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 inject the plastic to form the product and the inserts will be embedded in the plastic tightly.

Type of inserts

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

Why use metal inserts in products?

Metal inserts embeded into injection molded plastic products can improve the structural integrity of plastic components and enhance their functionality. This includes improvements in electrical conductivity or simplifying connections between the products and other components.

How Does Insert Molding Work?

Insert molding works similar with plastic injection molding. use 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.

Steps of Insert Molding:

  • Step 1: Position the instert into 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.  

Insert Molding materials

To choose the proper Insert Molding materials, you need to consider chemical bonding, mechanical bonding, and the product’s wall thickness. Here is some common used insert molding materials.

Thermoplastics materials

The usual thermoplastics in the market for insert molding are HDPE and LDPE which is chemical resistant with high toughness. Another common used thermoplastics is Nylon which is a strong and sturdy plastic type that can resist chemicals.

Thermoset materials

there are two thermoset materials, Phenolic and Epoxy, are used for inert molding. Phenolic are used in highly specialized operations when certain restrictions apply, Epoxy have the best adhesion as well as high resistance to heat and chemicals.


Natural Rubber and Polyurethane are two main Elastomers used for insert molding. Natural Rubbe is used in tire making and various other household appliance manufacturing, Polyurethane has exceptional abrasion resistance, and wear and tear tolerance

Insert molding operation

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 low volume injection molding or when multiple inserts are required for a single part.

Automated loading inserts

Automated loading can lower the unit cost of parts when it is used for large-scale production becasue this method can reduce operating cost and Increase production efficiency.

Advantages of Insert Molding

  1. More robust part when compared to two independently molded parts, It eliminates secondary processes like heat melting, welding, and riveting.
  2. Reducing assembly time and part costs.
  3. Integrating metal inserts into plastic make component stonger.
  4. Less part to part variability.
  5. More design fexibility

Disadvantages of Insert Molding

  1. Plastic may deformed and break as below pictures due to thermal expansion coefficients difference between inserts and plastic.
  2. More complext mold structure lead to a high mold cost and high scrap rate of production.
  3. Labor costs are higher when inserted by hand.

Applications of Insert Molding

Custom insert molding applications are widely use in different industries, with few limitations on material combinations. some of applications 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

Considerations Before Insert 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 modling process:

  1. Types of Inserts: choose the inserts which can achieve 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 you know tha application to select proper plastic materials for the applications.
  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 need to be used.

Design guideline for inset molding  

1.Brass is a commonly used insert material due to its resistance to rust, 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 be conducive to 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.

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.

7. The distance between the insert and the product’s side wall should be at least 1mm to ensure that the mold has a certain level of strength.

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


Insert molding and overmolding are two different processes for molding plastic. The main difference between these two processes is that overmolding means molding plastic layer over an existed plastic component while insert molding means molding plastic around metal component.


Insert molding is used in many applications. 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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