Polycarbonate sheet readily absorbs moisture from the air. Eventually the water content will reach 0.2% by weight.
In most applications this water content is not a problem, however, in applications where you need to process the sheet above a temperature of 120C, this water can vaporize within the sheet during processing and lead to small bubbles forming.
As little as 0.05% water can cause these bubbles.

Two processes that require the sheet to be heated above 120C are lamination of the Polycarbonate and thermoforming of the Polycarbonate.
Both of these processes can have problems with bubbles if the sheet is not dried correctly.

This article discusses drying polycarbonate sheet before either lamination or thermoforming.

To dry the sheet there is a wide range of recommendations that have been published. To dry 3mm thick sheet it is normally recommended to use an oven set at 120C. The drying time suggestions can range from 6 to 12 hours.
We would suggest that the longer time the sheet is dried the better and we would use 12 hours. Other recommendations suggest that a lower oven temperature of 80C can be used but the time must be increased to 24 hours.
If your oven is only capable of reaching 80C rather than 120C, you could certainly try this method – however, we are very skeptical of this approach as to drive the water off effectively you really need to be above the boiling point of water.
As the thickness of the sheet increases, the drying time will increase significantly as the water needs to be removed from the center of the sheet.

For 6mm thick sheet we would recommend 30 hours of drying at 120C and for 10mm thick sheet we would recommend 40 hours.
Drying polycarbonate sheet for shorter times may not successfully remove enough of the moisture.

One question that we are asked is “Do you need to remove the masking before drying?” In general the Polyethylene or paper masking is not a very good moisture barrier, so it should not hinder drying very much. There is generally more risk of damage to the sheet if the masking is removed prior to drying, so unless you have good handling conditions to prevent damage, the marginal improvement in drying time is usually not worth the risk.

Once the sheet has been dried, it should either be used immediately or stored in a dehumidified area with a dew point less than -10C.
To illustrate why this is the case, on a hot, humid day a dried sheet can absorb 0.5% water within 3 to 4 hours and at this level, bubbles could occur during processing.