Precision glass moulding

Technical requirements for precision glass forming equipment

Process Principle

Heating and Softening: The glass rod (preformed into a cylinder, sphere, or preform close to its final volume) is heated to a specific viscoelastic state (typically a viscosity between 10⁸ and 10¹⁰ dPa·s) above its transition temperature (Tg) and below its softening point. At this point, the glass possesses sufficient fluidity to deform under pressure.

Pressure Molding: In a protective atmosphere (typically nitrogen or vacuum), the softened glass is placed in an ultra-hard, ultra-precision, and ultra-clean mold cavity, under precisely controlled pressure (several to tens of tons).

Precision Filling: The glass viscously flows under pressure, completely filling the mold cavity and replicating the mold’s optical surfaces (including aspheric surfaces and microstructures).

Temperature Control and Pressure Holding and Annealing: After molding, the glass is cooled to room temperature at a strictly controlled cooling rate (annealing curve) to eliminate internal stresses and stabilize dimensional and optical properties.

Requirements for the operation of precision glass forming machines

Core Requirements:

Ultra-High Precision: Mold positioning accuracy and mold parallelism are at the submicron level.

Precision Temperature Control: Temperature control accuracy of the mold and molding area reaches ±1°C to ±5°C.

Precision Pressure Control: Stable, uniform, and controllable pressure application (pressure, speed, and dwell time).

Clean Environment: Prevents contamination of optical surfaces.

Atmosphere Control: Provides an inert gas or vacuum environment.

Precision Annealing Capabilities: Integrated or linked precision annealing processes.

Types:

Servo-Electric Molding Press: High precision, fast response, clean, and easy to control, the mainstream choice.

Pneumatic/Hydraulic Molding Press: High pressure, but relatively challenging to achieve precision and temperature control, used for applications with less demanding requirements.

Precision glass forming process

1. Raw Material Selection

Material Selection Criteria: Select glass grades based on optical properties (refractive index nd, Abbe number vd, transmittance band) and processability (softening point, thermal expansion coefficient, and crystallization tendency), with preference given to materials with low softening points (e.g., K9: 560°C, BK7: 720°C).
Bar Preparation:

Use commercial standard straight bars (φ5-50mm cylindrical/square bars, Schott/Ohara brands);

Cut to the desired length using a diamond wire saw (tolerance ±0.1mm);

Grind and chamfer the end faces (Ra ≤ 0.8μm, R 0.2-0.5mm);

Anneal at 300°C for 1 hour to eliminate cutting stress.

2. Bar Cleaning and Mold Treatment

Cleaning Process:

Ultrasonic ultrapure water dust removal (residual particles ≤ 0.5μm);

Anhydrous ethanol vapor degreasing (grease residue 0.05μg/cm²);

Drying with hot nitrogen purge (humidity < 15% RH). Mold Anti-Stick: DLC/Pt coating (0.2-0.5μm) is sputtered only on the mold cavity surface. Reduced demolding force by 40-60%, mold lifespan 50,000 cycles.

3. Heating, Softening, and Pressing Molding

Heating Control:

Infrared radiation gradient heating (inert gas N₂ protection).

Target temperature: 580-610°C for K9 glass, 720-750°C for BK7 glass (temperature control accuracy ±0.5°C).

Maintaining viscosity at 10⁴-10⁵ dPa·s ensures flow and filling properties.

Pressing Molding:

Precision mold clamping (coaxiality ≤5μm).

Three-stage pressure: low-speed pre-pressing (2MPa) → high-speed main pressure (5-30MPa) → holding pressure (10-60s).

Laser displacement sensor monitors fill level in real time (≥99.8%).

4. Annealing and Demolding

Annealing Process:

Slow cooling with mold to Tg – 50°C (rate 2-5°C/min);

Slow cooling in a programmed annealing furnace to room temperature (rate 0.5-1°C/min);

Residual stress ≤ 5 nm/cm (measured by polarimeter).
Demolding and Removal:

Use vacuum pick-up at temperatures ≤ 300°C (contact pressure ≤ 0.1N);

Operate in a Class 100 clean environment (temperature and humidity ±1°C/5% RH).

5. Post-Processing and Inspection

Required Steps:

Gateless treatment (no sprue formation during bar pressing);

Remove release agent residue with a neutral detergent (pH 6.5-7.5);

Optical functional coating (e.g., magnetron sputtering AR coating). Full inspection:

Surface accuracy (PV ≤ 0.2λ @ 632nm using interferometer);

Dimensional tolerance (±0.01mm using three-dimensional coordinate measurement);

Transmittance (90% of nominal value using spectrophotometer).

