HDPE Injection Grade
HDPE is an excellent material for a wide range of injection molding applications. Due to its chemical resistance and high strength, high quality products can be produced from HDPE using various types of injection molding.
To understand where injection grade HDPE comes from, and how to obtain the best quality HDPE for your purposes, some understanding of HDPE is necessary.
HDPE can come from one of two sources, and is either freshly made virgin HDPE, or recycled from scrap HDPE. HDPE can then be separated into various grades, largely differentiated by their molecular weight, or their melt flow index. Molecular weight refers to the density of the molecules in a compound, and how heavy they are, and melt flow index refers to the viscosity of HDPE, ie, how fast or slow it flows.
These two factors are inversely related, and HDPE with a higher molecular weight will therefore possess a lower melt flow index. Injection grade HDPE has a higher melt flow index and lower molecular weight than, say, the type of HDPE used for milk bottles, which is blow molded. A high melt flow index means that the HDPE flows more quickly and has a thinner consistency than other forms of HDPE.
As the majority of scrap HDPE comes from milk bottles, this can potentially present a problem for the manufacturer who wants to use scrap HDPE as a source for injection molding applications. Fortunately, melt flow index is not a static property, and with the correct blending, blow molding grade HDPE can be transformed into injection grade HDPE.
It is useful to know the methods by which blow molding grade HDPE is transformed into injection grade HDPE, as some of the additives may cause problems for certain manufacturing applications. In some cases, polypropylene is added to HDPE to lower the molecular weight, and in others, certain amounts of LDPE may be blended with HDPE to create a more viscous product.
Injection mold grade HDPE is suitable for the production of products such as children’s toys, wide mouthed containers, and any product that has an intricate shape. The quickly flowing nature of injection grade HDPE allows the HDPE to not only be easily molded into a wide range of shapes, but it is also able to be molded very quickly indeed, which saves the manufacturer money when it comes to labor costs, and allows more items to be produced with greater speed than other methods.
HDPE Injection Molding Parameters
HDPE is suitable for injection molding, generally to a minimum thickness of between .010 and .040 inches. .020 inches is considered to be the absolute minimum thickness that should be attempted. Thinner product walls are likely to result in faults and flaws, including buckling and cracks.
HDPE has a density of .941 to .959. It has a linear molecular structure, and it is highly crystalline. If this sort of language seems too technical, it is not important. What is necessary to be known is that these characteristics amount to a high strength product that can withstand higher temperatures than lower density polyethylene, are harder, and have a greater chemical resistance.
Injection Molding Process
The injection molding process is very simple. Raw material HDPE is generally purchased in refined pellet or granule form, and is then melted prior to injection molding. The actual injection molding involves the process of injecting the molten HDPE into a die which is permeated with many small holes, which allow the HDPE to flow in quickly. The die is then cooled, and the newly molded product is ejected from the die, and the process begins anew. This method allows for the quick and efficient production of a wide range of products from bottles, to garden furniture.
Virgin HDPE, Or Scrap HDPE?
If you are looking to produce products that are brightly colored, it is recommended that you purchase virgin injection grade HDPE. Recycled injection grade HDPE is, generally speaking, just as sound as virgin HDPE, but it is generally a murky color due to the various colors of HDPE blended in the recycling process. Recycled injection grade HDPE is recommended for applications where the products produced are to be black in color, or hidden away out of sight. There are however sources of scrap HDPE where scrap is sorted by colors. One can for example get HDPE made of pure white milk bottles scrap, or clear milk bottles scrap, etc. In that case HDPE made of scrap would be equal in all characteristics to Virgin HDPE.
An ideal mix for many manufacturers is a pellet or granule containing low amounts of recycled HDPE, and mostly made up of virgin HDPE. Recycled HDPE lowers the overall cost of the raw material, and brings an elevated tensile strength to the product, whilst the virgin HDPE ensures that the correct melt flow index is achieved without the blending of foreign plastics such as polypropylene.
If you are going to use recycled HDPE for injection molding applications, it is imperative that you obtain your injection grade HDPE from a reputable purveyor. There can be a large variance in quality between suppliers even when dealing with more simple scrap HDPE products, and due to the added blending that is required to create reliable injection grade HDPE, there is even more potential room for error. An inferior raw material will invariably translate to an inferior end product, so reputable and experienced purveyors are worth their weight in gold.