Extrusion defects

Surface defects in the plastic extrudate can occur due to flow instabilities or degradation of the plastic during the extrusion process. Some common defects and possible causes are hsted in Table B.2. 

Blister Raised or layered zone on the surface of the part Plastic material temperature may be too high, causing thermal degradation of the plastic 

Color streaks Localized change of color in plastic part Masterbatch was not mixed properly or the LJD of the screw is not high enough 

Gels Small round surface defects that resembles distorted plastic Poor mixing of plastic, thermal degradation of plastic at the barrel walls, un-melted plastic 

Melt fracture Severe distortion of the extrudate leading to defective appearances of spiraling, bambooing, ripples, and random fractures Occurs when a critical shear stress is exceeded in the die. Causes can be too small, an entry angle in the die or pressure oscillations 

---

A special advantage of this method is that the high shear rate range becomes available. It appears that one can measure nu — n33 up to the critical shear stress, at which extrusion defect (melt-fracture) occurs. On the other hand, entrance effects can also be studied, when the windows are located sufficiently close to the entrance. With the aid of the stress-optical coefficient, the corresponding normal stress difference can be... 

In this respect, additional results, as obtained on polymer melts 40), seem of interest. Fig. 1.9 shows results obtained on a polystyrene melt (Styron 666 at 190° with the aid of the discussed slit-apparatus. In this figure the total path difference is plotted in the form (P/d) against apparent shear rate Ds. Extrusion defect occurs only at the highest... 

There are some interesting points to be noted. First, it seems that also for polymer melts the normal stress differences (fin — fi22) and (fin—fi33) are practically equal. (Similar results have been obtained for melts of several polyethylenes.) Second, for the investigated polystyrene a practically quadratic dependence of nn — n33 on the shear stress is found up to the point of the inset of an extrusion defect. It is noteworthy that Fig. 1.9 shows no quadratic dependence of Pjd vs Ds, as would be expected for a second order fluid. Third, the measurements in the cone-and-plate apparatus have to be stopped at a shear stress at least one... 

The appearance of extrusion defects eind instabilities [1-3] in polymer melts flowing through sudden contractions is usuedly related to fluid slip at the wall [4, 5]. It is therefore understandable that this phenomenon has been widely investigated in many studies. However, it is importeint to start with, to note the existence of various types of slip. In the field of polymer melt rheology, the following are referred to ... 

Boger D. V. and K. Walters, Rheological phenomena in focus (Elsevier 1993). El Kissi N., Leger L., Piau J. M. and Mezghani A., "Effect of surface properties on polymer melt slip and extrusion defects," J. Non-Newtonian Fluid Mech., 52, 249-261 (1994 a). 

Robinson GC (1978) Extrusion defects. In Onoda GY, Bench LL (eds) Ceramic Processing before Firing. John Wiley Sons, New York, Chis-ters, Brisbane, Toronto, pp 391-407... 

Furthermore, it must be taken into account that the realizable property level of self-reinforced fiber composites not only strongly influences processing, but also the material quality (weaving technique, mono-/co-extrusion, defects per area, etc.) and, consequently, the price of the basic material. The material costs essentially follow the development of the above mentioned elasticity modules of self-reinforced composites. 

---