Polyethylene Injection Moulding: HDPE and LDPE Guide
Polyethylene (PE) is the world's most produced plastic, and both its high-density (HDPE) and low-density (LDPE) forms are fully processable by injection moulding. HDPE provides a rigid, chemically inert structure suitable for containers, pipe fittings, and industrial components; LDPE delivers a softer, more flexible result used for squeeze applications and snap-fit housings. Nordmould moulds both grades for orders from 100 pieces.
What are the mechanical and thermal properties of HDPE and LDPE?
| 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 (very tough) |
| Heat deflection temp (0.45 MPa) | 70–90 °C | 40–50 °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 |
| Shrinkage | 1.5–3.5 % | 1.5–4.0 % |
Both grades have near-zero moisture absorption, making them dimensionally stable in humid environments — a significant advantage over nylon or POM in outdoor or wet-contact applications.
What are typical applications of polyethylene injection moulding?
PE's combination of chemical inertness, toughness at low temperatures, and very low cost makes it the go-to plastic for high-volume commodity parts. At lower volumes, Nordmould moulds PE for specialised components where no other common plastic offers the same combination of properties.
HDPE applications:
- Bottles, caps, and closures in food and household chemical packaging
- Pipe fittings, manifolds, and valve bodies for water and gas distribution
- Industrial trays, bins, pallets, and material-handling components
- Fuel tanks and fluid reservoirs (HDPE has low permeability to hydrocarbons)
- Cutting boards and food-contact components in kitchen equipment
LDPE applications:
- Squeeze bottles and flexible lids where repeated deformation is required
- Snap-fit protective covers where stiffness is not critical
- Low-pressure seals and gaskets in non-elastomeric applications
- Liners and bags where injection moulding is used for fittings and connectors
How is polyethylene processed in injection moulding?
PE is one of the easier plastics to process — it flows readily and tolerates a fairly wide processing window. The main challenges are managing high shrinkage and avoiding sink marks in thick sections.
| Processing parameter | HDPE | LDPE |
|---|---|---|
| Melt temperature | 200–260 °C | 180–240 °C |
| Mould temperature | 20–60 °C | 20–40 °C |
| Injection pressure | 70–105 MPa | 55–90 MPa |
| Shrinkage | 1.5–3.5 % | 1.5–4.0 % |
| Recommended draft angle | ≥ 1° per side | ≥ 1° per side |
Shrinkage and sink management: PE's high shrinkage is the primary processing challenge. Thick sections must be cored out or redesigned to a uniform wall. Holding pressure and hold time are the principal levers for controlling sink marks. Nordmould recommends wall thickness of 1.5–4.0 mm for most PE parts.
Warp and distortion: flat PE parts warp predictably unless the part is symmetric and gating is central. For lids, trays, or panels, a multi-gate or fan-gate strategy is typically required. Nordmould evaluates gate placement during DFM to minimise post-mould distortion.
Ejection: PE's flexibility usually aids ejection; however, very deep draws with insufficient draft can cause drag marks. A minimum of 1° draft per side is recommended, with 2° preferred for textured surfaces.
Hot runners: suitable for HDPE; the low viscosity makes LDPE equally compatible. Both grades respond well to valve-gated hot-runner systems, which reduce waste and eliminate sprue grinding.
What grades and variants of polyethylene are available?
| Grade | Key feature | Typical use case |
|---|---|---|
| HDPE natural | General rigid applications | Caps, closures, containers |
| HDPE UV-stabilised | Outdoor durability | Garden equipment, infrastructure |
| HDPE carbon black | Maximum UV protection | Pipe fittings, outdoor bins |
| HDPE GF (glass-filled) | Higher stiffness | Technical fittings, structural parts |
| HDPE food-contact | FDA/EU 10/2011 certified | Food packaging components |
| LDPE natural | Flexible, soft touch | Squeeze parts, snap covers |
| UHMWPE | Extreme wear resistance, very low friction | Bearing liners, wear pads (limited moulding) |
Ultra-high-molecular-weight polyethylene (UHMWPE) is difficult to injection-mould due to its very high melt viscosity; it is more commonly compression-moulded or machined from stock. Nordmould's team will advise if UHMWPE is specified — compression-moulded blanks machined to shape is often the more practical route.
What are the advantages and limitations of polyethylene?
