1. What Is Insert Molding?
An insert is an item embedded in plastic, such as metal, glass, wood, fibers, paper, rubber, or a pre-molded plastic component. Metal is the most common insert material. Insert molding is a process in which an insert is first secured in the appropriate position inside an injection mold. Plastic is then injected into the mold. After the mold opens, the cooled and solidified plastic encapsulates the insert, producing a part containing features such as threads or electrodes.
In insert molding, inserts can be placed in the mold in two ways:
1) Manual placement
which significantly increases the injection molding cycle time and is suitable for small production runs or products with complex structures;
2) Automated placement
which is better for mass production, helping reduce human error, improve efficiency, shorten cycle time, and increase product reliability.
2.Advantages and Disadvantages of Insert Molding2.1 Advantages of Insert Molding
1) Combining the moldability and elasticity of plastic with the rigidity, strength, and heat resistance of metal makes it possible to manufacture robust, intricate, integrated metal-plastic products while reducing product size and weight.
2) Combining the electrical insulation of plastic with the electrical conductivity of metal enables molded parts to meet the basic functional requirements of electrical products. It can also meet special requirements such as magnetic conductivity, wear resistance, and fastening.
3) Adding metal inserts to plastic can increase the strength of the plastic part.
4) Insert molding provides greater design flexibility and creates new possibilities for combining plastic, metal, and other components.
5) It can eliminate secondary processes such as heat staking, welding, and riveting, potentially reducing assembly time and cost.
6) Inserts are not limited to metal; they may also include fabric, paper, wires, plastic, glass, wood, coils, electrical components, and molded plastic parts.
7) For rigid molded parts and flexible curved parts on rubber sealing sheets, molding the parts together into an integrated product can eliminate the complicated task of arranging sealing rings and make subsequent automated assembly easier.
8) Because molten plastic bonds with metal inserts, the clearance around metal inserts can be designed smaller than with press-fit methods, improving product reliability and making the product more likely to pass vibration tests.
9) With suitable plastic materials and molding conditions, even parts that are prone to deformation or damage, such as glass, coils, and electrical components, can be sealed and secured in plastic.
10) With an appropriate mold design, an insert can be completely encapsulated in plastic.
11) By combining vertical injection molding machines with robots and insert-arranging devices, most insert-molding operations can be automated.
2.2 Disadvantages of Insert Molding
1) Insert placement often makes the mold structure more complex, lengthens the molding cycle, increases manufacturing costs, and makes automation more difficult.
2) Because the coefficients of thermal expansion of the insert and plastic differ, residual stress may develop in the finished product and cause cracking. This issue is particularly noticeable when molding nut inserts.
3) The difference in thermal expansion between the insert and plastic can also cause the product to warp or deform.
4) Inserts, especially nut inserts, often need to be preheated or dried to reduce residual stress.
5) Inserts must be securely fixed in the mold; otherwise, the impact of the molten material may cause them to shift or deform.
6) If defects occur during insert molding—for example, incomplete molding, missing inserts, or incorrect insert positioning—the entire product may need to be scrapped, resulting in significant losses.
7) Insert molding makes product recycling and disposal more difficult.
3.Applications of Insert Molding
Insert molding is widely used in industries such as automotive, medical, electronics, and connectors.
4.Design Guidelines for Insert Molding
When designing for insert molding, the characteristics of the process must be fully considered and the design should follow Design for Manufacturing (DFM) principles.
The guidelines include:
1) Insert material selection. Copper, aluminum, steel, hard dissimilar plastic parts, ceramics, glass, and plastics can all be used as insert materials. Brass is commonly used because it is corrosion-resistant, easy to machine, and moderately priced.
2) Insert shape. Use circular or axially symmetrical shapes wherever possible. Avoid sharp corners and acute angles to promote uniform shrinkage and prevent local stress or stress concentrations.
3) Apply DFM principles to the insert itself. Metal inserts are manufactured by machining or stamping, so their geometry must be suitable for the manufacturing process.
▲ DFM design of the insert itself
4) To facilitate placement and positioning in the mold, the portion of the insert that protrudes (the part located in the mold) should be cylindrical, since round holes are easier to machine in the mold.
