LDPE, LLDPE, MDPE, HDPE, XLPE, and UHMW are not interchangeable polyethylene labels; structure, molecular weight, branching, and crosslinking change how each material behaves. The right choice depends on stiffness, impact, flexibility, chemical exposure, wear, temperature, processing route, and the part’s function.
One important production distinction is that not every polyethylene family is suited to conventional injection molding. XLPE is cross-linked and is normally processed through a different route, while UHMW grades can have extremely high melt viscosity and may require specialized processing. Do not select a material from a label alone.
Polyethylene families at a glance
| Type | General behavior | Typical design or process implication |
|---|---|---|
| LDPE | More branched, flexible, and softer than HDPE | Useful where flexibility and impact matter; confirm stiffness and creep limits |
| LLDPE | Linear chain with short-chain branching; good toughness and flexibility in many grades | Grade-specific flow, shrinkage, and film versus molding suitability must be checked |
| MDPE | Intermediate density and balance of stiffness, toughness, and stress-crack resistance | Application and grade data should decide whether it is a molding candidate |
| HDPE | Higher crystallinity, stiffness, chemical resistance, and shrinkage than lower-density grades | Allow for shrinkage, warpage, cooling, and gate/part design |
| XLPE | Cross-linked polyethylene with a network that does not remelt like a normal thermoplastic | Generally not a conventional injection-molding substitute for PE grades |
| UHMWPE | Ultra-high molecular weight, high wear resistance, and very high melt viscosity | Often requires specialized forming or compression/sintering rather than standard injection molding |
What matters for part selection
- Stiffness and load: density and crystallinity affect modulus, but temperature and time also matter.
- Impact and flexibility: lower-density grades may suit flexible parts, while additives and copolymers change the balance.
- Chemical and environmental stress cracking: confirm the exact grade and exposure conditions.
- Shrinkage and warpage: semi-crystalline behavior requires mold compensation and balanced cooling.
- Wear: UHMWPE can be valuable in wear applications but its processing route is different.
- Temperature and creep: check continuous-use conditions rather than only short-term strength.
Injection molding considerations for PE grades
For a conventional injection-molded PE part, verify melt flow, density, shrinkage, mold temperature, gate design, wall thickness, cooling, and ejection. PE can shrink significantly as it crystallizes, so thick sections, uneven walls, and asymmetric gates can create dimensional drift or warpage. The material supplier’s data sheet must control the process window.
Use a DFM review to check draft, ribs, bosses, living hinges, snap features, gate witness, and cooling. The engineering team can compare the resin choice to the part’s load, environment, and production volume before the tool is released.
How to choose between grades
- Define the part’s function, temperature, chemical exposure, impact, stiffness, wear, and appearance.
- Decide whether the part will be injection molded, extruded, rotomolded, compression molded, or machined.
- Shortlist specific supplier grades and compare data sheets, not only resin family names.
- Review shrinkage, wall design, cooling, gate, ejection, and inspection implications.
- Validate with the actual grade, color, process, and environmental conditioning.
RFQ information
Provide the part model, application temperature, chemical exposure, load, impact, wear, color, annual volume, and required compliance. State whether XLPE or UHMW is being considered for a specialized process rather than assuming conventional injection molding. Contact Cavity Mold for a material and manufacturability review.
Technical references
The LyondellBasell polyolefin guide explains differences among polyethylene families. The Protolabs material-selection guide reinforces the need to choose a specific grade from functional and environmental requirements.