HDPE (High Density PolyEthylene) is a polyethylene plastic commonly used in injection molding applications because of its high tensile strength, impact resistance, and melt flow index, not to mention its relatively low cost.

HDPE is just one type of polyethylene, and is usually defined as being polyethylene with a density of 0.941 grams per centimeter cubed or greater. Under the umbrella of HDPE, there are several different grades of product. In most instances, the grade of the HDPE is determined by its MFI, or melt flow index. This is a measurement that describes the viscosity of the HDPE when melted, and it is a crucial element where injection molding applications are concerned.

Injection molding HDPE must have a high MFI, as injection molding involves squirting HDPE resin into a die through many holes, filling it within seconds, and setting it quickly. Slow flowing HDPE would be disastrous in an injection molding application and would undoubtedly result in malformed and misshapen products.

Injection molding is commonly used to produce items which have unusual shapes, such as children’s toys. Due to its translucent color, virgin HDPE can be colored brightly, which adds to the appeal of using HDPE in applications where aesthetics are important. HDPE injection molding can be carried out to a recommended minimum thickness of about .020 inches. Anything thinner than 0.010 inches is likely to crack and otherwise degrade too quickly to be commercially viable.

Recycled HDPE can also be used in injection molding applications, though it may not possess the translucence of virgin HDPE. Recycled HDPE is usually obtained from post consumer and post industrial sources, and therefore has been exposed to many contaminants and colorants during its life. In the case of high quality recycled HDPE pellets and granules, contamination is a negligible factor, but coloration can be an issue. It is common for manufacturers to add a black or other dark dye to recycled HDPE to overcome this problem, and recycled HDPE is considered to be safe for all injection molding applications with the exception of any products designed for food contact applications.

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HDPE is a relatively low cost high, strength plastic which is used in a wide range of applications, one set of these being blow molding applications. HDPE is an acronym which stands for High Density PolyEthylene, a type of polyethylene which is defined as having a density equal to or greater than 0.941 grams per cubed centimeter.

Blow molding grade HDPE is a type of HDPE which possesses specific physical properties which make it suited to blow molding applications, the main one of these being a low melt flow index. Melt flow index is a measurement used to describe how fast the plastic flows when melted. Blow molding grade HDPE has a very low MFI, and flows quite slowly when melted. This makes it ideal for blow molding applications where the resin must be thick, flow slowly, and withstand the force of the compressed air which blows it up to the shape of the mold without forming areas of weakness.

Blow molding is commonly used to make bottles, tanks, and other receptacles. HDPE is an excellent material for these types of applications as it lends itself not only to the manufacturing process, but the end products have good tensile strength, high impact resistance, and good chemical resistance as well. When virgin blow molding grade HDPE is used in the production of products, the result is strong, translucent white containers and vessels which can also be colored or otherwise appropriately labeled. Because HDPE is not very reactive, HDPE vessels are popular for the storage of some chemicals and are quite commonly found in science labs.

Recycled HDPE is quite often used in blow molding applications, especially in the making of tanks and other such items where appearance is not key. Recycled HDPE is often colored, and it is common practice to melt recycled HDPE pellets, granules, or regrind down and simply add a dark color, such as black to cover whatever shade the recycled regrind may have been. In some instances it is possible to obtain translucent recycled HDPE granules or pellets, which lend themselves to the creation of translucent products, or being colored brightly, however where appearance is important, virgin blow molding grade HDPE comes most highly recommended.

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Off Grade HDPE is normally produced when a production plant is switching from making one grade of HDPE to another. If a plant is making injection grade HDPE, and must switch to pipe grade HDPE, there is likely to be at least one batch which does not meet the specifications for any grade. The HDPE produced in these batches is known as Off Grade HDPE.

Off Grade HDPE is still of a high quality, and structurally sound, though it may not suit the rigors of high stress applications. Manufacturers who are producing products that do not need to perform under extreme conditions may find that Off Grade HDPE may be an excellent way of lowering their material costs, whilst still producing a quality product.

Off Grade HDPE pellets are made in exactly the same way as other forms of HDPE Pellet. If the Off Grade HDPE pellets are being made from virgin HDPE (virgin HDPE being the term for freshly manufactured HDPE), then the freshly polymerized HDPE, which is a liquid, is extruded through a die, cut into pellets by a sharp blade, and then cooled.

