I am making a cylinder of polypropylene that must be pressurized (2 bar). The rough dimensions of the current design (A) are 7 cm tall, 5 cm in diameter, and wall thickness of 1-2 mm. The issue is that the end plate (disc) begins to bulge when the pressure is increased.

I want to make a new design that is stiffer. Is there a shape/pattern that is better in terms of preventing bulging than naively increasing thickness?

I want to maximize stiffness (prevent bulging) while minimizing the thickness of the end plate. I don't care about how much material I use. I.e. if the stiffness is the same solution B and C are equally good.


  • The end plate must be flat on the inside, but there are no constraints on the outside shape.
  • The final product must be injection moldable.

enter image description here

  • $\begingroup$ Do the inside corners have to be sharp. $\endgroup$
    – joojaa
    Aug 5, 2021 at 19:08
  • $\begingroup$ 90-degree interior angles are stress concentrators that may precipitate failure. A spherical system is probably safest, but bulging will still likely happen. You need to consider all the temperature extremes the device is likely to see. Frozen PP is fragile on impact. $\endgroup$
    – Jim Clark
    Aug 6, 2021 at 15:53

2 Answers 2


"... while minimizing the thickness of the end plate. I don't care about how much material I use." Those statements are in conflict with each other. If the end plate must be flat on the inside, you have constraints on the design.

To reinforce the end plate without increasing thickness, consider a web of plates perpendicular to the end plate. If properly tapered, it can be injection molded. The thickness of the webs will determine strength, along with the depth that the web encompasses.

The photo below is of an experimental rats "maze" from ResearchGate.net but represents an exaggerated image of my suggestion.

rats maze radial reinforcement

You can set the height of the walls of the maze to meet your requirements, as well as add additional walls and/or extend them to the center, as your cylinder is not required to contain rats.

This suggestion does increase the thickness, however, but it also does not as as much material as would a flat thicker plate.

  • $\begingroup$ @TokeFaurby I agree that adding stiffeners radially (from the center to the rim) is the best solution to this problem. You can see the added stiffness that is produced in this answer. $\endgroup$
    – NMech
    Aug 5, 2021 at 14:47
  • $\begingroup$ Thank you for the answer. I have elaborated on what I mean by "... while minimizing the thickness of the end plate. I don't care about how much material I use." If you still think that the design with radial walls is the best design (better than simply increasing the thickness of the end plate) then I will accept your answer. $\endgroup$ Aug 5, 2021 at 15:25
  • 1
    $\begingroup$ If you have too much material, you might end up having issues with injection moulding. Solution B is what I was going to suggest, but actually radial stiffeners are probably a better solution. $\endgroup$
    – am304
    Aug 5, 2021 at 15:51

This is a minor addition to Fred_dot_u's answer.

Regarding the radial stiffeners the best shape for the stiffeners (minimizing the deflection for a given weight would be a stiffener with the following shape (in green).

enter image description here

This optimization only makes sense if you have large production numbers, otherwise you can use plain rectangular profile for the stiffeners if production is small.

It is best that the stiffeners are all connected in a central hub like the following.

enter image description here


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