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PE Injection Molding: HDPE vs LDPE Materials, Process, and Machine Selection
Created on 01.19
PE injection molding is one of the most widely used plastic manufacturing processes. From daily household items to industrial containers, polyethylene (PE) parts are everywhere. Thanks to its low cost, chemical resistance, and easy processing, PE for injection molding is a popular choice for manufacturers across many industries.
HDPE and LDPE injection molding are the PE materials you see used most often. They're both polyethylene, but they act differently, mold differently, and are used in different ways. Knowing the differences helps you get consistent results, a nice surface, and lower costs.
This guide will cover what sets HDPE and LDPE injection molding apart, how PE injection molding works, how to set things up right, and how to choose the bestPE injection molding machinefor the job.
HDPE and LDPE Material Property Comparison
1. HDPE (High-Density Polyethylene)
HDPE has a linear structure with highcrystallinityIt has high strength, stiffness, and good resistance to chemicals. HDPE injection molding is commonly used for containers, caps, crates, and industrial parts.
2. LDPE (Low-Density Polyethylene)
LDPE has a branched structure, which makes it softer and more bendable. LDPE injection molding works well for squeeze bottles, flexible lids, and soft packaging parts.
HDPE vs LDPE Injection Molding Differences
Property | HDPE | LDPE |
Density | Higher (0.94–0.97 g/cm³) | Lower (0.91–0.93 g/cm³) |
Melting Point | Higher (130–135°C) | Lower (105–115°C) |
Shrinkage Rate | Medium to high | High |
Mechanical Strength | High rigidity and impact strength | Soft and flexible |
Flow Behavior | Moderate flow | Very good flow |
Typical Applications | Rigid containers, caps, industrial parts | Flexible bottles, thin-walled products |
HDPE offers better dimensional stability, while LDPE flows more easily and fills thin cavities faster.
PE Injection Molding Process
The PE injection molding process is simple and efficient due to PE’s low moisture absorption and excellent flow performance.
Basic Process Steps
- Material feeding: PE does not require drying under normal conditions.
- Melting and plasticizing: Pellets are melted in the barrel by heat and screw rotation.
- Injection: Molten PE is injected into the mold cavity.
- Cooling: PE cools and solidifies quickly but shrinks noticeably.
- Ejection: The finished part is removed from the mold.
Because injection-molded PE materials have very good flow, they are suitable for thin-wall parts, but mold and parameter design must compensate for shrinkage.
Key Processing Parameter Settings for PE Injection Molding
In PE injection molding, stable quality depends on proper control of temperature, pressure, speed, and cooling. Both HDPE and LDPE injection molding share a similar processing logic, but their parameter ranges and sensitivities differ. The table below summarizes the most important settings.
1. Recommended Parameter Ranges
Parameter | HDPE Injection Molding | LDPE Injection Molding | Process Notes |
Melt Temperature | 180–230°C | 160–210°C | Too low causes short shots; too high increases shrinkage |
Injection Pressure | Medium to high | Medium | Adjust according to wall thickness |
Injection Speed | Medium | Medium to high | Faster speed helps fill thin-wall parts |
Holding Pressure | Medium | Low to medium | Compensates for PE shrinkage |
Holding Time | Medium | Short | Excess time increases internal stress |
Mold Temperature | 30–60°C | 20–50°C | Higher temperature improves surface quality |
Cooling Time | Medium | Short to medium | Depends on part thickness |
This table helps operators quickly set initial values for injection molding PE parts, which they can then adjust based on mold design and product structure.
2. Wall Thickness and Parameter Adjustment
When injection molding PE parts, pay close attention to wall thickness. Thin walls need faster filling and slightly higher injection pressure to ensure complete filling. Thicker parts need longer holding and cooling times to prevent sink marks and internal voids.
PE flows well, so thin-wall designs are possible. However, PE shrinks quite a bit, so holding pressure must be well balanced. Too little holding can cause surface defects, but too much increases the chance of warpage.
3. Shrinkage Control in PE Injection Molding
Shrinkage is a key thing to consider when injection molding PE. HDPE usually shrinks less than LDPE. Compensation is needed through both mold design and process settings. A stable melt temperature, steady injection speed, and enough holding pressure will lower dimensional variation and make results more repeatable.
For precise PE injection molding, keeping parameter control consistent matters more than pushing the machine to its limits.
PE Injection Molding Machine Selection Guide (for HDPE / LDPE)
Selection Item | Recommended Range / Requirement | Why It Matters for PE Injection Molding |
Screw L/D Ratio | 20:1 – 30:1 | Ensures stable plasticizing and uniform melt for PE |
Screw Type | General-purpose or PE-optimized screw | Deep flights improve conveying of low-viscosity PE melt |
Compatible Materials | PE / PP standard materials | Avoids excessive shear from engineering-plastic screws |
Compression Ratio | 2:1 – 3:1 | Prevents PE degradation caused by over-shearing |
Rated Plasticizing Capacity | ≥ 1.2–1.5 × actual material consumption | Allows low screw speed and stable melt temperature |
Screw Speed Control | Wide adjustment range, stable rotation | Improves melt temperature and mixing consistency |
Barrel Temperature Range | 160 – 260 °C | Covers LDPE, LLDPE, and HDPE grades |
Temperature Control Accuracy | ±1 – 2 °C | Reduces oxidation, discoloration, and silver streaks |
Achievable Melt Temperature | Stable operation at 180 – 240 °C | Meets most PE injection molding requirements |
Mold Temperature Capability | 10 – 80 °C (typical use: 10 – 60 °C) | Balances cycle time and surface quality |
Rated Injection Pressure | 100 – 180 MPa | Provides sufficient margin for thick or complex parts |
Injection Speed Control | Multi-stage control (≥ 3–5 stages) | Adapts to thin-wall and thick-wall PE parts |
Holding Pressure Control | Pressure/time adjustable in stages | Compensates for PE shrinkage during solidification |
Clamp Force Estimation | 30 – 50 kN/cm² of projected area | Prevents flash and ensures mold stability |
Clamp Force Margin | 10% – 20% reserve | Improves long-term production stability |
Platen Size & Tie Bar Spacing | Matches mold size with upgrade margin | Supports future mold or product changes |
Ejection System | Adequate stroke and force, adjustable | Helps demolding PE parts with higher shrinkage |
Control System Capability | Independent setting & data recording | Enables optimization of PE process window |
Conclusion
To get quality and keep production smooth with HDPE and LDPE injection molding, it's key to know the differences. You should also learn the important processing steps and pick the right machine.
Whether you're making hard HDPE containers or bendy LDPE thin-wall parts, good control of the process will help. Need input on PE injection molding or prices for a machine? Reach out now for help from pro machine sellers.
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