|1. How to feed
bulk solids materials to an extruder ?
|2.Mixing of raw
feeding systems to an extrusion line
This page is focusing on how to design and operate the bulk solids feeders at the inlet of an extrusion process. The success of an extrusion process is indeed as much due to the design of the raw material dosing system as it is due to the subsequent screw profile of the extruder. Feeding the materials in an uncontrolled way will indeed lead to formulation problem in the extruder and a process difficult to control or even generating out of spec materials.
The supply of raw materials (powder or pellets) to an extruder must be metered. This can be done through one or several dosing systems, which are dosing the material either volumetrically, when the accuracy required is not too high, or gravimetrically (loss of weight) when the accuracy is critical. This operation can be done by different types of feeders such as screw conveyors or belt conveyors.
The material dosed is typically channelled to the inlet of the extruder thanks to a funnel. As gases (resulting from the compression happening in the extruder, or through some chemical reaction) are flowing back to the entrance of the extruder it is important to have a vent to offload the excess air / gas and avoid disturbing the dosing and flow of material to the machine.
Dosing of a single material is not very interesting (apart maybe to compact a defined powder) thus several ingredients must be dosed to an extruder in order to get a formulation meeting specific application needs for an industrialist (reinforced plastic compounds, reactive extrusion, enriched food recipes...).
A mixing step is thus often performed prior to the extrusion step in order to ensure a good homogeneity of the raw materials. A poor homogeneity will indeed lead to an inconsistent quality of the extrusion product as the extrusion has a limited back mixing ability. The mixing can be done either in batch, in which case the mixed product will be dosed to the extruder by a single feeder, or continuously in which case several feeders, dosing each an ingredients, are dosing the product to a continuous mixer, which discharges in turn continously to the extruder inlet. Of course, some more complex processes are possible where one of the ingredients is actually a preblend that was mixed ahead of dosing to the continuous mixer.
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One simple method to control the ratio of the raw materials to the extrusion line is to have a blending step ahead of the dosing to the extruder. This can be done with a conventional dry-mix system that will weight the different raw materials in the desired proportions, load them in a mixer (typically a ribbon blender, or a paddle mixer), mix for a certain duration to ensure the required homogeneity is reached, then discharge the blended materials to a hopper from where a volumetric or gravimetric dosing system will take the blended materials from and introduce them to the extruder inlet.
Figure 1 : Batch blending system and dosing to an extrusion line
This type of process has the advantage to be simple to control
both in terms of mixing, and in terms of dosing (single doser) to
the extrusion line. On the other hand it requires space, equipment
and could be limited in capacity.
The alternative is to avoid having a batch blender up in the process, but in the contrary include a continuous mixer right above the extruder. Continuous mixers are indeed very compact while able to reach high volumetric flows. The mixer is supplied in powder or granules by several feeders (typically 1 by raw materials) operated preferably in loss in weight (gravimetric feed).
This option is more compact and more adapted to extrusion processes reaching high capacity. It can however be more difficult to control as the accuracy of several feeders must be ensured.
Some alternatives can be considered :