Processors occasionally find black spots in extruded products such as films and sheets. Thin film creates very conspicuous black spots that are obviously not only unacceptable, but can create holes in the product when the film is subsequently oriented.
The two basic sources for black spots either originate in the polymer raw material or gets introduced during processing. The p olymer can also contain formulation defects that char instead of melting, becoming a dark speck surrounded by a gel.

Contaminants in raw material could be one of the possibilities. Inherent gel due to presence of very high molecular weight could also cause a gel but not necessarily a black speck. Specks are more likely to occur in off-spec or post-industrial recycled material. Better quality control of incoming material could identify the problem of presence of a contaminant in the polymer, which should be addressed to supplier.
If the incoming material is clean, a speck is introduced during processing due to several factors:
* Small amounts of polymer being overheated, exposed either to high temperatures for a short time or to moderately high temperatures for longer periods. When an area of very high temperature occurs in the barrel, downstream plumbing, or die, typically the problem could be caused by a problem in the control system�a bad thermocouple, runaway heater band, or a relay stuck in the wrong position. Any material passing through the affected zone is potentially vulnerable to degradation.
* Small amount of polymer can hang up and be exposed to normal process temperatures for abnormally long periods in a worn or pitted screw, barrel or die, or in cracks in chrome plating. This material de­grades over time, breaks loose with thermal cycling and the drag of polymer flow, and can make lots of black specks.

The source of the specks can be identified by the maintenance department, or by configuration or control equipment. Process conditions are also required to be critically examined. The screw and barrel rarely cause degradation, though vents or complex mixing or barrier sections may do so. Degradation usually arises downstream from the extruder� in plumbing that forces abrupt changes in the polymer flow path or in components such as breaker plates, screen packs, static mixers and melt pumps. These can have potential hang-up areas, like an abrupt taper into an adapter fitting.

Large or complex dies for film or sheet, containing low-flow areas where polymer can overheat, can also be one of the causes. Experience from past teardowns and an inspection is the best indicator of whether degradation is developing in areas of slowed or stagnated flow. In blown film dies, it could be the point where each port is fed or where a splitter feeds a spiral. In cast film and sheet dies, trouble could arise where the shoulder starts to spread out or in the relaxation chamber at the land just before the die lips. Once identified, these areas should be given particular attention in future cleanings. They may require local use of higher temperatures plus chemical purging compounds.

Heat tolerance of the polymer is also a contributing factor. Heat-sensitive materials like PVC, ABS, and EVOH, or engineering resins like acetals, PC, nylon, or polyesters are more likely to degrade than heat-tolerant polyolefins. An extrusion system that processes LDPE with no problem might degrade heat-sensitive EVA in a matter of minutes. Residence time should be not excessive. Melt temperature should be below critical level.

Higher throughput rate can also hasten formation of black speck, as running too hot can lead to degradation. Forcing a cool material to flow can generate excessive shear energy and localized degradation in the screw's flow channel. Polymer that is shear sensitive is easily prone to generate black speck.

Finally, stoppage and shutdown for adjustments, die changes, or maintenance often extend residence time and can cause material degradation. In a system operated five days a week, small amounts of residual material in the extrusion system acquire substantial heat history as the machine slowly cools and starts up again. So it's common for a system on five-day operation to begin producing black specks only a few weeks after a complete cleaning.

If black speck cannot be eliminated after trying all the above care then purging with right & good purging compound is a must. The frequency of purging can be decided by experimentation.