extrusion anomalies - plate-out

8/14/2019 Extrusion Anomalies - Plate-out

1/11

Extrusion AnomaliesPlate - Out

By: Dwight A. Holtzen and J. A. MusianoDuPont White Pigment and Mineral ProductsChestnut Run Technical Service Laboratory

Wilmington Delaware

Abstract: Relentless deposition of debris on extruder die surfaces negatively affect productivity during extrusionprocesses. Removal of these unwanted deposits is often the source of much down time. This paper will reviewpreviously published reports (1, 2) and present new data regarding the accumulation of compound remains on

extrusion dies and extruder screws. A proposal of how these relate to simple rheological properties of the materialsbeing processed will be given.

Deposition of residues on extruder screws will also be discussed and possible mechanisms proposed for thedeposits accumulated in the melting zone of the machine.

Introduction : Imperfections affecting film smoothness, gauging or appearance are often caused by unwanted diedeposits. How these deposits form has been the subject of a number of works which were reviewed in a recent

publication(3)

. In these works two suspected mechanisms are elucidated showing how die deposits can result frommismatches of rheology or because of inherent rheological properties of the material bulk being extruded throughchanging geometry. One common factor in both of these mechanisms is that the debris adhering to the die lands andeventually exuding from the exit almost always shows evidence of oxidative degradation. These two mechanismscause different plate-out composition and rate. Die plate-out can occur in a very short time frame and may be of acomposition that is similar to only one component of a complex blend of materials being extruded. However, plate-out resulting from another mechanism may require a long induction period and be of a composition similar to thebulk of the material(s) being extruded.

The first mechanism we will explore simple mixing of a multi-component system, it concerns the degree of mixing and relative viscosity and other parameters such as component viscous flow and activation energy (Ea) thatcan be related to unwanted die deposits. This mechanism usually causes appearance defects rather than gauging.The second mechanism assumes that the mixture is homogeneous. It relates plate-out formation to the relative timeconstant for the fluid being extruded through changing geometry relative to the time the fluid is exposed to thechanging conditions. This second mechanism can be related to the ratios of these times which is a dimensionlessquantity named the Deborah number (D e). The Deborah number is usually written as:

De = / t

Where " " is defined as a characteristic (maximum) time constant for the fluid being extruded and "t" is the time

over which flow occurs through a particular geometry. As the De number increases magnitude so does thepossibility of unstable flow and die deposit accumulation.

Experimental: To investigate the formation of plate-out due to rheological differences and inadequate distributivemixing a number of commercial concentrates containing 50% by weight of rutile Pigment White 6 (CI# 77891) wereobtained. The carrier resin in all concentrates was high pressure low density polyethylene (LDPE). Theseconcentrates were let down into LDPE in a 2.54 cm single screw extruder fitted with an 24:1 L/D Maddock mixing


2/11

however, special dies were constructed to study the effects of relaxation. Results of this investigation demonstratethe pronounced effect of Deborah number on plate-out.

Extruder screw plate-out was investigated by mechanically removing the unwanted deposits andmicroscopic examination. SEM was the primary mode of analysis coupled with Energy Dispersive X-ray Analysis(EDXA).

Results and Discussion: If complete mixing of all phases, e.g. letdown resin and concentrate is not completed inthe screw section of an extruder, flow in transfer piping and dies can result in phase segregation. This is especiallytrue if there is a great disparity in rheology (and compatibility) of the phases. Thus, two separate issues need to beaddressed; A) mixing of all individual phases in the extruder and B) separation of those phases in a Poisseuille flowregime.

Complete mixing of all components in the extruder is key to producing homogeneous film. However,operation of extrusion equipment beyond its design limits sometime results in insufficient residence time in theextruder barrel to thoroughly homogenize all of the components of a blend. Mixing components of differentviscosity have been studied by a number of researchers (4,5,6) . While most of the systems we work with are non-Newtonian, how rapidly two phases will mix while under simple shear can be visualized by examination of a plot,Figure I, of equation 11.4-1 from reference (4). A similar analysis is given in an earlier reference (6). ThisNewtonian approximation must be used with caution, however, it does show the importance of relative viscosity andvolume fraction on the degree of mixing. While one would assume that a very low viscosity pigment concentrate

would mix easily into a higher viscosity let down resin this is not always true. Relative volume fractions of eachphase must be considered. Optimization of the relative viscosity of phases is good practice and the value of thispractice will explained in the discussion that follows.

If all phases are not completely mixed when the material exits the extruder and flows into the die thepossibility of radial segregation exists. As multiple phases are pumped into the transfer piping and ultimately to thedie by pressure flow alone, phases begin to separate. Lower viscosity components migrate toward the region of highest shear rate (the tube wall or die land) while the higher viscosity material will move toward the region of lowest strain and center itself in the tube or channel. This is shown diagrammatically in figure II. To furthersupport this mechanism Figure III shows the appearance of an elongated "football" shaped inclusion centered in thefilm. The viscosity (measured at 500 sec .-1 at 180 C) of the inclusion was about 5 times higher than the surroundingmaterial. Figure IV shows a high concentration of pigment in a thin layer at the surface of a similar film. In this casethe relative viscosity of the pigment concentrate to the letdown resin is the reverse of that shown in the previousfigure. More detailed studies are to be found in reference (2).

