Troubleshooting: preventing lines or streaks on the surface of the filament

Streaks can appear on the surface of the extrudate that exits the nozzle of the extruder. This phenomenon makes the surface rough and can worsen over time, unless the problem is fixed.

Die lines are straight lines in the machine direction that remain in the same location on the extruded filament during operation (an example is shown in Figure 1 after an extrusion test with the polymer TPU).

Figure 1: Die lines/streaks on TPU filament

Potential causes and corrective actions to eliminate die lines are:

1. Surface defects

Surface defects in the die land area can be caused by scratches, burrs, or foreign material in the die/nozzle. The defect causes an indentation in the filament that remains in the same location over time. If the problem is due to a foreign particle on the nozzle edges or in the nozzle, clean the nozzle and die to eliminate the problem. Any scratches in the nozzle area have to be mechanically removed to eliminate the problem, but often it is easier to order a new nozzle package at

2. Nozzle build-up

Lines/streaks on filament might also be caused by a phenomenon called nozzle lip build-up (Figure 2). This is when material remains around the edges of the nozzle, and seems to keep returning even after it is physically removed. This usually because the nozzle temperature is too low for the material which is being processed. This is especially problematic with semi-crystalline polymers because if the nozzle is below the materials melting point, it is very likely that the material that gets in contact with the nozzle solidifies again.

Figure 2: Nozzle lip buildup

3. Streaks caused after extrusion

A machine direction streak may be caused outside the extrusion unit and nozzle, by dragging the product across something in the sizing operation (filament sensor and puller wheel) or on the take-up unit (positioner and winder). This is usually fairly easy to detect and correct. If the scratch is not present as the filament exits the nozzle but is seen partway down the line or on the spool, the problem is not in the nozzle. Determine where or what is scraping or damaging the filament surface after it leaves the nozzle and eliminate the problem. The most common cause here is a damaged puller wheel (Figure 3). If the puller wheel’s surface is not smooth, this may cause streaks/lines on the filament. The only way to fix this is by replacing the puller wheel with a new one which can be bought at

Figure 3: Damaged puller wheel

4. Chart of potential causes and corrective actions

Figure 4 summarizes the potential causes and corrective actions to employ in the event die lines or other lines appear in the extrusion direction and remain in a fixed location on the extruded part.

Figure 4, Die lines/straight lines in extrusion direction problem solving