Reprocessed Off Grade HDPE Pellets

It is also possible to obtain reprocessed off grade HDPE pellets. Reprocessed off grade HDPE pellets are pellets formed from scrap HDPE which is also off grade. At this point, it is worth noting that the term “off grade” does not denote lower quality. When purchasing reprocessed HDPE, care should be taken to ensure that contamination levels fall within an acceptable range for your application. Scrap HDPE is generally exposed to a wide range of contaminants during its life time, including dirt, metals, rocks, labels, and other plastics. During reprocessing every effort is made to ensure that these contaminants are washed away, and then later on filtered out of the HDPE, however no contamination removal process is 100%, which is why reprocessed HDPE is not approved for food contact applications.

Reprocessed HDPE may also be weakened structurally due to undergoing several heat cycles. It is possible to heat and reheat HDPE many times over, however, with each heating it becomes more likely that the molecular structure of the HDPE will begin to break down, as the molecular bonds within the HDPE weaken. This weakening is generally not significant for most applications, however, and reprocessed HDPE is used with confidence by thousands of manufacturers world wide.

Another factor to take into consideration is that reprocessed off grade HDPE pellets are also likely to be colored, due to label dyes and colored HDPE that was included in the scrap HDPE stream during processing. For this reason, reprocessed off grade HDPE is generally better for applications where appearance is not of huge importance. To save costs, some manufacturers use reprocessed off grade HDPE for inner layers of a product, and then put a layer of virgin reprocessed HDPE on the outside. This creates a product that is low cost to produce, but is still visually appealing to the consumer.

Why Off Grade HDPE Pellets?

If you choose reprocessed off grade HDPE, you may have the choice of using off grade HDPE pellets, or using even lower cost shredded HDPE scrap. For most manufacturing applications, off grade HDPE pellets are recommended because by being in pellet form, the HDPE forms a homogeneous HDPE supply to your production line when it is melted down. HDPE pellets are formed from shredded HDPE scrap that is melted down, mixed thoroughly, and then filtered to remove as much contamination as possible. This means that when you melt down HDPE pellets, you are left with liquid HDPE that has a consistent quality throughout the mass.

Shredded HDPE is not filtered in the same way that HDPE pellets are, and when shredded HDPE is melted, the manufacturer is left with a product which contains some high quality areas, and other low quality areas within the liquid mass. This then passes through the production line, creating some products that meet standards, and others which do not. The manufacturer must then spend time identifying and removing the defective products, and is also risking defective products being missed and sent out to consumers, something that can result in costly recalls. Generally speaking, off grade HDPE pellets are the best sources of off grade HDPE when all costs are taken into account.

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HDPE pipes have replaced concrete, steel, and iron pipes in many cases due to the fact that HDPE is lighter and more chemically resistant than these more traditional materials. Pipe Grade HDPE pellets create pipes with a minimum tensile strength of 4550 psi, and many blends of pipe grade HDPE pellets have even greater strength. HDPE also possesses a flexibility often referred to as the “trampoline effect”, this property of HDPE allows it to change shape when under pressure, changing perhaps from a circular pipe shape to a more oval one, without breaking or cracking. This is highly important for applications where some weight may be exerted on pipes, for example, in underwater or underground applications.

As has been indicated, HDPE’s strength does not only fall in the physical camp, HDPE is also resistant to many chemicals, including most acids and bases. HDPE retains its integrity when exposed to solvents, oils, ketones, and aldehydes. HDPE maintains its integrity when exposed to PH 1.5 < pH < 14, a range which makes it suitable for a wide range of applications.

Metal and concrete are also infamous for having problems with temperature variances. Unlike these substances which can burst or crack in extreme temperatures, HDPE can withstand most normal temperature ranges, demonstrating a range of -148 degrees Fahrenheit (-100 degrees Celsius), to 248 degrees Fahrenheit (120 degrees Celsius).

Pipe grade HDPE pellets are made from pipe grade HDPE, which comes in both virgin and reprocessed form. Reprocessed HDPE is quite often used in pipes because it is a low cost material that does the job that more expensive virgin HDPE. Because reprocessed HDPE is often discolored from the dyes that have seeped into the scrap HDPE from labels, and other colored pieces of HDPE, it is not favored for applications that require a pretty end result. Pipes for underground applications, for instance, rely mainly on physical strength and chemical resistance, appearance is not of great importance, so reprocessed pipe grade HDPE pellets make excellent raw material for these applications.

