Polyethylene (HDPE & LDPE) Injection Molding: A Buyer's Guide

Polyethylene (PE) is the highest-volume commodity thermoplastic globally. It dominates caps, containers, crates, and industrial packaging because it combines low resin cost with strong chemical resistance and cold-temperature toughness.

Writing “polyethylene” on a drawing is not enough. PE properties vary widely by density. HDPE versus LDPE sets stiffness, structural capacity, and processing behavior. Use this page with the material selection guide.

What Polyethylene Is

Polyethylene is a semi-crystalline thermoplastic. For injection molding, it is primarily categorized by density:

• HDPE (High-Density Polyethylene): Stiffer, stronger, and more chemically resistant due to a linear molecular structure.
• LDPE (Low-Density Polyethylene): Highly branched, resulting in a softer, more flexible, and compliant material.

Both grades are non-hygroscopic (no drying), lightweight (they float), and highly resistant to acids, bases, and everyday solvents. They also share PE’s limits: low tensile strength, poor heat resistance, high mold shrinkage, and a surface that rejects adhesives.

Why Buyers Choose PE

Characteristic	Engineering and Commercial Value
Very low cost	Among the lowest-priced resins per pound.
Chemical resistance	Withstands continuous exposure to aggressive industrial and household chemicals.
Cold toughness	Retains impact resistance at sub-zero temperatures where ABS and polycarbonate become brittle.
Low density	Reduces part weight and shipping costs.
Non-hygroscopic	Skips the drying step in molding, saving energy and process time.
Regulatory compliance	Readily available in FDA and USP Class VI grades for food and medical contact.

HDPE vs LDPE

The application defines the density requirement:

• HDPE is selected when the part must support a load or hold a rigid shape. Common uses include industrial crates, pails, structural housings, and threaded closures.
• LDPE is selected when the part must flex without cracking or provide a compliant seal. Common uses include flexible lids, snap-on caps, and living hinges requiring high cycle counts without rigid load-bearing capacity.

Where PE Falls Short

• Low structural strength. Even HDPE yields under moderate sustained loads. Structural parts need extensive ribbing or a switch to a stiffer resin.
• Thermal limits. PE softens and distorts at relatively low temperatures. It is not suitable near engines, heaters, or hot-fill processes.
• High, anisotropic shrinkage. Because PE is semi-crystalline, it shrinks significantly during cooling, and differently along the flow path versus across it. Flat, wide parts are highly susceptible to warpage.
• Unbondable surface. PE has extremely low surface energy. Glues, paints, and inks will not stick unless the part gets secondary surface treatment (corona, flame, or plasma).
• UV degradation. Unmodified PE degrades rapidly in sunlight. Outdoor applications need carbon black or specific UV stabilizers.

Common Applications

Polyethylene is the default for packaging and fluid handling. Typical molded parts include beverage caps, chemical pails, food storage containers, laboratory ware, drainage fittings, and durable consumer goods where chemical inertness matters.

What Buyers Should Know About Molding PE

• It processes easily. PE has a wide processing window, does not require drying, and fills thin walls easily.
• Tooling must account for warp. High shrinkage (often 1.5% to 3%) means moldmakers must plan gating and cooling layouts to combat warpage.
• Specify the density grade. Never leave a drawing marked simply “PE.” Call out HDPE, LDPE, or LLDPE, and ideally a specific melt flow index (MFI).
• Design for mechanical fastening. Because adhesives fail on PE, the CAD must include snap-fits, threaded features, or geometries suited to ultrasonic or spin welding.

Typical Processing Window

PE runs at relatively low temperatures and supports fast cycle times.

Parameter	Typical range (unfilled PE)
Drying	Not required
Melt temperature	~180–280 °C (LDPE runs at the lower end, HDPE higher)
Mold temperature	~20–60 °C
Mold shrinkage	~1.5–3.0% (Highly dependent on wall thickness and flow direction)

These ranges represent unfilled, general-purpose grades. Always confirm processing parameters against the specific resin datasheet.

How PE Compares

Compared to polypropylene (PP), PE offers better impact resistance at low temperatures and slightly better chemical inertness. PP offers higher stiffness, better heat resistance, and better fatigue resistance for living hinges. Choose PE when the part must survive the cold; choose PP when it must hold shape under heat.

Compared to ABS, PE is significantly cheaper and chemically resistant, but ABS provides better dimensional stability, structural stiffness, and cosmetic finish.

Buyer FAQs

What is polyethylene used for in injection molding?

Specify polyethylene when low cost, chemical resistance, and toughness matter most: caps, lids, industrial totes, pails, fluid-handling components, and consumer packaging. Do not use it as a default for precision structural housings.

What is the difference between HDPE and LDPE for molded parts?

Density sets stiffness. HDPE is rigid and strong, suited to structural crates and hard caps. LDPE is highly branched and soft, suited to squeeze-bottle caps and pliable seals. Put the density grade on the drawing; “PE” alone is not a material callout.

Does polyethylene warp like polypropylene?

Yes. Both are semi-crystalline and shrink a lot as they cool. Asymmetric cooling or uneven wall thickness will warp a PE part. Require DFM review and optimized mold cooling before you lock flatness tolerances that the resin cannot hold.

Can you glue or paint polyethylene?

Not without secondary operations. Low surface energy blocks adhesives and inks. Design for mechanical assembly (snaps, screws) or thermal welding, and use molded-in colorants instead of paint. If bonding is mandatory, budget corona, flame, or plasma treatment.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Material behavior is grade-specific; confirm properties and suitability against the manufacturer’s datasheet and your supplier’s engineering review.