Because polymer melts typically do not flow as Newtonian fluids, consideration of other rheologicalproperties must be taken into account. It is well known that relaxation characteristics of polymers can cause flowanomalies in dies (7) and it is proposed that these may be responsible for certain types of die lip plate-out (1). Aconvenient dimensionless number, the Deborah number (D e), can be related to flow anomalies. Increasing De leadsto more pronounced vortex formation in converging flow, it was found that die lip plate-out increased as D eincreased. Figure V shows the effect of D e number on vortex formation in a converging capillary die. Plate-outrelated to this mechanism has several characteristics; A) a relatively long induction time was observed before plate-out appeared at the die exit as a chain of droplets, B) exuded material is highly degraded, and C) the composition of the droplets is very near that to the bulk material being extruded.

Tests show that increasing the D e will result in more pronounced plate-out. The D e number can be


3/11

of materials can leave a detailed record of what has been run through the extruder. Figure VIII shows a crosssection of the layer deposited on the screw from an extruder that alternately processed unpigmented and TiO 2pigmented compositions. The image in the figure is SEM / EDXA showing alternate layers of carbonized resin anda composite of carbonized resin and pigment. Pigment concentration in the layers was found to be near 85% weight,not typical of actual concentration that was being extruded. As the TiO 2 layer is deposited, any excess resin is beingejected leaving a random densely pack layer of pigment.

As the cake of residue increases in thickness, it grows weaker in physical strength and can fail under thestress found in the melting section. When this occurs a packet of debris is transported down stream and can appearas a mass of small unwanted fragments.

Conclusions: Relatively simple rheological properties such as viscosity and activation energy of viscous flow havebeen found to correlate to the appearance of certain types of imperfections in extruded film including "streaking"and "specking" (2). Streaking is caused by one or more of the components in a blend being forced to the die land.Specking results from a component aligning in the center of the flowing resin. Once the material arrives at the dieland it slowly creeps toward the exit and can appear an elongated surface imperfection.

A mechanism involving a relationship between relaxation properties of a composition to plate-out has beenshown. In some cases fillers and pigments have been shown to change plate-out potential and the reader is referredto reference (1) for more detailed information.

Condensation of material and / or thermophoretic transport of particles to the screw root in the meltingsection can explain the build up of layers of debris. This cake of degraded materials periodically breaks freeresulting in packets of imperfections in the final product.

References:

(1) Dwight A. Holtzen & J. A. Musiano "Die Lip Plate-Out, A Proposed Mechanism", ConferenceProceedings , SPE RETEC , St. Louis, 1996.

(2) Dwight Holtzen "Mixing Masterbatch and Resins - Rheological Implications", Conference Proceedings,SPE, RETEC , New Orleans LA, 15 - 17 October, 1991.

(3) Jesse D. Gander and A. Jeffery Giacomin "Review of Die Lip Buildup in Plastics Extrusion", Polymer Engineering and Science , July 1997, Vol. 37, No. 7, pp 1113-1126.

(4) Z. Tadmor and C. G. Gogos, Principles of Polymer Processing , John Wiley & Sons, 1979, Chapt. 11.

(5) H. F. Mark et.al. ed., Encyclopedia of Polymer Science, Supplement Volume , John Wiley & Sons, NY,1989, pp 481-492.

(6) James M. McKelvey, Polymer Processing, John Wiley and Sons, Inc. NY, 1962, Chapter 12.

(7) Arthur S. Lodge, Michael Renardy and John A. Nohel Ed., Viscoelasticity and Rheology, Academic Press,Inc. NY, 1985. (see: K. Walters, "Overview of Macroscopic Viscoelastic Flow", pp 47 - 79)


4/11

2

3

45

8

48

Note: contours ploted at one unit intervals between 2 and 9

2 0.0

0.1

0.20.3

0.40.5

0.00.20.4

0.60.81.0

-2

0

3

5

8

10

VolumeFraction of MinorPhaseRelative Viscosity of Minor to Major Phase

Relative Strain

Figure I: Relative Strain of Minor Phase vs. Total Strain and Volume Concentration of Minor Phase


5/11

>> >>


6/11

Elongated High ViscosityInclusion

Film Layer A

Film Layer

B

Film Layer C

Figure III: Inclusion of Unmixed Resin in Multi-Layer Film

Inclusion has a higher viscosity than the bulk of resin in layer B


7/11

Film Layer

A

Film Layer B

Figure IV: Bi-Layer Film with Top Layer Pigmented via Concentrate

Pigment Concentrate atsurfaces of layer A

Layer B pigmentedwith regrind


8/11

Die Exit

Figure V: Vortex Formation in a Converging Die for High and Low Deborah Number Fluids


9/11

Figure VI: Die Plate-Out With Increasing De Number180 grams of 12 MI LDPE + 5% Pigment White 6 Extruded at 190 C

800 psi. ~ 0.5 De 1600 psi. ~1.0 De

2050 psi. ~ 1.5 De2700 psi ~2.0 De


10/11

98 Grams of 2 MI LDPE with 5% Pigment White 6. Extruded at 190 C at 3000 psi. Estimated De of > 3

Figure VII: Plate-Out with Higher Molecular Weight Resin


11/11

Figure VIII: Residue Removed from Melting Zone of The Extruder Screw

Figure shows the alternating layers of pigmented (white) and un-pigmented (black) layers

Pigment Layers

Carbonized ResinLayers

extrusion anomalies - plate-out

Documents