Page 27 - Plastics News April 2022

The use of a proper purging compound • In extrusion, start rotating the ing parts, not standing around.)
can also prevent the need for screw pulls. screw slowly, and then vary the Purging for Extended Shutdowns
And when screw pulls are required, screw speed throughout the pro-
purging compounds can significantly cess. Because the use of normal resins results
reduce the time and effort required for • During the purge process, stop the in oxidation over time, causing carbon
the procedure. screw and allow the purge to settle contamination, it is not recommended
In order to choose the correct purg- into any dead areas. to leave material in the machine barrel
at shutdowns. Purging with a CPC is
ing compound, you first must evaluate • For both injection molding and ex- recommended for cleaning out machin-
your process, which should start with trusion, make sure to purge at the ery for shutdowns since it is more ther-
the equipment and/or tooling and any temperature of the current material mally stable and can take the extreme
limitations they may have. Just like tools you are trying to remove. heat variations.
in a toolbox, each type of purging com-
pound has a function, and they are typi- • For best results, repeat the process After a thorough cleaning with a me-
cally broken down into the following when introducing the next produc- chanical or hybrid purge, a “shutdown
categories: tion material. and barrel sealing” procedure is highly
• Mechanical Purge: Cleans by By changing the screw speed and recommended for extended shutdowns,
scouring with a particulate such as velocity of the material flow, you to minimize material oxidation or build-
glass or mineral. are providing a highly fluctuating up of carbon, particularly with a non-
environment of pressure and agi- glass-filled grade of purging compound.
• Chemical Purge: Cleans using tation that allows a purging com-
chemical reaction. pound to work at its best. This is Sealing the barrel ensures that no fur-
ther oxidative degradation occurs and
• Hybrid Purge: Cleans using the done until the purge is visibly free provides further cleaning. When to seal
components of both mechanical from any contamination. or not seal may depend on employee
and chemical purge types. As a best practice, a periodic purg- comfort and confidence level. You don’t
While mechanical purging compounds ing routine or preventive-maintenance want to damage machines at startup; if
can be effective, they may contain parti- purge program should be established you rotate a screw that is not properly
cles that will lodge or block areas of the to avoid contamination buildup, which heat-soaked, you could possibly break a
process such as small hot-runner tips could lead to more difficult purging screw tip.
or extruder screens. While a mechani- situations requiring greater time and ef- Implementing a preventive-mainte-
cal purging compound is often a great fort. A preventive program is essential nance purging program is essential to
choice to clean the screw and barrel, a to any plastics-processing facility for the increasing efficiency and profitability in
chemical-type purge, or even one that following reasons: any plastics-processing facility. Whether
foams or expands, may prove more use- Reduces scrap. (Each scrap part is a lost experiencing black specks caused from
ful in cleaning the “dead areas ” that are sale.) resin degradation or color contamina-
common in certain screws and tooling.
• Increases machine utilization. tion from a previous production run,
Maximize Purging Process Dynamics (More business opportunities.) most part esthetic issues are related to
Here are some steps that will ensure you • Reduces screw pulls by an average the lack of a better purging program
maximize the dynamics of the purging of 50% or greater. that includes the extended shutdown.
process: • Typically decreases production Purging with a CPC is recommended
• Ensure that the barrel is fully downtime by an average of 65%. for cleaning out machinery for shut-
charged. downs. Incorporating the proper CPC
• Typically reduces natural/virgin
• Run the screw speed in injection resin consumption for purging by into a preventive-maintenance purging
molding between 70 and 120 rpm an average of 45%. program can substantially reduce layers
once the purge starts to come out of carbon buildup, eliminate the po-
of the machine. Use short bursts • Typically increases production ef- tential of cross-contamination of resins
throughout the process to clean the ficiencies by an average of 35%. and/or colors, and lessen the need for
check ring and mixing nozzle. (Operators spend more time mak- screw pulls.

27 PLASTICS NEWS April 2022