The Ultimate Guide to Polyethylene Injection Molding

Whether its squeeze bottles in your kitchen or the bulletproof vests shielding your law enforcement officers, polyethylene (PE) is everywhere. Successful polyethylene injection molding is more than the simple melting of a plastic. It has a somewhat more complex set of technical challenges involving temperature, control, shrinkage, and more.

This guide is oriented toward product design, engineering, and procurement, with a focus on the streamlining of manufacturing processes. We will examine the primary distinctions between LDPE and HDPE, the most pertinent material characteristics, and the advantages and disadvantages, as well as the essential, often unrecognized, applications that sustain the world around us.

What is Polyethylene (PE) Injection Molding?

As its name suggests, polyethylene molding is the process of creating a part out of polyethylene and involves the conversion of a raw plastic material, or plastic resin, into a usable part. During this process, polyethylene pellets are fed into a barrel, which is then heated, and a reciprocating screw that mixes the plastic until it is in a molten state. The molten material is then injected into a mold cavity under high pressure, where it cools and solidifies.

While the fundamental engineering and mechanics of the injection molding process are largely the same across the different types of plastics used, each material has its own specific characteristics that shape each molding process. For polyethylene, the choice is highly a question of LDPE and HDPE, the two titans of the polyethylene industry.

LDPE (Low-Density Polyethylene)

LDPE is a manifestation of a high degree of molecular branching in its structure. Think of a tree with many limbs (branching), which prevents the molecules from packing together. This results in a material that flexible, tough, and even transparent.

Due to its lower density and weaker intermolecular force, tensile strength is also weaker when compared to its high density counterpart. LDPE, however, does well when it comes to ductility. It can stretch withouth breaking. This is why it is one of the first choices when it comes to being the go to part for applications that need flexibility, like a lids that snap on, or tubing that is flexible. It is also very exellcent in flexiblility.

HDPE (High-Density Polyethylene)

HDPE is on the other side. It also has a molecular structure that is linear, with a little amount of branching. This also causes the polymer chains to pack to closely and effeciently to that of a stck of lumber. Because of the dense structure of the chains, its also dense overall. It’s rigid, opaque, and has a high strength to density packed ratio.

Because of its stifffness and increased strength, it is also the workhorse of the PE family. It is also suitable for structures that require overall strengths. If it is that you need to crate an extra thick tool set or a chemical tank that need to resist bulging making HDPE is the best option.

Critical Properties of Plastic Polyethylene

To truly master the Injection Molding of Polyethylene, you must understand the material science behind the pellets. The behavior of PE during processing and its performance in the field are governed by several critical properties.

Density

Density is perhaps the most defining characteristic of polyethylene. It directly affects the weight of the final part and its buoyancy.

• HDPE:Typically has a density greater than 0.941 g/cm³. This higher density contributes to its rigidity and barrier properties.
• LDPE:Generally falls below 0.940 g/cm³. This lower density correlates with flexibility and clarity.

Understanding density is crucial for cost estimation (since material is bought by weight but used by volume) and for applications where weight savings are critical.

Tensile Strength

Tensile Strength is defined as the amount of “pull” the materials can handle. Due to the added tightly bonded crystals structures formed inside of it, an HDPE is much more dense and stronger than LDPE and is much better for components/handles that are carrying heavy loads, such as crates and industrial components. LDPE, while weaker, offer elasticity that allows it to stretch and absorb rather than snap under other behind it.

Flexural Strength

Flexural Strength is another word for the stiffness of the material and its resistance to power deformation (change of shape). Here, HDPE is much better than its counterpart, with high and consistent stiffness enabling its parts to keep their shape, even when the parts are filled with thick and heavy liquids or when the parts are stacked on one another. On the opposite end, LDPE is used for squeezable bottles precisely because of its low flexural strength. You want the material to easily squeeze and deform, and be able to return to its shape once the squeezing pressure is removed.

Coefficient of Thermal Expansion (CTE)

Every injection molder must recognize the coefficient of thermal expansion, or (CTE). Polyethylene has one of the most (if not the most) high CTE rates when its heated and cooled which means it expands and so does the plastic. This expansion is not so critical. What is of concern is when the plastic is being cooled in the mold. If mismanaged, expansion and contraction can lead to parts which are warped or have their dimensions out of tolerance.

Moisture Absorption

One of the distinct advantages of polyethylene is its extremely low moisture absorption. Unlike materials such as Nylon or Polycarbonate, which are hygroscopic and absorb water from the air, PE is hydrophobic.

• Processing Benefit:You generally do not need to pre-dry PE pellets before molding, saving energy and time.
• Performance Benefit:Parts made from PE will not swell or degrade in wet environments, making them perfect for marine applications or outdoor use.

Chemical Resistance

Polyethylene is very resistant to many chemicals. Most strong acids, alcohol, bases, and esters are no match for polyethylene. This resistance is why PE is often seen in the construction of containers for bleach and gasoline. However, strong oxidizing agents and some types of hydrocarbons can still affect polyethylene and so there are always exceptions to be considered in choosing polyethylene.