HDPE and LDPE are both polyethylene, the world’s most produced plastic. Same monomer, same chemistry, yet one ends up as a rigid chemical tank and the other as a bread bag. The difference comes down to how the polymer chains are arranged, and it is worth understanding if you are choosing material for anything more demanding than packaging.
Branching: the whole story in one word
LDPE (low density polyethylene) is made under very high pressure, and the process leaves the polymer chains heavily branched. Branched chains cannot pack together tightly, so the material ends up less dense, less crystalline, softer and more flexible.
HDPE (high density polyethylene) is made with catalysts at low pressure, producing long, essentially linear chains. Linear chains pack tightly, which means higher density, higher crystallinity, and a material that is stiffer, harder and stronger.
The density gap looks small on paper, roughly 0.91 to 0.925 g/cm³ for LDPE against 0.94 to 0.965 g/cm³ for HDPE, but the difference in crystallinity roughly doubles the tensile strength and dramatically raises stiffness and temperature tolerance.
Side by side
| Property | LDPE | HDPE |
|---|---|---|
| Resin code | 4 | 2 |
| Density | 0.91 to 0.925 g/cm³ | 0.94 to 0.965 g/cm³ |
| Chain structure | Highly branched | Linear, tightly packed |
| Feel | Soft, flexible, waxy | Rigid, hard wearing |
| Tensile strength | Lower | Roughly double LDPE |
| Maximum service temperature | About 60 to 65°C | About 80°C, higher intermittently |
| Chemical resistance | Good | Excellent |
| Typical products | Films, bags, squeeze bottles, cable insulation | Sheets, pipes, tanks, drums, liners, chopping boards |
| Fabrication use | Rarely | The standard polyethylene for fabrication |
Those resin codes matter beyond the spec sheet. Code 2 is one of the most widely recovered plastics in Australia, and we cover what that means in practice in Is HDPE recyclable?
Where you meet each one
LDPE is everywhere in flexible applications: cling wrap, bin liners, squeeze bottles, protective film, irrigation tube. Its virtues are flexibility, toughness in thin sections and low sealing temperature, which is exactly what packaging wants.
HDPE is the engineering side of the family. It arrives in our workshop as certified sheet and becomes tanks, bunds, hoppers, ducting, pond and planter liners, cutting boards and process equipment. The stiffness carries structural loads, the chemical resistance handles acids, caustics and salts, and the linear structure welds reliably, which is what makes proper HDPE fabrication possible.
There are middle siblings too. LLDPE (linear low density) toughens films and is the resin behind most rotomoulded poly tanks, and MDPE sits in gas pipe territory. That matters commercially: rotomoulded tanks are the main alternative buyers weigh against fabricated HDPE, and the two are not interchangeable. A moulded LLDPE tank comes in the shapes, sizes and wall thicknesses the mould dictates, while welded HDPE sheet lets the tank be engineered to the duty: calculated wall thickness, custom geometry, certified virgin resin and documented welds. At the far end of the same family sits UHMWPE, with chains so long it stops behaving like ordinary polyethylene altogether.
Which one should you specify?
For anything fabricated, structural, chemical bearing or load bearing, the answer is HDPE, and in practice the real choice is not HDPE vs LDPE but which HDPE grade: PE100 for pressure duties, food grade for processing plants, UV stabilised black for outdoor service. Specify LDPE when the part must flex constantly or seal at low temperature, which in industrial settings usually means films, membranes and packaging rather than equipment.
Unsure which polyethylene, or which grade, suits the job? Send us the duty: contents, concentration, temperature and location. We have fabricated polyethylene equipment in Brisbane since 1971, and material selection advice costs nothing.
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