HDPE pellets come in a wide range of grades. Injection molding grade HDPE pellets are pellets with a high MFI, or melt flow index. Melt flow index is a term relating to how viscous HDPE is when melted. HDPE used in injection molding must be very runny when melted, so that it can be injected quickly into the mold.
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.