TIPS FOR EFFICIENT RECYCLING: HDPE materials #3

A clear consequence of difficulties with your screen changer is the reduced throughput of the extruder. Bearing in mind, in fact, that the screen changer is a “bottleneck” which the melt must flow through.

Here’s why FIMIC manufactures 4 different melt filter types and each in different sizes, because it’s important to be able to adapt the filtration surface of the melt filter based not only on the material being treated, but also the required level of filtration.

In the case of HDPE, FIMIC has succeeded in reaching very high throughput rates (up to 3 tonnes) on a single screen, with stable (low) pressures and temperatures corresponding to that of the extruder. But why are all these things so important in a melt filter?

Let’s consider temperature, for example. If the filtration surface of the screen changer is too small, the operator will need to increase the temperature of the extruder and/or filter to make the material more fluid and therefore reduce the operating pressure on the mesh screen.

This causes, however, the degradation of the plastic polymer. This can clearly be seen in the case of white or clear HDPE, which turns yellow at the end of the line!

It is therefore extremely important to use a melt filter with a large filtering surface in order to maintain a low operating pressure and temperature suited to the acceptable operational temperature of the melt.

If you have using a twin-screw extruder, you’ve been (or you will be) forcedly using a gear pump to allow the extruder to work with pressures higher than 100 bar.

Given that twin screw extruders are mainly intended for the mixing of compounds, often they are unable to manage pressures (generally) higher than 100 bar.

This often means the return of the melt to the extruder degassing unit in the event this pressure limit is exceeded.

And a melt filter, whether manual or automatic, works starting from 100 bar! Especially in the case of HDPE, where the stiffness of the material makes this a necessity.

However, in order to protect the gear pump against contaminations, it is generally necessary to install a protection screen changer with 400/500-micron filtration.

In this case, recyclers normally install a manual screen changer before the gear pump, which they change approximately every half hour, and then an automatic screen changer after the pump.

In case of single-screw extruders, the pump isn’t needed instead, and the screen changer is therefore installed directly without a pre-filter.

Many recyclers, whether using single- or twin-screw extruders to recycle HDPE, have adopted the FIMIC automatic technology, using laser filtration.

This screen is not mesh but rather a laser-drilled steel disc, which guarantees distinctly superior filtration with respect to mesh thanks to a circular rather than a square or rectangular hole. 

The rectangular holes used in mesh weaving technology, due to the pressure itself of the material flowing through, in fact tend to widen slightly and may allow various contaminants to pass through, even bigger in size than the filtration in use.

In case, instead, of circular holes drilled on a thick steel structure, such issue is not taking place.

FIMIC, therefore, in addition to easing the operator’s workload and guaranteeing lower power consumptions thanks to low operational pressures with respect to a manual filter, also guarantees a superior quality of the final pellet, at a melt temperature that doesn’t degrade the material while naturally maintaining the maximum hourly production rate of the extruder.

Of course, a FIMIC melt filter can be installed on any extruder type and used for many different types of materials, not just in the case of HDPE.