Molded Thread on a Plastic Bottle

You are right on the blended thread end. I end mine with a radius that matches the cutter diameter set at the pitch angle. Generally on a water bottle type neck:

1. I start the thread helix near the open end of the neck specified by a Plane1 parallel to the end of the bottle opening.
2. The sketch for the thread profile is put on a Plane2 which is intersecting Plane1 and the longitudinal plane of the bottle. That way the helix and the thread profile start on exactly the same spot in

1. The sketch for the blend can be done by creating a plane on the flat start of the thread.

Bo

I will say something about "Absolute Truth & the Designers Way".

Step 3 is a construction as someone would do using a small diameter milling cutter or grinding wheel. The blended end done that way will not actually be an end done as a helix.

If you wanted an end where the cutter was withdrawn toward the centerline of the thread, you would have to create another tapered helix, and I've not done that.

In practice many of these external threads are cut in with EDM into the tool steel cavities, and I suspect that the toolmaker may blend the end of the electrode in to the specification written in on the print. In some cases, given electrode wear, I am guessing the toolmaker may have the end formed by the worn down electrode, but that is a pure guess.

I have drawn many of these thread ends for various reasons involving caps, but I've not used a toolmaker to make a bottle mold.

Bo

Here's an example..

references.....

Thank you all, and thank you zxys for the invaluable data. G. De Angelis

references.....

The industry examples are good for bottle neck specs, but I did NOT see any description of the treatment of the ends of threads, meaning it is often left to the designer & toolmaker.

Examining ZXYS' sample file illustrates a feathered end on both ends of the thread much like I use. Doing feathered out ends on both ends is only practical for molded threads with split molds or side cores if done on both ends.

The higher the helix angle, the more subtleties can pop up in modeling & machining. A very high lead thread milled or ground on an ID, needs to have a cutter/grinding wheel angled at the helix angle, and that is not generally practical, at least for small diameter deep threads.

Bo