THERMAL TEMPERING

The main limitations are: (1) It cannot be cut, drilled, or edge-worked after tempering—all fabrication must happen beforehand. (2) Slight roller wave distortion may be visible from the horizontal tempering furnace, which can matter in optical applications. (3) Nickel sulfide (NiS) inclusions can cause rare spontaneous breakage; a heat-soak test per EN 14179 reduces this risk but adds cost and lead time. (4) Tempered glass shatters entirely into small granular fragments when broken—beneficial for safety but it means the panel is completely lost and cannot be salvaged. For projects where post-breakage retention matters, laminated tempered glass or heat-strengthened glass in a laminate is the standard workaround.

Rapid cooling solidifies the surfaces first, locking them in compression. The core stays in tension. Any force must overcome that compressive layer before a crack can start.

Thermal tempering uses rapid air cooling after heating to 620–700 °C, producing about 80–150 MPa of surface compressive stress. It works best for glass ≥3 mm thick and is cost-effective at high volumes. Chemical tempering (ion exchange) submerges glass in a molten potassium nitrate bath at 380–450 °C, swapping smaller sodium ions for larger potassium ions. The ion-exchange process yields 600–1,000 MPa of surface compression and handles glass as thin as 0.1 mm. Chemical tempering also preserves better optical quality and allows post-tempering cutting in some cases, but costs more per piece and takes longer. For example, a 0.7 mm smartphone cover glass would use chemical tempering, while a 6 mm shower door panel would use thermal.

4–5× stronger than annealed glass, 150–200 °C thermal shock tolerance, safe granular breakage pattern, and low per-unit cost at volume.

Most soda-lime float glass, low-iron glass, tinted glass, and some borosilicate compositions can be thermally tempered, provided the glass is at least 3 mm thick. Patterned and coated glass can also be tempered if the coatings withstand 620–700 °C. Fused silica and other ultra-low-expansion glasses do not benefit from the process.

Tempered glass typically costs $5–$40 per square foot depending on thickness, size, edge work, coatings, and order volume. Standard 6 mm clear tempered panels in volume orders sit near $8–$15/sqft. Custom shapes with CNC cutouts, polished edges, AR/AF coatings, and silk-screen printing push the price toward the higher end. Freight, MOQ, and import duties also affect unit cost. As a practical example, a 200-piece order of 6 mm clear tempered glass at 300 × 400 mm with polished edges might land around $10–$12/sqft direct from a manufacturer like Saiwei, compared to $18–$22/sqft through a domestic distributor—a savings of 30–50%.

No. Heat-strengthened glass is about 2× stronger than annealed glass; fully tempered glass is 4–5× stronger. The trade-off is breakage pattern: heat-strengthened glass breaks into larger pieces, which is useful in laminated assemblies where post-breakage retention matters.

Tempered glass handles continuous operating temperatures up to about 250–300 °C and thermal shock differentials of 150–200 °C. Prolonged exposure above 300 °C can relax the tempering stresses. For sustained high-temperature use, borosilicate or ceramic glass is a better choice.

Tempered glass is used wherever safety, strength, and thermal resistance are needed: smartphone cover glass, shower enclosures, glass doors, automotive windows, oven doors, industrial HMI touch panels, medical device displays, and architectural facades. In B2B contexts, it is commonly specified for electronic display covers and control panel overlays.

No. Cutting, drilling, or grinding releases the stress balance and shatters the panel. All fabrication happens before tempering.