DIE DROOL

ABSTRACT

Die drool is a commonplace, sometimes chronic, processing problem that can have serious effects on

the final quality and appearance of extruded products, including wire and cable constructions. A laboratory

scale test for assessing the severity of die drool for a given compound has been developed. In this study, the

effects of several extrusion parameters on the die drool generation of an automotive wire insulation

compound were examined. It was found that die condition, particularly the presence of damage and defects

in the die surface, strongly influenced the rate of die drool. In addition, the rate of die drool was found to

increase at higher extrusion rates and temperatures. Die cooling was found to be effective in reducing the

rate at which compounds generated die drool.

INTRODUCTION

The undesirable accumulation of material around the exit of an extrusion die is commonly referred to as

die buildup or die drool, and it is observed in a wide variety of polymer systems and extrusion processes. Die

buildup can be particularly problematic in processes employing the continuous vulcanization (CV) technique,

such as the insulation of automotive primary wiring. This buildup occurs because the die is enclosed by a

tube filled with high pressure steam and, thus, is inaccessible during the wirecoating operation, preventing

removal of the accumulating material by the operator of the line.

Eventually, die buildup releases from the die face in one of two ways. The buildup can attach to the wire

insulation in the form of a complete or partial ring of material (as shown in Figure 1), commonly referred to in

the wire and cable industry as a die ring, horsecollar, donut, or fuzzball. In addition to causing tears or other

flaws in the wire insulation, these rings can create problems in the automated harnessing operation, causing

wire breaks and downtime. Die buildup which does not adhere to the insulation eventually falls into the CV

tube and is washed down the tube by steam condensate. If the die drool is only partially crosslinked when it

detaches from the die face, numerous particles can adhere to one another, forming large pieces which can

clog steam traps, screens and water seals and result in unscheduled shutdowns.

In general, die buildup is a widely experienced, seldom studied phenomenon. While a wide variety of

causes and aggravating factors for die buildup have been proposed, the basic mechanism is still not

understood. A number of factors which have been proposed as sources of die buildup are 1:

1. Low molecular weight polymer species

2. Volatiles, including moisture

3. Presence of a filler material

4. Poor dispersion of pigments

5. Draw down and take off rates

6. Amount and rate of die swell

7. Atmospheric conditions including humidity

8. Die exit angles, land length, and land entrance size

9. Dissimilar component viscosities

10. Die condition (including cleanliness, presence of damage, defects, etc.)

11. Pressure fluctuations in screw channel

12. Improper melt temperature

Although many of these factors appear to aggravate die buildup, it appears that none of them is the

actual root cause.

While it is true that unfilled polymers will produce die buildup under certain circumstances, highly filled

compounds generally produce die buildup at a substantially higher rate and at much lower temperatures than

the unfilled base resins. The level of filler in the compound appears to have a strong influence on the rate at

which the compound generates die drool, probably due to interactions between the filler and the polymer.

Whether these filler-polymer interactions are physical or chemical in nature is unknown. The high filler levels

present in automotive primary wire insulation compounds make these compounds highly prone to die buildup

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