In plastics processing, quality problems are often caused by the presence of angel hair, impurities or dust. Angel hair can be recognized as very fine, hair-like plastic threads that can be formed during material transport. Dusts are fine solid particles of different sizes and can be made of plastic, but also of other materials. But what are the causes for the formation of angel hair and dust in the conveyor system and how can the resulting problems be avoided?
In today’s production environment, central silo systems are very common, resulting in comparatively long conveying distances for the plastic material (bulk material). When conveying granules, various challenges arise in which it is important to master them. In most situations, the bulk material is conveyed pneumatically. The material is conveyed in pipes or hoses by generating a gas flow (usually air) in this system. The bulk material is entrained by the resulting volume flow and transported in the pipe.
The volume flows and thus the conveying speeds can vary and are usually in the range of more than 20m/s for flight conveying.
These high speeds, with which the granulate grain is moved through the tube, can lead to high impulse forces or friction in case of contact of the granulate with a wall of the conveyor system. All those points in the conveyor system in which the granulate is deflected are therefore particularly critical. The friction forces arising here in turn lead to wear.
This can have the following effects:
- the granulate grain is subjected to so much stress that small particles break out or the whole pellet is broken up
- the forces acting on the pipe system are so great that the pipes are abrasively worn out
The two situations mentioned above then have further consequences. On the one hand, plastic dust (by breaking out small particles from the granulate) can develop. On the other hand, dust from other materials (by breaking out small particles from the material of the conveyor system) can also arise. Another problem that must also be mentioned is that dust may already be present in the granulate or bulk material, for example from upstream processes or even already contained in virgin material.
If the occurrence of very small plastic particles is accompanied by a thermal effect (increased temperatures, high frictional heating), the dust may melt, which, in combination with the high conveying speeds, allows this dust grain to pull a thread – the angel hair has formed.
How can the formation of dust and angel hair be avoided?
Various options are available to avoid the formation of dust. However, it must first be clarified where the dust comes from and in which process step it is produced:
1. Dust is produced in our own conveying system
a) Dust formation from the material of the bulk material (plastic granulate)
b) Dust formation from the material of the conveyor system
2. Dust is already present in the delivered material
Depending on where the dust comes from and what kind of dust it is, there are different ways to avoid it.
To 1a:
Provided that the dust is generated in your own system and the dusts are made of the same material as the bulk material itself, you can try to avoid the formation of dust. For this purpose it is necessary to keep the forces acting on the bulk material as low as possible.
Influencing the flow system
Due to the fact that the greatest frictional forces occur in the area of the deflection, special attention should be paid to these areas. Within the deflection, the material bounces against the arc as shown in the figure below.
Various geometric alternatives are offered to avoid the impact force and the associated friction. These include, for example, baffles or flow-optimised deflection geometries. The aim of these special deflection geomtries is to reduce the forces acting on the granulate grain and thus reduce the formation of dust and angel hair.
Influence of surface roughness
An increase in the roughness or waviness of the inner surface of the conveyor system can also make a positive contribution, as this makes the formation of a laminar flow more difficult and promotes the occurrence of turbulent flow.
Influenced by process parameters
If possible, the formation of angel hair can be achieved by lowering the temperature of the bulk material to be conveyed, as this prevents the material from softening/melting in the system.
An alternative variant that can also make a positive contribution to avoiding the formation of dust is lowering the conveying speeds to the lowest possible level.
Influencing the flow
Another way to avoid dust formation is to reduce the forces occurring by changing the conveying principle. While constant flows with air speeds of over 25m/s are used in flight conveying, there is also the possibility of using slowed down or pulsating air flows which can then lead to significantly lower flow speeds.
In this case we speak of slow or dense phase conveying, where only speeds of less than 10m/s occur. In this type of conveying, the material is conveyed through the pipe in a surge-like, pulsating manner, although comparatively high pressures are required for this purpose.
An equally interesting alternative is the use of a conveyor that uses a flow velocity range between slow and dense phase conveying. Air velocities of 10-25m/s occur and the material is moved out of a “hybrid flow” of dense phase and flight conveying. This compromise solution requires lower pressures than dense phase conveying, but also achieves higher flow rates.
(Note: Interesting animations about the different funding principles can be found in a Google search for the keyword: Strandphase Förderung on the websites of Pelletroneurope)
To 1b:
If the resulting dust comes from the material of the conveyor system, the use of alternative materials with a higher hardness is a possible solution. Glass arches or ceramic arches as well as hardened steels are used for this purpose, often. This avoids abrasion and thus also wear of the pipe bend. In addition, the use of the geometrically modified pipe bends shown above is also an alternative, as the forces occurring and thus also the abrasion on the deflection system can be reduced here.
To 2:
If the material is already contaminated, the only way to avoid causes is to contact the supplier in order to find a solution or to change the supplier.
However, if it is not possible to avoid the cause, there are also technical possibilities to remove the dust or angel hair later.
What can be done if the formation cannot be avoided?
If it is not possible to avoid the formation of dust and angel hair, further measures can be taken to clean the bulk material from dust and to remove the dust from the material. Different systems can be used to clean the material from dust.
The most common cleaning systems (dedusting devices) include:
- gravity classifier
- cyclone separator
- systems with induction coils
A functional description of the dedusting systems will follow in a later article.
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