Molded Thread on a Plastic Bottle

If someone here would send me a Solidworks file for a molded thread on
a plastic bottle, I would appreciate it. I would like to roll the
file back to see the technique of offsetting the radius at both ends
of the thread, and have the end blend to nothing, into the bottle
neck. I am guessing that a plane has to be made paralell to the pitch
and offset a circle to sweep? Any bottle, any size thread is ok. I
just want to see any basic concept that works.
Thanks for your help in advance.

G. De Angelis
Valhalla Grafix LTD
www.valhallagrafix.com

 

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
3D space and the sweep will work every time.

3. 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If you wanted an end where the cutter was withdrawn toward the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..
http://zxys.com/misc/thrd-sw2006-zxys.zip

And, here are some spec references.....
http://www.all-pak.com/capneck.asp?navid=35
http://www.all-pak.com/plasticgloss.asp?navid=42
http://www.kenplas.com/project/pet/neckfinish/
http://www.alphap.com/basics/anatomy.html
http://www.westbridge.ca/products/pre.html
http://www.westbridge.ca/tech/neck.html
http://www.plasticsindustry.org/search/index.htm

...

 

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

 

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