There is no lint screen that catches ALL the dryer
lint. Some lint will always get exhausted through the dryer vent exhaust ducting along with the moisture from your clothing as it dries. If the exhaust vent remains unobstructed, all the air and lint will be blown out the end of the dryer vent exhaust.
However, if the air meets any resistance from kinks, excessive bends, or sags, moisture will build up inside the dryer vent exhaust causing the lint to stick to the interior walls of the vent hose. Over time this lint builds up and forms a clog. With a clog comes condensation and longer dry times. The dryer will continue to run inefficiently and cause the heating circuits to work harder and overheat. This will eventually lead to a failure of the heating circuits. If you are seeing condensation inside the dryer, it is strongly recommended the you inspect and clean the dryer exhaust vent hose. You should repeat this a couple of times a year to ensure it remains obstruction free.
The most frequent causes of dryer vent clogs comes from the following:
1. Ducting that runs in an upward direction in homes that have an attic exhaust. This is a stupid design that gravity will always win. When the dryer shuts offf, anything left in the vent will fall down the ducting to the base of the wall and accumulate. Over time, this forms a clog.
2. Ducting that runs under the home in a crawl space. If not correctly hung from the rafters, the ducting will develop sags causing choke points where lint can accumulate. Leaving it on the ground is not the answer, either. This gives opportunity for rodents to possibly chew through it. This will cause leaks which exhausts warm moist air under your home resulting in mold and mildew.
3. Using plastic dryer vent hose. This type of hose is not recommended because it kinks easily and can get crushed, causing an obstruction where lint can clog. Rodents can also chew through it easily. Pushing the dryer up against the wall and crushing the hose is a common cause. Use the semi-rigid metal type ducting that resists crushing, kinks and rodent infestation.
4. Rodents. Mice love lint. If given the access to it, they will build inside the dryer vent hose which provides a nice warm place to live with lots of bedding
material. Make sure you exhaust vent on the exterior of your home is about 12 inches from the ground.
5. Exhaust vent screens. I know there are many types of exhaust vents on the market that you can purchase that have screens on them to prevent birds and rodents from entering them. The screen can actually become and obstruction, though. The smaller the opening, the more resistance the blower fan meets and lint will clog at the end of the exhaust. If you chose to use a protected exhaust vent, the ones with louvers work better. You will still need to periodically check the ensure it does not become clogged.
6. Excessively long vent hose. The rule of thumb when it comes to dryer vent ducting is: The SHORTER and STRAIGHTER the run, the BETTER. Excessively long dryer vent hoses will clog due to the fact that the blower fan is not able to push all the air and lint all the way to the exhaust.
7. Kinked, Excessively Bent, or Crushed vent hoses. If the vent has any choke points due to kinks, bends, or gets crushed behind the dryer, you will develop ponts where the exhaust vent will clog.
The following link may also help in providing some basic guidance on how to install dryer vent hose:http://www.fixya.com/support/r389357-dryer_ducting_installation_tips
I know it may seem that I'm beating this point to death, but it is important to provide good air flow for your dryer. The number one cause of dryer failures and house fires comes from poorly maintained and poorly installed ventilation ducting.
If you have any questions, please let me know. I hope you find this information helpful.