Part Flatness: How to Control warpage?

[Stymie]

I designed a part that looks like a plastic wheel with ribbed spokes but rather than round like a wheel, it's more oval-shaped. It's symmetrical across the parting line.

The nylon part is being molded in China by a manufacturing partner so I don't have many details and likely won't be able to answer some of the terrific detailed questions I see in other posts.

The problem they are having is that the part is warping slightly such that it's concave on the ejection B-side.

1. A couple of weeks ago they determined that the cooling channels on each half weren't well balanced around the perimeter of the part so they added additional cooling channels on each mold half so the part would cool more evenly. That didn't help.

2. They brought in tech support from DuPont who determined that the mold temperature on the B-side was approx. 30 deg. C higher than the temperature on the A side. This seemed like the obvious source of the warping. After balancing the temperatures, they found the part flatness didn't improve.

The nominal wall thickness is 0.150", perimeter wall thickness is 0.125", rib thicknesses are 0.090". The outer wall and rib thickness are consistent on each side of the part.

My original spec was 0.1mm flatness. They are asking for 0.3mm.

My question: is there a way to control the direction of the warpage such as this by altering a parameter that can be controlled during the molding process, e.g., increase cooling one one side of the mold, etc. Or, alternatively, increase the rib thickness on one half..., etc. Obviously, I'd rather not modify the cavities.

Both parties are frustrated and we need to move forward.

Thanks!

[rickbatey]

How/where is this part gated? How many gates? Is it a cold runner? These answers will provide the root cause but I will just about bet you have one gate in the part, the cold runner sizing isn't optimized for fill/packing the part, and the mold is too cold and more than likely tge melts too high just to get the part to fill.
DuPont has technical info concerning part design for their resins plus best practices for molding and part designs. In the past I molded cooling fans for hedge trimmers. The mold used three gates into the hub, cold runner, with a three plate tool design. We only had to add water to the parts to get the impact resistance back where it should be but the parts ran like a champ.
Rick.

[Stymie]

(I replied but it didn't post)

It is a 3-plate mold with cold runner. Originally, there were three gates located on the interior face of the part, evenly spaced away from each other - one on each end of the squished circle and one near the center, but all near the outer perimeter.

After much trial and error, they suggested going to a single edge gate without explaining the benefit. They made that mold modification which didn't improve flatness.

Material is Zytel 101L NC010.

They brought in DuPont only after I insisted a couple of times. I was confident/hopeful they would find a solution.

Can you suggest other questions I can ask them to help find answers?

Thanks so much for your reply.

Brett

Edit: I should add that the parts look very good and seem to be filling nicely with good finish. If it wasn't for that slight warpage, the parts would easily be approved.

[brentb]

Yes,temp differential could help. Autodesk (Beaumont/Mold Flow) can do warp analysis and prediction. You could do a DOE using software from Suhas (Nautilus) or Launsby (DOE Wisdom)

KOM
brent

[chrisprocess]

What is the OD of the wheel? And how exactly are you measuring it?
Have you physically held parts yourself to investigate flatness?
Is the design intent critical in maintaining +/- .1 mm spec?

Sounds like it's temperature/cooling related.
Is the water temperature controlled with thermolators? Both A and B side flow independently or from the same unit?
If possible, you may need to run them at different temperatures.

You could try increasing mold temp and increasing injection speed, allowing material to flow more uniformly.
Faster injection speeds allow plastic molecules to orientate the same direction, this would help with warpage.

Additional gates would probably be the best solution.

Also the ejector pins may be "warping" the part during ejection if plastic isn't cooled enough, or if it's a 'rough' ejection.

[Stymie]

What is the OD of the wheel? And how exactly are you measuring it?

The "wheel" is about 9.5 in. x 6.5 in. x 5/8 in.


Have you physically held parts yourself to investigate flatness?
Is the design intent critical in maintaining +/- .1 mm spec?

I have. The first production samples were warped much more than the 0.3 mm than they are currently proposing and more than would be accepted by my customers. I wish I could be less vague about the part but 0.1 mm is tighter than truly necessary but was specified to raise the bar on QC. My goal is to understand the problem so that they can control it better to produce the best part possible - repeatedly.



