HDPE vs LDPE vs Polypropylene (PP) Injection Molding: Polyolefin Selection Guide

Polypropylene (PP), High-Density Polyethylene (HDPE), and Low-Density / Linear Low-Density Polyethylene (LDPE / LLDPE) make up the polyolefin family—the highest-volume, lowest-cost ($1.30–$2.10/kg) commodity thermoplastics in the world. All three float on water (density < 1.00 g/cm³), require zero pre-drying before molding, resist almost all acids, bases, and aqueous solvents at room temperature, and share a waxy, low-surface-energy skin (29–31 dynes/cm) that cannot be glued or painted without flame/plasma treatment.

Yet confusing HDPE, LDPE, and PP on a part print leads to immediate functional failure:

• Specify PP Homopolymer for a dairy crate or outdoor bin used at -20 °C, and it shatters like glass (PP's glass transition temperature is ~0 °C to -10 °C, whereas HDPE and LDPE stay ductile down to -50 °C to -70 °C).
• Specify HDPE for a thin-wall cup that must survive 105 °C steam/microwave heating or a 1,000,000-cycle living hinge, and it softens, sags, or stretches out earlier than PP.

Use this quantitative comparison matrix alongside our material selection guide to pick the exact polyolefin grade for your thermal, mechanical, and dimensional requirements.

1. Master Engineering Matrix: LDPE / LLDPE vs HDPE vs Polypropylene (PP)

Property / Processing Metric	Low-Density Polyethylene (LDPE / LLDPE)	High-Density Polyethylene (HDPE)	Polypropylene Homopolymer (PP-H)	Polypropylene Impact Copolymer (PP-ICP)
Specific Gravity (g/cm³, ASTM D792)	0.915 – 0.930 (branched chains, 40–55% crystalline)	0.945 – 0.965 (linear chains, 70–85% crystalline)	0.900 – 0.910 (lightest commercial plastic!)	0.895 – 0.905 (PP matrix + 8–18% ethylene-propylene rubber)
Crystalline Melting Point ($T_m$)	105 – 122 °C (220–250 °F)	128 – 135 °C (262–275 °F)	160 – 166 °C (320–330 °F — +30 °C above HDPE)	160 – 165 °C (320–329 °F)
Heat Deflection Temp (HDT @ 0.45 MPa)	40 – 50 °C	65 – 82 °C	95 – 110 °C (survives boiling water & dishwasher)	80 – 95 °C
Tensile Yield Strength (23 °C, MPa)	8 – 15 MPa (soft, flexible squeeze)	22 – 31 MPa (stiff, tough)	32 – 38 MPa (stiffest unfilled polyolefin)	24 – 30 MPa
Flexural Modulus (23 °C, GPa)	0.15 – 0.35 GPa (pliable)	0.90 – 1.45 GPa	1.40 – 1.85 GPa (30–50% stiffer than HDPE)	1.05 – 1.35 GPa
Sub-Zero Cold Drop Impact (-20 °C to -40 °C)	Outstanding (No Break down to -70 °C)	Excellent (Ductile down to -50 °C in low-MFI grades)	Poor (Brittle below 0 °C to +5 °C!)	Good (Ductile down to -20 °C)
Living Hinge Flex Life (180° Fold)	25,000 – 100,000 cycles (tether straps)	10,000 – 50,000 cycles	> 1,000,000 cycles (Gold Standard)	300,000 – 1,000,000 cycles
Environmental Stress Crack Resistance (ESCR)	Very Good	Grade-dependent (specify Copolymer HDPE for detergents/surfactants)	Excellent (resists detergents, bleach, and hot water without stress cracking)	Excellent
Mold Shrinkage (mm/mm)	1.5% – 3.5%	1.8% – 3.5% (highest warpage risk on flat walls!)	1.2% – 2.2% (lower than HDPE)	1.2% – 2.0%
Melt Temperature Window	160 – 220 °C	190 – 250 °C	200 – 250 °C (400–470 °F)	200 – 250 °C
Mold Temperature Window	15 – 40 °C	15 – 50 °C	15 – 50 °C (60–120 °F)	15 – 50 °C

2. Head-to-Head Selection Rules: HDPE vs LDPE vs Polypropylene

1. HDPE vs Polypropylene (PP): Stiffness & Heat vs Cold-Impact Toughness

Buyers sourcing storage totes, caps, industrial pails, and fluid reservoirs constantly weigh HDPE against PP:

• Specify Polypropylene (PP) when the part requires:
  • Higher Stiffness at Thin Walls: PP-H has a flexural modulus of 1.4–1.8 GPa (30%–50% stiffer than HDPE) and a 5% lower density (0.90 vs 0.95 g/cm³), yielding more parts per pound of resin with thinner walls;
  • Elevated Temperature Resistance (> 70 °C up to 105–121 °C): Dishwasher-safe containers, hot-fill food cups, automotive under-hood coolant surge tanks, and steam-autoclavable medical trays require PP because HDPE softens above 75–80 °C;
  • High-Cycle Living Hinges (flip-top dispensing caps, one-piece clamshell cases);
  • Better Flatness / Lower Warpage: While both are semi-crystalline, PP shrinks 1.2%–2.0% compared to 1.8%–3.5% for high-crystallinity HDPE, making flat PP lids much easier to hold flat than HDPE lids.
• Specify HDPE when the part requires:
  • Sub-Zero Drop Impact (-10 °C to -50 °C): Outdoor trash carts, freezer totes, cold-chain pallets, marine hardware, and construction barricades use HDPE (or PP Impact Copolymer down to -20 °C) because homopolymer PP shatters in freezing weather;
  • Heavy Sliding Wear & Impact Cushioning: High-molecular-weight HDPE absorbs blunt impacts without stress-whitening as severely as PP.

