Polymer Extrusion Instabilities
Polymer extrusion instabilities can come as a result of many reasons. They may come as a result of improperly prepared feedstock, solids conveying problems, insufficient melting and or mixing capacity, barrel temperature fluctuations, improper screw design, melt temperature and viscosity non-uniformities in the die region, as well as improperly designed or operating ancillary downstream equipment. The following article, the first in a series, will address a very well known process instability known as ?melt fracture?
Melt fracture may be described as a severe distortion of the extrudate which is capable of being caused by a variety of reasons. There tends to be agreement that the primary cause of melt fracture comes as a result of the imparted shear stress onto the polymer exceeds the critical shear stress value for that particular polymer. Slip-stick flow conditions in the die are quite capable of producing melt fracture as well.
Various Forms of Melt Fracture
It is the purpose of the article to give the reader some helpful hints to address this problem when encountered. The following remedies have been proven to be of help to the processor in eliminating or reducing this process instability. They are not necessarily ranked in order of effectiveness or ease of implementation.
- Reduction of haul-off rate or line speed. This is typically not an option with many processors. Product throughput is of the highest importance. However in some instances where high line speeds are not essential, sometimes a small reduction in line speed will sufficiently reduce the amount of imparted shear onto the extrudate to provide for a defect free surface finish.
- Increase of the melt temperature in the die region. Quite often a small temperature increase in the die region(s) of the extruder, will sufficiently reduce the polymer?s viscosity in that region, and thus reducing the shear stress imparted onto the extrudate.
- Reduction of the land lengths of the die as well as the pin or mandrel. A reduction of the land lengths of the die and pin will reduce the shear rate / stress onto the extrudate. Long land lengths are at times desirable as they tend to provide for an increase of molecular alignment in the machine direction (MD), and simultaneously reduce the amount of extrudate swell, also known as die swell which is a misnomer as the die does not swell. A reduction of the land lengths will cause an increase in the extrudate swell as a result of the elastic response caused by the deformation of the polymer while in the die.
- Reduction of the polymer?s molecular weight. As in item 1, sometimes this is not an option, as this could possible adversely affect the mechanical properties of the final product. It has been demonstrated though, that switching to a material with a higher melt index (MI) value will allow a processor to attain higher throughput rates than with a similar polymer with a lower MI value before encountering melt fracture.
- Reduce the coefficient of friction in the die region. Nickel / Teflon tm , diamond like carbon, and other coatings or impregnations have shown promise in reducing the slip-stick phenomena in the die regions as well as reducing the imparted shear onto the polymer. One must be cognizant of the facts that coatings and or impregnations must be carefully selected as to withstand the prevalent operating temperatures as well as the possible chemical effects of the polymer. In addition, coatings will alter the physical dimensions of the treated parts.
- Incorporation of low molecular additives to the base resin. Low molecular weight additives to the base polymer will act as an internal lubricant, reducing the shear stress on the polymer and facilitating flow through the die. Typically small amounts, 5-10% of a compatible additive added to the base resin are sufficient to reduce or eliminate melt fracture. One must be aware that low molecular components added to the base resin may be capable of adversely affecting the mechanical properties of the final product. Typically, additions of 5% or less will have no adverse affects. Chemical compatibility is also a must, as additions of an incompatible additive will have adverse affects on the mechanical and possibly the cosmetic properties of the final product.
- Streamlining of the die. Reducing the entrance angles, degree of convergence and compression in the die, in other words facilitating the flow of the melt through the die, will be conducive in reducing and possibly eliminating melt fracture.
- Delivery of medium frequency ultrasonic energy in the die region. Research has shown that delivering ultrasonic energy (KHz range) to the die region via externally mounted transducers will not only reduce the degree of extrudate swell by facilitating molecular alignment, but is also an effective means of reducing or eliminating melt fracture.
Hans Kramer Ph.D
Applied Polymer Research.