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plastic pipe design for vacuum

plastic pipe design for vacuum

plastic pipe design for vacuum

(OP)
Hopefully there is an engineer out there who can help.
I am the program manager for a project that started out underdesigned.  

We are blowmolding plastic pipe sections, 6" in diameter to be used in vacuum application at 100C.  So far these tubes have failed at the prescribed 2.5 in HG at 100C.

We are supposed to supply product to the customer for a Sept 2nd build and do not have a viable product.  We need to redesign the tool used to blow mold these tubes but do not have time for FEA analysis.  

Does anyone know of a program for pipe or Pressure vessels that can get us in the ball park for adding structure to the tubes to prevent collapse. The tubes are about 4 feet long and we have the ability to change the outside shape through change of the tool used to blow mold.

thanks
 

RE: plastic pipe design for vacuum

Can't you just increase wall thickness, although isn't it  the temperature that's the real problem?

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

RE: plastic pipe design for vacuum

What is the plastic you are using, what wall thickness?

You may be able to add some circumferential ribs to help prevent collapse (buckling), but as BigInch says, it may be the plastic you are using just can't take the temperature (not many can).
 

RE: plastic pipe design for vacuum

Try duplicating schedule 80 or 160 wall thickness/diameter ratios.  2.5 inches of Mercury is less than 2 psi pressure change.  What type of CPVC is used?

RE: plastic pipe design for vacuum

That's why I said temperature is the problem.  I don't really think any wall thickness is gonna' help, but what else can you do?

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

RE: plastic pipe design for vacuum

(OP)
We are planning on adding 6 mm wide x 3 mm high ribs at a spacing of 13 mm.  We are at the limit of our shot size so this may cause thinning of the wall but as a couple of you have said we have few options.

We are using Dupont Hytrel and supposedly were told this material is good at these temperatures.  Thanks for you comments.  It sounds like you are of similar opinions about what the solution is.  

I was thinking that this tube is similar to a pressure vessel under vacuum and was hoping there was a program we could purchase or a general rule of thumb formula that we could use for future designs.

Any suggestions?

RE: plastic pipe design for vacuum

There is a general pipe colapse formula for pipe sections, but without ribs.  You might be able to correlate with some average diameter.

thread378-223498: Calculation of collapse pressure of steel pipe
 

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

RE: plastic pipe design for vacuum

Ugh.  That's not plastic, it's an elastomer (rubber).  Predicting the collapse pressure is even more difficult because it has viscous (viscoelastic) behavior rather than nice, clean elastic/plastic behavior.  Hopefully there is a time limit on your test...

Ribs will help, and the formula BigInch refers to is listed also (along with others) in Roark's Formulas of Stress and Strain.  But they all assume elastic materials...you will have to do some testing to find a value for "E" at 100 C that you can use in the calculation.

As for the part design, ribs help, as will anything that gives a higher effective bending moment of inertia to the cross section.  If your shot size is running out of headroom, can you use a foaming/blowing agent, and create a foam-core part with greater wall thickness?

 

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