The Different Types of Polyethylene Resins for Rotational Molding

Low Density Polyethylene (LDPE) is flexible and tough, easy to process, has excellent warp resistance and consistent shrinkage. Low Density Resins range from 0.925 g/cm³ and lower. LDPE resins are typically used in toys and consumer products that don’t require a lot of stiffness and / or load carrying ability. Disadvantages would be that it has both low stiffness and environmental stress crack resistance (ESCR).

Linear Low Density Polyethylene (LLDPE) or Medium Density Polyethylene (MDPE) resins have better mechanical properties than LDPE as well as higher stiffness, good warp resistance, excellent low temperature impact strength and excellent environmental stress crack resistance. Medium Density Resins range from 0.926 g/cm³ to 0.940 g/cm³. MDPE resins are typically used in recreational, consumer, or agricultural goods that require the higher temperature resistance or stiffness that low density resins do not provide. Disadvantages would be that these are less stiff than HDPE and have a lower temperature resistance.

High Density Polyethylene (HDPE) is the stiffest resin of the polyethylene family. HDPE has excellent

chemical resistance, higher heat deflection temperature and good processability and impact strength.

High Density Resins range from 0.941 g/cm³ to 0.959 g/cm³. HDPE resins are typically used in recreational and consumer products, bulk containers, marine products, and also agricultural and chemical storage tanks. Disadvantages would be that these have low ESCR and warpage/shrinkage is not as consistent.

Melt Index

In Rotational Molding, a resin needs to have very good flow when molten. With polyethylene, the flow is measured by the melt index. The higher the melt index, the better the flow. Most standard rotomolding resins have melt specifications ranging from 2g/10 minutes to 7g/10 minutes. The term “g/10 minutes” refers to the weight of molten resin moving through an orifice of a predetermined size in 10 minutes.

Density

Density is a measure of the specific gravity of a resin. The density of polyethylene is classified by types such as LDPE, MDPE and HDPE according to the American Society of Testing and Materials (ASTM). In addition to increasing the overall stiffness of a part, increasing density raises the melting point, permits higher operating temperature limits and improves barrier properties in the end product.

The goal of this resin information sheet is to better educate designers and manufacturers and provide them with the necessary information to enable them to choose the correct resin for their roto-molded products and applications.

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