Unscrewing core draft analysis

I'm designing a part that needs an unscrewing core. For a straight pull
part, we have draft analysis that works like a charm. How do you do an
equivalent "draft analysis" for an unscrewing core? All I can think of
is copying surfaces and moving them incrementally at the pitch/turn rate
of the unscrewing core to see if they collide with the inside of the
part. The problem is that I need to design some non-axisymmetric
features inside the part.

Any ideas welcome,

Matt

 

Boy, that is hard to image what you are doing.

I have designed many "Bottle Cap" thread type things, mostly medical
Luer fittings, but nothing which I might call assymmetrical, so I am
trying to imagine that.

For short distances (.25 - .50" long threads) & pliant plastics like
ABS, PP, PE, I have routinely left zero draft. For things in
Polycarboante, Acrylics or Styrenics on Luer Fittings, I tend to leave
0.5 degrees per side draft on the helix cone in the Sweep feature in
SolidWorks, and I use a constant thread "Profile", and that has worked
well. I would also trust experienced toolmakers who have seen what
does and does not work in specific situations.

Beyond that, I would be in experimental territory.

Bo

 

Do your features have edges or is it highly organic? If you've got edges to
play with, then you should be able to generate 3D sketches from some of them
and sweep them to make surfaces. Check if the surfaces intersect with your
part.

If the part is very organic, you might want to put curves on the surfaces
(intersection curves, split lines, or your favorite other method), then
sweep them the same way. Careful picking of the curves should give you a
pretty good idea how well the core will clear.

Jerry Steiger
Tripod Data Systems
"take the garbage out, dear"

 

Thanks, guys,

Here's a picture of the part I'm working on.

www.dezignstuff.com\images\screwcap.jpg

The part isn't complex, but I'm hoping I can design a part without huge
masses of plastic between the finger grips, and I would prefer not to
have the separations between grips be as narrow as ribs. I've already
considered making the finger grips asymmetrical, but they turn out
asymmetrical the wrong direction (they would make it easy to tighten and
difficult to loosen).

I know the indentations at the bottom of the core are too tall as shown
to be screwed out of the mold, but how will I know when they aren't? I
did the bit with the copied surfaces and the move bodies. It's very
tedious, but it gives results you can understand. Modeling a solid core
and automating movement and interference checking would be useful, but
probably a little more tedious, although you could reuse the macro for
the next time this problem comes up.

I guess I could do the sweep Jerry suggested and use that to construct
faces of the part. That approaches the problem from the other
direction, making it a modeling problem instead of the way I've been
looking at it as an analysis/verification problem. This is the first
time I've ever come up against this. I guess most people just have more
common sense and make unscrewed cores completely circular.

 

I'd need the model to be certain but you should be able to push/blow this
part off if it's propylene or PE.
The thread would need altering but not to much. What is it going to be
molded in?

 

I'm going to recommend PE. The part is very chunky, lots of material.
You're suggesting not unscrewing it, but just pushing it off the core?
Yikes. It's a 4-lead screw thread. The OD of the part is about 2.5".

 

I know and that was my first reaction before I had done one this way on a
baby bottle top for Cherubs/Playtex. That stuff was 30 melt PP and unless
there is a good reason to use PE I wouldn't. Interesting story behind
Cherubs BTW.
I think Platex still make the line of Juice cups and bottles but the tools I
built in the 80's are definitely toast by now.
Little GD hearts 5 color pad printed absolutely every F'ing where on the
bottles! Sheesh...
The most stubborn part of the project was designing a seal on the juice cup
that would seal properly after a couple of months.
The baby bottle tops worked like a champ right from the start and I was
pleased and relieved not to have ended up with a couple of 8 cavity
stainless boat anchors if you know what I mean.

You might also want to consider giving Paul Catalanoti a call at Roehr. This
really looks like a good collapsable job if you can't fiddle the thread and
he's both a great guy and the King of collapsables and expandables. You can
rely on him 100 percent. I've got a number for him here if you want it
privately.

 

John, does Roehr do collapsible tubing per chance.

I designed both the first collapsible tubing used for infant breathing
tubing in the NICU (10mm ID) and 22 mm tubing for adult ICU (about 26
mm ID) and a complete extruder corrugator system back in the 80s. We
sold that company, and now on a new project, I am looking for the right
collapsible tubing supplier again. There were only a few companies
doing collaspsible medical tubing I know of so far. I really don't
want to even think about building or buying a new extruder corrugator
system today, as they have to cost the better part of $300k+ by now,
probably from Corma in Toronto (www.corma.com).

