HDPE is popular for injection molding applications because it’s a durable, versatile, and cost-effective thermoplastic. Parts made from injection molded HDPE have high tensile strength, impact resistance, and overall toughness.
In this complete guide, as a professional plastic injection molding manufacturer, we’ll cover everything you need to know about using HDPE plastic in injection molding, including:
- What Is HDPE Plastic?
- HDPE Material Properties
- Can HDPE Be Injection Molded?
- Why Use HDPE for Injection Molding?
- The HDPE Injection Molding Process
- HDPE Injection Molding Applications
- Tips for Designing HDPE Injection Molded Parts
So if you want to leverage the power of HDPE injection molding, you’re in the right place. Let’s get started!
What Is HDPE Plastic?
HDPE stands for “high-density polyethylene,” and it’s one of the most popular plastics used in injection molding today.
HDPE is a thermoplastic polymer derived from petroleum. It has a high strength-to-density ratio, which gives it excellent durability and versatility.
Raw HDPE plastic pellets are opaque and white. But colorants can be added to produce HDPE plastic parts in various vibrant colors.
Fun fact: If you look at the bottom of plastic bottles, containers or other products, you might see the resin identification code number “2”. This indicates that the product was made from HDPE plastic!
HDPE Material Properties
Now let’s take a closer look at some of the unique properties that make HDPE such a great choice for injection molding applications:
Mechanical Properties
- High tensile strength
- Good impact strength
- Excellent chemical resistance
- High fatigue rating
Thermal Properties
- Melting Point Range: 130°C – 135°C
- Heat Deflection Temperature: 42°C – 80°C
Electrical Properties
- Volume Resistivity: >10^16 ohm·cm
- Dielectric Strength: 20–40 kV/mm
Processing Properties
- Easy flow with uniform properties
- Fast cycle times
- Low clamping forces
The well-balanced physical, mechanical, thermal and electrical properties allow HDPE to be easily processed while retaining excellent performance characteristics.
Can HDPE Be Injection Molded?
Yes, HDPE (high-density polyethylene) can be injection molded to create custom plastic parts and products. HDPE is actually one of the most common plastics used in injection molding due to its versatility, durability, and relatively low cost.
Why Use HDPE for Injection Molding?
Now that you know what HDPE is and its impressive material properties, let’s look at some of the reasons it’s such a popular choice for injection molding applications:
1. Cost-Effective
HDPE resin is affordable compared to engineering plastics, making it ideal for high volume production runs where per-part price is critical.
2. Customizable
The injection molding process allows HDPE to be molded into complex geometries and custom shapes not possible with other manufacturing techniques.
3. Durable
HDPE has remarkably high tensile strength and fatigue resistance compared to other commodity resins. Parts stand up well to daily wear and tear.
4. UV-Resistant
HDPE retains its color and mechanical properties even after prolonged exposure to sunlight and harsh weather conditions.
5. Recyclable
One of the most environmentally friendly plastics, HDPE is 100% recyclable. Products made from recycled HDPE resin perform similarly to those made from new resin.
6. FDA Approved
HDPE meets FDA requirements for direct food contact. Certain medical grades also meet USP Class VI standards for medical devices and healthcare applications.
7. Range of Finishes
From high gloss to matte, HDPE can be produced in a spectrum of aesthetic finishes to meet application requirements.
Thanks to these useful characteristics, HDPE injection molded components serve a diverse range of applications across industries.
The HDPE Injection Molding Process
Now let’s take a high-level look at the typical injection molding process using HDPE plastic:
Step 1: Plasticizing
- HDPE resin pellets are gravity fed from a hopper into the injection molding machine’s barrel
- The reciprocating screw feeds the pellets forward and shears them into a molten state
- Barrel temperatures are typically set between 360°F and 575°F
Step 2: Injection
- Once sufficiently melted, the screw acts as a plunger to force the molten HDPE through the nozzle at high pressure
- The plasticized HDPE flows through sprues and runners into the closed mold tool
- The mold is usually kept between 50°F to 250°F depending on the specific resin grade
Step 3: Packing
- Additional molten plastic is packed into the mold under pressure to account for material shrinkage
- Packing pressure is maintained until gate freeze off
Step 4: Cooling
- Cooling channels circulate chilled water or oil to extract heat and solidify the molded components
- Once adequately cooled, the mold opens automatically via tie bars and ejector pins remove the parts
Step 5: Ejection
- Ejector pins push the finished HDPE parts out of the mold
- Robotic arms, conveyors or handlers remove parts for trimming and secondary operations
- The mold closes and the injection molding cycle repeats
Maintaining proper processing parameters like temperature, pressure, cooling rate and cycle time is key to producing high quality HDPE injection molded components.
HDPE Injection Molding Applications
Thanks to its versatility, HDPE injection molding is used across a broad range of industries.
Here are some of the most common applications:
Automotive – Fender liners, truck bed liners, mudguards
Construction – Pipe fittings, plastic lumber, traffic barriers
Industrial – Pallet crates, drums, handles, rollers, spacers
Consumer – Caps and closures, bottles, housewares, toys
Medical – Diagnostic device housings, biopsy forceps
And many more! Almost any application that calls for a rigid plastic can leverage the benefits of HDPE injection molding.
Now let’s take a look at some design tips and best practices when injection molding HDPE plastic…
Tips for Designing HDPE Injection Molded Parts
To avoid manufacturing defects and produce high quality HDPE molded components, keep these design guidelines in mind:
Wall Thickness
- Uniform wall thickness preferred
- Gradually transition thick and thin areas
- Typical range: 1.5mm – 5mm
Draft Angles
- Include minimum 2° draft on all vertical walls
- Increase draft to 4-6° for deep cavities
Radii
- All internal radii should be ≥ wall thickness
- External radii should be 0.4 x wall thickness
Ribs and Gussets
- Add thickness to prevent sink marks
- Allow proper mold venting
Textures and Logos
- Minimum depth around 0.1mm
- Polish or sand blast to reduce ejection force
Also, providing your manufacturer with a proper CAD model makes part production much more efficient compared to using drawings alone.
Pro tip: Work with your injection molder during the part design phase to leverage their molding expertise. They can provide design for manufacturing (DFM) feedback to ensure your HDPE parts meet requirements while avoiding processing issues.
The Bottom Line
HDPE injection molded components offer the perfect balance of performance, customization, and value across a wide spectrum of applications.
HDPE is a remarkably versatile thermoplastic that can be injection molded into durable and complex parts in virtually any industry. With excellent strength, chemical resistance and cost-effectiveness, it’s easy to see why HDPE is one of the most molded plastics globally!
We hope this guide gave you a helpful overview of using HDPE plastic resin in the injection molding process. Reach out if you have any other questions about the capabilities or advantages of HDPE injection molding!
