Determining Polyethylene Specific Gravity Thresholds for Tariff Entry
Polyethylene tariff entry requires density testing per ISO 1183-1 Method A on annealed plaques, where crossing 0.940 g/cm³ shifts classification from 3901.10 to 3901.20.
Medium Density Polyethylene
Meaning
Thermoplastic resin grades represent polymers characterized by a specific density range that determines their mechanical and thermal performance. The material known as medium density polyethylene is defined by a density of 0.926 to 0.940 grams per cubic centimeter. This material balances the flexibility of low density resins with the strength of high density resins.
Molecular Structure
Chain branching is controlled during polymerization to achieve the target density. For medium density polyethylene, the level of comonomer incorporation is kept at a moderate level. This produces a polymer with fewer short chain branches than low density polyethylene but more than high density polyethylene.
The molecular structure results in a material with good shock resistance and stress crack resistance, making it ideal for pressure pipes.
Molding Performance
Processing parameters must be optimized to accommodate the material’s crystallization rate. When injection molding medium density polyethylene, moulders experience moderate shrinkage and warp. The material is often used for rotational molding of large tanks or extrusion of gas pipes.
This is because the resin has good melt strength and does not sag during processing. In addition, the material is highly resistant to chemical attack, extending the life of the pipes.
Sourcing Cost
Sourcing decisions depend on the cost of the resin and the required part performance. While medium density polyethylene is more expensive than standard low density polyethylene, it provides superior resistance to cracking and impact. This saves money in the long run by reducing part failures.
Moulders must weigh this cost difference when selecting the resin grade for a specific application.