plastics today extrusion expert webinarnb45 august 2010 v3

8/9/2019 Plastics Today Extrusion Expert Webinarnb45 August 2010 v3

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It Ain't Necessarily So

Some Extrusion Beliefs Too Easily Taken for Granted

A One-Hour WebinarHosted by Modern Plastics Worldwide

andPlastics Today

and presented by

Allan L. Griff

Extrusion Consultant and EducatorAugust 18, 2010


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The Right Screw will solve all problems

(the heart-transplant syndrome)The real world is seldom right vs wrong -- more often its worse, OK,better, best ..

Who decides what the right screw is, and on what basis? Rememberthat it depends on output rate, back pressure, product thickness, meltindex, barrel temps, scrap %, other factors, even for the same resin.Remember that screw design is not only dimensions and special features,but also materials, coring, driven-end design, ease of removal, tolerancesand delivery time.

Dont forget the barrel: the screw may be OK, but the barrel may not.

Financial: Is existing equipment already paid for? What dont you buy if you get a new screw? How much new money (not just a percentage)would be earned if the screw were replaced?


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More on screws ...

Screws and surging: a common belief is that worn screws cause surging (linear

mass and thickness variation). It is sometimes used to justify replacing orrebuilding a screw. There are many explanations of surging, from erratic feedto cycling heaters to stick- slip at the start of the compression zone , but I cantsee how wear can cause variation in the extrusion direction. Dont let thisbelief distract from measuring the surge and seeking its real cause(s).

The way to justify worn-screw replacement is to show how much money islost if the screw is NOT replaced, and that usually means the following: (a)worn screw reduces pumping capacity in lb/rpm, (b) screw must turn faster topump the same as before, (c) melt comes out hotter and cooling limits force aslowdown, and (d) the lost production could have been sold at a profit.

Compression ratio, the ratio of the volumes of the first and last flights, is often

used to define a screw e.g., to run this resin, you need a CR of 2.8 to 3.1.This may be true with certain machinery and conditions, but it isnt enough todefine the screw: one screw can have channel depths of 0.200 metering and0.600 feed, and another may have 0.100 and 0.300 they have the samecompression ratios but are very different screws. Get the numbers!


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The Right Profile will solve all problems

By profile I mean the shape of the graph of barrel settings: flat profile = all

settings the same, rising profile = cooler in back, inverted profile = hotter in back,there are valley profiles, hump or camel profiles, etc.

More wishful and simplistic thinking.

There is no right profile, even for a given resin. No neat and magic formulas. Thereare a thousand PE resins sold around the world for paper coating, milk bottles,wire jacketing, garment bags, etc. How can there be one right profile for PE thatapplies to all these, or even just all film? However, like Compression Ratio, it is asimplifying concept that wont go away.

The best profile depends on much the same things as the screw design: rate, temps,back pressure, product thickness, melt index/viscosity and its shear ratedependence, barrel temps, scrap %, barrel heat-cool efficiency, other factors.

One more thing left out of the profile concept is the importance of rear barreltemperature, which controls inpush and thus affects the screw speed needed toachieve a given rate. It should be selected and changed independent of the others.

There is no substitute for trial based on experience and sometimes backed up bycomputer simulation, if viscosity data are reliable.


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Speed Killsin a car as a drug but in extrusion?

Doesnt everyone want to run as fast as possible? Yes, but

if running faster means looser thickness tolerances, it pits product failure vs wasting material

alsoMore rpm may mean hotter melt (the 4 evils)

Older machines may fall apart sooner Can you store it? Can you sell it?


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Power costs are killing us!

Displacement of anxiety about gasoline costs, problems with Iraq, gang war inMexico, and the big oil spill

But not only aint it necessarily so, its almost impossible ..because

An extruder is a very efficient machine it converts mechanical energy (themotor) into the heat needed to melt the plastic

If we put too much energy in, we risk the four evils of overheating:1. Polymer breakdown

2. Cooling problems (lower rate?)3. Sizing problems (sticking, ovaling, sagging)4. Additive effects: yellowing, losing volatile additives, reactions


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So why do we have such a high energy bill?

Look at other equipment, especially dryers, stretching systems, air chillers,thermoformers, and power use elsewhere in the plant (especially air conditioning).

Are the necks and heads of the extruders shamelessly exposed? Insulate (Pyropel,magnets, Velcro).

Get an energy audit or do it yourself. Deal with power factors, reuse of cooling airto heat up feed, water-cool efficiently (old rags, do you really need refrigeration?)

But dont blame the extruder until you do the numbers(heat needed to get a lb or kg from feed temp to melt temp)

x (lb or kg per hour) = energy/hour

1 Kilocalorie = 4186 joules = 0.001163 kilowatt-hour


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Up Against the Plate

Screen Packs are put in with the coarsest screen next the breaker plate, right?

Yes, but they also work when put in backward, with the finest screen next to the plate!

Doesnt that fine screen get easily blown through the big breaker plate holes?

No, because the bigger pieces of contamination are caught on the coarserscreens, and the pressure differential across the fine screen isnt very much and its the differential that blows screens, not the absolute pressure.

