Page 46 - Plastics News August - 2023

FEATURE

FIG 2 Screw with solidified ABS resin from a Maddock-solidification experiment.

nderstanding how polymer melts for some single-screw extruders. For fragments. At slightly higher screw
Uin a single-screw extruder could these situations, the extruder drive speeds, solid polymer fragments start
help you optimize your screw design train, extrudate temperature and to appear in the extrudate, causing
to eliminate defect-causing solid poly- downstream equipment are not rate defects in the product.
mer fragments. limiting. As the screw speed and rate If fresh resin and masterbatch resins
The melting (or devitrification) pro- are increased, a maximum rate will oc- or color concentrates are fed to the
cess can be a rate-limiting process cur where the extrudate is complete- extruder, the defects will be solid
ly molten and free of solid polymer

FIG 3 Cross section in the early portion of the screw’s melting section. White areas were solids at
the time screw rotation was stopped. Black areas were molten resin.

polymer fragments from one of the flux has units of kg/(hr m2) where the area, eventually some solids will not
resins. But since the fresh resin is unit area term is the area of the barrel be melted and will instead discharge
added at the highest level, statistically wall where melting is occurring. with the extrudate. A photograph of
the defects are likely to be from the The melting capacity is defined a black- tinted HDPE thin-wall pipe
fresh resin. If the drive train is not lim- as a melting flux times the barrel cross section is shown in Fig. 1. Here
iting, this defect can be eliminated via area available for melting. the large white areas do not con-
screw design. tain pigment because they were dis-
The melting capacity is defined as
Solid polymer fragments are likely a melting flux times the barrel area charged from the extruder as a solid
to be discharged from the extruder available for melting. As the screw fragment. The fragments exited the
at high screw speeds because of the speed is increased and because the extruder because the rate was higher
processes governing solid conveying, melting flux does not increase at the than the melting capacity of the ma-
melting, and the metering sections. same rate as solids conveying and me- chine. Short term, the only solution
For example, if the screw speed is tering, additional area at the barrel to mitigate these fragments is to de-
doubled the pumping capacity of sol- wall is needed for melting. Thus, the crease the extrusion rate.
ids conveying and the metering sec- solid bed is forced downstream such The Maddock Solidification Ex-
tions will also double. The melting that more area at the barrel wall will periment
flux, however, will only increase by a be used to melt the additional resin. The melting process can be studied
factor of about 1.4 times. The melting using a Maddock solidification experi-
Because of limited barrel surface

46 PLASTICS NEWS August 2023
46 PLASTICS NEWS
August 2023