Sanseking Science Series: How to Better Protect Your Cards?

A Scientific Analysis of Film and Sleeve “Crystal Spots” and Card Damage

From time to time, we receive feedback from collectors who, after placing their cards into card sleeves or binder pages without first checking the quality of the protective materials, discover that crystal spots on the inner surface of these products have caused significant damage to their cards. This often leads to a drastic reduction in card grading scores and substantial financial loss.

It’s not only card sleeves and pages that may contain crystal spots. Even the most commonly used 35pt cardholders and semi-rigid card savers (frequently used for submitting cards for grading) can also carry this risk. That’s why it is essential to choose professional-grade protection products from reputable brands.

For valuable and important cards, we always recommend inspecting sleeves and other card protection products for crystal spots or impurities before use. It’s important to understand that no manufacturer can guarantee a 100% crystal-spot-free product. The frequency and size of these spots depend on material density and production conditions.

In this article, we’ve invited the Sanseking Engineering R&D Department to provide a scientific explanation of how crystal spots form during the manufacturing process of card films and sleeves.

What Are Crystal Spots?
In physical terms, crystal spots are often described as “over-polymerized” material. This means that the molecular weight of the polymer at the spot is higher than that of the surrounding material.

Due to this higher molecular weight, the melting point of the polymer in these spots is also higher. During the melting process, the viscosity of the melt is greater in these areas. When the film is blown, the spot cannot disperse or blend evenly with the rest of the polymer. It solidifies earlier than the surrounding material, forming arrow-shaped or spherical polymer condensates commonly referred to as “crystal spots.”

In the production of polymer films, these defects—sometimes also called “fish eyes”—are a common visual issue. They appear as visible specks, granules, or areas with abnormal light transmission, seriously affecting the appearance and quality of card films and pages. In severe cases, they can even lead to failures in printing or coating processes for artistic card sleeves.

Today we’ll break down the issue of crystal spots into three sections:

1. What is the fundamental nature of crystal spots?
2. How to scientifically analyze crystal spots when they occur?
3. How to systematically prevent crystal spot issues?

01 The Nature of Crystal Spots: Not Just “Crystallization,” but “Optical Abnormalities”

It’s commonly thought that crystal spots are formed due to crystallization during cooling. This is not entirely accurate. In essence, a crystal spot is a visible optical discontinuity—a localized anomaly with diverse causes.

Based on their origin, crystal spots can be divided into two main types:

1. Endogenous Crystal Spots (Material-Inherent)
These are caused by structural or morphological abnormalities within the material itself. Causes include:

• Localized Crystallization: Incomplete melting or uneven cooling leading to localized over-crystallization.
• Unmelted Particles/Gel Particles: Cross-linked polymer chains (“gel particles”) or thermally degraded material that fails to melt.
• Poor Formula Dispersion: Uneven distribution of additives (e.g., slip agents, anti-blocking agents), colorants (e.g., pearl pigments), or cross-linking agents, leading to localized differences in refractive index.
• Local Orientation Abnormalities: Molecular chain or microstructure distortion during casting or stretching.
• Equipment Temperature Variance: Localized temperature fluctuations in the screw or die.

Characteristics:

• Chemical composition is the same as the base material.
• Under microscope, often appear semi-transparent with strong refraction.
• Exhibit birefringence under polarized light (indicating ordered structure).
• DSC thermal analysis may show abnormal melting or crystallization peaks.

2. Exogenous Crystal Spots (Contamination-Induced)
These are caused by foreign particles introduced during production. Sources include:

• Undispersed Fillers: Agglomerated TiO₂, CaCO₃, SiO₂, etc.
• Environmental Contaminants: Dust, fibers, or hair from the air.
• Equipment Wear Debris: Metal particles from wear of screws, barrels, or dies.
• Thermal Degradation Particles: Carbonized burnt material or aged peeling particles.

Characteristics:

• Chemical composition differs significantly from the base material.
• Usually darker in color (e.g., black spots) or have abnormal reflectivity.
• Under microscope, show clear and sharp boundaries.
• Often accompanied by surface protrusions.

Key Insight:
Labeling all crystal spots simply as “crystallization” greatly limits troubleshooting. A broader perspective is needed, considering both material-inherent abnormalities and foreign contamination.

02 Scientific Analysis: A 5-Step Method to Identify the Root Cause

When encountering crystal spots, avoid guesswork. Follow this systematic analytical process:

Step 1: Optical Microscopy (Including Polarized Light)

• Purpose: Preliminary classification.
• Observations:
  • Morphology: Clear boundaries? Spherical, flake-like, or irregular?
  • Color: Same as the base? Transparent, white, or black?
  • Polarized Effect: Does it show birefringence patterns (e.g., “Maltese cross”)?
• Initial Judgment:
  • Birefringence + no clear boundary → Likely endogenous crystallization or orientation.
  • Clear boundary + black/gray color + strong reflection → Likely exogenous contaminant.

