Extruder surging

Thickness variations can be categorized as either time-dependent or position-dependent. For the case of time-dependency, thickness varies in the machine direction (MD) of the film, which may be caused by extruder surging (as described above). Also, temperature drifts on the extruder can lead to longer-term changes in the flow rate of polymer, hence thickness variations. It may be beneficial to document melt temperature on a chart recorder over several hours if this cause is suspected. 

Extrusion. The effects of excess moisture during extrusion may include a foamy melt, lower melt viscosity, bubbles in the extrudate, surging that results in arrow heads and wave forms in the extrudate, surface roughness, and reduced mechanical properties. 

Modifying the screw design, material or operating conditions can result in a completely different solid bed profile as shown in Fig. 12.1(b) [4]. In this case, the simulation predicts that the solid bed width reduces much slower and that melting is not completed until the very end of the screw. This is clearly a less desirable condition than the one shown in Fig. 12.1(a) because there is the potentiai for some materiai not to melt and mix properly before leaving the extruder. An extreme case of this can result if the reduction in the solid bed is slower than the compression rate of the screw. In this case, the volume of the channel is reducing faster than the voiume of the solid bed. This forces the solid bed to expand, accelerate, or break-up. Premature solid bed break-up can lead to extrudate surging and a poorly mixed material. 

Spalding, M.A., Powers, J.R., Wagner, P.A., and Hyun, K. S., Flow Surging in Single-Screw, Plasticating Extruders, SPEANTEC Tech. Papers, 46, 254 (2000)... 

Figure 10.10 Filtered and unfiltered discharge pressure data. The unfiltered data were collected using a portable data acquisition system while the filtered data was the pressure that was observed by plant personnel. The plant personnel believed that they had a very stable process even though this extruder was flow surging. The transducer was positioned in the transfer line and the extruder was operating at a screw speed of 60 rpm...

An extrusion trial was performed at the processor s plant using a 38.1 mm diameter production extruder, a proprietary screw design, and resin that had previously exhibited flow surging and reduced rate. The extruder was equipped with three barrel zone heaters with control thermocouples (labeled Tl, T2, and T3) and two pressure sensors. One pressure sensor was located in the midsection (zone 2) of the barrel (P2) and the other at the end of the barrel near the tip of the screw (P3). Both transducers were positioned over the top of the screw such that a pressure variation due to screw rotation would be observed. 

Figure 12.10 Barrel pressures and temperatures for the 38.1 mm diameter extruder operating at a zone 1 barrel temperature condition that caused the extruder to flow surge.

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