Today, Polyethylene is one of the most sought-after thermoplastics in the injection molding industry. Polyethylene is a popular injection molding material in the manufacturing industry due to its affordability, machinability, and compatibility with other materials. 

It is applied to various industries and can be categorized into several subcategories. The sub-categorization of polyethylene plastic is based on its molecular structure and each of which demonstrates certain unique characteristics that make it suitable for use in particular applications. The most common types of polyethylene are

  • Low-density polyethylene (LDPE)
  • High-density polyethylene (HDPE)
  • Linear low-density polyethylene (LLDPE) 
  • Very-low-density polyethylene (VLDPE)
  • Ultra High-molecular-weight polyethylene (UHMWPE) 

Among the above 5 forms of polyethylene, the two most common kinds of PE are high-density polyethylene (HDPE) and low-density polyethylene (LDPE). In this article, we discuss everything you need to know about these two materials, so you can decide which type of polyethylene is best for your next project. 

Table of Contents

Low-density polyethylene (LDPE)

LDPE, abbreviated for low-density polyethylene, is a soft, flexible, and lightweight translucent plastic. 

Low-density Polyethylene is a chemically inert and semi-rigid polymer with low levels of crystallinity at about 50-60%. It is known to demonstrate excellent physical properties such as flexibility, chemical, and impact resistance. Additionally, LDPE is also stain-resistant, electrically insulating, and waterproof.

Chemical Structure of LDPE

LDPE is a branched version of polyethylene. It is produced through a single monomer, ethylene which makes it a homopolymer. 

The chemical structure of LDPE includes many short branches and is composed of 4,000-40,000 carbon atoms. Its molecules are more loosely packed than other variants of Polyethylenes.  It’s still durable and strong but it is less dense than other linear polyethylenes such as HDPE. You should note that LDPE has a higher degree of short and long-side-chain branching. 

The above image is a cross-sectional picture of a vacuum void formed at an area where the outer section of the plastic has a small support ridge. 

LDPE chemical structure

LDPE Properties

  • The molecules in LDPE are loosely packed and are characterized by a 50-60% crystallinity. As a result of its crystallinity, it is translucent in appearance, however, very thin foils of LDPE can be transparent.
  • LDPE is not very easy to bond. It is sufficiently malleable and elastic and is thus, able to undergo significant elongation before breakage.
  • Although LDPE is durable and impact resistant, it is susceptible to stress-cracking beyond a point.
  • It is easily flammable since it has a low oxygen barrier rate at 450 – 500 cc/100 in2/24 hr. You can understand more about the oxygen barrier/oxygen transmission rate here.
  • LDPE softens at a relatively low temperature of approximately 100°C / 212℉. 
  • It has a reduced heat resistance due to its low melting point. This gives it a significant improvement in heat sealing which makes LDPE easy to process. 
  • It also undergoes high thermal expansion and can be processed by various thermoplastic methods.
  • LDPE also exhibits puncture resistance between the temperatures -40℃ to 90℃ / 104℉ to 194℉.  
  • LDPE exhibits excellent electrical insulating properties.
  • LDPE is FDA-compliant and available in various food grades.
  • LDPE is a low-cost polymer with excellent processability. 
  • The presence of more branching in the polymer chains brings certain drawbacks such as low strength and stiffness. 

LDPE Mechanical Properties

Melting point 105 – 115°C / 221 – 239°F 135°C / 275°F
Tensile strength 1,400 psi / 9.65 MPa 4,000 psi / 27.47 MPa
Tensile elongation 500 psi / 3.44 MPa 600 psi / 4.13 MPa
Flexural modulus 30,000 psi / 206.84 Mpa 200,000 psi / 1378.59 MPa
Water Absorption rate 0.10% over 24-hour immersion 0.10% over 24-hour immersion
Density 0.910–0.940 g/cm3 / 910-940 Kg/m3 0.93 to 0.97 g/cm3 or 970 kg/m3
Coefficient of Friction 0.12 – 0.70 0.200 – 0.280

LDPE Chemical resistance

  • It exhibits great resistance to alcohols, dilute alkalis and acids. 
  • It has limited resistance to aliphatic and aromatic hydrocarbons, mineral oils, oxidizing agents, and halogenated hydrocarbons. 
  • It has a high gas permeability, particularly to carbon dioxide.  
  • It exhibits poor UV resistance.

