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Drilling holes for plastite screws

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Harry Crowther31/05/2019 15:32:03
18 forum posts

Hi,

I'm thinking about using 'Plastite' screws to save time tapping holes.

Has anyone got any experience using these please?

Harry,

Jeff Dayman31/05/2019 17:20:03
1621 forum posts
40 photos

Hi Harry, I have a great deal of experience with self threading screws (Plastite and several other types) into thermoplastics and soft diecast metals, in industry. They work great in thermoplastics IF the boss walls are thick enough and IF the hole dia draft angle and the entry chamfer dia and angle are the right dimensions as recommended by the screw makers and IF the recommended drive torque is not exceeded. Use in soft diecast metals are more challenging due to risk of head breakoff during driving, but again if manufacturer's recommendations are followed for the hole, they usually work fine. Taptite screws (finer thread, tougher steel ) are better for diecast soft metal.

Trilobular self threading screws like Plastite are far better than Hi-Lo or other types of self threading screws in plastics. Drive stress is far lower than other types especially Plastite 45 (45 degree thread flank angle) and the trilobular screws have better resistance to breakoff during driving and resistance to backout under vibration after install.

Plastite screws do not work too well into thermoset plastics like Bakelite or other phenolics, including Tufnol , they tend to cause stress cracking. In Tufnol they can initiate delamination. I usually design for through fasteners in thermoset plastic parts, no threads in the plastic, and plastic only in compression. Less trouble.

Plastite screws do not work well for joining mild steel, stainless steel plates or sheets, or machined metal parts with drilled holes. (and are not intended for that, but people try it anyway, usually with bad results).

If you have any specific questions about this, post them and I'll do my best.

Harry Crowther04/06/2019 16:41:06
18 forum posts

Hi Jeff,

Thanks very much for the information. I have a few applications where I'm going to start using plastite screws.

Do you happen to know if the pilot hole depends on the type of plastic that I use, or is it ok if I just make a pilot hole that is the same size as the core diameter of the plastite screw?

Thanks

Harry

Clive Foster04/06/2019 17:59:02
1840 forum posts
59 photos

Harry

Have a search around the web for datasheets from the screw manufacturer and use their suggested hole size as a basis for experiment. Different suppliers do vary.

To accommodate cold flow of the material the pilot hole diameter usually translates to about halfway up the thread. But it varies with material for best results. Some materials don't flow well and you have to accept a shallower thread. Measuring the core diameter on a tri lobe can be tricky too. Easier with things like Pas-Tech 30 where the core is a true cylinder. Plas-Tech 30 have a shallower thread angle than Plastite, 30 ° as opposed to 45°, so less material has to flow out of the way of the thread but the thread form is relatively "wider".

Make sure you hve time and materials to experiment.

Be careful not tao get mixed up with specifications for Taptite screws, made for metal. I know a man who did!

Clive

Edited By Clive Foster on 04/06/2019 18:33:41

Jeff Dayman06/06/2019 14:40:11
1621 forum posts
40 photos

Hi Harry, if going into softer resins like PE or PP, the manufacturer's recommended hole size is a good starting point. If going into hard resins like HIPS or PC, or into glass fibre or carbon fibre reinforced resins, a slightly bigger hole may be needed to avoid breaking off screws or splitting screw bosses. Hope this helps.

Harry Crowther10/06/2019 13:10:38
18 forum posts

I've found this guideline document for plastite screws: **LINK**

I've noticed that there is a range of recommended pilot hole sizes rather than just one. Does this mean that I should start at the lower end of the range for soft resins and the larger end of the scale for hard resins?

Jeff Dayman10/06/2019 13:58:00
1621 forum posts
40 photos

The min and max hole sizes in the charts are intended to be used as manufacturing tolerance limits for the holes people are making in their products that will receive the screws. The screws will work OK at any hole size in this range, but the mean of the min and max is the best hole size to start with, generally.

What plastics exactly are you working with? what are you holding together with the plastite screws?

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