Heat alone does not break glass. A difference in temperature across the wall does, and your process creates that difference every time hot product meets a cool bottle or a chilled bottle meets a warm room.
Why a Gradient Breaks Your Bottle
Glass expands when it warms. When one surface warms faster than the other, the warm side tries to grow while the cool side holds it back, and the two layers pull against each other.
That tension has to go somewhere. It finds the nearest flaw, which is usually a surface scratch or a thin spot, and the crack starts there rather than where the heat arrived.
Where Your Process Creates the Gradient
| Moment | What happens to your glass | How to reduce it |
|---|---|---|
| Hot filling | Inner surface heats while the outer stays cool. | Warm your bottles before filling rather than after. |
| Pasteurising | The tunnel heats and cools in stages. | Keep each stage within the differential your glass survives. |
| Hot washing | Water hits a cold bottle at speed. | Step the temperature instead of jumping it. |
| Cold chain to warm port | A chilled 330 ml (11 oz) bottle meets humid heat. | Let pallets equalise before opening a container. |
How the Test Actually Runs
ASTM C149 gives the method. Bottles sit in a hot bath long enough to equalise, then transfer within seconds into a cold one, and each sample either survives or cracks.
The differential is what you specify. SGSBOTTLE runs the test on samples from your production run across the 14 lines in Shandong and Xuzhou, so the figure on your report describes your own glass.
Hot fill and cooling both create a gradient. SGSBOTTLE tests output from 14 lines in Shandong and Xuzhou.
Why Heavier Glass Is Not Safer Here
Thickness works against you in thermal shock, which surprises most buyers. A thick wall holds a bigger difference between its inner and outer surface, so it builds more tension for the same change in temperature.
Even walls matter more than heavy ones. A 750 ml (25 oz) bottle with consistent wall distribution outperforms a heavier one with a thin shoulder, because the thin spot is where the crack starts anyway.
What Annealing and Scratches Add
Residual stress from poor annealing sits in your bottle using up capacity before the gradient arrives. A bottle graded properly under ASTM C148 keeps that capacity for the thermal load instead.
Surface damage does the same from outside. Scratches picked up on your conveyor concentrate the stress, which is why coating condition and thermal shock performance are linked more closely than they look.
What Goes on Your Enquiry
Your fill temperature, the format in ml and oz, whether the pack is pasteurised or washed hot, and the temperature swings in your supply chain. SGSBOTTLE quality engineers in Shandong come back with the thermal-shock result and the annealing grade for your design.