Die Lines: Tracing a Longitudinal Mark Back to Its Source

A die line is a scratch, copied

A die line is a longitudinal mark on an extruded profile that runs parallel to the extrusion direction and continues without interruption for as long as the line runs. That continuity is the diagnostic: anything that appears and disappears, or that wanders across the face, is not a die line, it is a downstream mark or a melt defect.

The mechanism is a copy. The polymer slides along the die land, the final parallel section of the flow channel, at a shear rate that can exceed 100 per second. Any irregularity on that land, a scratch, a pit, a burr, a patch of degraded material stuck to the wall, disturbs the flow locally and leaves a corresponding ridge or groove in the surface of the extrudate. The mark then passes through the calibration sleeve, the water bath and the haul-off unchanged, because nothing downstream smooths it.

This is why die lines are so persistent. A scratch of a few micrometres on a hardened steel land produces a visible line on a glossy surface, and the line is regenerated continuously. Polishing it out once fixes it permanently, provided the cause of the scratch is also removed, which is usually a piece of grit, a burr on the die lip, or a cleaning tool used too enthusiastically.

The distinction that matters most is between a die line and a melt fracture line. Both are longitudinal. A die line is sharp, has a defined position that does not move with output rate, and is visible even at low speed. A melt fracture line is a roughness or a helical pattern that appears above a critical shear rate and disappears when the output is reduced. One is a surface problem, the other is a flow problem, and they have different fixes.

The mark depth follows the land roughness

The chart on the left plots mark depth against die land roughness for a typical rigid PVC profile. At Ra 0.2 micrometres the mark is about 2 micrometres deep, at 0.4 it is 5, at 0.8 it is 12, at 1.6 it is 26 and at 3.2 it is 48. The visible limit on a glossy white surface is around 10 micrometres, which the dashed line marks.

The relationship is steeper than linear because the mark is not simply a scaled copy of the scratch. A rougher land disturbs the flow over a larger region and the elastic recovery of the polymer after the die amplifies the disturbance. Doubling the roughness roughly triples the mark depth in the visible range.

The practical target is Ra 0.4 to 0.8 micrometres on any land that produces a visible face. That is a polished finish, achieved with a sequence of abrasives down to 800 or 1000 grit followed by a felt wheel and a polishing compound. It is a two hour job for a competent toolmaker and it is the difference between a profile that sells and one that is discounted.

The right hand panel shows where die lines actually come from: die land damage 30 percent, carbon build-up 24, poor polishing 18, melt fracture 14, a foreign particle 8. Land damage and carbon together are 54 percent, and both are surface conditions rather than design faults. Poor polishing at 18 percent is the original sin, the die that was never finished properly and has produced marginal profiles since day one.

Carbon build-up is a slow die line

Carbon build-up, sometimes called die drool or plate-out depending on the material, is degraded polymer that has stuck to the die wall and partially charred. It starts as a thin film, grows into a ridge, and eventually a fragment breaks off and leaves a comet-shaped defect that is much worse than the line it preceded.

The cause is a combination of residence time, temperature and formulation. Polymer that sits in a low-flow region near the die lip, or in a dead spot behind an insert, degrades slowly. PVC is particularly prone because it degrades by dehydrochlorination, which is autocatalytic: once it starts, it accelerates. A die that runs continuously for weeks without cleaning will build up carbon regardless of how good the formulation is.

The control measures are practical rather than theoretical. Purge the die on a schedule rather than when the line looks bad. Keep the die lip temperature above the melt temperature so the polymer at the wall does not freeze. Avoid dead spots in the flow channel when the die is designed. And check the stabiliser package, because a formulation that is marginal on thermal stability will build carbon faster than one that is not.

Diagnosing which one you have

The first test is to change the output rate. If the mark stays at the same position and the same depth at half speed, it is a die line. If it fades or disappears, it is melt fracture or a related flow instability, and the answer is a higher melt temperature, a lower shear rate, or a different land length.

The second test is to change the material. If a different compound on the same die produces the same line in the same place, the die is the cause. If the line moves or changes, the material is implicated, usually through its lubricant balance or its filler content.

The third test is visual and it is decisive. Stop the line, clean the die face, and inspect the land under a bright light with a magnifier. A scratch, a pit or a carbon ridge will be visible. Most die lines are found this way in ten minutes, and the argument about formulation ends there.

Table 1 — Telling a die line from a melt fracture line.

Polishing a land back

The repair is a sequence, and skipping a step makes it worse. Start with the finest abrasive that will remove the defect, not the coarsest available. A 400 grit stone on a 0.5 micrometre scratch creates a 400 grit scratch, which is deeper than what you were trying to remove.

Work along the flow direction, never across it. A scratch across the land produces a mark that is continuous around the profile rather than along it, which is a far more visible defect. Keep the abrasive flat and use a backing block on a flat land so the surface stays planar; a rounded land changes the flow and the die no longer produces the intended dimensions.

Finish with a felt wheel and a diamond or aluminium oxide compound, then clean the die thoroughly. Polishing compound left in a corner of the flow channel becomes a source of contamination and, in PVC, of degradation. A die that is polished and not cleaned produces a line from the residue within a shift.

Table 2 — The polishing sequence for a die land.

Preventing the next one

The most common cause of a repeat scratch is the cleaning method. A brass scraper, a screwdriver or a wire brush used to remove carbon from a die face will scratch a polished land every time. The correct tools are soft, and the correct method is a soak in a proper cleaning salt or solvent followed by a soft brush.

The second is handling. A die that is removed and set down on a bench face-first will pick up a scratch from the bench. Dies should be stored face-up on a soft surface, with the land protected by a cover, and moved with a lifting frame rather than by hand.

The third is the screw and barrel. A worn screw delivers unmelted particles that damage the land as they pass, and a barrel with a groove or a dead spot delivers degraded material that sticks to it. If die lines recur on a die that is polished and clean, look upstream at the screw and the barrel before polishing again.