LDPE Limitations

LDPE has various benefits, but it also has some drawbacks. We have listed some of the limitations below: 

  • LDPE and HDPE are both prone to stress cracking but LDPE is more susceptible than HDPE. 
  • It has poor UV resistance and is highly flammable. 
  • It is less heat resistant than HDPE, and highly permeable to gasses like carbon dioxide. 
  • LDPE is also highly flammable, greatly limiting its use in high-temperature applications.

LDPE Applications

LDPE is widely applied to manufacture packaging material for various industries. Some of the most popular applications include toys, grocery bags, plastic films or wraps, flexible packaging material, and food and beverage containers. 

Since its lightweight, LDPE is commonly used to manufacture light-packaging materials such as six-pack rings for canned beverages, waterproof carton lining, and plastic wraps.

Low-density polyethylene is also used to manufacture water pipes and hoses for the pipes and fittings industry due to its plasticity and low water absorption.

High-density polyethylene (HDPE)

HDPE, abbreviated for high-density polyethylene is also referred to as Polyethylene High-Density (PEHD), alkathene, polythene, and #2 plastic. It is a thermoplastic polymer made from petroleum and it offers greater rigidity and durability than LDPE. 

HDPE exhibits an outstanding tensile strength and is known for its large strength-to-density ratio. HDPE plastic also has a high-impact resistance and a relatively high melting point compared to other polymers.

It is also a highly adaptable plastic and can be applied to a variety of industries that ranges from pipes to storage bottles.

Chemical Structure of HDPE

HDPE has a linear structure as the branching degree is relatively low when compared to other categories of polyethylene. It’s a polymer made up of repeating units of ethylene monomer, and its chemical formula is (C2H4) n.

HDPE Chemical structure

Since its molecules are packed together tightly, HDPE is an extremely strong material with high tensile strength, rigidity, and impact resistance.

HDPE Properties

  • It has a higher tensile strength compared to other forms of polyethylene.
  • HDPE is a low-cost polymer with good processability.
  • It exhibits good low-temperature resistance.
  • Demonstrates excellent electrical insulating properties.
  • It has a very low water absorption.
  • It is available in translucent to opaque variations. 
  • HDPE displays excellent chemical resistance. It is resistant to various chemicals and can be UV-resistant. 
  • Along with its durability, HDPE is extremely versatile and easy to fabricate.
  • HDPE is recyclable and reusable. In addition, recycled HDPE is extremely versatile and weldable. The below diagram depicts the recycling codes of LDPE and HDPE:
recycling codes LDPE and HDPE

HDPE Mechanical Properties

Melting point 105 – 115°C / 221 – 239°F 135°C / 275°F
Tensile strength 1,400 psi / 9.65 MPa 4,000 psi / 27.47 MPa
Tensile elongation 500 psi / 3.44 MPa 600 psi / 4.13 MPa
Flexural modulus 30,000 psi / 206.84 Mpa 200,000 psi / 1378.59 MPa
Water Absorption rate 0.10% over 24-hour immersion 0.10% over 24-hour immersion
Density 0.910–0.940 g/cm3 / 910-940 Kg/m3 0.93 to 0.97 g/cm3 or 970 kg/m3
Coefficient of Friction 0.12 – 0.70 0.200 – 0.280

HDPE Chemical Resistance

  • HDPE exhibits excellent resistance to most solvents. 
  • It has very good resistance to alcohols, dilute acids, and alkalis. 
  • It demonstrates moderate resistance to oils and greases. 
  • HDPE has poor resistance to hydrocarbons (aliphatic, aromatic, halogenated).

HDPE Limitations

HDPE is susceptible to stress cracking under intense pressure and low-to-moderate heat resistance

  • It has a lower stiffness and poor UV resistance. 
  • It has a lower heat resistance compared to other polymers. 

HDPE Applications

HDPE has a linear structure, which makes it an ideal material for applications that require high-tensile strength such as vehicle fuel tanks, boat parts, pipes, and tubing. 

HDPE Applications
by unsplash.com

HDPE is malleable and due to its dimensional stability, it is often used in outdoor equipment such as playgrounds. 

HDPE is also applied in various industrial projects such as pipes, tubings, and chemical tanks. It is also applied to various consumer goods such as food and beverage bottles, and cutting boards. It is also used in applications that require a strong packaging material such as bottle caps, plastic milk bottles, drums, and bulk containers. 

HDPE and LDPE Comparison

HDPE and LDPE are the two most common polyethylenes that have similar properties with differing structures. HDPE has a linear structure and is opaque, while LDPE is a relatively transparent, branched version of PE. 