2. HDPE vs LDPE / LLDPE: Rigid Structure vs Flexible Snap & Squeeze

Within the polyethylene family, polymer chain branching controls crystallinity and stiffness:

• LDPE and LLDPE (0.915–0.930 g/cm³): Long or short side-chain branches prevent chains from packing tightly (only 40–55% crystalline). The result is a soft, flexible, translucent plastic (0.15–0.35 GPa modulus) used for snap-on coffee-can lids, squeeze closures, flexible dust caps, medical tubing ports, and tear-off tamper bands that must stretch over an undercut rim without cracking.
• HDPE (0.945–0.965 g/cm³): Unbranched linear chains pack into 70%–85% crystallinity, creating an opaque, 4× to 6× stiffer structural material (1.0–1.4 GPa modulus) for rigid bottle caps, milk crates, 5-gallon pails, and utility housings.

3. Why HDPE Warps More Than PP (and How to Control It)

A frequent surprise at T1 sampling occurs when a buyer asks a molder to shoot HDPE in a mold originally cut for Polypropylene (PP):

1. Dimensional Undersize (0.5% to 1.2% Smaller): Because HDPE is 75%–85% crystalline (versus 50%–60% for PP), HDPE shrinks 2.0%–3.5% compared to 1.3%–1.8% for PP. An HDPE part pulled from a PP mold will measure 0.5%–1.2% smaller (e.g., 1.5–2.5 mm shorter on a 250 mm box).
2. Severe Differential-Cooling Warpage: Linear HDPE chains align strongly along the melt flow vector and contract violently as they crystallize. If the gate side of an HDPE lid runs 10 °C hotter than the perimeter—or if corners have thick fillets—the lid twists into a severe saddle warp.
   • Remedies for Flat HDPE Molding: Choose a narrow-molecular-weight-distribution (low-warp injection grade) HDPE copolymer, use multi-point pin or valve gating to shorten flow length, design strictly uniform wall thicknesses (2.0–2.8 mm), and run separate cooling water circuits (15–30 °C) in the gate zone versus the perimeter.

Buyer FAQs

Is HDPE or Polypropylene (PP) stronger and stiffer?

At room temperature (23 °C) and above, Polypropylene (PP Homopolymer) is 20%–35% stronger in tensile yield (35 MPa vs 26 MPa) and 30%–50% stiffer in flexural modulus (1.6 GPa vs 1.1 GPa) than HDPE, while also weighing 5% less per cubic centimeter (0.90 g/cm³ vs 0.95 g/cm³). However, at freezing temperatures (below 0 °C down to -50 °C), HDPE is far tougher in impact because homopolymer PP passes below its glass transition temperature and becomes brittle.

What is the difference between HDPE and LDPE in injection molding?

HDPE (High-Density Polyethylene, 0.945–0.965 g/cm³) has linear, unbranched polymer chains that pack into 70%–85% crystallinity, producing a stiff, opaque, higher-heat (130 °C melt point) plastic for crates, pails, and threaded caps. LDPE (Low-Density Polyethylene, 0.915–0.930 g/cm³) has branched chains that limit crystallinity to 40%–55%, producing a soft, flexible, stretchable (0.2 GPa modulus, 110 °C melt point) plastic for snap-on overcaps, flexible spouts, and protective dust plugs.

Can HDPE and Polypropylene (PP) be molded in the same injection mold?

For small parts (< 50–75 mm) with loose commercial tolerances (±0.3–0.5 mm), HDPE and PP can often be sampled in the same mold. For larger parts (> 100 mm) or tight-fitting lids and threads, running HDPE in a PP-shrunk mold usually fails because HDPE shrinks 2.0%–3.5% compared to 1.2%–2.0% for PP—leaving the HDPE part undersized and much more prone to twisting warpage.

Which is better for outdoor sun exposure: HDPE or Polypropylene?

Unmodified grades of both HDPE and PP degrade in UV sunlight, but HDPE degrades significantly more slowly than PP because PP’s tertiary carbon atoms oxidize rapidly under UV radiation (causing surface chalking and embrittlement within months). For multi-year outdoor service in either resin, always specify 2.0%–2.5% well-dispersed Carbon Black or a Hindered Amine Light Stabilizer (HALS) UV additive package.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Polyolefin shrinkage, warpage, and ESCR vary widely by melt flow index (MFI) and comonomer content; verify parameters against the resin supplier’s grade-specific datasheet.