Introduction
Imagine a car bumper that absorbs impact like a sponge on the outside but stays rigid and strong underneath. Or a smartphone case that feels soft in your hand yet protects the device like armor. These aren’t compromises—they’re the result of co-injection molding.
Also called sandwich molding or two-component injection, co-injection molding injects two or more materials into a single mold—one after the other—to create a part with distinct layers. Typically, an outer skin provides aesthetics, texture, or chemical resistance. An inner core delivers strength, stiffness, or cost savings.
At Yigu Technology, we’ve used co-injection to solve problems that single-material molding simply couldn’t. In this guide, we’ll explain how the process works, where it delivers the most value, and how you can decide if it’s right for your next project.
How Does Co-Injection Molding Work?
A Simple Breakdown
The process happens in three carefully timed stages. Each stage builds on the previous one.
Step 1: Injecting the Skin Material
First, the skin material enters the mold cavity. This material forms the outer layer of your part. It determines surface appearance, texture, and resistance to environment or chemicals. Injection pressure typically ranges from 50 to 100 MPa, depending on material and mold complexity.
The skin material doesn’t fill the whole cavity. Instead, it flows along the mold walls, creating a hollow shell. Think of it like inflating a balloon—the material pushes outward, leaving an empty space inside.
Step 2: Injecting the Core Material
Next, the core material injects into the center of that hollow shell. The timing is critical. If the core injects too early, it mixes with the skin. If too late, the skin may have cooled and won’t bond properly. Injection speed for the core is often 30 to 60 mm per second—slightly slower than the skin to avoid breakthrough.
The core material fills the interior space. This material provides the bulk of the part’s structural properties. It can be a different color, a different material, or even a recycled grade that wouldn’t look good on the surface.
Step 3: Final Skin Encapsulation
Finally, a second shot of skin material seals the core completely. This ensures no core material shows through on the surface. It also reinforces the bond between layers.
When the mold opens, you have a single part with three layers: skin, core, and skin again. The core is fully encapsulated, invisible from the outside.
What Equipment Does Co-Injection Require?
Specialized Machines
Co-injection demands an injection molding machine with at least two injection units. Each unit feeds a different material. The machine coordinates their timing with precision.
Small machines—with clamping forces around 50 to 100 tons—work well for electronics components and medical parts. Large industrial machines exceed 1,000 tons of clamping force and produce automotive bumpers or appliance housings.
Complex Molds
Molds for co-injection are more sophisticated than standard molds. They contain multiple flow channels that guide each material to the right place at the right time. The mold must also handle sequential injections without cross-contamination between materials.
Most co-injection molds use hardened steel—often P20 or 718 grades—to withstand repeated high-pressure cycles. Polished cavities ensure the outer skin comes out with a clean, consistent finish.
Why Choose Co-Injection Over Traditional Molding?
Enhanced Product Performance
Layered construction allows properties that no single material can offer.
| Property | Single-Material Molding | Co-Injection Molding |
|---|---|---|
| Impact resistance | Uniform throughout | Soft outer layer absorbs shock; rigid core prevents penetration |
| Surface feel | Limited to material choice | Soft-touch skin possible without sacrificing structural strength |
| Chemical resistance | Entire part must resist | Skin provides resistance; core can be a different, less expensive material |
| Thermal insulation | Uniform conductivity | Skin can insulate; core can conduct heat away |
Real-world example: A Yigu Technology client producing industrial equipment housings needed a part that resisted harsh cleaning chemicals on the outside but remained lightweight and affordable. Single-material options forced a compromise. Co-injection allowed a chemical-resistant skin over a glass-filled nylon core. The result: a housing that lasted three times longer than the previous design.
No comments:
Post a Comment