▲ Design of the protruding portion of the insert
5) Inserts should include sealing shoulders or similar features to prevent plastic from flashing during injection molding.
▲ Design of the insert shoulder
6) To prevent the plastic beneath the insert from becoming too thin and developing wavy sink marks that affect appearance and strength, the minimum distance T from the bottom of the insert to the plastic wall should be T > D/6.
▲ Distance between the insert and the bottom surface
If the distance is too small, the design shown below can be used.
▲ Maintaining uniform wall thickness
7) The clearance between the insert and the product sidewall must not be too small, so that the mold retains sufficient strength.
▲ Clearance between the insert and the sidewall
8) When an insert is placed in a boss, it should extend to the bottom of the boss (while maintaining the minimum bottom thickness) to ensure a secure bond and sufficient strength in the plastic body. Round the head of the insert.
▲ Insert in a boss
9) Small cylindrical inserts can be anchored in the plastic body using a central slot or diamond knurling on the surface. The knurl grooves should be 1–2 mm deep.
▲ Securing small cylindrical inserts
10) Plate- or sheet-shaped inserts can be secured using holes or windows. For thin inserts (less than 0.5 mm thick), notches or bending are recommended.
▲ Securing plate- or sheet-shaped inserts
11) Rod-shaped inserts can be secured by flattening, notching, bending, or splitting the end, or by flattening the middle section of a round rod.
▲ Securing rod-shaped inserts (deformed end)
▲ Securing tubular inserts (deformed middle section)
12) For stamped tubular inserts, a raised bulge can be formed during stamping to improve retention.
▲ Securing stamped tubular inserts (deformed middle section)
13) In connectors, ensuring that terminal inserts are firmly anchored in plastic is a key consideration for product design engineers. Common methods to greatly improve terminal retention include:
- Punching holes in the terminal embedded in the plastic
- Giving the terminal an irregular profile
- Bending the terminal
- Adding surface roughness or raised/recessed features to the terminal
- Increasing the plastic wall thickness
▲ Terminal inserts with holes, irregular profiles, and bends
5. Design Guidelines for Nut Inserts
Nut inserts are among the most common types of inserts. The nut insert and its corresponding plastic boss must be designed correctly; otherwise, quality problems such as insufficient pull-out force or boss cracking may occur.
5.1 Three Methods of Installing Nuts
There are three methods for embedding nuts in plastic parts:
1) Heat-staked nuts
Heat staking is the most common installation method and is generally performed using a heat-staking machine or a manually operated soldering iron.
2) Insert-molded nuts
This method requires very tight control of the hole diameter in the plastic boss, typically within 0.05 mm. The nut is held in place by a molding pin and positioned in the injection mold, so the hole diameter must be controlled according to the size of the pin on the injection molding machine.
3) Ultrasonic insertion
Ultrasonic vibration creates friction between the nut and the workpiece, raising the temperature at the interface. When the temperature reaches the softening temperature of the workpiece, the nut is embedded in the plastic part. Once vibration stops, the workpiece cools and solidifies under pressure. Each method has advantages and disadvantages; this article focuses on insert-molded nuts.
5.2 External Geometry of Insert Nuts
The external geometry of insert nuts is specifically engineered to create strong mechanical interlocking with plastic materials. Key features typically include diamond or straight knurling to maximize anti-torque and pull-out resistance. Longitudinal grooves or undercuts are often added to prevent rotation. A chamfered guide at the bottom allows for easy and accurate placement into the mold or pre-molded hole. Additionally, a top flange or collar limits insertion depth and provides axial support. Some designs feature a sealing ring to prevent molten plastic from overflowing into the threads. Together, these geometric features ensure a highly reliable and durable connection.
5.3 Examples of Insert Nut Specifications
5.4 Plastic Boss Design and Nut Selection
1) Basic dimensions of the nut and plastic boss
- Dimension d is the base of the nut, also called the guide/locating section. Before insertion, it fits with section C of the plastic boss. Therefore, dimension d at the bottom of the nut should be smaller than the inner-hole diameter C of the plastic boss to facilitate positioning.
