Basic Properties
HDPE is an opaque, white, waxy material with a density lighter than water (0.941–0.960 g/cm³). It is soft and tough—stiffer and slightly less extensible than LDPE. It is non-toxic and odorless.
Combustion Characteristics
Easily flammable and continues burning after the flame source is removed. The flame is yellow at the top and blue at the bottom. It melts while burning with dripping, produces no black smoke, and emits a paraffin-like odor.
Key Advantages
- Resistant to acids, alkalis, and many organic solvents
- Excellent electrical insulation
- Retains toughness at low temperatures
- Higher surface hardness, tensile strength, and rigidity than LDPE; close to PP; tougher than PP but with poorer surface gloss than PP
Main Limitations
- Inferior mechanical properties compared with engineering plastics; poor gas barrier
- Prone to deformation, aging, brittleness (though less brittle than PP), and stress cracking
- Low surface hardness; easy to scratch
- Difficult to print (requires surface corona treatment); cannot be electroplated; surface lacks gloss
Typical Applications
- Extrusion: Packaging films, ropes, woven bags, fishing nets, water pipes
- Injection molding: Low-end daily goods and housings, non-load-bearing parts, totes, turnover crates
- Extrusion blow molding: Containers, hollow articles, bottles
Injection Molding
HDPE is used in countless applications, from reusable thin-wall drink cups to 5-gal cans, accounting for about one-fifth of domestic HDPE consumption. Injection grades typically have a melt index (MI) of 5–10. There are tougher, lower-flow grades and more processable, higher-flow grades. Uses include thin-wall consumer and food packaging; tough, durable food and paint cans; high ESCR applications such as small-engine fuel tanks and ~90-gal garbage bins.
Characteristics:
Typical melting point ≈ 142 °C; decomposition ≈ 300 °C. Broad workable injection temperature window. Processing temperatures are usually 180–230 °C. Being an olefin, HDPE is non-hygroscopic and generally needs no drying; for quality, it may be dried at 60 °C for 1 h to remove surface moisture. The melt viscosity is high and the flow-length ratio is low—thin-wall parts may short-shot—so gates and runners should be relatively large. Parts are prone to static and surface dust pickup. Typical molding shrinkage ≈ 16‰ (1.6%); flash limit ≈ 0.05 mm.
Performance:
HDPE offers good heat and cold resistance, chemical stability, high rigidity and toughness, and solid mechanical strength. Dielectric properties and ESCR are good. Compared with LDPE, HDPE has higher hardness, tensile strength, and better creep resistance; abrasion resistance, electrical insulation, toughness, and low-temperature performance are all good (overall insulation slightly inferior to LDPE). Chemically stable—insoluble in organic solvents at room temperature; resistant to acids, alkalis, and various salts. Films have low permeability to water vapor and air and low water absorption. Aging resistance is poor and ESCR is inferior to LDPE; thermal-oxidative degradation reduces properties, so antioxidants and UV absorbers are added. HDPE film has a relatively low heat-distortion temperature under load—consider this in application.
Manufacturing Processes
PE is most commonly produced via slurry or gas-phase processes, less often via solution-phase. These exothermic reactions involve ethylene monomer, α-olefin comonomers, catalyst systems (often multi-component), and hydrocarbon diluents. Hydrogen and certain catalysts are used to control molecular weight.
Slurry reactors are typically stirred tanks or, more commonly, large loop reactors with circulating slurry. Upon contact of ethylene/comonomer and catalyst, PE particles form. After diluent removal, the particles/powder are dried and dosed with additives to make pellets. Modern lines with twin-screw extruders can produce >40,000 lb/h of PE. New catalysts have improved the performance of newer HDPE grades.
Two common catalyst families are chromium oxide (Phillips) and titanium compound/alkyl-aluminum (Ziegler-Natta). Phillips catalysts typically give medium MWD; Ziegler-Natta catalysts give narrow MWD. Catalysts used for narrow-MWD polymers in dual-reactor setups can also produce broad-MWD grades. For example, two reactors in series making markedly different molecular-weight products can yield bimodal polymers with very broad MWD.
Molecular Weight, Melt Index & MWD
- Molecular weight (Mw): Roughly 40,000–300,000 for typical HDPE grades; viscosity also depends on test temperature and shear rate. Mw is characterized by rheology or molecular-weight measurements.
- Melt index (MI, 230 °C/2.16 kg): Roughly 100 → 0.029 g/10 min across grades; higher Mw (lower MI) increases melt strength, toughness, and ESCR but makes processing more demanding (higher pressure/temperature).
- Molecular-weight distribution (MWD): Varies from narrow to broad depending on catalyst and process. The common index is polydispersity (HI = Mw/Mn), typically 4–30 for HDPE. Narrow MWD reduces warpage and improves impact in molding. Medium-to-broad MWD improves extrudability; very broad MWD can boost melt strength and creep resistance.
Additives
- Antioxidants: Prevent degradation during processing and oxidation in service.
- Antistatic agents: Used in many packaging grades to reduce dust/soil adhesion.
- Specialty packages: E.g., copper inhibitors for wire/cable.
- UV/weathering: Add UV stabilizers or carbon black for outdoor use. PE without UV stabilizers or carbon black is not recommended for continuous outdoor exposure. High-grade carbon black gives excellent UV resistance for outdoor wire/cable, liners, or pipes.
Injection Molding: Parameter Guide
Barrel temperature profile (typical setpoints in parentheses)
- Feed zone: 30–50 °C (50 °C)
- Zone 1: 160–250 °C (200 °C)
- Zone 2: 200–300 °C (210 °C)
- Zone 3: 220–300 °C (230 °C)
- Zone 4: 220–300 °C (240 °C)
- Zone 5: 220–300 °C (240 °C)
- Nozzle: 220–300 °C (240 °C)
General settings & notes
Equipment: Standard three-zone screw (typical L/D ≈ 25:1). For packaging, use screws with specialized mixing and shear sections; open (straight-through) nozzle; non-return valve.
Shot-size utilization: 35%–65%
Flow length-to-wall-thickness (L/t): 50:1–100:1
Melt temperature: 220–280 °C
Barrel hold/soak: ≈220 °C
Mold temperature: 20–60 °C
Injection pressure: 80–140 MPa (800–1400 bar); thin-wall packaging may require up to 180 MPa (1800 bar)
Packing/holding pressure: 30%–60% of injection pressure (HDPE shrinks significantly; hold long enough when precision matters)
Back pressure: 5–20 MPa (50–200 bar); too low can cause part weight/color dispersion inconsistency
Injection speed: High for thin-wall packaging; medium is often suitable for other parts
Screw speed: High linear speed (≈ 1.3 m/s) is acceptable; ensure plasticizing completes before cooling ends; low torque demand
Metering stroke: 0.5–4 D (min–max); 4 D provides adequate residence time
Cushion (residual): 2–8 mm, depending on shot size and screw diameter
Pre-drying: Not required; if storage is poor, dry 80 °C × 1 h
Regrind: Up to 100% reusable
Shrinkage: 1.2%–2.5%; prone to warpage; most post-mold shrinkage stabilizes within 24 h
Gating/runners: Pin gate; heated hot runner, insulated runner, or internal gate bush; relatively small cross-section is adequate for thin sections; no special purging needed during downtime; PE tolerates temperature increases