This reverse order also makes it unnecessary to put in a cover screen to protectthe finest screen from exposure to the spiral flow that gets broken in thebreaker plate. (That is where the name came from.)

Of course, if you are concerned that the pack might be put in backward, you canuse a sandwich pack, coarse on the outsides, fine inside, such as 20-50-100-50-20 (wires/inch) and pay more for screens but gain peace of mind.


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What a Difference an A Makes!

The difference is between the MELT temperature and the MET ALtemperature.

The temperature of the metal (the walls of the barrel, head and die) may ormay not be representative of whats inside.

Measure the melt as well, with an immersion thermocouple, maybe withvariable depth.

Also, it may be useful to measure with an infra- red gun directed at the meltas it emerges. Know the angle that it measures, scan round extrudates

for max reading, avoid reading die surfaces.

No thermocouple or IR gun? You can get an idea of internal melttemperature by changing a control value in the adapter so that the heater

just goes on and right off again.


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Trust the supplier?

Yes, but.

Trust is a way of shifting responsibility.

Yes , you may be able to get them to take it backbut at what cost (time, money, relationship)?

Are you testing what comes in?

Are you recording test results, performance data, and labelmarkings (photos?)

The Lords of the Flies (a story)


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Once is Not Enough

At least, not when you are doing physical testing. It's best toget the average of three (better, five) tests: if all the values

are clustered closely, you can trust the average more. If oneor two values are very high or very low, test a few more; itmay be contamination (stress concentrators) or a poorlymade blend.

It takes some courage to do more tests when the results of a

few show you what you want to see. Dont rock the boat!But responsible testing means reliable results.


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$CRAP I$N'T ALWAY$ A LO$$

We save it for economic reasons. If we can work it right back into the feed, that isbest of all.

Some scrap enhances physical properties (HDPE where cross-linking occurs fasterthan chain breakage)

Keep it cleaned and screened, to avoid contaminants that can act as stressconcentrators (promote cracking).

Use colors to mask yellowing that is expected with reprocessed resin. Oil of violetis good, carbon black is best of all.

True recycling is of greatest value when it replaces the purchase (hence themaking) of the equivalent new material.ULS-FOS-TANA = Use Less Stuff, Fix Old Stuff, Throw Almost Nothing Away!


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Ten Key Principles of Extrusion

1. Screws for single-screw extruders are usually right-hand thread , and turn counter-clockwise (looking from the rear), as if to unscrew themselves back-ward out of thebarrel. A fixed thrust bearing takes the backward force, while the matching forwardforce (Newton) pushes the plastic melt out of the die. In twins, the screws may turnin either direction or both, but the principle is the same.

2. Most of the heat that melts the plastic comes not from the heaters but from themotor, whose energy is converted to frictional heat in the barrel, as the turning screwovercomes the resistance of the viscous plastic mass. Exceptions, where heaterenergy is significant, are very small extruders, slow-moving twin-screws, some high-temperature plastics, and extrusion coating.

3. Motor speed is reduced by a factor of 10 to 30 to get reasonable screw speeds lowest for very small machines (need residence time) and very large machines ((toomuch frictional heat).


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4. The entering plastic acts as a coolant, as it melts by absorbing heat from the motorand heaters. Normally, the front (output) end is hotter than the rear (feed) end, but if feed stops, the system levels out and the front transfers heat to the rear, with the risk of sintering, bridging and sticking to the screw root.

5. In the feed zone, particles must stick to the barrel and slip on the screw, and also stickto each other as much as possible for maximum feed rate (inpush ). However, more isntalways better, as a screw can bite off (take in) more than the front end wants to chew

(pump out).

6. Material is by far the biggest component of manufacturing cost. This means re-using asmuch scrap and trim as possible, and the holding of tight tolerances.

7. Energy is a small proportion of the cost of extrusion. Extruders are very efficient

machines, and excess energy would overheat the plastic and make it unextrudable.

8. Pressure at the screw tip is important, as it relates to safety, mixing, buildup on thescreens, and thrust bearing wear. This pressure is the cumulative demand of the headfrom screens to die lips, and is not something generated independently in the barrel.


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9. Output rate in a full (not starved) extruder is(a) the drag flow displacement of the last flight, less(b) the effect of resistance (pressure demand of the head), plus(c) effect of overbite at the feed.Leakage over the flights may have an effect, too, negative or positive depending onthe pressure gradient.

10. Typical shear rates are around 100 reciprocal seconds (rsec) in screw channels,from 100 to 1000 rsec in most die lips, and well above 1000 in flight to wall clearancesand tiny-hole dies. However, ASTM melt index is tested in the 1-10 rsec range.Proper comparison and description of a melt requires at least two viscosities (not aratio), with measurement or extrapolation into practical extrusion range.

11. Heating and cooling through the barrel is always opposed by the motor.Heating lowers viscosity at the barrel wall, so the motor turns easier and develops lessfrictional heat. Cooling is the reverse, thickening the melt at the wall so the motormust work harder and generate more heat.


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plastics today extrusion expert webinarnb45 august 2010 v3

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