What Is Insert Molding Process: All You Need To Know

There are many plastic products in our lives that have metal parts inside them. these kinds of products are manufactured by Insert molding process which is a special kind of injection molding process. After introducing What is overmolding in our blogs, Today, in this article, we will introduce what is insert molding and how insert molding works.

What is insert molding？

Insert molding is an injection molding process that involves adding inserts, particularly metal, into an injection molded part. inserts are pre-positioned in the injection mold, then the plastic is injected into the mold to form the product and the inserts will be embedded in the plastic tightly.

Types Of Inserts for Insert Molding

Inserts refer to the embedding of metal, glass, wood, fibers, paper, rubber, or pre-formed plastic components into plastic products, metal is the most common insert. Inserts usually are used for fastening and locating plastic parts with other assemblies. some common metal inserts are as below:

• Female Threads
• Male Threads
• Dowel Pins
• Electrical Contacts
• Spring-Loaded Clips

Advantages Of Insert Molding

• Insert molding creates More robust parts when compared to two independently molded parts, It eliminates secondary processes like heat melting, welding, and riveting.
• Reducing assembly time and part costs.
• Integrating metal inserts into plastic makes components stronger.
• No need to make a fixed structure that can give plastic parts more design flexibility.

Disadvantage of Insert Molding

• Plastic may deform and break as below pictures due to the thermal expansion coefficients difference between inserts and plastic.
• More complex mold structures lead to a high mold cost and high scrap rate of production.
• Labor costs are higher when inserted by hand.

Steps for Insert molding process

Insert molding works similarly to plastic injection molding. use the same injection molding materials as traditional plastic molding. The primary difference between traditional injection molding and insert molding is that the metal is inserted into the molten resin before the mold closes.

• Step 1: Position the insert into the mold. this operation can be done manually or with the assistance of robots.
• Step 2: Inject the Molten plastic into the mold.
• Step 3: After Cooling, Open the mold and take out the component.
• Step 4: Quality inspection follows a quality control process.  

How To Put Insert In Mold?

Manual Insert Loading

In a manual insert loading process, an operator inserts components into the mold manually while the molding process is underway. This approach is usually used in injection molding prototyping or when multiple inserts are required for a single part.

Automated Loading Inserts

Automated loading can lower the unit cost of parts when used for large-scale production because this method can reduce operating costs and increase production efficiency.

Considerations before injection molding

Insert molding is a complex process used to combine plastic parts with other materials. Here are some factors you need to consider before you start the inset molding process:

1. Types of Inserts: Choose the inserts that can meet your function requirements.
2. Part Size and Depth: The size and depth of parts will determine how long the molding process will take. 
3. Product Application: When planning a design for insert molding, it is better to know the application to select proper plastic materials.
4. Budget: Evaluate insert costs, operator expenses, and the potential increase in piece price due to added inserts to balance budget and cost.
5. Production Volume: The production volume of plastic molded parts determines whether automated or manual loading needs to be used.

8 Design guidelines for Insert Molding 

1. Brass is a commonly used insert material due to its resistance to rust, and corrosion, ease of processing, and moderate cost. if no special requirement, choose brass as the insert material.

2. Use circular or axially symmetric shapes for inserts, avoiding sharp corners or angles to prevent deformation and breakage.

3. Metal inserts are typically produced through CNC machining or stamping processes. Therefore, the shape of the insert must work for proper manufacturing processes.

4. The extended portion of the insert should be designed as a cylinder for ease of mold processing, as cylindrical holes are the easiest to machine in molds.

5. Inserts should feature sealing protrusions or structures to prevent material overflow during the injection molding process as the below picture shows.

6. To prevent the appearance of ripple-like shrinkage marks on the bottom of the plastic, which can affect the appearance and strength, the minimum distance “T” between the insert’s bottom surface and the plastic wall surface should be greater than D/6, where “D” is the diameter. Here is the picture to show each dimension.

7. The distance between the insert and the product’s side wall should be at least 0.5mm to ensure that the mold has a certain level of strength as below picture.

8. When setting the insert in the boss, to ensure the stability of the insert’s attachment and the strength of the plastic matrix, the insert should extend to the bottom of the boss (while ensuring a minimum bottom thickness), and the top of the insert should have rounded edges. below picture is an example.

Common Examples of Insert Molding Parts

Custom insert molding applications are widely used in different industries, with few limitations on material combinations. some of the applications are as below:

• Medical devices and instruments such as needle hubs and tube valves
• Encapsulated electronic devices
• Electrical plugs
• Threaded fasteners
• Encapsulated bushings, tubes, studs
• Aircraft seats

Conclusion

Insert molding is used in many applications and requires skills. At KUSLA, We have rich experiences of insert molding projects. We provide support during the design phase to ensure your product is optimized for production. Contact us today for your insert molding project.

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