VIII.  PELLETIZING

Issue:  While it is possible to manufacture products directly from reclaimed, post-consumer, clean HDPE regrind, most end product manufacturers require that the regrind be converted by extrusion into pellets. Pellets yield a more homogenous, less contaminated feed stream for manufacturing. Proper selection of extrusion equipment is important for optimal throughput, consistent material properties, and production of high-quality post-consumer HDPE pellet feedstock that can be  competitive with virgin resin in manufacturing applications.

Best Practices Summary

·        Materials should be thoroughly dried and mixed prior to entering the extruder feed system

·        Reduce moisture content to between 2000 and 5000 ppm

·        Use an auger feed between the hopper and the extruder to provide a consistent flow of material into the extruder and prevent bridging
·        Control temperature and shear rate, the two most critical factors in preventing material degradation
·        Use extruders with extruder length to diameter (L:D) ratios in the 28:1 to 34:1 range to impart lower shear, good mixing and adequate degassing of moisture and volatiles
·        Systems using melt filtration should maintain pressures within the extruder barrel below 4000 psi to avoid material degradation and prevent screen “blowout”
·        Reclaimed HDPE resin generally needs a greater flight depth than virgin resin to lower shear rate and prevent material degradation
·        Timely screen change out is needed to avoid pressure increases in the extruder

Background 

Recycled pellets containing post-consumer HDPE regrind are formed by an extruder, which mechanically forces the HDPE through a heated cylindrical barrel by means of a screw.  The compression that occurs in the extruder barrel creates friction, which assists in melting the regrind.  The extruder mixes, devolatilizes, and filters the melted material to remove remaining contaminants.  The molten HDPE is then pushed, or extruded, through a die consisting of a series of small holes in a steel plate, and is cut to form pellets.

Types Of Pelletizers

There are two basic types of pelletizers that are commonly used when pelletizing post-consumer HDPE regrind.  The first are known as “hot face” pelletizers, and the second are known as “cold-cutting” systems.

Hot Face Pelletizers

Hot face pelletizers are probably the most common, and are considered the simplest to operate.  The molten extruded material is forced through holes in a circular die.  An attached blade at the discharge end cuts pellets to a specific size.  There are two basic types of hot face pelletizers used to process post-consumer HDPE regrind into pellet: air and underwater.

Air Pelletizers

In air pelletizers, air circulates through the cutting chamber to begin initial cooling of the pellets that are then conveyed to fluidized bed dryers for further cooling.  Alternatively, the cut pellets are discharged directly into a water bath and later dried in fluidized beds or centrifugal dryers.  These types of machines can produce pellets at rates up to 10,000 pounds per hour.

Underwater Pelletizers

Underwater pelletizers use a cutting blade located under a stream of water where the extruded material is discharged. The use of underwater pelletizers can reduce operating noise levels and prolong cutting blade life.  In addition, underwater pelletizers require less horsepower to operate and occupy less floor space than air systems. Recent developments in underwater pelletizing technology have led to machines that are simpler to operate than earlier systems. Today’s underwater pelletizing systems can produce pellets at rates up to 50,000 pounds per hour.

Cold Cutting Systems

Cold cutting systems include dicers and strand pelletizers.  Both differ from hot face systems in that the pellets are cut after the plastic material has been extruded into a continuous strand, air or water cooled, and then dried.  However, cutting extruded resin in solid form increases noise levels and reduces cutting blade life.  In addition, these systems generally require more floor space than hot face systems.

Extruder Design And Selection Considerations

The most important design considerations for extruders are screw design, screw diameter, flight depth, and screw length to diameter ratio (L/D).  All of these factors will affect the throughput rate of the extruder.  Throughput can range from a few pounds to as much as 25 tons/hour.  Additional extruder features may include such items as wear-resistant screws, bimetallic barrels, aluminum heaters, zone heater burnout indicators, easy access covers for barrel and heaters, water-cooled feed hoppers, swing-gate die changers, digital instrumentation and solid state control systems. 

The choice of extruder will depend upon budget considerations, space availability, required throughput and production rates, and the specific properties of the reclaimed HDPE or blend being processed.  What follows are brief descriptions of these design features and how they impact pellet production.

Screw Design

The extrusion screw is an important component in pelletizing extruders.  The screw is a shaft with helical flights that rotates within a cylindrical housing called a barrel. The purpose of the screw is to mechanically melt, mix and advance the material being processed to the die plate for extrusion into pellet. Screw design is important for proper materials mixing.  Optimizing screw design for use with reclaimed HDPE resins enables manufacturers to obtain consistent processing characteristics and help maximize desired material properties in the finished pellet.

