Page 34 - Plastics News June 2018

FeAtures

4. Recovery or plasticating time with identical cycle is important for consistency. There are also cooling
plastic backpressure. Not a set rpm, but the same parameters and still others, but those will have to wait
plasticating time, achieved by adjusting the rpm to for another column. The six parameters above are a good
replicate that time. Replicating circumferential screw start, but remember: They are process results not machine
flight velocity is more logical, but better results are setpoints.
obtained by replicating plasticating time to replicate Bottom line: Replicate actual volumes, times,
melt temperatures.
temperatures, and pressures with machines that are
5. Cycle time, replicating pack and cooling time. capable and calibrated. This may sound complicated,
but it is simpler than fighting Murphy on every other run
6. Whole-part IR temperature picture, measured as soon
as possible before or after ejection. or working with the idea that molding is art rather than
science.
Are there other parameters that are important? Absolutely,
like melt temperature, but interrupting a production cycle John Bozzelli provides training and consulting services
to measure that temperature is a hassle and staying on to injection molders, including LIMS, and other specialties

Optimize Feeding to Get More Money in Twin-Screw Compounding

- ALEX UTRACKI

Every business wants to increase profits. Companies can deploy many strategies to do so that can fall across
various areas of the business, such as reducing inventory, negotiating lower prices from suppliers, or increas-
ing marketing efforts, to name a few.

When it comes to operations, the capital equipment the downstream equipment. For example, difficult-to-
is often already in place and is therefore a sunk cost. handle materials may stick or flow poorly through feeding
Increasing profit- ability in this case usually depends on equipment, limiting the rate at which they can be
maximizing productivity of these assets. For twin-screw discharged into the twin-screw extruder. In the process
extrusion processes, this is most easily achieved by section, an improperly designed melting section may
increasing throughput rate, thereby spreading operating not be able to fully melt the polymer above a certain
costs (depreciation, labor, maintenance, etc.) over a throughput rate.
greater amount of product produced in the same unit
of time. Another option to reduce cost is by increasing Conversely, materials with a high energy requirement to
energy efficiency in converting raw materials into melt may be limited in rate by the amount of power that
the motor and gearbox combination is able to supply.
finished goods. Since energy is relatively inexpensive
in the U.S., the dollar amount of these savings tends Limitations in the downstream equipment include high
to be minor compared with productivity increases. discharge pressure in the case of high-viscosity materials
But there are other benefits businesses can realize by and restrictive dies, and high pellet temperature in the
cutting energy consumption, such as reducing their case of inadequately sized cooling equipment. Only once
carbon footprint and enhancing public image. In some the limitation is understood can steps be taken to remove
cases, though, it is possible to achieve both increased it. This process is one of continuous improvement, where
if one limitation is removed, another will be uncovered
productivity and reduced specific energy consumption.
at a higher throughput rate.
When it comes to increasing throughput, first find out FEED SYSTEM
what is causing the current limitations. For twin-screw
extrusion processes, limitations can exist anywhere To achieve the highest throughput rates, feeding
from the feed system through the process section to equipment must deliver an accu- rate and consistent

Plastics News June 2018 34