretical answer for the dome. I need the easiest way to do it and of course not very expensive (less than USD 100) as I am just a student. Thanks again.

[One of Many]

Injection molding only pays off for high production.

If your shool campus has a any machine shop training. Contact someone there to see if they would produce the part or a die to make the part.

Metal spinning and vacuum forming would be your cheapest bet for the desired result off inexpensive tooling. If you can have a print dimensioned properly and some materials, I would think another student in the trades could make it. 40mm diameter is not that large.

With metal spinning, the die can be made to exact size. With vacuum AKA Thermal forming plastics, the die would need to be a bit larger than finish size to compensate for some shrinkage in the finished part. Depends on how accurate the part needs to be for analysis. In either type of die, there will need to be some excess height beyond the trim line of the cap. For metal spinning you need something to grip on too, for plasic forming, the die can be mounted on a plate of aluminum.

If you made the die to form the material on, some local shops in your area may be willing to form the part for you in support of education.

DC

[cklow01]

Originally Posted by One of Many Injection molding only pays off for high production. If your shool campus has a any machine shop training. Contact someone there to see if they would produce the part or a die to make the part. Metal spinning and vacuum forming would be your cheapest bet for the desired result off inexpensive tooling. If you can have a print dimensioned properly and some materials, I would think another student in the trades could make it. 40mm diameter is not that large. With metal spinning, the die can be made to exact size. With vacuum AKA Thermal forming plastics, the die would need to be a bit larger than finish size to compensate for some shrinkage in the finished part. Depends on how accurate the part needs to be for analysis. In either type of die, there will need to be some excess height beyond the trim line of the cap. For metal spinning you need something to grip on too, for plasic forming, the die can be mounted on a plate of aluminum. If you made the die to form the material on, some local shops in your area may be willing to form the part for you in support of education. DC

What do you mean by metal spinning and vacuum forming? Is it metal spinning will be used to produced the die for plastic forming or the metal spinning can directly produced my 40CM plastic torispherical dome (metal spinning can shape plastic as well?) ? If I use plastic forming, does that mean that I need both male and female dies, and both of them will be made with metal spinning? What is the estimated time needed to make the dome from zero to fully completed if I spend 3-4 hours on it everyday? Thanks again for your patience in helping me.

[One of Many]

Let say that the last 1" business end of a baseball bat were your die for either type of forming. Which is pretty close to a torispherical dome.

Both processes us a simple single sided male tool. The material you start with is sheet near to the proper thickness of the finished part. Material strength can be altered just from cold working it. Not so much with plastics.

Metal spinning is done in a lathe type of machine. A metal sheet starts out flat and is worked onto the form as it is spinning. Search for examples on the web.

Vacuum forming uses the same type of form. Only a sheet of hot plastic is placed over the die and vacuum is applied under the material and die to suck the hot plastic to the form/die. Again, search for examples on the web.

DC

[lerman]

How did you select your material? Why is it plastic rather than metal? Why is it 1cm thick? Why not thinner? What is its purpose? How precise does it have to be?

To spin it from metal, a form is made (of wood). The form (called a mandrel) is mounted on a special lathe. Then a blank of metal is placed and held against the mandrel while both are spun in the lathe. The metal is then pushed against the mandrel and the metal takes the shape of the form.

Ken

[ViperTX]

Why don't you just buy a large hollow metal ball and slice off the portion you need...let's see 40cm.....that's only about 15.7 inches in diameter.....also you might check those large stainless mixing bowls....

[One of Many]

Oops! I read that as mm and never looked back. My sincere apologies!

Centimeters is a whole different story. You could still have it vacuum formed but not realistic to spin form it. 1mm yes, 1cm not even remotely possible. I'd think that would take a huge stamping press and possibly hot formed. Maybe plasma or Hydro-formed.

I'd look for a scrap tank and cut off the end, IF you can find one 1cm thick.

If you have a boilermaker pressure vessel fab shop in your area, maybe they can offer you something?

DC

[cklow01]

Originally Posted by lerman How did you select your material? Why is it plastic rather than metal? Why is it 1cm thick? Why not thinner? What is its purpose? How precise does it have to be? To spin it from metal, a form is made (of wood). The form (called a mandrel) is mounted on a special lathe. Then a blank of metal is placed and held against the mandrel while both are spun in the lathe. The metal is then pushed against the mandrel and the metal takes the shape of the form. Ken

The dome will be used to perform stress analysis when it is subjected to various load such as internal pressure. Plastic is chosen because the load subjected to it for destructive test wouldn't be that high if compared to metal. The thickness of the dome is only my estimation until I discuss with my supervisor, it might less than 1CM. I hope the precision of its size is as high as possible so that its answer can agree very well with my theoretical solution.

[cklow01]

Originally Posted by One of Many Oops! I read that as mm and never looked back. My sincere apologies! Centimeters is a whole different story. You could still have it vacuum formed but not realistic to spin form it. 1mm yes, 1cm not even remotely possible. I'd think that would take a huge stamping press and possibly hot formed. Maybe plasma or Hydro-formed. I'd look for a scrap tank and cut off the end, IF you can find one 1cm thick. If you have a boilermaker pressure vessel fab shop in your area, maybe they can offer you something? DC

If my area have a boilermaker, isn't them fabricate the pressure vessel's cap with metal? But what I want is only a plastic dome, is that they can do it as well? If I vacuum form it, how shall I make my male and female die? Is it by metal casting? Thank for giving me a lot of idea.

[One of Many]

Torispherical domes from metal might be readily available for about $100usd.

One done from plastic is going to take some time to create the tooling and find someplace to get it formed, trimmed to size, and ready for testing. If you do not have much time to find access and complete either portion of the tooling or plastic forming process, you have nothing to work with. As an alternative option, you might even find a vessel tank end cap to use as a die to form your plastic one on.

Do your theoretical answers require a specific material strength? Can that theory be scaled to suit what you end up with, or is the dome shape of your own design that you expect to test prove your pre-calculated theory based on a torisphere?

Would laying up fiberglass cloth and resin work?

In the US, $100 might get you between 1- 2 hours of a professionals shop time. I'd say those with experience that could make this as a custom plastic cap for you may have 1-2 days of labor to do the die and at least 1/2 day setting up and forming it. A minimum of 12hrs shop time blows your budget here in the States. Your only choice is to find out what that 12hrs might cost you if you do find a plastics shop in your home town.

Doing it on your own? You will need to locate a plastic sheet supplier and find out how many 1cm x 50cm sqaure blanks of plastic $100 will buy. How many of those can you afford to waste trying to get a usable one? PVC forms nicely and is one of the cheaper ones.

I see this taking some networking on your part to find the plastic sheet supplier, who may know a place that thermal forms and those that thermal form know who makes thermal forming tools.

DC