Mold maker blowing smoke at me?

I'm having to deal with a difficult person -- a mold maker favorite of
my client who likes to make me jump hoops to please him. (Actually he's
not such a favorite any more, because our common client now understands
how difficult he's being.) He's telling me something that might be
accurate, and might just be his way to make life difficult for me.
Basically his story is that when metal is passing metal in a core/cavity
that they need 10 degrees draft between the two surfaces, otherwise
there is a high degree of danger that the mold will lock up at some
point. I've got no special difficulty believing him, on the one hand,
but I'm suspicious. I wouldn't have thought quite that much draft would
be necessary. I'm going to have to do some thinking about how to
accomplish it in the particular application I have. You can see the
area of the part by going to:

http://www.h2omarkdesign.com/img/connector_ring.jpg

I have annotated the area in question. Any comments? Experiences to
the contrary? Doubtless I'll give him his requested draft anyway since
we don't want to give him an excuse to do a poor job, but we're probably
going to be moving to a different mold maker soon just because this guy
is so difficult (not specifically because of this issue, but on a number
of different issues).

TIA
Mark 'Sporky' Stapleton
Watermark Design, LLC
www.h2omarkdesign.com

 

Ho Sporky,
I usually get away with 5 degs for designs like that. Obviously the 5
degrees only needs to be wehre metal meets metal, so it doesnt affect the
standard draft for the plastic sides. what natural angle does it work out to
with the gap you have given hime between the 2 vertical faces?
I have occasionally negotiated even smaller drafts with the toolmakers for
those situations, but if there's no reason not to, I generally try to give
them as much as possible as long as it doenst affect the design. Sounds like
he may just be difficult, if he's not willing to compromise.
Olaf

 

Aside from the fact that I got this when I tried to look:

403 forbidden

Server configuration does not allow access to this page. Please go back
and try again.

Send it by email.

The tangent of 10 degrees is .17, very close to the general coefficient
of friction for steel on steel. So you can't argue that 10 degrees
wouldn't be generally safe against lockup. But there are many other
factors to consider too.

 

Spork,

I use 5 degrees as well.

After looking at your picture, it should be plenty.

Regards

Mark

 

Funny -- it does that to me also sometimes. But then if I just paste
the link into my browser it comes up fine. I don't really get it.
Guess I'll ask the folks at my Web hosting service.

 

Mark, I too, have used 3-5 degrees, but I try to keep 5 degrees so that
I don't have as much chance for the ever-so-slight misalignment on
close that can cause frettting wear.

On mating surfaces of conical stripper rings I keep 10-15 degrees.

Fretting with materials of the same hardness is common, so the
toolmakers I have worked with always have recommended nitride on one
surface and a different hardness on the other surface makes for longer
life.

I must admit I have left these decisions to the toolmaker's discretion
except when the angle was critical to achieving design intent. I have
an exact equivalent of that in one design right now where the flow area
I need given the limiting ANSI standard fittings required will demand I
use 3 degrees on core shutoff. I will ask for a carbide face on one
surface or the whole core pin.

Bo

 

That's some interesting info about the different surface treatments --
makes perfect sense to consider that to help prevent galling. I'm not
going to tell this guy his business (although he tries to tell me mine),
but I'll remember the concept. Thanks (to all who reply also).

 

I've never heard of this. What does the tangent of an angle have to do with
the coefficient of friction of a material? Static or sliding?

Hoping to learn something new.

-Martin

 

I design molds and I would say you could use 5 degrees in that
situation. We have similar parts that are single thread nuts and we
have had to do the same thing. We have so far ran about 10 million
parts with no trouble. But again with not doing the whole design I
could be missing something. I would say it is good he is at least
giving you a heads up. Maybe ask to go see his design and talk it over.


The biggest problem I had before I designed our molds is that my molds
were being made by very good machinist who did not understand molding
our how the material flowed in the mold. Did not understand venting and
cooling. I was a Plastics Process Engineer before I designed molds. In
fact I still am the one to dial the molds in after we make them.

