HDPE comes in several different grades, which delineate the purposes that the HDPE is best suited for.
The grade of HDPE should not be confused with its quality. It is entirely possible to have the choice of high quality virgin HDPE that is designated as being injection molding grade, or a low quality contaminated scrap HDPE source that is also injection molding grade.
The ‘grade’ of HDPE actually refers to a measurement known as Melt Flow Index, or MFI. The MFI is simply a measure of how fast or slow a HDPE product flows when melted, how viscous it is. MFI is inversely related to the molecular density of the HDPE. The more dense the HDPE, the lower the melt flow index, ie, the slower it flows when melted.
For injection molding applications, a high MFI is required, as it is essential that the HDPE flows quickly into the mold and fills every nook and cranny. In blow molding applications, a thicker, slower, and more gravity resistant HDPE product is required in order that the HDPE will retain its shape and a uniform thickness during the blow molding process.
Meet The HDPE Grades
There are three main grades of HDPE, listed in order of molecular density from highest to lowest, and therefore also in reverse order of melt flow index, from lowest to highest.
Blow Molding Grade HDPE
In blow molding, HDPE flows into a mold and is then blown up into shape with compressed air, not unlike a ballon. As the HDPE expands, it conforms to the shape of the mold. When the HDPE has finished expanding, and has cooled it is ejected as the finished product. This method is generally used for bottles and other products where strength is needed.
Film Grade HDPE
HDPE film is often made by melted HDPE being blown through a mold with compressed air, rising into a column of film. The film is then flattened and rolled up, ready to be processed into things such as carrier bags for groceries and the like.
Injection Molding Grade HDPE
In injection molding, HDPE flows into a cooled die through many small holes, quickly filling up the mold before cooling and being ejected as the finished product. Injection molding is suitable for a wide range of applications, including children’s toys and other products of irregular shape.
How Useful Are Grades?
It is worth noting that there are very few set standards when it comes to HDPE gradings. There is no absolute authority on the subject, and whilst grade may give an indication of suitability for various manufacturing applications, it is certainly not the only element that should be given attention when purchasing HDPE in virgin or scrap form.
It is also worth noting that grade is not a static value. Blow molding grade HDPE is not destined to forever remain blow molding grade HDPE. Through a process of blending HDPE with other grades, or even with LDPE, or perhaps other plastics or agents, the grade of HDPE can be changed. There really is no hard and fast rule when it comes to grades, although they are useful as a rule of thumb. The best way to go about purchasing HDPE is to be aware of the MFI you require, and seek a product that best approximates it.
What cannot be changed as easily as the grade is the quality of the HDPE. You might have the perfect grade HDPE for your application, but there is little point in having a nicely flowing HDPE source for say, injection molding, if the resulting products are structurally unsound due to contaminants in the HDPE product. This is especially of concern for manufacturers using scrap HDPE, HDPE regrind, HDPE flake, or recycled HDPE pellets or granules. Scrap products always contain a risk of contamination, so the source of the product should be weighed carefully before large orders are made and an entire production line put at risk.