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Proper Screw Threading Design

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  1. Dec 4, 2014 #1
    Hi,
    My question regards a plastic bottle that I'm designing. For the threading that fits the two parts together, will and Acme thread work? This is how I've designed my part, but I can change the thread profile if need be. Also, is there anything that I need to know regarding the pitch? I have not chosen a specific plastic, but it will be similar to a pill bottle, and a little larger than an inch in diameter.

    Thank you,
    Mike
     
  2. jcsd
  3. Dec 4, 2014 #2

    Danger

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    If I'm understanding you correctly, there is absolutely no need for an Acme thread. It's far more complicated and expensive to produce than you need. A simple half-round bead with a matching groove should be sufficient. Of course, pressure requirements and other factors will make a difference.
     
  4. Dec 4, 2014 #3
    Thanks for the response. Regarding the location of the threading, how much room on the piece should I leave before the threading begins?
     
  5. Dec 4, 2014 #4

    Danger

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    I don't quite understand that question. Do you mean how much "neck" between the "shoulder" and the threads? If so, there is no minimum requirement. Leaving some bare is more aesthetic than functional, unless your manufacturing process (may I assume "blow-moulding"?) will benefit from it.
     
  6. Dec 4, 2014 #5
    Yes, that was what I meant when asking. Thank you, that makes sense. Sorry for the lack of nomenclature, I've never prototyped before,
     
  7. Dec 4, 2014 #6

    Danger

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    Don't sweat it; I don't know what I'm talking about either. I just learned to fake it a long time ago. :approve:
    If you have specific structural requirements, though, wait for someone else to help you. I'm not really educated.
     
  8. Dec 7, 2014 #7

    Baluncore

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    There is no need for a matching groove. You can use a half round profile on both the internal (cap) thread and the external (bottle) thread. By using only 35% of the area for thread you will reduce the manufacturing cost by increasing clearance and make fitting the screw cap easier.

    The pitch of the thread and the number of starts will be decided by the number of turns you require to open and close the screw cap.

    How will your bottle seal? Threads leak, so you will need some non-threaded section where the lip of the bottle can seal against the cap. Those surfaces should be well defined and not include the area that is cut or trimmed after moulding as cut quality is unpredictable. Bottles with poorly sealing lids are useless.

    Take a look at similar examples on supermarket shelves. Pay attention to the sealing section inside the cap.
     
  9. Dec 7, 2014 #8

    Danger

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    Excellent post, Baluncore. I'm glad that you showed up when you did, because I really was at the end of my "knowledge". (Actually, it appears that I was somewhat beyond the end... :redface:)
     
  10. Dec 7, 2014 #9

    Baluncore

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    @Danger. You took the first step by recognising that a precision thread is not needed.
    If we did not challenge ourselves with new problems we would never learn. Having to answer another's questions forces us to crystallise our understanding.

    After encountering three different plastic thread problems last week, I can now summarise my experience in just two paragraphs.

    1. The tensile strength of an injection moulded cap was insufficient, so it tended to split over time. The manufacturer used progressively cheaper material. It was doomed to fail in the end because material quality was never defined or tested. Design of the seal so as to not require high tension in the cap material would allows a cheaper plastic to be used. Tension in the cap must be opposed by compression in the neck of the bottle, so a better seal design leads to savings in the bottle also.Thread profile can magnify tension in the cap. Too fine a thread can also lead to high cap tension.
    2. Solve a sealing problem with an unusual large diameter screw cap. It came down to final dimensions and how the product was trimmed after moulding. Different materials for cap and bottle lead to thermal size variation, that requires larger sealing cones and relatively bigger threads.
    3. Find an alternative cap for an existing bottle. This got me to wonder what standards there are for plastic bottles and their caps. I googled 'plastic bottle thread standards' for enlightenment.

