Study Notes: Glass Packaging Chapter

Chapter 4: Glass Packaging

After completing this chapter, a student should be able to accomplish the following objectives:

  • Define glass.
  • List the general advantages and disadvantages of glass as a packaging material.
  • List the components of packaging glass and discuss the function of each
  • Diagram a glass furnace and explain what is accomplished at each stage of the process.
  • Define “jargon” terms relating to the glass furnace (bridgewall, shadow wall, gob, etc.).
  • Define “jargon” terms relating to the container-making process (settle blow, parison, counter blow, etc.).
  • Describe or define a brittle material.
  • Compare and contrast press and blow molding with blow and blow molding.
  • Identify points of weakness in a glass container and judge the process (press and blow or blow and blow) that was used to manufacture a particular container.

I. Definition of Glass

  • Definition: Glass is an inorganic, noncrystalline, brittle solid formed by cooling from a liquid state.

    • Does not show discontinuous changes in properties at varying temperatures.
    • Becomes more rigid as temperature decreases and less rigid as temperature increases.

  • What Does It Mean to Be Brittle?

    • Materials can be categorized as ductile (metals like steel, aluminum, plastics) or brittle (glass).
    • Ductile Materials: Deform gradually under load, recover to their original form if unloaded before the proportional limit.
    • Brittle Materials: Fail suddenly when stress exceeds yield stress, leading to a break or shatter.
    • Example: A rubber band returns to its original size when the load is removed unless overstressed, causing either permanent deformation or breaking.

  • What Does It Mean to Be Noncrystalline?

    • Crystalline materials have orderly, repeating molecular structures (e.g., salt, sugar).
    • Noncrystalline materials, like glass, possess random, unordered molecular arrangements (amorphous), leading to:
    • Clearer sections compared to crystalline regions.
    • Increased strength; crystalline glass may shatter upon shock due to weaker regions.

  • What Does “No Discontinuous Change at Any Temperature” Mean?

    • Unlike water, which changes phases abruptly at freeze and boil points (0°C/32°F and 100°C/212°F), glass gradually becomes more viscous as temperature decreases.
Everyday Language Summary
  • Glass is unique—has liquid molecular structure but solid physical properties.
  • Can be formed, cooled, used, remelted, facilitating recycling.
  • Inorganic; sometimes referred to as a supercooled liquid.

Advantages and Disadvantages of Glass vs. Other Packaging Materials

Advantages:
  • Moderate cost per container.
  • Resistant to most chemicals.
  • Inherently strong, especially in compression.
  • Low coefficient of thermal expansion.
  • Microwave safe.
  • Rigid, nonpermeable, tasteless, and odorless.
  • Can be transparent (flint glass) or protective against UV (amber glass).
  • Strong, high-quality image.
Disadvantages:
  • Heavy compared to other materials.
  • Requires large, expensive manufacturing facilities.
  • Brittle and easy to break.
  • Importance of implications to be understood throughout the course.

II. The History of Glass

  • Natural Occurrences:

    • Formed by heat from volcanoes (obsidian) and lightning (fulgurite).
    • Obsidian, used in tools by prehistoric people.

  • Historical Significance:

    • Roman historian Pliny attributes glass making to Phoenician mariners active around 3000 BC who discovered heating sand and natron.
    • Notable dates in glass history:
    • 12,000 BC: Glass beads used in Syria.
    • 700 BC: Glass jewelry in Egypt.
    • 3000 BC: Egyptian glass bottles using baked clay forms.
    • 300 BC: Introduction of blowpipes for glass making.
    • 200 BC: Romans produced flat glass.
    • 1608 AD: Glass manufacturing in Jamestown, Virginia.
    • 1800s: Automation introduced in glass manufacturing.
    • 1903: Michael Owens invented the first automatic bottle-making machine, revolutionizing the industry.

  • 2004 U.S. Census Report: Produced 239,654,000 gross glass containers.

III. Three Selected Types of Glass

1. Glass Made from Pure Silica
  • Made solely from silica, limited usability (mirrors and telescope lenses).
  • Characteristics:
    • Inexpensive raw material, durable, high energy requirements, hard to form.
2. Glass Made from Soda and Silica
  • Characteristics:
    • Faster melting due to soda.
    • Chemically resistant but some formulations (water glass) dissolve in water.
3. Container (Packaging) Glass
  • Comprised of silica, soda, calcium carbonate, aluminum oxide.
  • Desirable Characteristics:
    • Low devitrification tendency.
    • Good durability, moderate cost, low viscosity, chemically durable, inherently strong, impermeable, tasteless, odorless, and can be transparent or opaque.
Key Manufacturing Insights
  • Modern Manufacturing: Complex, energy-intensive, specialty glass plants.
  • Raw materials stored to facilitate feed into the glass-making process.

IV. Modern Glass Manufacturing Processes

Batch House and Glass Furnace

  • Batch House: Location where ingredients are mixed before entering the furnace. Utilizes automation for measurement.

  • Glass Furnace Components:

    • Melter: Largest furnace section; melts ingredients, operates at extremely high temperatures (2,600 to 2,900°F).
    • Bridgewall/Throat: Allows controlled flow of liquid glass and captures impurities (slag).
    • Refiner: Polishes glass, floating impurities removed.
    • Forehearths: Stabilize glass temperature for container manufacturing.
Container Manufacturing
  • Gobs and Molds: Controlled flow of molten glass into pre-formed molds; key to forming shapes and ensuring consistency.
  • IS Machines: Contain multiple independent sections for flexibility in production, often controlled by computerized systems.
  • Press and Blow vs. Blow and Blow Molding:
    • Processes defined by mold types and application specifics for jars and bottles.
Preventing Surface Damage
  • Surface Considerations: Scratches can reduce glass strength, necessitating careful handling.
  • Annealing Process: Gradual reheating and cooling in a lehr to relieve internal stresses.
  • Coating Applications: Enhances durability and reduces friction during production.

V. Strength of Glass Containers

  • Natural glass is strong under compression (~150,000 psi) but weaker under tension.
  • Manufacturing processes create surface defects—final effective strength about 15,000 psi.
Factors Affecting Strength
  • Distribution and thickness of glass, shape, surface conditions, and load type.
  • Strategies to improve strength through annealing, limited surface damage, and design optimization.
References
  • Introduction to Packaging by Harold Hughes. Chapter 2 Glass Packaging. Pages 17 to 31.