Contents
Introduction
Imagine a power tool that feels comfortable in your hand, doesn’t slip when you sweat, and survives drops on concrete. Now imagine that same tool costs less to manufacture than older versions. That’s the power of over molding.
Over molding—also called two-shot or multi-shot molding—combines two or more materials into a single, integrated part. Typically, a rigid substrate (like ABS or polycarbonate) forms the structural core. Then, a softer material (like thermoplastic elastomer or silicone) bonds directly onto it. The result? Parts that perform better, last longer, and feel more premium.
At Yigu Technology, we’ve used over molding to solve design challenges across industries—from medical devices to automotive interiors. In this guide, we’ll walk you through how the process works, why it outperforms traditional methods, and where it delivers the most value. Whether you’re designing a new product or improving an existing one, understanding over molding can open doors you didn’t know existed.
What Exactly Is Over Molding?
A Simple Definition
Over molding is a manufacturing process where one material is molded directly onto another. The first material—called the substrate—is formed using standard injection molding. Then, a second material is injected over it in a separate mold or cavity. The two materials bond chemically or mechanically, creating a single, unified part.
Think of a toothbrush. The hard plastic handle gives you structural support. The soft rubber grip on the back provides comfort and control. That grip isn’t glued on afterward. It’s over molded directly onto the handle during production.
One-Shot vs. Two-Shot Processes
There are two main ways to perform over molding:
| Method | How It Works | Best For |
|---|---|---|
| One-Shot Over Molding | Both materials inject into the same mold but at different times or through different gates. The substrate forms first, then the over mold material flows over it without moving the part. | Simpler geometries, lower tooling costs, moderate volumes. |
| Two-Shot Over Molding | The substrate forms in the first mold. Then the part transfers—often robotically—to a second mold where the over mold material is added. | Complex parts, high-volume production, materials with very different processing temperatures. |
At Yigu Technology, we often recommend two-shot molding for clients who need consistent quality at scale. The upfront tooling cost is higher, but the per-part cost drops significantly once production ramps up.
How Does the Over Molding Process Work?
Step-by-Step Breakdown
The process requires precision at every stage. Here’s what happens behind the scenes.
Step 1: Substrate Creation
First, we create the foundation. The chosen substrate material—say, ABS or polycarbonate—is dried, melted, and injected into a mold. Injection temperatures for ABS typically range from 200°C to 270°C. The mold itself is machined to exact specifications, often with cooling channels designed to shorten cycle times.
Once injected, the part cools and solidifies. Cooling time depends on wall thickness and material, but for small to medium parts, it usually takes 10 to 30 seconds. The substrate then ejects from the mold.
Step 2: Mold Transfer or Cavity Rotation
In two-shot molding, the substrate moves to the second mold or rotates to a second cavity. This step must be precise. Even a 0.1mm misalignment can cause flash or weak bonding. Many modern machines use rotary platens or robotic arms to ensure exact positioning.
Step 3: Over Mold Injection
Now, the second material—often TPE (thermoplastic elastomer) or silicone—injects over the substrate. TPE injection temperatures are lower, typically 150°C to 200°C. The material flows into cavities designed to cover specific areas—like the grip zone on a handle or the sealing edge on a connector.
The bond forms through thermal fusion or mechanical interlocking. If the materials are compatible, the heat from the second shot melts a thin layer of the substrate surface, creating a molecular bond. If not, the mold design includes undercuts or holes that allow the over mold material to physically lock into place.
Step 4: Cooling and Ejection
The combined part cools again, allowing the over mold material to set fully. Then the mold opens, and the finished part ejects—ready for use or minimal post-processing.