Why do the cross-sectional shapes of the die orifice and the ...

[Diane]

Why do the cross-sectional shapes of the die orifice and the final (cooled) extruded part sometimes differ?
 
  What will be an ideal response?

Question 2

You are responsible for the extrusion of a thin PE rod and find customers are complaining because it is splitting longitudinally after a relatively brief time in service. Indicate two changes you would make in each of the following: resin, extrusion conditions, postextrusion treatment. Explain.
 
  What will be an ideal response?

[deja]

Answer to Question 1

The polymer material flows unevenly through thick and thin sections. Therefore, if a sharp corner is cut in the orifice, the corner will restrict the flow and the part will have a rounded corner. To obtain a part with a sharp corner, the orifice should have some restriction in the flow of the middle of the part that will push the flow to the corner relatively.

Answer to Question 2

Use LLDPE, use less filler or different fillers, run the material processing speed slower, crosslink the material, stretch as little as possible. Splitting is probably because the molecules are highly oriented with very little intermolecular strength across the alignment so if there is a material with better knitting it will increase the strength. If the process is run at a lower speed, it doesn't induce as highly oriented molecules as it does at higher speeds. Post-extrusion stretching of the material also induces further orientation and decreased cross-directional strength, crosslinking will allow for that strength to be returned.

[Diane]

Excellent

[epscape]

Gracias!