Is the water temperature controlled with thermolators? Both A and B side flow independently or from the same unit?
If possible, you may need to run them at different temperatures.

I don't know the type of temperature control but believe that the two sides are NOT independent since it was a revelation to them that they were running at such different temps. I had hoped that was the cause and that their reviews with DuPont would have remedied that.



You could try increasing mold temp and increasing injection speed, allowing material to flow more uniformly.
Faster injection speeds allow plastic molecules to orientate the same direction, this would help with warpage.

Additional gates would probably be the best solution.

Perhaps they could revert back to the three gates they started with.

Thanks very much for your suggestions. I'll pass them along.

Brett

[rickbatey]

Brett the three gates is perfect as long as they are done into the same area of the part. The only way to use a single gate is to use a gate that feeds the entire center of the part like into the hub, as a ring or I think some folks call them diaphragm gates. It's like a thin cone with a thin area around the hub. Only issue with this style is the gate removal after molding. It usually requires a drill or end mill with a locating fixture.
The molder must run two water temp controllers on the mold so they can have a chance to regulate steel temps. Now the out of round is from differential shrinkage of the part. Short hold pressure stepping down pretty rapidly is your best shot. Trying to use scientific molding will make that part look more like a potato chip. You know the B-side having ejector pins in it may the whole issue. Where there's pins, you can't place water lines.
Rick.

[JayDub]

“The first production samples were warped much more than the 0.3 mm than they are currently proposing and more than would be accepted by my customers. I wish I could be less vague about the part but 0.1 mm is tighter than truly necessary but was specified to raise the bar on QC.”

Let me correct that last part for you:

“… 0.1 mm is tighter than truly necessary but was specified to raise the cost of manufacturing the part.”

The suggestions given are all good, but the bottom line is you have a relatively large part molded from a material not best suited to holding tight tolerances. DuPont’s molding guide for Zytel nylon suggests flatness tolerances of .015” (commercial) or .007” (tight) on a six inch part – so expecting .004” on a nine inch part may be overly optimistic. In fact the .3 mm (about .012”) that the molder is asking for sounds more than reasonable.

[chrisprocess]

“The first production samples were warped much more than the 0.3 mm than they are currently proposing and more than would be accepted by my customers. I wish I could be less vague about the part but 0.1 mm is tighter than truly necessary but was specified to raise the bar on QC.”

Let me correct that last part for you:

“… 0.1 mm is tighter than truly necessary but was specified to raise the cost of manufacturing the part.”

The suggestions given are all good, but the bottom line is you have a relatively large part molded from a material not best suited to holding tight tolerances. DuPont’s molding guide for Zytel nylon suggests flatness tolerances of .015” (commercial) or .007” (tight) on a six inch part – so expecting .004” on a nine inch part may be overly optimistic. In fact the .3 mm (about .012”) that the molder is asking for sounds more than reasonable.

JayDub brings up a very good point, .3mm isn't all that much - and in most cases I'd expect for this to be OK for most applications this large.

However if you open up spec to .3mm you had better be sure their measurement methods are verifiable!
This is why I asked if you measured flatness yourself, or how exactly they were measuring it.
They did in fact agree to the design spec, right?

A compromise could be to open it to .2mm (or .3mm if absolutely necessary) and have them beef up their QC sampling size - in addition to comparing their measurements with an outside source (maybe you).
Ultimately I would be cautious of their quality procedures more than the spec itself.


On the other hand lay the parts out in the sun, or warm them up with a hair dryer to see if they relax a little. If they're under stress they may straighten out, if not then maybe they're just molded that way.
Note the direction of the warp in relation to the gate or any other feature of the part. Is it consistent shot to shot? Or is it random?

Also may want to explore how the product is handled when ejected from the mold. Is is stacked in a box? Assembled? Laid out on a flat surface? Can a fan be used to help cure parts? Can parts be orientated a certain way under some condition to help with warpage?

Can this process be documented?

Just food for thought :)

Good luck!