 

Never had a moldmaker squawk about anything down to 7 degrees for
passing cores. 3-5 degrees was almost always doable in situations that
required it.

 

Never had a moldmaker squawk about anything down to 7 degrees for
passing cores. 3-5 degrees was almost always doable in situations that
required it.

 

grantmi1, I was glad to see your comment on knowing about material
flow, venting and cooling.

I have had a situation where a toolmaker-designer (so-called) claimed
he was doing a proper cooling design on a 4 cavity mold and in fact
after the mold ran, it was probably 35% longer cycle time than everyone
thought it would be (& experience showed possible).

Is there a book on mold cooling design where I can gain more insight
into the subtleties? I know that there are mold cooling analysis
software, too, but that only analyzes what you throw up on screen, so
that comes later, if needed.

Thanks - Bo

 

Dig up your old physics book.

The friction coefficient is the ratio between the friction force and
the normal force that generates it. The two forces are perpendicular so
that ratio is a tangent (rise over run) if you draw the force vectors
out. So if you put a box on an incline and raise the incline angle to
greater than the arctan of the friction coefficient the box will begin
to slide.

In this case we want the box (mold halves) to slide past each other so
the draft angle should be greater than the arctangent of the friction
coefficient. Steel on steel typically is taken as .2 which is a little
more than 10 degrees. Polished hard steel on hard steel, perhaps with a
little chrome plate or electroless nickel will probably be less.

 

The SPI website has tons of books and might be worth joining if you
want to network with plastic mold types.

 

Tapers that are more shallow than a certain angle (for example the
Morse taper in the quill of a drill press) are self-holding. They're
around 2.8-3 degrees.


Best regards,
Spehro Pefhany

 

Our mold is harndend steel. I will check it out monday, and see if we
used 10 degrees I could have recalled wrong. Other then that I have
read many books and also just learned from experience on the cooling,
venting, and material flow.

The manufacture of the plastics sometimes have some very detailed
callouts on the required flow paths, cooling, and venting and they
specify it for the exact material being used. Another good book 'On the
Road with Bob Hatchet," he has some good insight to the issues he ahs
found and solved in the field.

When I design the part I have molding in mind and also mold making
because I also create the NC code for cutting the mold in the CNC. When
I am designing the mold I always have the cycle of the part in mind.
Say for instance, I have had molds that I have had to slow the cycle on
because the part was getting pin push from not have enough or large
enough ejector pins. So I design with plenty of pins. Another popular
issues is the mold designer's desinging small sprues and runners
thinking that it saves material. Saving the slight bit of material does
not come close to saving money if the cycle time is increased to make
good parts, or the bad part scrap rate is higher. If the sprue is
small, and the gates are small then the pressures are very high on fill
and usually you have a smaller window for molding. If I save material
on the sprue but my scrap percentage on the part is 10% then I have not
saved anything. Anyway I could go on forever, I am passionate about
this issue.

 

Spork, As you know, I worked in the toy industry for quite a while and it
was standard prctice to have 7 degrees draft for any shutoff and .040"
"land".

/
/
/
_.040_
/
/
/
But these were for Kenner and Tyco (two querky companies) so I think that 5
degrees is probably resonable, especially in light of the posts from peeps
who have been doing just that.

My 2 cents,
Muggs

 

Sporky;

Rule of thumb is any angle less than 7 degrees is a locking taper. That's
for steel. It does vary for other materials, consult the Machinery's
Handbook.

On that part, any angle will suffice for the noted shut off and not lock
up the tool, but it is best to use as much as possible for longevity.
(i.e. minimize galling, fretting and wear which will eventually lead to
flash).

Your mold maker is just being sensible, sounds like a good one who wants
to build a quality tool. If he wants 10°, give him 10°. Shouldn't
be any sweat off your back. Remember, it's not about how easy it is to
design the tool, it's about the tool itself.

 

"Modern Plastics Encyclopedia".
Published each year for the subscribers to Modern Plastics
IIRC.

 

Hot runners.