Screw/Threaded connection

Hello to all

I am fresh to plastic part design, especially regarding the about the ways parts should or could assemble each other.

At this particular moment I am most interested in screw/thread joints, like a cap for the bottle, cover, or two parts assembled togheter by “screw joint”. By this I do not mean parts joint by screws - >

I can find the mechanical equation for this issue but I missed general info.

Can you tell me what, where and why?

  • Which type of thread fits best for plastic application?

  • how to develop best number of rotation for “nut part”?

  • what should be the height of “nut part”

please follow thread types introduced in attached picture

thx in advance

It’s a bit difficult to answer with just looking at your sections of pitch and revolutions. Do you have a concept or a sketch you can show to better illustrate your problem. A lot of plastic screw thread combinations also depends on resin choices as well. Throw up your problem so we can offer more solutions.

As masood1224 mentioned, it’s impossible to answer without seeing what you are looking to accomplish. Thread aren’t very difficult to design, if it’s your first time:

1 - Just buy a product that has a similar thread to what you want
2- Cut in half
3 - Measure the pitch, revolutions, direction (clockwise or counter-clockwise) and teeth geometry of the thread
4 - Integrate in your CAD the dimensions you found above
5 - If you have access to a rapid prototyping machine just print and validate your design.

For a lot of technical details like this, reverse engineering is the way to go. Why bang your head against the wall when the solution is already out there.

VanDeBar is absolutely correct. Don’t reinvent the wheel if you don’t have to, and if you have to then your probably need experienced engineers to join you on the project (or, more likely, you’re over-complicating things). All I have to add is that sometimes your molder can be an asset if they do work in this market already and if you find it difficult to measure the tooth geometry exactly try looking in Machinery’s Handbook (Machinery's Handbook - Industrial Press). If you get rough measurements you should be able to find the thread type and get the exact measurements from that.

I am not sure if this is what you are looking for, but for plastic packaging there are specific standards that are used for most rigid packaging you see. Look up SPI neck finish specifications. Different neck finish threads are referred to primarily by the diameter of the neck in mm and the number of revolutions of the thread, not by the type of package they are on (“TE cap” does not identify the type of thread used) . SPI neck finish charts and/or drawings should give you all the critical info you would need to build a standard bottle/cap thread.

thank you all for the response

at this particular moment I am especially interested in:

  • type of thread considered for coupling same one can find in the garden(see attachment)
  • type of thread considered for connection between bottle and nozzle (check attachment)

I`ve just started reading about SPI specification so it can be possible I will find the answers there.

Anyway please do not hesitate to reply :wink:

With a quick google search I found a forum thread (Redirecting to Google Groups) that indicated that garden hose threads are type NH or NHR for thin walled couplings (ANSI/ASME B1.20.7-1991). That spec is in Machinery’s Handbook under Hose Coupling Screw Threads, and looking at them it appears to match the description offered by wikipedia of the Garden Hose Thread (GHT) standard. GHT probably isn’t really a standard, but I have seen it used as a term for fittings.

The bottle and nozzle could be lots of things. If there’s a gasket, which I expect there is, it could be the same as above or NPSM (a straight mechanical thread). It may also be a buttress thread, which are strong in one direction only, and are sometimes used to get more strength from plastic. Without a gasket you’d need an NPT (which are tapered and seal on the threads) and some teflon tape.

There is a difference between bottle cap screw threads and other threaded nut-style connections. Bottle cap threads have separate specifications which have been described and linked to elsewhere here.

The Gardena hose fitting you scratched off uses standard V-profile threads. They are produced two different ways:

  1. A continuous threaded tool insert with electrical actuator to unscrew in tool prior to ejection. This is very expensive, slow, requires maintenance, but produces very accurate, fine threads.

  2. Tool halves are machined separately with half diameter thread and the part ejects normally. This requires some careful tool machining to ensure the thread halves align, but is no problem for competent tool shops. These parts of course mold cycle very fast but will show occasional part - part thread unevenness due to process variance over time, tool wear, or flash. Looking carefully at the Gardena hose fitting you can see the tool witness line across the thread V profile, this part was molded from separate half-threaded tool cavities.

General rule for thread engagement like you illustrate is .5 - 1X thread pitch engagement minimum, i.e. 1-16 UN thread should engage between 8 - 16 full threads . Of course you know threads work via competing tension and compression force, therefore the more thread surface engagement your parts have the stronger, better seal, they will be when engaged. Also, as material thickness is different in the thread area, you must be very careful with geometry in the thread relief - neck area to prevent typical plastic molding phenomena.