Tiny Bubbles in the wine (I mean part)

[joeprocess]

Have this very unusual issue with a specific Lexan material. It shows itself in a few different parts that we mold for this project. It shows in the thicker sections of the part. It does not happen if using other PC materials. The material is tested as dry. I believe the bubbles are small voids, but the melt stream has these bubbles as well.

What I'm scratching my head about is that when the part is filled to 100% on 1st stage without pack/hold psi/time there are no bubbles. When the part is packed out the bubbles show up. The fill only part does not show any voids or sinks anywhere on the part either. See attachment.

[chrisprocess]

Hah that's odd.
Look like voids given part thickness and even spacing between the bubbles..

Given the information u provided my best guess is the added pressure is influencing the heat exchange from the mold steel into the part.. ?
Like if you were to touch a stove top oven with 1lb of force it would be hot, but if you were to touch it with 50 lbs of force it would feel even MORE hot

That's funny though, maybe u could just run fill-only parts

[joeprocess]

Part isn't bad because of the bubbles/voids, but it was just really strange to me that the unpacked part looked so good. I've never witnessed this in the 205+ years I have been molding.

[chrisprocess]

Thanks for sharing, this sorta stuff makes us learn more! Just curious do you transfer on pressure or on position?
Maybe transfer pressure is enough to fill the part but yeah I hear you some sinks would definitely be expected in that thick wall section area.

[joeprocess]

We transfer by position.

[iautry1973]

What type of machine? I know Sumitomos do some odd ball things with their "Flow Control" It stops between hold and inj to balance cavities.

[nemanja]

Hey Joeprocess,

As i understand that you have problem only with that type of material. Do your material have any additional additives? Are you sure that drying parameters are correct? Maybie you can try to go on different temperature for drying.
For one material that needed to satisfy some special customer requests i needed to go way out of "recomended" settings for drying and it solved my problems with similar problems on surface. My parts have much thiner walls so is not that much visible.

[brentb]

Heat part with heat gun. If the bubble or void gets bigger and pops, it is air bubble. If it collapses, you can surmise it is a void. You can then take appropriate corrective action!

KOM

brent

[joeprocess]

It's a Toshiba, but the issue does not show up with other PC parts ran with this part.

[JayDub]

Have you done a gate-freeze study? If the gate is too small it may be sealed before the part is fully packed.

What is the maximum pressure when the part is filled 100% on first stage?
What is the maximum injection pressure when the part is filled & packed normally? Hold pressure?

My first SWAG here is that the first-stage fill part is seeing higher pressure, plus the shear heat (from faster screw speed on fill versus pack) is holding the gate open longer.

[chrisprocess]

Curious what the thermal conductivity of this particular PC is in comparison to other PC you are running..