posted March 19, 2003 09:12 PM               

Is this gas destructive (i.e. eat up barrel material, etc.)?

Is this gas crucial to the extrusion, or a by-product of extruding this particular material?

Most barrels have a rupture disc and perhaps you could adapt that, but it depends on what is going on...

posted March 20, 2003 08:40 AM               

I don't think you have any recourse except to purge the offending material out of the extruder when you shut down.

posted March 21, 2003 12:20 AM               

What material are you extruding, and what sort of extrusion are you doing. Is the gas that's being given off a product of decomposion. Or is it the by product of an additive.

How do you shut your extruder down?

posted March 21, 2003 12:17 PM               

quote:

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Originally posted by Steve H:
What material are you extruding, and what sort of extrusion are you doing. Is the gas that's being given off a product of decomposion. Or is it the by product of an additive.

How do you shut your extruder down?

Steve H

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Hi Steve, the gas is given off as a result of an additive into the melt via an injection point mid way along the extruder. The gas should be there, but I am trying to install a safety device so that if power is lost, and the extruder shuts down, then this gas can be vented so as to not create a saftey hazard due to pressure buildup.

Also, the gas is crucial to the extrusion process and seperates out of phase, and is not corrosive to the barrel/adaptor.

posted March 21, 2003 02:02 PM               

In the event that power to the extruder is cut, will this also cut the gas injection system?

My first thought is the latent heat in the die adaptors would allow the release of accumulated gas in the melt. So this shouldn't be a problem on shut down.

There could be some problems restarting after power has suddenly been lost. You would want to reheat the line from either end toward the point at which the gas is injected, when the line is reheated.

As Bob pointed out, burst discs(rupture plugs) should be fitted to relieve any extreme pressure build up. Replacing bolts on adaptors with studs and using brass nuts could be another option.

posted March 21, 2003 04:43 PM               

This is an emergency device only, but perhaps will provoke some thoughts to help solve your problem.

posted March 25, 2003 11:09 PM               

quote:

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Originally posted by louis33:
Shouldn't your machine be equipped with a blow out plug?

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posted March 27, 2003 07:14 AM               

Sintered metals do this - just as brass does this when exposed to high temperatures for a long period of time.

I poosted this as a concern for others due to an incident many years ago, when a flange blew off and the disc did not rupture. Fortunately, no one was injured, as I was heading up the Safety committee.

There was no, and is no "warning", but there was a message in the instructions of installation stating (perhaps not in the exact words) that said something to the effect that "this device is subject to case hardening when exposed to high temperatures over a period of time."

posted March 27, 2003 08:37 AM            

OK

OK

"Couldn't resist it Zeb"

Thanks for that info.

posted March 27, 2003 09:11 AM               

For normal startup and shutdown:

1) the first thing in the extruder, and the last thing in the extruder should be stable polymer. On shutdown all reactive, or potentially destrutive, components should be purged out, including easily degraded polymer.

2) When starting, once the screw, barrel, and die are full and extruding at full speed, then start adding other components in a logical order. (I prefer one by one.)

3) Dosing devices should have some type of positive shut off or injection screw to prevent polymer from flowing into the dosing device.

4) Dosing device interlocks should be considered which will not allow the device to operate unless conditions under #2 are being met.

5) A dosing extrution processes should go into an automatic shutdown procedure *before* any component of the formulation runs out, or if a dosing interlock trips.

For an unplanned shutdown:

1) As suggested proper setup of blow out plugs and die head clamps is a must.

2) It is possible that high pressures may be relieved out the throat of the extruder.

3) After some bad expereinces, I would generally recommend having a maintenance crew check out the extruder and dosing devices in order to bring them into a condition where thay can be restated in a normal condition, safely as described above.

Tom C

posted March 28, 2003 07:18 AM               

In other words, don't heat up the screen changer or adapter pipe without heating the die, or you will get tremendous pressure in that adapter pipe because the polymer is trying to expand and has nowhere to go. (Yes, adapter pipes can explode)...

posted March 28, 2003 07:42 AM               

The problem however, is usually money and not common sense decisions.

It does not sound like they want to really change anything significantly - and spend money on.

posted March 29, 2003 05:51 AM               

If you were able to tap a hole in the barrel for some sort of a pneumatic valve or something (assuming you could find one to handle the pressure), I would think you'd still have a dead spot where material would build up and burn, until it flakes off back into the flow and causes problems with the finished product. I really can't imagine a device that will allow pressure to be bled off.

I can't think of another solution besides suppose the downstream equipment could be kept warm (using heater bands and an emergency generator) so that if the gas does expand, it just pushes material out through the die.

posted April 03, 2003 03:57 PM               

quote:

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Originally posted by zabielski:
Something to seriously consider with blow-off plugs. They are designed with sintered metal, and case harden within a year. Becoming case hardened, the designed pressure rating (i.e. 8,000 PSI) increases (i.e. 10,000 - 12,000 PSI) over the coarse of normal operating temperatures.

You should replace them annually during a scheduled maintenance program. You can send out the old one to have it tested, and replaced. By keeping these records, you can then fairly estimate the point at which you should replace them - based on your operating temperatures. FLUKE has this service available, and it's a SAFE thing to do.

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All of our machines have at minimum 2 transducers, with machine shutdown wired through the alarms.