A Nice article on self-tapping screws for plastic

[JosephS]

I was looking for some spare screws for some toy grade rc cars, as well as my son jegs stinger. I ran across the below article when looking for information on thread count/pitch.

https://engineeringproductdesign.com/knowledge-base/self-tapping-screws-for-plastics/

It's a nice read but some key points are

Thread cutting

Thread cutting screws for plastics, also known as a thread tapping screws, has a sharp cutting edge that cuts plastic when they are screwed into the pilot hole, forming a threaded joint. This produce reduced internal stresses compared to thread forming screws, making them suitable for stiffer plastic material with higher flexural modulus.



It is important to have the correct pilot hole diameter and depth. Hole depth has to be deeper than the screw engagement length to allow space for the removed material to move. For fibre or glass-filled plastics, these provide high thread engagement, high clamp loads & reduced stress. The biggest drawback of the thread cutting screw is thread stripping during disassembly.

...
Disadvantages

Only up to 10 disassembly and reassembly cycles
Stress concentration or raiser is likely

I was surprised on the low disassembly/reassembly count. It would think folks here could have exceeded 10x times rebuilding parts on a car.

As a note when reading the plastics table
Polyamide (PA) I another name for nylon.
ABS is what Tamiya parts are made of.


I typically replace tread-cutting screws with hex head 4-40 screws without thinking much about holding force. I'm guessing that I am weakening part connects where I do that.

https://www.stanleyengineeredfastening.com/-/media/web/sef/resources/docs/other/threaded_fasteners_for_plastics.ashx Seems to go a bit deeper and has more pictures.

[juicedcoupe]

Whoever wrote that has clearly never owned a Tamiya or early Kyosho.

I still use a tap to cut proper threads in plastic when using machine screws.

[ZED32]

Idk, I back thread before threading in, I've never stripped a screw hole with one of these. Seems like if you exercise some amount of caution you'll be fine on the reassembly front.

[TRX-1-3]

Idk, I back thread before threading in, I've never stripped a screw hole with one of these. Seems like if you exercise some amount of caution you'll be fine on the reassembly front.

As with alot of these newer 1:1 cars/trucks that have moved away from flip clamp/bail type secured air cleaner covers, and switched to steep threaded screws into plastic (Ram trucks come to mind). If you don't get those into the same thread track, it's over, hole is railed. You might get away with it once, but not twice.