Manufacturing the Mold for Precision Glass Molding (PGM)

The mold in Precision Glass Molding (PGM) acts like a master template—its geometry and surface finish are directly replicated onto the final glass optic. Unlike plastic injection molding, PGM molds must withstand extreme temperatures (often above 1500°C) while maintaining sub-micron accuracy.

Cost & Complexity

The mold represents the largest upfront investment in PGM production:

Simple molds (for spherical lenses or low-volume runs) start at $10,00–$30,00.

High-precision molds (for aspheric lenses, freeform optics, or mass production) can exceed $5,000, depending on complexity and material.

This high cost comes from the ultra-precise machining, specialized coatings, and exotic materials required to endure repeated thermal cycling without deformation.

Materials & Manufacturing

PGM molds are typically made from:

Stainless steel or silicon carbide (SiC) for hardness and thermal stability.

Ultra-low-expansion (ULE) materials for extreme precision optics.

The molds are CNC-machined to nanometer-level tolerances, then polished to a mirror finish (often <1 nm Ra roughness) to ensure optical clarity.

Additional features include:

Precision alignment systems to maintain positioning during pressing.

Integrated heating/cooling channels for controlled thermal management.

Protective coatings (e.g., diamond-like carbon, DLC) to prevent glass adhesion and extend mold life.

The Mold for Precision Glass Molding (PGM) – Design & Functionality

1. The Basic Two-Half Mold (Core & Cavity)

The simplest PGM mold consists of two halves:

Upper mold (Cavity side) – Defines the front optical surface (e.g., aspheric or diffractive profile).

Lower mold (Core side) – Defines the rear optical surface (often spherical or flat).

Like in injection molding, straight-pull molds are preferred for cost efficiency, but they require:

No undercuts or overhangs (glass cannot flex like plastic).

Draft angles (minimal, but sometimes needed for mold release).

2. Complex Geometries Require Additional Components

If the lens design includes non-rotationally symmetric features (e.g., off-axis optics, prismatic elements), side-action inserts or segmented molds are used. However:

Side cores increase cost significantly (due to ultra-precision alignment requirements).

Thermal expansion mismatches must be controlled (glass and mold materials expand differently at high temps).

3. The Two Sides of a Glass Lens Mold

Unlike plastic parts, both sides of a glass lens are optically functional, but they still differ:

A-Side (Cavity, Front Surface)

Defines the critical optical performance (e.g., aspheric curvature, anti-reflective texture).

Requires super-polished finishes (1 nm Ra roughness).

Often coated with DLC (Diamond-Like Carbon) to prevent glass sticking.

B-Side (Core, Rear Surface)

May have ejection features (though glass lenses are often removed manually or with gentle vacuum pickups).

If the lens has mounting flanges or alignment features, these are machined here.

Surface finish is still high-precision but slightly less critical than the A-side.

4. Cooling vs. Heating – The Opposite Challenge

In injection molding, 50% of cycle time is cooling. In PGM:

Heating is the slowest step (glass must reach 500–800°C before pressing).

Cooling must be controlled precisely (too fast → stress fractures, too slow → poor throughput).

Mold materials must resist therma

Key measures to prevent defects

1. Uniform Wall Thickness Design

Principle: Avoid sudden changes in wall thickness, with a maximum thickness difference of ≤ 1.5 times.

Solution:

Replace thick-walled areas with hollow structures (e.g., hollowing or internal arches).

Add smooth fillets (radius ≥ 0.5 times the wall thickness) at wall-thickness transitions.

2. Optimize Mold Temperature and Cooling

Principle: Maintain mold temperature uniformity (within ±10°C).

Solution:

Installate cooling channels within the mold, prioritizing thick-walled areas.

Slowly cool the mold during the holding phase (especially for borosilicate glass).

3. Exhaust System Design

Principle: Ensure smooth exhaust of gases.

Solution:

Installate exhaust grooves (depth 0.01–0.03 mm) at the parting line or at the end of filling.

Avoid enclosing air pockets (e.g., adding a breathable steel insert at the bottom of a blind hole).

4. Optimize the Release Structure

Principle: Reduce release resistance and prevent sticking.

