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When are Metal Inserts Required in Injection Moulding?

Metal inserts are typically required in injection moulding under the following circumstances:

Reinforcement: Certain parts or components may require additional strength and rigidity. Metal inserts are used in such cases to enhance the structural integrity of the final product.

Enhanced heat conductivity: When a component needs to dissipate heat efficiently, metal inserts can be used to improve the heat conductivity of the part (i.e. electronic devices).

Threaded features: Plastic materials are not always suitable for directly creating threaded features. Metal inserts provide a reliable solution for creating threads in moulded parts.

Assembly or fastening purposes: Parts may need to be securely attached to other parts or fastened together. Metal inserts provide reliable attachment points or mounting features.

Wear resistance: Plastic materials, while versatile, may not always offer sufficient resistance against wear, friction, or abrasion. By incorporating metal inserts strategically, injection-moulded parts can gain enhanced durability and longevity.

Electrical conductivity: Some plastic parts require electrical conductivity or grounding capabilities. Metal inserts can create conductive paths within the plastic part, ensuring proper electrical functioning and preventing static buildup or electrostatic discharge.

The Various Types of Metal Insert for Injection Moulding

There are different types of metal inserts which can be used in injection moulding, each with its own distinct characteristics and applications:

Threaded Inserts: These inserts have internal or external threads and provide a threaded hole or bolted connection in the moulded part. They are commonly used in applications which require assembly or disassembly.

Heat Inserts: Heat inserts (heat-staked inserts) achieve a strong and permanent connection between the metal insert and the surrounding plastic material. They are heated and then inserted into the mould, allowing the plastic to mould around them as it solidifies. They are commonly used in applications which have high stress or load-bearing requirements.

Press-fit Inserts: Press-fit inserts (interference fit inserts) are designed to be press-fitted into a pre-moulded hole in the plastic part. These inserts have knurls, grooves, or other features that grip the plastic material, providing a secure connection.

Self-tapping Inserts: Self-tapping inserts have cutting or tapping features which allow them to be screwed directly into plastic without requiring pre-drilled holes. They are often used in applications which need strong threads or where disassembly/reassembly may be required.

Key Factors When Choosing What Type of Insert to Use

Material compatibility: The metal insert should be compatible with the material. It should not react chemically or cause degradation or contamination to the injected material.

Thermal conductivity: The metal insert should have sufficient thermal conductivity to facilitate efficient heat transfer during the injection moulding process. This ensures proper cooling and solidification of the part.

Mechanical properties: The metal insert should possess the required mechanical properties to withstand the moulding pressures and forces without any deformation or failure. Factors like strength, hardness, toughness, and wear resistance are important considerations.

Surface finish: The surface finish of the metal insert can impact the final product’s aesthetics, functionality, and ability to release from the mould. It should be smooth and non-porous to prevent defects and achieve the desired surface quality.

Corrosion resistance: If the injection moulding process involves corrosive materials or environments, the metal insert should be resistant to corrosion to ensure long-term performance and durability.

Cost: The cost of the metal insert, including its manufacturing, material, and maintenance, should be considered to ensure it fits within the project’s budget.

Production volume and cycle time: High-production volumes may require more durable and cost-effective inserts, while shorter cycle times may benefit from inserts with better thermal conductivity and efficiency.

Complexity and design flexibility: The complexity and design of the moulded part can influence which type of metal insert is selected. Some designs require specific metal insert configurations or features to achieve desired part geometry, wall thickness, or precision.

It is crucial to consult with materials engineers, injection moulding experts, and experienced manufacturing suppliers to select the most suitable metal insert for the required application.

When to Add the Metal Inserts

Metal inserts can be added to the plastic part during the injection moulding process or afterwards as a post-processing operation.

There are several factors which will influence the decision on when to add these inserts:

Functionality: If the metal insert is crucial for providing strength, conductivity, or specific mechanical properties, it may be better to add it during the injection moulding process to ensure robust integration.

Design complexity: If the part has complex geometries, it may be difficult to add the insert during moulding. Adding the insert as a post-processing option may be more feasible.

Cost: Injection moulding with metal inserts can increase tooling and manufacturing costs, especially if specialised equipment or techniques are required. Post-processing options may be more cost-effective if they can achieve intended functionality without significant expense.

Production volume: For high volumes, it may be more efficient to add the metal inserts during the moulding operation, as this can be achieved in a streamlined, automated manner. For low volume production or rapid prototyping, post-processing may offer greater flexibility.