It happens that factory operators sometimes notice that the pressure drop in the conveying pipe is steadily increasing over time leading to a stoppage of the conveying line. When the pipe is opened, an accumulation of material is often found, which it is necessary to remove. This kind of pipe blockage is very different than a sudden stoppage of the line, often due to a surge in the quantity of product in the pipe, since it happens progressively. This product accumulation is said to be a build-up,
Actually, there can be many different root causes leading to a build-up of product in the pipe. The most common causes are to have a very cohesive material or a sticky material. Under the impact on the pipe, especially at bends, the powder is getting compacted - in the case of a cohesive material - or adhere - for a sticky one. Some materials may also change properties just at the impact, due to the energy involved.
Build-up will most probably happen at bends, as particles will all impact in a reduced area, although some powders can also cause issues in straight pipe sections.
Note that the conditions of temperature and humidity in the pipe may make sticky a material that is not causing problem in ambient conditions.
Some sources as well report issues created by very fine particles < 10 microns. These particles are indeed so fine that they can settle close to the pipe where there is a very fine layout of laminar fluid flow. One they enter this area, they adhere to the pipe wall and initiate the build-up. 
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As seen, some materials are by their properties causing build-up, thus build-up may not be totally avoided but controlled through different measures :
- Convey at the minimum possible speed : as build-up happen at bends and is often related to the strength of the impact, reducing the conveying air velocity should have an impact
- Change the pipe bends radius : too sharp bend radius may be change for long radius pipe, it should however be noted that long pipe radius may be detrimental in other way for some materials (angel hairs for polymers pellets for example)
- Use flexible pipes : having flexible pipes allow to break the build-up and avoids that the accumulation grow to the pointg of blocking the pipe. It is not always possible to use this method in some industries.
- Review the conveying conditions : adapting air temperature and humidity may be a solution for some materials
- Convey dense phase : conveying dense allows to reach very slow air conveying velocity, thus avoiding impact and build-up.
As mentionned above, some other phenomena may be involved in the build-up, one could be static electricity which accumulates fines.
 Powder Bulk Engineering, Ask the Expert, Managing fine material build-up in conveying line, 2017