    Since the cap is fundamentally used to provide a seal, the seal should be designed first to meet the requirements specified. Next, the screw thread requirements can be considered. Final material specification follows. All of those must have the cost of manufacture in mind. The container is there to protect a more valuable product. The quality of the product is finally dependent on maintaining the quality of the packaging.
     
  11. Dec 7, 2014 #10

    Danger

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    Thanks. That's really what I love most about being here on PF (well... that and the joys of occasionally pissing off Russ, of course... :olduhh: :devil: :biggrin: Honestly... fish in a barrel sometimes... :oldeyes:)
    Anyhow, your focus upon sealing brought something else to mind, which I'm also sure that you know much more about than I do. It's related to the "shelf-life" of the product after initial opening, I believe. Even though it is probably just an anti-tampering safeguard, a lot of medicines that I buy (not the prescription ones) have a plastic/aluminum disk heat-glued to the rim of the neck under the cap. Having such would eliminate any need for sealing by the threads, given a low-pressure situation, as long as the product isn't expected to be stored for any significant length of time once accessed. That might be more expensive than well-sealed threads, though.
     
  12. Dec 7, 2014 #11

    Baluncore

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    Therein lies the problem. Threads are very hard to seal if used multiple times. In hydraulic and pneumatic applications the seal surface is almost always isolated from the thread. The common tapered pipe threads require a sealant and so can really not be cycled often.

    Food and medicine bottles have tamper detection caps so as to transfer the liability for injury by malicious contamination from the manufacturer and distributor to the end-user. If it looks like it has been tampered with it must be refused or returned without further interference with the evidence. If you touch the damaged seal you have destroyed the evidence of prior interference.

    The seal below the cap prevents atmospheric moisture exchange due to cyclic “breathing” of the container in transport or storage. That reduces the mass of desiccant needed to protect the product. The seal also prevents the theft of a small amount of material from each container, which reassures the paranoid customers, (maybe about 5%), that the quantity is correct. The seal significantly reduces unresolvable complaints flowing back up the supply chain.
     
  13. Dec 7, 2014 #12

    Danger

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    Thanks again. That's a very informative post.

    Yeah... we used Lock-Tite or locking washers on mechanical bits like in our cars, silicone sealant on gaskets, Teflon tape for pipes... you name it, we used it.
     
  14. Aug 3, 2015 #13
    Do more threads increase strength?
    I am designing a bottle to fit with a standard closure. I will likely use 70mm x 8TPI. I will have an aluminum threaded cap made that can be reused, this is needed because it will have a hose connected to the metal cap. I am not so concerned about the standard closure leaking as I am my machined aluminum cap. I chose 8TPI over 6TPI to increase the amount of torque that can be applied to the aluminum cap to keep it from leaking. Is the choice of 8TPI over 6TPI valid?

    Thank you.
     
  15. Aug 3, 2015 #14

    Baluncore

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    Welcome to PF.
    Why mix metric and imperial systems?
    What is the thread profile proposed?
    Is there an O-ring or flat elastomeric seal? or how and where does the seal occur?
    What is the bottle material and what are the contents?
    Repeated use of Al screw threads may tend to gall the surface. What are the lubricant properties of contents?
     
  16. Aug 3, 2015 #15

    Thank you for the reply.
    Bottle caps are measured in mm ID and threads are given in TPI. Not sure why.
    This is HDPE plastic cap and the threads are buttress style. Here is a link to the typical cap. http://www.riekepackaging.com/pail-and-can-products-print/6
    Caps seal between the top of the bottle and the underside surface of the cap. Most caps have a gasket ring that the top of the bottle sits into. The thinking in choosing 8TPI over 6TPI is to be able to increase torque and provide more resistance to internal pressure.
    Bottle will be blow molded polyethylene. Contents of the bottle are a variety of construction adhesives and caulks.
    The machined aluminum cap would be used repeatedly, the bottle is a one time use package.
    Thank you in advance for your thoughts in this regard.
     
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