Screw design is important for proper materials mixing.  Optimizing screw design for use with reclaimed HDPE resins enables manufacturers to obtain consistent processing characteristics and help maximize desired material properties in the finished pellet.

Post-consumer HDPE reclaimers and converters generally use a screw design that is composed of three distinct sections in which the flight depth of the screw varies.  Each section of the screw provides a specific function.  The feed section has a large flight depth to begin the process of melting and mixing, and to ensure proper forward movement of the melt into the next section.  The transition section gradually decreases the flight depth to increase compaction, melting and mixing of the flake material.  Finally, the metering section further reduces the flight depth of the shaft to create additional compression and finalize the melting process.   The melted material is then pumped forward through filtration screens and extruded through the die plate.

Single-screw extruder units are the most common for converting clean HDPE regrind into pellet.  Twin-screw extruders are also available, but are specialized equipment used primarily when loading high levels (>20%) of fillers or reinforcements.  They are far more costly and have higher maintenance requirements than single screw units.

Screw Diameter

Screw diameters generally range from 1 to 8 inches, and are sized according to desired production throughput rates.  Extruders with a larger diameter are capable of higher throughput rates.

Extruder Length To Diameter (L/D) Ratio

The extruder screw L/D ratio can range from about 6:1 to 48:1.  The higher ratio indicates a longer extrusion barrel.  The L/D ratio affects the mixing shear on and the residence time of the HDPE material in the extruder.  Most HDPE reclaimers and converters use extruders in the 28:1 to 34:1 L/D range to impart lower shear, good mixing, and adequate degassing of moisture or volatiles. 

Since recycled HDPE resin has already had one or more heat histories, it is generally recommended not to exceed a 32:1 to 34:1 L/D, which implies longer residence time in the extruder.  The longer the residence time, the longer the material is exposed to melting temperatures that can degrade the molten HDPE.  Excessive exposure to high temperatures can break down the HDPE molecular structure, causing variations in its properties, particularly melt index.

Flight Depth 

Flight depth is measured as the perpendicular distance from the tip of the flute to the core of the screw.  The lower the bulk density of the feed stream, the deeper the flight depth required to maintain consistent throughput levels.  Flight depth affects the shear rate that material is exposed to and they have an inversely proportional relationship. Reclaimed HDPE resin generally requires a greater flight depth to lower shear rate and prevent material degradation.  Greater flight depth also allows a larger volume of materials to be processed, increasing throughput.

Feed Systems

Feed systems for extruders vary, but most use rectangular, gravity-feed hoppers which discharge clean dry HDPE regrind from directly above the opening to the extruder screw.  However, material with excessive moisture can build up on the sides of gravity feed hoppers causing bridging and discontinuous feed of material to the extruder.  To prevent this, some systems use an auger feed between the hopper and the extruder to provide a consistent flow of material into the extruder, which is necessary for efficient operation and to prevent possible damage to equipment. There are also feed systems available that pre-heat and densify HDPE regrind or blends prior to entering the extruder.  Such systems can help reduce the L/D ratio required for pelletizing, thereby reducing residence time and material exposure to elevated temperatures, reducing material degradation.

Venting

While all facilities should follow the relevant federal state and local regulations, atmospheric venting is usually adequate to remove moisture, gasses, and volatiles from clean HDPE regrind during pelletizing.  Gasses and volatiles usually originate from product residues not sufficiently removed during cleaning.  More sophisticated venting systems, such as closed-loop, vacuum systems are available, but are far more costly and generally not necessary for clean HDPE regrind that meets purchaser moisture and residue level specifications.  However, inadequate melting and mixing of materials in the extruder may cause material to clog vents, preventing release of vapors, resulting in possible loss of material through the vent or bubble formation in the pellet.  Bubble formation in pellets can result in inferior end products.

Melt Filtration

Melt filtration removes unmelted particulate contaminants at the end of the extruder barrel, just before the melt is extruded through the die plate and cut into pellets.  The melt is passed through a series of screens, which must be replaced as contaminants build up on the screen.  Timely screen change out is required to avoid pressure increases in the extruder.  When clogged screens raise the melt pressure in the extruder, it decreases production throughput, and increases the melt temperature and shear on the melt material.  Increased temperature and shear degrade the HDPE molecular structure and can result in a change of the resin’s melt index at the die face, leading to defective product.

Optimal Processing Parameters

Several factors must be considered to optimize processing throughput and material quality of the finished pellet. Establishment of optimal processing parameters depends upon the material characteristics of the reclaimed HDPE resin to be pelletized.  These characteristics include temperature and pressure sensitivity, melt index and polymer density, bulk density, and levels and types of contamination.

