injection molded part design rule question: thickness variation 1

[eks32]

Hi,

All the design rules I've read on injection mold design suggest
maintaining uniform thickness to avoid sinking.

On my first design, I toiled for hours to get a uniform thickness
(within %10) , the mold house was happy and the prototypes came back
with all sorts of sinking. So the mold house says "let's add some
blowing agent" and voila sinking goes away; A classic example of the
real world finding a work around while the engineer toils in the
closet.

Left unspoken is my thought that the slight cost of the blowing agent
would have, in my case, been a very small price to pay to avoid all
the fancy cad work to maintain close to uniform thickness.


My 2nd design is an extrusion where the local material thickness
__must vary.

What is the __real __world rule of thumb for the amount of thickness
variation that an extrusion can get away with to avoid sinking
(without blowing agent) ? With blowing agent?

Thanks,

 

[MikeHalloran]

It depends on the _exact_ design of the part.

This runs contrary to Purchasing 101, but my real world rule of thumb is to select a molder (or extruder) first, have him participate heavily in the part design process, and have him buy and maintain the mold for you. Need a second source? Buy a second mold.

A lot of purchasing managers think that you can buy a mold to your specs, and move it around among a bunch of molders based on the part price. I've never seen that strategy actually work. The parts always come out different, until the new molder makes substantial changes to the mold. Two vendor changes, and the mold is scrap anyway.

There's a lot of stuff that individual molders 'know' they can do, but they can't put it into nice neat rules for you to use. Okay maybe they could, but they'd be stupid to tell you about it.



Mike Halloran
Pembroke Pines, FL, USA

 

[plastx]

Within 10% is typically a good design so there may be other issues that are causing the sinks. The problem with blowing agent for a non-structural foam part is the cost of the blowing agent and the mixing step, part surface appearance and longer cycle time (sometimes). If the molder can't "pack out" the sink then the gate(s) might be under-sized with a large pressure drop across the part. The flow should also be from thick to thin sections but if the part is within 10% wall variation then this shouldn't be an issue.

Rule of thumb for wall thickness variation is 25% for amorphous materials and 15% for semi-crystalline materials w/o foam. As with any rule of thumb there are exceptions so as Mike H says the best way is to work closely with your molder to use their expertise in this area.

 

[eks32]

Hi,

I'm talking generic ABS, and probably with some blowing agent if I'm
reading the replies correctly: The extrusion has a left and right
side, so that the the same part mates into itself. When joined,
there's a rectangular hole in the middle. I'm trying to do this
without any slides- a straight pull out.

Thanks to an ascii art web site,

http://www.bornsquishy.com/flump/howto.html

here's a pretty good picture.
[tt]
_.-""""-._
.'z ______ `.
/ _| |_ \
| | |q |
|__| |__|
_ _
|z| |z|
__| | | |__
| | | |
| q | | q |
\ |______| /
`._ _.'
`-....-'
[/tt]
The "thin" spots are marked with a "z" and the thick parts with a "q".
After adding taper to some internals it's better, but the portion
marked "p" is still going to be thicker than the "z" spot here:
[tt]
_.-""""-._
.' ______ `.
/ / \ \
| | | |
|__| |__|
/| |\
__| | | |__
| | | |
| p | | p |
\ |______| /
`.z _.'
`-....-'
[/tt]
three quick q's

Does the fact that my thicknesses vary slowly. make a difference ?

Does the 25% variation spec apply to ABS ?

How much more thickness variation can you "buy" with blow agent ?

Thank you,

 

[TheTick]

Just how thick was this constant thickness? Even uniform thickness parts sink if they ae too thick.

 

[eks32]

Absolute and vary thicknesses ?
At present, the worst case is this section
[tt]
/| |\
__| | | |__
| | | |
| p | | |
\ |______| /
`.z p _.'
`-....-'
[/tt]

where
min thickness @ z is ~ .2"
max thickness @ p is ~ .30"

if that's really bad, I'll have to scoop it out somehow, but I'd rather assume that the blowing agent can
help.

Thanks again

 

[MikeHalloran]

We've discussed your first part, which was injection molded, had sinks despite wall variations, and the sinks were cured with blowing agent. That's an unusual cure, but maybe there's something unusual about the part, too. Let's put that part in a shoebox and forget about it for now, because it worked, and you know how to get more.

