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Scientific Analysis of Fisheyes in Card Sleeves and Pages and Their Impact on Card Damage
We occasionally receive feedback from friends who, without checking the quality of their card sleeves beforehand, find their cards severely damaged by fisheyes present in the sleeves or pages. This damage can lead to a significant drop in card grading scores, ultimately resulting in substantial financial loss. It's not just card sleeves and pages that can have fisheye issues; even commonly used 35pt card holder and semi-rigid card holders (soft card holders specifically for submission) can carry this risk of card damage. Therefore, it's crucial to choose professional-grade card accessory brands. For valuable and important cards, always inspect sleeves and other accessories for fisheyes and impurities before use. *No brand can guarantee a 100% fisheye-free product!* (This depends on the frequency of fisheye occurrence, the density of material forming the fisheyes, and their size within the product).
For this reason, we've invited our Sanseking Engineering R&D Department to provide a scientific explanation of the causes of fisheye formation during the manufacturing process of card sleeves and pages!
Fisheye: A common physical explanation is an "over-polymerized mass." This means the molecular weight of the polymer at the "fisheye" location is higher than that of the surrounding polymer of the same type.
Due to the higher molecular weight, the polymer at the fisheye location has a higher melting point and higher viscosity when melted. During film blowing, this fisheye polymer cannot disperse and mix evenly with the surrounding polymer. It solidifies before the surrounding polymer after the melt is blown into a film. Therefore, these solidified masses of over-polymerized material, often appearing "arrow-shaped" or "spherical," are conventionally called "fisheyes." In the production of polymer films, "fisheyes" are a troublesome visual defect. They often appear as visible small spots, particles, or areas of abnormal light transmission, severely affecting the appearance grade of the base material for card sleeves and pages, and can even lead to printing or coating failures for artistic sleeves.
Today, I will explain it clearly in three points:
① What exactly is a fisheye in card sleeves? What is its essential nature? ② In practical engineering, how should we scientifically analyze fisheyes when we encounter them? ③ In engineering practice, how can we systematically prevent fisheye issues?
01 What is a card sleeve fisheye? What is its essential nature?
The general understanding of "fisheye" is often biased. Many people simplistically think: "A fisheye is just polymer that crystallized during the cooling process," but this is only partially correct. Let's systematically break down the nature of fisheyes.
My view is: A "fisheye" is not a specific material structure, but rather a locally anomalous phenomenon that is optically discontinuous and visible to the naked eye. It can originate from the material itself or be caused by foreign matter.
In other words: "Fisheye" is a phenomenon, not a substance.
Based on their cause, fisheyes can be divided into two main categories:
Endogenous Fisheyes: Caused by the material's own structure. These are typically caused by:
Localized Crystallization: Insufficient melting, uneven cooling rates, etc.
Unmelted Particles / Gel Particles: Insoluble particles formed from cross-linked polymer chains or thermal degradation.
Uneven Formula Distribution: Uneven distribution of additives, resins, cross-linking agents, etc., leading to local differences in refractive index. (Similar to how pearl pigment powder in pearlized sleeves, if not fully diffused when fused with PP, can easily form fisheyes).
Local Orientation Abnormalities: Microstructural distortion during casting or stretching processes.
Machinery: Abnormal local temperature differences in the screw.
Characteristics:
Composition is the same as the base matrix.
Under a microscope, often appear translucent with strong refraction.
May show birefringence under polarized light (indicating an ordered structure).
Thermal analysis might reveal new melting points or crystallization peaks.
Exogenous Fisheyes: Caused by foreign object contamination.
These fisheyes are essentially foreign particles mixed into the film or adhering to its surface, such as:
Undispersed fillers (TiO₂, SiO₂, CaCO₃).
Dust or fibers from the operating environment.
Metal shavings from equipment wear.
Carbonized particles, aged gel particles, etc.
Characteristics:
Composition is distinctly different from the base matrix.
Usually darker in color or have abnormal reflection.
Have clear boundaries under a microscope.
May be accompanied by 3D surface protrusions.
Therefore, it's essential to emphasize: Foreign objects certainly belong to one type of fisheye. We cannot limit the definition of "fisheye" to just "crystallization," as this restricts troubleshooting (thought processes).
02 How do we analyze fisheyes? Step-by-step guide to breaking down the cause.
When encountering fisheyes, avoid blind guesses or relying solely on experience. Follow a systematic analysis process:
1. Optical Microscopy: Preliminary identification of fisheye morphology.
Tools: Standard optical microscope, Polarized Optical Microscope (POM)
Key points to observe:
Is the fisheye color different from the surroundings?
Are there defined boundaries, sharp contours, or protrusions?
