Thread forming vs thread cutting in plastic

[BitMag]

Hello,

I`m currently developing a new kind of product, which (hopefully) will be used in the construction industry. It is an hand-held tool/accessory that should make the job of the construction workers a little bit more comfortable.
Anyway, as the initial production volume will be rather small, I have to use machined plastic parts (rather than moulded ones). The plastic used is POM-C (Ertacetal).
In the first prototypes I made, I used standard metric metal screws to assemble the tool and it actually performed quite well (the forces that work on the threaded connection are marginal).
But on the longer run I wan`t to use screws made specifically for plastic applications. These should offer a even better holding force and they do not require a tapping operation in the machining process.

But the question is which kind of screws (for plastic) to use because there are many different options on the market? Or a more fundamental question would be whether to use thread-forming or thread-cutting
screws, taking to account that the tool needs to be dissassembled about 10 times during its life cycle (for cleaning and maintenance)?

Any kind of recommendations and comments on the subject are welcomed!

Best regards,
Peter

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[RCaffin]

I have used both sorts. I currently use standard 'self tappers' into acetal and carefully calibrate the size of the drill bit for the hole.

This was for some very small self-tappers (#0). I drilled two different sized holes, inserted the STs, and then milled the block in half to look at the threads formed. There was not much difference in the appearance of the threads, but driving the ST into the 1.2 mm hole was MUCH harder than into the 1.3 mm hole. So for production we use the 1.3 mm size. So far, no problems.

Cheers
Roger

[BitMag]

I have used both sorts. I currently use standard 'self tappers' into acetal and carefully calibrate the size of the drill bit for the hole.

This was for some very small self-tappers (#0). I drilled two different sized holes, inserted the STs, and then milled the block in half to look at the threads formed. There was not much difference in the appearance of the threads, but driving the ST into the 1.2 mm hole was MUCH harder than into the 1.3 mm hole. So for production we use the 1.3 mm size. So far, no problems.

Cheers
Roger

Hi Roger,

Thanks for the information. I guess I just have to carry out some testing myself. It is the safest way to tell which type of solution works best in this situation. I´m also considering push-in type inserts.

Best regards,
Peter

[RCaffin]

Yes, brass push-in inserts work very well. I use them down to about M3 in plastic when the screw is going to be used a number of times. The thread lasts a whole lot longer. They should be inserted from the back though. If you push them in from the screw side they can pull out sometimes.

Cheers
Roger

[BitMag]

Yes, brass push-in inserts work very well. I use them down to about M3 in plastic when the screw is going to be used a number of times. The thread lasts a whole lot longer. They should be inserted from the back though. If you push them in from the screw side they can pull out sometimes.

Cheers
Roger

Yeah, as it happens I`m dealing with blind holes so the "from the back" option is unavailable. Anyway I`m gonna do some tests and see how it goes.

Best regards,
Peter