Publish Time: 2026-04-21 Origin: https://www.bakwayplastic.com/
I’ve spent the last three days on a commercial roof in 35°C heat, inspecting a series of "oil canning" failures on a 5,000sqm warehouse skylight. The client was convinced the sheets were "warping" due to poor extrusion quality.
They weren't.
The failure was purely mechanical, born from a total ignorance of the Coefficient of Thermal Expansion (CTE) and the brutal reality of thermal cycling. If you are an architect or a contractor still using standard self-tapping screws and rigid washers on large-span PC sheets, you are essentially building a structural time bomb.
Here is the engineering reality of managing thermal movement in 2026, and why the "drill and bolt" method is officially dead for professional-grade installations.
Polycarbonate has a CTE of roughly 0.065 mm/m°C. That sounds small until you do the math on a 6-meter multiwall sheet. Between a cold winter night (-5°C) and a scorching summer afternoon where the sheet temperature hits 65°C, that panel is going to move nearly 28 millimeters in length.
If you have fixed that sheet with standard fasteners every 600mm, the material has nowhere to go. It buckles. This creates the "oil canning" effect—those ugly, structural waves that not only ruin the aesthetics but also stress the internal ribs of the multiwall structure. Over hundreds of thermal cycles, this constant flexing leads to fatigue failure at the fastener points.
Most installers use standard HSS drill bits. If the bit is slightly dull, it leaves microscopic "chatter marks" or jagged edges on the inside of the hole.
Polycarbonate is notoriously notch-sensitive. Under the stress of thermal expansion, those microscopic nicks become stress concentrators. The sheet "pulls" against the rigid fastener, and the notch propagates into a full-blown crack. By the time the first heavy wind load hits, the sheet has zero fracture toughness left. It shears off the screw head and you’ve got a 6-meter projectile heading for the parking lot.
At Suzhou Bakway, we tell our clients: Stop pining the sheets.
If you must use traditional fasteners, you need to over-bore the holes by at least 10mm and use wide-diameter EPDM-backed washers (never PVC—the plasticizer migration will cause ESC). But even then, you’re relying on the installer's "feel" for torque. One over-torqued screw and the thermal movement is locked.
This is why we’ve pivoted our production toward the U-Lock Standing Seam System.
By extruding a vertical flange onto our multiwall panels, we allow the sheets to be secured via a concealed sliding clip. The panel is never punctured. It "floats" within the aluminum or PC joiner profile. This decouples the sheet’s thermal expansion from the building’s sub-structure. You get a 100% leak-proof seal and zero "oil canning" because the 28mm of movement is absorbed by the sliding mechanism, not the polymer matrix.
When a sheet is constantly expanding and contracting, the length of the polymer chains determines its fatigue resistance. Cheap sheets made with "regrind" have shortened chains due to multiple heat cycles in the extruder. They might pass an initial impact test, but they fail the Thermal Fatigue test within three years.
We run our 5-layer Italian OMIPA co-extrusion lines using 100% virgin Covestro (Makrolon®) and SABIC (Lexan™) resin for a reason. Longer polymer chains mean the material can handle the "accordion effect" of 10+ years of seasonal shifts without developing micro-fissures.