The bulk of scrap HDPE bought and sold comes in the form of HDPE Pellets. This article outlines what HDPE pellets are, how they are created, and provides insight into various aspects of the pelletization process that should arm the HDPE consumer with enough information to be able to make informed decisions when purchasing HDPE pellets.
To begin with the basics, HDPE pellets are created from HDPE regrind. HDPE regrind is the result of the HDPE recycling process whereby scrap HDPE is taken, washed, sorted, and shredded into a form that is suitable for further manufacturing applications.
Whilst regrind is able to be used for the production of new products, many manufacturers prefer to use HDPE pellets due to the fact that pellets provide a cleaner, continuous stream of HDPE, are generally of a higher quality than regrind HDPE, and unlike regrind HDPE which is simply shredded and cleaned scrap HDPE, and therefore varies from piece to piece in quality, size, and other factors, HDPE pellets are uniformly reliable and make for a homogeneous HDPE source which is important in many manufacturing processes.
The greatest advantage that HDPE pellets have over HDPE regrind however, is the additional filtering that the HDPE undergoes during the pelletization process. Essentially, HDPE pellets are the most refined form of scrap HDPE, and therefore the most reliable source of scrap HDPE from an end user perspective.
Creating HDPE Pellets
Once the HDPE regrind has been shredded and cleaned, it can either be sold as such, or be put through what is known as an extrusion process. This normally involves the HDPE regrind making its way through a heated barrel with the assistance of a screw, which carries it along and ensures uniform heating and melting. It is in the extrusion barrel that the HDPE is compressed, and further filtered of contaminants
When the melted HDPE reaches the end of the barrel it is forced out (extruded) through a die, which is a steel plate containing holes, and is then cut to form the HDPE pellets used in manufacturing.
Types of HDPE Pelletizers
There are many difference pelletizing systems, but most fall into one of two categories, either hot face pelletizers, or cold cutting pelletizers. Hot face pelletizers are generally preferred in most operations, as they run faster, quieter, and use less power than cold cutting systems.
Hot Face Pelletizing Systems
These are the most commonly used type of pelletizers, and work simply as outlined in the text above. The melted HDPE is carried through a heated chamber, filtered, extruded through a die, and is then cut to length by a blade. What happens next is the main difference between pelletizing systems.
Air Pelletizers
Much as the name suggests, air pelletizers use air to cool the cut HDPE pellets. Air is circulated through the cutting chamber, which causes the pellets to start cooling immediately, and they are then transported to fluidized bed dryers. In some air pelletizer systems, the HDPE pellets fall into a water bath, and are then dried in either a centrifugal dryer, where they are essentially spun dry, or on fluidized bed dryers. This is a relatively slow method of HDPE pellet production, but nevertheless these types of systems are still more than capable of producing over 5,000 kilograms of HDPE pellets every hour.
Underwater Pelletizers
Still a member of the hot face pelletization family, underwater pelletizers utilize a stream of water running over the cutting blade and the extrusion point. Underwater pelletizers are distinctly less noisy than air pelletizers, take up less floor space, require less power to run, and the machine blades last much longer. In addition to these advantages, underwater pelletizers are also capable of producing 25,000 kilograms of HDPE pellets every hour,.
Points Of Importance In The Production of HDPE Pellets
While HDPE pellets are an excellent source of scrap HDPE, there are several points in the pelletization process where things can go wrong. This is a brief guide to three common problems that can occur in the production of HDPE pellets.
Melt Filtration
The melt filtration stage of HDPE pellet production is invaluable as it is the primary step in producing a high grade HDPE pellet. If it were not for the melt filtration stage, much of the pelletization process would be merely cosmetic.
Melt filtration generally takes place towards the end of the extrusion barrel, before the melted HDPE is pressed through the die holes and cut into pellet form. It is here that several filters are placed in the form of screens that the HDPE must pass through. The care and maintenance of these screens is vital, and as contaminants build up on the screens they must be replaced regularly.
Screens that are not properly attended to can form a build up of contaminants, which naturally lowers the quality of the HDPE produced. Not only do clogged screens allow more contaminants through to the final product, they also put a strain on the entire extrusion system due to a build up of pressure in the extrusion barrel. The temperature inside the barrel rises, elevating the temperature of the melted HDPE past the point where it retains structural integrity. Though the HDPE pellets produced under these circumstances might look satisfactory, they are unsound, and products produced with them are likely to be unsound also.
Venting
Proper ventilation allows for gases and moisture to be released from the HDPE regrind as it undergoes processing. This is particularly important if the HDPE regrind being used has high moisture levels, or is not as clean as it should be. In addition to this, even good quality HDPE regrind can be problematic in cases where it is not well mixed and melted. If the HDPE regrind is not being well mixed and melted, it can clog the vents, preventing the escape of gases and moisture, and possibly leading to the formation of bubbles in the HDPE pellets. HDPE pellets containing gaseous bubbles produce much lower grade products, and should be avoided by reputable manufacturers concerned with the production of high quality goods.
Temperature Control
Although HDPE can be heated and reheated, and is a highly recyclable product, it is possible for the HDPE to become overheated during processing, which will lead to degradation of the material. Scrap HDPE has already undergone several heatings in its lifetime, and with each subsequent melt, it becomes more and more likely that structural instabilities on a molecular level will result. Thus it is of great importance that the melt temperature is rigorously controlled during the extrusion process.
The optimal temperature for extruding regrind HDPE is between 375 – 390 degrees Fahrenheit. This is a relatively narrow temperature range, but being exposed to higher or lower temperatures can either degrade the product, or make for inadequate mixing, which disturbs the uniformity of the HDPE pellets produced.
Conclusion
The production of HDPE pellets is rather a delicate business at times. As this article has shown, there is much more to the process than simply melting some regrind down and cutting it into pellet form. If produced by inexperienced or inept manufacturers purchasing HDPE pellets can be a worse investment than simply purchasing regrind HDPE. On the other hand, if produced by a skilled operator, HDPE pellets are the finest form of scrap HDPE that money can buy. It is therefore important to do your research and ensure that you are obtaining your HDPE pellets from a reputable, experienced source before proceeding with an investment that could make or break your product.
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