Injection moulding is a versatile manufacturing process that produces rather complex components with very high precision and minimal scrap. Its two main sub-processes are two-shot injection moulding and overmoulding. However, individuals often confuse these terms as they are used interchangeably. Product designers and engineers should know the differences between them for choosing the proper process with respect to material compatibility, design needs, and production scale.
Here, let us take a look at what sets two-shot injection moulding (which includes double-shot or multi-shot injection moulding) apart from overmoulding, and what benefits or limitations the two bring.
What is Two Shot Injection moulding?
Two-shot injection moulding is a process in which two different materials or colours are moulded into a single part using one machine, one mould cycle, and two sequential injection steps. It is also sometimes referred to as double-shot injection moulding or multi-shot injection moulding.
The injection of the initial material fills a cavity in the mould, created for a substrate. After cooling and solidification, the mould either rotates or is moved to a second cavity (usually with a rotary platen or a robotic arm) so that the second material can then be injected over or around it to create the completed part. Bonding takes place either mechanically or chemically at this time.
Key Examples
- Power tools handles
- Seals and gaskets
- Casing two colours
- Hinges and snap-fit assemblies
Benefits of Two-Shot moulding
- High accuracy and repetitiveness
- Mechanical bonding plus strength in bonding materials
- Avoids secondary operations and assembly
- Allows for complex geometries as well as multi-colour parts
- Economical to manufacture even in mass production runs
Disadvantages
- Higher costs of investments for tools and machines
- Mould design should be more convincing, and control of tolerances important.
- Most materials that are compatible with each other are limited to those that bond well within the same cycle.
With that in mind, let’s dive into the complete injection moulding process and how it affects quality and cycle time in parts.
What is Overmoulding?
Overmoulding involves moulding over a substrate that was already moulded or processed before with one relatively hard plastic or metal material into another material. Unlike two-shot injection moulding, overmoulding usually uses two separate cycles or even different machines.
The substrate is put into a second mould, and the overwound material is injected around it. When the moulding process is completed, that part will have an overall structure with both materials bonded together.
Common Applications
- Ergonomics grips
- Medical devices.
- Domestic trim and harness power electronic parts
- Wearable equipment and military gear
Overmoulding Advantages
- Best suited for low to medium volume production materials
- Cheaper tooling than that for two-shot moulding
- Compatible with most conventional injection moulding machines
- Best for soft-touch or non-slip surfaces
Disadvantages
- Weak bonding strength compared with the two-shot method
- Careful materials compatibility planning is required
- Slightly less precision in complex geometries
- More handling can increase cycle time
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Key Differences Between Two-Shot Moulding and Overmoulding
| Feature | Two Shot Injection Moulding | Overmoulding |
| Materials | Injected sequentially in one cycle | Injected separately, often in different machines |
| Bonding | Strong chemical or thermal bond | Chemical or mechanical bonding |
| Equipment | Specialized two-shot machine | Standard injection moulding machine |
| Production Volume | Best for high-volume | Best for low-to-mid volume |
| Precision | High precision and alignment | Slightly lower tolerance |
| Cost | Higher initial cost, lower per-part cost at scale | Lower setup cost, higher per-part cost |
Which One Should You Choose?
Production requirements would primarily govern the decision between two-shot injection moulding and overmoulding:
Two-shot injection moulding should be invoked when a high volume of production is required for tolerances that are much tighter on the parts and designs requiring utmost precision for multi-material integration.
On the other hand, overmoulding is being preferred where the volume is low to medium, or it is done on metal substrates, or some ergonomic features are introduced to a rigid part.
Also, from a general perspective, material compatibility, tooling costs, and aesthetic requirements from any of these processes should be taken into account. With this basic understanding of the pros and cons of injection moulding in general, you can better pinpoint which among these subprocesses matches your goals.
Conclusion
In today’s product-development processes, both two-shot injection moulding and overmoulding are being increasingly employed as techniques. While both target the integration of materials functionally and aesthetically, they are nevertheless different in their approach.
If you work with an injection-moulding partner with vast experience in the technical aspects, this will provide you with certainty about the successful outcome and guide you toward the most efficient solution for your product.
Ready to Make the Right Choice for Your Mold?
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