Virgin pipe grade HDPE pellets are made from virgin HDPE, which is freshly created HDPE. They have several advantages over reprocessed pipe grade HDPE pellets, though these advantages may or may not be relevant to specific applications. Because they have not undergone previous heat cycles, which weaken the molecular bonds, virgin pipe grade HDPE pellets may be a little stronger than reprocessed pellets, though if one purchases high quality reprocessed pellets, the differences should be negligible.

Virgin HDPE also has a nicer color than reprocessed HDPE, being a translucent white, however if appearance is that important, one can always apply a coating or special outer layer to reprocessed HDPE pipes to improve their appearance.

Virgin HDPE has also never been exposed to any form of contamination, unlike reprocessed HDPE which is exposed to a wide range of contaminants during its life. Most of these contaminants are filtered from the HDPE however, and their effect is negligible at best in most instances.

If you are searching for a high quality raw material for HDPE pipes, purchasing HDPE pipe grade pellets from a reputable seller is the surest way to get the best value for your money. Be sure to familiarize yourself with the specifications of your project before contacting us to ensure that the pipe grade HDPE pellets you purchase meet your specific requirements.

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This is more true with film grade HDPE pellets than most other grades. Whist blow molding applications require a very viscous HDPE product to be used, and injection molding applications require a very thin, runny HDPE product, film applications represent the middle ground. Like Goldilocks, most film applications require a mid range product to work best, but it is also possible to use other grades successfully, depending on the specific application.

In some cases, HDPE film has even been created from bottle grade HDPE (generally known as blow molding grade HDPE). It is relatively easy to increase the melt flow index of HDPE (and therefore make it more runny) with the use of additives, or by blending LDPE or other plastics into the feedstock.

Film grade HDPE pellets are made in much the same fashion as other HDPE pellets. They are either created from scratch by the catalytic reaction of ethylene (in which case they are known as virgin HDPE pellets), or they are created from recycled scrap HDPE. In the former instance, the pellets are made by taking the freshly created liquid HDPE and extruding it through a die, then cutting it into shape and allowing it to cool into its solid form. In the latter instance, scrap HDPE is washed, melted, filtered, and then extruded and cut to length in much the same fashion.

The choice between virgin film grade HDPE pellets and scrap film grade HDPE pellets will largely be influenced by the intended use of the finished product. Virgin HDPE is the only type of HDPE that can be used in food packaging, and has a nice translucent color that takes dyes well, whereas scrap HDPE is more environmentally sound and cheaper, but is generally less structurally sound, more contaminated, and is likely to be tainted with colors from the various dyed forms of HDPE which are recycled.

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Viscosity of HDPE is determined by its molecular structure, which is determined during the polymerization process by which HDPE is created. HDPE stands for High Density Polyethylene, which is created from ethylene gas. Ethylene gas is extracted from naphtha, and then processed in a catalytic reaction which bonds the ethylene molecules together. In HDPE the ethylene molecules form long straight chains which are tightly packed together. Lower density polyethylene contains branching molecule chains which make the polyethylene less dense.

HDPE has a molecular weight of between 100,000 and 500,000. As you can see, there is quite a variance of possible molecular weights even in high density polyethylene, and correspondingly, there is a fair degree of variance in the melt flow index.

When HDPE is produced via catalytic reaction, it emerges as a white liquid ooze, which is then extruded through a die and cut to shape before it cools to a solid at room temperature. This is how injection molding grade virgin HDPE pellets are made. These are the most pure and highest quality injection molding grade HDPE pellets available.

Injection Molding grade HDPE pellets can be made from virgin HDPE, created as outlined above, or from scrap HDPE. It will make no difference to the MFI whether virgin HDPE is used, or if scrap HDPE is the base material, but it may make some difference to the quality and appearance of the product. Virgin HDPE pellets have a lovely translucent quality and when melted down, color can be added to produce bright shades. Virgin HDPE must also be used if the product manufactured is to fall into the category of food packaging, as only virgin HDPE products are approved for food contact purposes.

Injection molding grade scrap HDPE pellets on the other hand may contain contaminants resulting from their previous uses. Scrap HDPE is carefully treated to remove most contaminants, but it is impossible to produce completely clean scrap HDPE pellets, which is why this form of HDPE is not approved for food packaging. Injection molding grade scrap HDPE pellets may also be colored, due to labels and colored HDPE being included in the HDPE stream during recycling. Obviously this will be of concern to manufacturers who must be concerned with the appearance of their product.