Under polarized light, does it show colorful birefringence patterns? (Suggests crystallization or orientation)
Typical Judgments:
Phenomenon
Inference
Rainbow patterns, no clear particle boundaries
Likely crystallization or oriented structure
Gray/black small particles, strong reflection
High probability of foreign matter: carbon particles, metal debris
2. DSC Thermal Analysis: Determine if crystallization behavior is involved.
Tool: Differential Scanning Calorimeter (DSC)
Sample both the fisheye area and a defect-free area, observe differences:
New melting point → Unmelted particles or crystals.
Appearance of cold crystallization peak → Local structural or formulation abnormality.
No difference → Could be foreign matter or difference in structural density.
3. IR / Raman Spectroscopy: Determine if the chemical structure is consistent.
Analyze the functional group composition of the fisheye area:
If spectra are identical: Could be a structural issue, like crystallization or cross-linking.
If new peaks appear: Consider foreign matter, contamination, degradation, or reaction residue.
Common anomalies:
Hydroxyl/Carboxyl peaks → Material degradation.
Siloxane/Metal-Oxygen peaks → Additives or inorganic impurities.
Isomer/Cross-linking groups → High-temperature side reactions.
4. Scanning Electron Microscope + EDS (SEM-EDS): Determine if it's inorganic contamination.
Tool: Scanning Electron Microscope + EDS (Energy Dispersive X-ray Spectroscopy)
Observe fisheye morphology at high magnification and obtain its elemental composition:
Ca/Ti/Si → Aggregation of common fillers.
Fe/Cr/Ni → Equipment wear debris.
Inorganic impurity aggregation → Uneven mixing or storage of materials.
This step is particularly crucial for identifying "exogenous fisheyes."
5. 3D Topography Measurement: Determine if there are surface protrusions.
Tools: Laser Confocal Microscope, AFM (Atomic Force Microscope), White Light Interferometer
Used to determine if the fisheye causes local thickness abnormalities or roughness changes, especially important for:
Optical films, screen protectors, and other applications extremely sensitive to surface uniformity.
Cases where it's necessary to identify if the fisheye penetrates the film.
6. Combine Process Data to Trace the Source.
Includes, but is not limited to:
Screw temperature, shear speed, residence time.
Melt pressure fluctuations, filter system cleaning frequency.
Raw material batch changes, additive sequence.
On-site cleanliness and electrostatic control measures.
Many card sleeve fisheye problems are not due to one parameter being absolutely wrong, but rather an imbalance in the relative matching relationship between parameters. For example:
Declining filter precision + Raw material batch change → Foreign matter not filtered out.
Slightly higher extrusion temperature + Shear-sensitive formula → Gel particle formation.
03 In engineering practice, how does Sanseking systematically prevent fisheye problems in card sleeve materials?
To control fisheyes, efforts must be made from two directions:
1) Control the source at the "Generation End":
Sufficient Filtration Precision: Reasonable selection of mesh size, regular replacement.
Thorough Screw Cleaning: Prevent carbon buildup and gel particle formation in dead zones.
Incoming Raw Material Inspection: Pay attention to batch dispersion, thermal stability.
Formula Optimization: Reduce crystallization-inducing components, avoid shear-sensitive additives.
Sufficient Additive Dispersion: High-speed pre-mixing or masterbatch addition is more reliable.
Improved Environmental Cleanliness: Especially in the film blowing zone, control electrostatic adsorption.
2) Establish an interception mechanism at the "Outflow End":
Install melt filtration systems (dual-column switching, pressure difference alarm).
Online monitoring of fisheye density (optical camera) with alarm and rejection capabilities.
Perform polarized light inspection or lightbox inspection on finished products for visual quality control.
Timely quarantine of problematic batches + trace back to the source when issues are found.
<u>Best practice: Establish a "Fisheye Map" record</u>: Record frequency of occurrence, location, size, and batch correlation.
Final Words
Fisheyes are not mysterious, but the reasons behind them can be very complex: uneven composition, abnormal shear, insufficient dispersion, cross-linking, crystallization, foreign object contamination... all can cause similar manifestations.
You are not just solving a spot; you are addressing a协同 (synergistic) problem of the entire process system.
Only by establishing a clear understanding and forming systematic analytical capabilities can you calmly respond and truly keep such defect problems at bay.
As a 14-year Chinese manufacturer of card sleeves and products, Sanseking's commitment to higher product standards has always been the core value of our brand. The content of this article is a very professional sharing of production processes, which can bring some thoughts and serve as a reference for exchange within the industry. From a consumer's perspective, it might seem complex and difficult to understand. However, understanding the underlying production logic and craftsmanship of a product you frequently use helps you make independent judgments when selecting products and evaluate their quality grade through your own perspective!
