Q: During a cold startup, what is the optimum soak time, particularly when there is resin in the extruder/die? Second, what types of resins are likely to cause plateout, and how to I fix it? I would also like to better understand how different types of gels are formed and controlled.
—Global film supplier
A: Soak time depends on the extruder size. Soak time for a 10-in. extruder can be as long as a day, for instance. Typically, though, 30-60 min after the highest zone reaches its setpoint is adequate soak time before starting the extruder drive. “Cold slugs” between screw tip and breaker plate require more heat-up time.
Second, “plating” or “plateout” is typically due to processing aids that are added to the neat resin, such as slip and antiblock agents. Screw cooling will help reduce the amount of plateout that builds on the root of the screw.
Third, any defect in molten polymer is a gel. It could be a crosslinked gel, a black speck, or unmixed (unmelted) material. On barrier-type screws, if the melt channels do not have the proper aspect ratio and the channels are too deep, downstream velocity will be reduced and as a result crosslinked gels will build up and cause “showers.” For optical-quality film, a tighter barrier gap in the barrier section should be considered.
Tim Womer, TWWomer & Associates LLC
Edinburg, PA (724) 355-3311 • twwomer.com
Q: I have an old Foremost HD-2 granulator with a 7.5-hp motor, and it keeps getting jammed. I have had the blades sharpened. Would putting a 10-hp motor help?
—California custom molder
A: Before you increase the horsepower on your HD-2 you should put an ammeter on the unit and see what amp draw you are seeing when the machine jams. The first thing you should check is the rotation on the knives. If the rotor is going backwards it will still grind, but not efficiently. Changes in part weight may also be an issue.
Drew Schmid, Foremost Machine Builders
Fairfield, NJ (973) 227-0700 • foremostmachine.com