Undercure Rubber - Molding Tool

[Beorges]

I working to troubleshoot an Undercure NR Oring with Injection Molding,

Any suggestion on potential root cause with the Mold Tool Sprue, Gates, Cold deck.

From what i read, mostly this issue is caused by process parameters (Time Pressure Temp), or compounding.

But what about the tool itself, this was producing goog parts after, so, what migh have change ??

Can this undercure spot, be cause by a tool issue ??

 

[btrueblood]

What is used to heat the mold?

I.e. check for blocked fluid channels if heated by steam or hot fluid.

Check for shorted or open heater coils if heated by cartridge heaters.

Check for dents or other loss of flatness on the mold faces, or loss of heating on the platens if heated via press platens.

 

[GrahamBennett]

If the undercure is just in the same one location on every cure cycle then it's a machine problem. As BTB says, check the heaters (and thermostats).

 

[btrueblood]

To amplify a bit, both Graham and I are assuming that what you think is an undercure is correctly diagnosed, and that you are not, e.g. seeing an area where the rubber is scorched, or an underfill/nonfill defect.

 

[SiliconeAurora]

Undercure can be a tooling design issue as well. Even if everything is working properly and nothing is broken or damaged, depending on where your heat sources are in the block, certain areas of the mold may be receiving more heat than others. Also, you'll be losing your heat to the atmosphere from the edges of the mold rather than the center. Cavities on the edge might be cooler than ones near the middle. It's something to consider. If you have some sort of handheld pyrometer, scan around the block and see if any areas are noticably cooler. Even ten degrees or so can be a big difference.

Other than that, temp and time are the two most common factors. When in doubt, turn 'em up and see if the problem goes away. Hopefully you won't introduce new ones in the process.

 

[GrahamBennett]

10 deg C lower roughly doubles the cure time. 10 deg C higher roughly halves the cure time.

 

[GrumpyPlasticBastard]

Has this mold run these parts before without issues? Or is this an ongoing nightmare?

If you've run this before without issue, check that you do not have DIRTY MOLD SURFACES.

Take your mold out of the press. Clean your platens, clean the top and bottom of the mold where they contact the heat transfer platens.

If you are directly heated by fluid circulating in the mold, flush the mold with cleaner to clean the transfer surfaces inside the mold.

Electric heaters in the mold itself... pull them out verify operation and use a LONG wire brush to clean the channels.


While that is going on, attack the mold cavity with solvent, De-Solv-It and vigorous scrubbing. Scrape with brass tools if needed.


No kidding we get an invisible film buildup in our mold that causes heat transfer issues. It starts manifesting as cracked parts, as soon as I get a cracked part I stop and scrub all six cavities. Problem goes away.

Since the problem is repeating in one location odds are you got some buildup there that needs to be remedied.




Now... if this doesn't work look at the mold surface finish. If it is ok, look to altering your injection speed and your stock temp. Small incremental adjustments and document EVERYTHING you change.





----
BFL Dreamworks
"If we don't have an answer we'll get one"

 

[edine]

Unless this is a really large o-ring is it unlikely to be undercured in just one spot. Without a picture, I suspect what you have is air trap with a bubbly ugly lump that left gunk in the tool. Inject time and/or speed changes can help as can adding vacuum. On thing to be careful with molding NR is reversion. It is better to increase cure time than to increase temperature to check for undercure.