Screw/Threaded connection

[test1screwdriver]

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 - >

Screws for Plastic.pdf

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 advance12ptFEATUREtips1.jpg

[Kelly_Bramble]

All of the threads in the attachment will work just fine as they are designed very specifically for plastics. The only real decision you have is which head style do you need/prefer.

This document provides the detail design suggestions for these screw threads - Installation Design requirements Thread Forming Screws:

tb_pf_installation_06_28_12.pdf

[Kelly_Bramble]

Here is a calculator that will determine the pull-out load and required torque for the screws.


Self Tapping Screw Pull-Out and Torque Calculator

[JAlberts]

With respect to your list of questions, for most of these the general answer for most is thjat each of these parameters depends upon the type of application; except, in the case of the length of the thread engagement, or as you stated the height of of the internally threaded part.
The length of thread engagement for threaded plastic joints is the same as for any threaded joint of any material and is determined by the required resistance of the joint to thread shear failure when subjected to the required tensile load or anticpated maximum joint assembly torque. The one additional factor that also must be considered for thin walled internally threaded plastic components is the elastic expansion of the component due to the wedging effect of the screw thread's angle.

As far as the "number of turns", that is a tradeoff between the need for a compact component thread height vs the desire for a quick assembly and disassembly of the threaded connection. For example, with a general bottle cap application where the actual thread loading and applied assembly torque are relatively low, a primary consideration is the ease and speed of use by the consumer, no user wants to have to turn a bottle cap a half of dozen times to open and close a bottle. On the other hand, in other industrial application the strength of the joint will probably be focused on providing adequate thread strength and resistancs to shear or pull out failures.
Beyond those issues, a primary consideration in threaded plastic components is also the mold design molding procedures, which can be extensive for an internal threaded component.