Q: How do you reduce ampere "wiggle"
A: Find the cause. Small extruders may have feed particles too big to smoothly enter the feed channels. Feeders may be erratic. On-off heaters on small dies may cause corresponding surges in output. Motor regulation may be faulty. Those are the “easy” solutions, as the cure is obvious from the diagnosis. The remaining major cause, second-zone surge, is more difficult – amps cycle with period of 30-90 seconds, caused by melt pushed up into solid pellets at entry to compression zone, clumps form and don’t convey easily, eventually they melt and break up, and the cycle repeats. This sometimes goes away if feed is warmer or rear or next-to-rear barrel temps are changed, but sometimes it persists so that slowing down the screw is the only escape. Barrier screws are often resistant to this problem.
Q: I notice that there is more output with a screen than without one. Could you explain this?
A: If you mean output per rpm, it should be the other way round, more/tighter screening raises pressure in the screw and output/rpm should go down. If cooling is the limit, it may also cut output because melt temperature may be higher. However, practical output may be higher if the higher pressure helps mixing enough to allow faster rpm.
Q: Premixing feed. Do most people mix additives in the extruder now?
A: Most additives are added as concentrates at the extruder entry at the bottom of the hopper, unless they are already compounded into the feed.
Q: About the heat that materials absorb in the extruder, are there many differences between pellets or powder?
A: This question refers mainly to the feed zone, and once melted, any material is the same no matter what form it was when it entered the machine. In the feed zoneThe amount of heat absorbed by the plastic in the extruder depends on the internal friction. Some powders need a lot of power, so twin-screws are usually used, which require less energy for forward conveying and thus produce less heat. Barrel heaters make up what the motor and internal friction don’t do. As for pellets, a lot depends on the lubricity of the pellets (slippery = not so good). Once melted, heat input for all systems depends on viscosity of the melt
Q: How important is the die temperature in controlling size stability ?
A: It’s always important, but I can’t be more specific without knowing the product. In blown film, a hotter dielip means a thinner film; in most other products, a hotter die lip means more flow = thicker. Also, air currents and irregular dies lead to temperature differences that might affect flow, so consider insulation in spots or all around as an alternate to heating.
Q: What typically causes a purple or blue discoloration of a screw, assuming no mechanical damage to the flights (no indication of contact friction from alignment issues)?
A: This is caused by a pressure peak and corresponding overheating at this point (see reply to Vallee question).
Q: Our extruder's output fluctuates as it runs. Is this typical?
A: Depends on what you mean by “fluctuate.” A small variation, say +/- 2%, can be tolerated in most applications, but some are more critical (use a gear pump) and some are less (compounding into pellets, for example).
Q: Our barrel is worn unevenly, could this be a factor?
A:
Not likely, as the wear is there all the time. Wear has been accused of causing output fluctuations but I’ve never seen a reliable experimental proof on commercial scale.
Q: Which area of the barrel is the clearance most critical and is that a factor?
A:
Clearance at entry to metering zone is typical high-wear point, may be a factor in reducing total output and thus requiring higher rpm, which may lead to higher melt temperature, which may lead to degradation, cooling limits, sizing troubles. Be sure the wear is causing these troubles before rebuilding, and remember that wear can be on the screw as well as (and often more likely than) the barrel. If wear is highest at screw tip, suspect misalignment, requires prompt attention to avoid bearing/gear damage.
Q: Please explain Blue Screw Syndrome and what to do about it.
A: See reply to first Stewart question, as well as Vallee question.
Q: What is the best way to control pressure peak?
A: Pressure peak results when the feed takes in more than the ftont end wants to put out. It can often be ignored, unless:
A) there is too much frictional heat generated in the barrel, AND the heat is causing a serious problem (degradation, lower output because of cooling limits, sizing problems). In such cases, try to reduce inpush (feed, bite) via rear barrel temperature, or a controlled feeder, or making the head less resistant (lighter screens, hotter metal, larger passages).
B) Desired operation requires more power than the extruder has available. In such cases, perhaps reduction ratio can be changed, or feed preheated, or head resistance lowered (see A).
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