HDPE and LDPE are both polyethylene, but they are not interchangeable in injection molding. HDPE is generally more linear, stiffer, and more crystalline; LDPE is more highly branched, softer, and more flexible. The correct choice depends on the part’s stiffness, impact, flexibility, chemical exposure, dimensional requirements, appearance, and the exact resin grade—not only on the name of the polymer family.
What is the difference between HDPE and LDPE?
| Property or decision | HDPE | LDPE |
|---|---|---|
| Molecular structure | More linear and closely packed | More branched and less densely packed |
| Typical part behavior | Higher rigidity and chemical resistance | More flexibility and easier deformation |
| Crystallinity and shrinkage | Crystalline behavior requires shrinkage and cooling review | Also crystallizes, but the grade and geometry determine the result |
| Design fit | Rigid containers, caps, housings, and industrial components where stiffness matters | Flexible closures, squeeze features, and parts that need compliance |
| Processing rule | Use the exact injection grade’s data sheet and shrinkage information | Do not assume a film or extrusion grade will process like an injection grade |
These are general tendencies, not acceptance criteria. Branching, molecular weight, melt-flow rating, additives, colorants, and reinforcement change how a grade fills and shrinks. The material supplier’s data sheet should control the approved processing window.
How does polyethylene behave in the mold?
Polyethylene is a semi-crystalline material, so cooling conditions affect crystallization, shrinkage, stiffness, and dimensional stability. Autodesk’s HDPE material guidance notes higher shrinkage than many amorphous materials and provides grade-dependent processing information. Do not use a single shrinkage percentage for every HDPE or LDPE part.
Processing checks for HDPE and LDPE
- Confirm the exact grade, melt-flow value, color, additive package, and whether the resin is intended for injection molding.
- Follow the supplier’s drying and storage instructions. Polyethylene is less moisture-sensitive than some engineering resins, but surface moisture, contamination, and poor storage can still create appearance or process problems.
- Set melt and mold temperatures from the grade data sheet, then validate the actual temperature at the machine and mold.
- Use a fill profile that avoids jetting and premature freezing, then set packing from part weight, dimensions, and gate-freeze evidence.
- Allow enough cooling for the thickest feature and measure the part after it reaches the agreed inspection condition.
How should the mold be designed?
Design for uniform walls, gradual transitions, proper draft, adequate radii, and a gate that fills the part without unacceptable weld lines or orientation. Polyethylene shrinkage makes cooling balance and packing especially important. Thick ribs, bosses, and local mass can create sink or warp if they are not cored or cooled appropriately.
Common HDPE and LDPE molding risks
| Symptom | Possible cause | First review |
|---|---|---|
| Warp or dimensional drift | Uneven cooling, crystallization, packing, or wall thickness | Cooling circuits, part geometry, gate, and measurement timing |
| Short shot | Low melt or mold temperature, long flow path, restricted gate, or slow fill | Fill pressure, gate, venting, and resin grade |
| Sink mark | Thick section, insufficient packing, or early gate freeze | Wall transition, hold profile, gate seal, and cooling |
| Flash | Excess pressure, mold mismatch, high temperature, or weak vent | Parting surface, clamp condition, and process window |
What should you provide for a polyethylene quote?
Send the CAD file, drawing, exact polyethylene grade, color, annual volume, order quantity, tolerance, surface finish, flexibility or stiffness requirement, and any chemical or temperature exposure. Cavity Mold can coordinate DFM, tooling, and production molding. Send the material data sheet and part requirements for a practical review.
Choose polyethylene from the part’s failure mode
Start by identifying what must not happen in service. Shape retention, chemical exposure, sealing, flexing, and compliance can point in different directions. These are screening decisions only: density, molecular weight, melt-flow behavior, additives, color, and the supplier’s injection grade can change the result.
| Application question | Design consequence |
|---|---|
| Will the part flex repeatedly? | Review hinge geometry, stress concentration, cycle life, and conditioning. |
| Is a seal or flat interface critical? | Review shrinkage direction, cooling balance, pack response, and inspection state. |
| Will it contact chemicals or outdoor exposure? | Confirm the exact compound and validate the actual environment. |
Do not transfer settings from film, blow-molding, or extrusion grades to injection grades. Material references frame questions; the approved grade data sheet controls conditions and shrinkage. Include grade, color, wall map, flexing or sealing requirements, volume, and critical dimensions in the engineering request; use the materials page for the shortlist.