- Dimension D is the outer diameter of the nut and corresponds to the inner-hole diameter C of the plastic boss. For mobile-phone nuts, the plastic hole diameter is generally about 0.25–0.3 mm smaller than the nut’s outer diameter.
- Dimension L is the length of the nut and corresponds to the hole depth Y in the plastic boss. The plastic hole is generally designed 0.5–1.0 mm deeper than the nut length to provide space for excess plastic.
- W is the plastic wall thickness around the hole. The wall thickness of a plastic boss is generally at least 0.8–1.0 mm. Larger nuts require thicker walls. See the parameter table below.
| Thread | Outer Diameter D | Length L | Plastic Boss Hole | Plastic Wall Thickness W | |
| Diameter C | Depth Y | ||||
| M1.2*0.25 | 2.3 | 2.0 | 2.0 | 3.0 | 0.8 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| M1.2*0.25 | 2.5 | 2.0 | 2.2 | 3.0 | 0.8 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| M1.4*0.3 | 2.3 | 1.8 | 2.0 | 2.8 | 0.8 |
| 2.0 | 3.0 | ||||
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| M1.4*0.3 | 2.35 | 2.0 | 2.1 | 3.0 | 0.8 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 4.0 | 5.0 | ||||
| M1.4*0.3 | 2.5 | 2.0 | 2.2 | 3.0 | 0.8 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| M1.4*0.3 | 2.7 | 2.0 | 2.3 | 3.0 | 0.8 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| M1.6*0.35 | 2.5 | 1.8 | 2.2 | 2.8 | 1.0 |
| 2.0 | 3.0 | ||||
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| 4.0 | 5.0 | ||||
| M1.6*0.35 | 2.7 | 2.0 | 2.3 | 3.0 | 1.0 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| 4.0 | 5.0 | ||||
| M1.6*0.35 | 3.0 | 2.0 | 2.6 | 3.0 | 1.0 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| 4.0 | 5.0 | ||||
| M1.7*0.35 | 3.0 | 2.0 | 2.6 | 3.0 | 1.2 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| 4.0 | 5.0 | ||||
| M1.8*0.35 | 3.0 | 2.0 | 2.6 | 3.0 | 1.2 |
| 2.5 | 3.5 | ||||
| 3.0 | 4.0 | ||||
| 3.5 | 4.5 | ||||
| 4.0 | 5.0 | ||||
5.5 Common Problems and Solutions
1) Correct sizing of the hole in the plastic boss is critical. If the hole is too large, the plastic will not adequately engage the nut after embedding, resulting in insufficient torque and pull-out resistance. If the hole is too small, flash or cracking may occur.
2) If the nut dimensions and plastic dimensions are correct but problems persist, consider improving the design.
Because the plastic boss is shallow, using a double-oblique-knurl nut can result in a short knurled section after the nut dimensions are proportioned. Once embedded, too little plastic engages with the knurling, leading to insufficient torque and pull-out resistance. It is therefore generally recommended that the plastic boss hole be at least 2.5 mm deep and the nut be at least 2.0 mm long.
Improvement: If modifying the mold is difficult, consider changing the nut knurl pattern to a single-oblique BS1 pattern, as shown on the right. This increases the knurl step height and the amount of plastic engaging with the knurling, thereby improving torque and pull-out resistance.
3) Flashing or cracking around the hole in the plastic boss
When the nut style shown on the left is used, areas A1 and A2 expand after the nut is embedded in plastic, while area B contracts sharply. This makes it difficult for the plastic to flow away, and flash can form at the upper and lower ends of the nut, seriously affecting product appearance.
Improvement: Use section C as a guide and locating feature so the nut can be placed securely in the plastic hole, improving efficiency and yield. The guide section at C allows some plastic to be displaced from area A2 while leaving enough plastic for area A1. In addition, the A1 and A2 sections have oblique knurls with a 90° included angle and 45° slopes. Their intersection forms a strong node that resists circumferential torque, greatly increasing torque and pull-out resistance.
