Spiral Mold | Benefits?

[chrisprocess]

There's a Spiral Mold sitting on the shelf from some time ago.

Anyone have experience with this?
I've read people can use them to test resin lots, gate types (shear), and even machines.

I understand you assign a Spiral Mold# to determine how far the melt travels.
What's baffling to me is whether you inject with a fixed injection speed or fixed injection pressure. (and their relationship by introducing other variables)

Is this a hidden gem to help solve processing issues!?


I'm curious to hear people's thoughts and how Spiral Mold helped your team.

Thanks,

Chris

[brentb]

Good for testing melt flow of similar resins for comparison. Like PE of different grades, you can see flow in "real life" and not just go by specs.

KOM

brent

[chrisprocess]

Thanks brentb.

Seems like to measure flow you'd need to inject with a fixed injection pressure, say 1000 psi.

I'd suspect flow would be very similar for both resin types if you used a fixed injection speed. -What might differ is the amount of pressure required by the machine to achieve that fixed speed. (Just a theory - I could be way off)

[brentb]

There are ASTM tests that dictate protocol.

KOM

brent

[JayDub]

Thanks brentb.

Seems like to measure flow you'd need to inject with a fixed injection pressure, say 1000 psi.

I'd suspect flow would be very similar for both resin types if you used a fixed injection speed. -What might differ is the amount of pressure required by the machine to achieve that fixed speed. (Just a theory - I could be way off)

What you are measuring is essentially viscosity, which for a polymer melt is dependent on shear rate (injection speed). Lower viscosity gives longer flow length, other conditions being equal. Pressure is just resistance to flow, so if you impose a fixed pressure you are restricting the shear rate to whatever produces that fixed resistance, which in turn controls the viscosity. But viscosity is what you were trying to measure in the first place – it’s supposed to be an input to the test, but your test conditions (fixed pressure) make it a (controlled/limited) output. Or to put it more simply – you wouldn’t run an actual process pressure-limited, so why run your test that way?

The spiral mold is a sort of low-tech rheometer – it gives a comparative measure of viscosity at high shear rates (as opposed to MFI, which is measured at low shear rate). One thing this will show up is differences in molecular weight distribution. Given two batches of the same grade of resin with the same MFI, the one with the tighter molecular weight distribution will flow better.

[brentb]

There are ASTM tests that dictate protocol.

KOM

brent

I think I was wrong about ASTM tests for thermoplastic, for thermoset,they do exist. I think every molder develops their own protocol.

KOM

brent

[chrisprocess]

I think you're both right.

Interestingly the ASTM D3123 injections with 1,000 psi at transfer. Though this is for thermoset plastics.
It probably depends on what output you're looking for!

On a side note

1) Let's say you have a benchmark resin lot# 'X' with with a spiral number of 15 and another resin lot# 'Y' with spiral number 12.
2) You discover by increasing mold temp 10 degrees resin lot 'Y' will now reach spiral number 15 too.

Will you get the same quality as resin lot 'X' ?

I ask because I have no idea - I'd suspect they'd be pretty darn close.