When purchasing injection molding grade scrap HDPE pellets it is important to keep two things in mind. One, that the quality of scrap HDPE pellets will vary from supplier to supplier. Reprocessing scrap HDPE can be a delicate business, especially as there is potential for producting unsound material if the temperature is too high, and a capacity for contaminants to sneak through if cleaning sorting and filtering processes are not carried out with enough efficiency. Always chose a reputable dealer when purchasing injection molding grade HDPE pellets, as a bad feed stock can destroy an entire production run.

HDPE Pellets

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Blow molding grade HDPE pellets are produced through an extrusion process, where melted blow molding grade HDPE is pressed through a steel plate with holes cut in it (known as a die), and a cutting blade at the end of it which slices the HDPE into the right length. Blow molding grade HDPE pellets are an excellent source of homogeneous blow molding grade HDPE, and are easily transported due to the density with which they can be packed.

How Are Blow Molding Grade HDPE Pellets Graded?

The grading system is mostly based on a measure known as the Melt Flow Index, or MFI. The MFI simply refers to how viscous the HDPE is when melted. Some forms of HDPE are very runny, and flow quickly. These forms have a high MFI. Other forms of HDPE are very thick when they are melted, and flow much more slowly. These forms of HDPE have a low MFI.

Blow Molding Grade HDPE is HDPE with a relatively low melt flow index. This means that when melted, the HDPE is fairly viscous, flows slowly, and is resistant to the forces of gravity. This makes it perfect for blow molding applications where HDPE is blown up into shape inside a mold through the forces of compressed air. It is essential that if one is undertaking blow molding applications, that one obtains a source of blow molding grade HDPE, as other grades of HDPE are likely to simply flow to the bottom of the mold and produce deformed or structurally flawed products.

Sources Of Blow Molding Grade HDPE Pellets

Blow molding grade HDPE pellets can be manufactured from virgin HDPE, or it can be produced from reprocessed HDPE. The source makes little difference when it comes to the melt flow index and the resulting grade of the HDPE. The source may make a considerable difference to the quality of the HDPE however. If you require a very clean and pure source of HDPE, to be used in toys or food packaging applications, for instance, then virgin blow molding grade HDPE is the better option. Virgin HDPE is the only form of HDPE allowed in food contact applications, and it also has the added advantage of being nice and translucent, which makes for nice translucent products, or if you would like to add color, very brightly colored products. Reprocessed HDPE is more suitable for the manufacturing of products that are hidden, or black products. It is also worth mentioning that reprocessed HDPE, whilst having been very carefully screened, cleaned and filtered, is still slightly contaminated and less strong than pure virgin HDPE. For many manufacturing applications, the slight deviances are immaterial however, and reprocessed blow molding grade HDPE pellets can be used with confidence.

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The HDPE emerges from the polymerization process as a very hot liquid, however, as it begins to cool, it quickly begins to solidify. This is due to one of the properties that make HDPE so very useful, that property being the fact that HDPE is a solid at room temperature. In order to make the Virgin Grade HDPE useful for manufacturing applications, before it is cooled entirely, it is extruded through a die with many small holes in it, and cut to length in the form of small pellets. The resulting product? Virgin Grade HDPE Pellets.

HDPE is therefore much stronger than other forms of HDPE, has a high level of chemical resistance (it is fairly inert), and is also lightweight and flexible, as well as having a pleasing translucent quality that takes color well and produces beautiful products such as children’s toys, garden furniture, and food packaging, amongst other things.

Caveat

Most of the time “grade” is a term used to refer to the Melt Flow Index, and is generally expressed as a function of the operations that are most suitably undertaken with that particular type of HDPE. For example, Injection Molding Grade HDPE has a high melt flow index, which means that it is highly viscous and will flow quickly into molds.

The title “Virgin Grade HDPE” is therefore a bit of a misnomer, which may be confusing to some who lump it in with other grades, such as Blow Molding Grade. It is possible to have Virgin HDPE that is Blow Molding Grade, or Virgin HDPE that is Injection Molding Grade. Virgin HDPE is simply a term to describe new HDPE that has freshly been created from a polymerization process and has never been used in any manufacturing applications whatsoever. Recycled/reprocessed/scrap HDPE can never be correctly termed as being virgin grade HDPE.

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