The negative tolerance of laminated glass (i.e., the extent to which it is allowed to be thinner than the nominal thickness) does not have a globally uniform fixed value. It mainly depends on the total thickness of the glass and the standards it follows (such as national standards, industry standards or specific customer requirements).
Chinese National Standard
The latest relevant national standard at present is GB 15763.3-2009 "Safety Glass for Buildings - Part 3: Laminated Glass".
This standard does not directly stipulate the negative tolerance of the total thickness, but rather specifies the thickness deviation of laminated glass, which is jointly determined by the thickness deviation of the original sheet glass and that of the intermediate layer.
The thickness deviation of the original sheet glass: It should meet the requirements of its respective standards (for example, float glass complies with the provisions of GB 11614). Generally, for float glass used in construction, the allowable deviation of its thickness is between ±0.2mm and ±0.6mm, depending on the thickness and quality grade of the glass.
Thickness deviation of the intermediate layer (such as PVB/SGP, etc.) : The thickness deviation of the intermediate layer material is usually very small. For instance, the thickness tolerance of standard PVB film (0.38mm per layer) is typically between ±0.03mm and ±0.05mm per layer.
Therefore, the negative tolerance of the total thickness of laminated glass can be approximately understood as: the negative tolerance of the original sheet glass + the negative tolerance of the intermediate layer.
For example:
A laminated glass of (5+0.38+5)=10.38mm, composed of 2 pieces of 5mm glass and 1 layer of 0.38mm PVB.
Suppose the thickness tolerance of 5mm float glass is ±0.2mm (in accordance with GB 11614).
The thickness tolerance of PVB film is ±0.04mm.
Then, the minimum possible thickness of this laminated glass (i.e., the negative tolerance limit) is: (5-0.2) + (0.38-0.04) + (5-0.2) = 4.8 + 0.34 + 4.8 = 9.94mm
Its nominal thickness is 10.38mm, so the negative tolerance reaches -0.44mm.
From this example, it can be seen that the thicker the glass and the more layers it has, the greater the absolute value of the cumulative tolerance may be.
Common industry practices and agreements
In actual orders and production, to facilitate communication and acceptance, both the supplier and the buyer usually agree on a more concise and clear total thickness tolerance range. The common practice is:
For laminated glass with a total thickness of no more than 10mm: The tolerance is usually agreed upon as ±0.3mm
For laminated glass with a total thickness greater than 10mm: The tolerance is usually agreed to be ±0.5mm or ±1.0mm (for very thick structural glass)
This is merely an empirical value. The most authoritative basis is always the tolerance requirements clearly stated in the order contract or technical agreement.
Other important influencing factors
Intermediate layer type: Different types of intermediate layers (such as standard PVB, high-strength PVB, SGP, EVA, etc.) are used, and their thickness and tolerance characteristics are slightly different, which will affect the total thickness.
Production process: Laminated glass needs to undergo high-temperature and high-pressure treatment. During this process, there will be slight changes in thickness. Excellent equipment and control processes can better ensure thickness consistency.
Customer requirements: Some high-end or special projects (such as curtain walls, anti-theft glass, bulletproof glass) have stricter requirements for thickness, which must be clearly stipulated in the contract.
Summary
Strictly speaking, the negative tolerance of laminated glass should be calculated based on the cumulative tolerance of the original sheet and the intermediate sheet, in accordance with standards such as GB 15763.3-2009.
In actual operation, the most commonly used simple reference is:
Thickness ≤ 10mm: ±0.3mm
Thickness > 10mm: ±0.5mm to ±1.0mm
