A comparison: LDPE vs HDPE

Despite both materials deriving from polyethylene, differing chemical structures mean Low-Density Polyethylene (LDPE) and High-Density Polyethylene (HDPE) have their own unique characteristics and properties.

When you’re deciding which thermoplastic might be best for your injection molded part, it’s key that you understand the key differences between the two materials.

To help make your decision-making process hassle free, we’ve put together all the information you need to know about LDPE, HDPE and which materials are best for which parts.

What is LDPE?

One of the first types of polyethylene to be created, LDPE is produced by putting ethylene gas under high pressure in an autoclave or tubular reactor. This forces the molecules of the monomer to link together into polymer chains. The fact that it’s formed from a gas means that the polymer chains of LDPE branch off from each other. This creates an irregular molecular structure (known as an amorphous structure) which gives LDPE its unique properties.

The characteristics of LDPE

As its name suggests, LDPE is of a lower density than other types of polyethylene. This is thanks to its irregular structure, which leaves plenty of space between molecules and makes LDPE lightweight, transparent and flexible.

This spacious structure also means that the chemical bonds between molecules are relatively weak, this means that LDPE has a relatively low melting point at 115C and is less durable than other types of polyethylene. Despite this, compared to other types of plastic, LDPE is fairly resistant to impact, moisture and other chemicals.

How is HDPE produced?

High-Density Polyethylene (HDPE) is primarily produced through a process known as catalytic polymerization of ethylene gas, often using either the Ziegler-Natta catalyst or metallocene catalysts. This process typically occurs at low temperatures (20–80°C) and low pressures (10–80 bar), which contributes to HDPE’s highly crystalline structure. These conditions allow the polymer chains to arrange tightly, resulting in a dense and strong material with minimal branching. HDPE can be manufactured using methods such as slurry polymerization, gas-phase polymerization, or solution-phase polymerization, each impacting its properties slightly. The resulting polymer is then typically extruded, molded, or blow-molded into products. Due to its high tensile strength and rigidity, HDPE is widely used in applications like piping, containers, and plastic bottles.

How is LDPE produced?

Low-Density Polyethylene (LDPE) is produced through a process known as high-pressure polymerization, typically carried out at pressures of 1,000 to 3,000 atmospheres and temperatures between 80–300°C. This high-pressure approach often involves free radical polymerization, where ethylene molecules react in the presence of initiators, such as oxygen or peroxides, to form LDPE. The polymerization process generates a highly branched molecular structure, resulting in a lower-density material with flexibility and transparency. LDPE’s unique branching allows it to be softer and more pliable compared to HDPE, making it ideal for applications like plastic bags, film wrap, and squeeze bottles. The polymerized resin is then cooled, extruded, and shaped into the desired product through various molding or casting methods.

What is HDPE?

To produce HDPE, petroleum is heated to very high temperatures in a process known as ‘cracking’. This turns the petroleum into a gas, these gas molecules then join together to create a raw form of HDPE, this raw material is molded and set into its usable form. This different manufacturing process gives HDPE a regular molecular structure (known as a crystalline structure) with closely connected polymer chains.

The characteristics of HDPE

A tight molecular structure gives HDPE a higher density than other types of polyethylene. The lack of space between molecules also means that the thermoplastic is highly durable, rigid and opaque.

Strong chemical bonds between molecules gives HDPE a higher melting point at 135C and is very strong. It also means that HDPE is inflexible compared to other types of polyethylene, as well as being very resistant to heat, moisture and other chemicals. In fact, HDPE is so resistant that it cannot join to other materials or other HDPE parts very easily.

Which material is best for your part?

Although their molecular structures mean HDPE and LDPE have some material differences, there are some common characteristics between the two, including:

• Lightweight compared to other thermoplastics.
• Good level of resistance to impact, chemicals, moisture and weather.
• Low manufacture and production costs.

These properties mean both LDPE and HDPE can be injected into molds which to make parts that can be used for a huge range of industrial purposes.

Protection parts

The chemical resistance of HDPE and LDPE make them a great option for protection parts, such as spiral wrap hose protectors which have both PP and HDPE parts. Able to resist other chemicals, moisture and weather conditions, HDPE is the material of choice for any parts that require rigidity and strength whereas LDPE is more suitable for protection that requires some flexibility.

Thanks to its semi-rigidity, HDPE is typically used for hydraulic application or pipe protection caps and plugs. Threaded O-ring plugs and protection caps are typically made of HDPE and provide excellent protection from the ingress of dirt, moisture or other foreign objects. Its strength also makes it an appropriate material for some push-to-fit plugs and caps.

However, the flexibility of LDPE makes it ideal for push-to-fit caps and plugs that are easy to apply and remove. Stretch fit cover caps, side release or compression fit plugs made of LDPE are the perfect solution for tricky forms of protection, particularly in pipes or flanges.

Both HDPE and LDPE are used to protect hoses and tubing thanks to their flexibility, impact and abrasion resistance.

Hardware parts

Although LDPE and HDPE are fairly resistant, they don’t provide as much durability as other plastics meaning they’re not often used as the main component of hardware parts. However, they can be used as a component part of feet, castors and gliders alongside nylon or rubber. The qualities of LDPE are ideal for tube inserts and glides, offering secure fitting and covering sharp edges.

Fasteners

Similarly to hardware parts, LDPE and HDPE are not as durable or strong as other plastics and therefore not commonly used in fasteners. The strength and high resistance of nylon means this plastic is more often used in washers, screws and nuts. It’s very rare to find LDPE or HDPE fasteners.

Which material is the most sustainable?

Although both LDPE and HDPE are recyclable, their most common uses mean they’re often contaminated with food and dirt. This means extra cleaning stages need to be included in the recycling process. However, recycling is the only way to sustainably dispose of these materials, as neither are biodegradable.

Just like other plastics, both LDPE and HDPE are made from oil, a fossil fuel which makes them fundamentally unsustainable. However, as the technology improves, more environmentally friendly alternatives to LDPE and HDPE will be developed.

During the past 12 months, thanks to considerable investment in injection molding machinery by Essentra Components, nearly all LDPE products are being manufactured with at least 40% recycled plastic content at its plant in Oxfordshire, UK. The new machinery can mix virgin plastics resins with post-consumer resin, attaining the same level of quality, integrity and durability.

This is a key part of Essentra Component’s sustainability strategy and its pledge to the Circular Plastics Alliance. The long-term aim is to achieve a minimum of 20% sustainable sourced material used in processing.

So, which is the best material for your injection moulded part?

Whatever industry you’re in, if you’re looking for protection or electronic parts, then LDPE or HDPE might be suitable options. Although both have similar fundamental properties, LDPE offers flexibility combined with durability whereas HDPE provides more rigidity and strength. Although LDPE or HDPE aren’t materials that are typically used for hardware parts or fasteners, they are suitable if you’re looking for protection or electronic parts.

If you’re still not sure whether LDPE or HDPE are the most suitable materials for your part, then our expert agents can help. With specialist training and an in-depth knowledge of our product catalogue, our team will be able to help you find the ideal solution and make your part decision making process truly hassle free.