Page 59 - Plastics News September 2024
P. 59
TECHNOLOGY NEWS
Retrofit Homogeneity - Melt Blenders to Step up
Efficiencies in Extrusion Lines
rates. To meet these re- quirements, the poly-
mer needs to melt within very short time, with
driving tem- perature differences at a high level.
Due to the nature of the process, high tem- per-
ature differences within the melt are obtained.
The difference between high- est and lowest
melt temperature mea- sured in the extruder
outlet may well be above 15C. And downstream
devices such as melt pumps or screen changers
also generate temperature differences. The vari-
ation in temperatures within the melt may cause
in many cases quality problems. This is due to
the fact that a temperature gradient in the melt
means flow and cooling behaviors will vary, too.
The flow profile in the case of laminar plastic melt
flow is parabolic – meaning the melt flows much
faster in the center than in the boundary section.
any extrusion processes cannot be This leads to significant differences in melt resi-
operated at the maximum output rate dence times. High residence times and tempera-
Mspecified by the producer. The maxi- ture peaks may then yield the undesired effects
mum production capacity is rather determined of polymer degrading and building of deposits.
by the quality requirements of the extrudate.
High output rates then lead to unacceptable var- These are the challenges the static mixer tack-
iations in layer thicknesses, to uneven or rough les.
surfaces, sink marks or other quality problems. The Effects of a Melt Blender
With quality requirements constantly increasing,
and with polymer processing growing more dif- A well laid out melt blender is able to equalize
ficult, start-up and adjusting the optimum oper- temperature differences effec- tively. A melt
ating point takes more and more time. Both fac- blender by Promix Solutions AG, Winterthur,
tors diminish productivity, cause expenses, and Switzerland, can diminish temperature differ-
can sometimes be quite unnerving, too. ences by a factor of 5 to 12, subject to layout.
Correspondingly, it reduces a 15C temperature
Quality Problems Due to Temperature Differ- variation over the channel cross section down
ences to 1.25 to 3C. Figure 1 presents typical tempera-
Extruders are supposed to melt the poly- mer ture distributions in the melt at the extruder out-
and convey it under high pressure at a constant let with and without a Promix melt blender. The
rate. The extruder itself should be as compact melt blender’s geometry decides on the ques-
as possible as well as gen- erate high output tion whether the maximum benefit is achieved.
The Title fig- ure shows a static mixer of the lat-
PLASTICS NEWSASTICS NEWS
September 2024
September 2024 PL 59 61