Thanks - Bo Clawson

 

Could it be a two part solution? The screw out portion with no draft,
and a heavy spring loaded internal plunger that goes into the finger area
with standard draft. If both portions were made of materials having the
same thermal expansion coeffecient, the tolerances between them would
preclude flash of any signifigance.

The sprung plunger would only have to move far enough so that when
unthreading occurs, there are no clearance issues. Visually, it looks as if
about 1/2 the height of the finger knobs would be enough.

 

Depending on material, I'd definitely look at bumping it off as the
easiest first option. I've worked w/ PP, cap diameters ranging from
38mm to 120mm, single thread lead, 1 to 1.5 turns, and thread depth
about .040 - .045. Successful bump-offs for the entire product line.

 

SolidBug, your comment is very interesting and rings true for something
I've been trying, though for the opposite of threads.

I'm stripping PP off of undercuts so I can do two parts that snap
together with undercuts & be virtually a locked joint that a person
can't take apart (think Tupperware that you can't reopen).

Are you able to use a flat stripper plate or ejector sleeve, without a
chamfer on the parting line, and still get good parts off without
warping and rounding edges?

My first parts were done with a bit of a chamfer to aid "stretching"
the part outward to come off an OD undercut without deformation, by
stretching about 1.5% (or .0275" in that case) radially, and that
worked.

Any help would be appreciated here. I certainly have NOT found much on
this technique in print, and have learned by experience to date.

Thanks - Bo Clawson

 

I should add that in order to keep the product compact, that my joint
is "thin" and the undercuts are "sharp" in order to make them very
resistant to being pulled apart, as opposed to a normal part with a
rounded undercut that is striped off a core with a stripper-sleeve.

Successfully dealing with a thin sharp connection that is stripped is
where the tricks lay hidden, particularly when the part is not purely
round.

Bo

 

Matt

I like this idea if the threaded portion can spin or rotate around a
post the supports the "Finger Grip" core. How are you going to actuate
the threaded core? At 2.5" OD there should be plenty of room inside
and it does not look like it needs to be timed to the mold opening.

Interesting problem, I'd like to know how you end up solving it.

Mike Butler
Volex Inc.

 

I think we're going to go with a collapsible and a PP material. It's
big enough to do it, the molder suggested it, and it solves our design
problem, so everyone's happy.

It was that trying to core the thing out in a helical direction was
tough to visualize. Even with a relatively steep thread angle, the
angle is too shallow to really do much coring.

 

Matt, the last Polypropylene (Huntsman 13T25A) part I did with a .0275"
undercut on a 3.5" diameter stripped off over a 45 degree angle
shoulder without a problem. It was a bit of a gamble, but it worked.

Looking at your part, I can't tell the height of the thread segments.
I'll bet if they are below .05" and have a 45 degree ACME thread type
side angle on at least the interfering faces, that you ought to be able
to strip it fine. Dupont's design manual recommendation is that you
should be able to strip a 5% PP undercut. 5% of 2.5" = .0625" on the
radius.

Dupont's manual notes the book where they got their recommendations
from & the pdf is named "H76838.pdf", and you can find it on Google.

Bo

 

Dupont did mention that to strip higher % undercuts, you want to keep
your mold temperature up to around 105-110 degrees F as opposed to 90
degrees, to keep the elasticity of the part up for the stripping
action, as I recall their manual.

I found similar things with my mold.

Bo

 

Bo,

The thread is about .055 all around, and the angle is far less than 45,
more like 15. If it were an outside thread, I'd go for it because it
would shrink away from the cavity, but as an inside thread it shrinks
onto the core. The molder wasn't crazy about trying to just strip it
off. It would introduce some pretty high wear on a surface critical for
function. I appreciate the encouragement, but the mold base is too big
to use as a door stop or paper weight, we're gonna play it safe with
this one and go collapsible.

Thanks for the discussion on this, it has been very useful!

Matt

 

I would agree 15 degrees will not easily slide off. I have succeeded
with as low as 30 degrees, but not at 4% undercut.

Given the strength of PP, and its use with threads having a high side
angle, like many bottle caps and drum caps, I would bet a hundred bucks
that a 45 degree holding face on the thread would hold the cap on the
container mating threads just fine with hand tightening and strip just
fine off the core, particularly with the multi-lead thread.

Bo

 

The way to know if this works is to use a single cavity MUD insert set
with aluminum plates and just the thread portion, if you want to be
quick.

Stick a quicky P20 ejector sleeve in it or buy the stripper plate and
just stick on a 35-45 degree chamfer on the threads and shot a few test
shots and see what you get.

The savings in mold complexity and cost would be considerable with a
simple stripper.

The Cost of an 8.4 x 9 MUD insert set is less than a split core
assembly.

Bo