What Causes Crystal Spots in Black Blown Film?
Short Answer
"Crystal spots" is a shop-floor name for small, hard points in film that look clearer, lighter or glossier than the surrounding film; in black film they show as glossy or lighter dots or bumps. They are usually polymer particles that did not melt or mix fully, such as high-molecular-weight, crosslinked or incompatible material, coming from the base resin, the masterbatch carrier or degraded material in the line. Undispersed carbon black agglomerates look different: they appear as dark, hard specks. One film can contain several defect types, so the source has to be traced by comparison rather than assumed.
Crystal Spots, Fish-Eyes, Gels and Black Specks Are Not the Same
| Defect | Appearance in black film | Typical nature |
|---|---|---|
| Crystal spot | A small, clearer, lighter or glossy point that may feel hard | Unmelted, high-molecular-weight or incompatible polymer particle |
| Fish-eye | A lens-shaped point, often with a halo or tail where the film stretched around it | A gel or particle around which the film thinned |
| Gel | A soft or hard lump of polymer that did not flow with the rest | Crosslinked or oxidized polymer, or a very high-molecular-weight fraction |
| Black speck | A dark, hard point, sometimes rough | Carbon black agglomerate, carbonized polymer or contamination |
| Hole or pinhole | A perforation in the film | Often a large particle, contamination or bubble that tore the film |
These names are not used consistently across plants and suppliers, so describe a defect by what it looks like and where it appears, not only by its name. A 2025 review of blown film processing lists fish-eyes among the main defects of polyolefin films, alongside gauge variation, melt fracture and other optical defects.
Where the Defects Come From
Unmelted, High-Molecular-Weight or Incompatible Polymer
Gels and crystal-type points often come from the base resin itself, for example a lot with a higher gel level. They can also come from a masterbatch carrier whose melting behavior or viscosity differs from the base resin, so that some carrier does not melt or mix completely, especially at low melt temperature or short residence time.
Thermally Degraded Polymer
Material that stays too long in dead spots of the screw, adapter or die can crosslink or oxidize and later break away as gels or brown and black specks. Such defects often increase after a stop, a restart or a long run without cleaning.
Carbon Black Agglomerates
Agglomerates that were not broken down appear as dark, hard specks and can raise the pressure in front of the screen pack. They are a dispersion problem rather than a crystal spot. Dispersion of concentrates can be assessed with the film test in ISO 23900-6:2015; that standard evaluates pigment dispersion and is not a general test for gels or fish-eyes. See our guide to evaluating black masterbatch dispersion.
Fillers and Contamination
Agglomerated fillers in a filled masterbatch, moisture held by fillers, foreign polymer, dust and, in recycled feedstock, residues from earlier use can all create points in the film.
Screen and Die Issues
A torn or wrong screen lets particles through, and a heavily loaded screen can release accumulated gels. Die lip build-up, temperature profile changes and output changes can also produce or release defects.
Masterbatch, Resin or Equipment?
| Observation | Points toward | Next check |
|---|---|---|
| Defects appear only when the black masterbatch is added | Masterbatch carrier, dispersion, filler or moisture | Compare with an approved masterbatch lot; compare carrier and base resin melt behavior |
| Defects are also present in natural film without masterbatch | Base resin or equipment | Check the resin lot, screens and die |
| Defects increase after a restart or during a long run | Degraded material, dead spots or screen loading | Purge, clean the die, change the screen and record the time |
| Defects change with a new resin lot | Base resin | Retain samples and contact the resin supplier |
| Only dark, hard specks | Carbon black agglomerates or carbonized material | Section a speck; check the screen pressure trend |
A Practical Investigation Sequence
- Collect film samples, mark the machine direction and photograph the defects in transmitted and reflected light. Count defects per area so that changes can be measured.
- Run the natural base resin alone, then add the masterbatch at the same settings.
- Exchange the masterbatch lot for an approved reference, then exchange the resin lot.
- Inspect the screen pack after each run and note the pressure trend.
- Review the temperature profile, output and any stops or restarts before the defects appeared.
- If the source is still unclear, send marked samples for laboratory analysis.
When Laboratory Analysis Helps
Specialist laboratory methods can help where comparison trials are not conclusive. Optical microscopy of a cross-section can show whether a defect contains dark particles, voids or clear polymer. FTIR spectroscopy can indicate whether the defect material differs chemically from the surrounding film, for example oxidized or a different polymer. DSC can show differences in melting behavior. The usefulness of each method depends on the sample and the laboratory; these are options to discuss with a qualified laboratory, not tests that are performed routinely on every complaint.
Frequently Asked Questions
Are crystal spots caused by black masterbatch?
Sometimes, for example when the carrier does not melt or mix fully with the base resin. Equally often they come from the base resin or the process. Comparing natural film with film containing the masterbatch is the quickest way to find out.
Are crystal spots the same as carbon black agglomerates?
No. Crystal spots are polymer-related points that look clearer or glossier. Carbon black agglomerates look like dark, hard specks.
Will a finer screen pack remove crystal spots?
A finer screen can catch some particles, but it raises pressure and does not remove the source. Soft gels can also deform and pass through screens.
Does ISO 23900-6 test for gels and fish-eyes?
No. ISO 23900-6 evaluates the dispersion of pigments and extenders using a film test. General film defects such as gels are outside its scope.
Related Guides
- How to evaluate black masterbatch dispersion
- Choosing masterbatch for blown film
- How MFI (MFR) relates to black masterbatch processing
Black Masterbatch for Blown Film
For polyethylene blown film, see our standard PE/EVA black masterbatch. If you are investigating crystal spots or specks, contact ZFH with marked film samples, photos, the base resin grade and lot, and the processing conditions.
Technical References
- ISO 23900-6:2015 — Pigments and extenders: film test
- Structural Defects and Processing Limitations for Polymer Film Blowing Applications, Polymers 2025, 17(24), 3314
These references provide technical background. They are not analyses of any specific customer film or ZFH grade.