Step 2: DSC Thermal Analysis

• Purpose: Determine whether crystal spots involve differences in crystalline behavior.
• Method: Compare samples from spotted and normal areas.
• Judgment:
  • New melting/crystallization peaks → Unmelted particles or abnormal crystalline structure.
  • No thermal difference → Likely foreign contaminant or density/orientation variance.

Step 3: FTIR / Raman Spectroscopy

• Purpose: Determine whether chemical structure is consistent.
• Method: Perform micro-area analysis.
• Judgment:
  • Presence of hydroxyl/carboxyl peaks → Material oxidation degradation.
  • Detection of Si, Ti, Ca, etc. → Filler agglomeration or inorganic pollution.
  • Spectrum matches base material → Structural issue (e.g., cross-linking, crystallization).
  • Abnormal peaks → Foreign chemical contamination.

Step 4: Scanning Electron Microscopy + Energy-Dispersive X-Ray Spectroscopy (SEM-EDS)

• Purpose: Observe microscopic morphology and analyze elemental composition—the “gold standard” for identifying exogenous spots.
• Method: Examine cross-sections under high magnification and conduct EDS analysis.
• Judgment:
  • Detection of Ca, Ti, Si → Filler agglomeration.
  • Detection of Fe, Cr, Ni → Metal wear debris.
  • Primarily C and O, consistent with base → Endogenous cause (e.g., gel, carbonized material).

Step 5: Process Data Review

• Tools: Laser confocal microscope, AFM, white light interferometer.
• Purpose: Correlate lab findings with production data to identify the root cause.
• Key Points:
  • Process parameters: Were there fluctuations in screw temperature, melt pressure, or motor speed?
  • Raw material batches: Was there a change in material or additive batch? Was pre-drying performed?
  • Equipment status: Were filters replaced on time? Was equipment cleaned regularly?
  • Environmental factors: Was cleanliness/humidity controlled in the production area?

Often, crystal spots result from multiple overlapping factors:

• “Clogged filter” + “new raw material batch with slightly more impurities” = Contaminants penetrate the filter.
• “Slightly elevated extrusion temperature” + “shear-sensitive additive” = Additive degradation forming gel particles.

03 Systematic Prevention: Full-Process Control from Source to Output

Preventing crystal spots requires a systematic approach covering both source control and end-point interception.

1. Source Control (Prevention)

• Strict Raw Material Management:
  Establish inspection standards for incoming materials and additives, focusing on impurity content, melt index, and thermal stability.

• Optimized Formulation and Process:
  Use masterbatch for shear-sensitive or migratory additives.
  Improve mixing processes to ensure uniform dispersion.
• Precise Process Control:
  Set and monitor appropriate screw temperature profiles to prevent local overheating or poor plastification.
  Avoid excessively high shear rates and prolonged material residence time.
• Equipment Maintenance:
  Regularly clean screws and dies to prevent material buildup and carbonization.
  Establish strict filter replacement schedules and standards.
• Environmental Control:
  Ensure cleanliness in material handling, conveying, and film-forming areas. Use electrostatic neutralizers to prevent dust adsorption.

2. In-Process Interception (Detection)

• Online Monitoring:
  Install surface defect detection systems at the end of production lines to monitor crystal spot size and density in real time.

• Efficient Filtration:
  Use dual-column melt filters allowing continuous operation during screen changes. Set pressure difference alarms.
• Final Inspection:
  Implement sampling inspections using polarized microscopy and backlight inspection stations.
• Quality Traceability:
  When issues are detected, immediately quarantine affected batches. Use a “crystal spot map” to trace back to specific production lines, time periods, and material batches.

Conclusion
Crystal spots are the visible “result” of multiple underlying causes—often a combination of process and material failures. The solution lies not only in “removing a spot” but in restoring the entire production system (people, machinery, materials, methods, environment) to a stable, coordinated, and controlled state.

Superior quality does not come from rigorous final inspection alone. It is achieved through precise design and control of every step in the production process. Addressing crystal spot issues is ultimately an exercise in improving technical understanding, analytical capabilities, and management systems. This is the foundation for confidently tackling complex quality challenges.

As a 14-year-old Chinese manufacturer of card films and sleeves, Sanseking has always adhered to high standards as a core brand value. While this article may be highly technical and more suited to industry professionals, we believe that understanding the production process and material science behind the products you use daily can help you make more informed decisions and develop a critical eye for quality.

Thank you for reading. Protect your collections with knowledge and care.