Tips for efficient recycling: HDPE materials

HDPE, High Density Polyethylene, is a widely used material for different applications.

We are all familiar with the HDPE used to produce pipes, but HDPE is also used to produce detergent bottles or shampoo bottles, hence applications forming part of our everyday life. Few people know that in some cases, HDPE can also be used in film applications.

However, all these different types and origins of HDPE differ in terms of their recycling process.

And above all, depending on the stage of the recycling process, certain types of machines work better with HDPE while others are less suitable to recycling such material stream.
Let’s start for example with washing: in this case, the specific weight of HDPE regrinds is easier to wash compared to, for example, HDPE film, therefore different technologies need to be used.
For the extruder too, there are different opinions on what the best type or configuration is.While some Customers prefer to consider that for HDPE it is extremely important to use a twin screw extruder in order to improve the mixing and homogenization of the material, others prefer, instead, to go for a single screw extruder solution with agglomeration system and enhanced degassing.

And what about the melt filter? Should it be manual or automatic type?

It’s common knowledge when recycling especially post-consumer materials, that manual (slide plate type) melt filters have several limitations. Given that post-consumer materials undergo washing, the residual contaminations that need to be eliminated by the filter are very small in size, thus creating the need for an extremely fine mesh in order to ensure efficient filtration. A manual type melt filter implies only manual operations, essentially requiring an operator to stand by the extruder and change the screen each time it gets clogged by the contamination in the melt. Since HDPE regrinds are “easier” to wash, many Customers initially install manual screen changers to filter this plastic type, based on the initial theory that there’s only a small percentage of residual contamination, thus beginning their journey in the world of recycling with a manual screen changer.

So why, after a couple of years at most, do they change their mind and switch to an automatic type?

The contaminations may be lower than what can be found in other materials, but have you ever considered the percentage of waste and the operational costs?
If you can’t measure it – maybe because laboratories that specialise in this type of measurement can be not always around the corner — it can be efficiently calculated by measuring how often you need to change a mesh screen in your manual melt filter.
If the change interval is less than 20 minutes (on average, of course), in terms of operational costs it’s worth switching to an automatic filter; for several reasons. Financial reasons! First of all because your operator is naturally forced to spend most of their time at the filter rather than perhaps feeding the extruder with new material, mixing, monitoring degassing, verifying the hourly throughput rate, or doing general cleaning. Either way, the need for an operator represents a limitation in terms of the quality of the material, because human error is always a possibility and the return of non-compliant material represents a problem for all recyclers worldwide, in addition to damaging your company image.

The second point to be considered is the cost of the mesh screens.

In this case, we need to consider the tendency of certain countries to burn these meshes with the flame and then reuse them. This is considered standard practice, but in reality we need to take into account not only the environment and the damage caused by burning plastic in the open air, but also the fact that the mesh screens are made of woven metal wires. These metal wires allow filtration thanks to the barrier effect given by the woven structure. Heat causes these metal wires to loosen and open slightly. Which therefore entails an alteration of the final quality. In case the final application of the r-pellets is injection moulding this may not necessarily cause any evident alteration to the final product, but in the case of film or pipes extrusion, the difference is quite significant. This is why the calculation of spare parts required when using a manual screen changer needs to be realistic, without considering the possibility of reusing mesh screens, as this would mean not to achieve a high quality standard for the very near future, expectation expressed by all government and political authorities.
Therefore, looking at the best-case scenario, if we consider 1 Euro per mesh screen for a manual screen changer, we need to calculate (considering a changeover at best every 15 minutes), a cost of 4 Euros/hour for consumables alone. Across a 24-hour period of operation, this translates into 96 Euros per day for screen filters alone. Based on your monthly operating shifts, you can easily calculate the cost of the mesh screens per month, not to mention their disposal and the waste plastic created each time the screen needs to be changed, as well as the cost of the operator. And obviously you’re limited in the material you can buy… this is a necessary variable to consider when using a manual screen changer, to avoid the need for screen change in less than 20 minutes. When processing more contaminated materials, the screen might even need to be changed every 3 minutes, depending on the plastic you want to recycle and its contamination. 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.