Moisture Content

The first best practice regarding moisture, is that materials should be thoroughly dried and mixed prior to entering the extruder feed system, particularly if blended mixtures are being pelletized.  Improperly mixed materials may disrupt the homogeneity of the finished pellet.  The second best practice is to reduce moisture content between 2000 and 5000 ppm, as materials that contain moisture above these levels can inhibit proper venting and lead to inferior quality products in end-use manufacturing.

Temperature And Pressure Sensitivity

When pelletizing post-consumer HDPE regrind, controlling temperature and shear rate are two important factors in preventing degradation of the post-consumer HDPE resin or blend.  Since recycled resin already has one or more heat histories, it is more prone to thermal degradation.  Therefore, controlling melt temperature is critical to pellet quality. 

The melt index of the recycled resin or blend of resins must be considered when determining temperature profiles.  Recycled blends with a lower MI than virgin blends may decrease throughput and require increased mixing to obtain consistent mechanical properties in the finished pellet.  Extrusion temperatures are generally in the range of 375-390 ^oF.  Temperatures outside this range can adversely affect the melt index or homogeneity of the finished pellet, resulting in inferior quality end products.

Temperature and pressure profiles for extruders are fairly standard, but the profiles may require iterative experimentation to find the optimal settings for a given material or blend.  This is especially true for blends containing materials with varying polymer densities.  It is recommended that systems using melt filtration maintain pressures within the extruder barrel below 4,000 psi to avoid material degradation and prevent screen “blowout.”

Shear Rate

Material degradation can also occur when maximum shear rates are exceeded.  Shear rate is defined as the “surface velocity at the barrel wall divided by the flight depth.”  All plastics have maximum allowable processing shear rates.  Reclaimed HDPE resins are more heat sensitive than virgin HDPE and have lower permissible shear rates. Shear rate is inversely proportional to flight depth, therefore, reclaimed HDPE resin generally requires a greater flight depth to lower shear rate and prevent material degradation.  Greater flight depth also allows a larger volume of materials to be processed, increasing throughput.

Bulk Density

Bulk density, related to regrind or blend size, impacts throughput and feed efficiency.  As the flake size of HDPE regrind decreases, bulk density increases.  Flake sizes of 3/8" to 1/2" are common in the HDPE recycling industry.  Larger flake sizes feed less efficiently, decreasing extrusion throughput.

Meeting Recycled Pellet Market Specification

As with other forms of recycled HDPE, specifications for reclaimed HDPE pellet vary between pigmented and natural HDPE.  As a general rule, natural HDPE pellets can be used in higher-valued applications and typically have more stringent specifications.  This is particularly true for melt index, polymer density, particulate contaminants, polypropylene (PP) and non-bottle grades of HDPE resin.

End-users making thick-walled extruded profiles may allow for higher levels of certain contaminants because the manufacturing process is less demanding than other types of product applications.  In addition, end-users may require compounded pellet products that have incorporated colorants or other additives, based on the end-product properties and requirements.  Specifications for compounded pellet products are established by mutual agreement between buyer and seller, based on specific product applications.

The sample specifications for pelletized reclaimed HDPE attached as Appendix D assume that all other quality and processing Best Practices presented in this document have been followed.  The allowable levels of the contaminants listed in the specifications an example of types of contamination that may be permitted.  By omission, any other contaminants are =typically unacceptable at any level.

Finally, most reclaimed HDPE pellet purchasers require certification that each shipment of pellet conforms to specifications in terms of post-consumer “pedigree,” melt index, density, color, odor, and contamination.  While industry-accepted test methods for certain properties are listed in the specifications, the sampling procedures and testing requirements for certification are generally established by mutual agreement between seller and buyer.

The following general best practices are recommended for preparing reclaimed HDPE pellet for shipment to converters or end-users:

Packaging: Boxes

·        Package and ship pellets in clean, corrugated “gaylord” boxes placed on pallets
·        Use boxes and pallets of sufficient quality to maintain their integrity throughout loading, shipping, unloading and storage
·        Use new plastic liners in boxes to protect pellet from contamination
·        Strap boxes to pallets
·        Mark or label boxes of pellet with weight information (gross, tare and net weights)
·        Mark or label boxes to trace conditions of manufacture, including information to identify the processing equipment used, the operator, the date produced, and any other available quality data

Packaging: Bulk (Truck/Railcar)

·        Clean and seal bulk trailers from contamination and moisture
·        Clean and adequately seal railcars from contamination and moisture by using plastic caps and metal seals on bottom hatches, and clean “shower” caps and metal seals on top hatches