Solution:

Draft angle: ≥ 3° (higher than the 2° used for injection molding), increased to 5° for deep cavities.

Surface treatment: Mold polished to Ra ≤ 0.1 μm, or hard chrome plated for anti-sticking.

Ejector design: Ejector pins are placed in uniformly stressed areas (such as the base of ribs).

5. Edge and corner reinforcement

Principle: Avoid cracks caused by stress concentration.

Solution:

All internal corners are rounded (radius ≥ 1.5 mm), and external corners are enlarged to match.

Edge design: Micro-chamfer (0.2 mm × 45°) or filleted.

6. Matching material and process parameters

Principle: Adjust the process according to the glass type.

Solution:

Temperature: 50–100°C above the softening point.

Pressure: Apply pressure in stages (low-pressure filling first, followed by high-pressure compaction).

Dwell time: Calculated based on wall thickness (≥ 1 second/mm) to avoid shrinkage.

Reduce Post-Processing: Optimize mold surface quality to avoid polishing (such as mirror plating).

Material Utilization: Design a chute to collect excess material for reuse on low-precision parts.

Core Mold Technical Requirements

Reaction-sintered silicon carbide (hardness 2800 HV) must be used to resist glass corrosion. Optical mold surfaces must be diamond polished to Ra ≤ 0.05μm (mirror-grade), while industrial molds must be sandblasted to Ra = 0.8-1.6μm (anti-glare). Mirror molds must have a lifespan of ≤ 5,000 cycles (requiring hydrofluoric acid cleaning every 50 cycles), while textured molds can reach 20,000 cycles.

Application Scenario Design Specifications

Laboratory Equipment: Observation window and other components: Wall thickness ≥ 3mm, edge radius ≥ 0.5mm

Optical Devices: Aspheric surfaces: Compensate for 8% thermal shrinkage, draft angle ≥ 1°

Medical Consumables: Microchannel aspect ratio ≤ 1:1.2, draft angle ≥ 2°

Industrial Windows: Thickness tolerance ±0.1mm, flatness λ/4 @ 632nm

Five Secrets to Cost Reduction in Borosilicate Glass Pressing

Tip 1: Minimalist Mold Design (Reduces Mold Costs by 15-25%)

Traditional Problem: Stainless Steel Mold Solution
Side-core pull mechanisms are prone to jamming (high viscosity of hot glass). Eliminate moving parts → Adopt the following design:
✅ Natural draft angle ≥ 5° (higher than the conventional 3°)
✅ Hough parting (vertical parting to avoid lateral forces)
✅ Threaded locking ring (replaces hydraulic side-core pulls)
Case Study: Microfluidic chip mold cost reduced from $12,000 to $8,500, with lifespan increased to 30,000 cycles.

Tip 2: Wall Thickness Gradient + Rapid Cooling (40% Energy Consumption Reduction)

Exclusive Solution for Stainless Steel Molds:

Mathematical Formula
Cooling Time (s) = [Wall Thickness (mm)]² × 0.8 (Thermal conductivity of stainless steel is approximately 15 W/m·K)
Design Optimization Results
Main body wall thickness ≤ 3.5mm: Cooling time reduced to 60% of traditional solutions
Rib thickness = Main body thickness × 0.5: Avoid sink marks and reduce re-pressing time
Copper alloy cooling pipes embedded in the mold: Increase heat exchange efficiency by 200% (cooling rate up to 15°C/s)

Title 3: In-Mold Function Integration (50% reduction in post-processing costs)

Secret 4: Innovative Annealing Process (30% Energy Consumption Reduction)

Process Parameters:
620°C × 15min (hold) → Gradient Cooling:
650°C → 500°C @ 5°C/min → 500°C → 300°C @ 3°C/min → Natural Cooling

Advantages:

Total annealing time reduced to 45min (conventional process requires 90min)

With microwave-assisted annealing, energy consumption is reduced to 1.8kWh/kg (previously 2.6kWh/kg).

Secret 5: Precise Bar Matching (18% Material Savings)

text
Bar Length L = [V Finished Product × 1.1] / [π × (D Bar/2)²] V Finished Product: Part volume (including flash)

D Bar: Bar diameter (tolerance ±0.05mm)

Factor 1.1: Compensates for the flash rate of stainless steel molds (5-8%, higher than silicon carbide molds)

Implementation: Purchase pre-cut bar stock (diameter tolerance ±0.05mm), and increase scrap recycling to 20%.