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What causes paper curl in laser printers?

What do we mean by paper curl? It’s when paper exits the printer and instead of lying flat, it has a curl to it.

What causes paper curl?

This is a common question we get when the weather starts to heat up and humidity rises. Paper curl is caused by excess humidity in the paper. When paper passes through the fuser, the combination of heat and pressure required to bond toner to the paper also causes curling because of excess moisture in the paper. All laser printer are susceptible to this. You may be thinking, that’s ridiculous, my paper is dry, and it works fine in other printers. It can’t be the paper. Yes it is. I’ve had this conversation way too many times. It’s always the paper. Some printers may be more prone to this, especially fast laser printers. Fast printers need to run paper through the fuser quickly and with enough heat and pressure to fuse toner. The variables that differ between laser printers would be: fuser temperature, fuser pressure, print speed, and paper path. These can all cause a printer to have more or less curl.

Here is what HP suggests along with a few of our suggestions to solve problems with paper curl:

  • Change the fuser mode (fuser temperature). Many HP Laserjets have the ability to lower the fuser temperature. Consult your user’s manual.
  • Store paper in a drier environment. Paper reams generally have a moisture-proof barrier on the inside of the wrapper. Keep the paper sealed until you need it.
  • Buy paper from a different vendor or a different manufacturer.
  • Print out of a different output bin on the printer, for example a side, top, or rear output bin.
  • Flip the paper stack over in the tray or turn 180 degrees.
  • Use long-grain paper. You may be using paper with a short grain construction, or the grain might be in the wrong direction.

Some people have made the observation that paper seems hotter when it comes out curled. This is because the paper has more moisture in it. Water does a pretty good job retaining heat; it has a high heat capacity. Heat dissipates more quickly in paper with a lower moisture content. So even though you may be thinking this is a problem of the fuser getting too hot, it most likely is not. Fusers have controlled operating temperatures and if the thermistors in the fuser (the temperature sensors) read too hot the printer gives a fuser error. If the thermistor fails, then a thermoswitch opens up and shuts down the fuser preventing it from getting too hot.