Polyethylene Injection Moulding: HDPE and LDPE Guide
Polyethylene (PE) is the most-produced plastic in the world. In injection moulding, it splits into two main families: HDPE for rigid components and LDPE for flexible ones. Neither absorbs moisture to any meaningful degree, both resist nearly every common acid and alkali, and both are among the cheapest resins available. The challenge is always shrinkage—PE shrinks more than almost any other injection-moulded material, and managing that is the central design and tooling task.
Mechanical and Thermal Properties
Polyethylene is a polyolefin with near-zero water absorption (<0.01% in 24 hours). That makes it exceptionally stable in wet environments—a contrast to Nylon, which absorbs 1–3% water and changes dimension accordingly.
| Property | HDPE | LDPE |
|---|---|---|
| Tensile strength | 20–35 MPa | 8–20 MPa |
| Flexural modulus | 800–1,500 MPa | 200–400 MPa |
| Elongation at break | 100–1,000% | 200–600% |
| Notched Izod impact | 20–80 J/m | No break |
| Heat deflection temp (0.45 MPa) | 60–85°C | 35–50°C |
| Continuous service temp | 90–110°C | 60–80°C |
| Water absorption (24 h) | <0.01% | <0.01% |
| Density | 0.94–0.97 g/cm³ | 0.91–0.93 g/cm³ |
| Mould shrinkage | 2.5–5.0% | 1.5–3.0% |
HDPE remains impact-resistant at −50°C—a temperature at which many engineering plastics become brittle. For cold-chain equipment and outdoor components in northern climates, that toughness matters.
The shrinkage range for HDPE (2.5–5.0%) is the widest of any commonly moulded material. Wall thickness, gate design, and packing pressure all shift where a given part lands within that range.
Typical Applications
HDPE:
- Rigid containers—heavy-duty bins, industrial chemical bottles, and household storage.
- Pipe fittings—manifolds and connectors for water, gas, and chemical distribution.
- Consumer goods—cutting boards, appliance bases, and utility parts.
- Automotive—fluid reservoirs and protective shrouds where cold-weather toughness matters.
LDPE:
- Flexible covers—snap-on lids and dust caps that need to deform and recover repeatedly.
- Squeeze components—soft-touch triggers or deformable housings.
- Medical—flexible tubing connectors and disposable protective caps.
Moulding and Design Guidelines
PE melts and flows easily, but its high shrinkage makes it one of the hardest commodity materials to hold to tight tolerances.
- Wall Thickness: Aim for 1.5–4.0 mm. Thick sections in PE sink badly—the surface depressions are disproportionate compared to other plastics.
- Shrinkage management: HDPE shrinks 2.5–5.0% as it crystallises. Flat parts like trays and panels will warp unless gating is central or multiple gates are balanced. Asymmetric wall sections almost always bow; design them out at the CAD stage.
- Melt Temperature: HDPE processes at 200–260°C; LDPE at 180–240°C. PE tolerates somewhat longer residence times in the barrel than POM, but degradation still occurs above 300°C.
- Draft Angles: Minimum 1° is standard. For LDPE, which is softer, less draft can sometimes work, but more is always better for clean ejection.
- Ejection: LDPE's flexibility means parts can often be stripped off cores without cracking—an advantage for undercut features that would require side actions in a stiffer material.
Material Grades
| Grade | Feature | Typical Use |
|---|---|---|
| HDPE Natural | Rigid, low cost | General housings, caps |
| UV-Stabilised HDPE | Resists sun damage | Outdoor fittings, bins |
| Glass-Filled HDPE | Higher stiffness | Industrial structural parts |
| Food-Contact HDPE | FDA/EU certified | Food bins, bottle caps |
| LDPE Natural | Soft, flexible | Protective caps, lids |
UHMWPE (Ultra-High-Molecular-Weight Polyethylene) is technically a polyethylene but cannot be injection moulded—its viscosity is too extreme for conventional screws. Components requiring UHMWPE properties are machined from stock rod or sheet.
Advantages and Limitations
Why choose Polyethylene:
- Very low material cost.
- Excellent resistance to acids and alkalis—suitable for chemical-contact applications.
- Stays tough at sub-zero temperatures (HDPE to −50°C).
- Naturally food-safe and the most widely recycled plastic in the world.
What to watch for:
- Low stiffness: Not suitable for structural parts under significant load.
- Bonding problems: Surface treatment (flame, corona) is mandatory before adhesive bonding or painting.
- High shrinkage: Large, flat PE parts will warp unless designed and gated carefully.
- UV degradation: Specify UV-stabilised grade for any outdoor application.
Choosing PE vs Alternatives
- PE vs PP: Choose HDPE for cold-weather toughness (−50°C). Choose PP for living hinges or if the part needs to survive boiling water or autoclaving.
- PE vs TPE: Choose LDPE for a low-cost flexible part. Choose TPE if you need true elastic recovery or a rubber-like tactile feel.
Sustainability
Polyethylene carries resin codes 2 (HDPE) and 4 (LDPE)—among the most widely recycled in both industrial and consumer streams. For non-critical industrial parts, recycled content can often be incorporated to reduce cost and carbon footprint. Bio-based HDPE derived from sugarcane ethanol is available for projects with specific environmental credentials.
Nordmould provides tooling from €3,000 and low-volume production starting at 100 pieces. Send us your STEP file for a free DFM review—we'll assess wall thicknesses and shrinkage risks to ensure your PE parts are production-ready.