Your second part is an extrusion. Well, the rules are different, because you ooze the product out by the mile and cut it to length. For one thing, you could make the extruded part with the rectangular hole in it, like a hose, so there would be no joining step, and no features associated with the joint, because there would be no joint.

Or, you could extrude it in two pieces, as you have illustrated. But there is no concern about 'pulls' or draft in the plane of the illustration, because there are no moving parts in an extrusion die; it just squeezes out a profile. The only thickness concerns have to do with buckling or distortion as the extruded part cools.

You might be able to sink both halves of your part in the same extrusion die, if they're symmetrical, but most extruders would resist the urge because if you're not lucky, you can't optimize one part without making a continuous stream of scrap from the other hole.

There's another choice. You can extrude a single G- section part that is closed by the end user, like Zippertubing. That would have just one set of closure features, and some sort of hinge or flexure section opposite. Not every resin allows that to work, but it's common enough that you can find examples in lots of places.

I think you need to visit an extruding house.



Mike Halloran
Pembroke Pines, FL, USA

 

[eks32]

Mike,

I really value the time anyone spends on this forum
answering my gimmes.

Analytically ( in solidworks) it's an extrusion,
but the total length is only 14"-and it's on a curve,
but the extrusion path radius is pretty large compared to
the x-section, so I strongly believe the sinking problem
can be characterized as a 2D issue.

I apologize for the confusion.

 

[MikeHalloran]

I worked with a gifted molder, now retired, who managed to make a part with sections that big, in Delrin, without visible sinks.

He put the sinks on the _inside_. When you sectioned the parts, you'd find a substantial void running right down the middle. Sort of like structural foam with just one big bubble. But the outsides were perfect, and the parts were strong enough.

Your parts are pretty much doomed to sink. If you can't disguise it with texture, or need good surfaces for functional reasons, then define exactly where you need the good surfaces, make them 1 to 1.5mm thick, skeletonize everything else, and use thin (1mm) ribs, sparingly.



Mike Halloran
Pembroke Pines, FL, USA

 

[TheTick]

We make some pretty thick parts here. These are made w/ glass or talc filler in the resin (about 30% by volume). Also, cooling is painstakingly controlled. Definitely not a shoot-and-ship process.

Like Mr. Halloran said, reduce shell thickness. Use ribs to strengthen (most designers call this "coring", especially if ribs form a repeating criss-cross pattern a la ice cube trays). Ribs' thickness should be 2/3 shell thickness or less to avoid sink.


Resorting to tricks of pressure or chemistry to compensate for sink is usually a sign of a design in need of improvement.

I could be the world's greatest underachiever, if I could just learn to apply myself.

http://www.EsoxRepublic.com

-SolidWorks API VB programming help

 

[eks32]

Folks,

I'm reading that people view the blowing agent (BA) as a "last measure" sort of thing;
Something you can't rely on in a design.

I guess I'm trying to tie my experience of the BA "magically" fixing a
design that "should have worked" back into the next rev.

Relating to this picture, Let me ask a two last q's

[tt]
_ _
|t| | |
__| | | |__
| | | |
| q | | |
\ |______| /
`.z p _.'
`-....-'
[/tt]

If sinking is a "local" effect where the material that sees a "sharp"
cooling gradient experiences distortion... then,

1) If the part had to stay in the mold while segment "p" cooled off,
wouldn't "t" be just a bit cooler , but not distorted?
I can't see a mechanism for section "p" to distort the thinner
section "t" unless they are adjacent. (I can see how "q" would distort "t").


2) wouldn't the gradual transition of thickness that I've shown from
the "p" point to the "z" point, reduce the severity of sinking ?
Especially with the blowing agent ?

I'm appreciative of the time advice from MH and TT. Can you cite any
good books/references that discuss these points ?

Thanks

 

[MikeHalloran]

Shrinkage and sinks derive from freezing of the resin. Sections p and q _can_ influence section t by pulling liquid resin away from its interior. Most likely each will just affect itself, but there's no guarantee. Remember the thermal environment includes the mold.

The mold temperature distribution may be controlled by means of heaters and cooling lines, , or by choreographing operation of the machine and throwing away product until the machine stabilizes.

All of this stuff used to be covered very well in books that you could get for free from DuPont.



Mike Halloran
Pembroke Pines, FL, USA