Advantages:
- Very low material cost — among the cheapest engineering resins available
- Near-zero moisture absorption; dimensionally stable in wet or humid environments
- Excellent chemical resistance to acids, alkalis, and many solvents
- Good low-temperature toughness, including at sub-zero temperatures
- Food-safe in certified grades; widely accepted for packaging applications
- Easily recyclable (PE is one of the most recycled plastics globally)
Limitations:
- Low stiffness compared with PP, ABS, or nylon — not suitable for structural load-bearing
- High shrinkage (1.5–4.0 %) requires careful process control to avoid warp and sink
- Difficult to bond, print on, or paint without surface activation
- Poor UV resistance in unmodified grades
- Low service temperature for LDPE (softens above ~60 °C)
- Cannot be easily solvent-welded; ultrasonic or hot-plate welding is required
When should you choose polyethylene over other materials?
Choose HDPE over PP when the part must resist very low temperatures (HDPE retains impact toughness down to −50 °C), when minimal water vapour transmission is needed, or when the cost differential justifies it in very high volumes.
Choose LDPE over TPE when a low-cost, flexible, food-contact part is needed and the snap-back force is not critical. TPE provides better elastic recovery and defined Shore A hardness; LDPE is simply soft and flexible without controlled elastomeric properties.
Choose PE over ABS or PC when cost is the overriding driver, chemical exposure is aggressive, or moisture is constant. ABS and PC are both attacked by some solvents that PE resists, and both absorb more moisture.
Avoid PE when dimensional precision matters (its high shrinkage makes it one of the hardest plastics to hold to tight tolerances), when the part operates above 110 °C, or when a strong adhesive bond is needed.
Recyclability and sustainability
Polyethylene carries the most mature recycling infrastructure of any thermoplastic. HDPE (resin code 2) and LDPE (resin code 4) are collected in kerbside programmes across the EU. Post-consumer HDPE is widely available as a recycled resin and can be incorporated into non-food-contact applications. In moulding, PE regrind from sprues and runners can be reblended at up to 20–25 % in non-critical applications. Bio-based HDPE (from sugarcane ethanol) is commercially available and offers carbon-neutral or carbon-negative material options for brands with sustainability commitments.
Upload your STEP or PDF file and Nordmould will return a free DFM review within one business day, covering grade selection, wall analysis, shrinkage risks, and an indicative quote.
Frequently asked questions
What is the difference between HDPE and LDPE in injection moulding?
HDPE has a more ordered crystalline structure, giving it higher stiffness (tensile strength 20–35 MPa), better heat resistance (up to 110 °C), and lower moisture permeability. LDPE is softer, more flexible, and has lower shrinkage predictability. Nordmould recommends HDPE for structural containers and LDPE where flexibility or soft touch is needed.
Why does polyethylene shrink so much and how is warping controlled?
PE is highly crystalline and can shrink 1.5–3.5 % — among the highest of common injection moulding plastics. Uniform wall thickness, balanced filling, and adequate holding pressure are the primary controls. Nordmould's DFM review flags any thin-to-thick transitions or asymmetric geometry that would cause differential shrinkage and bow.
Is polyethylene food-safe for injection moulded parts?
Yes. HDPE and LDPE are FDA 21 CFR and EU 10/2011 compliant in standard food-contact grades. They are among the most widely used plastics in food packaging and are considered safe for repeated contact with food and beverages. Nordmould can supply food-contact certified resin — confirm this requirement during the DFM review.
Can polyethylene be used for outdoor applications?
Standard PE degrades under prolonged UV exposure, yellowing and becoming brittle over 1–3 years outdoors. UV-stabilised HDPE grades containing carbon black or HALS stabilisers are available and provide significantly extended outdoor life. Nordmould can source UV-stabilised grades for outdoor applications on request.
Does polyethylene bond well with adhesives and paints?
PE has a very low surface energy (~31 mN/m) and is notoriously difficult to bond or paint without surface treatment. Flame treatment, corona discharge, or primers specifically formulated for polyolefins are required for adhesive bonding. Nordmould can advise on this during DFM if bonding or painting is part of your assembly process.
What is the minimum order quantity for polyethylene parts at Nordmould?
Polyethylene injection-moulded parts are available from 100 pieces. Tooling starts from €3,000 for bridge aluminium tooling, with lead times of 4–11 weeks depending on tool complexity and material. PE's low material cost makes it one of the most cost-effective options for low-volume production runs.
How does HDPE compare with PP for injection moulding?
Both are polyolefins with similar chemical resistance and low cost. PP offers better fatigue resistance (living hinges) and slightly higher service temperature. HDPE has better impact resistance at low temperatures and lower water vapour transmission. Nordmould recommends PP as the default polyolefin for most consumer products, reserving HDPE for pipe fittings, containers requiring low gas permeability, and cryogenic applications.