Overview
LDPE (low-density polyethylene) is the flexible polyolefin — the plastic that built the film and bag industry. It’s branched (the “low density” comes from short and long chain branches that prevent close packing), soft, extraordinarily ductile (>600% elongation), and chemically inert in aqueous service. It blow-molds into squeezable bottles, extrudes into flexible tubing, and blows into film at scale that dwarfs every other commodity polymer combined for film volume.
Within the polyethylene family, LDPE occupies the flexible end of the spectrum: HDPE is rigid (cutting boards, pipe), LLDPE is the “linear-branched” middle (tougher film, stretch wrap), and LDPE is the softest with the highest melt strength for film blowing.
Pick LDPE when you need: flexibility, chemical resistance, low cost, and FDA compliance, with no expectations of stiffness or temperature capability. Outside those constraints, HDPE or PP is almost always the better polyolefin.
Where LDPE actually lives — film and squeeze bottles
The vast majority of LDPE production goes into:
- Blown film — plastic bags (grocery, produce, bread, trash), shrink wrap, stretch film, agricultural mulch and greenhouse film. LDPE’s high melt strength makes it uniquely suited to film blowing.
- Squeezable bottles — ketchup, mustard, lotion, lab wash bottles. LDPE’s flexibility plus elastic recovery (memory) is the selling point.
- Wire and cable jacketing — low-voltage cable insulation and jackets where flexibility and dielectric performance matter more than mechanical strength.
- Sealant components — the soft inner cap liner of threaded bottles (the felt-like white disc), squeeze-bottle nozzles.
Sheet and rod LDPE for machining exists but is uncommon — when the application calls for machined polyolefin, HDPE or PP is almost always the right choice. LDPE is rarely the answer to a CNC question.
Machining notes (rarely needed)
LDPE is a difficult material to machine cleanly. It deforms under tool contact pressure, gums easily under friction heat, and offers poor workholding stability. Most “machining” of LDPE in practice is film slitting, kiss-cutting, and trimming of molded parts rather than full material removal.
If you must machine LDPE bar or sheet:
- Sharp HSS or carbide, very positive rake, generous relief
- Slow speeds (200–500 SFM)
- Light feeds (0.003–0.010 in/rev)
- Flood coolant mandatory
- Minimal clamping pressure — vacuum tables or fixture jigs
- Expect 32–63 Ra finish
Joining: thermoplastic hot-air welding works for LDPE-to-LDPE fabrication. Solvent welding does not work. Adhesive bonding requires plasma, flame, or corona surface treatment.
Failure modes worth designing around
Creep is the dominant design issue. LDPE cold-flows under any sustained load — including its own weight in tall standing parts. Never specify LDPE for parts under continuous compression or tension. Spring preload, expansion gaps, or accept-and-replace strategies are required.
Environmental Stress Cracking (ESC) is real but less commonly seen than in HDPE because LDPE is rarely under sustained tensile load. Detergent contact in squeeze bottles is the classic ESC trap — design relief into bottle geometry to keep walls below yield strain.
Thermal limits — LDPE’s continuous service temperature is ~60–75°C, the lowest of the polyolefins. Vicat softening is only ~50°C. Hot-fill applications (>80°C) require HDPE or PP. Steam sterilization is out of the question.
Thermal expansion at 200+ × 10⁻⁶/°C is among the highest of any polymer. A 1 m LDPE part changes 8–10 mm over a 40°C swing. Combined with low stiffness, this means LDPE parts bridging temperature gradients deform freely. Use that flexibility, or design with floating mounts.
UV degradation is fast in unstabilized LDPE — clear agricultural mulch film is single-season; greenhouse film with UV package gets 3–5 years. Most outdoor LDPE is carbon-pigmented (UV stabilizer + pigment in one).
Flammability — UL94 HB only, and LDPE drips burning material. Flame- retardant grades (FR-LDPE for cable jacketing) reach V-2 typically; V-0 is difficult to achieve while retaining LDPE’s processing characteristics.
Variant selection guidance
- Film resin — the default for blown film, bags, shrink wrap.
- Squeeze-bottle grade — slightly stiffer for blow-molded containers where shape recovery (memory) matters.
- LMDPE (Linear Medium Density) — the LDPE/HDPE bridge for parts that need slightly more stiffness than LDPE but more flexibility than HDPE.
- Cable jacket grade — flame-retardant, UV-stabilized for low-voltage insulation and jacketing.
- Agricultural film — UV-stabilized, often EVA-blended for tear resistance. Greenhouse and mulch grades.
Applications by industry
- Packaging (dominant industry) — blown film bags (grocery, produce, bread, garbage), shrink wrap, stretch film, lamination layer in juice boxes and milk cartons. LDPE is the volume leader by far.
- Squeezable bottles — condiments, sauces, lotions, personal care products, lab wash bottles. Blow-molded LDPE with shape memory.
- Agriculture — drip irrigation tubing, greenhouse film (3–5 year UV grade), mulch film (single-season), silage wrap.
- Electrical — low-voltage cable jacketing, primary insulation for building wire and communication cable.
- Consumer products — squeeze toys, flexible toy components, plastic film products, sealant cap liners.
- Construction — vapor barriers, pond liners (heavier-gauge LDPE geomembrane), temporary protective film.
- Lab and medical — wash bottles, flexible tubing, disposable ID bracelets, low-cost disposable items.