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A Simple Introduction of Pressing Glass Techniques

As a primary production process for glass product forming, pressing stands out as one of the most efficient forming techniques due to its speed, efficiency, and precision. It involves pressing molten glass into a mold cavity under pressure to create complex glass products. Glass formed by this method is termed "pressed glass."

1. Introduction

With rapid advancements in electronics, lighting, instrumentation, and other industries, demand for glass products continues to rise—especially for precision glass forming. Applications include glass bulbs, tube glass, glass tubes, precision flat glass, precision mirror glass, high-end architectural decorative glass, insulated glass, and various glassware and instruments.
Pressing has become one of the most effective glass-forming processes due to its speed, efficiency, and accuracy.

2.Pressed Glass Production Process

In pressing, raw materials like sand, limestone, and soda ash are melted in a furnace at high temperatures (typically 1400–1600°C) to form liquid glass. After homogenization and purification, the molten glass is stored in a feeder. During forming, the glass is extruded and cut into gobs of specific mass. These gobs are transferred to a cooled mold, where compressed air or plunger pressure forces the glass to fill the mold cavity. After cooling and solidification, the product is demolded .

3.Challenges in Pressed Glass Production

Pressing is a precision-forming process where molds are critical. Key issues include:

1. 1.Mold temperature control: Excessive temperature causes sticking and deformation; low temperature leads to cold streaks.

2. 2.Cooling rate: Rapid or uneven cooling (due to poorly designed regulators) creates temperature gradients and internal stress, causing cracking during forming.

3. 3.Traditional mold design: Relies heavily on intuition, experience, and trial-and-error adjustments. Complex products (e.g., glass bulbs with high curvature accuracy) require repeated testing to optimize parameters like ambient temperature, cooler structure, pressing speed, and pressure. This extends development cycles, increases costs, and compromises quality.

4. 4.High trial costs: Continuous production lines (e.g., furnaces consuming >¥1 million daily) make new product trials and mold modifications prohibitively expensive. Traditional methods also result in ~10% defect rates, impacting both quality and costs.

4.Evolution of Pressed Glass

Current precision glass mold and process design remain experience-driven. However, rapid industry evolution (e.g., electronics) and global competition demand faster product updates, higher quality (e.g., for HDTV and flat-panel tech), shorter development cycles, and cost reduction.

Since the 1990s, Western countries have prioritized research on pressing theory and design methods. Companies like Philips collaborate with institutions (e.g., UK universities, Netherlands' TNO Applied Physics Institute) to advance pressing technology.

5. Pressed Glass Cup Production

Glass cups are made by blowing or pressing, each with manual and machine variants. Pressing uses a plunger to force molten glass into molds, distinct from blowing (which uses air pressure).

Pressing process:

1. 1.Molds rotate on a machine base while plungers move vertically.

2. 2. Molten glass enters the mold via a feeder tube.

3. 3.The plunger rapidly descends, with an upper plate ensuring a flat rim.

4. 4.The mold base rises to eject the red-hot cup blank.

5. 5.Workers remove the blank (rim may require fire polishing to smooth edges).

6. 6.Blanks undergo annealing to relieve internal stress (preventing cracks).

7. 7.Finished cups emerge from the annealing furnace.