Both materials demonstrate excellent strength, weldability, impact, and chemical resistance.

LDPE Versus HDPE Material Properties

Although both, LDPE and HDPE are derived from the same ethylene monomer, they demonstrate a wide variety of unique properties due to the difference in their chemical structure. 

As the name suggests, low-density polyethylene (LDPE) has a lower density, strength, and temperature resistance whereas high-density polyethylene (HDPE) is characterized by higher specific strength and heat resistance.

The table below depicts the various differences between LDPE and HDPE: 


The linear structure of LDPE has more branches whereas that of HDPE is packed together more tightly which is the reason for its impressive material characteristics. 

LDPE versus HDPE linear structure

Benefits and Limitations

Both materials have various benefits but they also have some limitations. The table below depicts the benefits and limitations of using LDPE and HDPE:


Production and Processing of LDPE and HDPE

LDPE and HDPE can be processed through injection molding and additive manufacturing techniques however; they are both best suited for different techniques. LDPE is best suited for injection molding and HDPE is best-suited for CNC machining as it helps to achieve tight tolerances. 

It’s imperative for the product teams to research and determine the best-suited material for your upcoming project. A trusted manufacturing partner can guide you to make the most optimum decision for your project.

LDPE Production

LDPE is produced at a high pressure of 1000-3000 bar at 80-300°C via a free radical polymerization process. 

LDPE is manufactured either through a stirred autoclave or a tubular reactor. Its manufacturing process includes ethylene gas compression, material polymerization using an initiator, and gas separation.

HDPE Production

HDPE can be produced either through slurry polymerization or gas-phase polymerization. The process starts with polymerization from a solution of ethylene monomers, followed by separation and drying. HDPE is made through a 3-step process which is listed below:

  1. HDPE is derived from petroleum through a linear polymeric chain reaction. The first step of HDPE manufacturing includes applying high heat to petroleum-based hydrocarbons which breaks the hydrocarbons into ethylene monomers.  (In the case of HDPE, ethylene monomers are collected in the form of gas)
  2. The ethylene monomers (in the form of gas) are mixed with metallic catalysts such as titanium tetrachloride or diethylaluminum chloride to allow the formation of polyethylene slurry.  
  3. In the next step, the HDPE is recovered by extraction, filtration, and drying by cleaning the slurry.  HDPE is further purified and processed for commercial purposes. 

Processing Solutions of LDPE and HDPE

Once LDPE and HDPE are produced, they can be processed through either of the following methods:

Injection Molding

Injection molding is an efficient manufacturing process that converts LDPE and HDPE resins into plastic parts that are defined by the mold. 

Plastic resins are sent to a hot barrel, in which the resins melt through a screw barrel and heater bands. The plastic resins melt here, which is further injected into a pre-designed mold. The material also cools down here, and after solidifying, the plastic material is ejected out of the molding machine.


Extrusion is similar to injection molding that is conducted by incorporating heat to melt the plastic granules. The only difference between extrusion and injection molding is at the end, where the melted resins go through a pre-designed opening and are then cooled down to solidify.

Blow Molding

Blow molding is often applied to manufacture hollow-shaped plastic goods. Instead of injecting the molten plastic, the process uses compressed air to blow the material into the mold.

Choosing Your Polyethylene Type

Understanding the difference between LDPE and HDPE is imperative as it can help you choose the best material for manufacturing your plastic part.  If you have any queries about LDPE, HDPE or any other type of polyethylene, you can reach out to us. VEM Tooling has the expertise for your PE injection molding project. Our team has an experience of over 20 years with manufacturing and mold making. We can surely help you to make an optimum decision for your project.

LDPE vs HDPE Frequently Asked Questions (FAQ)

The polymer chains of LDPE are highly branched whereas the polymer chains in HDPE are more linear. This branching in LDPE reduces the intermolecular forces of attraction and this is not the case with HDPE. . This results in a plastic that is softer and more flexible but also has lower tensile strength.

The molecules are tightly packed together during crystallization, making HDPE dense and possessing higher resilience than LDPE.

HDPE is an ethylene-based polymer plastic that is created by processing petroleum under extreme heat and pressure.

Yes, HDPE is appropriate for waterproofing projects.

OTR, abbreviated for oxygen transmission rate, is the steady-state rate at which oxygen gas permeates through a film at specified conditions of temperature and relative humidity. The values of OTR are expressed in cc/100 in2/24 hr in US standard units and cc/m2/24 hr in metric (or SI) units.